textproduct: Pendleton

KEY MESSAGES

- Warm, dry conditions through the weekend

- Breezy to windy through the Cascade gaps Saturday and Sunday

- A low-pressure system and cold front are likely (80 percent confidence) to arrive Monday, but details are still uncertain

DISCUSSION

Dry northwesterly flow aloft today will transition to zonal flow over the weekend, resulting in warm, dry conditions, with temperatures of around 5-10 degrees above normal for our population centers.

Breezy to locally windy westerly winds are forecast Saturday through the Cascade gaps as cross-Cascade pressure gradients increase in magnitude to 6-10 mb between PDX and GEG per latest NWP guidance. Don't have high confidence in the need for wind headlines, but NBM probabilities suggest localized areas with a 50-70 percent chance of reaching advisory-level wind gusts (45 mph or greater) through the eastern Columbia River Gorge, Kittitas Valley, the Simcoe Highlands, and wind-prone portions north-central Oregon. Surface pressure gradients are expected (80 percent confidence) to slacken by several millibars on Sunday, with breezy winds forecast through the same region as Saturday.

Monday through the remainder of the week, uncertainty in forecast details becomes noteworthy as ensemble guidance suggests a low-pressure system with origins in the Gulf of Alaska will dive southeastward into the Pacific Northwest, resulting in breezy to windy conditions, cool temperatures, and precipitation chances region-wide (highest for the mountains). While all clusters of ensemble members show the low tracking into the Pacific Northwest by Monday, there are some differences in the depth of the closed low as it approaches the coast. Differences in timing of the low's arrival also translate to some uncertainty in timing of the cold frontal passage; 00Z ensemble guidance takes the front from west to east across the forecast area sometime between late Monday morning and late Monday evening. Widespread breezy to windy conditions are forecast with the passage of the front. The ECMWF EFI is painting much of the forecast area with values of 0.6-0.9 for wind, and the NBM shows medium-high (50-90 percent) probabilities of advisory-level gusts across wind-prone portions of all of our non-mountain zones.

By Tuesday, ensemble systems begin to diverge in their solutions regarding the track and evolution of the closed low. Roughly 40 percent of members show a deeper low centered over western Oregon while the remaining members have a slightly shallower low centered over central Oregon to central Washington. Differences really begin to grow by Wednesday as 13 percent of members show the low transitioning into an open wave while the remaining members keep a broad closed low over the Great Basin extending into the Pacific Northwest. Solutions continue to diverge Thursday with about 70 percent of members retaining the closed low somewhere over the Great Basin while the remaining members favor a progressive solution with either near-normal heights (16 percent of members) accompanied by cool temperatures or above- normal heights (15 percent of members) and warmer, dry conditions. Of the members that advertise a closed-low, roughly half show the low extending far enough north into the Pacific Northwest to result in cool, wet weather while the remaining members take it far enough south into the Great Basin to have warmer temperatures and mostly dry conditions for the Washington side of our forecast area.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

VFR conditions continue to prevail for all TAF sites, with light 5-15 mph winds. With general subsidence aloft, cloud coverage will be limited to SKC to SCT. Not expecting any CIG or VIS issues.

Preliminary Point Temps/PoPs

PDT 80 49 83 51 / 0 0 0 0 ALW 80 53 84 56 / 0 0 0 0 PSC 85 52 89 52 / 0 0 0 0 YKM 84 53 88 51 / 0 0 0 0 HRI 83 52 86 54 / 0 0 0 0 ELN 78 50 79 48 / 0 0 0 0 RDM 81 44 83 42 / 0 0 0 0 LGD 77 45 82 46 / 0 0 0 0 GCD 80 43 85 44 / 0 0 0 0 DLS 85 56 83 53 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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