textproduct: Pendleton

UPDATE AVIATION...06Z TAFs

Thanks to the low stratus layer, this will drop KDLS/KPSC to MVFR with KYKM at IFR and KPDT/KALW to LIFR conditions. KDLS/KPSC may briefly see patchy fog overnight into early Thursday morning (35% confidence), but dense fog could redevelop for KYKM/KALW while continuing for KPDT thus dropping VSBYs to 1/2sm or less. The fog should then dissipate around 12Z for KDLS/KPSC and later in the afternoon for KYKM/KALW. KPDT may have lingering fog/mist through the day Thursday (40-50% confidence). In addition to the fog and low cloud decks, a weak system will begin approaching early Thursday morning with light rain starting KDLS first before other sites. KRDM/KBDN will be VFR with light rain Thursday morning until MVFR conditions return early Thursday evening when low clouds redevelop. Winds will remain less than 10kts. Feaster/97

DISCUSSION

Rest of Today through Thursday Morning: A weak shortwave moving over the region has helped to lift fog impacting areas of the Columbia Basin and adjacent valleys/foothills this afternoon, though cloud decks remain fairly low across these areas. This reprieve is expected to be brief, as there is mod-high confidence (55-85%) that dense fog will redevelop in these areas by this evening. That said, confidence is low(25-40%) in the exact timing and areal extent of the fog redeveloping. Regardless, opted to extend the current fog advisories through 12PM PST tomorrow for the Kittitas/Yakima valleys and the Northern Blue Mountain Foothills of OR/WA.

By Thursday morning, the first in a series of coupled shortwaves and surface lows will arrive to the PacNW, while a plume of moisture originating from the Hawaii region will aid precipitation amounts with each incoming system. As for impacts Thursday morning, dense fog is expected to persist across portions of the Columbia Basin and adjacent foothills/valleys, with rain and high mountain snow or rain/snow mix starting developing along the Cascade Crest, then spreading southeast to the remainder of the forecast area. One concern with incoming precipitation will be the potential for freezing rain to develop tomorrow morning if the cold pool strengthens and drops to or below freezing tonight. Areas that would be most likely impacted would be portions of the Kittitas/Yakima valleys, and along the northern Blue Mountain foothills. That said, confidence is very low (<15%) in this scenario playing out with onset of precipitation tomorrow.

Thursday Afternoon through Saturday: Ensemble guidance is in great agreement through Saturday night that a persistent upper level ridge in the northeast Pacific will force several surface lows and attendant shortwave troughs to dive southeast across the forecast area. While each system will be approaching from the northwest, which would generally mean their airmasses would be fairly cool/cold, a plume of tropical moisture with IVTs in the 150 to 250 kg/m/s range will ride up and over the ridge into the PacNW, keeping the airmass generally saturated with well above normal temperatures than normal this time of year (confidence 70-90%). As the airmass infiltrates the PacNW, snow levels will slowly rise above pass level, keeping any snow or wintry mixes to the higher mountain ridges and peaks through this period.

By Thursday afternoon, the first in the series of systems will glide over the forecast area, resulting in widespread rain and high mountain snow chances the remainder of Thursday. By Friday, a more potent surface low with a strong cold front will approach the PacNW, resulting in a tightening of pressure gradients at the surface. Westerly winds will increase as early as Friday morning, with winds forecast to peak Friday evening as the surface low and cold front move across the forecast area. Wind gusts will increase to 25-40mph across much of the area(confidence 60-80%), with a 50-80% chance that wind gusts will exceed 45mph across portions of the OR Columbia Basin, Blue Mountain Foothills, North Central OR, Simcoe Highlands, as well as ridges in the WA Columbia Basin to the WA Cascade east slopes. Winds will gradually decrease through Saturday as the low departs the region to the east.

Otherwise, during this period, there is a 60-85% chance of the Cascade crest and east slopes receiving 1.5 inches of QPF while the northern Blues will only see a 30-55% chance. Looking at 0.5 inches of QPF, the northern Blue Mountain foothills and the remainder of the eastern mountains will see a 35-70%. For the remainder of the areas, there is generally a 20-60% chance for 0.25 inches of precipitation, with the lowest chances in the WA Columbia Basin and across portions of central OR.

Sunday through Tuesday: Guidance continues to be in good agreement Sunday through Tuesday as all but about 10-13% of ensemble cluster members show the ridge axis moving inland and flattening Sunday and Monday, with zonal flow developing over the PacNW. Meanwhile, ensemble members of the GFS and ECMWF continue to depict a 60-80% chance of at least a weak atmospheric river (AR) with IVT of 150 kg/m/s filtering into the area. Under the zonal flow, weak shortwaves with surface fronts will continue to impact the forecast area through Tuesday, and with the accompanying AR, this will result in periods of moderate to heavy precipitation in the mountains with lighter amounts further down along the Cascade east slopes and the northern Blue Mountain foothills. Of the ensemble cluster members, the solutions made up of the earlier mentioned 10-13% favor lower 500mb heights over the PacNW, which would translate to lower snow levels and potentially snow at or just above pass level in the mountains. Otherwise, the remaining members favor mostly rain with high mountain snow during this period. Lawhorn/82

PRELIMINARY POINT TEMPS/POPS

PDT 28 43 37 53 / 0 80 70 90 ALW 31 42 37 49 / 10 80 80 90 PSC 28 42 32 51 / 10 70 50 60 YKM 27 42 31 52 / 10 40 40 70 HRI 29 43 36 53 / 0 80 60 70 ELN 27 43 31 46 / 20 50 50 80 RDM 25 47 35 55 / 0 50 40 60 LGD 26 40 35 48 / 0 90 90 100 GCD 26 42 37 49 / 0 80 90 90 DLS 36 48 42 56 / 10 70 70 90

PDT WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 11 AM PST Thursday for ORZ507.

WA...Dense Fog Advisory until 11 AM PST Thursday for WAZ029.

Freezing Fog Advisory until 11 AM PST Thursday for WAZ026-027.