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FXUS66 KPDT 071759 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 959 AM PST Thu Dec 7 2023

AVIATION
18Z TAFs...A weather system will bring periods of 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 spread north and east through the day and are highest at BDN/RDM/DLS/PDT/ALW through this evening with mostly dry conditions expected for Friday at all sites. Aside from temporary MVFR CIGs/VSBYs in passing showers (highest chances at BDN/RDM/DLS this morning and early afternoon), prevailing VFR conditions are forecast with <25% chance of MVFR CIGs/VSBYs returning overnight. Plunkett/86

PREV DISCUSSION
/issued 802 AM PST Thu Dec 7 2023/

Update...Current satellite and radar shows that most of the forecast area is between weather systems this morning as we await the next system over western Oregon to move across the forecast area today and overnight. Have adjusted POPs for today through tonight to account for the current break in precipitation. Will see an increase showers across the forecast area today and then taper off overnight as the western Oregon system transits the forecast area. Still looking at some snow over the Cascade and eastern mountains with accumulations in the low end advisory range. Current advisories remain in effect through Friday morning.

SHORT TERM
Today through Saturday night
Current radar and infrared satellite imagery showing light to moderate showers passing across the Lower Columbia Basin from the southwest under cloudy skies. These conditions are a result of an upper level trough that has approached the coast and will slowly push onshore later this morning before driving through the Pacific Northwest later today and digging to our southeast overnight into Friday. A weak atmospheric river is associated with this troughing feature, which will continue to bring ample mountain snow and periods of rainfall over lower elevations through the day today and into Friday morning. This has warranted the issuance of Winter Weather Advisories across the east slopes of the Washington and Oregon Cascades above 3500 feet as 5-10 inches of snow is anticipated to fall through Friday morning. There is also an active Winter Weather Advisory across the Northern Blue Mountains above 4000 feet, with between 6-10 inches expected. The NBM lends additional confidence in these snow totals, as an 80-100% chance of 6 inches of snow or more exists along the Oregon Cascades, 50-70% chance over the Washington Cascades, and a 30-50% chance along the Northern Blue Mountains. All Advisories are active through 10 AM Friday. Rain amounts should stay below 0.10 inch across the area, with the best chance of higher amounts between 0.10-0.30 inch along the Northern Blue Mountain foothills. The other weather concern today resides with breezy winds along the east slopes of the Washington Cascades and the Simcoe Highlands area as gusts up to 40 mph will be possible through much of the day. Elevated winds are also likely along the Blue Mountains and foothills late this afternoon through the evening with gusts up to 35 mph will be possible. The NBM highlights a 70-80% chance of gusts reaching 40 mph along the east slopes of the Washington Cascades and Simcoe Highlands and a 50-70% chance along the Blue Mountain foothills.

The upper level trough's axis will pass through our area late this afternoon, which will coincide with the highest rain/snow rates during this event. High temperatures will be about 5 degrees cooler today than on Wednesday, with highs peaking in the upper 40s to low 50s across the Eastern Gorge, Lower Columbia Basin, Blue Mountain foothills, and the Yakima Valley. Highs will stay cooler and in the low to mid 40s through Central Oregon and the Kittitas Valley. Rain chances will begin to diminish over lower elevations of the Basin and the Yakima/Kittitas Valleys through the afternoon, expanding to include Central Oregon and the Blue Mountain foothills overnight into Friday morning. Conditions continue to slowly dry along the Blue and Cascade Mountains Friday afternoon as an upper level ridge begins to build offshore in the wake of the departing system. Northwest flow aloft will attribute to a cooler conditions through the remainder of the short term period, which correlates to highs peaking into the low to mid 40s for lower elevations both Friday and Saturday. An embedded shortwave will approach the coast early Saturday, allowing showers to return during the late afternoon and extend into Sunday. This synoptic feature allows for a pressure gradient of 4-6mb between Baker City and Meacham, which relates to gusts of up to 48 mph.
This would incur a marginal wind advisory event, but will hold off until guidance comes into better agreement with these wind gust values. Snow levels will be increasing quickly through the day from the southwest, initially between 500-1500 feet Saturday morning and increasing to between 2000-4000 feet north-to-south by the late afternoon/early evening. Lower elevation rain and mountain snow continue through the remainder of the weekend as cooler temperatures are anticipated into the extended period. 75

LONG TERM
Sunday through Thursday
The long term will be somewhat active to begin with as a shortwave trough moves across bringing low elevation rain and mountain snow, followed by a ridging that should quiet conditions through the end of next week.

The long term begins as a shortwave trough is already moving into the region, with the warm front lifting across. The cold front and main section of the trough will then move through, exiting by Tuesday at the latest as ridging then builds back in. Models remain in good agreement that this system should be mostly inconsequential in terms of overall impacts. Snow levels will be relegated to above 5-6k feet, with overall QPF totals around a tenth to a third of an inch in the lower elevations, and 1-2 inches in the mountains. With the heightened snow levels, this will keep snowfall of around 5 to 10 inches above 5k feet. A tightened pressure gradient from this system could also produce breezy winds, but remaining below advisory criteria.

Into Tuesday on the backside of the system with ridging building in, a cooler northerly flow is expected, bringing temperatures back down. Highs Sunday and Monday will range in the 40's to low 50's for the population centers of the CWA, but should only max in the mid 40's by Wednesday. For lows, this will bring back sub- freezing temperatures, with widespread 20's to low 30's outside of the Columbia Gorge. An approaching system will start to shift the ridge eastwards, but model discrepancy builds into the end of the long term as deterministic guidance struggles to resolve this feature. Ensemble guidance generally favors ridging through the end of the period, with only 12% of members expecting the ridge to break down on Thursday. The NBM paints generalized slight to chance of precip during the later part of the week, likely due to these outliers, but the expected outcome is that we remain on the drier side as the ridge holds on until the weekend after next.
Goatley/87

PRELIMINARY POINT TEMPS/POPS
PDT 49 35 41 27 / 40 30 20 0 ALW 50 36 42 28 / 50 50 20 0 PSC 51 37 46 31 / 20 10 0 0 YKM 48 29 45 25 / 20 10 0 0 HRI 53 37 46 29 / 20 20 10 0 ELN 44 30 40 23 / 30 20 0 0 RDM 42 26 39 21 / 80 30 20 0 LGD 45 31 40 22 / 90 80 50 0 GCD 44 28 38 20 / 90 60 40 0 DLS 50 38 48 33 / 40 40 10 0

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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