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Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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370 FXUS66 KPDT 142212 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 312 PM PDT Wed Aug 14 2024 .SHORT TERM...Today through Tomorrow Night...A transient ridge is crossing the region today. Mostly clear skies and near normal high temperatures today. However, few cumulus clouds are developing within Wallowa county, the crest of WA Cascades including south central OR, observed from satellite imagery. Confined smoke aloft still remains in central OR and Ochoco-John Day Highlands from local wildfires. Winds will be around at 20 mph or less across the forecast area this afternoon. Tonight through tomorrow afternoon, an upper level trough makes its way through the PacNW which is now offshore the OR/CA border. There are chances (40-60%) of thunderstorms starting late tonight over central OR that then ramp up tomorrow afternoon and evening in terms of both coverage and lightning potential. A bit more spread/uncertainty in terms of elevated instability overnight that spreads across south central OR northward. Best signal is focused across northern Klamath-Lake. Thereafter, environment becomes more conducive for storms tomorrow. Model soundings indicated MUCAPEs between 500-1000 J/kg, Lifted indices (LI) of -3, and lapse rates 6.5-8 C/km across central OR to eastern OR. Thus a moderately unstable environment. Of note, HREF probability of thunder is forecast around 30-60% overnight that then ramp to greater than 80% across portions of eastern OR. Of note, a mix of wet and dry storms are expected in this event with storms likely to be more wet than not across the Southern Blue and Strawberry Mountains north and east. Of which, HREF LPMM highlights this nature. A reasonable high-end is expected to be approaching one inch or more. HREF shows raw probs of exceeding 0.5" at around 40-50% in spots. Wetting rains are further supported by NBM probs. Of which, a tenth or more exceed 60% across a large area, roughly Crook county southward and northeast into Wallowa through 06Z Friday. While storms are wet there, the threat for new fires is sufficient between fuels and lightning activity. Thus Red Flag Warnings were issued for ORZ644-645. Elsewhere, the Fire Weather Watches were replaced with a warning with these zones having two windows of lightning potential, albeit storms more isolated-widely scattered in character compared to the storms in the east. Overall, models are in good agreement with trough returning over the PacNW with a cyclonic low for tomorrow. Breezy winds will also develop up to 20-30 mph tomorrow around the Cascade Gaps extending to Columbia Basin (>50% confidence). This is due to increasing surface pressure gradients up to 5-8 mb. Tomorrow night, the worst will come to an end as thunderstorms begin moving out of the area. Gusty winds will then return at 15-25 mph with the surface pressure gradient decreasing to 3-5 mb. Clear skies will return as well as clouds decrease. Temperatures will then steadily increase by a few degrees with relative humidity above critical levels. Confidence enhances with the Extreme Forecast Index (EFI) showing temperatures to be near-normal through the forecast period. Feaster/97/80 .LONG TERM...Friday through Wednesday...The previous area of low pressure continues to lift northeast out of the forecast area on Friday morning. Lingering moisture will provide a slight chance (15- 20%) for continued shower activity, mainly over Wallowa County in Oregon and Columbia County in Washington. This system should generally lift out of the forecast area by the late morning hours, with another area of low pressure moving over the Pacific Coast. Models continue to indicate instability, especially over the Eastern Mountains of Oregon. With atmospheric moisture over the area, showers and isolated thunderstorms (15-25% chance) will be possible through Friday afternoon. The best opportunity for thunderstorm development will be over eastern Union County, with showers otherwise. The previously mentioned area of low pressure off the Pacific Coast will advance east with moisture moving over Washington and Oregon between Saturday and Sunday. Ahead of the arrival on Saturday, temperatures will warm around 5 degrees across the area, with cooler conditions anticipated on Sunday. The best moisture will be found west of, and including, the Cascades, though moisture will spill over into the central portions as well (30-50% for favored locations). Guidance indicates the afternoon and evening hours to be the favored timeframe for shower activity . Instability is generally faced in the western reaches of the moisture with slight chance (10-20%) of development. The area of low pressure will drift far enough northward to lose influence over area weather by Sunday afternoon/evening. Drier conditions expected to persist through the remainder of the forecast period, though pockets of moisture will produce isolated showers and thunderstorms over the Washington Cascades through Wednesday. Temperatures through this period will remain near to slightly above normal. Overall, clusters are demonstrating fair agreement with the progression of these systems. Intensity differences to exist, as well as how far east the weekend system moves, but significant differences are not currently present. Branham/76 && .AVIATION...18z TAFs...VFR included through TAF period with low pressure moving across the area. Guidance indicates the development of showers across central Oregon (KBDN, KRDM) after 05z this evening with the potential for showers lasting through much of Thursday. There is a low chance (less than 5%) for nocturnal thunderstorms over those terminals. Cloud cover increases after 06z with a approach of the incoming system. Any shower development over terminals could produce periods of MVFR conditions. Branham/76 && .PRELIMINARY POINT TEMPS/POPS... PDT 54 80 54 81 / 0 30 20 10 ALW 57 84 59 85 / 0 20 30 10 PSC 59 87 62 86 / 0 10 20 0 YKM 55 85 54 84 / 0 20 20 0 HRI 59 85 59 86 / 0 10 20 0 ELN 57 85 56 83 / 0 10 20 10 RDM 52 75 47 80 / 40 40 20 10 LGD 52 80 50 83 / 0 50 60 20 GCD 54 80 50 83 / 40 70 40 20 DLS 61 80 59 83 / 10 20 10 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Red Flag Warning from 11 AM to 11 PM PDT Thursday for ORZ644-645. Red Flag Warning from 11 PM this evening to 11 PM PDT Thursday for ORZ611-640-642. WA...None. && $$ SHORT TERM...97/80 LONG TERM....76 AVIATION...76