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FXUS66 KPDT 022315 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 415 PM PDT Thu May 2 2024

Updated for Aviation

AVIATION
00Z TAFS...VFR conditions are expected through the TAF period.
Due to the presence of some isolated -SHRA winds in some areas are a bit gusty, but these should subside in the next hour or two.
Winds are expected to be 10 kts or less through the period.



PREV DISCUSSION
/issued 249 PM PDT Thu May 2 2024/

SHORT TERM
Tonight through Sunday
Deterministic models and ensembles are in good agreement on the 500 mb longwave pattern through the short term though subtle differences in the position of an upper low on Saturday night and Sunday will determine where the axis of heaviest QPF will set up.

For this evening scattered rain and snow showers over the central mountains (20-45% chance) will diminish this evening before ending after sunset. Friday will be dry as a shortwave ridge moves across the area downstream of an approaching upper low.
High temperatures will be in the mid 60s to mid 70s with mid 50s to mid 60s mountains.

The GEFS and ECMWF ensembles are in good agreement on moving an upper low from off the Washington coast Friday night to southwest Oregon into NRN CA and then to NRN NV on Sunday. The ensemble clusters agree for the most part on this track though a couple of the clusters are a bit farther north with the low. The ultimate track of this low will modulate the QPF across the region. The GFS ensemble shows a Weak AR with this system so there is a good potential for moderate to heavy QPF. At this time it appears that the heaviest precipitation will occur over northern and eastern Oregon, southern Washington and along the Cascade east slopes.
Used the WPC QPF for this forecast which yields 48 hour amounts (ending 12Z Monday) of 1-2 inches across the northeast mountains and east slopes and .7-1.1 inches elsewhere. Areas downwind of the Oregon east slopes are forecast to have a bit less QPF (.4-.5 inches) due to rain shadowing. These values are close to the 90th percentile NBM values and seem reasonable to use given that the EFI is depicting the potential for a climatologically outlier event.

Initially there will be little concern for snow with snow levels Friday night and Saturday above pass level. However Saturday night snow levels across central Oregon will lower to 3500-4500 feet and remain there through the day on Sunday. This will result in some higher mountain snow with the probability of 4 or more inches of snow Sunday through Sunday night of 40-65% for the eastern mountains and the Oregon east slopes (highest probabilities over the southern Blue Mountains).

The other sensible weather concern is strong and gusty winds. The first period of interest is Friday night and Saturday along the northern Blue Mountain Foothills of Oregon. NBM is showing 45-50 kt SELY wind gusts along the base of the Blue Mountains.
This could be a bit high given that the KBKE to KMEH pressure gradient is forecast to be only 5-6 mb . Nonetheless there is moderate potential (40-50% chance) for at least wind advisory level winds along the base of the Blues during this period. The second period of more widespread stronger winds across much of the lower elevations will be on Saturday night and Sunday when winds become westerly. Deterministic NBM is predicting 50 mph wind gusts at KPDT at 12Z Sunday morning. NBM 24 hour probabilities (ending 06Z Monday) of 50+ mph gusts are 30 to 70% for much of the lower elevations. 78

LONG TERM
Sunday night to Thursday
Early next week will be marked by somewhat unsettled weather with the greatest precipitation chances tied to the mountains followed by a drying, warming trend that is becoming increasingly more likely than not Thursday onward.
The main sensible weather highlights include: - Continued mostly beneficial rain that will be tied mainly to the mountains Sunday night through Tuesday, - Cooler than normal highs, 5-15 degrees below normal, Monday to Wednesday, and - Near seasonable temperatures Thursday with low chances (15-30%) in the high mountains.
- Modest rises on mountain rivers and streams with high confidence (>85%) in flooding not occurring.

Ensemble guidance is in fair agreement initially with the synoptic pattern to start the long term. Early Sunday night will be characterized by a weakening upper-level closed low over southern ID that will be ejecting east as an open wave over the Northern and Central Rockies Monday. Initial clustering scenarios shows the spread is driven by position/timing of this feature more so than amplitude Sunday night-Monday with nearly double the spatial variance (33%) amongst members. Much of the disagreement is centered in this open wave’s position as it evolves east from near the Central Great Basin to the Northern Rockies area. While the greatest 12-hr QPF in the long term falls on Sunday night, much of this rainfall stems from residual moisture in place and the weak AR event that will be winding down by early Sunday night. Of which, confidence in wetting rain (>0.1) is moderate (50-70%) and confined mainly to the upper eastern slopes of the WA/OR cascades and the Blues Sunday night. Elsewhere, chances are low (15%-40%) for wetting rain across the Lower Basin into the foothills of the Blues.

