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FXUS66 KPDT 072233 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 233 PM PST Thu Dec 7 2023

SHORT TERM
Tonight through Saturday...The upper level trough moving into the Pacific Northwest today will pass across the forecast area overnight then exit to the east on Friday. Will see some showers across the forecast area this afternoon and overnight mainly over the mountains then decreasing and ending on Friday. Some marginal to very low end advisory level snow accumulations are expected along the crest of the Cascades and over the northern Blue Mountains through Friday morning where snow advisories are in effect. A temporary ridge of high pressure will pass over the region late Friday through early Saturday before the next weather system arrives Saturday afternoon and overnight. This will arrive as a warm front followed by a weak cold frontal passage on Sunday night into Monday. Initially snow levels on Saturday will be low around 1000 to 2000 feet then rise overnight to 3000 to 6000 feet. This will turn precipitation over to rain by early Sunday everywhere except far eastern areas near Idaho where snow levels remain near 3000 feet. 24 hour NBM snow probabilities are indicating only a 40-60% chance of 4 to 6 inches of no at Snoqualmie pass between 18Z Sat to 18Z Sunday indicating a marginal advisory event at this time. No other mountainous zone is showing any probabilities over 10%. Will also see some increasing south to southeast winds over the weekend associated with the warm front. This could lead to advisory level winds in the southern Grande Ronde Valley area Saturday into Sunday.

LONG TERM
Sunday through Thursday...The period will start out active over the weekend as a trough slides across the Pacific Northwest Sunday morning, lingering through Sunday night/Monday morning. The main weather with the weekend system will be mountain snow and southeasterly winds through the Grande Ronde Valley. 12Z EPS EFI values for QPF, valid 00Z Sunday - 00Z Monday, are 0.6-0.9 for the Blue Mountain foothills, northern Blues, and 0.5-0.7 all of the lower Columbia Basin/Gorge and adjoining lower elevations of Washington. 12Z EPS EFI values for snow, valid 00Z Sunday - 00Z Monday, are 0.6-0.9 for the northern Blue Mountains and 0.5-0.7 for the northwest portion of our forecast area including Yakima and Kittitas counties. These values highlight reasonably good ensemble agreement in a climatologically unusual snowfall event for the mountains with lower agreement in QPF and snow for the Washington Basin/valleys.

Tuesday through Wednesday, ensembles are in excellent agreement that upper-level ridging will build over the Pacific Northwest with mostly dry conditions.

12Z EPS EFI values for wind, valid 00Z Wednesday - 00Z Thursday, highlight the Grande Ronde Valley for a wind event. While values are not high (0.5-0.6), this is the windy season for the Grande Ronde so it takes excellent ensemble agreement in a very strong forecast wind event for the EPS forecast distribution to exceed the climatological distribution in a noticeable way such that EFI values approach 0.9- 1. While ensemble clusters do show some uncertainty in the details of the longwave pattern for Wednesday, agreement in the overall pattern (upper-level ridging over the Rockies and northern Plains with a closed low or open wave for the Southwest US) is very good so confidence in some form of a wind event is high. Regarding the aforementioned wind for the Grande Ronde Valley, NBM 4.2 probabilities indicate a 25-45% chance of exceeding 39 mph gusts Wednesday.

Model uncertainty grows by Thursday as ensemble clusters show 57% of members form some flavor of a closed low over the Pacific Northwest while 35% keep offshore troughing with southwest flow aloft over the Pacific Northwest and ridging over the Rockies and northern Plains, and the last 8% show a ridge centered over western OR/WA. Plunkett/86

AVIATION
00Z TAFs...A weather system will keep periodically breezy southwest to west winds at TAF sites through Friday morning with sustained winds of 10-15 kts and gusts of 15-25 kts. Chances of -RA taper off this evening except for the mountains where upslope showers are forecast to persist through Friday morning. Mostly dry conditions are then forecast Friday at all sites. Aside from temporary MVFR/IFR CIGs/VSBYs in passing showers, prevailing VFR conditions are forecast with <25% chance of MVFR CIGs/VSBYs returning overnight. Plunkett/86



PRELIMINARY POINT TEMPS/POPS
PDT 35 42 27 44 / 30 20 0 10 ALW 36 43 28 43 / 50 30 0 10 PSC 37 47 30 41 / 10 10 0 20 YKM 29 46 27 38 / 10 0 0 40 HRI 37 47 28 43 / 20 10 0 20 ELN 30 41 25 37 / 10 0 0 50 RDM 26 40 21 42 / 30 20 0 10 LGD 31 39 25 38 / 90 50 0 10 GCD 28 39 20 41 / 60 40 0 10 DLS 38 48 31 42 / 40 10 0 40

PDT WATCHES/WARNINGS/ADVISORIES
OR...Winter Weather Advisory until 10 AM PST Friday for ORZ502-509.

WA...Winter Weather Advisory until 10 AM PST Friday for WAZ030-522.

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