FXUS66 KPDT 262131 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 231 PM PDT Mon Aug 26 2024 .SHORT TERM...Tonight through Wednesday...Water vapor imagery this afternoon shows a high pressure ridge aloft pushing east into the northern Rockies, while an upper trough approaches Vancouver Island in the northeast Pacific. Into tonight, the upper ridge over the inland PacNW will continue to produce dry and warm conditions with light winds across much of the forecast area. By tonight, the upper trough with a cold front at the surface will arrive to the PacNW. Ahead of the frontal passage, surface pressure gradients across the Cascades will tighten tonight, resulting in breezy winds developing through the Cascade gaps and into the Columbia Basin. Winds will increase further across most of the forecast area starting tomorrow morning when the frontal boundary and axis of the upper trough move across the inland PacNW. Wind advisories have been issued for the Kittitas valley, Simcoe Highlands, the Eastern Gorge, and for the Hanford area, as these areas will see the strongest winds mix down in the afternoon. In fact, the NBM shows probabilities of 65-90% for the 24 hour max wind gusts greater than 45mph in those areas. Blowing dust will also be a concern in these areas throughout the day tomorrow. While no fire weather highlights are planned at this time, fire weather concerns will be elevated as well. Relative humidities will be marginal throughout the afternoon in the Columbia Basin, with a few localized areas meeting wind and relative humidity thresholds. While most of the forecast area will remain dry through the trough/frontal passage tomorrow, the WA Cascade crest and a portion of the northern Blues will see a chance of light showers developing tonight. Cooler temperatures will also develop Tuesday and Wednesday as the trough pushes a cooler airmass into the area. That said, this airmass will be very dry as well, with areas of northeastern OR looking at poor overnight relative humidity recoveries Wednesday night. Otherwise, winds will diminish overnight Tuesday, with light winds continuing throughout Wednesday. Lawhorn/82 .LONG TERM...Thursday through Monday...Models are in good agreement through much of the long term period with some differences starting to develop Sunday and increasing on Monday. Thursday through Saturday will see a ridge centered over the Pacific Northwest and this will be a quiet period for weather aside from a warming trend yielding above normal temperatures Friday and Saturday. Sunday will see a cutoff low in the eastern Pacific approach the southwest Oregon/northern California coast and begin nudging the ridge eastward. A southerly flow will lead to a slight chance of showers in the Oregon Cascades in the afternoon. By Monday, models bring the weakening low and a trough ashore though exactly where and the strength of the trough is unclear. It does give the Oregon Cascades, central Oregon and the eastern mountains a slight chance of showers with just light amounts. The trough will bring a few degrees of cooling and some breezy winds through the Cascade gaps in the afternoon. The Extreme Forecast Index (EFI) has just some minor highlights. Thursday and Friday, temperatures have values of 0.64 to 0.74 over the mountains and central Oregon. Then with warming temperatures Saturday and Sunday, values rise to 0.69 to 0.79 over the entire area. On Saturday and Sunday, CAPE has values of 0.82 and 0.93 highlighted over the Cascades. Thursday through Saturday, with the ridge over the area, skies will be mostly clear with no chance of precipitation and winds will be light. Temperatures will rise about 10 degrees on Thursday with highs in the upper 70s to mid 80s. NBM probabilities give a 95-100 percent chance of 80 degree temperatures but less than a 20 percent chance of 90 degrees even in the warmest parts of the Columbia basin. Friday will see temperatures warm another 5-7 degrees to the mid 80s to mid 90s and then another 1-2 degrees on Saturday. Chances for 80 degrees is near 100 percent both days in the lower elevations. Saturday has a 75-90 percent chance of 90 degree temperatures and Sunday has an 85-100 percent chance. On Sunday, the cutoff low that had been well offshore, arrives at the coast near the Oregon/California border in the ECMWF and Canadian while the GFS keeps it about 100-200 miles off the coast. A southerly flow develops in the afternoon as the ridge moves east over the Rockies. This allows a slight chance of showers to develop over the Oregon Cascades, central Oregon and the Ochoco-John Day Highlands in the late afternoon. As mentioned above, the EFI, which is based on the ECMWF, highlights CAPE and would indicate possible thunderstorms. However, the NBM, which is a multi model blend, has thunderstorm chances of less than 10 percent, so have left the precipitation as rain showers for now. Temperatures drop about 5 degrees in central Oregon to the upper 80s though further north and east, temperatures will be in the lower to mid 90s. Chances for 80 degrees remains near 100 percent, but chances for 90 degrees drop to 55-75 percent in central and north central Oregon and remain at 80- 100 percent over most of the rest of the lower elevations. On Monday, models agree in bringing the low and trough ashore but differ as to whether the low will be over the Washington Columbia Basin, central Oregon or the Willamette Valley. In any case, it should bring more widespread showers with light rain amounts to the Oregon Cascades, central Oregon and the eastern mountains. The NBM continues to have low chances of thunderstorms except over the southern Blue Mountains, so have a slight chance of showers in those areas and a slight chance of thunderstorms only in the southern Blue mountains. The whole area drops about 5 degrees to the mid 80s to lower 90s with mainly 80s in the mountains. Chances of 90 degree temperatures drops to 50-80 percent in the Columbia Basin and less than 30 percent elsewhere. Perry/83 && .AVIATION...Previous Discussion...18Z TAF...VFR conditions are expected for the next 24 hours. A disturbance moving through the area will bring a few light showers mainly to the mountains. Have included a PROB30 with -SHRA for KYKM from 09Z-13Z with MVFR vsby but confidence is not high for that. Otherwise will see increasing mid to high clouds late this afternoon and tonight with skies beginning to clear after 15Z. Winds will be light through this evening then begin to pick up after 07Z at KDLS with northwest winds at 15-20 kts gusting to 30 kts. KPDT, KYKM, KPSC and KALW will increase to 10-15 kts with gusts to 25 kts after 14Z. Perry/83 && .PRELIMINARY POINT TEMPS/POPS... PDT 56 74 43 73 / 0 10 0 0 ALW 61 77 49 76 / 0 10 0 0 PSC 65 80 52 78 / 10 10 0 0 YKM 58 72 44 77 / 10 0 0 0 HRI 62 78 48 78 / 10 10 0 0 ELN 56 66 43 77 / 10 10 0 0 RDM 50 73 38 76 / 0 0 0 0 LGD 53 75 41 74 / 0 10 0 0 GCD 52 80 39 78 / 0 0 0 0 DLS 62 74 51 80 / 0 10 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...Wind Advisory from 11 AM to 9 PM PDT Tuesday for ORZ041. WA...Wind Advisory from 11 AM to 9 PM PDT Tuesday for WAZ024-028-521. Wind Advisory from 9 AM to 9 PM PDT Tuesday for WAZ026. && $$ SHORT TERM...82 LONG TERM....83 AVIATION...83