Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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714 FXUS66 KPDT 170530 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1030 PM PDT Mon Sep 16 2024 .EVENING UPDATE...Isolated rain showers are ongoing across the Blue Mountains and their adjoining foothills. Farther south and east, thunderstorms in Baker, Malheur, and Harney counties are diminishing as steering winds take them slowly west-southwest into an environment less conducive to maintenance of convection. Best synoptic forcing remains south and east of our CWA line, though Grant, Union, and Wallowa counties do have a slight chance (15%) to likely (60%) rain showers as well as a slight chance (15%) of isolated thunderstorms overnight. Elsewhere, a slight chance of nocturnal activity extends westward across the rest of the Blue Mountains and along portions of the northern foothills. Plunkett/86 && .AVIATION...06Z TAFs...Isolated rain showers across the Blue Mountains and adjoining foothills are ongoing this evening, though the likelihood of rain at terminals is too low (<30%) to mention. Another round of rain showers may affect ALW/PDT (20-30% chance) late night into Tuesday morning; chances elsewhere are very low (<20%). CIGs mostly 080-120 kft through the period. Winds will increase in magnitude from the west Tuesday. Plunkett/86 && PREV DISCUSSION... /issued 304 PM PDT Mon Sep 16 2024/ SHORT TERM...Tonight through Wednesday...With one low swinging across the region and a second dropping down into Tuesday, the weather pattern remains unsettled through the short term period. Ongoing showers and some radar returns are noted mostly along the foothills of the Blues and and encroaching on the Columbia Basin, but have limited precipitation to just east of the Tri-Cities as we`re struggling to find ground truth that these echoes are creating true rain that is hitting the ground. As the low continues to shift eastwards, flow will spin this activity to a southwestward movement which should help to keep it just east of the region. Overall just not confident enough to bring a slight chance of precipitation into this area only for it to fizzle out or dive more southwards before reaching the area. Otherwise, the low will continue to spin showers and some possible storms through the overnight hours, with activity shifting from across Central Oregon through the eastern mountains and even foothills of the Blues, to primarily the eastern mountains. Instability is present enough to allow for a few lightning strikes with this activity, with mesoanalysis showing MUCAPE values of around 500 J/kg creeping up into Wallowa and Grant Counties, but the axis of best instability still lies to the southeast where ongoing storms are already noted. Satellite shows a weak little cumulus field also popping up, so expect a few lightning strikes but stronger activity is not anticipated otherwise. Moving into the overnight hours and into Tuesday, a deep low from the Gulf of Alaska will drop down and ridge the PacNW coastline, skimming our area and once again bringing the bulk of it`s energy to California and just missing the region. As this occurs, breezy conditions begin to pickup overnight and peak into tomorrow morning and afternoon. Analysis of 850 mb winds and the pressure gradient shows that stronger sustained winds and gusts should be possible, but the majority of which should remain just below advisory criteria (sustained winds 30-39 mph, gusts 45-57 mph). Don`t be surprised if we see a few sites hit these criteria briefly, but sustained for an hour or longer isn`t expected. Meanwhile, precipitation will continue to shift with rain lining our CWA from the north to south along both the Cascades and eastern mountains between the dancing lows. Because of this, rain along the Cascades should be moving from the northwest to southeast into the mountains, and rain along the eastern mountains from the northeast to southwest. Sadly, this means that by Tuesday afternoon, portions of our interior CWA will likely see little to any precipitation. As the second low continues to descend, a low probability (<20%) of rain could make it into the foothills of the southern Blue Mountains of Oregon, but otherwise the rain should be limited to our higher terrain through it`s departure into Wednesday. On Wednesday as mentioned, precipitation departs and winds calm as a ridge extending from the Pacific extends into the PacNW. This will calm the overall weather for the remainder of the day, allowing for clearing skies and benign, seasonable conditions. Goatley/87 LONG TERM...Thursday through Monday ... Key Messages: 1. Rain chances return over the weekend. 2. Slightly cooling temperatures through the period. The extended period is characterized by a blocking pattern through the workweek as a closed low slowly tracks across California and the Desert Southwest ahead of a potential shortwave passing over the weekend. Ensemble guidance is in good agreement with the synoptic pattern early in the period, but struggles with the weekend system. Current consensus leans to zonal west-northwest flow setting up over the weekend before turning more from the southwest early next week associated with an incoming shortwave. This flow would allow for cooling temperatures and slight chances for showers over mountainous terrain, potentially including the foothills and east slopes if the shortwave is able to tap into some Pacific moisture. At this time, that looks to be a low probability (20-30%) as QPF ensemble clusters are indicating a 65% chance of drier conditions than what is currently in the forecast. This coincides with 55% of ensemble members advertising zonal flow versus a weak shortwave moving onshore on Saturday. At this time, minimal rain amounts are expected over mountainous terrain, with the NBM suggesting a 50-70% chance of measurable (>.01") of rainfall along the Washington Cascades, 30- 50% chance over the northern Blue and Wallowa Mountains, and 20-30% elsewhere. Due to this zonal upper level flow, temperatures will slowly drop 1 to 3 degrees each day and stay below normal for this time of year. The NBM showcases a 70-90% chance of temperatures staying below normal through the duration of the week and into the early part of next week, so confidence is high in these cooling temperatures persisting. 75 && .PRELIMINARY POINT TEMPS/POPS... PDT 51 70 48 72 / 20 10 10 0 ALW 54 72 52 75 / 20 30 10 0 PSC 56 77 56 79 / 0 10 0 0 YKM 47 79 47 78 / 0 0 10 0 HRI 55 75 53 77 / 10 10 0 0 ELN 51 74 52 76 / 0 0 10 0 RDM 43 66 43 69 / 20 10 60 0 LGD 50 66 46 70 / 30 40 20 10 GCD 47 67 44 69 / 40 30 20 10 DLS 55 71 54 75 / 0 20 40 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ EVENING UPDATE...86 SHORT TERM...87 LONG TERM....75 AVIATION...86