Weather Ready Nation

NWS Des Moines Now on Periscope

By Kenny Podrazik, Journey Forecaster and Brad Small, Senior Forecaster

Periscope is a fairly new feature available through Twitter that was released earlier this year.  It allows you to experience events that you wouldn’t normally be able to experience. In the weather community, this has tremendous potential. You’ll be able to experience a live broadcast of a tornado in western Kansas or large hail in South Dakota or white-out blizzard conditions in northern Iowa. All you need is a Twitter account and you can discover the advantages of using live streaming video via Periscope.

There are two ways to utilize Periscope, as a broadcaster or as a viewer.  If you are broadcasting video, this allows your followers to see what you are seeing in real time as they are immediately linked to your live feed. As a viewer, this provides you with the opportunity to see through someone else’s eyes.

For us at the National Weather Service, we see the benefit of both. The benefit of being a broadcaster allows our followers to see what we’re seeing on radar or maybe take a quick look at a storm survey or even a brief tour of the office.  As a viewer, this has an even more impactful benefit to the National Weather Service.  It allows us to view live broadcasts from our storm spotters and have a better sense of what they are reporting to us. It doesn’t just end at spotters though.  We’ll be able to follow storm chasers’ live feed and see the evolution of a supercell and potentially the formation of a tornado.  This will aid tremendously in warning decision making because there is nothing more beneficial than real time reports on the ground.

We highly recommend that if you have a Twitter account to utilize Periscope if you are out storm spotting, chasing or encounter any interesting weather.  Please utilize the #iawx or #nwsdmx hashtags when you title your Periscope stream  to make it easier for us to track your video.  You can follow us on Twitter and Periscope at @NWSDesMoines.

June 24-25 Raccoon River Flooding Review

Blog post by Kenny Podrazik – NWS Des Moines
SVRDMX-0082

Figure 1

SVRDMX-0098

Figure 2

Two periods of storms affected central Iowa during a 24-hour period from June 24 to June 25, 2015.  The first round of storms occurred during the morning hours on June 24 that affected portions of Iowa along and south of Highway 20. This was the “primer” for significant flash flooding and river flooding that would result from the second round of storms to affect the same area later that evening.  The second round of thunderstorms began to develop over west-central Iowa late in the afternoon and quickly became severe by the early evening. The first Severe Thunderstorm Warning (Figure 1) issued by the National Weather Service in Des Moines was at 6:20 p.m. CDT on June 24 for Guthrie and Dallas Counties.  The final Severe Thunderstorm Warning (Figure 2) expired at 1:50 a.m. CDT on June 25 for Marion and Jasper Counties.  There were multiple reports of large hail that ranged from the size of a quarter (one inch) to hen egg (two inches) throughout the night. Winds to 60 mph caused numerous trees to become damaged and there was even a report of an 80 mph wind gust in Guthrie County early in the evening.  Extremely heavy rain brought widespread flash flooding from west-central Iowa to southeast Iowa, including the Des Moines Metro, late in the evening into the overnight hours Thursday morning. The video below shows the entire second round of storms unfold on radar from the first development of a thunderstorm around 6 p.m. CDT on June 24 to the final drop ending around 2 a.m. CDT on June 25.

Widespread rainfall amounts of three to seven inches were reported after the event, with the highest report of 7.25” near Dawson, Iowa in northwest Dallas County.  From Bagley to Jamaica, Iowa, radar estimated near nine inches of rain fell within this area of the upper reaches of the Raccoon River Basin. The majority of the heaviest rain fell within the Raccoon River Basin (Figure 3).  As a result, major to near record flooding occurred along the Raccoon and Des Moines River Basins, including several tributaries (Figure 4).  In fact, the Walnut Creek recorded its highest crest ever at two river gauge locations. The Clive I-80/35 (CLVI4) and Des Moines 63rd Street (DOSI4) gauges crested at 13.41 feet and 18.82 feet respectively, both breaking the previous records set back on August 9, 2010.  Near record flooding occurred on portions of the Raccoon River, with Van Meter (VNMI4), Des Moines Highway 28 (DMWI4) (Figure 5), and Des Moine Fleur Drive (DEMI4) all cresting at their third highest crest in history. All the flooding on the Des Moines and Raccoon subsided by June 29, 2015.

