FXUS66 KPDT 291017 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 317 AM PDT Wed May 29 2024 .SHORT TERM...Today through Friday... Bottom Line Up Front: (BLUF) 1. Breezy to windy conditions through the Cascade Gaps today. 2.Mountain rain with thunderstorms in far WA Cascades and far eastern mountains of Wallowa County. 3. Dry and warm conditions to return tonight. Models are in firm agreement with the upper level trough that remains overhead. The front has moved past the region which has allowed the winds to tapper off ever so slightly. However, much like ahead of the front, winds will still remain elevated through this evening becoming a bit more regular. Raw ensembles show 60% to 80% chances of >25 mph through the Simcoe Highlands, lower Columbia Basin of OR, foothills of the Blue Mountains as well as Kittitas Valley. Ensembles also show 60-80% probabilities of >39 mph gusts in the same areas. After the trough traverses the remainder of the region, models are in firm agreement with the leading edge of an upper level trough moving overhead. Clusters agree with the positioning and timing of the ridge and favor the EURO ensembles with the only variance being with the amplitude. With the upper level trough continuing overhead, mountain rain and isolated thunderstorms will continue to linger. Especially across the WA Cascades around Stampede Pass and Snoqualmie and then the far eastern mountains of Wallowa along the Snake River. Looking at model derived soundings, instability will remain across the areas with CAPE values lingering between 150-250 J/kg, lapse rates of 7.5 C/km and precipitable water values of near 0.50 inches. While these ingredients are not stellar for thunderstorm development, the orographic lift provided along the mountains will help enough that some isolated thunderstorms cannot be ruled out. However, ensembles show By tonight the models show the leading edge of an upper level ridge making its way into the region. The upper level trough will be to the east across Idaho and western Montana. Northwesterly flow aloft will dominate the region bringing the daily diurnal winds through the Cascades Gaps. Dry conditions will be ahead of the upper level ridge with warm temperatures making their way back to the region. Temperatures today will be cooler today due to the frontal passage. However, a warm up will begin Thursday and continue through Friday. 60% of the raw ensembles show the foothills, central and north central OR to be in the upper 50s/low 60s and the Basin, Gorge, Kittitas and John Day Basin in the low 70s. Temperatures will increase each day with raw ensembles showing over 50% the majority of the region in the mid to upper 70s. Bennese/90 .LONG TERM...Saturday through Wednesday... Key Messages: 1. Breezy afternoon winds, peaking Saturday and Monday. 2. Afternoon thunderstorms possible Saturday and Monday. 3. Near normal temperatures through Monday before warming. The extended period is characterized by a strong upper level low pressure located over the Gulf of Alaska, spinning multiple shortwave troughs into the Pacific Northwest that will lead to breezy afternoon winds, widespread showers Sunday and Monday, and the potential for thunderstorms along the Blue Mountains and east. The primary concern through the period resides with breezy winds associated with the passage of a cold front both Saturday and Monday, as areas experiencing the highest gusts include the Simcoe Highlands, Kittitas Valley, and portions of the lower Columbia Basin of Oregon. Gusts of 25-35 mph are forecast on Saturday and 30-40 mph on Monday. Confidence in these values is moderate to high (60-80%) as the GFS suggests a pressure gradient between Portland and Spokane of 8-9 mb on Saturday and 9-10.5 mb on Monday, which is shy of the normal advisory threshold of 12 mb. There is slightly higher confidence in Monday's winds as the ECMWF EFI showcases unclimatologically elevated winds and gusts through the Basin and along the east slopes, with 60-80% of ensembles in agreement. In comparison, the ECMWF EFI does not indicate any unclimatologically high wind speeds or gusts on Saturday. The NBM is also in agreement with higher winds Monday afternoon and early evening as the probability of 40 mph wind gusts is a 75-95% chance over the Simcoe Highlands, a 60-80% chance across Kittitas Valley, and a 55-75% chance along the Lower Basin of Oregon and the northern Blue Mountains foothills; compared to a 70-85% chance for the Simcoe Highlands, a 55-75% chance for the Kittitas Valley, and a 50-70% chance along the Lower Basin of Oregon and the northern Blue Mountains Saturday afternoon and early evening. At this time, there is low to moderate confidence (40%) in advisory level winds Monday afternoon/evening as the NBM indicates a 30-50% chance of 50 mph winds or greater over the Simcoe Highlands and across the Basin, and a 10-20% across the Kittitas Valley. Breezy afternoon winds are also anticipated Sunday and Tuesday afternoon, but gusts are only expected to stay between 15 and 25 mph, with pockets of up to 30 mph in elevated and exposed areas in the Simcoe Highlands, Kittitas Valley, and Basin/foothills. The secondary concern in the long-term period resides with developing thunderstorms Saturday and Monday afternoon as the cold front and associated upper level trough pass through the area. The primary areas of concern include Wallowa County on Saturday and the Blue Mountains, John Day-Ochoco Basin/Highlands, Grande Ronde Valley, and Wallowa County Monday afternoon/evening. Much like with winds, the upper level trough and cold front on Monday look stronger. However, guidance is not in agreement with trough strength on Monday as the GFS hints at a weaker trough and less instability over the eastern mountains. The ECMWF indicates very little surface based CAPE (<50 J/kg) Saturday and more substantial CAPE values (150- 350 J/kg) on Monday compared to the GFS CAPE of 100-200 J/kg Saturday and 50-100 J/kg on Monday. Currently, more confidence is in a weaker system on Saturday as 64% of ensembles are advertising this outcome, which are highly favored by the ECMWF and CAN. The main uncertainty with Monday's system is timing, as indicated by 57% of ensembles with the trend in a later arrival (32% versus 25%). This is also visible when viewing the 12Z and 00Z runs of the GFS and ECMWF, which have both slowed down about 6 hours and have strengthened slightly. In regards to trough strength, ensembles are still in equal disagreement with 22% hinting at a weaker shortwave and 21% aligning with a stronger shortwave. Thus, it is expected that the strength of Monday's system is less likely to change in relation to the timing of the system. Behind the initial cold front and shortwave Saturday, flow aloft will turn more from the southwest as the next system approaches and drops from the parent system located in the Gulf of Alaska. This will enhance moisture transport to lead to widespread showers starting Sunday afternoon as the next system approaches the coast. Rain chances will peak overnight into Monday morning before slowly tapering off through the afternoon and evening. At this time, rain chances peak between 5 PM Sunday and 5 PM Monday. During this timeframe, the NBM highlights a 25-35% chance over the Lower Columbia Basin (including Tri-Cities), Eastern Gorge, Central Oregon (Bend/Redmond), and the Yakima/Kittitas Valleys; a 55-70% chance across the northern Blue Mountain foothills; and a 65-85% chance at elevation over the Cascade and Blue Mountains of 0.25 of an inch of rain or more. As indicated in the previous section, guidance is in disagreement with timing of this system. This leads to the expectation that the peak timing of rain chances may arrive earlier Sunday afternoon as observed via ensembles. This will have to be further monitored as the event nears. Due to the overall agreement in guidance in Saturday's system and more concern residing with timing associated with the second system on Monday, confidence is moderate to high (50-70%) regarding temperatures in the extended period. Overall, high temperatures will stay near to slightly above normal until late in the period, with temperatures briefly dipping below normal on Monday. The warmest temperatures are anticipated to occur on Wednesday as an upper level ridge builds in the wake of the departing upper level trough. There is high confidence (80-90%) in this building ridge, as 85% of ensembles are indicated this pattern midweek. This would lead to highs breaking into the low to mid-80s across the Basin, northern Blue Mountain foothills, Central Oregon, and the Yakima Valley. 75 && .AVIATION...(previous discussion)...06Z TAFs...Variable mid and high level clouds that are SCT-BKN 5-10 kft and cirrus clouds at 20-25 kft will remain tonight and most of Wednesday. Winds have decreased over the terminal airports but will increase again Wednesday morning and afternoon. Westerly winds will increase to 15-25 kt gusting to 25-35 kt in the afternoon. Wister/85 && .PRELIMINARY POINT TEMPS/POPS... PDT 64 38 68 43 / 10 10 0 0 ALW 66 42 70 45 / 20 20 0 0 PSC 71 44 74 48 / 20 10 0 0 YKM 68 35 73 41 / 10 0 0 0 HRI 69 41 74 45 / 10 0 0 0 ELN 62 37 68 43 / 20 0 0 0 RDM 61 32 70 38 / 10 0 0 0 LGD 60 36 65 39 / 10 10 0 0 GCD 63 35 67 40 / 10 0 0 0 DLS 66 43 73 48 / 10 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...90 LONG TERM....75 AVIATION...85