Area Forecast Discussion
Issued by NWS Spokane, WA

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000 FXUS66 KOTX 231730 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 1030 AM PDT Sat Mar 23 2024 .SYNOPSIS... The mild spring weather will come to a close. Expect cooler and wetter weather this weekend. As snow levels gradually fall this weekend minor impacts from winter weather are forecast in the mountains, as well as north Idaho Sunday morning. An active weather pattern continues through next week with a series of storms bringing mountain snow, and mainly rain in the valleys except some snow at times overnight for the northern valleys. && .DISCUSSION... Today through Sunday night: A deep upper level low pressure system is spinning off of the Oregon coast with the ridge axis centered over the Idaho Panhandle. Southerly flow across eastern Washington will pump moisture up into the region. An occluded front is beginning to push up into central Washington and this will supply the lift along with upslope flow into the east slopes of the Cascades. The heaviest precipitation this morning will occur closer to the east slopes of the Cascades where on the order of 0.25-0.50 inches is expected around Wenatchee, Waterville, Chelan, Leavenworth, and the Methow Valley. Some areas in the Cascades will see 20-30% chance of between 0.50 and 1.00 inch. Snow levels will be right around 4,500 feet and Stevens Pass may see some slushy snow, but not expecting snow to accumulate on the road itself. The frontal band will spread across eastern Washington and into the Idaho Panhandle through the morning hours. Dynamics aloft will spin up a weak surface low over the northern portion of the Columbia Basin. This will result in a TRough Of Warm Air aLoft or TROWAL like feature that extends across the northern portion of the forecast area by this afternoon into this evening. The surface low will then migrate essentially across the Highway 2 corridor into the Spokane- Coeur d`Alene area late tonight into Sunday morning. The forming deformation band (or TROWAL) will wrap in around this low and begin to shift east with precipitation being focused into the Idaho Panhandle by Sunday afternoon. The most challenging part of this forecast has been snow levels and snow accumulations in the lowlands. Model ensembles continue to waver on the potential for snow in the Northern Panhandle. The 00Z ECMWF Ensemble run shows 2 inches of snow as the 75th percentile at Coeur d`alene and around 3.5 inches at Sandpoint using a 10:1 snow to liquid ratio (SLR). A 10:1 SLR probably is a bit high, but there is a higher than average bust potential here with the most likely scenario painting the potential for up to an inch of snow an less for the lowlands across the Purcell Trench. Road temperatures figure to be too warm for much snow to accumulate, but that could be overcome if snow falls heavier than anticipated under the deformation band. Grassy surfaces will have the best potential for seeing upward of 2 inches to as much as around 4 inches or so. Sherman Pass and Lookout Pass stand to see the best potential for snow and winter travel conditions. Snow accumulation over these passes look to be in the range of 3-6 inches. Confidence is only moderate in these accumulations right now and holding off on issuing a Winter Weather Advisory due to this being a marginal event and lack of confidence at this point. Below are updated probabilistic guidance on snow accumulations for areas of concern off of the National Blend of Models (NBM): *Chance of at least 1" of snow: Sandpoint = 60%, Colville = 15%, Spokane = 8%, Coeur d`Alene = 30% *Chance of at least 2" of snow: Sandpoint = 30%, Colville = 5%, Coeur d`Alene = 10% *Chance of at least 4" of snow: Sandpoint = 5%, Stevens Pass = 10%, Lookout Pass = 60%, Sherman Pass = 35% *Chance of at least 8" of snow: Lookout Pass = 10% Temperatures will cool below normal for this weekend with highs in the 40s and 50s. Cooler temperatures will be felt more so across northeast Washington and into the Idaho Panhandle where more steady precipitation is expected. Low temperatures over extreme eastern Washington and into the Idaho Panhandle will be dipping near to and below the freezing mark, especially as the low pulls away Sunday night. There will be breezy north to northeast winds down the Okanogan Valley and Purcell Trench over the weekend. Expect sustained winds of 10-20 mph with gusts up to between 25-30 mph. /SVH Monday and Tuesday...The cool and relatively strong upper level low which will impact the area this weekend will migrate south and east of the Inland NW resulting in drier weather, at least for a good portion of the area. The precipitable water values plummet as upper level west-northwest flow moves into the area. Despite the drying, we will be left with relatively unstable atmosphere and given some daytime heating we will see some light showers develop Monday afternoon. The best chances will occur over the SE quarter of WA and into the ID Panhandle with a rain shadow over central WA. The depth of the instability isn’t sufficient to generate charge separation and thus no thunderstorms are expected, however graupel showers certainly are possible given the cool cloud base temperatures. Although