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FXUS66 KPDT 251754 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 1054 AM PDT Thu Apr 25 2024

Updated Aviation Discussion

AVIATION
18Z TAFS...VFR are currently favorable for most sites, but MVFR conditions for KBDN due to low CIGs and VSBY this afternoon until VFR again tonight. Due to the arrival of the cold front moving over PacNW, rain and wind gusts become the highlight for this TAF period. Winds will be moderate at 10-15 kts for sites KBDN/KYKM/KPSC through morning/afternoon, with wind gusts at 20-22 kts. These conditions should improve later this evening into tonight. Feaster/97

MORNING UPDATE
Very little changes to the forecast this morning. Just an update with regards to the timing of the incoming precipitation. Current radar is showing the upper level trough has made its way onshore along the west side of the Cascades.
Current observations show very little to no precipitation along the eastern slopes of the Cascades with the exception of Warm Springs seeing 0.03 inches within the last hour. Models show precipitation will steadily creep across to the east side with POPs increasing to 35-65% through central OR along the eastern mountains and through the Basin, while the eastern slopes of the Cascades seeing 70-90% POPs. Raw ensembles do show only 30-40% probabilities of the lower elevations seeing 0.05 inches of rain through today. NBM shows temperatures to be seasonal with the Basin, John Day Basin and the Gorge seeing temperatures in the low to mid 60s, elsewhere in the low to mid 50s. Bennese/90

PREV DISCUSSION
/issued 414 AM PDT Thu Apr 25 2024/

SHORT TERM
Today through Saturday
Overnight satellite imagery shows cloud cover building into the forecast area as an oncoming trough moves in to encompass the PacNW over the next couple of days. Radar already showing shower activity over most of the west side of the Cascades, and as the trough continues its onshore progression, those showers will move in to our neck of the woods by the afternoon hours at the latest. PoPs were drawn across our area based on the latest HRRR, which puts relatively light amounts across our population centers of less than a tenth of an inch through early Friday morning, with higher amounts over the foothills of the Blues. Our mountain zones look to see closer to 0.2 to 0.5 inches, with heavier amounts over the Cascades, however guidance does paint quite the bullseye over the Oregon Cascades over the next 24 hours, due to a favorable orographic lift pattern aloft. WPC QPF is anywhere from 1 to 2 inches as a result. Snow levels will remain elevated at generally above 4500 feet, but if model QPF verifies, areas around the Three Sisters and Mt Bachelor could see over a foot of snow. Did make an effort to trim down snow amounts from the last forecast, however, by tuning down SLR's, as it is a bit late in the season to see widespread 10:1's and higher.

This ongoing trough will begin to dig down into the western CONUS by Friday, leaving us under its axis during the daytime hours. Light orographic showers may spawn intermittently throughout the day, but with us under the dry slot of the low, not expecting anything too impactful. NBM does produce some slight chances for thunderstorms across primarily the Strawberries and Wallowa County, with MUCAPEs around 400 J/Kg and moisture just ample enough at the mid-levels, however shear is pretty weak and overall column moisture is lacking, so not too enthusiastic about storm chances Friday for these areas.

Forecast looks drier heading into Saturday as the low dips down into the south central CONUS, leaving zonal flow in its wake. Light orographic showers are expected across our mountain zones under this pattern, with the best chances in the Cascades, but guidance QPF is light at only a couple hundredths at best. Expect generally seasonable temps for the period. Evans/74

LONG TERM
Sunday through Thursday
There remains decent agreement amongst ensemble members that a progressive weather pattern will bring daily chances of mountain rain/snow showers through the long term period. However, rain/snow amounts will be light as current ensemble products indicate little moisture support with each system. Locally breezy conditions will also develop with each system passage early next week.

Ensemble guidance is in great agreement that Sunday will start the week off with an upper low centered off the BC coast ejecting a shortwave trough and cold front across the PacNW. In the lower elevations and through the Cascade gaps, breezy winds of 15 to 25 mph with gusts up to 25-40mph will develop with the cold front passage in the afternoon Sunday, and will continue into Monday as surface pressure gradients will be slow to weaken post cold front passage. As for precipitation, ensemble members of the GFS, ECMWF, and Canadian all favor at least 0.25 inches of QPF in the WA Cascade crest Sunday, with at least 0.1 inches along the east slopes of the Cascades and OR Cascade crest. However, only about 55% of cluster members favor around 0.1 inches in the northern Blues with a more southerly passage of the shortwave trough; the rest of the eastern mountains will see QPF values less than 0.1 inches. By Monday morning, the shortwave trough is advertised to exit into the Northern Rockies, leaving a low off the BC coast and a westerly flow aloft. The westerly flow aloft will result in the heaviest rain/snow occurring along the Cascade crest with good agreement amongst ensemble members that at least 0.25 inches of QPF will fall in this area. As for the rest of the mountain areas, ensemble members keep QPF below 0.1 inches throughout Monday. The lower elevations Sunday/Monday will remain fairly dry, though a light rain shower or two may impact the immediate Blue mountain foothills.

Tuesday into Thursday, ensemble guidance begins to come into disagreement with the progression of the upper low off the BC coast. There is general agreement amongst ensemble members that the low off the BC coast will deepen, but open into a trough in the northeast Pacific by Wednesday morning, and begin a gradual push towards the PacNW the second half of Wednesday into Thursday (moderate confidence 60-70%). However, there is disagreement amongst these members as to how far offshore and how deep the trough develops, with cluster solutions (~20% of ensemble members)
even depicting a transient ridge developing over the PacNW Tuesday and early Wednesday. The closer the trough develops to the PacNW and the deeper the trough becomes results in a wetter solution across the mountains, whereas the eastern mountains will see drier conditions with a shallow and more offshore trough. Currently, there is mod-high confidence (70-80%) that the Cascade crest will continue to see rain/snow showers develop during the middle of the week, but low-mod confidence (30-40%) in rain/snow showers continuing in the eastern mountains Tuesday through early Wednesday. While ensemble members are in general agreement that afternoon temperatures will remain near to below normal (60s to low 70s...50s mountains) through the beginning of next week, the uncertainty in the trough/ridge development results in wider temperature ranges, with the ridge favoring afternoon highs in the upper 60s to lower 70s, the shallow, more offshore trough resulting in mid to upper 60s, and the deeper/closer trough resulting in temperatures in the upper 50s to lower 60s. Current confidence in the going temperature forecast is low (25-30%).
Lawhorn/82

PRELIMINARY POINT TEMPS/POPS
PDT 62 45 62 42 / 60 70 30 10 ALW 65 47 65 47 / 60 80 40 10 PSC 65 49 68 46 / 40 40 10 0 YKM 61 42 65 39 / 70 20 10 0 HRI 65 48 67 45 / 50 40 10 0 ELN 58 41 62 40 / 70 30 20 10 RDM 56 40 54 34 / 60 50 20 0 LGD 61 43 57 41 / 50 90 50 20 GCD 61 41 57 38 / 70 80 70 30 DLS 61 48 62 44 / 90 70 20 10

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.

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