textproduct: Pendleton

KEY MESSAGES

- Very low to slight chance (5-15%) of thunderstorms today (Blue Mountains) and Saturday (Cascade and Blue mountains).

- High confidence (99%) that a low-pressure system will track over the Pacific Northwest Saturday into early Sunday, resulting in below-normal temperatures and widespread breezy westerly winds.

- Periods of elevated fire weather conditions through the weekend due to the breezy winds, but low confidence (<40%) in widespread Red Flag conditions.

- Low-medium probability (20-50%) of near-freezing morning low temperatures in cold-prone valleys of central and northeast Oregon Sunday and Monday.

DISCUSSION

Satellite imagery early this morning shows southwesterly flow aloft, with a shortwave trough noted upstream just offshore in the Pacific. Meanwhile, a closed low is located over far southeast Alaska, moving southeast along the coast. The latter low will be the primary driver of our weather over the weekend.

Today, diurnally breezy winds are expected (99% confidence) through the Cascade gaps. Weak forcing and instability over the Blue Mountains (mainly Elkhorns, northern Blues, and Wallowa County) may facilitate isolated showers and thunderstorms this afternoon and evening, with stronger synoptic-scale forcing associated with the aforementioned low along the Alaska coast leading to isolated showers overnight into early Saturday along the Blue Mountains and their adjoining foothills as it approaches the Pacific Northwest. PoPs are quite low (5-15%) during this period.

Ensemble NWP is in excellent agreement (99% confidence) that the low will track inland over the Pacific Northwest on Saturday, then exit to the east through Sunday. Widespread breezy westerly winds are forecast on Saturday, strongest though the Cascade gaps. NBM probabilities suggest medium (30-70%) chances of reaching advisory-level wind gusts through our most wind-prone areas such as the Kittitas Valley and the eastern Columbia River Gorge and vicinity. Despite the breezy winds, chances of widespread Red Flag conditions are low (<40%) due to below- normal temperatures.

Rain showers are likely (55-75% PoPs) along the Washington Cascade crest later tonight into Saturday as the low swings inland. Chances of precipitation are lower (10-40%) over the Blue Mountains Saturday afternoon, but still noteworthy. Chances of thunderstorms are low (5-15%) for the Cascades and Blue Mountains on Saturday.

Looking ahead, there are low-medium chances (20-50%) of near- freezing morning temperatures (lower to middle 30s) in cold- prone valleys of central and northeast Oregon Sunday and Monday in the wake of the cold upper low. NBM probabilities are likely too aggressive Sunday morning due to some lingering breeziness, though forecast PWATs and temperatures aloft are supportive of chilly conditions. Thinking the coldest temperatures in valleys will fall on Monday morning with light winds areawide under a shortwave ridge, something NBM probabilities do not highlight well.

Analysis of ensemble clusters suggests a continued troughing pattern for the Pacific Northwest by the middle of next week with all four clusters exhibiting some flavor of trough/upper closed low and near-normal or below-normal 500-hPa heights. Will the region see its first widespread wetting rain with the fall-like pattern? NBM probabilities are quite optimistic for the 48-hr period ending 5AM Friday. In general, it shows 40-70% chances of 0.10" of precipitation for non-mountain areas with 50-90% chances across the mountains (highest chances for the Washington Cascades).

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

VFR conditions are forecast at all sites through the period. Smoke across central Oregon terminals presents the best chance of sub-VFR VSBYs/CIGs this morning, but confidence was too low (<50%) to have a mention in the 12Z TAFs.

Winds will be diurnally breezy, strongest through the sites downwind of or within the Cascade gaps (DLS, RDM, BDN) with some breezes expected to arrive at PDT later this evening.

Mostly clear skies are forecast outside of FEW-SCT cumulus and bands of cirrus that are currently draped across far eastern Oregon and northwest Oregon to central Washington.

Preliminary Point Temps/PoPs

PDT 82 47 73 45 / 0 0 0 0 ALW 84 54 75 50 / 0 10 0 0 PSC 85 51 78 45 / 0 0 0 0 YKM 85 53 76 48 / 0 0 0 0 HRI 85 52 76 47 / 0 0 0 0 ELN 77 47 69 43 / 0 0 0 0 RDM 82 39 72 34 / 0 0 0 0 LGD 86 47 75 40 / 0 0 0 0 GCD 89 49 77 41 / 0 0 0 0 DLS 82 56 74 51 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until noon PDT today for WAZ026-522-523. OR...Air Quality Alert until noon PDT today for ORZ041-505-506- 509>511.