textproduct: Pendleton

KEY MESSAGES

- Isolated to scattered thunderstorms through Thursday night across northeast Oregon and southeast Washington

- Critical fire weather conditions from abundant lightning and outflow winds through Thursday night

- Moderate to locally major HeatRisk through Thursday, and a return of widespread moderate HeatRisk Sunday into next week.

DISCUSSION

Today through Thursday: Satellite and radar imagery shows plenty of activity across the mountains this afternoon, with widely scattered thunderstorms developing across the OR Cascades and ElkHorn Crest down to the Strawberries. Meanwhile, an isolated thunderstorm has developed in far northwest Yakima county, well ahead of the incoming system.

Through tomorrow morning, a shortwave impulse currently lifting across northeast OR will continue to initiate isolated to scattered thunderstorms, with main activity expected to ride up the Cascade east slopes through tonight. A few isolated thunderstorms will also remain possible across the eastern mountains into northern Wallowa county. With plenty of surface moisture in place (pwats up to 1.2 inches), dry mid- layer(inverted V-sounding profile), steep mid-level lapse rates, and MUCAPE up to 1000J/kg, a few thunderstorms will have the potential to produce strong to severe outflow gusts (50 to 60 mph) this afternoon into this evening. Expect moderate to locally heavy downpours(0.25 to 0.4 inches/hr) with these stronger storms as well.

Thursday, an upper low offshore will cut across northwest WA as it kicks north. This upper low will cause a moderate pressure gradient to develop across the Cascades, leading to breezy winds (25 to 40mph gusts) through the Cascade gaps (confidence 65-75%). However, the low will also bring cooler temperatures west of the Cascades, creating a strong thermal gradient (20-30 degrees F) between the Columbia Basin and I-5 corridor, which will aid in strengthening winds through the gaps. Otherwise, another shortwave impulse rotating north across far eastern OR/WA will bring another round of isolated thunderstorms from the Strawberry mountains, to the northern Blues in WA (including Wallowa County). A strong thunderstorm or two will also be possible with activity tomorrow, as MUCAPE will be 500-1000 J/kg, with moderate mid-level lapse rates (7-8 C/km) and low level sheer between 20-30kts. Expect outflow winds and heavy downpours to be the main threats.

Friday through Sunday: Dry conditions will redevelop across the area as upper level riding begins to build into the PacNW from the east. Winds will become light and diurnally driven across the area. Behind the low exit Thursday, temperatures will drop about 3 to 5 degrees across the area, resulting in mainly low to mid 90s in the Columbia Basin. However, with the ridge building back over the weekend, expect temperatures to warm 1 to 2 degrees day to day, with a return of the upper 90s in the Basin by Sunday.

Monday and Tuesday: Fairly good agreement amongst ensemble cluster solutions of weak troughing/low pressure develop offshore the PacNW while upper level ridging amplifies poleward. This will bring a deep south to southwest flow into the area, resulting in increased instability and monsoonal across the intermountain PacNW (confidence 50-60%). As a result, the mountains and central OR will see a return of at least isolated thunderstorm chances through Tuesday. The southerly flow will also bring increased temperatures across the rest of the region, with NBM showing a 35-55% chance of breaking the 100 degree mark in the Columbia Basin Monday and Tuesday. Lawhorn/82

FIRE WEATHER

Main threat through Thursday will be strong isolated to scattered thunderstorms developing across the Cascades, eastern mountains, western portions of the Columbia Basin, and the Yakima/Kittitas valleys. With unseasonably dry fuels in place, thunderstorms with frequent lightning will produce a critical fire weather threat. Breezy winds will develop through the Cascade gaps tomorrow afternoon, but an associated marine push will keep fire weather conditions from becoming critical. Warm and dry conditions develop Friday into the weekend, with isolated thunderstorm threat returning early next week.

Preliminary Point Temps/PoPs

PDT 62 93 57 90 / 20 20 10 0 ALW 67 97 63 92 / 20 20 10 0 PSC 66 99 62 95 / 20 20 10 0 YKM 67 97 60 94 / 70 40 10 0 HRI 67 96 61 93 / 20 20 0 0 ELN 64 93 57 89 / 70 50 10 0 RDM 55 87 47 90 / 30 0 0 0 LGD 61 97 58 93 / 20 30 20 0 GCD 58 97 55 95 / 20 10 10 0 DLS 65 85 59 89 / 70 20 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory until 5 AM PDT Thursday for WAZ024. Heat Advisory until 9 PM PDT Thursday for WAZ026>029. Red Flag Warning until 10 AM PDT Thursday for WAZ691-694. Red Flag Warning until 10 AM PDT Thursday for WAZ695. Red Flag Warning from 8 PM this evening to 10 AM PDT Thursday for WAZ690. OR...Heat Advisory until 5 AM PDT Thursday for ORZ041. Red Flag Warning until 5 AM PDT Thursday for ORZ691. Heat Advisory until 9 PM PDT Thursday for ORZ044-049-050-507. Red Flag Warning until 11 PM PDT this evening for ORZ700- 703>705.