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FXUS66 KPDT 021751 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 951 AM PST Sat Dec 2 2023

Updated for Aviation


AVIATION
18Z TAFS....Latest radar showed a swath of precipitation, some of it moderate, from just west of ALW to just east of PSC extending south into Oregon and affecting pdt and further south into the Blue Mountains. This area should continue moving eastward and will be east of the airports in the next 2 to 3 hours and out of the region by 03/00Z. There should be a lull in the precipitation...some low (30-40%) chances for BDN and RDM and not much more than a passing shower elsewhere, until rain chances increase overnight and especially toward morning on Sunday for most locations.

For the most part, conditions will be VFR, once the RA moves through until later tonight/early Sunday morning when conditions will deteriorate again. MVFR is likely and IFR can not be ruled out.

Gusty winds 25 to 35 kts with and occasional gust to 40 kts are likely through the afternoon, before decreasing to 10 kts or less for the remainder of the period.



MORNING UPDATE...Broad swath of precipitation and a front continues to push across the region this morning. The main shield of rain/mountain snow is currently entering the eastern-most portions of the CWA, and should exit through the late morning hours, with lingering activity continuing through the weekend.
This front also brought the strongest winds with many locations reaching wind advisory/high wind warning criteria in those respective products. Of note, a couple of stronger gusts seen at the Hanford Site near wind advisory criteria (45 mph), but bulk of that zone remains in the 30-35 mph range, below advisory criteria. Not expecting any stronger activity, and will continue to hold off on a wind advisory for the zone including the Tri- Cities area.

Temperatures this morning for the most part ran a bit warmer than forecast, generally by a few degrees, so made some minor adjustments to today's high temperatures, but not expecting a major deviation from the overnight forecast package. Also made some minor adjustments to winds based on ongoing conditions. The rest of the forecast package remains on track. Goatley/87

PREV DISCUSSION... /issued 354 AM PST Sat Dec 2 2023/

SHORT TERM
Today through Monday
An active weather pattern continues this morning as another shortwave trough in northwest flow aloft pushes across the PacNW. The main band of precipitation with this system has already arrived to the Cascades, with light to moderate snowfall in upslope flow across the northern Blues.

Today, the shortwave trough passage will continue to produce several impacts in the form of icy precip, snow, rain, and wind across much of the forecast area. The first of these impacts will be the next round of moderate to heavy snowfall across the Cascades, northern Blues, and the Wallowas, and a rain/snow mix in the eastern mountains. Across the Oregon half of the forecast area, yesterday's system was able to scour out the cold air at the surface, allowing today's precipitation type to be mostly rain. Across the Washington half, the colder airmass has been a bit stubborn, and has only slowly dissipated across the eastern half of the Columbia Basin; expect mostly rain in this area today.
Meanwhile further west the cold airmass is still settled in the Yakima/Kittitas valleys, and at onset of precipitation, a warm tongue is expected to override this airmass, resulting in chances of icy precipitation in these areas for several hours this morning, though light snow will be possible initially in the Kittitas valley. A transition to just rain is expected by late morning as increasing westerly winds will help to scoop out the colder airmass. The incoming shortwave trough will also produce breezy to gusty west to southwest winds across the forecast area today. Winds early this morning through the afternoon will be aided by 55-70kt low level jet at the southern edge of the trough, which is expected to mix down sustained winds around 40mph and gusts up to 60mph across areas of the north central OR and the southern Blue Mountain foothills. Across the rest of the lower elevations, CAMS indicate that this jet will be hard-pressed to mix down to the surface with generally 20-30 mph sustained winds and gusts up to 45mph, except sustained winds of 25-35mph and gusts up to 50mph are expected in the OR Columbia Basin and along the northern Blue mountain foothills.

Tomorrow, flow aloft will become more zonal as a broad, flat ridge with a warm front at the surface lifts across the PacNW. By late morning, snow levels will have risen to above 6kft-7kft across the forecast area, bringing the threat of heavy snow to end across most mountain locations. That said, precipitation impacts will not end as an atmospheric river (AR) accompanies the warm front passage tomorrow. As the warm front lifts north across the forecast area, areas of freezing rain will be possible in the mountain valleys along the WA Cascades east slopes in the early morning, which will transition over to rain by the late morning.
Ice accumulations in these areas are expected to be light, with a few hundreths of an inch possible. Otherwise, outside of the highest Cascade/Wallowa peaks, moderate to heavy rainfall across the mountains and light to moderate rain in the lower elevations is expected through much of Sunday. Breezy west to southwest winds will once again redevelop in the lower elevations, though winds are anticipated to weaker, with sustained winds of 15-25mph and gusts up to 40mph possible across the wind prone areas in central and north central OR, OR Columbia Basin, and the Blue mountain foothills.

