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FXUS66 KOTX 101830 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1030 AM PST Sun Dec 10 2023

SYNOPSIS
After the widespread precipitation that fell last night into this morning, a ridge of high pressure will settle over the region for much of the upcoming week, save for a weak system passing around later Wednesday into Thursday.

DISCUSSION
Morning update: Most of the advisories and warnings were allowed to expire at 10 AM PST this morning. This is due to a combination of both decreasing precipitation and moderating temperatures. The Idaho Panhandle advisories and warnings continue with web cams and precipitation still showing snow impacting roads around the region. The advisory for the North Idaho Panhandle runs through 1 AM PST this afternoon while the warning for the Central Panhandle Mountains runs through 4 AM Monday due to upslope flow keeps snow showers persisting.
However the warning may be downgraded or ended early as snow levels increase and precipitation intensity decreases this afternoon into tonight as the moisture plume shifts south of the region. JW

Previous discussion:

Today through Monday tonight: A rather complex winter storm is ongoing over the region continuing to deliver moderate snow, areas of rain and freezing rain, and even some reports of sleet. Each model seems to be initializing a piece of the puzzle but tough to say any one piece of guidance is winning. Based on the 2AM reports and observations, the HRRR has a decent handle on the rain-snow line and was utilized going forward into Sunday morning.

As of 2AM, the warmer air has crossed into southern Chelan County, the lower Columbia Basin, and inching into the Palouse. P-type has remained snow so far for Pullman, Rockford, Spokane, Davenport, and Wilbur. It becomes a bit messy around Grand Coulee, across the Waterville Plateau, and into Wenatchee and along the Wenatchee River Valley toward Stevens Pass. Spotters reported freezing rain at Grand Coulee earlier this evening which didn't match well with any modeled warm tongue and it wasn't until 12AM-2AM when Wenatchee, Leavenworth, and Stevens Pass transitioned to freezing rain and sleet. So over the next 3-5 hours, the warm air will make very slow progress northward across Highway 2 and a better push to the east into the Palouse, Camas Prairie, and L-C Valley. This is was something I believe the GFS has been hinting over the last few days which is allowing Spokane- Cd'A to remain snow longer into the event and is in response to a shortwave (currently dropping in from the northwest over the San Juan Islands) denting the ridge from the north. The satellite derived precipitable water is showing a wedge of higher PWATs with values near 0.60" stretching from Chelan County to Whitman County and back into northern Oregon. Deeper into this plume, PWATS increase closer to .70" or higher. This aligns very well with the aforementioned rain-snow line. As this plume continues to punch inland, it is inevitable that snow levels will not come up. The challenge is mainly what orientation will the plume take (mainly east to west or southwest to northeast). This is something that models have struggled with and our 12z balloon data will give us a better idea which model may be initializing best.

So for the remainder of this morning, light to moderate precipitation will continue across the Idaho Panhandle mainly south of Sandpoint, and southeastern WA mainly south of Spokane. An additional 0.15-0.30" of QPF will be likely for these areas by 10AM falling as snow in the Idaho Panhandle outside the L-C Valley and possibly the higher benches of the Palouse into the Spokane Area. Per the HRRR, we should see Pullman, Rockford, and Spokane switch to rain by 6AM and this trend slowly work eastward into Post Falls-Cd'A and Moscow by 8AM. With the low-level flow still easterly, I am skeptical in this timing but eventually the air mass will become too moist given the incoming PWATS pushing snow levels above 3000 feet. After 18z, these areas are likely to receive an additional 0.10-0.30" in the form of very wet snow or rain. In Central WA, westerly flow resulting in some shadowing.
Showers or light precipitation is not completely over but amounts will be minimal and generally under a tenth of an inch through the afternoon. This is great news given the wintry mix ongoing. QPF increases again on the Cascade Crest due this terrain intercepting the rich plume of moisture and another half inch of liquid will be possible. For our northern communities, I delayed the warm air through much of the day which will keep mention of snow until later in the day but additional snow amounts will also be minimal and under an inch.

