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Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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854 FXUS66 KPDT 120524 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1024 PM PDT Sun Aug 11 2024 Updated Aviation Discussion .AVIATION...06Z TAFs...VFR conditions expected over the next 24 hours outside of YKM where brief periods of MVFR could be possible due to localized haze and smoke. Otherwise look for breezy winds to be possible again tomorrow all sites except YKM/PSC, with expected gusts between 20-30 knots, strongest DLS. Goatley/87 && .PREV DISCUSSION... /issued 425 PM PDT Sun Aug 11 2024/ SHORT TERM...Tonight through Tuesday...A troughing pattern is expected (>95% chance) to persist through Tuesday. Current satellite imagery shows a mid/upper-level low over Vancouver Island with a modest mid/upper-level ridge axis placed over eastern Oregon into western Idaho and northeast Washington. This translates to weak southwesterly flow aloft. Some mid-level moisture is apparent on satellite imagery with clouds increasing in coverage. Additionally, cumulus fields have developed over the Blue Mountains and Washington Cascades. Confidence in any showers and thunderstorms is low (5-15%) this afternoon and evening due to weak synoptic forcing. This evening through Monday morning, there is a non-zero chance (5-15%) that elevated showers and an isolated thunderstorm will develop across the Blue Mountains and adjoining foothills. While some weak synoptic forcing and elevated instability will be present, mid-level moisture appears marginal (50-70%) in forecast soundings. Have maintained a "slight chance" mention of showers in the forecast, but actual chances of measurable precipitation are likely on the low end of the aforementioned range. Attention shifts to the evolution of a compact closed low centered off northern California for Monday afternoon through Tuesday. Guidance is nearly unanimous in advertising the closed low transforming into an open-wave shortwave trough as it swings northeast across northern California, northwest Nevada, and eastern OR late Monday evening through Monday night. The broad forcing for ascent (evident in PVA and q-vector fields) associated with this shortwave, coupled with ample mid-level moisture (700-500 mb RH >70%) and negative mid-level (700-500 mb) theta-e lapse rates should facilitate nocturnal showers accompanied by embedded thunderstorms. The best chances for convection will be across the Blue Mountains, perhaps extending as far west as the foothills of the Blue Mountains. Notably, chances of wetting rain (0.10" or greater) for eastern Grant, Union, and Wallowa counties are 20-40% from Monday evening through Tuesday morning. Looking west, probabilities drop to 5-20% for the western Blue Mountains and adjoining foothills. Breezy to locally windy onshore winds are forecast Monday afternoon/evening and Tuesday afternoon/evening through the Cascade gaps. Confidence is currently too low to advertise any wind highlights, but the pattern is conducive to sustained winds of 15-25 mph and gusts of 30-45 mph through climatologically windy locations. Plunkett/86 LONG TERM...Tuesday Night through Sunday...Models are in good agreement in having a trough influencing our weather through the long term period though there are differences as to the exact location and strength of the trough for any particular time. Initially, the trough will be fairly shallow and weak, giving us quiet and benign weather but by Thursday, an upper low over British Columbia will start deepening and strengthening the trough and this will give us a slight chance of showers and mountain thunderstorms through next weekend. With the trough over the area, temperatures will be below normal, especially next weekend, where high temperatures may be 10 degrees below normal. The trough will also bring stronger pressure gradients and it is expected that there will be breezy Cascade gap winds each afternoon. The NBM gives 70-95 percent chance of 20 mph winds in the Columbia Gorge and Kittitas Valley each afternoon through the period. The Extreme Forecast Index is indicating little in the way of unusual weather except for below normal temperatures Saturday onward with values of -0.70 to -0.75 centered over the Oregon Cascades and central Oregon. Clusters show a shallow trough centered offshore Tuesday night and Wednesday in 75 percent of the model ensemble members though a minority of the Canadian and ECMWF members favor slight ridging and a westerly flow. Weather will be generally quiet though a slight chance of showers and thunderstorms will linger in the early evening over Wallowa county. The trough will also provide tight pressure gradients and 15 to 25 mph winds are expected in the Columbia Gorge and Kittitas Valley. Lows will be in the 50s in the Columbia Basin and in the 60s elsewhere though higher mountain valleys will dip into the 30s. Wednesday`s highs will be a little warmer than Tuesday with highs in the mid 70s to mid 80s. Not expecting much in the way of showers though a slight chance of showers is possible south of Bend. Thursday will see energy diving southwestward out of northern British Columbia and beginning to deepen the trough. Models differ as to the location of the trough axis though about two thirds have it along the coast and the rest further inland. A generally southwest flow will warm most of the area a couple more degrees to the upper 70s and 80s. A southwest flow will bring moisture and increased instability. The NBM shows 100-300 j/kg of CAPE and have a slight chance of thunderstorms over the eastern mountains and central Oregon with a slight chance of showers over the Washington Cascade crest, eastern Columbia Basin and the Blue Mountain Foothills. The Cascade gaps will see breezy winds of 15-20 mph in the afternoon. Friday will see the arrival of a closed low off the Pacific Northwest coast. 85 percent of the model ensemble members have the trough axis along the coast or just offshore with the remainder having it further offshore. Colder air with the low will cool temperatures about 5 degrees to the lower 80s in the Columbia basin and in the 70s elsewhere. There will again be enough instability for a chance of showers and a slight chance of thunderstorms in central Oregon and the eastern mountains with rain amounts of up to a tenth of an inch. The rest of the area will have a slight chance of showers with generally a few hundredths of an inch. Saturday, model clusters continue to show a strong closed low and trough over the area though they differ as to whether the trough will be centered offshore or onshore. This will continue the showery pattern with a slight chance of afternoon thunderstorms in the eastern mountains. By Sunday, the low moves north to near Vancouver Island and this reduces showers over most of the area. Temperatures will be mainly in the 70s Saturday with a few 80s in the Columbia basin. The NBM is showing a 60-85 percent chance of reaching 80 degrees in the Columbia Basin and only about 30-50 percent elsewhere. With low moving north on Sunday, temperatures warm a few degrees to the mid 70s to mid 80s though NBM chances of reaching 80 degrees are only slightly higher. Perry/83 && .PRELIMINARY POINT TEMPS/POPS... PDT 58 85 54 79 / 10 20 20 10 ALW 64 89 58 83 / 10 20 20 10 PSC 66 90 61 84 / 0 0 10 0 YKM 58 88 53 82 / 10 0 0 0 HRI 63 88 58 83 / 0 0 10 0 ELN 59 85 55 80 / 10 0 0 0 RDM 50 83 45 78 / 0 0 10 0 LGD 55 88 53 81 / 20 20 40 20 GCD 55 90 53 83 / 10 20 20 20 DLS 61 81 59 77 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...86 LONG TERM....83 AVIATION...87