919 FXUS66 KPDT 251624 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 924 AM PDT Sun Jun 25 2023 .MORNING UPDATE...Rain showers have moved northeast out of Wallowa County in the last hour though partly to mostly cloudy skies remain over the eastern mountains. There will be a break for the next few hours before another round of showers and thunderstorms develop again this afternoon. The HREF composite reflectivity paintball plot shows a chance of thunderstorms developing in the afternoon over the eastern mountains, especially over the higher terrain of Grant County northeast into Wallowa County. A few thunderstorms will be possible in central Oregon from Bend southward and westward into the Cascades, as well as in the Washington Cascades from Cle Elum up to Snoqualmie Pass. Thunderstorms activity will drop off rapidly around sundown. Thunderstorms don't look like they will be severe, but a few could pulse up and become fairly strong before falling apart, similar to yesterday afternoon. Have made a few adjustments to POPs and thunderstorm coverage this afternoon, but nothing too substantial. Rain amounts will generally just be a few hundredths of an inch, but the Strawberry and Elkhorn mountains could get up to a quarter of an inch. Temperatures should be up a degree or two from yesterday with highs in the 80s to lower 90s with mainly 70s in the mountains. Did adjust temperatures a degree or two in places, mostly downward. Likewise, low temperatures tonight should be up a degree or two from this morning. The Columbia Gorge and Kittitas Valley will have west to northwest winds of 15 to 25 mph this afternoon and early evening with northerly 10 to 15 mph winds in central Oregon. Otherwise, winds will be fairly light. Forecast update already out. Perry/83 && .PREV DISCUSSION... /issued 436 AM PDT Sun Jun 25 2023/ AVIATION...12z TAFs...VFR conditions expected for the period. The same weather system that spawned localized showers and thunderstorms will linger around today, however expectation is that all shower and thunderstorm activity on Sunday will remain across the eastern mountains of Oregon. Sct midlevel clouds with mostly NW/N winds less than 15 kts throughout the day. Evans/74 PREV DISCUSSION... /issued 212 AM PDT Sun Jun 25 2023/ SHORT TERM...Today through Tuesday night...Currently, there are some nocturnal showers over southern Union County, which is moving into southwest Wallowa County at this time. These are just showers with no lightning strikes being detected. These will likely persist through the rest of the morning. The synoptic weather pattern is not expected to change much during the short term period, with a persistent elongated upper trough still over the region from central CA northeast into SW Canada. There is a southwest flow aloft over the forecast area, which is bringing a continuous supply of unstable air into the CWA. This is resulting in diurnally driven afternoon and evening showers and pulse type thunderstorms. However, during the past two days, there have been some outflow boundaries developed from thunderstorm downdrafts. These do have the potential to initiate new storms. However, so far that has not happened, but there is a potential of it occurring. Otherwise there will be no other triggers such as any cold fronts. Thunderstorms are mostly developing and affecting areas from central OR to the eastern and northeast mountains, as well as along the Cascades. The amount of instability has been slowly increasing each day, with now instability parameters indicating unstable enough conditions to warrant the mention of heavy rain, gusty winds and small hail in areas where the chances of storms are greatest. Other areas will just see a slight chance of thunderstorms, mainly on the edges of areas with a higher chance of storms. The NBM 4.1 is and has been fairly consistent showing the best chances for thunderstorms to form each day in central OR northeast across the NE mountains southward. Thunderstorm probabilities from the NBM 4.1 has been as high as 30-40 percent along the eastern border of the CWA, in the eastern mountains, and probabilities as high as 20-30 percent over the Cascades and east slopes, especially in WA. The HREF is showing probabilities of thunder as high as 70-80 percent in small areas over eastern OR, but with most areas in the 30-50 percent range. MUCAPE values will be as high as 1000-1500 J/kg, and LIs of -3 to -4 in areas with the greatest chances of storms. The deterministic and ensemble