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FXUS66 KOTX 251713 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1013 AM PDT Thu Apr 25 2024

SYNOPSIS
Light rain will spread into the Inland Northwest this afternoon and evening. More seasonal temperatures are expected Friday into the weekend with on and off showers and a few weak thunderstorms.
Unsettled conditions continue into next week.

DISCUSSION
Today through Saturday: A shift in the weather pattern is ongoing this morning with a transition from warm and dry conditions toward seasonal and unsettled. A weak cold front that moved through on Wednesday was the initial blow to the upper ridge with mean upper-level troughing now established. A stronger shortwave that is evident on the water vapor along 130W will enter the PacNW today and depart on Friday. This system has a deeper axis of moisture to work with with PWATS increasing closer to three- quarters of an inch or 180% of normal for this date. However, the best dynamics will split around the Inland Northwest.
Additionally, some of the moisture will be reserved to saturate an antecedent sfc-700mb dry layer. Nonetheless, widespread light rain will spread from west to east through the afternoon and evening. Rainfall amounts through Friday morning will be light with less than a tenth in the Basin and between a tenth to quarter of an inch in northern mountains and Idaho Panhandle. The mountain valleys could be closer to a tenth or less. HRRR supports this notion with 5-15% chance for a tenth in the Basin and 60-100% chance northern mountains and Idaho Panhandle. The system, lacking strong westerly flow, should deliver a few hours of light rain to all locations including the lee of the Cascades. This will not be a very cold system allowing temperatures on Thursday to sneak into the low 60s and upper 50s.

Steady rain will transition to showers on Friday afternoon. Breaks in the clouds will result in mediocre CAPE in the afternoon and convective showers and few weak thunderstorms. There is 20-40% for 200 J/kg; 15% chance for 300 J/kg; and 0% chance for CAPES of 400 J/kg. 500mb temperatures only cool near -24C which is not overly impressive. Cells will mainly produce brief downpours and potential for infrequent lightning strikes. At this time, the highest probability for lightning will be in two areas: 1) across the northern mountains (roughly between Hwy 2 and the Canadian Border) and 2) across the far southeast (Blue Mountains, Camas Prairie, L-C Valley, and Palouse). A lot will be dependent on where the best breaks in the clouds occur. Rainfall amounts Friday afternoon will vary from one location to another given the showery nature but generally around a tenth or less outside thunderstorm cores. Temperatures on Friday will remain in the upper 50s to lower 60s.

Another ripple in the flow will arrive on Saturday. At this time, 500 mb temperatures warm a few degrees and mid-level westerly flow becomes a bit stronger. As the next wave comes through, another band of showers or clouds will accompany the midlevel front with light showers. There should be more shadowing in the lee of the Cascades with this wave keeping the highest shower chances across Idaho and the eastern third of WA as well as the Cascade Crest. A few weak thunderstorms will be possible (15% chance) over Northeastern WA and far North Idaho. Rainfall amounts will be less than a tenth of an inch outside any t-storm cores.
South to southwest winds do increase with speeds near 10-15 mph within the Okanogan Valley and Columbia Basin gusting near 25 mph at times. Temperatures warm a degree or two with readings in the mid to low 60s. /sb

Sunday through Thursday: There is high confidence for unsettled conditions to continue into next week. A deep low off the BC coast Sunday will keep precip chances around for Sunday with west- southwest flow into eastern WA and north Idaho. 850 mb temperatures in the Cascades cool enough to support snowfall by Sunday evening with a frontal passage (80% of the global ensemble colder than -1C). There is a 70% chance of 3 inches of snow or more Sunday evening through Monday night at Stevens Pass (25% at the recently opened Washington Pass).

There is a brief window where a couple snowflakes are possible in the eastern third of WA and north Idaho Monday morning with a 40% chance of 850 mb temperatures colder than -1C. However, the shadow effect will prevent significant precipitation accumulation east of the Cascades next week. With the front passage Sunday afternoon, there is a 20-40% chance of wind gusts 35 mph or greater, raising blowing dust concerns in the Basin and Waterville Plateau. Winds remain elevated Monday as the PDX-GPI pressure gradient is modeled around 12 mb with similar probabilities of wind gusts 35 mph or greater.

The parent trough stays off the BC coast through at least Wednesday and starts to migrate inland Thursday. Thus, the cooler and moist pattern remains through late week. /Butler

AVIATION
18Z TAFS: A front passing through the region will increase chances for precipitation by noon in central WA and later this afternoon in eastern WA and north ID. Light rain is expected across all airports with the potential for MVFR conditions into the evening and overnight hours.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Moderate confidence in MVFR cigs around 01-06z light rain band.
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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 60 44 58 42 61 42 / 30 80 60 20 30 20 Coeur d'Alene 58 44 54 41 59 42 / 30 90 70 30 50 30 Pullman 58 43 56 40 57 40 / 20 90 60 30 30 20 Lewiston 66 48 62 46 64 45 / 20 80 60 20 20 20 Colville 61 43 57 39 61 37 / 40 90 90 50 70 40 Sandpoint 55 44 53 41 56 42 / 40 90 90 50 80 50 Kellogg 55 43 51 43 55 43 / 40 90 90 50 80 50 Moses Lake 63 43 65 42 67 40 / 50 40 20 10 10 0 Wenatchee 60 45 62 44 64 43 / 70 40 30 10 10 0 Omak 63 45 62 44 66 41 / 60 80 60 30 20 10

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

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