2021 Spring in Review

Temperatures did not depart significantly from normal. Precipitation was below normal region-wide, with a large portion of our area receiving less than 50 percent of normal spring precipitation.


Typical of early spring, March was a month of changeable weather and wide temperature variations.  Temperatures averaged near normal. It was a dry month, with much of the region receiving fifty percent or less of average March precipitation.  On the 5th, southeast winds ahead of a slow-moving cold front raised temperatures to record highs at some locations. The high of 69 at Ontario broke the old record of 66 set in 2012.  The front weakened as it moved inland, bringing only light precipitation.  A second cold front crossed the area on the evening of the 7th ahead of a cold low pressure trough from the Gulf of Alaska. Again only light precipitation fell as the cooling trend continued.  Skies cleared as the cold air moved inland on the 10th, resulting in some of the coldest lows of the month:

Baker City 20
Boise 25
Burns 22
McCall 5
Mt. Home 19
Ontario 22
Rome 16
Twin Falls 19 (11th)

An upper-level high pressure ridge building over the Pacific Northwest brought a warming trend from the 11th through the 14th.

A weak cold front crossed the area on the 15th. After brief cooling on the 16th, high pressure provided gradual warming, resulting in the warmest high temperatures for the season so far.  On the 19th, moderate amounts of rain accompanied a cold front. A record daily rainfall of 0.27 inch was set at Twin Falls.  An upper-level low pressure trough with much cooler air followed the front, but an even colder system was on the way.  On the 22nd, a strong cold front and upper-level trough from the Gulf of Alaska brought more precipitation.  A record daily rainfall of 0.46 inch was set at Boise, including half an inch of snow during the evening. Most of the precipitation was in the form of thunderstorm rain which preceded the snow.  Many locations experienced some the coldest highs of the month on the 22nd and 23rd:

Baker City 49 (23rd )
Boise 46
Burns 49
Jerome 46
McCall 35
Mt. Home 49
Ontario 54
Rome 49 (23rd )
Twin Falls 44 (23rd )

Another cold upper-level trough from the Gulf of Alaska continued the cold wet weather on the 24th and 25th.  An upper-level ridge brought much warmer weather. On the 28th, a record high of 70 was set at Twin Falls, and a record high of 75 was tied at Baker City. But very windy and much colder weather would soon arrive from the Gulf of Alaska.  During the evening of the 28th, a very strong cold front crossed the area.  Northwest winds increased rapidly behind the front, driven by a steep surface pressure gradient and enhanced by a powerful jet stream on the southern periphery of the upper-level trough which followed the front.

A gust of 61 mph was measured at the Boise airport shortly after midnight on the 29th.  No major damage was reported, but a number of shallow-rooted trees were toppled, and there were numerous broken branches. Downed lines left around 23,000 Boise customers without power, and a semi trailer was blown over on I-84 near Gowen Road, blocking most of the west-bound lane for several hours.  Afternoon highs on the 20th were 10-15 degrees below normal, and breezy northwest winds made it feel even colder.  A warming trend commenced during the afternoon of the 31st as a high pressure ridge moved in from the west.

precipitation percent of normal Marchtemperature departure from normal March



April 2021 is memorable for dry windy weather and temperature extremes, although the average temperature for the month was close to normal.  It was an unusually dry April, with about half of the area receiving less than fifty percent of normal precipitation.  A high pressure ridge kept temperatures warm from the 1st through the 4th.  Record highs were set at Twin Falls on the 1st, 2nd , and 3rd with 73, 72, and 75.  On the 3rd Ontario tied a record high of 80, originally set in 1990.  Temperatures were below normal on the 5th and 6th following a cold front on the 4th. Another cold front from the Gulf of Alaska initiated a cool spell which persisted through the 14th. A record low of 17 was set at Baker City on the 7th.  Record lows were set on the 9th at Burns, Ontario, and Baker City, with 14, 22, and 12.  A record low of 15 was set at Burns on the 11th. Record lows were set at Twin Falls and Baker City on the 12th, with 23 and 16.  Early on the morning of the 19th, a dry cold front from western canada crossed the area. After the front passed, northwest winds increased through the day, and gusts around 50 kts were observed at a few locations.  A high pressure ridge near the coast maintained dry weather from the 20th through the 23rd.  The ridge moved inland and weakened on the 24th, allowing moist and unstable air to spread inland ahead of a low pressure trough.

Locally moderate amounts of rain fell with showers and a few thunderstorms on the 24th and 25th:

Baker City .19 .25
Boise .53
Burns .07 .27
McCall .25 .37

Then on the 26th, a disturbance from the Gulf of Alaska moved down the coast and into the trough, generating more rain.

On the 27th, a strong high pressure ridge building inland from the northwest coast kicked the trough south to the Mexican border.  The ridge brought a warming trend which culminated on the 30th.  A record high of 83 was set at Burns on the 29th.  A record high of 86 was set at Twin Falls on the 30th.

precipitation percent of normal April temperature departure from normal April




May continued the dry pattern of April. Precipitation was below normal across most of the area, but some locations in Malheur, Canyon, and northwest Owyhee Counties reported near or above normal precipitation.  This was probably the result of isolated convective showers.  Much of the area had no measurable rain from the 2nd through the 20th.  Cool spells alternated with warm spells, resulting in an average temperature for the month that was very close to normal for most of the area.  There were no record highs or lows.  Early on the morning of the 1st, a Pacific cold front crossed southeast Oregon and southwest Idaho. That afternoon the moist and unstable air which followed the front spawned thunderstorms with gusty outflow winds. Gusts around 60 mph were observed in Ada and Elmore Counties. Damage was minimal, but 5500 customers in north Meridian were without power for awhile.  At Boise, a gust of 62 mph at 3:15 pm MDT, was the strongest gust for May on record going back to 1980.

A high pressure ridge brought a warming trend which started on the 2nd and culminated on the 6th with highs of 89 at Mountain Home, 90 at Boise, and 91 at Ontario.  Highs on the 7th were around 20 degrees lower following a strong Pacific cold front. Highs on the 8th were 10 additional degrees cooler under the cold low pressure trough which followed the front.  Temperatures slowly recovered starting on the 9th, maxing out on the 17th under a high pressure ridge.  Another Pacific cold front crossed the area on the 18th ahead of a cold wet low pressure trough from the Aleutians. By the 20th the trough had settled over the Intermountain Region.  The system produced the first significant rain of the month at Boise and Burns, where nearly half an inch fell on the 22nd. Mountain Home got a third of an inch, but only two tenths of an inch was measured at Ontario. A record rainfall of .32 inch was set at Twin Falls on the 21st, breaking the old record of .23 inch set in 2016.

The trough finally exited into Canada on the 24th, but close on its heels was another trough which brought more rain along with scattered thunderstorms on the 25th and 26th. Around a quarter inch of rain fell at Boise, Burns, and Ontario. Baker City got over a third of an inch, and nearly three-quarters of an inch fell at McCall.  A weak high pressure ridge brought brief warming on the 27th.  During the early morning hours on the 28th, the cold front preceding the last trough of the series crossed our area. No measurable rain accompanied the trough at lower elevations east of the Cascades, but temperatures were 10-15 degrees cooler.  On the 29th a high pressure ridge started a warming trend as it began to build over the region. Temperatures were above normal for the last three days of the month.

precipitation percent of normal May temperature departure from normal May