model forecasts have been in very good agreement with this persistent weather pattern. Therefore overall forecast confidence is high (80-90 percent). Since these storms have been and will continue to be slow moving, there is some chance that heavy rain could cause flash flooding over burn scars, especially over recent burn scars from 2021 and 2022. However, PWAT values, and QPF amounts are not quite high enough to warrant enough confidence for any Flash Flood highlights at this time. However, this could change and it needs to be closely monitored. Temperatures will be almost steady during the short term, with high temperatures about 4-6 degrees above normal. For example, the normal high for today at the Pendleton Airport is 81 degrees, and the forecast high is 87 today, or 6 degrees above normal. Highs will be in the mid 80s to lower 90s lower elevations and upper 60s to upper 70s mountains for today and Monday, and then a degree or two of cooling on Tuesday. This cooling on Tuesday will likely be due to increased cloud cover and a greater area of thunderstorm coverage. Overnight lows will also be fairly steady, with readings in the mid 50s to mid 60s lower elevations, and mostly mid 40s to lower 50s in the mountains, with just a degree or two of possible difference from night to night. It will become locally breezy to windy each afternoon and evening, mainly through the Cascade gaps, the eastern Columbia River Gorge and the valleys adjacent to the WA Cascade east slopes. These winds will combine with afternoon minimum RHs of 15 to 20 percent today, and then mostly 20-30 percent on Monday, and then mostly 25 to 35 percent on Tuesday to create elevated fire conditions. However, with the trend of RHs increasing each day, the fire weather conditions will become less of a concern each day in the short term. Elsewhere, winds will be light through the short term forecast period. 88 LONG TERM...Wednesday through Sunday...The last bit of chances for afternoon showers and thunderstorms across primarily our mountain zones will occur Wednesday, before the rest of the period looks to be dry and warm. Ensemble guidance remains in solid agreement up until the latter half of the work week, however despite the discrepancies, overall consensus is that the synoptic pattern will shift and support benign weather through next weekend and beyond the long term period. The axis of an upper-level low will cross the forecast area during the day Wednesday. The consensus across ensemble members suggest that the center of the upper-level low will fall right over the Oregon Basin, meaning our lower-elevation zones may be too moisture- starved to see any precipitation out of this system. QPF ensembles suggests that perhaps the wraparound moisture from this low afflicts areas such as the Kittitas Valley, but that the eastern mountains and central WA Cascades would see the best chances for showers and thunderstorms. Bulk shear continues to be weak across deterministic guidance, although recent warmth and instability should be enough to trigger orographic convection. Will gauge forecast confidence for mountain showers/thunderstorms in the aforementioned areas at around 50% and less than 20% for the Kittitas Valley, while confidence in the rest of the forecast area seeing dry weather hovers around 60%. Models then split between depicting zonal flow and ridging after Wednesday's system shifts eastward, with the NBM leaning toward a warming solution of some kind. Temperature ensembles across the NBM, GEFS, and EPS all suggest a warming trend, with temps in the lower Basin inching toward the triple digits by the end of the period. The obvious spread exists across ensemble members after day 5, but given this, despite the divergence in how the synoptic pattern is depicted, will gauge forecast confidence at around 75% for an overall warming trend and dry weather across the entire forecast area Thursday onward. Biggest question as of now is the magnitude of the warmth the Interior NW stands to experience through the first full week of July. Evans/74 && .PRELIMINARY POINT TEMPS/POPS... PDT 85 56 86 57 / 0 0 10 10 ALW 89 59 89 62 / 10 0 10 10 PSC 91 62 92 64 / 0 0 0 10 YKM 88 59 89 60 / 10 0 20 10 HRI 90 60 91 62 / 0 0 0 10 ELN 85 57 87 59 / 10 0 20 10 RDM 81 50 81 49 / 20 10 20 10 LGD 81 55 80 54 / 30 10 20 20 GCD 80 51 77 52 / 40 20 20 20 DLS 88 59 86 60 / 0 0 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ MORNING UPDATE...83 SHORT TERM...88 LONG TERM....74 AVIATION...74