Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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512 FXUS66 KPDT 291731 AAA AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1031 AM PDT Sat Jun 29 2024 Updated Aviation Discussion .MORNING UPDATE...Ridging will move out today as a trough begins to nose its way into the region. The SW flow accompanying this trough has brought widespread cloud cover to the forecast area this morning, with mountain showers following once moisture advection takes better hold later in the day. Still looking at primarily the Cascade crests and eastern mountains as far as shower chances are concerned for today, however the cloud cover this morning will likely dampen storm chances in the afternoon. Instability is there, however mid-level moisture is not terribly impressive and wind shear is already weak. Did scale back storm mentions as a result, while adding in some mention along the central WA Cascade crests as the latest HREF did tap into a pocket of instability in that area. Otherwise, PoPs were kept pretty similar to the previous forecast package, and overall messaging remains the same: cloudy and cool conditions today with a 30-40% chance of showers across most of our mountain zones this afternoon and evening. Would currently suggest that storm chances fall around 10-20% in the eastern mountains and WA Cascades given current trends. Evans/74 .AVIATION...18Z TAFs...VFR conditions are highly likely (90% chance or greater) for all sites through the period. A weak system passing over the region this afternoon and tonight will result in widespread BKN-OVC mid and high cloudiness. Cloudiness will decrease late tonight and Sunday morning. The SPC HREF is developing light showers over KPDT and KALW from roughly 06-10Z so included a tempo group for -SHRA in the TAFS for those two sites. Winds will be mostly 12 kt or less but some gusts to 20-25 kt are expected this afternoon and evening at KDLS, KRDM and KBDN. 78 && .PREV DISCUSSION... /issued 253 AM PDT Sat Jun 29 2024/ SHORT TERM...Today through Monday...Nighttime satellite imagery tonight shows mid to upper level cloud cover spreading across WA and western OR. Meanwhile, water vapor imagery shows a transient ridge exiting into ID/MT while an upper trough develops in the northeast Pacific. Today, the transient ridge will continue to push east of the region, while a developing trough offshore places the PacNW under an increasing southwest flow aloft. In this flow, a shortwave trough will lift across the PacNW this afternoon and evening. While previous runs showed this feature being relatively dry, most current runs of short-term and global deterministic guidance have switched course, with scattered showers developing across the WA Cascades this afternoon, then spreading to the eastern mountains and Blue Mountain foothills later in the evening and overnight hours. With the support of the left exit region of an upper jet and modest low level lapse rates, there will be slight chances (15-20%) of isolated thunderstorms across the eastern mountains through early Sunday morning. Winds through the Cascade gaps will also be on the increase later this afternoon the cross Cascade pressure gradient tightens with the shortwave passage. Sunday, the trough offshore will push across the PacNW, resulting in lingering scattered shower chances across the central WA Cascade crest and Wallowa county. Lingering surface based instability will also result in the development of an isolated thunderstorm or two across eastern Wallowa county Sunday afternoon. An attendant surface cold front with the trough passage will help to strengthen a marine push through the Cascade gaps, resulting in winds increasing further through the Kittitas valley, eastern Gorge, Simcoe Highlands, and the OR Columbia Basin. Winds in these areas will generally be 20-30mph with gusts up to around 40mph throughout Sunday, though probabilities of 24-hr max wind gusts exceeding 45mph in the aforementioned areas is 60-80%. While the breezy winds will bring elevated fire weather concerns, the marine push will help to raise minimum relative humidities above critical levels into the 20s to mid 30s, keeping conditions from reaching red flag criteria. The upper trough will slowly make it`s way into ID/MT through Monday, with northwest flow aloft developing over the PacNW by Monday evening. Surface pressure gradients will remain tightened across the Cascades as the trough continues to transit the region, resulting in another day of breezy winds between 15-25mph with gusts up to 40mph across the lower elevations. Locally windy conditions will also be possible through the Gorge, Kittitas valley, Simcoe Highlands, and the OR columbia Basin, where probabilities of 24-hr max wind gusts exceeding 45mph are 75-90%. Afternoon relative humidities will remain above critical levels throughout Monday, though fire weather concerns will remain elevated. Temperatures will moderate through the beginning of next week, with afternoon highs generally in the upper 70s to 80s in the lower elevations, and 60s to mid 70s in the mountain areas. Lawhorn/82 LONG TERM...Tuesday through Saturday...Ensemble clusters show little spread in solutions on Tuesday. Confidence is high (>95% chance) that the synoptic pattern will consist of downstream troughing over the Northern Plains and Northern Rockies, with an anomalously strong upper-level ridge in place upstream in the Pacific Ocean. This will place the forecast area under a dry northwest flow aloft. Precipitation chances are extremely low (<5% chance for the Wallowas and Washington Cascade crest). Westerly winds are forecast through the Cascade gaps, strongest for the Kittitas Valley and eastern Columbia River Gorge. NBM probabilities show a 50-90% chance of reaching advisory-level gusts through the Kittitas Valley on Tuesday, while probabilities are low (<20% chance) elsewhere. Wednesday, little overall change is anticipated in the longwave pattern. Deterministic guidance advertises a shortwave sliding southeast across eastern WA and northeast OR during the day. The dry air mass should preclude any significant chances of precipitation (<5% area-wide) as the shortwave traverses the region, but another day of breezy to locally windy gap winds is likely (>90% chance) through the Kittitas Valley and eastern Columbia River Gorge. Currently, no Red Flag conditions are forecast, and NBM probabilities of advisory-level wind gusts drop to 30-70% through the Kittitas Valley. Ensemble clusters begin to show diverging solutions by Thursday. While the favored solution is an eastward-propagating ridge (56% chance), a minority of members advertise a return to troughing for the PacNW (11%), and the remaining 33% keep the ridge axis well offshore. The result is reasonably high confidence in warming temperatures accompanied by Moderate HeatRisk for the lower elevations on Thursday. Moreover, with the ridge axis favored to shift eastward over the forecast area, onshore gap winds would diminish in magnitude. By Friday and Saturday, ensemble clusters reveal continued divergence in member solutions. 85% of ensemble members are advertising an amplified upper-level ridge in the vicinity of the PacNW. The unlikely solution (13-15% chance) is troughing over the PacNW. Should troughing materialize, much cooler temperatures relative to what is forecast would result. Moreover, it would support light precipitation for the Cascade crest and northern Blue Mountains. The strong ridging solutions would support continued warming across the region. This anticipated warming trend would facilitate widespread Moderate HeatRisk for the lower elevations Friday and Saturday. To provide some tangible values and illustrate uncertainty based on the aforementioned scenarios, the NBM is advertising a 60-85% chance of exceeding 90 degrees, and a 10-30% chance of exceeding 100 degrees for our main population centers Friday and Saturday. While very unlikely at this point, the lower elevations have a 1-7% chance of reaching or exceeding 110 degrees on Saturday. Plunkett/86 && .PRELIMINARY POINT TEMPS/POPS... PDT 83 58 83 54 / 10 20 0 0 ALW 86 60 86 57 / 10 30 10 0 PSC 87 63 89 59 / 10 10 0 0 YKM 85 57 85 54 / 20 20 0 0 HRI 86 61 88 58 / 10 20 0 0 ELN 81 56 78 54 / 20 30 0 0 RDM 79 50 79 47 / 10 10 0 0 LGD 83 56 79 50 / 10 40 10 0 GCD 86 53 81 49 / 10 40 10 0 DLS 86 60 83 59 / 10 10 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ SHORT TERM...74 LONG TERM....86 AVIATION...78