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FXUS66 KPDT 081130 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 330 AM PST Fri Dec 8 2023

SHORT TERM
Today through Sunday Night...Current radar and infrared satellite imagery showing light to moderate returns over the Cascade crest and the Northern Blue Mountains under mostly cloudy skies. This is in response to the upper level trough that passed through our area late Thursday afternoon as it continues to dig to our southeast and approach the Rockies later this morning. This will promote upslope rain/snow showers through much of the morning for our mountain zones as an upper level ridge begins to build offshore. However, so far snow amounts have been lacking and are not expected to reach advisory criteria between 3500-5000 feet along the Cascades, so have elected to cancel the advisory early. The Winter Weather Advisory over the Northern Blue Mountains is still active, as 5 to 10 inches of snow is still expected to fall through the expiration time of 10AM. Confidence in these snow amounts are marginal (40-60%) as NBM probabilities are lacking for advisory snow amounts over the Northern Blue Mountains, but recent upslope enhancements over the last several hours have provided enough confidence to keep the advisory active.
Snow showers will continue over the Cascade and Blue Mountains through the morning before tapering off by early afternoon. Breezy winds across the east slopes of the Washington Cascades and the Blue Mountains/foothills, associated with the passing system, are also continuing to subside in the wake departing system. In accordance with the digging trough to our southeast and a building ridge offshore, northwest flow aloft will continue through the day today, providing a 2-6 degree drop in afternoon temperatures.
The real difference will come Saturday morning as mostly clear skies tonight will allow low temperatures to drop into the upper 20s to low 30s across lower elevations of the Basin and Blue Mountain foothills, and in the low to mid 20s across Central Oregon.

The upper level ridge approaches the coast tonight and moves onshore early Saturday, continuing to cool high temperatures another 3-8 degrees to reach values in the low to mid 40s across the Basin and Central Oregon. This synoptic feature will lead to breezy conditions through the afternoon and evening through the Grande Ronde Valley, as gusts up to 40 mph will be possible.
Confidence in these wind values is high (~80%) as the GFS, NAM, and SREF all highlight a 4-6mb gradient between Baker City and Meacham. An upper level shortwave will approach the coast Saturday afternoon, suppressing and weakening the ridge to allow a return to mountain snow and lower elevation rain. Rain/snow chances will accumulate along the Cascades and east slopes through the afternoon before spilling into the Basin during the evening hours.
Another weak Atmospheric River (AR) is associated with this system, which will allow for 4-10 inches of snow above 4000 feet along the Washington Cascades and 3-6 inches over the North Blue Mountains. These amounts are marginal advisory criteria, so highlights are likely to be issued for this event over the next 24 hours. Confidence in these snow amounts is moderate to high (60-80%) as the NBM shows a 50-60% of 4 inches of snow or more over the Northern Blue Mountain, and a 60-70% chance of 8 or more inches over the Washington Cascades. The upper level ridge continues to weaken into Sunday as the shortwave slowly passes through the area over the late morning and early afternoon, keeping mountain snow and lower elevation rain in the forecast through the weekend. This will also allow flow aloft to shift toward the west, helping to increase both low and high temperatures on Sunday. Low morning temperatures will drop into the mid to upper 30s as highs peak in the mid to upper 40s across the Basin, Blue Mountain foothills, and Central Oregon. 75


LONG TERM
Monday through Friday...The long term remains a problematic period as models see significant discrepancies in the expected pattern, but base agreement through at least Wednesday gives a level of confidence to what we can expect by the end of next week.

Monday begins with our region on the backside of a passing shortwave trough. This system will continue to the east, with high pressure ridging controlling the pattern by Tuesday. This will allow rain and mountain snow to exit through the day, with a cool northerly flow then moving in as high pressure ridging overtakes by Tuesday. A glancing shortwave could pass across and bring a mountain snow shower or two across the eastern ranges of Oregon and Washington, but impacts will be minimal. Into Wednesday, ridging continues to hold the PacNW, but by the afternoon and evening is where the model discrepancies build significantly. An approaching front will be the source of the differences, as the European model believes this will pass across the region, bringing a quick hit of precipitation, whereas the GFS expects the majority of this feature to be shielded off up north into Canada, with the ridging holding firm. This causes the model solutions to then completely diverge from each other, with the GFS keeping us under the ridging, and the ECMWF placing us in between the aforementioned system and ridging building back in.

Analyzing the ensembles of the major models gives us a little bit more confidence to the overall forecast. When the expected trough is supposed to pass across Wednesday through Thursday, ensembles only indicate around 20% of members in agreement with this solution. Meanwhile, cluster phase space diagrams indicate that by the last few days of the forecast period, the GFS has better agreement with its ensemble members and less overall spread than compared to the ECMWF solution. All of this together places a bit more confidence in the GFS's expected outcome, and collaboration with neighboring offices also agree that this is the more likely solution. Still, the NBM places a swath of general precipitation chances Tuesday through Friday, but these are likely overdone thanks the other wetter solutions. Some adjustments to the precipitation chances on Tuesday were necessary to temper down the expected rain and limiting it to the eastern side, but otherwise have kept the more widespread slight to chance in during the later period to account for uncertainty in the forecast. Overall there is moderate to high confidence in the forecast through Wednesday (60-80%), with low confidence thereafter (30%). Goatley/87



AVIATION
12Z TAFs...VFR conditions expected over the next 24 hours. Rain is exiting the region, with breezy winds continuing for PDT/ALW/PSC through the beginning of the afternoon.
Thereafter, winds less than 10 kts all sites anticipated. CIGs lifting with high clouds by the nighttime hours. Goatley/87



PRELIMINARY POINT TEMPS/POPS
PDT 42 29 44 35 / 20 0 10 70 ALW 44 30 44 37 / 30 0 10 90 PSC 47 31 41 36 / 10 0 20 70 YKM 46 28 38 33 / 0 0 50 70 HRI 47 29 43 36 / 10 0 20 70 ELN 41 26 37 31 / 0 0 60 90 RDM 39 21 43 33 / 20 0 10 30 LGD 39 25 39 33 / 60 0 10 80 GCD 38 21 42 33 / 40 10 10 50 DLS 48 33 42 37 / 10 0 60 80

PDT WATCHES/WARNINGS/ADVISORIES
OR...Winter Weather Advisory until 10 AM PST this morning for ORZ502.

WA...Winter Weather Advisory until 10 AM PST this morning for WAZ030.

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