A second round of precipitation is anticipated Monday with chances ramping up in the afternoon. This is attributable to a weak surge of moisture associated with a leading shortwave trough ejecting east across WA out of a larger longwave trough that is centered over the Gulf of Alaska. Increased chances of 50-90% across the crest of the Cascades and eastern mountains for more than a tenth of an inch Monday afternoon/early Monday night. That said, there is modest potential for higher amounts as denoted by EPS EFI exceeding 0.6 across the Blues. Raw ECMWF ensemble shows the 50th percentile at Meacham near 0.25” over a 24-hr period ending 12Z Tuesday. Meantime, a modest corridor of 500 mb winds will be adjacent south and west of the area with a decent westerly 850 mb jet stream directed across the CWA Monday. Current thinking is winds will stay elevated Sunday night and carry/strengthen some Monday. The winds aloft in tandem with a modest cross-Cascade pressure gradient during the day will help promote another breezy day on Monday with the deterministic GFS and mean GEFS showing a cross-Cascade pressure difference of at least 10 mb. This is increasing confidence in sub-advisory wind gusts (50-70%; up to 40 mph). Of which, max 24-hr daily peak gusts for the NBM portray chance for exceeding 45 mph is 40-70% across the eastern Gorge, OR Lower Basin into the foothills of the Blues and Klickitat. However, raw EPS has scant-limited areal coverage for chances of exceeding 34 kts/39 mph Monday.

Northwest flow is expected to remain in the wake of the passing shortwave trough Tuesday. Of which, high chances (60-90%) will return late confined mainly to the Cascade crest and northern Blues.
This is in part due to potential for embedded disturbances within the mid-level flow. That said, moisture is limited and QPF is light overall.

A drying trend is anticipated to start with only moderate confidence in the forecast (50%) at this time. This revolves around the strength of the building ridge in the eastern North Pacific and the potential for lingering cyclonic flow Wednesday. This stems from the scenario of Monday’s shortwave trough elongating the upper trough across the central CONUS into the southwest/western CONUS. A weaker upper trough, stronger ridge scenario would be slightly warmer compared to the cyclonic flow case; impacts will be scant regardless of what pans out. Ultimately, this uncertainty translates then to Thursday with gudience and 500 mb means showing a leaning for this upper ridge to build and evolve east to extend across Vancouver Island. However, there is appreciable disagreement with the location of the upper ridge axis based on clustering scenarios with over 8 dm of spread amongst ensemble member across the western CONUS. The scenarios breakdown into near 50:50 split between a more elongated/stronger upper trough extending southwestward across the western CONUS and a stronger upper ridge displaced more east into the West Coast. The latter warmer and offers low chances for precip, while the former would be cooler and promote increased chances (>30%) for precip in the mountains.

AVIATION
18Z TAFS
VFR conditions during this period with currently cloudy skies (5-25 kft), except for KYKM having clear skies. However, cloud coverage will decrease for most sites tonight into Friday morning, excluding KYKM where it will increase.
Confidence was very low (10-15%) at first to include wind gusts for KRDM/KBDN, but due to the surface pressure gradient weakening as the low passes, confidence level rose higher for that possibility (70%).
Winds will be less than 10kts. Feaster/97

PRELIMINARY POINT TEMPS/POPS
PDT 35 70 47 61 / 0 0 10 60 ALW 41 73 53 69 / 0 0 10 50 PSC 42 74 53 68 / 0 0 10 50 YKM 37 70 46 61 / 0 0 40 80 HRI 39 75 49 64 / 0 0 20 60 ELN 37 68 45 61 / 0 10 30 70 RDM 30 65 41 48 / 0 10 90 80 LGD 31 65 47 63 / 10 0 10 50 GCD 32 63 45 56 / 50 0 30 80 DLS 42 71 49 55 / 10 10 70 80

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

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