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FXUS66 KPDT 101143 AAA AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 350 AM PST Sun Dec 10 2023

Updated Aviation Discussion

SHORT TERM
Today through Tuesday night...Current radar and infrared satellite imagery showing widespread light to moderate returns streaming into the area from the west, providing mountain snow and lower elevation rain under cloudy skies. This is a result of an upper level shortwave trough riding over and suppressing the broad upper level ridge that is centered over the Pacific Northwest. This shortwave does have a weak Atmospheric River associated with it, adding to the complexity of the forecast as warmer air present with this system has led to periods of freezing rain along the east slopes of the Washington Cascades and the Blue Mountains. This threat of freezing rain and mountain snow accumulations have warranted the issuance of multiple Winter Weather Advisories across our mountain zones. A Winter Weather Advisory remains in effect across the upper slopes of the Washington Cascade crest until 10AM Sunday as 4 to 10 inches of snow and ice accumulations of up to two tenths of an inch are expected. There is also a Winter Weather Advisory for the lower slopes of the Washington Cascades until 10AM Sunday with 3 to 7 inches and ice accumulations of up to two tenths of an inch likely. A Winter Weather Advisory has been issued for the Kittitas Valley until 10AM Sunday for 1 to 3 inches of snow and ice accumulations of up to one tenths of an inch. The Yakima Valley also has an active Winter Weather Advisory through 10AM Sunday for 1 to 2 inches and ice accumulations of up to a tenth of an inch.
Winter Weather Advisories also extend across the Blue Mountains, with the Northwest Blue Mountains of Washington's advisory in effect until 10AM due to snow accumulations up to 6 inches. The remaining Blue Mountain zones also have active Winter Weather Advisories that extends until 4PM Sunday as snow accumulations of 3 to 9 inches and ice accumulations of up to two tenths of an inch are likely over the Northern Blue Mountains, and up to 3 inches and ice accumulations of up to a two tenths of a inch will be possible over the Southern Blue Mountains. Confidence in these snow amounts is high (75-85%) as the HRRR highlights an 80-90% chance of 6 inches of snow or more across the Northern Blue Mountains and across the east slopes of the Washington Cascades.
Confidence is ice accumulation is moderate to high (60-80%) as the HRRR advertises a 70-80% chance of ice accumulations of two tenths of an inch or more across the Blue Mountains (including along the I-84 corridor), the Washington Cascades (including along the I-90 corridor), and a 30-40% chance over the Southern Blue Mountains.

There is also a Wind Advisory until 4AM for the Grande Ronde Valley (specifically the Ladd and Pyles Canyons) due to wind gusts of up to 45 mph. Currently, winds are still consistent between 40-45mph at the Charles Reynolds Rest Area along I-84 so this advisory may need to be extended past 4AM. The temperature gradient between Baker City and La Grande is only a couple degrees, but the pressure gradient is still 4-5mb and is expected to slowly decrease through the morning hours. This will be monitored over the next hour in case and extension is warranted.

The upper level ridge will continue to weaken and suppress through the day today as the shortwave slowly passes through the area.
This will keep precipitation chances at elevation over our mountain zones as chances for lower elevations of the Basin will wane through the afternoon and evening. Snow levels have already soared into the 6500-7500 foot range through Central Oregon, the Eastern Gorge, Lower Columbia Basin, and the Yakima Valley to allow temperatures to slowly warm since Saturday afternoon. Low temperatures will only drop into the upper 30s to low 40s across the Basin and along the Blue Mountain foothills as today's highs peak into the upper 40s to low 50s. The upper level ridge attempts to rebuild as a dry, backdoor cold front slides down the front side of the building ridge on Monday. This brings slightly cooler temperatures our way through Tuesday with afternoon highs in the low to mid 40s for lower elevations of the Basin, Central Oregon, and across the Blue Mountain foothills. The upper level ridge and associated northwest flow aloft infiltrates into the Pacific Northwest on Tuesday, keeping conditions dry, winds light, and temperatures 2-4 degrees warmer than normal. 75


LONG TERM
Wednesday through Sunday...No major weather concerns are anticipated at this time during the long term period--at least nothing that looks like a big winter or wind event. Models are in great agreement through Friday and almost mirror each other in the synoptic pattern along the eastern Pacific and the PacNW. Therefore confidence is high in the forecast through the end of the work week.
A high pressure ridge on Wednesday will take on a NE-SW orientation due to an increasing SW flow aloft off the coast. The forecast will show dry conditions on Wednesday but an increasing chance (50-60% north and 40% south) along the WA/OR Cascades Wednesday night. A Pacific front is progged to gradually travel across the region late Wednesday night into Thursday, but models advertise the front stretching apart. Any chance of precipitation (only 15-30%) will be primarily be over the mountains on Thursday with less than 0.10" QPF and snow levels around 4000 feet. After the passage of the front, high pressure will return with mostly dry conditions (confidence around 60%). There is a hint of a warm front along the WA coast spreading precipitation as far as the WA Cascades, mainly north of the I-90 corridor.

Confidence in the long term forecast is fairly low over the weekend, but none of the models are showing any strong storm systems. The differences are mostly with a shortwave trough over WA. The deterministic ECMWF is considerably faster than the GFS, and the Canadian is somewhere in between. The deterministic EC quickly pushes the trough across WA on Saturday, but the GFS is about 24 hours slower. The resolution of the ensembles are too low to show details of the shortwave but the ensemble means mostly show a flat ridge over the weekend. The main impacts with the shortwave will be precipitation over the mountains and breezy winds, but only slight chance based on the limited upper level support. Wister/85



AVIATION
12Z TAFs...Periods of light to moderate rain will be observed for the next 24 hours at all TAF sites. VSBYs will vary from 3-6SM and the CIGS will generally range from 3-6K feet.
Therefore, there will be MVFR conditions observed, but a chance (40%) of IFR, mainly at KDLS, KPDT and KALW. Current temp at KYKM is 33F, so I can't rule out light freezing rain near the airport.
Winds today will mainly be from the SW-W 5-15kts, although KBDN will have stronger SW winds 20G30kts this morning. Wister/85



PRELIMINARY POINT TEMPS/POPS
PDT 49 39 43 34 / 90 60 30 10 ALW 48 39 43 35 / 100 60 40 10 PSC 46 37 43 36 / 80 30 10 0 YKM 44 31 45 30 / 40 20 10 0 HRI 48 38 45 34 / 80 40 20 0 ELN 41 30 42 30 / 50 10 0 0 RDM 48 37 46 28 / 50 50 30 0 LGD 39 34 42 31 / 100 90 60 10 GCD 45 36 45 30 / 80 90 60 10 DLS 48 41 50 39 / 90 60 20 10

PDT WATCHES/WARNINGS/ADVISORIES
OR...Wind Advisory until 4 AM PST early this morning for ORZ049.

Winter Weather Advisory until 4 PM PST this afternoon for ORZ502- 503.

WA...Winter Weather Advisory until 10 AM PST this morning for WAZ026- 027-030-522-523.

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