Storm Total Radar Estimated Precipitation ending 7 am June 25, 2015. The yellow oval received 6-9 inches of rainfall in the headwaters of the Raccoon River Basin.

Figure 3: Storm total radar estimated precipitation ending 7 am June 25, 2015. The yellow oval received 6-9 inches of rainfall in the headwaters of the Raccoon River Basin.

June 2015 crests along central Iowa Rivers.

Figure 4: June 2015 crests along central Iowa Rivers.

Figure 3: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple) on June 26, 2015.

Figure 5: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple horizontal line) on June 26, 2015.

Summer Weather Review

By Craig Cogil, Senior Forecaster

Temperatures:

Temperatures across Iowa were generally favorable this summer with limited bouts of heat and, in most instances, readings at or below normal. Departures were generally one to two degrees below normal with the largest departures along the Highway 30 corridor. The warmest reading in Iowa during the summer was 99 degrees in Sioux City on June 9. This year was the second year in a row that no summertime 100 degree readings were recorded in the state. Otherwise, temperatures were at or below normal for long stretches, especially in early July and again in mid to late August.

Summer temperature departure from across Iowa – generally slightly cooler than normal

Summer temperature departure from across Iowa – generally slightly cooler than normal.

Precipitation:

Moisture was more than sufficient for much of the state during the summer. The exception was across northeast Iowa where deficiencies were common for much of the growing season. Abnormally dry conditions did creep into the northeast by the second half of the summer but the area still received enough rain to keep widespread drought conditions from developing. Farther south and west, thunderstorms were more common with localized heavy rain from June into August. A couple of events produced flash flooding in and around the Des Moines metro area as well as far south Iowa near the Missouri border. Rainfall frequency did decrease into August, but remained common enough to support crop maturation.

Summer rainfall departures – Drier in the far northeast, quite wet south half.

Summer rainfall departures – Drier in the far northeast, quite wet south half.

SummerTable

A Recap of 2015 Central Iowa Tornadoes

By Kevin Skow, Meteorologist Intern

Overall, the 2015 Iowa tornado season has been relatively quiet up through the end of August. A preliminary total of 30 tornadoes have been recorded for the year throughout the state, which is below the average of 46 tornadoes typically seen in a given year. Iowa’s tornado season historically peaks in the months of May and June, though late season outbreaks can occur well into November. With the bulk of the season behind us, here is a recap of some notable tornadoes to strike central Iowa this year.

May 10: Carroll and Calhoun Counties

The first significant tornado of the season to hit central Iowa occurred on the late afternoon of May 10. A small and compact supercell overrode a warm front and, aided by high low-level instability and shear, produced when has been up to this point the longest-tracked tornado of the season for Iowa. This very visible EF1 tornado touched-down at 7:10pm northwest of Lidderdale in northern Carroll County and tracked to the NNE. The broad but thankfully weak tornado made a direct hit on the town of Lake City at 7:32pm, damaging the roofs of a number of homes and businesses, including the school. The tornado continued churning northeastward across rural Calhoun County, hitting several farmsteads along its way, but damage was relatively light. The dying tornado passed just to the west of Rockwell City shortly after 7:50pm and finally dissipated northwest of town at 8:00pm. It travelled for 23.5 miles during the 50 minutes it was on the ground.Radar

The tornado quickly lofted dust and other debris high enough to be sampled by the Doppler radar in Des Moines, 70 miles away and at a radar beam height of 5,000 ft. This shows up as the yellow and green shaded region on the correlation coefficient radar image at right, also provided with a velocity image (left) to show the location of the tornado. This tornadic debris signature reached up to 16,000 ft into the storm! Videos shot of the tornado, as well as satellite imagery of the track in the days afterwards, showed that this tornado was multi-vortex in nature, with the worst damage concentrated in small streaks.

Mid-May to Mid-June

The middle part of May to the middle of June, typically the peak of Iowa’s tornado season, was only characterized by a few weak, but somewhat rare, early morning tornado episodes. The first event early on the morning of May 17 was also responsible for producing a downburst that derailed an 80-car train near Osceola. The five short-lived tornadoes that occurred afterwards were weak but did damage a few farmsteads in Madison and Dallas counties.

AmesJune7

Another early morning tornado event took place between 1 and 2 am on June 7 across Webster, Boone, and Story counties. Four very brief tornadoes touched down along the leading edge of a squall line, one of which moved through far southern Ames and caused tree (photo on right) and light building damage. A squall line was also responsible for strong EF1 tornado on the afternoon of June 20 near Eddyville.

