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FXUS66 KOTX 030447 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 947 PM PDT Thu May 2 2024

SYNOPSIS
With the exception of widely scattered showers along the Canadian border Friday afternoon, the Inland Northwest will experience mild and dry day. A slow moving frontal system will bring increasing chances for rain to central Washington Friday night into Saturday. Sunday will be a chilly and rainy with portions of north Idaho and eastern Washington experiencing highs in the 40s Sunday. Cool and showery weather will be in store Monday through Wednesday followed by warmer temperatures late next week.

DISCUSSION

Tonight: Afternoon temperatures are several degrees warmer than they were at this time yesterday which is an indication that our air mass is moderating. Even though are dewpoints continue to be very dry (mid 20s to low 30s), warming aloft should prevent widespread freezing temperatures overnight. It will still be chilly with light winds and mostly clear skies, but we don't see a need for a Freeze Warning tonight. The National Blend of Models (NBM) morning lows in the mid 30s for the valleys of northeast Washington, the West Plains, Palouse, and Moses Lake area. There will be pockets that dip below freezing, so folks that have planted sensitive vegetation and live in cold spots should take measures to protect their gardens.

Friday: A slow moving front will bring widespread rain to the Inland Northwest this weekend, and we will begin to experience effects of that front by Friday afternoon. In the morning, skies will be mostly clear and winds will be light. By the afternoon, southeast winds will increase with the most notable increases over the Waterville Plateau and up the Okanogan Valley. Guidance from the High Resolution Ensemble and the NBM produce sustained winds of 15 to 20 mph with gusts up to 30 mph for Waterville, Mansfield, Tonasket, and Omak. Similar winds can be expected on exposed ridges in the north Washington Cascades. Frontal precipitation will be slow to arrive with only a 50 percent chance after midnight at Stevens Pass and a 20 percent chance of light rain at Wenatchee by daybreak Saturday morning. There will be enough residual instability over the mountains along the Canadian border Friday afternoon for a 20 percent chance of light rain showers over the high terrain of near Republic, Kettle Falls, Metaline, and Bonners Ferry. With the reversal of our mid-level flow (ahead of the front), these showers will move from south to north...the opposite cell motion we have had the last 3 days.

Saturday: The eastward progress of the front on Saturday will be slow. The upper low pushing it east will slide down the coast and into Oregon during the day. North Idaho and the eastern third of Washington should experience a relatively decent day. Morning temperatures will be significantly warmer with overnight lows in the 40s. Chances for measureable precipitation through 4PM Saturday are only 20 percent for Spokane, Coeur d'Alene, Lewiston, and Colville with highs in the 60s (low 70s in Lewiston). Ensemble based guidance suggests a different story for Wenatchee, Leavenworth, Chelan, and the Methow Valley with 50 to 70 percent chance of rain and highs in the 50s. The NBM spreads a band of much needed rain across the Columbia Basin, West Plains, Palouse, and L-C Valley Saturday night. The month of April was quite dry.
Pullman fell nearly a half inch below average, Spokane almost seven tenths below average, and Moses Lake nearly four tenths below average. It's unfortunate that rain will fall over the weekend, but the regional wheat crop can certainly use the moisture. /GKoch

Sunday through Thursday: Transition to wet and cool conditions is well underway at this point, and very much apparent in numerous tools utilized to view the NBM data. Very apparent there will be significant cooling in daytime highs Sunday however the members show a large spread with 1D viewer showing whisker plots (spread) between 44 and 67 for KGEG's Max temperatures for Sunday and is very similar elsewhere in the CWA The whiskers show the 10th and 90th percentiles)...even the boxes showing the 25th and 75th percentiles are rather tall varying between 47 and 57....SO its still reasonable to say Sunday will be quite cool and wet. NBM is noting Saturday's high temperature for Spokane (KGEG) could fall very close to past record low max of 47 degrees set back in 1961. Precipitation amounts in the NBM have increased and as such our rain amounts in the forecast for Sunday and Sunday night have increased. They area approaching near one half inch overall accumulation for Spokane area if one adds the rain amounts expected Saturday night, Sunday, and Sunday night. This is due to a wet frontal zone wrapped up along the northeast edge of an upper level area of low pressure centered over the northern great basin moves slowly east. This frontal zone has a moderate, yet very effective, tap into subtropical moisture and this is reflected in high forecast precipitable water values approaching 160 percent of normal for the forecast area. The moisture plume and the associated upper level trof moves east, however any resulting shortwave ridging is still embedded in a general longwave trof continuing to linger over the Western US. With this in mind a rain-shadow of sorts can be expected in places between the lee of the Cascade Crest and the higher terrain of North Idaho that expands with time heading into Thursday with remaining periphery holding onto varying pops and QPF as well as minor mentions of short-lived weak low topped thunderstorms and all are favoring the afternoon and early evening hours. /Pelatti

AVIATION
06Z TAFS: VFR conditions will prevail at the TAF sites through Friday evening as the next weather system spreads increasing high clouds across the region on Friday. A quieter day in terms of mountain convection is forecast, with a 20% chance of showers limited to areas near the Canadian border in the afternoon.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high confidence in VFR conditions at all TAF sites through 06z Saturday. 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 65 46 68 41 47 / 10 0 0 20 50 70 Coeur d'Alene 36 63 44 68 41 46 / 10 0 0 10 40 70 Pullman 35 62 46 65 37 44 / 0 0 0 20 70 80 Lewiston 40 70 47 73 45 52 / 0 0 0 20 70 80 Colville 34 65 41 67 41 52 / 20 10 10 20 40 70 Sandpoint 37 61 42 66 44 48 / 50 10 0 10 40 80 Kellogg 37 61 44 65 44 45 / 20 10 0 10 50 80 Moses Lake 37 71 52 68 44 54 / 0 0 10 50 60 60 Wenatchee 43 67 52 59 47 56 / 0 0 20 60 60 40 Omak 40 70 50 67 48 59 / 10 0 10 40 40 50

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

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