textproduct: Pendleton

KEY MESSAGES

- Widespread rain today, heaviest over the Cascade and Blue mountains

- Breezy to windy today through Wednesday, strongest on Tuesday

- Warmer, drier weather late week through the weekend

DISCUSSION

Water vapor imagery shows a closed mid/upper-level low rotating in the Gulf of Alaska this morning. Meanwhile, a shortwave is evident southeast of the low, diving towards the Pacific Northwest. 00Z ensemble NWP guidance is in excellent agreement that the shortwave will move inland later this afternoon through tonight, ushering in widespread shower chances. For the lower elevations, precipitation totals are currently forecast to be light (a few hundredths of an inch to a couple tenths of an inch) while the mountains will see higher precipitation amounts of several tenths of an inch (Blues) to locally over an inch (Cascades, especially in Washington).

Ahead of the precipitation, warm temperatures in the 70s coupled with locally breezy winds and low relative humidity in the teens to lower 20s are forecast for roughly the eastern half of our area of responsibility. Elsewhere, cooler temperatures and higher relative humidity are expected beneath thicker cloud cover and/or in light rain.

Per latest 00Z NWP guidance, breezy southerly to southwesterly 850-hPa winds of 15-35 kts are expected (90 percent confidence) to overspread the region this afternoon, becoming southwesterly to westerly and increasing in magnitude to 35-55 kts overnight through Tuesday morning, then becoming westerly to northwesterly and slackening to 25-40 kts Tuesday afternoon and 15-25 kts on Wednesday.

NBM probabilities of exceedance suggest very high (80-99 percent) chances of reaching or exceeding advisory-level wind gusts (45 mph or greater) 11 PM PDT tonight (Monday night) through 11 PM PDT Tuesday across the foothills of the Blue Mountains, north-central Oregon, the Simcoe Highlands, and through the Kittitas Valley and eastern Columbia River Gorge. Forecaster confidence in these probabilities is a little lower (70-90 percent) based on the timing of the strongest winds aloft not coinciding with peak mixing (i.e. strongest winds are at night), but forecast cross-Cascade pressure gradients are sufficiently strong (8-12 hPa difference between PDX and GEG) Tuesday afternoon to support wind headlines. Will note the NBM indicates a low (20-40 percent) chance of reaching warning-level wind gusts (58 mph or greater) for our most wind-prone locations.

The remnants of the aforementioned low in the Gulf of Alaska are expected (80-90 percent confidence) to slide over the Pacific Northwest late Tuesday through Wednesday, resulting in continued mild temperatures and rain showers (mainly for the mountains). Weak instability (CAPE of up to a couple hundred J/kg) coupled with forcing from the shortwave may facilitate an isolated thunderstorm over the eastern mountains Tuesday afternoon, but chances are low (5-15 percent).

Warmer, drier conditions are then forecast Thursday through the weekend as an upper-level ridge of high pressure will likely (80-90 percent confidence) build offshore in the Pacific and eventually shift east into the Pacific Northwest. Ensemble cluster analysis reveals some differences in the amplitude and location of the ridge through the weekend, and a low (10-20 percent) chance of a cooler, wetter pattern with the ridge axis located well offshore and shortwave troughing digging into the Pacific Northwest from the north.

While outside of the current forecast period, temperatures are forecast to peak on Monday, with a 50-80 percent chance of >= 90 degrees, and 5-15 percent chance of >= 100 degrees for the lower elevations. Moreover, probabilistic HeatRisk guidance indicates a 15-40 percent chance of Major (level 3 of 4) HeatRisk for the majority of the lower elevations within the greater Columbia Basin region on Monday. This level of heat affects anyone without cooling/hydration.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

VFR conditions are forecast for all sites through the period. An incoming low-pressure system will bring light rain to all terminals by afternoon or evening. MVFR CIGs are not forecast, but cannot rule them out (5-30 percent confidence, highest at YKM). Winds will become breezy later today, especially overnight into Tuesday morning when gusty winds are likely (90 percent confidence), but confidence in exact timing/magnitude of winds at terminals is low (30 percent).

Preliminary Point Temps/PoPs

PDT 71 48 68 43 / 20 90 40 0 ALW 75 52 67 47 / 0 90 60 30 PSC 75 49 73 44 / 20 90 30 0 YKM 70 48 71 42 / 50 80 10 0 HRI 73 48 71 44 / 20 80 20 0 ELN 66 45 64 39 / 50 90 40 0 RDM 64 43 65 32 / 80 60 10 0 LGD 71 46 63 41 / 0 100 80 20 GCD 70 44 66 37 / 40 100 40 10 DLS 68 53 68 49 / 80 90 30 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.