 

June 22: Marion, Lucas, and Monroe Counties

EF3MonroeCountyThe streak of weak tornadoes came to an end on June 22. A powerful supercell spawned a tornado over far southern Marion County shortly after 5:00pm, which then clipped northeastern Lucas County before strengthening and tracking into northwest Monroe County. While it thankfully stayed out over rural areas of the county during its half hour long life, this now 500 to 600 yard wide EF3 tornado did make a direct hit on one farmstead, completely destroying the house and several outbuildings (photo). Thousands of trees were destroyed by this rain-wrapped tornado as it traversed the hilly and wooded regions of Monroe County on its 11 mile track. The tornado lifted eight miles northwest of Albia, but a second EF2 tornado developed just to the west of town and hopscotched through the southwest part of the city, heavily damaging several businesses and homes.

August 2: Adair and Adams Counties

Following a quiet July, a cold front swept through Iowa on the afternoon of August 2. Thunderstorms erupted along the leading edge of the cold front, aided by over 6000 J/Kg of surface based CAPE. Despite the lack of low level or deep wind shear, the cold front generated enough localized spin along its forward flank to produce a very picturesque tornado just after 6:20pm when this broad circulation was stretched by a developing thunderstorm updraft in southern Adair County. With little in the way of steering winds near the surface, the tornado first drifted southwestward for about a mile, then interacted with another storm outflow boundary and turned southeast and strengthened. Widening to 300 yards, the tornado struck the hamlet of Williamson and inflicted EF1 damage to several buildings. Scouring its way southward, the tornado paused for several minutes two miles south of Williamson before turning to the northeast and dissipating. See the satellite image below that shows this unusual path of this tornado. In the 36 minutes the tornado was on the ground, it traveled only 7.5 miles.AdamsCountyTornado

Additional information on 2015 tornadoes for the state of Iowa can be found here!

Earliest Fall Freeze in Iowa – September 1883

On September 8-9, 1883, an unusually cool weather pattern resulted in damaging frost across northern Iowa on September 8.  The very next morning on the 9th, the earliest general fall freeze on record occurred in Iowa and frost was observed nearly statewide. This included frost at Des Moines, even though the low temperature on the 8th was 38 F (Left Image Below). Back then, some sensors may have been placed on top of buildings, resulting in a 1-2 F difference than areas with grass.  But, even in southeastern Iowa at Muscatine, the observer noted that it was ‘quite a severe frost’ with crop damage and a low temperature of 32 F. Likewise at Keosauqua the frost reportedly killed all corn on the bottom lands and the observer lamented that ‘there will be a great deal of soft here this winter.’ Across northern Iowa a hard freeze occurred in some areas, with serious crop damage observed at Concord in Hancock County and a low of 24 F reported at Wolfdale in Woodbury County, where the observer wrote that ‘there will be thousands of bushels of soft corn in this part of Iowa.’ Typically the normal date of the first frost is late September into the first week of October (Right Image Below).

Sept1883

August 2015 Iowa Monthly Climate Review

Temperatures

The statewide average temperature for Iowa in August 2015 was 69.2°F which was 2.3°F below normal (See Figure 1). August 2015 became the 27th coolest August out of 143 years of records for the state of Iowa. This is the 4th consecutive month with the temperatures being below normal. Actually, throughout the month of August 2015, there were no long duration heat waves, just a few isolated days reaching the lower 90s early in the month. The hottest temperature in the state during the entire month was only 94°F at Sheldon on the 2nd. Des Moines recorded 93°F on the same day and that was 1 of only 2 days in which Des Moines reached 90°F or greater. The 2nd time was on the 7th when it reached 90°F on the nose. The coldest temperature throughout the month was 40°F in Spencer on the 25th. The coldest high temperature at Des Moines was 68°F on the 19th and the lowest temperature was 53°F on the 25th and 26th. Waterloo reached exactly 90°F only once and that occurred on the 2nd and its coldest temperature in August was 45°F on the 26th. On the 18th and 19th, Waterloo had back-to-back days of highs remaining in the 60s and from the 18th to the end of the month, only 2 days had a daily average temperatures that were above normal. Waterloo for the month was 2.4°F below normal for a monthly average temperature while Des Moines was 1.2°F below normal for a monthly average temperature. In Ottumwa, it was 2.7°F below its normal monthly average temperature for August.

