Sunrise 5:41 AM   Sunset 8:16 PM Moonrise 2:51 AM   Moonset 1:52 PM

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FXUS66 KPDT 031153 AFDPDT

Area Forecast Discussion...UPDATED AVIATION.
National Weather Service Pendleton OR 452 AM PDT Fri May 3 2024

SHORT TERM
Today through Sunday...A transitory ridge of high pressure will be passing over the region today. This will provide a mild Spring day with temperatures getting into the low 70s around the Basin and 60s in the mountains. High clouds will begin to increase and spread across the forecast area late today and overnight as the next weather system approaches. This closed upper level low pressure system is dropping SE out of the Gulf of Alaska with the center of the low taking a path into SW Oregon Saturday afternoon then continuing into the Great Basin on Sunday. This will bring the leading edge of a cold front into the Cascades tonight but its eastward progress overnight and into Saturday will be slowed as the low center moves into SW Oregon. Eventually the cold front turns into a deformation band of precipitation Saturday night into Sunday as moisture wraps around the low back across northern Oregon and southern Washington. 48 hour QPF amounts ending 12Z Monday continue to show a very high probability that lower elevations could see .5 - 1.0 inches of precipitation while the eastern mountains and Cascade crest could see 1 - 2 inches of precipitation. This amount of precipitation is expected to cause streams and rivers to rise and peak early next week but remain below flood stages.

Snow levels will initially be high Friday night and early Saturday around 6000 to 7000 feet. They will begin to lower Saturday afternoon and night as the low passes south of the forecast area.
Central Oregon will see snow levels down to 3500 to 4500 feet Saturday night but remain 4500 to 6000 feet across the rest of the forecast area. This could produce 4 to 10 inches of snow along the Oregon Cascade crest impacting travel over the passes so a Winter Weather Advisory has been issued Saturday morning through Sunday morning.

Other concern will be some increasing S-SE winds overnight and Saturday. This will produce some gusty winds over the higher terrain of eastern Oregon and cause some downsloping off of the Blue Mountains along the Foothills. A wind shift back to windy westerly winds is expected to take place late Saturday and continue through Sunday.

LONG TERM
Monday through Thursday...Another, weaker weather system will move across the Pacific Northwest on Monday and bring another round of precipitation, mainly to the mountains. However, there will be chances even in the lower elevations Monday afternoon.
QPF amounts will be fairly light in most areas. In the mountains QPF will be a bit higher, ranging from 1/4 to 1/2 inch along the crest of the Cascades to around 1/4 inch in the Blues and Wallowas.
Most other areas will see only a few hundredths.

Precipitation chances will end everywhere except over the mountains by Tuesday morning. Some additional shortwave energy could enhance precipitation chances again Tuesday afternoon, mainly over the eastern Oregon mountains, but overall QPF is expected to be very light...only a few hundredths in most locations.

Snow levels start out at 4000 feet or higher on Monday, but drop to between 2500 and 3000 feet by Tuesday morning. So, certainly the precipitation will mix with and change to snow in the higher elevations of the Cascades and Blues, and a few inches are possible along the crests with little elsewhere. The ECMWF EFI focuses on precipitation and snow over the Blue Mountains and nearby foothills with and area of 0.6 to 0.8 on Monday.

Also on Monday, there is at least some chance of thunderstorms mainly over the eastern Oregon mountains, as this next systems moves through. CAPE values are expected to be in the 100 to 300 J/KG range, though the best CAPES look to be further southeast. LI values look to be between 0 and -2 degrees C.

By Tuesday evening into Wednesday and for the remainder of the period, a ridge will build over the Pacific Northwest. This should bring drier weather and warmer temperatures. A few shwoers over the eastern mountains can't be ruled out on Thursday, but confidence is low (<30%) at this time.

