Winter 2020-2021 in Review

December

Temperatures were above normal in the mountains and below normal at lower elevations due to temperature inversions.  The entire region was drier or much drier than normal. The driest areas were the central Snake River Valley, the Camas Prairie, and the Boise Mountains.   An upper-level high pressure ridge dominated through the 10th, keeping our region dry. Burns was an exception, where a weak upper level low pressure trough brought light snow on the 6th.  From the 11th through the 16th, a temperature inversion resulted in areas of night and morning fog. A trough from the Gulf of Alaska brought light snow on the 13th and 14th. It was too weak to mix out the inversion, as most of its energy crossed the Intermountain Region well south of our area. 

The inversion finally broke as a stronger trough crossed the area on the 17th, bringing 5-10 inches of snow to the mountains, but only light precipitation in the valleys. An exception was the .55 inch rainfall at Jerome, which established a new record for the date.  Mild westerly flow aloft brought a warming trend from the 18th through the 22nd.  High temperature records were tied or broken at several locations on the 21st and 22nd

City New Record Old Record Year
Baker City 56 55 1972
Burns 53 51 1972
Jerome 58 58 1969
McCall 43 43 2019
Twin Falls (21st) 59 53 2014
Twin Falls (22nd) 50 50 2005

A strong Pacific cold front crossed the area on the 22nd, followed by a 10-20 degree temperature drop. Strong northwest winds gusting to 40-50 mph followed the front. A gust of 57 mph was measured north of Andrews in Harney County.  The trough which followed the front produced light precipitation at lower elevations. Moderate amounts of snow fell in the mountains, including 5 inches at Brundage.  

https://twitter.com/NWSBoise/status/1341508962311688197/

Following this system, a strong ridge kept skies mostly clear through Christmas eve.

A weak trough brought mainly light precipitation on the 25th and 26th. Boise was an exception, where a third of an inch of precipitation fell on the 26th, which included half an inch of snow. A few inches of snow fell in the mountains. 

Another temperature inversion formed on the 26th, and locally dense fog returned to the valleys.  With cold air trapped in the valleys, temperatures failed to reach the freezing mark at many of the usually warmer locations in the 28th and 29th

https://twitter.com/NWSBoise/status/1343759866368675842?s=20

The final trough of the month weakened as it moved inland on the 31st, but it was able to drop several inches of snow on the mountains, including 8 inches at McCall and 6 inches at Tamarack.

DecemberDecemberp

January

Temperatures were above normal across the region, and much above normal in the Snake River Valley and parts of southeast Oregon.  Most of southern Idaho and parts of Baker and Harney Counties in Oregon were drier than normal, while the Treasure Valley and west central Idaho received above normal precipitation.  

https://twitter.com/NWSBoise/status/1345922598413062145

Strong westerly flow across the Pacific was charged with abundant moisture as it swept inland with a warm front on the 3rd, resulting in heavy precipitation. Totals from a half inch to an inch of water were common, falling as rain at lower elevations and snow in the mountains. Bogus Basin accumulated 7 inches.  Strong west winds, mostly in the 40 to 50 mph range, followed a cold front on the 4th.  A gust of 63 mph was measured at Wagontire in Harney County.  On the 5th another warm front brought heavy snow to the mountains. Banner Summit got 14 inches. Bogus Basin and Tamarack received 10 inches. Atlanta also got 10 inches. Bogus Basin got 7 additional inches on the 7th as a trough crossed the area. Little if any precipitation fell at lower elevations from the 5th through the 7thOn the 12th a weak but moist trough embedded in the westerly flow brought more snow to the mountains, while relatively light precipitation fell at lower elevations. At Council 7 inches was measured, and 6 inches fell at Brundage and McCall.  A cold front crossed the area on the 13th, followed by wind gusts of 30-45 mph.