Figure 1: Average statewide monthly temperature departure from normal for August 2015. Image is courtesy of High Plains Regional Climate Center (HPRCC).

Figure 1: Average statewide monthly temperature departure from normal for August 2015. Image is courtesy of High Plains Regional Climate Center (HPRCC).

Precipitation

The average statewide precipitation was 5.42 inches or 1.22 inches above normal for the month of August 2015 (See Figures 2 to 4). This is the 21st wettest August among 143 years of records and was the fourth consecutive month with above normal rainfall. The average statewide precipitation during the first half of August was 1.30 inches which was 0.92 inches below normal. A few light rain events occurred during the first couple of weeks with only one significant severe weather event on the 2nd and a heavy rain event that transpired on the evening of the 8th into the early morning on the 9th. On August 2nd, a tornado brought EF1 damage to Adair and Adams Counties as it was on the ground for 36 minutes covering 7.54 miles (See Figures 5 & 6). During the 8th and 9th, a swath of heavy rain fell from Webster County through Story, Boone, and Jasper Counties and southeast into Poweshiek County where roughly 2 to 5 inches of rain fell (See Figure 7). Heavy rain also fell over portions of far southwest Iowa on the 8th. The highest preliminary rainfall report was 7.00” in Otho, Iowa in Webster County on August 8, 2015. More widespread rain events occurred during the final two weeks of the month when the state averaged 4.12 inches or 2.14 inches above normal during that time frame. Heavy rain fell on the 17th and 18th over central to northwest Iowa (See Figure 8). Des Moines picked up 2.46 inches on the 18th, well over half of its monthly total of 4.25 inches. However, the most notable heavy rain event occurred on the 28th when a widespread 3 to 7 inches fell over portions of central Iowa with an isolated amount over 9 inches reported in Dayton, Iowa in Southeast Webster County (See Figure 9). A major flash flood on the Skillet Creek affected much of the town of Dayton as a result of the large amount of rain (See Figures 10 & 11). One other notable event that was related to weather was a lightning strike on August 23 that struck a corn stover bale storage unit near Maxwell and set it on fire (See Figure 12). The fire blazed for roughly a full day before burning out and was seen on radar for a good portion of the day on August 23 (See Figure 13).

Figure 2: Total precipitation for the state of Iowa during August 2015.

Figure 2: Total precipitation for the state of Iowa during August 2015.

Figure 3: Total precipitation departure from normal for the state of Iowa during August 2015.

Figure 3: Total precipitation departure from normal for the state of Iowa during August 2015.

Figure 4: Total precipitation percent of mean for the state of Iowa during August 2015.

Figure 4: Total precipitation percent of mean for the state of Iowa during August 2015.

Figure 5: August 2, 2015 EF1 tornado track in Adair and Adams County.

Figure 5: August 2, 2015 EF1 tornado track in Adair and Adams County.

Figure 6: August 2, 2015 EF1 tornado track in Adair and Adams County. Photo is courtesy of Adam Amdor via Twitter (@Amdor_7).

Figure 6: August 2, 2015 EF1 tornado track in Adair and Adams County. Photo is courtesy of Adam Amdor via Twitter (@Amdor_7).

Figure 7: August 8-9, 2015 observed precipitation.

Figure 7: August 8-9, 2015 observed precipitation.

Figures 10 & 11: Major Flash Flooding on the Skillet Creek affected the Golf course and Campground in Dayton, Iowa. Photos are courtesy of James Hobbs and Jennifer Eckert.

Figure 10: Major Flash Flooding on the Skillet Creek affected the Golf course in Dayton, Iowa. Photo is courtesy of James Hobbs and Jennifer Eckert.

Figures 10 & 11: Major Flash Flooding on the Skillet Creek affected the Golf course and Campground in Dayton, Iowa. Photos are courtesy of James Hobbs and Jennifer Eckert.

Figure 11: Major Flash Flooding on the Skillet Creek affected the Campground in Dayton, Iowa. Photo is courtesy of James Hobbs and Jennifer Eckert.

Figure 8: Q3 radar estimated precipitation from August 18, 2015.

Figure 8: Q3 radar estimated precipitation from August 18, 2015.

