637 FXUS66 KOTX 222203 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 303 PM PDT Thu Oct 22 2020 .SYNOPSIS... An early season winter storm will bring snow to portions of the Inland Northwest on Friday. Biting north winds are expected on Saturday with the arrival of unseasonably cold air. Low temperatures in the teens will be common Saturday night and Sunday night potentially breaking records. Next week is expected to be dry and stable with temperatures gradually rebounding close to average by mid to late week. && .DISCUSSION... ...EARLY SEASON WINTER STORM TO BRING AREAS OF MODERATE TO HEAVY SNOW FRIDAY INTO FRIDAY NIGHT FOLLOWED BY MUCH COLDER TEMPERATURES... Tonight: Clouds invading the sky from the north and west late overnight mark the arrival of the next weather system to impact tomorrow more so than tonight with no pops for precipitation. The cold northwest/northerly flow should allow for reasonable cooling overnight before the cloud cover gets increases too substantially to allow for overnight lows to again drop below freezing with widespread 20s and low 30s with a few upper teens thrown in here and there. Winds will remain rather light and prevail mostly out from the east. /Pelatti Friday through Saturday: A tough forecast as a complex early season winter storm moves over the region on Friday into Friday night. An upper trough dropping out of British Columbia and Alberta will pick up a surface low dropping south off the BC coast. The low deepens to around 1007 mb over NW WA Friday afternoon before tracking southeast to near the WA/OR border possibly as far north as Lewiston Friday evening. This keeps the region on the cold side of the storm with low snow levels. Precipitation type though is tricky with many valleys areas near the freezing mark in the afternoon with highs in the mid 30s. Lower dew points today in the teens and 20s will aid in wet bulb cooling as precipitation develops will help keep the snow levels low. Precipitation type: With precipitation arriving Friday morning over North Central WA and the northern valleys when temperatures are at or slightly below freezing snow is expected. With cold temperatures holding into the afternoon an all snow event is expected. Exception is the lowest elevations such as downtown Omak, Chelan, and Wenatchee where afternoon highs may warm into the mid and upper 30s allowing snow to mix with or change to rain. For locations along and south of I-90 precipitation intensity should keep precipitation as mainly snow from Sprague through Spokane to Lookout Pass, possibly mixing with rain in the Spokane Valley. From George to Ritzville southeast through La Crosse, and LC Valley milder temperatures should keep precipitation as mainly rain or rain/snow mix. The palouse is a tougher call with some model solutions showing a potential change to rain. However with this area north of the low track (colder side) and moderate to heavy precipitation intensity, lower snow levels are preferred with most of the recent model guidance keeping precipitation as snow. Snow amounts/highlights: The majority of the Winter Storm Watches have been upgraded to warnings with valley snow amounts generally 3-6 inches except locally 6-8 inches in the Cascades. Slightly warmer temperatures in Spokane Valley should limit amounts to 1-3 inches with the higher amounts above 2000 feet (South Hill, Sunset Hill, West Plains). Will issue a winter weather advisory along the Highway 2 corridor west of Davenport as well for the Wilbur area with 2-4 inches expected. Impacts: The heavy wet snow this early in the season may cause issues with snow loading on deciduous trees leading to downed tree branches and power outages. Morning snow will have a better chance of accumulating on roads in North Central WA and northern valleys given earlier timing of snow onset. In the afternoon the mountain passes will likely continue to accumulate snow while in the lower elevations roads should be mainly wet or slushy with the higher late October sun angle and air temperatures in the low to mid 30s. The big question is will the increasing precipitation rates from Deer Park to Spokane/Coeur d'Alene/Kellogg and the northern palouse with rates as much as 1" per hour allow snow to begin accumulating on roads in the afternoon? This is possible and will continue to be monitored. The better chance of snow accumulation on roads comes Friday evening as the sun goes down and moderate to locally heavy precipitation rates continue with the highest rates focused from Spokane/Coeur d'Alene south through the palouse. As colder air arrives overnight Friday with widespread lows below freezing by Saturday morning, all the wet and slush on area roads will turn to ice where untreated. On