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

2014 Boise Weather Stats

2014BoiStats

2014 was the 5th warmest year on record, and the 11th highest yearly precipitation total at ‪Boise since records began in 1877.  There were 28 days in July that were above 90°F, including seven days above 100°F.  The 30-year average for days above 90 °F is 16.  August precipitation was below normal, while locations south and east of Boise saw record August precipitation, especially at Twin Falls and Jerome. October was the second-warmest on record, thanks to 18 days of 70°F or higher (30-year average is 10 days).  Even though November was 4.2°F below normal, there was still a daily record high of 65°F set on Thanksgiving, 3 daily record low temperatures set, the daily precipitation record for November (1.15″, 11/1) and the greatest 2-day snowfall (7.6″, 11/13-14).  The coldest temperatures of 2014 came on November 15th, when the mercury plunged to 1°F, while temperatures nine days earlier reached 69°F (November 6th).

September 2014 Climate Overview

September 2014 precipitation was well above normal for a large portion of southwest Idaho and southeast Oregon, thanks to the first major low pressure system of the season near the end of the month. During September 26 through 29, a closed low moved slowly east across the northern Great Basin and produced widespread rainfall of 0.75 to 1.50 inches from extreme southeast Oregon across most of southwest Idaho.  Portions of the Upper Payette Basin and Upper Boise Basin received over 2.00 inches of rain during the period. Aside from the end of the month system, September was rather dry and warm.

September 2014 Precipitation

September 2014 Precipitation

The temperature anomaly for the month was generally +2°F to +4°F across southwest Idaho and southeast Oregon.

September 2014 Temperature Anomalies

September 2014 Temperature Anomalies

Long-term drought continues to plague much of the area with the worst conditions across southeast Oregon.

October 21, 2014 Drought Monitor

October 21, 2014 Drought Monitor

August Precipitation in Context

August was an uneventful month in Boise, but another story south and east in the central Idaho Mountains and the Magic Valley.  While Twin Falls recorded 5.57″ in August, we don’t have a sufficient climate record to compare with other August years, with climate data back to only 1998.  Instead, let’s take a look at the climate data from Jerome, Idaho, which extends back to 1915.

While we often present historical data in chronological order by year, it can be helpful to view the data in ascending order.  Putting the data in ascending order helps us see how unusual the precipitation was for this area and particular month. Here we look at the precipitation data for Jerome, Idaho from 1915 through 2014.

August Precipitation (1915-2014) at Jerome, ID

August Precipitation (1915-2014) at Jerome, ID

The average precipitation in August for Jerome is 0.29″ and the median precipitation is 0.125″.  The median represents the 50% value at which years are drier or wetter.  An interesting fact from looking at this data is how skewed the data is: 72.4% of the time August precipitation is below normal, and  27.6% of the time August precipitation is above normal.  Thus, if you want to look smart in front of your friends, make a bet with them each year that August will be below average, you will have a 72.4% chance of being right. Notice how the average is skewed by a handful of wet August months in the past, most notably 1968 and of course this year. August 1968 is the only August in history which comes close to the 2014 precipitation record.  August 1968 was also the highest August precipitation recorded in Boise, with 2.37″.

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.

Happy Fourth of July! Hot and Dry weather to continue…

Forecast High Temperatures for the Fourth of July

Forecast High Temperatures for the Fourth of July

Ever wondered about the Fourth of July Weather History across Southeast Oregon and Southwest Idaho? Hot and dry weather will continue through next week.  Here is an illustration of air-masses and upper level moisture for Friday July 4, 2014 and for Tuesday July 8, 2014.  The air-mass temperatures are shaded with cooler being blue and warmer temperatures being red.  Air-mass moisture is shaded by dry in reds and oranges and moist using white, green, blue and pink (most likely rain) to highlight areas of cloudiness.

7/4/14 Weather Map

7/4/14 Weather Map

 

7/8/14 Weather Map

7/8/14 Weather Map

The jet stream will stay well north of our region, thus expect hot and dry temperatures to continue with only high clouds passing by.  Later in the week the Monsoon (sunny mornings, afternoon thunderstorms) begins to move northward as the high pressure shifts over Southern Nevada.

This will bring warmer temperatures as the warmest part of the air-mass moves closer to Southwest Oregon and Southeast Idaho Tuesday into Wednesday  and the possibility of thunderstorms beyond that if the monsoonal moisture is pushed into our area.