High pressure kept the area mostly dry from the 14th through the 20th.  From the 21st through the 26th, weak troughs moving down the coast from the Gulf of Alaska brought only light precipitation to our valleys and a few inches of snow to the higher elevations.  A record low temperature of 10 degrees was set at Twin Falls on the 25th, breaking the old record of 12 set in 2008.  On the 27th a stronger trough centered over southeast Alaska deepened southward just offshore. Southerly flow aloft on the east flank of the trough was felt at the surface as gusty south to southeast winds, generally in the 40 to 50 mph range. A gust of 68 mph was measured at Trail Gulch, 14 miles east-southeast of Hollister.   Also on the 27th, moderate amounts of snow fell in eastern Oregon and west-central Idaho, due mainly to lifting of the air by the mountains. At McCall 9 inches was measured, and 7 inches fell at Midvale. At Huntington in Baker County 5 inches was reported. 

Januaryp

January

February

February was a month of very active weather. Temperatures were near or above normal at most locations. February is normally warmer than January, but this year it was actually colder than January at Boise, McCall, Mountain Home, and Ontario. It’s not the first time this has happened, but at Boise it was the fourth year in a row.

On average, February is the driest winter month, but this year it was the wettest in the Treasure Valley and eastern Oregon. However, the Boise Mountains, Camas Prairie, and Magic Valley were drier than normal.  The month began with above-normal temperatures under southwest flow aloft ahead of an offshore trough. As the trough moved inland, a cold front crossed our area during the afternoon and evening of the 2nd.  This marked the beginning of a pattern change which would lead to long overdue winter weather for the valleys. Northwest flow aloft developed on the 4th and strengthened during the following days.  On the 5th, a fast-moving trough from the Gulf of Alaska brought a few inches of snow to the mountains, but its main impact was strong west to northwest wind. Gusts of 45-55 mph were common, but by far the strongest wind measured was 92 mph at Soldier Mountain.  Meanwhile on the 3rd, arctic air had plunged south across the Canadian border east of the Rockies. The mountains, and strong northwest winds aloft, kept it out of Idaho. A second invasion of even colder arctic air entered Montana on the 5th. By the 6th, its western margin had stalled along the Montana border.  Meanwhile, the “milder” western portion of the same arctic airmass was drifting south through the mountains of British Columbia. It entered Washington on the 8th and northeast Oregon on the 9th.  The front marking its leading edge reached eastern Oregon and southern Idaho on the 11th.  Moist air streaming inland ahead of an approaching trough was lifted over the cold air north of the front, resulting in snow which became heavier as the trough drew closer on the 12th and crossed our area on the 13th.  Snowfalls of 3 to 6 inches were common in the valleys on the 12th.  Up to 6 more inches fell on the 13th, with heavier accumulations in the mountains. By the time the snow ended, 7 to 10 inches had accumulated in the Treasure Valley. McCall, which nearly always gets way more snow than Boise, also reported a storm total of 10 inches. Totals of 13 inches were measured at both Bogus Basin and Magic Mountain ski areas, while Twin Falls reported only 2 inches.  The 4.4 inches which fell at the Boise airport on the 12th broke the old record for the date of 1.3 inches set in 1966, and the 5.5 inches on the 13th broke the old record of 4.3 inches set in 1995.  

On the 15th another trough from the Pacific brought more snow, with 1 to 4 inches in the valleys and 5 to 10 inches in the mountains. Wind gusts of 40 to 55 mph were observed in Harney County and the Magic Valley.  From the 16th through the 22nd, Pacific systems brought more snow to the mountains but generally light precipitation in the valleys. With afternoon temperatures above freezing, there was little if any snow accumulation in the lower valleys.  On the 23rd, a cold front was followed by a wind event which mainly affected southeast Oregon and areas east of Boise. There were many reports of gusts in the 45 to 55 mph range, but a gust of 67 mph was measured at Twin Falls.  On the 26th, another strong cold front swept across the region. One of the stronger gusts was 59 mph from the west-northwest at the Boise airport. A brief snow squall followed the front, but less than half an inch fell at the airport.  

On the 27th a trough from the Gulf of Alaska brought heavy snow to the mountains. Some of the heavier totals were 12 inches at Mores Creek Summit, 15 inches at Banner Summit, 20 inches at Tamarack, and 22 inches at Brundage.