Winds will start out quite gusty as a result of the pressure gradient between the departing trough in the Great Basin and higher pressure to the west. WInds will gust 25 to 35 mph and possibly as high as 40 mph in portions of the Columbia Basin, Blue Mountain Foothills, Columbia Gorge. Winds will decrease during the day but remain gusty and become more widespread as the next system approaches. Gusty winds look to continue into Tuesday, with speeds 25 to 35 mph, before decreasing into Wednesday.

The ECMWF EFI keys in on the Columbia Basin and Foothills of the Blue Mountains for winds on Monday, with values ranging from 0.6 to 0.8, with a core of 0.8 to 0.9 over the Blue Mountains.

The NBM probabilities of wind gusts >= 39 mph on Monday are 70 to 90% across much of the Basin and Blue Mountain Foothills Simcoe Highlands and Cascade Gaps.

On Tuesday, the NBM probabilities of wind gusts >=39 mph are 80% plus across much of the Basin, Gorge, Blue Mountain Foothills , Simcoe Highlands, and Kittitas Valley. The probabilities of wind gusts >=47 mph are 50 to 70% across portions of the Columbia Basin, Foothills of the Blue Mountains, Simcoe Highlands and 60-90% across the Kittitas Valley.

High temperatures on Monday will start of 10 to 15 degrees below normal and will mainly range from the 50s in central Oregon to the 60s across the Columbia Basin. By Wednesday, high temperatures will begin to rise and by Thursday, highs will be close to normal in most areas and above normal in the Basin.

The ECMWF EFI shows this trend as well for high temperatures. On Monday, there is a large area of -0.7 to -0.8 across most of Oregon, and -0.6 to -0.7 across much of southeastern Washington. On Tuesday, the area moves a bit eastward. There is a large area of - 0.7 to -0.8 across much of the region and even an area of -0.8 to - 0.9 over the Oregon Blue Mountains. By Wednesday, most of the anomalous temperatures are gone in the EFI.



AVIATION
12Z TAFS...VFR conditions and little concerns area-wide before elevated aviation concerns begin right at the end of the TAF period. Latest nighttime microphysics RGB imagery shows large clear skies, except for a few swaths of thing high clouds crossing the Cascades and areas of low stratus in the higher terrain of central OR and the central OR mountains.

The next weather system offshore in the eastern North Pacific will be approaching the OR coast tonight with its attendant cold front reaching the Cascades near the end of the TAF period. There will be increasing mid and high clouds out of the system late morning- afternoon before low-end VFR ceilings (less than 7 kft) materialize west to east early tonight as moisture gets transported from south to north in tandem with increasing large scale forcing for ascent.
High confidence in low-end VFR ceilings late tonight, >70%, with increasing potential for sub-VFR ceilings across central OR. Latest LAMP ceiling probs have been growing the last three runs with chances for MVFR now >30% at KBDN and KRDM after 09 UTC. Current thinking is MVFR flight categories may need to be introduced if this trend continues and timing of the system speeds up. Already high chances for rain that coincide with this potential after 09 UTC tonight (>70%). Otherwise, increasing southerly to easterly winds this afternoon ahead of the system as it evolves south and east the next 24-hrs, however, potential for frequent gusts up to 20 kts is low (less than 50%) at all sites.

PRELIMINARY POINT TEMPS/POPS
PDT 70 51 67 41 / 0 10 50 100 ALW 72 55 71 44 / 0 10 40 100 PSC 73 55 72 45 / 0 10 40 90 YKM 69 48 65 42 / 0 20 80 70 HRI 75 54 70 43 / 0 10 50 90 ELN 66 44 61 42 / 0 20 80 70 RDM 65 41 50 32 / 0 70 90 70 LGD 64 46 63 39 / 0 0 50 100 GCD 63 44 61 35 / 0 10 90 100 DLS 72 50 57 46 / 0 60 90 90

PDT WATCHES/WARNINGS/ADVISORIES
OR...Winter Weather Advisory from 5 AM Saturday to 11 AM PDT Sunday for ORZ509.

WA...None.

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