FXUS66 KPDT 250944 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 244 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 HREF, 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 && .AVIATION...Previous Discussion...06Z TAFs...Skies are currently SCT-BKN with variable bases 8-15 kft, and clouds will thicken to BKN-OVC early Thursday morning. After 17Z, CIGS will lower to less than 10 kt as precipitation increases from the west. Latest model runs are coming in about 2-3 hours slower on timing of precipitation. In addition, latest guidance does not show CIGS lowering below VFR at DLS. Therefore, will back off a couple of hours on precipitation and will have VFR (3.5 kft in some sites) for all terminal airports. Winds are decreasing to 15 kt or less for most TAF sites, although it remains breezy with NW winds gusting to 25 kt at DLS. By early morning and most of Thursday, winds will be westerly at 12 kt or less. Wister/85 && .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 / 60 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. && $$ SHORT TERM...74 LONG TERM....82 AVIATION...85