Winter 2016 and Spring 2017 Flood Summary

This past winter and spring had its share of flooding across southeast Oregon and southwest Idaho.  Not only did spring runoff bring flooding to rivers and streams, but ice jams and snow melt caused flooding during the winter as well.  The stage was being set for an active spring flood season as far back as October 2016, when 150 to 400 percent of normal precipitation occurred across much of the region which moistened the soil profile.  The winter storm track brought well above average snowfall to most of southeast Oregon and southwest Idaho, with extreme snowfall across lower valleys.  A relatively cool and wet early spring was the final piece of the puzzle to ensure abundant spring runoff.  An indicator of how wet this past winter and spring have been, water supply forecasts for the April through September period rank in the top 10 for most of southeast Oregon and southwest Idaho, dating back to 1970.  Additionally, all major reservoir systems either have filled or are expected to fill.  Record high precipitation was seen across many areas from December 2016 through June 2017, shown in the figures below.

pnw_cl (3) pnw_cl (4)

The map below shows March 1 snow pack along with areas where flooding had a significant impact.

flood

Although the threat of snow melt flooding has diminished, summertime thunderstorms can pose a serious flood risk.  Areas of steep terrain and areas burned by wildfire are at particular risk for flash flooding due to thunderstorms.  For flood safety information, visit http://www.floodsafety.noaa.gov/.  For the latest river conditions, see http://water.weather.gov/ahps2/index.php?wfo=boi.

Interesting southwest Idaho snow event

We had a very interesting terrain-driven weather pattern last night. You may have wondered why Bogus Basin didn’t get any snow but Boise did?

No snow at Bogus Basin but snow on Foothills and in Boise.

No snow at Bogus Basin, but snow on foothills and in Boise.

An arctic cold front moved across the region from the Northeast as a low pressure moved across the area into southern California. When this happens air cannot easily get over the Rocky Mountains so it goes through the paths of least resistance, hence it moves in from the northwest via Hells Canyon and Baker Valley in NE Oregon and from SE Idaho via the Snake River Plain.

Low Pressure moving into CA allowed cold air to set up convergence zone in Treasure Valley.

Low Pressure moving into CA allowed cold air to set up convergence zone in Treasure Valley.

Air in the lowest levels of the atmosphere collided, creating what we call a convergence zone.  Terrain-induced northwest winds collided with southeast winds near Boise and Mountain Home, causing lift. This lift essentially squeezed the moisture out of the air…much like wringing water out of a wet cloth.

Southeast winds collided with northwest winds causing a convergence zone along the Boise foothills to Mountain Home east to the Camas Prairie.

Radar Imagery: Southeast winds collided with northwest winds causing a convergence zone along the Boise foothills to Mountain Home east to the Camas Prairie.  Areas in blue and green indicate snow.

Bogus Basin was not subject to the convergence zone as the Boise foothills acted like a barrier, keeping the convergence zone in the Treasure Valley.  The areas where the snow-cover were visible on the MODIS imagery.  Notice the swath of snow along the Boise foothills, east Boise, and areas east of Lucky Peak and north of Mountain Home.

MODIS Imagery showing snow cover on the afternoon of 2/22/2015

MODIS Imagery showing snow cover on the afternoon of 2/22/2015

Severe Weather of August 13, 2014 Summary

Conditions were favorable on August 13, 2014 for large hail, damaging winds and heavy rain across portions of southeast Oregon and most of southwest Idaho.

Conditions were favorable for strong thunderstorms in the highlighted areas.

Conditions were favorable for strong thunderstorms in the highlighted areas.

With an upper level low pressure system situated on the Oregon Coast, the upper level dynamics were favorable for strong upward vertical  motion (thunderstorm development), especially over Idaho as the jet stream diverged from S. Idaho into N. Idaho, as seen in the following image.  Jet stream divergence promotes upward vertical motion.

Upper level Jet-stream at 250MB (34,000 ft)

Upper level Jet-stream at 250MB (34,000 ft)

We look at winds at different levels to examine wind shear, an important factor in strong thunderstorm development.  Wind shear is important because it helps sustain and organize the thunderstorms, which can lead to large hail development.  For more information on how thunderstorms, hail, tornadoes, and lightning form, click here.

