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FXUS66 KPDT 291057 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 257 AM PST Wed Nov 29 2023

SHORT TERM
Today through Friday...Currently, nighttime satellite imagery and surface observations show little change in the widespread stratus layer that covers the Columbia Basin, adjacent valleys, and north central OR. Patchy fog has been noted underneath this stratus layer, with visibilities down 4 miles being reported, however, looking at ODOT cameras in north central OR and near Athena show dense fog still present in these areas.
But that's not all, the stratus layer continues to create areas of dense fog where it intersects the Blues and the Cascades.

While a split flow aloft is expected to develop behind the exit of the upper level ridge today, little change in conditions is expected across the forecast area. The stratus layer over the Columbia Basin and adjacent valleys will persist through early tomorrow, with area temperatures underneath this layer continuing to moderate between the mid 20s to low 30s. Meanwhile, areas outside of the stratus layer will see mostly clear conditions and temperatures warming into the mid 40s, or mid 30s to low 40s in the mountains. Stagnate conditions will also persist across the lower elevations, as mixing heights will continue to be below 1.5kft and transport winds are less than 10 mph.

Thursday will see the beginning of a long term pattern change across the PacNW with the arrival of the first of a series of shortwave troughs and frontal passages. The first system will arrive late in the morning Thursday, with precipitation spreading east across the forecast area throughout the day. Most of the forecast area will see a chance of snow at onset of precipitation, but then as the day progresses snow will turn over to rain across central OR and the John Day Basin and into a rain/snow mix in those areas currently trapped underneath the stratus layer. Only exception will be portions of the Yakima/Kittitas valleys and the Horse Heaven hills where precipitation will fall mostly as snow.
That said, snow amounts will generally be under an inch across the lower elevations. There is a concern for freezing rain to also develop across the Blue mountain foothills and Yakima/Kittitas valleys if the stratus layer is slow to erode away through the first half of Thursday, though confidence in this occurring is low (20%). In the mountains, snow accumulations up to 5 inches below 5kft are expected through Thursday night, with generally a 35-45% chance of exceeding 5 inches across the Cascade east slopes and the interior northern Blues. At this time, there is no plan to issue any highlights for the mountains with this first system, however if liquid equivalent precipitation amounts increase, this will need to be reevaluated. Otherwise, outside of the higher mountain areas, much of the forecast area is expected to warm above freezing Thursday afternoon, albeit only by a 2 to 3 degrees in the Columbia Basin and adjacent valleys.

There will be little in the way of a break from precipitation overnight Thursday into Friday as the next shortwave trough and frontal boundary in a northwest flow aloft slide across the Columbia Basin. This next system will be capable of producing light snow in the lower elevations early Friday morning, but as snow levels rise above above 2.5kft to 3.5kft, most of the lower elevations will see a transition from snow to rain/snow to rain through Friday morning and early afternoon. In the mountains, Friday will be the start of a period of moderate to heavy snow, with a 80-90% chance of exceeding 10 inches in a 24-hr period through Friday night along the Cascade east slopes, and 50-60% chance of exceeding 8 inches in a 24-hr period through Friday night across the northern Blues and the Wallowas. Including the expected snowfall over the weekend, winter storm highlights will likely be needed as snow accumulations exceed warning criteria across the northern Blues and Cascade east slopes. While the southern Blues and rest of the eastern mountains is expected to see some snowfall Friday, unfavorable orientation to storm motion and rain shadowing from the Cascades will result in lesser snow accumulations, with only a 45-60% chance of exceeding 4 inches of snowfall through Friday night. Breezy west to southwest winds will also develop throughout Friday as surface pressure gradients tighten from south to north across eastern OR, though peak winds are not anticipated until overnight Friday and early Saturday.
Afternoon temperatures in the lower to mid 40s are expected in the lower elevations, with upper 20s to 30s in the mountains.
Lawhorn/82

LONG TERM
Saturday through Wednesday...Models are in pretty good agreement through the period. However, there are some minor discrepancies in the strength of a deepening trough over the central US and an amplifying ridge over the Pacific NW. This is reflected nicely in the WPC Cluster analysis. This pattern is expected to tap into some subtropical moisture producing a weak to moderate AR event over the area. There will also be periods of enhanced precipitation due to isentropic lift. Although this will primarily impact the mountains, there is moderate to high confidence (60-70%) that we could see 0.25" or more of rain along the Blue Mountain Foothills Saturday and Sunday. The Blue Mountains and Cascades will very likely (75+%) see precipitation amounts of 0.75" or more. This pattern is expected to continue into the middle of next week with periods of rain. Initially, snow levels will be around 2000 feet north and 4000 feet south, but will begin a rapid rise to 5500+ feet by Sunday evening. Depending on snow accumulations this could produce some hydrologic concerns early next week as rapid snow melt could occur (i.e. rain over snow).

It will be a breezy to windy Saturday, but pressure gradients are expected to slowly relax through the day. Daytime temperatures are expected to be 10-15 degrees above normal through the period.
Earle/81

AVIATION
12Z TAFS...All sites should remain MVFR or less through the period, except KRDM and KBDN which should remain VFR. Low CIGs and patchy fog will continue over KPDT, KYKM, KPSC, and KALW. A system will begin to approach our western CWA late tonight improving conditions just beyond this period. Winds will be 10 kts or less.
Earle/81

PRELIMINARY POINT TEMPS/POPS
PDT 31 25 33 26 / 0 10 40 80 ALW 30 26 32 27 / 0 10 40 80 PSC 32 28 34 27 / 0 10 30 50 YKM 34 24 33 21 / 0 10 30 20 HRI 34 26 34 26 / 0 10 40 70 ELN 33 22 32 21 / 0 10 30 30 RDM 43 24 42 24 / 0 10 50 50 LGD 42 25 37 27 / 0 10 30 70 GCD 45 25 43 24 / 0 10 50 80 DLS 38 30 39 30 / 0 10 80 70

PDT WATCHES/WARNINGS/ADVISORIES
OR...Air Stagnation Advisory until 11 AM PST Thursday for ORZ041-044- 050-505>508-510-511.

WA...Air Stagnation Advisory until 11 AM PST Thursday for WAZ024- 026>029.

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