Figure 9: The observed significant amount of rain on August 28, 2015.

Figure 9: The observed significant amount of rain on August 28, 2015.

Figure 12: Lightning started a fire of a corn stover storage unit near Maxwell, Iowa on August 28, 2015. Photo is courtesy of Melissa Spencer, Story County EMA.

Figure 12: Lightning started a fire of a corn stover storage unit near Maxwell, Iowa on August 28, 2015. Photo is courtesy of Melissa Spencer, Story County EMA.

Statewide climate statistics courtesy of State Climatologist Harry Hillaker: http://www.iowaagriculture.gov/climatology/weatherSummaries/2015/pms201508.pdf
Blog Post by Kenny Podrazik – NWS Des Moines

July 2015 Iowa Monthly Review

Temperatures

The statewide average temperature for July 2015 was 72.2°F or 1.4°F resulting in the 27th coolest July out of 143 years of statewide climate records (See Figure 1). Unseasonably cool temperatures resulted in below normal values during the first week and a half of July (See Figure 2), which was the primary culprit for the entire month being below normal. Des Moines saw 3 days when the average daily temperature was 10-11°F below normal and Waterloo had 4 days in which the daily average temperature was 11-12°F below normal. Low temperatures dipped into the lower 50s over much of central Iowa through the 10th. 44°F was the coldest temperature during the month and occurred in Cresco and Elkader on the 2nd and then again on the 3rd at Spencer. Lamoni and Ottumwa set new record low temperatures on the 8th with minimums of 57°F and 55°F respectively. Both records were previously set back in 2006 and Lamoni’s previous record was 61°F and Ottumwa’s was 56°F. Summer-like temperatures returned during the middle of the month (13th to 20th). The overnight lows were well above normal and was the main reason this period resulted in being above normal for average temperatures (See Figures 3 & 4). One of the hottest days of the month occurred on the 13th when temperatures rose well into the 90s. Des Moines had the warmest temperature during the month when it topped 97°F on the 13th, but just a few days later on the 17th, Bellevue, Lamoni, and Osceola rose 97°F on the 17th. Hot and humid conditions affected the state during the middle of the month when heat index readings of 105°F to 111°F were reported from the 11th to 13th (See Figure 5). Temperatures through the last 10 days were relatively normal with a couple of hot days occurring on the 24th and 28th when heat index readings jumped back to 100°F to 110°F.

Figure 1: Average Maximum Temperatures Departure from Mean during July 2015 shows maximum temperatures were below normal.

Figure 1: Average Maximum Temperatures Departure from Mean during July 2015 shows maximum temperatures were below normal.

Figure 2: Average Temperature Departure from Mean from July 1 to July 10, 2015.

Figure 2: Average Temperature Departure from Mean from July 1 to July 10, 2015.

Figure 3: Average Maximum Temperature Departure from Mean from July 13 to July 20, 2015.

Figure 3: Average Maximum Temperature Departure from Mean from July 13 to July 20, 2015.

Figure 4: Average Maximum Temperature Departure from Mean from July 13 to July 20, 2015.

Figure 4: Average Maximum Temperature Departure from Mean from July 13 to July 20, 2015.

Figure 5: Maximum Heat Index Forecast for July 13, 2015.

Figure 5: Maximum Heat Index Forecast for July 13, 2015.

Precipitation

The statewide average precipitation was 5.71 inches or 1.21 inches above normal which made July 2015 the 19th wettest July out of 143 years of statewide precipitation records (See Figures 6 & 7). However, dry conditions affected the state during the first couple of weeks as the statewide average precipitation was 1.64 inches (See Figure 8). Normal value is 2.28 inches during this time frame. In fact, the first half was so dry that ‘abnormally dry’ conditions crept into northwest Iowa by July 14 according to the Drought Monitor (See Figure 9). The last half of the month however was much more active and the statewide average precipitation was 4.07 inches (See Figure 10). Normal during that period is 2.22 inches. There were a few days during the first half the month where with locally heavy rain occurred. The 6th and the 15th all had locally heavy rain over portions of southern to southeast Iowa, while on the 11th more widespread rain cause some flash flooding over southern Iowa (See Figure 11). The most active week for rainfall was from July 24th to the 28th, with southern Iowa receiving the majority of the rain (See Figure 12). However, northern Iowa had its share of heavy rain on the 24th when Mason City picked up 4.13 inches. Spring Hill, in Warren County, received the month’s heaviest rain on the night of the 28th with 6.32 inches. The monthly rainfall totals ranged from a low of 1.20 inches in Dubuque to a whopping 13.33 inches in Knoxville. Des Moines totaled 7.17 inches which was 2.70 inches above normal and Waterloo totaled 5.56 inches or 1.29 inches above normal. In fact, Des Moines had 7 days with measurable precipitation from July 15-28 and had 3 consecutive days with 1 inch or more from 26-28.

