408 FXUS66 KOTX 080003 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 503 PM PDT Mon Oct 7 2019 .SYNOPSIS... Big changes arrive tonight into Tuesday as a strong cold front plows through the Inland Northwest. It will bring windy conditions, a chance of rain and snow and much colder temperatures to the region. Cool, dry weather is expected for the end of the work week with unsettled conditions returning next weekend. && .DISCUSSION... ...Gusty winds and rain changing over to snow with accumulating snow at mountain passes tonight through Tuesday night... Tonight into Tuesday: A strong cold front will bring the potential for moderate wind and snow impacts to portions of the Inland Northwest. The cold front is currently pushing southeast across central BC this afternoon. Moisture is advecting into the region behind a warm front. The only areas where moist isentropic ascent is occurring is across the Northeast Mountains and into the Northern Panhandle where light showers have been observed through the day. The rest of the region is still a bit too dry at lower levels with mainly just a veil of high clouds streaming in east of the Cascades. The prognosis is for the atmosphere to moisten up from the top top into this evening precipitation chances will also increase with strong frontal dynamics providing lift along with orographics playing a big role. The orographically component will result in a strong rain shadow signal in the lee of the Cascade Mountains with drying across the basin and precipitation picking back up again across the Idaho Panhandle. Convective showers will continue through the afternoon on Tuesday. The Idaho Panhandle stands to pick up between a quarter to a half of an inch of precipitation. The main concerns with the frontal passage will be for winds and snow impacts though: * Winds: Cold front passage will be overnight tonight. There should be enough cold air advection and instability at low levels to over come a weak surface inversion for momentum transfer of stronger winds aloft down to the surface. Exposed areas to westerly winds in the lee of the Cascades, mainly across the basin and into the Spokane Coeur d'Alene corridor and Palouse, will see sustained winds of 20-30 mph with gusts up to around 45 mph. Strongest winds will be with cold front passage, so not expecting a long duration of winds at the higher end of this range. Main concern will be with stronger gusts. * Snow: Snow levels will drop rapidly behind the cold front. This will result in rain at the Cascade crest transitioning over to snow. A general 2 to 5 inches of snow will be possible right at the crest. This will include Washington Pass and Stevens Pass. Higher resolution models also are showing a Puget Sound Convergent Zone (PSCZ) setting up across the Cascades between Steven Pass and Snoqualmie Pass. Where this feature sets up would like produce even heavier snow amounts. * Impacts: Loose items may become airborne if not tied down due to the gusty winds overnight. Small tree branches may also brake off and/or isolated power outages may occur. A Wind Advisory remains in effect for portions of southeast Washington for the overnight hours. Snow over the mountain passes may result in winter travel conditions late tonight into Tuesday morning. Brief heavy snow may occur over Stevens Pass. Tuesday Night: A second front of cold and very dry air will push down out of BC. A tight pressure gradient across the Okanogan Valley is expected to produce winds of 15-25 mph with gusts up to 40 mph possible. Winds will also be breezy through the Purcell Trench with gusts up 30 mph possible. A bigger impact may be the potential for snow though. Lookout Pass will see sow with 1 to 3 inches possible. Moderate travel impacts may be felt over the Camas Prairie where models are showing a low pressure circulating near the surface tracking down across this area. Timing would be in the evening into the early overnight hours. Lapse rates increase substantially to 8 C/km. That is quite convective and low level winds switch northwest, which is an upslope direction into the Camas Prairie. It's possible that 4-6 inches of snow falls here above 3,000 feet and a Winter Weather Advisory will be needed. The big question is if the low will track across the area, which is still a bit uncertain at this time. /SVH Wednesday through Monday...Good deterministic model agreement exists for the rest of the work week making for reasonably high confidence of a dry but rather cool weather period. By Wednesday morning the trough digging into the northwestern US will begin to eject eastward...with lingering snow showers mainly over the Idaho Panhandle as dry northerly flow aloft and at the surface begins to invade the Washington zones in the wake of this weather system. Breezy and gusty winds down the Purcell Trench and the Okanogan Valley on Wednesday will usher in dry Canadian continental air to set the stage for a pair of very frosty mornings around the region for Thursday and Friday morning. The initial northerly flow will evolve into a short wave ridge Thursday and Friday bringing mainly clear skies and light winds promoting strong radiational cooling during the overnight hours with daytime highs only poking into the 40s on Wednesday but gradually recovering into the mid to upper 50s by Friday afternoon. Switching over to the ensemble model mean fields of motion for the upcoming weekend there is loose agreement that a new short wave will approach off the Pacific on Saturday. This wave will be rather weak but none-the-less adequate foe wiping out the ridge and driving a moderate fetch of Pacific moisture into the area. At this time none of the models suggest a significant chance of precipitation except on the high terrain surrounding the basin this weekend...but they all depict this wave as a precursor to a more progressive pattern of subsequent disturbances off the Pacific...however snow levels will by this time retreat back into the mountains but valley and basin temperatures will remain a few degrees below normal. /Fugazzi && .AVIATION... 00Z TAFS: Increasing winds will be the main aviation impact during the TAF period as a strong cold front crosses the Cascades aft 06Z. Pecipitation along and ahead of this front will mainly impact the PUW and LWS TAF sites. Brief periods of low VFR ceilings between 08Z and 15Z with showers being more spotty and hit-and-miss for GEG-SFF-COE. EAT and MWH will be shadowed out and shouldn't see precip. Winds behind the FROPA will increase to westerly 15 to 25 kts with local gusts to 35-40 kts at the EAT...GEG...PUW TAF sites with LLWS after 07Z-10Z at the sheltered valley TAF sites especially SFF...COE...PUW...KLWS. Winds will decrease through the afternoon hours. && .PRELIMINARY POINT TEMPS/POPS... Spokane 40 50 28 45 24 47 / 20 30 30 0 0 0 Coeur d'Alene 41 49 27 43 23 47 / 40 40 30 0 0 0 Pullman 42 51 28 43 22 46 / 80 40 50 10 0 0 Lewiston 51 58 36 48 28 52 / 20 40 70 30 0 0 Colville 35 52 26 50 21 53 / 50 30 30 0 0 0 Sandpoint 42 49 27 43 22 47 / 60 50 50 0 0 0 Kellogg 43 47 28 40 23 46 / 90 50 50 10 0 0 Moses Lake 41 56 32 53 25 54 / 10 20 20 0 0 0 Wenatchee 43 53 33 50 31 53 / 20 20 20 0 0 0 Omak 38 51 30 50 27 53 / 20 20 10 0 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Wind Advisory from 11 PM this evening to 8 AM PDT Tuesday for Coeur d'Alene Area-Idaho Palouse. WA...Wind Advisory from 11 PM this evening to 8 AM PDT Tuesday for Lower Garfield and Asotin Counties-Spokane Area-Upper Columbia Basin-Washington Palouse. && $$