textproduct: Pendleton

KEY MESSAGES

- Critical fire weather conditions stemming from low humidity and breezy winds today across the Kittitas Valley and Oregon Lower Basin.

- Elevated to critical fire weather conditions Tuesday and Wednesday.

- Low thunderstorm chances in the central Oregon mountains Monday shifting to the eastern mountains Tuesday.

DISCUSSION

Latest satellite imagery shows clear skies though some cu apparent in east central OR with eroding stratus west of the Cascades. Water vapor shows cyclonic flow aloft at mid-levels with a westerly mid-level jet centered roughly across the WA/BC border. A sharpening thermal low is developing in the Columbia Basin/interior NW today wherein the developing cross-Cascade pressure gradient and modest low-level winds are contributing to gusts commonly 20-30 mph along gaps of the Cascades, eastern Gorge spilling into the Lower OR Basin, and the Kittitas Valley. The latter seeing the highest gusts with gusts reaching 40 mph. Winds will lessen tonight with loss of solar heating and cooling. The large scale pattern will see a similar zonal pattern tomorrow as today, and similar conditions with respect to temperatures and humidity with weak height rises owing to a shortwave ridge working into WA/BC. A lower surface pressure gradient will be in place though, thus limited weather concerns overall with light winds expected and highs largely 3-8 degrees above average for this time of year. Looking ahead, an upper-level closed low will be developing southward across the Gulf of Alaska promoting large-scale troughing in the northern eastern North Pacific with another closed low developing northeast of Hawaii. This will work to prompt a southwest flow aloft with increasing 850 hPa temperatures moving in, and increasing moisture into southern and eastern OR. The latest ensemble guidance shows precipitable waters around 125-175% of normal in place by Monday across the southern-third of the forecast area. The combination of orographic lift and large scale ascent with this reinforcing moisture and moderate instability (with deterministic guidance showing 500-800 J/Kg of MUCAPEs and MLCAPEs of 400-600 J/kg) will promote thunder chances across the central OR mountains and southern eastern Mountains. The weak forcing regime and quantity of moisture lowers the overall confidence though with present thunder chances around 10-20%, highest across southern Deschutes, Crook, and Grant counties.

This mid-level trough is forecast to become more amplified Tuesday with highs 6-12 above average under the southerly flow and the shower/thunderstorm threat shifting more to the eastern mountains/Blues and Wallowas. However, moisture remains a concern and the lack thereof under the weak forcing pattern again. Meantime, sharpening pressure differences will prompt breezy winds under hot, dry conditions on Tuesday with the upper-level troughs mid-level jet spreading into the region. This will promote gusts 25-40 mph across the Cascade gaps, Kittitas Valley, and the eastern Gorge spilling into the Lower Basin. Chance for peak daily gusts exceeding 30 mph is 70% or higher across these aforementioned areas; chances for 40 mph or higher around 50-90% localized across the Kittitas Valley and parts of the eastern Gorge. The upper troughs attendant cold front will then move through early Wednesday promoting windy conditions and highs about 8-15 degrees cooler than Tuesday. While precipitation looks limited with mostly dry conditions anticipated, higher gusts are more likely than Tuesday with 30-45 mph gusts forecast. There are better chances for areas seeing 40 mph across the Lower Basin and north central OR (50-60%) Wednesday than Tuesday. The combination of windy conditions and low humidities will prompt potential elevated to critical fire weather conditions Tuesday and Wednesday.

Increasing large-scale troughing in the eastern North Pacific looks likely then late this week with chances around 70-80% for this scenario. This will prompt similar to a slight rise in highs going into the early weekend with near-average highs forecast Wednesday-Friday. Overall dry conditions look more likely than not with an increasing cross-Cascade gradient alongside diurnal breezes on Friday and Saturday.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/

VFR conditions are forecast at all sites for the next 24 hours. Winds will be breezy this afternoon at most sites, strongest at DLS, then lighter and more terrain-driven overnight through Sunday.

FIRE WEATHER

Critical fire weather conditions are in place with low RH and gusty winds seen across Kittitas Valley and the OR Lower Basin. The former dipping down to around 15% with gusts 30-40 mph early this afternoon while the latter has seen winds 15-20 mph and RH 10-15%. Conditions will remain favorable for rapid fire spread into the evening with a sharp cross- Cascade pressure gradient evolving this afternoon. Looking ahead, weaker pressure differences are expected tomorrow with low humidity again but lighter winds. Thus fire weather concerns are lower compared to today, and this will continue Monday. That said, there are low thunderstorm chances Monday across the central OR mountains.

Tuesday and Wednesday hold the next weather concerns with broad troughing in the eastern North Pacific. This will promote hot, dry, and windy conditions with a sharp cross-Cascade gradient developing with increasing westerly flow. Main areas of concern is across Cascade gaps, eastern Gorge into north central OR, Kittitas Valley, and Simcoe Highlands. A cold front will cross the area then early Wednesday knocking temperatures down some with humidity increasing as much as 10% in the north. However, more widespread breezy conditions are expected with higher-end gusts more likely on Wednesday.

Preliminary Point Temps/PoPs

PDT 53 88 55 90 / 0 0 0 0 ALW 58 89 59 92 / 0 0 0 0 PSC 56 91 56 94 / 0 0 0 0 YKM 55 91 58 95 / 0 0 0 0 HRI 56 90 56 93 / 0 0 0 0 ELN 51 87 54 91 / 0 0 0 0 RDM 49 89 54 89 / 0 0 0 10 LGD 53 88 56 89 / 0 0 0 10 GCD 52 92 56 91 / 0 0 0 20 DLS 57 92 60 95 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning until 7 PM PDT this evening for WAZ690. OR...Red Flag Warning until 10 PM PDT this evening for ORZ691.