Severe weather was limited during the month of July with just a handful of localized areas that were impacted over central Iowa. In fact, the NWS Des Moines office did not issue a Severe Thunderstorm Warning until July 13! One notable event occurred on July 16 when several reports of funnel clouds were received late in the afternoon in Story County (See Figures 13 & 14).

Figure 6: Accumulated Precipitation during the month of July 2015.

Figure 6: Accumulated Precipitation during the month of July 2015.

Figure 7: Accumulated Precipitation Percent of Mean during the month of July 2015. Southern received the bulk of the rainfall.

Figure 7: Accumulated Precipitation Percent of Mean during the month of July 2015. Southern received the bulk of the rainfall.

Figure 8: Accumulated Precipitation Department from Mean from July 1 to July 14, 2015.

Figure 8: Accumulated Precipitation Department from Mean from July 1 to July 14, 2015.

Figure 10: Accumulated Precipitation Department from Mean from July 15 to July 31, 2015.

Figure 10: Accumulated Precipitation Department from Mean from July 15 to July 31, 2015.

Figure 9: U.S. Drought Monitor showed drought conditions creeping into northwest Iowa.

Figure 9: U.S. Drought Monitor showed drought conditions creeping into northwest Iowa.

Figure 11: Radar estimated precipitation on July 11, 2015 over southern Iowa. A widespread swath of 3-5 inches accumulated with a few locally higher amounts.

Figure 11: Radar estimated precipitation on July 11, 2015 over southern Iowa. A widespread swath of 3-5 inches accumulated with a few locally higher amounts.

Figure 12: 5-Day radar estimated precipitation ending 10:41 p.m. on July 29, 2015 where the most of the heavy rain fell over southern Iowa.

Figure 12: 5-Day radar estimated precipitation ending 10:41 p.m. on July 29, 2015 where the most of the heavy rain fell over southern Iowa.

Figures 13 & 14: Reports of funnel clouds on July 13 near Ames, Iowa were shown on Twitter.

Figures 13: Reports of funnel clouds on July 13 near Ames, Iowa were shown on Twitter.

Figures 13 & 14: Reports of funnel clouds on July 13 near Ames, Iowa were shown on Twitter.

Figures 14: Reports of funnel clouds on July 13 near Ames, Iowa were shown on Twitter.

A few statewide climate statistics were courtesy of State Climatologist Harry Hillaker: http://www.iowaagriculture.gov/climatology/weatherSummaries/2015/pms201507.pdf
Blog Post by Kenny Podrazik – NWS Des Moines

El Niño Impacts in Iowa

Prepared by: Craig Cogil, Lead Forecaster – NWS Des Moines

El Niño conditions currently exist across the equatorial Pacific Ocean and are expected to persist into the upcoming winter. These conditions are characterized by above normal sea surface temperatures near the equator in eastern and central portions of the equatorial Pacific Ocean. While these conditions exist many thousands of miles away from the United States, impacts from El Niño can be observed in the continental United States including Iowa. El Niño and La Niña conditions are determined by the Oceanic Niño Index (ONI) calculated at the Climate Prediction Center. For more information on the ONI, please click here. According to the ONI definition, water conditions meeting and exceeding the 0.5C threshold for 5 consecutive over-lapping seasons would be considered an El Niño.

Impacts to both temperature and precipitation become apparent across the United States, including Iowa when El Niño years are compared to the average conditions. This paper will look at the historical impacts in Iowa from late fall into spring during El Niño years versus the historical average from 1951-2010. During this time, eighteen El Niño events occurred of varying strength. Generally, events that range from ONI 0.5-0.9 are considered weak El Niño’s with moderate events having an ONI of 1.0-1.4. Strong events have ONI values of 1.5 or greater.

