FXUS66 KPDT 031506 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 806 AM PDT Tue Sep 3 2024 .UPDATE...The center of the upper level disturbance is currently over far northeast Oregon which will continue to move off to the east and out of the forecast area later this afternoon. Still expect some showers and thunderstorms associated with this disturbance through the afternoon over northeast Oregon and southeast Washington. Rest of the forecast area will see generally clear skies but with some haze from new fire starts. Morning forecast update was done to refine the timing and location of showers and thunderstorms for today. .SHORT TERM...Today through Thursday night...An upper low currently centered near Lexington, OR has moved very little over the past several hours, but the low will eventually travel east towards the ID border today. This system has made for a challenging forecast in terms of storm intensity and rainfall. The PWATs this morning are lower than models have advertised. SPC Mesoanalysis shows PWATs around 0.5-0.7" over most of eastern OR, therefore the air mass remains dry. This is also shown by the BOI and OTX 00Z RAOB with a classic inverted V profile and elevated moisture. Although CG strikes are less early this morning, there remains concern for new fire starts and outflow winds as storms approach Union and Wallowa Counties. On the other hand, radar is estimating heavy rain with some of the storms wrapping around the tight circulation over Lexington as well as heavy rain with a band of showers and thunderstorms over Kittitas and Yakima Valleys that are starting to decay. Dewpoints east of the Blue Mtns have increased from the 30s Monday evening into the 40s at this time. Low level moisture will deepen as the low sets up near Hells Canyon and provide wrap around precipitation, but this does not look to be a significant rainfall event. CAMS probs of 0.1-0.2" range from 60-90% but the chance of 0.2-0.3" is only 30-60% with highest probabilities over the high terrain. Even the CAMS have struggled with the present low pressure system, so I am not fully confident in the QPF. Forecasters will need to keep an eye on wrap-around precipitation with storms in far eastern Wallowa County. The rest of the forecast area will observe clearing skies behind the departing low pressure. Winds will also be on the downward trend, and the odds are likely that the current wind advisories for the Kittitas Valley and eastern CR Gorge will expire at 4 AM. Gusts from the WNW remain 35-40 mph for these two zones, so it's best to keep it going until expiration. It will be mostly sunny, locally breezy, and seasonal temperatures today. It will be warmer Wednesday and Thursday as high pressure returns and the inverted surface thermal trough makes its appearance once again. Temperatures will climb back into the mid 80s to mid 90s on Thursday with moderate heat risk. RHs will be on the downward trend and in the teens and 20s. Winds will be relatively light and terrain driven. It's hard to state at this time, but air quality issues may need to be addressed with the new wildfires from yesterday's storms. Wister/85 .LONG TERM...Friday through Tuesday...The long term forecast is coming into much better agreement compared to yesterday, as ensembles expect a prolonged period of well above normal temperatures through the weekend, with a quick moving system expected to disrupt the high and help alleviate some of this warmth, followed by an approaching deep upper trough/low that will bring us under a southerly to southwesterly flow pattern with widespread precipitation possible right as the period ends into Tuesday. As mentioned, models appear to have a better agreement on the overall pattern, with an upper level deep ridge of high pressure over the Desert Southwest controlling our region to start the period, with a weak low/trough just offshore of California. This system will weaken and become absorbed into the overall flow, but should help to migrate the ridge more eastward. Under this pattern, the entirety of the weekend looks to be well above normal in regards to temperatures. Highs on Friday and Saturday will range in the 90's for our population centers, with the NBM indicating a 15-40% chance of highs 100+ both days for portions of Central Oregon, the Columbia Basin, and the foothills of the Blue Mountains; meanwhile, there will be a 50-60% chance for these highs in the Columbia Gorge, which is the most likely location to see another round of triple digit heat. These temperatures will begin to become disrupted on Sunday though thanks to the weakening trough moving into the region. This low may bring a chance of some showers or possibly thunderstorms on Saturday and Sunday, but confidence in this activity remains low (<30%) as models show limited potential for any activity with how weak this system is and PWAT values generally being near normal, around 0.5-0.6 inch. Still, most models show some weak instability, and a few showers and storms could fire off of the higher terrain near Central Oregon on Saturday. Moving into early next week, ensembles show the ridge beginning to creep eastwards as very deep troughing begins to emerge from the Gulf of Alaska and move down towards the West Coast. This is where the greatest discrepancies lie in forecast though, as some ensembles show the troughing beginning to move inland by Tuesday, while others hold the system off until after the long term ends. There are even around 15% of ensemble members that just don't expect the troughing to be notable, and show the entire Western US under broad ridging; this is considered a high outlier though, as ensembles have been fairly consistent in a more progressive pattern beginning to emerge by just beyond our forecast period. Nonetheless, ensembles agree this brings southerly to southwesterly flow and much more abundant moisture, which should position us for shower and thunderstorm activity by early to mid next week. Meanwhile, this moisture brings widespread cloud cover to help temper our highs and bring us closer to near normal by Tuesday, with widespread highs in the 70's to low 80's - an encouraging sign that we might not only be able to break this late summer heatwave, but also that more beneficial moisture may be on the way. Overall with increasing confidence in the models, there is moderate confidence (50-60%) in the forecast. Goatley/87 && .AVIATION...12Z TAFs...VFR conditions expected over the next 24 hours. A few lingering showers/thunderstorms may bring some periods of lowered CIGs and VSBYs for PDT/ALW/PSC primarily, but these should be brief, with precipitation exiting the region by the beginning of the afternoon. Skies will be clearing, allowing for the widespread BKN 10-25k CIGs to become SKC this afternoon/evening. Winds remain breezy this morning, but by this evening expecting all sites to have sustained winds 10 knots or less. Goatley/87 && .PRELIMINARY POINT TEMPS/POPS... PDT 83 54 90 56 / 20 0 0 0 ALW 85 58 93 59 / 40 0 0 0 PSC 88 59 93 60 / 20 0 0 0 YKM 88 52 91 54 / 0 0 0 0 HRI 88 57 94 57 / 10 0 0 0 ELN 86 53 92 56 / 0 0 0 0 RDM 83 47 92 51 / 0 0 0 0 LGD 81 52 88 54 / 40 0 0 0 GCD 83 51 91 54 / 10 0 0 0 DLS 84 57 96 62 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...85 LONG TERM....87 AVIATION...87