083 FXUS66 KOTX 061132 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 332 AM PST Sun Nov 6 2022 .SYNOPSIS.... The next winter storm will bring snow into north-central Washington today. Snow will spread eastward into Idaho tonight into Monday morning impacting the morning commute. Brisk north winds with cold wind chills on Tuesday. Cold temperatures with lows down into the single digits and teens will continue through the work week before slowly moderating next weekend. && .DISCUSSION... Today and Monday: Convective rain and snow in NE WA and N ID have mostly moved out of the area. A compact mesoscale shortwave provided forcing for ascent and favorable low level thermodynamics allowed for some beefy convective rain and snow showers with graupel. Radar estimated QPE shows a small bullseye over a tenth of an inch over Schweitzer Mountain. Radar imagery has shown some transient bands of showers in southern WA as well. HRRR/HiRes-FV3 seem to be handling this area of rain and snow showers the best currently with lighter coverage then the ARW. Nonetheless, forecast 0-2 km AGL mean RH, wind and theta-e lapse rates support convective rain & snow showers through 4 AM. Winds have finally calmed down as the atmosphere has decoupled in most locations except the Palouse and Southern WA where low level instability remains. Details with the incoming snowstorm are below. *Strong upper level low to bring significant snow accumulations to Eastern Washington and north Idaho today and tomorrow* Strong synoptic lift and southerly 500mb flow and low level southeast winds will make for a potent snowfall event for north central Washington. One area of great uncertainty is the US-2 corridor through Wilbur, Creston, and Almira. A tight snowfall gradient will occur as southern locations will receive less than an inch while locations just 25 miles north will see accumulations above 6 inches. Wenatchee is seeing higher totals as well, but am not confident in this given the high certainty of the warm conveyor belt keeping them warm enough for lower end accumulations. Winter Storm Warnings look good and will be in effect from 4am Sunday through 10 AM Monday. Omak snow totals have been bumped to better reflect the latest global ensemble guidance around 12 inches. This is still below ensemble means but the warmer temperatures this afternoon will lead to lower snow ratios and compaction of the morning snowfall. Additional highlights are being discussed for sub warning criteria snowfall & flash freezing of rainfall behind the arctic front. Point snowfall forecasts can be found at weather.gov/spokane. Timing: Precipitation will spread east through the day today with a change to rain or a rain/snow mix as temperatures warm above freezing for the Wenatchee area, the Basin into the Spokane area, Palouse and southward. Stevens Pass and Loup Loup Pass will see the snow starting now through the next few hours as coverage increases. Sherman pass will start to see snow by noon or so according to guidance. Snow levels will increase to 1700 feet or so by 4 PM this afternoon but rapidly fall to the surface overnight in the northern counties with the arctic front passage. The transition to all snow will occur late this evening as the front begins to push south. Snowfall will ramp down Monday morning for the Okanogan Highlands and Monday afternoon for northern Idaho. A secondary wave will bring additional accumulations to the Cascades/Okanogan Highlands Monday afternoon. Accumulations are uncertain at this point but an additional couple inches is plausible in the lowlands. Snow intensity: Synoptic lift is strong and HREF guidance is hinting at greater than 70 percent probs of several hours of 1 inch snowfall rates today and tonight. This makes sense given several hours of strong isentropic lift from the 290-300K theta surfaces, favorable upslope flow, and 2+ inches of QPF in 36 hours in north central Washington. The Cascades and Okanogan highlands will first experience these snowfall rates through this morning then the higher elevations of Idaho this afternoon. Highs on Monday will be much colder with the Arctic front passage with highs dropping into the 20s to mid 30s and lows in the teens to low 20s with single digits across our northern valleys. Breezy northeast winds will result in very cold wind chills Monday morning. Lows Monday night will be quite cold in the single digits in N ID to low 20s in the lower Basin. /Butler Tuesday through Sunday: A cold upper-level trough will remain over the Western US Tuesday but the Inland Northwest will remain void of any shortwave energy as the Polar Jet digs southward into California. A robust