Overview:
This was Spring!? The night of Thursday, April 13, 2000 began the atmospheric collision of two different weather systems over eastern Washington and the Idaho Panhandle. The storm lasted into the afternoon hours of the 14th and produced winter-like amounts of snow, sleet, freezing rain. The Spokane Airport set an April record for 24 hour total rainfall with 1.53 inches between 8:00 a.m. Thursday to 8:00 a.m. Friday.

Some Highlights:
The storm produced a wide variety of weather including sleet, rain and snow. The rain/snow line extended from Metaline to north of Deer Park, WA to Hayden, ID.

Anywhere from 1 to 5 inches of valley snow fell north of this line with north winds gusting to 30 mph between Bonners Ferry and Sandpoint, ID.

Near the rain/snow line a wintry mix of rain, snow and sleet fell.

Heavy amounts of rain fell south of the rain/snow line with some areas just north of Spokane recording over 2.25 inches in a 24 hour period. The heavy rain would cause flooding of small streams and some main-stem rivers later that weekend.

The figure at right: Satellite water vapor picture of Thursday afternoon, April 13th, shows upper level lows/highs and moisture over Washington and northern Idaho.

Meteorlogical Overview:
This was an unusual storm. The most suprising aspect was the cold, Canadian borne air mass that swept down into the northern valleys of northeast Washington and the north Idaho Panhandle by Friday morning. The main north-south valley that runs through the Idaho Panhandle (Hwy 95 from Porthill to Coeur d'Alene) is geographically known as the "Purcell Trench". The Purcell Trench is very effective at channeling these cold air surges.

The cold air was key to producing the myriad of wintry weather. It is not unusual to get cold surges from Canada but many times in the Winter, the cold air will stay east of the Continental Divide in western Montana. This time the the Canadian Rockies were not able to hold the cold air back. The low pressure system off the California coast brought warm, moisture laden air from the south while the cold, low pressure cell over B.C. and Alberta, Canada forced cold air southward into the valleys (illustrated by figure above). Without the continuous feed of cold air, this storm would have been mainly rain in the valleys.

The deeper part of the "wedge" of cold air was in the Purcell Trench from Bonners Ferry to near Athol ID resulting in snow in this area. Between Athol and Coeur d'Alene, mainly rain, freezing rain and sleet fell. Near Spokane, the Purcell Trench becomes more shallow thus affecting the depth of the cold air. Either rain, or rain and snow mixed fell in the higher hills north of Interstate 90 of the Spokane Valley. A very cold rain fell in the lower valley of downtown Spokane.

The figure above: shows warm, moist air from the south riding over cold/dry air from Canada Friday morning. The red, dashed lines overlayed on the water vapor satellite image represent temperatures in degrees Celsius below freezing at a height approximate to 5000 feet above mean sea level..

Surface observations taken at 5:00 a.m. Friday morning show the stark line of the cold front's progress. Notice the strong north wind at Coeur d'Alene (KCOE) and the equally strong south wind at the Spokane International Airport. It is also apparent just how cold the air gets farther north when you look at Kalispell, MT (FCA) and Creston, B.C. Canada (CWJR) with temperatures in the teens and mid 20's respectively.

The frontal boundary would go on to make an impact at the Spokane International Airport in the afternoon where the winds would shift to the east and the temperature dropped 10 degrees in a matter of minutes in the middle of the day!

Recap and comments:
Cold, Canadian air forced its way into northeast Washington and the north Idaho Panhandle and created a wild, wintry mix of weather. This is nothing new to this area of the U.S. What was unusual about this storm is the time of year it occurred. Usually the Inland Northwest is prone to this kind of weather pattern in late December or January. This is not what you would expect in April when the normal maximum temperatures range in the 50s and morning minimums are usually in the 30s. With this storm, some areas maximum temperatures only reached the mid 30s with anywhere between 1 and 5 inches of snowfall in the valleys.

Nevertheless, this was an example of how the combination of mountainous terrain in the Pacific Northwest and a complex weather pattern can work together to present another interesting forecast challenge.

By
Todd Carter
Senior Forecaster, NWS Spokane