Temperatures

The following image shows temperature departures from the long-term average (1951-2010) of all El Niño events from 1950-2014. Temperatures anomalies across Iowa average from around 0.5 to 1.5 degrees above normal during the winter months with northern Iowa having the warmest departure.   However, not every year has above normal temperatures.   For both Des Moines and Waterloo, 8 out of the 19 El Niño years had below normal readings or 42% of the time. In fact, several very cold winters occurred during El Niño episodes including the winters of 1976-1977, 1977-1978 and more recently 2009-2010. In other words, El Niño does not guarantee a warm winter, but tends to shift the probabilities in that direction.

AllElNinoTempDepatureLargeImage

The next image shows the temperature anomalies from the strongest El Nino’s since 1950. An even stronger signal for above normal readings is apparent with Iowa temperatures ranging from about 2.0 to 3.0 degrees above normal. 5 out 7 years were above normal at both Des Moines and Waterloo or 71% of the time. StrongElNinoTempLargeImage

The next two graphs look at temperature departure for Des Moines and Waterloo respectively during El Niño cool seasons. The temperature averages are divided into three different categories: all El Niño events, only moderate and strong El Niños and finally just strong events. These temperature averages are then compared to the 1951-2010 normal average temperatures for the given three month period to come up with the departures for each category. These graphs appear to indicate that as the strength of El Niño increases, there is a greater threat for a warm departure during the winter months with a greater departure in northern Iowa than southern Iowa. The historical frequency of warm winters is also greater for strong El Niños at 71% versus 58% for all events at both sites.

DSMelNinoTempDep

ALOelNinoTempDep

 

The next image indicates that precipitation across the county also has a distinct pattern during El Niño. The southern United States sees above average rainfall along with portions of the High Plains and the East Coast. The Pacific Northwest, the Lower Ohio and Tennessee River Valleys generally see below normal precipitation. Iowa generally sees near normal precipitation in the west with slightly below normal precipitation in the east when all El Niño episodes are considered.

AllElNinoPcpnDepatureLargeImage

The next image below is for strong El Niño cases (ONI > 1.4) only. This indicates a stronger historical correlation for higher precipitation across the southern half of the Midwest. California, the Deep South and the East Coast all continue to have strong indications of above normal precipitation as well. The drier than normal areas remain in the Pacific Northwest and the Ohio River Valley. Iowa certainly trends wetter during the strong episodes with the highest departure across the south.

StrongElNinoPcpnLargeImage

The following graphs are similar to the previous graphs but are for precipitation departures in Des Moines and Waterloo. Both graphs show the precipitation departures for all El Niño events, moderate and strong events combined, and finally only strong events at both sites. A wet departure is seen at both Des Moines and Waterloo in fall and into early winter. However, the departures disappear later in Winter with a negative departure developing. This is especially true in Des Moines the following spring.

The historical frequency for wet conditions in the fall of all El Niño events for Des Moines is 53% and 68% in Waterloo. This jumps to 86% for both locations when considering only strong El Niños. The historical frequency of wetter than normal winters is 47% in Des Moines and 53% in Waterloo for all events. However, when looking at only strong El Niños, the frequency increases to 71% for both sites.

DSMelNinoPrecipDep

ALOelNinoPrecipDep

Quick Facts about El Niño in Iowa:

  • Moderate to strong El Niño events pose a greater chance of affecting Iowa weather compared to weak events.
  • The meaningful impacts in Iowa are most common from fall through spring.
  • Moderate to strong El Niño’s increase the probability for warmer conditions during the winter with the best threat in northern Iowa.
  • Moderate to strong El Niño’s increase the probability for wetter conditions in the fall and to a lessor extent into the winter.
  • Moderate to strong El Niño’s have a greater chance of seeing dry conditions during the spring.  

The upcoming seasonal outlooks for the nation can be found at the Climate Prediction Center website.

For the specific 3 – month outlooks across the nation, click here.

For local temperature outlooks for central Iowa, click here.

For any questions or comments, please contact craig.cogil@noaa.gov

A Look Back at Summer 2015 and A Look Forward to Autumn at Waterloo

Summer 2015, defined by meteorologists as June through August, is now in the books. At Waterloo the average temperature over these three months was 70.0 degrees, which is a little below the normal value of 71.6 degrees. That places this summer in a tie for the 24th-coldest with records going back to 1895, and makes it the 3rd-coldest in the last 20 years. Meanwhile 14.38 inches of rain fell, which is only 0.22 inches above the normal value of 14.16 inches. Remarkably, 4.10 inches of the 2015 total fell in just one day on August 28, making it the 8th-wettest day on record at Waterloo and the 2nd-wettest day in the last 35 years.

