Sunrise 5:11 AM   Sunset 8:35 PM Moonrise 7:05 PM   Moonset 4:02 AM

  Print   HELP   Reset   Save   Recall  New

FXUS66 KOTX 212150 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 250 PM PDT Tue May 21 2024

SYNOPSIS
A wet low pressure system will bring steady rain across the region into Wednesday. Snow levels will drop down to pass level Tuesday night with wet snow accumulations for travel over the Cascade crest. Another weather system will move in on Friday and Saturday bringing more unsettled weather into the Memorial Day weekend.
Monday will be the warmest day of the holiday weekend.

DISCUSSION

Tonight through Wednesday night: A warm front with a modest plume of moisture is making its march across the Northwest this afternoon.
Radar mosaic is filling in across the region as stratiform precipitation develops behind the front. The leading edge is beginning to move into the Washington Palouse, Spokane Area, and Northeast Mountains at the 2pm hour. Accumulations will be light but steady, and soaking rains are expected across much of the region through this evening. The trailing cold front will move into the Cascades around midnight tonight. This will shift the moisture plume eastward with drying occurring in the lee of the Cascades. Snow levels will crash down to around 3,000 feet in the Cascades at this time. Slop over moisture across the Cascade crest will result in a transition of rain to snow for Stevens Pass and Washington Pass. Lee side cyclogenesis will occur in central Washington with moisture wrapping around the center of the low across the northern mountains into the northern portions of the Cascades. This will be the axis where stratiform precipitation will continue into at least early Wednesday. A dry slot punching across southeast Washington will turn off the faucet for the basin into the Palouse and Spokane Area where a break in precipitation is anticipated. The upper level low with a minus 27 celsius cold pool will be squarely over the Inland Northwest by early Wednesday afternoon. Cold temperatures aloft will promote convection into the afternoon, but the coverage and intensity of convection will be limited by the cooler temperatures at the surface. It will also be limited by drier air being entrained into the region across the southeast portion of the forecast area.
We will then see showers diminishing through the evening on Wednesday from northwest to southeast. Light northwest flow into the southern and central Idaho Panhandle will keep showers persisting there into the overnight hours on Wednesday.

* Rainfall: accumulations have been trending drier with models indicating a good punch of dry air moving in behind the cold front. We will still see a decent amount of rainfall with much of the eastern half of the forecast area expected to pick up at least between 0.15 to 0.30 inches. The Okanogan Highlands, Northeast Mountains and Idaho Panhandle looks to pick up closer to a half of an inch, and some areas potentially picking up near 0.75 inches when all said and done. This will all be beneficial rainfall with base flows for our streams and rivers running low at this time.

* Snowfall: Main impacts from snow will be for travel over Washington Pass and Stevens Pass once the cold front moves through after midnight. Total snow accumulations looks to be between 2 to 4 inches. Road temperatures will start out warm, but snow intensity does look to be enough to overcome the warmer road temperatures with slushy accumulations possible over these passes by early Wednesday morning. Sherman Pass may also see light snow, but the coldest air will track south of this pass and may just see non-accumulating snowfall.

* Thunderstorm potential: Doesn't look great for Wednesday afternoon due to the cool surface temperatures. It will take breaks in the cloud cover for sufficient warming to spark off convection. Best potential for a 15-20% for thunderstorms will be over the Pasayten Wilderness to the Okanogan Highlands and over the southeast portion of the forecast area (Northeast Blue Mtns into the Palouse).

* Temperatures: Wednesday will be the coldest day this week. The precipitation, cold air advection, and cloud cover will dampen our warming with highs generally in the 50s. /SVH

Thursday through Saturday: The Inland Northwest will be under the influence of upper-level troughing over this multi-day span with several shortwaves dropping through from Canada. This will not only keep temperatures in check with readings near or cooler than average but also keep a threat for showers and thunderstorms just about every day. The highest precipitation chances will focus over the Idaho Panhandle and northern mountains but the Basin will carry at least a 20-30% chance at times. As different shortwaves drop into the mean trough, they will cool the upper-levels with 500mb temperatures near -21C at times supporting afternoon CAPES between 200-600 J/kg. This will be sufficient for a handful of thunderstorms with the main hazards being heavy downpours of rain and small hail along with lightning strikes. At this time, models show some of the heaviest rain cores with showers Thursday through Saturday where pockets of a quarter of an inch or more will be possible. Although Sunday will keep a mention of showers and storms, instability parameters will be on the decline as 500mb temperatures warm suggesting less rainfall with any cells.

85% of the ensemble solutions support a shift in the weather pattern late Sunday into early next week with a low pressure becoming reestablished over the Gulf of AK and heights building northward from the Great Basin into the Inland NW. The exact placement and amplification of these features carries moderate uncertainty.
Forecast is leaning toward dry conditions for Monday which is supported by 70% of the ensembles. This decreases to 50% by Tuesday and further later in the week. There is high confidence that these upcoming precipitation chances will come with a moisture trajectory from the southwest indicative of a warmer environment. This raises some concern that instability parameters could be on the rise with more thunderstorm activity. Stay tuned as we continue to dial this period in. /sb

AVIATION
18Z TAFS: A moist frontal system will push east of the Cascades today result in lower ceilings and light rain. Ceilings are expected to lower down to between 3-6 kft agl. in the lee of the Cascades for KEAT-KOMK-KEPH-KMWH. Rain moving into extreme eastern Washington and the Idaho Panhandle later today (after 22Z) will bring the potential for MVFR conditions tonight with ceilings down to between 1-4 kft agl and visibility down to 4SM. Low clouds will continue into Wednesday morning with numerous showers continuing and a low potential for isolated thunderstorms where sun breaks occur.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence remains high for ceilings lowering to MVFR after 02Z Wednesday over extreme eastern Washington and the Idaho Panhandle. HRRR shows at least a 70% chance of CIGS lowering to MVFR for KGEG-KSFF-KCOE- KPUW, and there is a 30% chance for IFR conditions developing at times. For KEAT to KMWH, there is a 80-90% chance of conditions remaining VFR through the TAF period. /SVH

-----------------------

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 45 56 41 64 44 65 / 100 70 20 20 20 20 Coeur d'Alene 45 54 41 59 44 62 / 100 90 30 30 30 30 Pullman 43 51 39 59 40 62 / 100 80 30 10 10 10 Lewiston 51 61 46 66 47 70 / 100 70 40 20 0 10 Colville 40 56 36 64 39 63 / 100 100 30 70 40 50 Sandpoint 43 52 39 57 44 59 / 100 90 50 60 60 60 Kellogg 45 49 42 56 45 60 / 100 90 70 50 30 40 Moses Lake 42 60 43 72 45 73 / 70 50 30 0 0 0 Wenatchee 45 59 46 71 48 70 / 50 50 20 0 0 10 Omak 43 62 42 73 45 70 / 70 90 20 20 10 20

OTX WATCHES/WARNINGS/ADVISORIES
ID...None.
WA...None.

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

Spokane, WA,