pressure gradient between low pressure to our south and a cold Canadian high to our northeast will create blustery north to northeasterly winds across the forecast area throughout Tuesday and to a lesser degree...Wednesday. Sustained winds of 10-15 mph with gusts of 25 mph will be common across the Columbia Basin, Purcell Trench, and Okanogan Valley. With afternoon highs only expected to top out in the lower 30s...be prepared for biting wind chills in the teens and twenties during the afternoon then teens to near zero or colder overnight. We will see increasing clouds by Tuesday night as midlevel warm air advection brings an influx of moisture into southeastern WA and the lower Idaho Panhandle. NBM, ENS, GEFS all introduce 30-50% precipitation chances around the Blue Mountains, Camas Prairie, L-C Valley, and into the Palouse and Central Panhandle Mountains. The ENS is the most aggressive model with the precipitation with several members expanding precipitation chances as far north as Spokane-Cd'A and Bonners Ferry. This does not have a lot of support from the other ensemble models but will need to be watched in the coming days as most precipitation types by this point will be snow. Our drier model guidance is barely producing a few hundredths of liquid but several ENS members support a tenth so this could be the difference of a dusting of snow to as much as an 1-1.5" following the wetter ENS. Thereafter, the region will get a well needed break from the active weather pattern as high pressure settles into the Northwestern US from Thursday into Saturday. This stable weather pattern following a few days of wet/snowy weather is a good setup for below normal temperatures within the valleys and potential for low clouds and fog, especially overnight and morning. Models tend to struggle with temperatures in these patterns. Locations with fresh snow that experience clear skies in the evening will likely radiate very rapidly and experience lows in the teens or even single digits. The low November sun angle typically cannot provide enough heating to fully mix out these inversions in the afternoon. However, if there are other layers of clouds present which is a strong possibility, it will be significantly warmer at night but continued cooler than normal in the afternoon with afternoon temperatures remaining in the 30s. All things considered, we see large spreads with the ensemble temperature forecast, sometimes on the order of 8-10 degrees for any individual MaxT/MinT point. Confidence in the forecast begins to tumble by mid to late weekend as the ridge starts to flatten. Roughly 40% of a 100 member ensemble system bring an influx of moisture in the region bringing our next chance for snow. Stay tuned. /sb && .AVIATION... 12Z TAFS: Strong upper level low will spread precipitation into the Cascades in the next hour and into Wenatchee, Chelan, and Omak between 14-17z. High confidence cigs will dip below IFR at EAT with light to moderate snow begins. Temps will rise and change to rain/snow mix then rain at EAT later this morning but cigs will remain low. Precip will shift eastward through the day but confidence is low- moderate on timing/intensity of the showers, so VCSH groups are the prevailing weather group for GEG, SFF, COE. An arctic cold front will change precip type to snow overnight for COE, SFF, GEG, PUW, EAT. MVFR conditions will prevail for those sites with lower cigs but a moderate burst of snow could induce IFR conditions briefly. MWH/LWS has rain as the dominant weather type overnight, but a brief period of snow may occur overnight as the arctic front passes through. Gusty winds up to 25 kt may occur with tight pressure gradients today and with the passage of the cold front. Confidence is moderate in gusts exceeding 20 kt during the day and high with the arctic front passage overnight. Butler Butler && .PRELIMINARY POINT TEMPS/POPS... Spokane 42 26 29 14 30 14 / 30 80 60 10 0 10 Coeur d'Alene 41 25 28 14 31 16 / 20 60 80 0 0 0 Pullman 42 33 38 17 35 16 / 20 80 60 10 0 10 Lewiston 50 37 44 26 38 25 / 10 60 50 10 0 10 Colville 34 22 28 4 32 4 / 60 80 70 10 10 0 Sandpoint 36 24 26 12 27 13 / 30 60 90 0 10 0 Kellogg 39 26 31 11 30 14 / 20 40 90 0 0 10 Moses Lake 42 33 37 21 34 14 / 60 70 30 20 10 10 Wenatchee 37 30 34 23 32 17 / 70 70 30 20 10 0 Omak 33 28 32 17 35 14 / 80 90 60 20 10 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Winter Storm Warning until 10 AM PST Monday for Northeast Mountains-Okanogan Highlands-Okanogan Valley-Waterville Plateau-Western Okanogan County. Winter Weather Advisory until 10 PM PST this evening for Central Chelan County-Wenatchee Area-Western Chelan County. && $$