The graph below shows the high and low temperatures (represented by a blue bar) for each day this summer, with the normal values (brown band) and record values (red and blue lines) also plotted. Note that for most of the summer, temperatures stayed within or close to the normal range and no daily records were broken.

WaterlooTempDataWhile summer 2015 saw a near-normal total rainfall at Waterloo, the graph below shows that precipitation was actually below normal for most of July and August until the impressive 4.10 inch rainfall discussed above. The graph also shows accumulation for 2014 and 2013, which were a little drier than normal, as well as 2012 which was a very dry year.

ALOPrecipDataThe 14.38 inches of rain that fell this summer, while only slightly above normal, makes it the wettest summer since 2010. Below is a graph of total precipitation for each of the last 30 summers, with 1993 standing out as easily the wettest during that time, and 2012 in particular depicted as remarkably dry.

ALOTotalPrecipLooking ahead to autumn, it is always a time of great transition particularly with regards to temperature. Daily normal temperatures will be falling steadily, accelerating downward from late October through November as depicted in the plot of daily normal and record temperatures below.

ALOTempNormalsExtremesAlong with the falling temperatures comes the question of when the first frost and freeze will occur. As can be seen below, half of the years on record at Waterloo have seen a freeze (temperature of 32 degrees or lower) by October 2nd, while 90 percent of years have seen a freeze by October 16th.

ALOProbabilityFinally, autumn also brings the first appearance of snow. At Waterloo the average date of the first flakes is November 2nd, but snow has fallen as early as September 25th during the remarkable storm of 1942. The average date of the first one inch snowfall is not until November 24th, but it has come as early as October 18th which occurred during the remarkably cold autumn of 1991.

Blog Post by Jim Lee – NWS Des Moines

 

A Look Back at Summer 2015 and A Look Forward to Autumn at Des Moines

Summer 2015, defined by meteorologists as June through August, is now in the books. At Des Moines, the average temperature over these three months was 74.1 degrees, which is right at the normal value. Meanwhile 19.82 inches of rain fell, which is an impressive 6.28 inches above the normal value of 13.54 inches and makes this the 11th-wettest summer on record. Remarkably, seven of the top 11 wettest summers (with records going back to 1878) have now occurred since 1990.

The graph below shows the high and low temperatures (represented by a blue bar) for each day this summer, with the normal values (brown band) and record values (red and blue lines) also plotted. Note that for most of the summer, temperatures stayed within or close to the normal range and no daily records were even approached.

DesMoinesTempData

While summer 2015 was wet at Des Moines with nearly 20 inches of precipitation, the rainfall was distributed pretty evenly over time. The accumulation graph below shows rainfall piling up this summer (green line) as compared to 2014, which ended up with nearly the same total amount largely due to a very wet late August, and 2013 and 2012 which were much drier.

DSMPrecipData

Again, the 19.82” of rain this summer made it the 11th wettest with records going back to 1878 in Des Moines. Below is a graph of total precipitation for each of the last 30 summers, with 1993 and 2008 standing out as easily the wettest during that time, and 2012 and 2013 depicted as remarkably dry. In fact, this summer Des Moines received almost quadruple the amount of rain that it did in either 2012 or 2013.

DSMTotalPrecip

Looking ahead to autumn, it is always a time of great transition particularly with regards to temperature. Daily normal temperatures will be falling steadily, accelerating downward from late October through November as depicted in the plot of daily normal and record temperatures below.

DSMTempNormalsExtremes

Along with the falling temperatures comes the question of when the first frost and freeze will occur. As can be seen below, half of the years on record at Des Moines have seen a freeze (temperature of 32 degrees or lower) by October 14th, while about 90 percent of years have seen a freeze by Halloween.

DSMProbability

Finally, autumn also brings the first appearance of snow. At Des Moines the average date of the first flakes is November 1st, but snow has fallen as early as September 16th during the remarkable storm of 1881. The average date of the first one inch snowfall is not until November 26th, but it has come as early as October 10th which occurred just back in 2009.

Blog Post by Jim Lee – NWS Des Moines