Monday, flow aloft will turn towards to the southwest, resulting in a more mild airmass filtering into the intermountain PacNW and snow levels rising to above 9kft. Another AR will also arrive to coastal PacNW under this southwest flow aloft, resulting in another round of rain area-wide. Due to the orientation of the AR, however, the heaviest precipitation will mostly occur across the WA Cascades and western WA, though light to moderate rainfall is expected to impact most of the forecast area. Breezy west to southwest winds will continue into Monday as surface pressure gradients will be slow to relax behind the frontal passage over the next two days.

A concern going into the start of next week will be the expected rises along area rivers and streams. The newly established snowpack in the mountains is likely to be reduced as snow levels rise and precipitation turns to rain. The rivers originating from the WA Cascades will be the most at risk for seeing elevated levels due to the heavier rain amounts expected, but expect most rivers and streams to see elevated rises. Lawhorn/82

LONG TERM
Tuesday through Saturday
Latest GEFS and EC ensembles are in good agreement on the evolution of the 500 mb height pattern through Thursday which provides high confidence in the forecast. Afterward the ensemble clusters show considerable variability in the 500 mb pattern for Friday and Saturday which results in lower confidence in the forecast for the end of the week.
On Tuesday both the GFS and GEFS Ensemble Mean show a weak atmospheric river penetrating into the inland northwest with the brunt of it aimed across the Washington and Oregon Cascades in fast southwesterly flow aloft. There will be high chances for precipitation Tuesday through Wednesday areawide (60-100%, highest in the mountains) with the 50 percentile NBM QPF for the 48 hours ending 12Z Thursday ranging from 1.5-2.5 inches along the Cascade crest, 1-1.5 inches for the Blue Mountains and .25-.50 inches for the lower elevations. Snow levels will start out at 9000 feet MSL on Tuesday morning and gradually lower to 4000 feet by 12Z Thursday, so most of the precipitation during this period will fall as rain.
Hydrologic conditions will need continued monitoring through midweek as rivers and streams will continue to rise.

There will be a chance of Showers on Thursday and Thursday night as an upper trough edges closer to the coast. The NBM was used for this forecast and POPS range from 20-40 Percent in the lower elevations and around 60 percent over the mountains. Because trajectories with this trough are more northwesterly an atmospheric river is not expected and QPF amounts will be lighter. Snow levels will also lower to 2000-3000 feet. Thus some light mountain snow is anticipated. On Friday and Saturday there is less certainty in the forecast as the ensemble clusters differ on the depth and speed of the aforementioned trough and a subsequent building ridge behind it.
However an overall trend toward cooler and somewhat drier weather is expected by next weekend. 78

AVIATION
12Z TAFs
An area of rain will move across the region this morning. -FZRA being reported at KYKM may continue until about 15-16Z. As a front moves through winds will pick up significantly with gusts 25-35 kt. Any IFR CIGS and VSBYS this morning will be scoured out by about 18-20Z. Tonight another storm system will approach from the southwest. CIGS and VSBYS will gradually lower and become MVFR/IFR at KRDM, KBDN, KDLS, and KYKM with -RA and BR redeveloping. 78

PRELIMINARY POINT TEMPS/POPS
PDT 49 38 54 45 / 80 70 100 30 ALW 49 38 54 45 / 90 40 100 40 PSC 52 38 53 43 / 70 30 90 20 YKM 48 30 45 34 / 80 30 90 20 HRI 52 40 56 45 / 80 70 90 20 ELN 43 29 43 33 / 90 20 90 10 RDM 47 37 54 45 / 80 70 80 30 LGD 41 33 45 41 / 100 80 100 60 GCD 44 34 48 40 / 90 90 100 50 DLS 49 40 53 46 / 100 90 100 50

PDT WATCHES/WARNINGS/ADVISORIES
OR...Wind Advisory until 6 PM PST this evening for ORZ044-507.

Winter Storm Warning until 10 AM PST Sunday for ORZ502.

Winter Weather Advisory until 4 AM PST Sunday for ORZ503-506.

High Wind Warning until 1 PM PST this afternoon for ORZ508-510.

Winter Storm Warning until 4 PM PST this afternoon for ORZ509.

WA...Wind Advisory until 6 PM PST this evening for WAZ029.

Winter Storm Warning until 10 AM PST Sunday for WAZ030-522.

Winter Storm Warning until 4 AM PST Sunday for WAZ523.

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