Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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702 FXUS66 KPDT 190408 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 908 PM PDT Sat May 18 2024 .EVENING UPDATE...Mid-level water vapor imagery clearly reveals a trough digging ESE across the PacNW this evening. Mid/upper-level PVA over the Columbia Basin and the foothills of the northern Blue Mountains is likely aiding the isolated to scattered showers that are apparent on radar, perhaps further enhanced by the left jet exit and modestly steep mid-level lapse rates (00Z HREF-advertised 7 C/km). Along the Cascade crest, any showers will be more orographically driven. While no significant lightning threat is anticipated with shower activity, GLM did briefly highlight some activity with the strongest cell of the evening northwest of Boardman. Have opted to keep a mention of thunder out of the forecast given low confidence in any further activity as any lingering weak instability wanes overnight. Plunkett/86 && .PREV DISCUSSION... /issued 457 PM PDT Sat May 18 2024/ AVIATION...00Z TAFs...VFR conditions are likely (80-90% chance) for all sites through the period. A trough approaching from the NW will produce increased cloudiness this evening, especially for PSC/PDT/ALW where temporary light showers are forecast. CIGs of 5-10 kft are expected. Westerly to northwesterly winds of 10-20 kts with gusts of 15-30 kts are expected to continue this evening before transitioning to 12 kts or less overnight for all sites. Plunkett/86 PREV DISCUSSION... /issued 247 PM PDT Sat May 18 2024/ SHORT TERM...Tonight through Monday night...Any concerns for the short term period are minor and not worthy of any highlights at this time. Currently, the PacNW is under a broad cyclonic flow aloft. Washington and northern Oregon are on the north side of an upper level jet that is rotating along the bottom of a shortwave trough. The shortwave trough will track across the region this evening. Doppler Radars are detecting increasing showers over the northern half of Washington at this time, and the showers will develop over the Washington and northern Oregon Cascades as well as the Blue Mountains and Wallowas this evening. HREF shows these as light showers in our CWA with QPF amounts less than a tenth and little to no snow accumulation with the exception of the east slopes of the WA Cascades. White Pass has about a 40% of 1" of snow but only 10% of 2" of snow on HREF. Since snow levels will be around 4000 feet, Snoqualmie Pass will have rain or a rain/snow mix. Any instability capable of thunderstorms with this approaching shortwave will be north of the forecast area. There have been mixing of winds aloft to the surface that will continue through the evening hours with numerous stations reporting gusts 25-35 mph...locally 40 mph. The probability of gusts greater than 40 mph lowers dramatically, so no wind advisories this evening. Any showers on Sunday will be widely scattered and primarily in Wallowa County. The chance of showers is only 20-30%. On Sunday, the forecast area will be on the backside of the trough and under a northerly flow aloft. There are no embedded waves or upper level support to speak of. It will be a chilly morning and the afternoon will be about 5-10 degrees below seasonal average. The weather pattern does not change much on Monday, but there will be a slightly better chance (30-40%) of rain and mountain snow in Wallowa County and a slight chance of rain over the northern Blues. I cannot rule out a thunderstorm or two in Wallowa County where SBCAPES are as high as 200 J/kg (Canadian GEPS mean is around 350 J/kg), but the mean shear is very weak. The upper low to the east is progged to strengthen with some wrap around moisture. Not all models agree, as the NAM is particularly dry over northeast Oregon. The deterministic GFS is the most bullish on precipitation, but nothing significant. Monday will be warmer than Sunday but still slightly below seasonal average. Wister/85 LONG TERM...Tuesday through Saturday...Unsettled weather is expected through the long term period as a couple of troughs move across the area with only a brief break in between. Model agreement has improved in the last couple of runs though the normal deviations grow by late in the week mainly about the strength and timing of the trough and upper low arriving Friday into Saturday. The Extreme Forecast Index indicates below normal temperatures in the long term with values of -0.70 to -0.75 Tuesday through Thursday though it only highlights temperatures in the Washington Cascades on Wednesday. Rainfall is highlighted with a value of 0.67 on Wednesday in the Washington Cascades, the eastern mountains of Oregon and the Blue Mountain Foothills. However, it does have a shift of tails of 0 over the Washington Cascades and the Blue Mountain Foothills, so there is some possibility of a more extreme rain event. Finally, it also highlights windy conditions mostly in Oregon on Wednesday with a value of 0.70. Tuesday starts out with models in good agreement in having an upper low and trough over the northern British Columbia coast then having it move south to near the US/Canadian border by Tuesday evening and into western Washington overnight. It will be pushing a front ahead of it into our area in the late afternoon. During the day, rain will be confined to the Cascade crest but Tuesday night will see a chance of rain over the entire area. The mountains will get a quarter to a half inch of rain since snow levels are 6000-7000 feet. The Blue Mountain Foothills will get 1 to 2 tenths of an inch while the rest of the lower elevations will just have light rain mountains. The Columbia Basin will have westerly winds of 10 to 20 mph. Temperatures will be a few degrees below normal with highs in the upper 60s to lower 70s in the Columbia Basin and in the upper 50s to mid 60s elsewhere. On Wednesday, 75% of model ensemble members will have the trough overhead before moving into Idaho Wednesday night. This will give us another chance of rain across the area and the NBM puts a slight chance of afternoon and early evening thunderstorms over the eastern Oregon mountains. The remaining 25% have a very weak crossing the area a few hours faster than the other ensemble members. Rainfall amounts will be up to a quarter inch in the mountains and a tenth of an inch or less elsewhere. Winds will again be breezy with 15 to 25 mph in the afternoon. Temperatures will cool around 5 degrees to a well below normal 60s in the Columbia Basin and in the 50s in the mountains. Thursday will be a bit of a break though 83% of the model ensemble members keep the trough over Idaho and close enough to cause some issues. Have kept a 30-40% chance of rain showers (snow levels around 6000 feet) in the mountains and a 10-20% chance in the Blue Mountain Foothills with the rest of the area dry. Rain amounts will be very light. Temperatures will warm 2-4 degrees from Wednesday. Models diverge on Friday with the ECMWF and Canadian deterministic runs having a trough with a deep closed low arriving either Friday afternoon or night while the GFS arrives a few hours earlier with a much weaker low. The NBM appears to favor the former though rain amounts are light. A southwesterly flow ahead of the trough will warm temperatures 3-5 degrees from Thursday to the 60s and lower 70s with mid to upper 50s in the mountains. By Saturday, 71% of model clusters and the ECMWF and Canadian deterministic runs keep the trough over the area while the GFS deterministic and the rest of the ensemble members have the low and trough having moved off to the east. Have kept a chance of light rain showers over the mountains next Saturday with temperatures a degree or two cooler than Friday. With a trough crossing the area, unstable conditions will allow for a slight chance of thunderstorms over the eastern mountains each day. Perry/83 && .PRELIMINARY POINT TEMPS/POPS... PDT 40 63 39 69 / 20 0 10 0 ALW 43 66 43 72 / 40 10 10 10 PSC 47 70 46 77 / 20 0 0 0 YKM 39 68 41 75 / 10 0 0 0 HRI 43 69 43 75 / 20 0 0 0 ELN 39 64 39 71 / 10 0 0 0 RDM 33 57 31 66 / 0 0 0 0 LGD 35 58 36 64 / 20 10 10 20 GCD 36 58 34 64 / 10 0 10 10 DLS 46 65 44 72 / 10 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ EVENING UPDATE...86 SHORT TERM...85 LONG TERM....83 AVIATION...86