Saturday cold and dry northeast winds taking over as a cold Canadian air mass overtakes the area. Some local drifting snow is possible over the North Idaho Panhandle from Sandpoint to Coeur d'Alene with wind gusts up to 40 MPH. Elsewhere north winds will be breezy down the Okanogan Valley and Columbia Basin. High temperatures in most areas will only reach the 30s except low 40s for Wenatchee and Moses Lake areas. JW Saturday night to Thursday: Well below normal temperatures start this period before some moderation Tuesday onward and conditions. It will also be mostly dry, with winds declining into the new work week. Saturday night to Sunday the cold upper trough sags south from the region. In the northerly flow behind it we will see some of the coldest air we have seen in a while. This cold is not unheard of this time of year, but forecast values are about 20-25 degrees below normal and it is possible to see some records broken. Lows are projected to drop into the single digits and teens by Sunday morning. Temperatures warm into the 20s and 30s Sunday afternoon, with a few low 40s toward the deeper Columbia Basin and lee of the Cascades. Winds could also result in some sub-zero wind chills, but at this time most areas do not warrant a wind chill advisory. Some areas that may come close will be the Purcell Trench and over the higher Palouse. The north-northeast winds will continue to blow at a pretty good clip for some areas, particularly down the Purcell Trench from Bonners Ferry south toward the Rathdrum Prairie where some blowing snow will remain a threat even after accumulations have ended and down the Okanogan Valley and over the deeper Columbia Basin. Speeds of 10-20 mph with gusts to 30 mph will be possible, while other locations will see speeds around 7 to 15 mph with gusts to 20 mph. These will be highest Saturday night into Sunday morning, then gradually decrease and become much lighter by Sunday night into Monday. From Monday to Thursday high pressure starts to nose in. A weak wave overtopping the ridge bring limited snow shower chance to the Cascades Tuesday into Wednesday, with some cloud cover elsewhere. Otherwise the weather is dry. Temperatures remain cold Monday, with lows in the single digits and teens and highs in the 30s with some low 40s again in the deeper Columbia Basin. Thereafter temperatures start to moderate back, with lows in the 20s and 30s and highs in the 40s to 50s, especially going into the second half of the week. The other thing that we will have to consider going into next week will be air stagnation. With the cold air and ridging, mixing heights across some of the region may be rather limited (1500 feet or less). However the details are still a bit murky, but stay tuned. /Cote' && .AVIATION... 18Z TAFs: Aviation area between weather systems with some low and mid level clouds hanging over the far east edge of the aviation area with some MVFR conditions and brief IFR ceilings at KLWS and near KPUW as the stratus/fog is slow to dissipate and/or rise there with VFR conditions to prevail otherwise. A significant winter storm is expected on Friday with a mix of rain and snow across the low lands resulting in widespread IFR conditions. && .PRELIMINARY POINT TEMPS/POPS... Spokane 24 35 23 33 12 33 / 0 100 100 10 0 0 Coeur d'Alene 24 35 22 33 12 33 / 0 100 100 10 0 0 Pullman 22 36 25 33 9 28 / 0 100 100 20 0 0 Lewiston 30 43 32 38 19 34 / 0 70 100 30 0 0 Colville 21 34 22 38 8 38 / 0 100 80 10 0 0 Sandpoint 23 34 22 32 12 31 / 0 100 90 10 0 0 Kellogg 27 35 20 29 10 30 / 0 100 100 30 0 0 Moses Lake 26 39 30 42 17 38 / 0 60 70 0 0 0 Wenatchee 33 37 30 41 20 37 / 0 90 80 10 0 0 Omak 30 34 25 39 17 39 / 0 100 90 10 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Winter Storm Warning from 5 PM Friday to 5 AM PDT Saturday for the palouse along and east of highway 195 for Idaho Palouse. Winter Storm Warning from 1 PM Friday to 5 AM PDT Saturday above 2000 feet for Coeur d'Alene Area. Winter Storm Warning from 1 PM Friday to 5 AM PDT Saturday for Central Panhandle Mountains-Northern Panhandle. WA...Winter Storm Warning from 8 AM to 11 PM PDT Friday for Northeast Mountains-Okanogan Highlands-Okanogan Valley. Winter Storm Warning from 5 PM Friday to 5 AM PDT Saturday for the palouse along and east of highway 195 for Washington Palouse. Winter Weather Advisory from 8 AM to 11 PM PDT Friday for the highway 2 corridor across the upper columbia basin for Upper Columbia Basin. Winter Storm Warning from 1 PM Friday to 5 AM PDT Saturday above 2000 feet for Spokane Area. Winter Storm Warning from 8 AM Friday to 1 AM PDT Saturday for East Slopes Northern Cascades. Winter Weather Advisory from 8 AM to 11 PM PDT Friday for Waterville Plateau. && $$