Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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171 FXUS66 KPDT 260519 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1019 PM PDT Tue Jun 25 2024 .AVIATION...06Z TAFs...VFR conditions are forecast for all sites through the period (>95% probability of occurrence). Increasing mid-level moisture and elevated instability will lead to increasing cloud cover and may facilitate high-based showers and isolated thunderstorms during the overnight hours (10-40% chance, highest across south-central WA). For TAF sites, the best chance of measurable precipitation will be at YKM (30-40%) during the late night and early morning hours. Confidence is lower at all other sites (<30% chance). Afternoon convection may affect KPDT/KPSC/KALW/KYKM Wednesday, but confidence on timing/location precludes mention in the 06Z TAFs. Westerly winds will increase to 12-22 kts with gusts of 20-35 kts for all sites Wednesday morning and afternoon. Plunkett/86 && .PREV DISCUSSION... /issued 208 PM PDT Tue Jun 25 2024/ SHORT TERM...Tonight through Thursday...Currently there is an upper level ridge of high pressure over the region while an upper level trough approaches the Pacific Northwest coast. This will provide slightly warmer than normal temperatures overnight. There is also some convective moisture over northern California and northwest Nevada lifting north into southern Oregon. So far this is mainly showers but expect convection to continue to intensify with some thunderstorms and showers moving into southeast Oregon overnight. Some of this could clip Grant county late. Will see a shortwave rotating around the bottom of the upper level trough late tonight approaching the Cascades and then swing across the forecast area through Wednesday morning and midday. This shortwave begin to trigger some convection beginning around 12Z along the east slopes of the Cascades roughly from Redmond northward then proceed northeastward through the rest of the morning and early afternoon. This will at least provide a slight chance (10-15%) of thunderstorms across the forecast area with the eastern slopes of the Washington Cascades being the most likely (15-25% chance). This convection will shift northeast and out of the Columbia Basin in the early afternoon while the east slopes of the Cascades will see some lingering convection through the afternoon. Convection will begin to increase over northeast Oregon and southeast Washington (mainly the mountains 20-30% chance) in the afternoon and then continue overnight. This includes a chance of thunderstorms (15-25%) through the evening. The shortwave will also usher in some breezy 15-25 mph westerly winds (70-90% probability) through the day bringing slightly cooler conditions. See more discussion regarding winds in the Fire Weather section below. The upper level trough will move inland and across the region on Thursday. This will further cool temperatures and maintain breezy conditions. Showers are expected along the Cascade crest as well as over the mountains of northeast Oregon and southeast Washington until the evening and then come to an end. LONG TERM...Friday through Tuesday...Weather in the long term period will be generally benign though a trough crossing the area Sunday and Monday will raise a few concerns. Temperatures will be within a couple of degrees of normal though a transitory ridge crossing the area Saturday will lead to temperatures 3 to 5 degrees above normal. Friday will be a quiet day as a departing trough moves through eastern Montana into North Dakota and Saskatchewan while another trough begin to move out of the Gulf of Alaska. Deterministic models are in good agreement in having a modest ridge develop off the coast and this will warm temperatures around 5 degrees from Thursday to the upper 70s to mid 80s with 70s in the mountains. It should be sunny without any chance of precipitation and winds will be no more than 10 mph. Models agree in having the ridge strengthen Friday night into Saturday with the ridge axis over Idaho and Montana while the trough moves to just offshore by Saturday afternoon. The ridge and the resulting southwest flow will warm temperatures another 2 to 5 degrees with highs in the 80s with 90 possible in the Tri- Cities (47% chance) and Hermiston (36% chance). The mountains will be in the mid 70s to lower 80s. This will be the warmest day of the long term period. Cloudiness will be increasing in the afternoon and there will be a chance of light rain showers along the Cascade crest. On Saturday night and Sunday, models are in reasonably good agreement in having the trough move over the area with a shortwave crossing the area. This will lead to a chance of showers over the mountains and the Blue Mountain Foothills and a slight chance of showers elsewhere. A slight chance of thunderstorms will be confined to Wallowa county. Rain amounts will be just a few hundredths of an inch though the highest terrain of the eastern mountains could get up to a quarter inch of rain. It will also be breezy to windy in the afternoon with 20 to 30 mph winds anticipated in the Eastern Columbia Gorge, the Kittitas Valley and Oregon Blue Mountain Foothills. In those areas, the NBM has a 55 to 85 percent chance of gusts to 40 mph. Temperatures will cool 3-4 degrees back to the upper 70s to mid 80s with 70s in the mountains. The shortwave will move to the east Monday and models have some differences about the location and strength of an upper low off the British Columbia. This leads to some differences about whether we will have a northwest or westerly flow overhead. Either way, it will be a sunny and dry day with temperatures a degree or so cooler than Sunday. Wind will again be the main concern with 20 to 30 mph winds in the Kittitas Valley and 15 to 25 mph winds in the Columbia Gorge, Simcoe Highlands and Blue Mountain Foothills. NBM probabilities show a 97 percent chance of 40 mph wind gusts and a 30 percent chance of 49 mph gusts in the Kittitas Valley while the other locations mentioned above have a 50 to 70 percent chance of 40 mph wind gusts. By Tuesday, deterministic models are widely differing. The GFS has a closed low and trough over the area, the ECMWF has a low and trough over central British Columbia with a zonal westerly flow over our area while the Canadian has the low and trough in the Gulf of Alaska with a northwesterly flow over our area. Have kept the forecast sunny and dry with highs a couple of degrees warmer than Monday. Perry/83 FIRE WEATHER...As mentioned in the Short Term discussion, westerly winds will increase on Wednesday becoming 15-25 mph (70-90% probability) along the east slopes of the Cascades and across the Columbia Basin. The strongest of these winds are expected across the Lower Columbia Basin from The Dalles to Hermiston. Meanwhile, minimum humidities are expected to range from the lower 20s to mid 30s with those lower range humidities mainly to be north of the Horse Heaven Hills. The minimum humidities between The Dalles and Hermiston are expected to be in the mid 20s to lower 30s. The combination of winds and low relative humidities does not quite reach Red Flag Warning criteria. But none-the-less, any fine fuel fires experiencing these winds will likely see rapid spread and precautions should be taken to avoid sparking or starting a fire. && .PRELIMINARY POINT TEMPS/POPS... PDT 58 83 52 74 / 10 20 10 10 ALW 59 88 55 78 / 10 20 10 20 PSC 61 89 59 80 / 10 20 0 10 YKM 60 85 53 76 / 10 40 10 0 HRI 61 87 56 80 / 10 20 0 10 ELN 57 79 55 71 / 10 50 10 10 RDM 57 80 46 72 / 10 20 0 0 LGD 58 84 50 70 / 10 20 20 20 GCD 59 90 49 73 / 10 20 0 10 DLS 61 81 57 75 / 10 20 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...91 LONG TERM....83 AVIATION...86 FIRE WEATHER...91