the precipitation chances should taper off over the evening hours, a weak band of moisture is expected to traverse eastward across the region overnight and into Tuesday associated with a weak upper level trough. The trough will lose steam as it heads to the ID/MT border and it looks like Tuesday will be the wetter of the two days. Precipitation amounts on Monday will generally be less than a tenth of an inch. On Tuesday precipitation amounts will generally range from 0.05-0.15” with up to a quarter inch possible over the central ID Panhandle. As for precipitation type most will fall as snow on Monday at elevations above 3000’ with a mix of rain/snow/graupel below. On Tuesday the snow levels will rise to 3500-4000’. On Wednesday there is good ensemble agreement that we will see a good influx of moisture moving into the area ahead of another trough digging off the coast. Precipitable water values will rise up to nearly 150% of normal with strong low-level warm air advection resulting in widespread precipitation during the day. The offshore trough will near the coast on Wednesday night into Thursday morning which will end the precipitation in the lee of the Cascades overnight. However the precipitation will be slower to end over the remainder of eastern WA and the ID Panhandle as upper level cooling will lead to a rapid destabilization of the atmosphere and numerous showers. There is some question whether the atmosphere will be unstable enough to support thunderstorms. Yesterday we saw a handful of lightning strikes with an atmosphere that was slightly more unstable that we suspect Thursday will be. As usual the Canadian ensemble members are by far the most unstable and are dragging the NBM numbers up with it. So for now we will leave it out of the forecast. Between Wednesday and Thursday we are expecting a significant amount of precipitation. Totals will likely range from 0.10-0.20” over central WA with amounts ranging from 0.20-0.40” over eastern WA and N ID. The warm air advection on Wednesday will push snow levels up to 4000-5000’ before dropping to 3000-3500’ on Thursday. So even though this is a wetter system, most of the snow will only impact the higher mountains. Stevens Pass could see up to a half foot of snow with slightly lower amounts at Lookout Pass. However I suspect that whatever falls during the daytime hours will have a hard time accumulating on the roads. For Friday into Saturday, yet another upper level trough moves south to southeast toward the NorCal coast with unstable conditions remaining over the Inland NW. That should return us to seeing afternoon and evening showers but nothing significant from a precipitation standpoint. For temperatures, we are looking at readings temperatures remaining slightly cooler than normal for this time of year. Highs will be in the mid 40s to mid 50s from Monday through Thursday with a subtle warming by the end of the week. Lows will generally be in the 30s. fx && .AVIATION...
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18Z TAFS: Low pressure moving into the region will bring rain and lowering ceilings down to 2,000-4,000 ft agl through this morning. Moderate rain at times into the east slopes of the Cascades may reduce visibility down to 4SM-6SM at times between KEAT to KOMK. A surface low will spin up near KMWH/KEPH this afternoon with a deformation band and stratiform precip forming across the northern mountains of eastern Washington into North Idaho. This portion of the region will continue to see MVFR conditions through the afternoon into Saturday evening. The surface low will push east into the Idaho Panhandle late Saturday night with stratiform precip wrapping in around the backside of the low. Snow levels will drop over northeast Washington into the northern Idaho Panhandle Saturday night with rain transitioning to snow across the Spokane-Coeur d`Alene corridor (KGEG-KCOE) and airports north (KDEW-KCQV- KSZT-K65S) Sunday morning. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high confidence of ceilings lowering and MVFR conditions with rain today. MVFR conditions will impact the KEAT-KMWH-KGEG-KSFF-KCOE- KPUW terminals today. JW ------------------------ Confidence descriptors: Low - Less than a 30 percent chance Moderate - 30 to 70 percent chance High - Greater than a 70 percent chance For additional probabilistic information for NWS Spokane airports, please refer to the Aviation Dashboard on our webpage: https://www.weather.gov/otx/avndashboard
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&& .PRELIMINARY POINT TEMPS/POPS...
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Spokane 49 34 44 30 49 33 / 70 60 60 10 30 30 Coeur d`Alene 47 32 41 26 46 33 / 80 60 60 10 30 30 Pullman 50 37 45 30 48 34 / 60 40 90 30 50 60 Lewiston 56 40 52 35 54 39 / 60 30 80 30 30 50 Colville 46 32 48 26 50 30 / 90 70 50 10 10 20 Sandpoint 44 30 39 25 44 31 / 90 90 60 10 30 40 Kellogg 47 34 40 28 46 36 / 90 70 80 20 40 50 Moses Lake 57 37 57 35 56 34 / 20 30 20 10 10 0 Wenatchee 52 40 56 36 54 36 / 90 50 10 10 10 10 Omak 50 37 57 35 56 35 / 90 80 20 0 10 10
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&& .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$

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