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FXUS66 KPDT 302309 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 309 PM PST Thu Nov 30 2023

UPDATED AVIATION DISCUSSION



PREV DISCUSSION
/issued 150 PM PST Thu Nov 30 2023/

SHORT TERM
Tonight through Saturday
Weather pattern continues to undergo a change from the dry ridging pattern to a more active and wet pattern through the weekend. This is currently taking place as the first in a series of systems is slowly moving into the forecast area switching us to a northwest flow pattern. This system is weakening as it pushes into the Pacific Northwest but will still produce some light precipitation as it moves across the forecast area late today and overnight before exiting Friday morning. Low snow levels combined with the cold air pool trapped in the Columbia Basin means the precipitation type will be primarily snow but with just light accumulations expected. Expect a dusting to up around an inch in the lower elevations while mountains could see 2 to 4 inches.

The next system in the northwest flow arrives Friday with more abundant moisture and increasing winds. These winds will aid in scoring out the cold air pool trapped in the lower elevations resulting in a change from snow to rain during the day. The mountains will begin seeing more significant snow accumulations associated with this system with snow levels 2000 to 3000 feet.

Yet another system is expected to pass over the area on Saturday into early Sunday as a warm front. In addition to the expected wet conditions this system will begin to raise snow levels through the day and overnight to around 3000 to 4500 feet Saturday afternoon and then 4000 to 6000 feet Saturday night. This will lift snow levels above most pass levels in the forecast area. Will also see some breezy to windy conditions which will approach advisory levels and will be monitored for the need for a possible advisory. Also, numerous Winter Storm Warnings and Winter Weather Advisories remain in effect through Sunday morning.

LONG TERM
Sunday through Thursday night
The long term will be characterized by a active weather pattern. Two separate atmospheric river events will bring plenty of precipitation to the area. These systems will also bring warmer temperatures back to the region with snow levels rising back to above 7000ft by midweek. This will allow the precipitation to transition back to rain with the exception of the higher peaks. With the rain on snow occurring, there may be some hydrologic concerns that arise. This will continue to be monitored over the course of the two atmospheric river events.

Sunday through Monday models are in firm agreement with the continuation of the atmospheric river event. The continuation of the event will bring with it more precipitation as well as warmer temperatures. Total QPF Saturday to Sunday will near two inches or more along the Cascades with 0.3 inches along the Cascades foothills, 0.10 inches through the Columbia Basin and increasing to 0.5-1 inches along the foothills of the Blues and Blue Mountains.
Probabilities of these accumulation amounts are above 85% for the Cascades and foothills of the Cascades, 75% through the Columbia Basin, and 50-80% for the foothills of the Blues and Blue Mountains.
The beginning of the period will see a rain snow mix along the Cascades, transitioning to rain as you head farther east then a rain/snow mix through the Blues as the snow levels will be around 4000ft and lifting to above 7000ft through Monday. EFI is showing the precipitation amounts to be well above seasonal (0.9-0.93) and signaling temperatures to be above average temperatures for this time of year (0.79-0.8) with the 39% of the ensembles showing temperatures in the 50-55 degree range and 26% showing temperatures of 55-60 degrees. These temperatures are 10-15 degrees above the average across the region.

As we move into Tuesday, models begin to diverge slightly but mostly with the amplitude of the next incoming system. Models show a brief ridge will move over the region however, precipitation is still expected with this system as another atmospheric river has set itself up to flow across the region. The western and northern sections of the CWA will be the areas that will see the highest chances of rain. Again, warmer temperatures will accompany the system and snow levels will crest 8000ft, so all precipitation will fall as rain with the exception of the peaks of Wallowa Mts. Total 24 hour QPF amounts show that there is an 82% chance that the WA Cascades will see an additional 2 inches of rain, 25-35% chance of the Columbia Basin and Gorge will see an additional 0.25 inches and 40-6-% chance that the foothills of the Blues and Blue Mts will also see an additional 0.25 inches. EFI continues to show above average QPF amounts as well as temperatures. 54% of the ensembles show that temperatures will be in the 50-55 degree range again on Tuesday.

Wednesday through Thursday night, models continue to diverge ever so slightly. These slight discrepancies are not enough to drop confidence in the forecast. The models show the continuation of the atmospheric river flowing into the region with the main variance being the positioning of the axis. Even with these variances, all the models as well as the ensembles show the continuation of precipitation. Precipitation will continue to be rain as the snow levels remain above 6000-8000ft with warm temperatures across the region. EFI is continuing to show above average temperatures as well as above average QPF for this time of year. However, rain amounts do dwindle and remains primarily locked along the mountains where there is above 50% chances the Cascades and above 40% chances the Blues could see an additional 0.25 inches of rain. Temperatures continue to be above normal however slightly cooler than previous days with 42% of the ensembles in agreement of 40-45 degrees. Snow levels will decrease again to 3000-5000ft so the precipitation along the Cascades will fall as a rain/snow mix while remaining rain elsewhere. Bennese/90

AVIATION DISCUSSION
00Z TAFs
All Taf sites will bounce between VFR-MVRF (BDN/RDM) and MVRF/IFR with chances of LIFR for the remaining sites. This is due to low CIGs of OVC009 and below, VIS below 2SM, snow and heavy mist. All sites will remain under these conditions through the period with light and variable winds below 5kts. BDN/RDM will see winds increase around 16Z with sustained winds of 15-30 kts with gusts of 25-30 kts likely. Bennese/90

PRELIMINARY POINT TEMPS/POPS
PDT 29 41 34 47 / 90 90 50 80 ALW 30 39 35 46 / 80 100 60 90 PSC 31 39 35 50 / 80 90 20 60 YKM 29 41 33 49 / 70 90 30 60 HRI 32 44 36 51 / 90 90 30 60 ELN 28 38 30 43 / 60 90 50 70 RDM 29 42 31 46 / 80 80 40 70 LGD 28 39 34 42 / 80 100 90 100 GCD 28 40 30 43 / 90 100 70 90 DLS 33 47 37 50 / 90 100 60 90

PDT WATCHES/WARNINGS/ADVISORIES
OR...Winter Storm Warning from 4 AM Friday to 10 AM PST Sunday for ORZ502.

Winter Weather Advisory from 4 AM Friday to 4 AM PST Sunday for ORZ503-506.

Winter Weather Advisory until 4 AM PST Friday for ORZ509.

Winter Storm Warning from 4 AM Friday to 4 PM PST Saturday for ORZ509.

WA...Winter Storm Warning from 4 AM Friday to 10 AM PST Sunday for WAZ030.

Winter Storm Warning until 10 AM PST Sunday for WAZ522.

Winter Storm Warning from 4 AM Friday to 4 AM PST Sunday for WAZ523.

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