Sunrise 5:18 AM   Sunset 8:38 PM Moonrise 6:08 PM   Moonset 3:09 AM

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FXUS66 KPDT 211123 AAA AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 423 AM PDT Tue May 21 2024

Updated Aviation Discussion

SHORT TERM
Today through Thursday...Little to note across the forecast area this early morning as a dry northwest flow aloft keeps conditions cool and quiet. While area radar imagery shows very little, nighttime satellite imagery is showing scattered bands of high clouds beginning to filter into the PacNW ahead of the next weather system.

The quiet and dry conditions across the forecast area will continue through the morning hours, but by the afternoon a warm front arriving ahead of an upper low will begin to spread light to moderate rain across the forecast area, with the heaviest rainfall occurring along the Cascade crest, Blues, and the Wallowas. Steady rainfall will continue through the overnight hours across the aforementioned mountain areas, with probabilities of at least 0.5 inches of rainfall between 50-60%, and at least 0.75 inches between 20-30%. Breezy to locally breezy winds will also develop with the frontal passage this afternoon, with sustained winds between 15-20 mph and gusts 25-30 mph in the lower elevations. As the upper closed low drops into the PacNW with the left entrance region of a stacked jet between 250mb-500mb over northeast OR, precipitation will switch from steady stratiform to persistent light to moderate showers during the morning hours Wednesday and continue into the evening hours. While the cold core of the upper low will allow for graupel to develop within the showers, increasing instability and steepening mid- level lapse rates across the eastern mountains will lead to a slight chance (15-20%) of isolated thunderstorms developing. Snow levels will drop from 6kft-7kft Tuesday night to 4kft-4.5kft Wednesday morning across the mountains, introducing chances of snow showers across mainly the higher mountain areas with accumulations up to 1 inch near Tollgate. The arrival of the upper low overhead will also lead to breezy to windy westerly winds developing through the Cascade gaps and into central OR, Simcoe Highlands, and the OR Columbia Basin. Along the Simcoe Highlands and the OR Columbia Basin, NBM probabilities for sustained winds 30mph or greater reach into the 60-75% range, with 70-80% probabilities for gusts to exceed 45mph in the afternoon hours.

As the upper low moves into central ID Wednesday night, a dry northwest flow aloft will begin to develop over the PacNW, pushing any remaining shower chances to far southeastern WA and northeastern OR. By Thursday afternoon, only a few isolated showers will be possible over Wallowa county, with dry conditions elsewhere. Winds will have also weakened area wide for Thursday, with only locally breezy west to southwest winds through the Columbia Basin and along the Blue mountain foothills. Lawhorn/82

LONG TERM
Friday through Tuesday...Deterministic models and ensembles are in very good agreement on the 500 mb pattern for Friday through the weekend then subtle differences develop early next week on how close an upper level trough will approach the PACNW coast. On Friday an upper low will move SEWD across British Columbia and then across WA/ORE on Saturday. This will result in increasing chances for showers with 30-50% POPS over the mountains and foothills and 10-20% POPS elsewhere beginning Friday night and continuing through Saturday afternoon/early evening. QPF amounts are forecast to be modest with NBM 50th percentile amounts of .10-.25 inches for the 24 hour period ending 00Z Sunday for the northeast mountains and Cascade crest. One caveat to this is that both the deterministic GFS and ECMWF are showing heavier amounts of QPF with this system. Thus QPF forecasts will likely need to be fine tuned as the event gets closer.

Best potential for thunderstorms (15-20%) will be over the eastern mountains on Saturday afternoon and early evening as the upper low passes over the region. Forecast soundings are not very impressive.
Though there are steep low to mid level lapse rates, CAPE values are only around 200 J/kg. Vertical shear is also quite weak so expect disorganized and isolated storms with only sporadic LTG strikes. It will also be breezy to locally windy on Saturday especially in those lower elevation locations that are windy under westerly flow regimes. Almost all of the 50 ensemble members of the ECMWF show peak wind gusts of 35-45 mph at KELN on Saturday afternoon with a mean of 40 mph. Likewise the NBM 4.2 shows a peak wind gust of 40 mph at KELN at 23Z on Saturday. Thus there is some potential (40- 45%) that a wind highlight may be needed for the Kittitas Valley on Saturday.

After the upper low exits the area Saturday evening the ensembles show a rather pronounced upper level ridge building over the Rockies with an upper trough gradually approaching the coast. This will put the region in a drier, warming SW flow aloft for Sunday into early next week. High temperatures will warm from a few degrees below normal on Sunday to as much as 10 degrees above normal by Tuesday.
On Tuesday high temperatures will be well into the 80s in the Columbia Basin. The NBM Probabilities of high temperatures exceeding 90 degrees on Tuesday for the Columbia Basin and Yakima Valley are currently 20-30%. The ensemble clusters mostly support this dry/warm scenario for early next week but one of the clusters that is heavily weighted by the Canadian model moves the offshore trough closer to the coast sooner which could result in increasing chances for showers as early as Monday afternoon or Monday night. The probability of this occurring is around 20%. Thus for now went with the NBM which holds off on any precipitation east of the crest until Tuesday evening. 78



AVIATION
12Z TAFs...VFR conditions highly likely (95-100% chance)
at all sites through Tuesday early afternoon, though an incoming weather system will usher in BKN mid and high-level clouds. This afternoon and evening, OVC CIGs and -RA will spread across eastern Oregon and eastern Washington. Chances of MVFR CIGs increase to 60- 70% tonight. Forecast soundings and NBM suggest the best chances of BKN-OVC MVFR CIGs would be at PDT/ALW/KBDN/KRDM. Winds will pick up this morning and generally be 10-15 kt sustained with gusts 15 to 25 kt through the remainder of the period, though the gustiness in the winds will tend to subside after sunset. 78

PRELIMINARY POINT TEMPS/POPS
PDT 68 46 57 42 / 20 90 70 50 ALW 70 47 60 45 / 20 90 80 50 PSC 71 53 64 48 / 20 80 50 30 YKM 67 42 61 41 / 50 50 20 10 HRI 71 50 63 46 / 20 90 50 30 ELN 61 39 56 43 / 60 60 30 10 RDM 65 43 53 33 / 20 70 20 10 LGD 66 43 53 39 / 10 90 80 70 GCD 66 43 53 37 / 10 90 80 70 DLS 66 48 62 47 / 50 70 30 10

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.

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