A stronger shortwave will drop in from the northwest Sunday night into Monday. This will continue to fuel showers across the Idaho Panhandle, eastern third of WA, and at times Central WA. The warmer, maritime air should be more established and mixed into the environment by that time such that p-type will mainly be rain for the lowlands and snow above 4000 feet. The exception will be across far NE WA and N ID where the cooler air will be deeper and struggle to fully erode today. The Central Panhandle Mountains will have an opportunity to pick of a few inches of wet snow. /sb

Tuesday to Saturday: Mainly dry weather at the start of the period is followed by increasing precipitation chances going into the second half of the week. Temperatures will be near to slightly above normal. Tuesday into Wednesday a ridge builds over the region, while the next long-wave trough deepens over the eastern Pacific and gradually approaches the coast. On Tuesday and Wednesday this will lead to mostly dry weather, but I do expect some fog. The threat will be across a broad area Tuesday morning.
Then as the easterly flow increases the fog potential will shift toward the north and western CWA, i.e. the northern mountain valleys and lee of the Cascades. Some light precipitation might be squeezed out in the upslope easterly flow near the Wenatchee Valley, Okanogan Valley and Methow near the Cascades as light rain/drizzle or light snow/freezing drizzle where colder air sits toward the Methow Valley. Confidence in any impacts is low as whatever falls should be light, if anything falls at all. Yet this will still be monitored should there be more of that wintry precipitation.

Later Wednesday into Thursday a shortwave disturbance pivoting around the long-wave trough pushes inland. This will bring the next precipitation potential to the region, though the models seem to have trended further toward the Cascades and northern CWA with the higher precipitation potential. Precipitation chances increase through the Cascades Wednesday afternoon and evening, then expand eastward throughout the region overnight into Thursday. However the potential for significant precipitaiton is low, with the main area where models paint >=0.10" of precipitation is toward the Cascade crest and near the Canadian border. Any precipitation that may fall looks like snow in the mountains and in the lowlands looks like snow during the night and morning hours, becoming mixed or changing to rain chances in the afternoon Thursday. With all that said, the forecast right now depicts only a couple inches near the Cascade crest and northeast mountains and little to no accumulation elsewhere. Precipitation chances wane Thursday night into Friday, then another wave moves in later Friday and Saturday to revitalize the precipitation potential mainly around the mountains. We will have to keep an eye on the system to potentially add some chances in the lowlands too, but confidence leans toward not right now. /Solveig



AVIATION
18Z TAFS: A deep, moist, and saturated boundary layer has led to widespread IFR/MVFR stratus over the region. Light winds will result continued stratus over the region for the next several days. Given multiple stratus layer confidence in precise visibility and ceilings is low for each TAF site, especially the Spokane area where conditions vary over short distances as of 17z this morning. Areas of light precipitation will continue over Southeast Washington into the Idaho Panhandle, falling as rain in Pullman and Lewiston. Lewiston has benefited from downslope boundary layer winds off the Blues with VFR conditions. As the downslope winds ease, CIGS are expected to lower to at least MVFR.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: There is high confidence of continued restrictions over the region with widespread stratus. Given multiple stratus layers confidence in precise ceilings and visibility is low. This is especially true around the Spokane and Moses Lake areas where ceilings and visibilities vary over short distances. There is moderate to high confidence that the VFR conditions over LWS this afternoon will drop to MVFR tonight as the downslope boundary layer winds ease, but low confidence with the timing and degree of restrictions. JW

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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

PRELIMINARY POINT TEMPS/POPS
Spokane 37 33 39 31 39 26 / 50 40 10 0 0 0 Coeur d'Alene 37 34 39 31 39 24 / 70 60 10 0 0 0 Pullman 38 36 39 33 40 26 / 80 80 40 10 0 0 Lewiston 44 40 45 37 45 30 / 70 50 40 0 0 0 Colville 35 27 39 24 37 23 / 30 20 0 10 10 10 Sandpoint 36 32 39 27 37 23 / 70 60 20 10 10 0 Kellogg 37 36 40 34 40 24 / 80 80 40 10 0 0 Moses Lake 39 32 40 30 40 29 / 10 10 0 0 0 0 Wenatchee 39 33 40 31 37 32 / 10 10 0 0 0 10 Omak 37 32 41 29 38 28 / 10 10 0 0 10 20

OTX WATCHES/WARNINGS/ADVISORIES
ID...Winter Weather Advisory until 1 PM PST this afternoon for Northern Panhandle.

Winter Storm Warning until 4 AM PST Monday for Central Panhandle Mountains.

WA...None.

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