Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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777 FXUS66 KPDT 250946 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 246 AM PDT Tue Jun 25 2024 .SHORT TERM...Today through Thursday night...Weather concerns for the short term period revolve around a developing low pressure trough off the eastern Pacific that will be pushing eastward and increasing the instability and moisture over the area. Winds will also increase ahead and behind the front, although winds will not be particularly strong (confidence 80%). The trough will track across WA/OR on Thurs bringing cool temperatures and mountain showers. Overall confidence in the short term forecast is moderate, as uncertainty lies in the convective activity on Wed. Latest models are trending stronger with the approaching trough and embedded shortwaves associated with it. Today will be a hot and dry day with the HeatRisk in the minor to moderate category. A large upper level ridge over the four corner states has amplified over eastern WA/OR. The inverted surface thermal trough has strengthened as well with 850mb temps rising to around 20-25C by the afternoon. It is highly likely for many of our lower elevations to observe mid-90s today and a few spots in the John Day Basin--Monument and Spray--are forecast to observe 100. The ridge will shift eastward tonight as a deep upper low approaches the coast. There is increasing instability aloft over southeast Oregon, and latest models are showing signs of a weak embedded disturbance that will bring a chance (30%) of high based thunderstorms south and east of John Day after midnight. As stated previously, the low is coming in stronger and is even showing signs of a negative tilt as it moves inland tomorrow. Confidence is high that showers and thunderstorms will develop as early as sunrise with the threat continuing through the day, but the questions remain where will convection likely occur, will storms be strong or severe, and how much precipitation to expect. Fortunately, the CAMs now run as far out as Wed afternoon and have proven helpful on the details. The areas with the best chance of showers and thunderstorms will be along the east slopes of the WA Cascades, the Blue Mtns, and the Wallowas where PoPs are 30-60%. There is a slight chance (10-20%) of showers and thunderstorms elsewhere. PWATs increase from around 0.7-0.8 in the morning to a little over an inch during the day, therefore wetting rain can be expected. NBM and HREF means probability of 0.1" over the Blues and Wallowas as well as the east slopes of the WA Cascades north of Mt Adams ranges 30-60%, and the probability of 0.25" or more in these same areas is around 20%. SBCAPES have increased to 300-450 J/kg which isn`t significant but has doubled from the CAPEs in previous model runs. 0-6km shear is strong--ranging from 45-60 kts. This will mean that some storms will be strong with sustained updrafts and will be capable of heavy downpours, hail and strong outflows. Wallowa and Union Counties are shown to have the highest CAPES and impressive bulk shear. Meanwhile, a tight pressure gradient will set up with a marine push increasing winds across the Columbia Basin and surrounding areas. The eastern CR Gorge will be windy on Wed. RHs will be in the 20s and 30s for most of the forecast which is not critically low to warrant fire weather highlights. The marine push will help to stabilize the atmosphere so that Thurs will be more showery rather than convective due to cool air at the surface. It will be breezy to windy, however. The Cascades, mainly over WA, will have numerous showers (60-70% chance) while showers over the northeast mountains will be widely scattered (20-30% chance). The upper trough will arrive into ID Thurs night followed by a drier northwest flow aloft. Wister/85 .LONG TERM...Thursday through Tuesday...The long term starts off quiet, but by the weekend we should see a bit more active weather including both the return of breezy winds as well as some lower elevation rain and mountain thunderstorms while temperatures generally hover around normal, varying by a few degrees each day. The overall pattern should be fairly simple, as we see transient ridging quickly moving across Friday into Saturday. Then by Saturday evening, a deep trough will move inland, keeping the weather active during the remainder of the weekend into early next week. Thereafter, we should remain in between separate weather features, with a deep ridge offshore of California and troughing to our east, causing zonal to northwesterly flow through the end of the long term. In regards to sensible weather, any concerns with temperatures should be fairly nil as highs remain mostly near normal through the period. This should mean a smattering of upper 70`s to low 80`s in Central Oregon, with more widespread low to upper 80`s for the remainder of our population centers. The warmest day is expected to be Saturday during the last day of the transient ridge, where the NBM shows a a 20-40% chance of seeing highs of or above 90 degrees for the Columbia Basin of Washington and down to the Foothills of the Blues including Walla Walla and Pendleton. Otherwise the passing of the trough over the weekend and then the westerly to northwesterly flow thereafter should help to keep us temperate and mostly near normal. With the passing trough, two concerns should occur with regards to breezy winds and showers/thunderstorms. For winds, breezy conditions can be expected both Sunday and again Monday. The NBM indicates around a 20-70% probability at gusts of 45+ mph for our usual breezy locations (Lower Columbia Basin/Foothills of the Blues, Simcoe Highlands, and Kittitas Valley) for both days but highest chances on Monday in the post-frontal environment. This system will also be conducive of precipitation, with PWAT values on Sunday around 100-150% of normal according to ECMWF ensembles. This combined with modest instability of around 300-600 J/kg in the eastern mountains of Oregon should promote thunderstorms for this region, while much of the rest of the CWA has a chance at a few showers. QPF amounts should still remain on the low side though, with the aforementioned eastern mountains of OR the only area likely to see more noteworthy amounts as the NBM expects a 20-40% probability of 12-hour QPF amounts 0.1 inch or more. Then through early next week, concerns for precipitation are minimal to nil, with the expected flow pattern causing rain shadowing from the Cascades, possibly allowing for some isolated activity into the eastern mountains. Goatley/87 && .AVIATION...12Z TAFs...VFR conditions expected over the next 24 hours. Upper level cloud deck moves in across the region tomorrow morning at FEW to BKN. Winds 10 knots or less all sites except BDN/RDM, where a few breezes 15-20 knots may be possible this afternoon. Goatley/87 && .PRELIMINARY POINT TEMPS/POPS... PDT 91 61 86 53 / 0 10 20 10 ALW 95 63 91 57 / 0 10 20 10 PSC 95 64 93 59 / 0 10 20 0 YKM 91 60 88 52 / 0 0 20 10 HRI 95 63 90 58 / 0 10 20 0 ELN 90 60 82 53 / 0 0 20 20 RDM 92 56 83 47 / 0 10 10 0 LGD 90 60 86 53 / 0 10 30 20 GCD 95 60 90 52 / 0 10 20 0 DLS 94 63 81 58 / 0 10 20 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...85 LONG TERM....87 AVIATION...87