Sunrise 6:02 AM   Sunset 7:50 PM Moonrise 2:40 PM   Moonset 4:25 AM

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FXUS66 KPDT 181115 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 415 AM PDT Thu Apr 18 2024

Updated Aviation Discussion

AVIATION
12Z TAFS...VFR for all sites with current clear skies.
KRDM/KBDN/KYKM might have cloud (FEW to BKN, 100-250 kft) come by this late morning into tonight (40-50% confidence). Winds will remain light while shifting mostly north and east throughout the day. Feaster/97

SHORT TERM
Today through Saturday night...Nighttime microphysics RGB imagery reveals mostly clear skies across the forecast area early this morning. The region is located under a dry northwesterly flow aloft on the back side of a broad trough downstream over the Northern Great Plains. Offshore, an upper- level ridge axis is apparent and is tracking slowly eastward.
Coupled with low PWATs, low surface dew points, and a relatively weak offshore-oriented surface pressure gradient translating to light winds (<10 kts for most non-ridgetop areas), temperatures have dropped to near freezing for most lower-elevation zones. The freeze warnings for the Yakima and Kittitas valleys, lower Columbia Basin of OR/WA, and foothills of the northern Blues of OR/WA remain in place through 9am this morning.

This afternoon, isolated showers are forecast to develop, supported by weak instability and some modest upper-level forcing, but chances of measurable precipitation are very low (5-14%)
forecast for the WA Cascades and Blues of NE OR and SE WA.

As the inverted surface pressure trough strengthens this afternoon and Friday, offshore east to northeast winds are forecast to become locally breezy for north-central OR and south- central WA especially along ridgetops.

Friday morning, chances of freezing temperatures are lower than this morning, but the same zones in this morning's freeze warnings have a 20-60% chance of freezing temperatures per NBM probabilities so additional highlights may be needed.

Saturday morning, the upper-level ridge axis is expected to move over the PacNW. Saturday afternoon through Saturday evening, ensemble guidance is showing a pattern change with an upper-level trough moving into the PacNW. While ensemble clusters are not available for analysis this morning, ensemble mean H500 and vorticity are exhibiting some differences in timing and location.
The best chances of precipitation will be across the mountains (25-50% for the Blues, 55-90% for the Cascade crest and eastern slopes) with lower chances (10-40%) for the lower elevations.
Breezy westerly winds are forecast with the trough passage, but confidence is currently low (<40% chance) in reaching advisory criteria. Plunkett/86

LONG TERM
Sunday through Tuesday...The long term will begin with the remnants of the weak upper level trough that will move over the region. The precipitation with this upper level shortwave will be light an mainly across the Cascades bringing a light dusting of snow. By Sunday night the shortwave will dissipate making way for the leading edge of an upper level ridge to make its way towards the region bringing in the dry and warm temperatures through Wednesday.

Sunday models are in relatively firm agreement through the period with the only variances being in the positioning of the upper level trough with a very slight timing variance. With this upper level system, a brief down tick in the snow levels will allow Snoqualmie Pass to see snow. 70-90% of the raw ensembles show up to 0.3 inches of snow along Snoqualmie Pass with higher amounts along the crests. Much of the energy associated with this system is poised across the WA Cascades with raw ensembles showing less than 30% chances of up to 0.10 inches along the OR Cascades and the Blues. Pressure gradients will continue to tighten as the trough slips over the mountains and raw ensembles show 30-50% probabilities winds will be sustained winds of 20 to 25 mph along the lower Columbia Basin and the foothills of the Blues.
Temperatures will remain in the upper 50s to low 60s across the region and mid 60s through the Basin, with the majority (90%) of the ensembles in agreement.

Monday through Tuesday the models remain in firm agreement with the upper level ridge making its way onshore. Models show the leading edge of the ridge to be encroaching the region Monday with the variances being in the timing and amplitude of the ridge. Dry and warming conditions will accompany the ridge with global ensembles all showing agreement to an uptick in temperatures. Raw ensembles show over 60% probabilities Central/N Central OR and the foothills of the Blues will see temperatures in the mid 60s, over 50% show the Basin in the high 60s to low 70s. Over 50% of the global ensembles show that Tuesday will see temperatures rise another 2-5 degrees. Winds will be mostly from the southwest and becoming diurnally breezy. Bennese/90

Wednesday morning to Thursday night, a closed low moves over from the OR coast with precipitation that will impact the PacNW as it continues to move southerly towards the CA coast. Mostly rain will be impactful in the low elevations with some very light snow in the Cascades crests and Blue Mountains, with snow accumulations less than 0.01 inches and QPF less than 1 inch (30-40% confidence). Snow levels will be around 5000-7000ft Wednesday into Thursday night as the trough passes. Temperatures continue being in the 50s along the crest of the Cascades and 60s to low 70s at the low elevations (>60% confidence). Breezy conditions will occur with wind gusts up to about 22 mph across the Simcoe Highlands Wednesday afternoon but might come and go as throughout Thursday.
Feaster/97

PRELIMINARY POINT TEMPS/POPS
PDT 59 31 63 35 / 0 0 0 0 ALW 62 34 66 38 / 0 0 0 0 PSC 65 38 68 40 / 0 0 0 0 YKM 64 34 65 34 / 0 0 0 0 HRI 64 34 68 37 / 0 0 0 0 ELN 60 33 61 33 / 0 0 0 0 RDM 59 28 62 33 / 0 0 0 0 LGD 57 29 60 34 / 0 0 0 0 GCD 58 30 63 36 / 0 0 0 0 DLS 66 39 67 42 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES
OR...Freeze Warning until 9 AM PDT this morning for ORZ044-507.

WA...Freeze Warning until 9 AM PDT this morning for WAZ026>029.

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