Winter 2019-2020 was much warmer than normal, with the greatest departures in the western portion of our County Warning Area (CWA). The average temperature for December through February at Ontario, Oregon was 7.4 degrees above normal, making it their third warmest winter on record.
Snowfall was generally below normal, mainly because of the unusually warm temperatures, but below-normal precipitation in some areas also played a part.
Most of our forecast area had near normal precipitation for the season. Camas, Twin Falls, Gooding, and Jerome Counties, and the Owyhee Mountains, were drier than normal. Southern portions of Harney, Malheur, Owyhee, and Ada Counties were wetter than normal, as was Baker County, with the notable exception of the Baker Valley, which was significantly drier than normal.
One of the large-scale controlling mechanisms for winter weather in the Northern Hemisphere is the Arctic Oscillation (AO). It is characterized by a counterclockwise circulation of winds around the Arctic, centered at about 55 degrees north latitude. This winter it was in its positive phase, meaning that the winds were strong and consolidated. This tends to hinder arctic air from moving south into the lower 48 states. Most of this winter the AO was strongly positive, especially in January and February. In fact it set all-time records twice in February.
New Daily Records
Dec 21 – McCall – record high temperature of 43 (old record 42 in 1906)
Dec 22 – McCall – record high temperature of 48 (old record 46 in 1940)
Jan 26 – Boise – record rainfall of .74 inch (old record .40 inch in 1970)
Feb 5 – Boise – record snowfall 2.6 inches (old record 2.1 inches in 1948)
Feb 5 – Baker City – record rainfall of .25 inch (old record .16 inch in 2010)
Temperatures were relatively mild, especially in the Treasure Valley. Ontario had its sixth warmest December on record. Baker City had its eleventh warmest.
Aside from portions of Harney County, most of our CWA was drier than normal. Factors contributing to the precipitation deficit included a split in the jet stream, leaving our area between northern and southern branches. Most of the Pacific moisture moved south over the southwest states instead of east over our area. During periods of west or southwest flow over our CWA, some valleys were in the precipitation shadows of mountain ranges. This effect was especially pronounced at Baker City and Mountain Home, downwind from the Elkhorn and Owyhee Mountains. However, precipitation at McCall was enhanced by lifting over the higher terrain to their east.
Despite widespread below-normal precipitation, there were several storms which produced significant snowfalls.
On the 1st and 2nd, 5 to 10 inches of snow fell in southeast Oregon and the mountains of southwest Idaho. Amounts were lighter in the Treasure and Magic Valleys, where only 1 to 3 inch amounts were reported.
On the 8th, heavy snow fell in the mountains of Baker County Oregon and the west central Idaho and Boise Mountains. The greatest reported amount was 13 inches at Atlanta Summit and Deadwood.
On the 12th, Bogus Basin received a 9-inch snowfall. Tamarack ski area got 14 inches, and Brundage accumulated 12 inches. The heaviest amount was 15 inches at Mores Creek Summit.
From the 13th through the 16th there were relatively light amounts of snow across most of the area. Exceptions included 7 inches near Frenchglen in Harney County, 12 inches east of Three Creek in southern Twin Falls County, and 8 inches near Murphy Hot Springs in Owyhee County, all on the 14th.
On the 19th, 6 inches was measured at Council in Adams County and at Placerville in Boise County, while 9 inches fell at Bear Saddle Snotel in Washington County.
On the 20th, 8 inches fell east of Meadows in Adams County and at Halfway in Baker County. Tamarack in Valley County picked up 11 inches.
Temperatures were above normal across nearly all of our CWA, and much above normal in the Treasure and Magic Valleys and southern Malheur County. Boise was +6.2° F above normal for the month – making it the fifth warmest January on record.
Strong westerly flow aloft maintained a progressive pattern which brought Pacific weather systems across our area at frequent intervals. This hindered the formation of long-lasting temperature inversions and discouraged arctic air from pushing south.
A persistent ridge of high pressure off the California coast steered storm systems, including a series of atmospheric rivers, into the Pacific Northwest. Atmospheric rivers are long narrow streams of air carrying large amounts of moisture. They are responsible for most of the water vapor transport outside the tropics.
Most of our region was wetter or much wetter than normal. The mountains of central and northern Idaho and northeast Oregon saw a significant improvement in snowpack.
Snow fell nearly every day in the mountains.
A storm on New Year’s Day dropped heavy snow on the mountains of Baker County and central Idaho. Storm totals included 14 inches at Tamarack and McCall, 15 inches at Brundage, and 5 inches at Bogus Basin.
Strong winds were reported at many locations that day, including gusts of 67 mph east of Van in northern Harney County, and 65 mph south of Rogerson in Twin Falls County. Elsewhere, gusts of 45 to 55 mph were common.
From the 9th and 14th, snowfall was heavy in the mountains. Even in the lower valleys moderate amounts fell.
A storm total of 18 inches was reported at Centerville in Boise County. In Valley County storm totals included 23 inches near Donnelly, 17 inches at Yellow Pine, and 9 inches at Cascade. In Baker County Halfway’s total was 17 inches. Featherville in Elmore County accumulated a total of 36 inches.
Total snowfall at Boise for the six days was 8 inches, more than half the total this winter. The most on the ground was 4 inches on the morning of the 15th.
A couple of places reported strong winds on the 14th, including a gust of 63 mph at Rome in Malheur County and 58 mph at Twin Falls.
February was colder than January at several locations. Normally February is around 5 degrees warmer than January. Still, Februrary was warmer than normal across most of our area.
On the 5th and 6th a warm front, which had entrained tropical moisture from east of Hawaii, brought heavy snow to the mounains and moderate amounts of rain and snow to the valleys.
In Baker County, 7 to 8 inches of snow fell near Baker City, and 12 inches was measured at Sumpter. In Valley County 17 inches fell near McCall and at Yellow Pine. Near Donelly 9 inches was reported. In Adams County New Meadows got over 25 inches. The storm total at Brundage was 40 inches.
A storm on the 16th dropped 4 to 5 inches of snow on Boise County. McCall received only 3 inches. Rain fell in the lower valleys. By far the greatest reported amount was 0.53 inch at Boise, which set a new record for the date.
During the afternoon and evening of the 23rd, gusty northwest winds followed a strong cold front. Some the the stronger gusts reported included 67 mph at Baker City, 77 mph at Wagontire in Harney County, 67 mph at Midvale Hill in Washington County, 53 mph at Burns in Harney County, 56 mph at Black Canyon in Payette County, and 65 mph near Hammett in Elmore County.