A special balloon launch was performed at Noon MDT (18 zulu). Normally, we do a balloon launch at 5am (12 zulu) and 5pm MDT  (00 zulu) daily to get an profile of the atmospheric temperature, humidity and wind up to a height of about 100,000 feet. The atmosphere profile provided from our balloon launches are used in our Numerical Weather Prediction Models to help us and other National Weather Service forecast offices forecast the weather.

Special Balloon Launch at Noon to determine the atmosphere profile.

Special Balloon Launch at Noon to determine the atmosphere profile.

BOI 8/14 00z Sounding

BOI 8/14 00z Sounding

The BOI sounding at 00z on 8/14 revealed high MU CAPE Values (1873), high SFC-6km shear (39kt), high precipitable water (PW) values of 1.12in. These values are favorable for supercell thunderstorms, storms which are favorable for large hail and damaging winds and heavy rain.  The precipitable water values also highlighted a flash flood risk from abundant moisture in the atmosphere.

Storms began to form around 2pm MDT south of our area in Nevada and spread northeastward into the Magic Valley by 3:15pm MDT.

Radar Imagery

Radar Imagery around 3:15pm MDT (Severe Thunderstorm Warnings in Yellow Boxes)

These cells quickly blossomed into severe storms knocking down trees and causing flash floods on McMullen Creek, washing out roads.  These storms produced wind gusts to 82mph at the Trail Gulch RAWS weather station, 8 miles north of Magic Mountain Ski Area.

As the afternoon progressed, more thunderstorms started to form along the southwest highlands of Idaho and the Owyhee Mountains.

Radar Imagery around 4:05pm MDT

Radar Imagery around 4:05pm MDT (Severe Thunderstorm Warnings in Yellow Boxes, Flash Flood Warnings in Green Boxes)

Severe thunderstorms southwest of Mountain Home began to form and move to the north towards SE Boise.  This storm produced wind gusts to 69mph just NW of Mountain Home as it trekked towards Boise.  Another storm by Murphy was moving NE and it was on track to merge with the storm just SE of Boise around 4:30pm MDT. By 5:00pm MDT the storms merged between Boise and Mayfield.

Radar Imagery around 5:00pm MDT (Severe Thunderstorm Warning in Yellow Box)

Radar Imagery around 5:00pm MDT (Severe Thunderstorm Warning in Yellow Box)

Supercell thunderstorm between Mountain Home and Boise (4:45pm MDT)

Supercell thunderstorm between Mountain Home and Boise (4:45pm MDT)

These storms moved into the mountains and produced debris flows across Highway 21, 2 miles southwest of Mores Creek Summit near the Hayfork Campground.  A spotter reported 0.87″ of heavy rainfall within 15 minutes with these storms just north of Boise.

Radar Imagery around 5:30pm MDT

Radar Imagery around 5:30pm MDT

Just after midnight on August 14, the Treasure Valley was treated to heavy rain, small hail and quite the lightning show!

Lightning and Radar Data at  12:25am MDT (Courtesy of Earth Networks)

Lightning and Radar Data at 12:25am MDT (Courtesy of Earth Networks)

Lightning and Radar Data at  1:00am MDT (Courtesy of Earth Networks)

Lightning and Radar Data at 1:00am MDT (Courtesy of Earth Networks)

Ada County reported 94 strikes on 8/13 which ranks as the 3rd highest count in August (since records began in 2000), and 15th highest all-time (since records began in 2000).  These storms produced heavy rain, small hail and abundant lightning throughout the night. Some areas received up to 2″ of rain according to the radar with these storms.

24 hour precipitation accumulation over southern Idaho and southeast Oregon.

24 hour precipitation accumulation over southern Idaho and southeast Oregon.

To recap, due to a unique atmospheric setup for our area, the storms of August 13th and 14th of 2014 produced wind gusts up to 82mph and 69mph in two separate storms, debris flows over Highway 21 in Idaho, flash flooding with some areas receiving up to 2″ of total rainfall and up to 0.86″ of rain fell in under 15 minutes, and a memorable lightning show for residents of the Treasure Valley.