textproduct: Pendleton

KEY MESSAGES

- Isolated thunderstorms today, mainly east slopes of the Washington Cascades

- Multiple Red Flag Warnings in effect Thursday due to breezy winds and low relative humidity

- Cooler, breezy, and showery weather is likely (95 percent confidence) Friday into the weekend

DISCUSSION

Water vapor imagery shows a weak shortwave moving over the Pacific Northwest this afternoon. While some cirrus is present, the main focus has been on cumulus fields over the mountains. The most robust of which developed across the east slopes of the Washington Cascades mid-morning and has resulted in showers and isolated thunderstorms. Elsewhere, vertical development has remained stunted over the higher terrain of the Oregon Cascades and Blue Mountains, but some recent vertical growth has been noted in western Wasco County east of Mt Hood. There is still a very low to slight chance (5-15 percent) of high-based showers and thunderstorms across the east slopes of the Oregon Cascades and Blue Mountains this afternoon and evening.

Widespread Moderate (level 2 of 4) HeatRisk is still expected across the lower elevations of the forecast area today, with afternoon temperatures peaking in the upper 80s to mid-90s. This level of heat primarily affects those who are sensitive to heat, especially those without cooling/hydration.

Breezy westerly winds in tandem with low relative humidity will facilitate another round of localized fire weather concerns this afternoon and evening (see Fire Weather section below for details).

By Thursday, the closed low currently located in the Gulf of Alaska is expected (95 percent confidence) to move southeast along the coast of British Columbia and southeast Alaska. A shortwave wrapping around the base of the low is forecast to move inland over the Pacific Northwest during the afternoon. Breezy to windy westerly winds and low relative humidity are anticipated and Red Flag Warnings have been issued (see Fire Weather Section below for details).

A transition to a cooler, wetter pattern is likely (90 percent confidence) by Thursday night as a second shortwave moves overhead, followed by the parent closed low over the weekend. Cluster analysis of 00Z ensemble members shows relatively small differences in timing/location and magnitude of the low during the period, so confidence is high in the current forecast of below-normal temperatures Friday through the weekend. However, even small differences in the track/timing of the low can have a significant impact on precipitation; the trend in ensemble guidance has been to keep the lower elevations relatively dry with precipitation focused over the mountains.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

VFR conditions are forecast at all TAF sites for the next 24 hours. Winds will become breezy and gusty through the Cascade gaps this afternoon and evening with lighter more variable winds for PDT/ALW/PSC. There is a low (10-30 percent) chance of high-based showers at YKM this afternoon and evening, with an even lower chance (10 percent) of thunderstorms. The chance of rain or thunder at all other sites is less than 10 percent.

FIRE WEATHER

Winds will transition to onshore (westerly) and become breezy during the mid/late afternoon hours as a weak shortwave trough moves over the Pacific Northwest. 12Z HREF and REFS NWP guidance both show medium probabilities (30-70 percent) of reaching localized Red Flag conditions across OR691 (lower Columbia Basin of Oregon), WA691 (lower Columbia Basin of Washington), WA690 (Kittitas Valley), and OR703 (Warm Springs Reservation) this afternoon and evening. Have held off on issuing any fire weather headlines due to insufficient confidence (need 80 percent or higher).

Showers and isolated thunderstorms are ongoing along the east slopes of the Washington Cascades this afternoon. Elsewhere, there is a very low to slight chance (5-15 percent) of showers and thunderstorms for the east slopes of the Oregon Cascades and portions of the Blue Mountains.

Thursday, fire weather concerns increase across OR691, WA691, WA690, and OR703 as breezy to windy westerly winds (10-20 mph with gusts of 20-30 mph) overlap with low relative humidity of 10-20 percent. 12Z REFS, 12Z HREF, and NBM probabilities paint a 60-100 percent chance of reaching RFW criteria across a good portion of these zones. Have opted to issue Red Flag Warnings, valid between 10 AM and 9 PM PDT Thursday due to wind and low relative humidity.

Cooler, breezy, and showery weather with isolated thunderstorms (mainly for the mountains) is then forecast Friday through the weekend as an upper-level closed low pressure system moves over the Pacific Northwest.

Preliminary Point Temps/PoPs

PDT 60 84 52 74 / 0 0 20 40 ALW 64 86 58 75 / 0 0 10 40 PSC 62 89 56 79 / 0 0 10 10 YKM 60 87 55 77 / 10 0 20 20 HRI 63 86 55 77 / 0 0 20 10 ELN 59 81 50 69 / 10 0 20 60 RDM 49 81 47 70 / 10 0 50 20 LGD 56 84 53 70 / 0 0 20 80 GCD 53 84 50 71 / 0 0 50 80 DLS 63 83 57 73 / 0 0 20 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning from noon to 9 PM PDT Thursday for WAZ690-691. OR...Red Flag Warning from 10 AM to 9 PM PDT Thursday for ORZ691. Red Flag Warning from noon to 8 PM PDT Thursday for ORZ703.