textproduct: Pendleton

KEY MESSAGES

- Above-normal temperatures today.

- Low (5-15 percent) chance of thunderstorms today, mainly over the Blue Mountains.

- Widespread breezy to windy conditions Monday night through Tuesday.

DISCUSSION

The upper-level ridge that is parked over the Pacific Northwest early this morning will flatten through the day. A weak shortwave is wriggling through the ridge, and it will be followed by more significant height falls later this morning and afternoon as a stronger wave evident in water vapor imagery treks southeast along the British Columbia coast before diving inland across the Northern Tier. Meanwhile, a deep closed low is cutting off from the jet stream well offshore in the Pacific around 34.5N, 143.0W; I'll talk more about the implications of the cut-off low later in the discussion.

Confidence is very high (90 percent) that afternoon high temperatures will be well above normal today. If forecast highs in the middle to upper 70s and isolated lower 80s verify, they will generally fall in the top 5 or 10 warmest April 6ths during the period of record at all of our climate sites.

Glancing at probabilistic output, there is a high (65-90 percent) chance the eastern Columbia River Gorge, Yakima Valley, lower Columbia Basin of Washington, and lower elevations of north-central Oregon will reach 80 degrees. Elsewhere, chances of reaching 80 degrees are lower at 50 percent or less, highest for the Walla Walla Valley and lower Columbia Basin of Oregon.

Guidance continues to show low (5-20 percent) chances of showers and very low (5-15 percent) chances of thunderstorms this afternoon and evening. Latest 00Z CAMs are advertising a modestly unstable atmosphere with MUCAPE/SBCAPE of 250-500 J/kg, locally higher, over the mountains. Best chances of showers and thunderstorms will be over the Blues, but cannot rule out more isolated activity for the Cascades of both Oregon and Washington. Thunderstorms, if any, should be of the air mass variety due to weak effective shear of 15-25 kts. My analysis of storm motion was incorrect yesterday, and for that I sincerely apologize. Based on latest 00Z HREF and REFS forecast soundings, storm motion of 15-20 kts to the east or east- southeast is expected, rather than the lethargic 5-10 kts that I stated in yesterday's discussion. Gusty outflow winds are still expected to be the primary hazard with convection. 00Z HREF and REFS suggest up to 40 kt gusts with the most robust cells, and DCAPE of 500-800 J/kg in forecast soundings supports these sub- severe outflow winds. In addition to outflow winds, any thunderstorms that develop will be capable of producing small hail.

Winds tonight (Monday night) and Tuesday are still the best chance of a headline-worthy event for the next week. That said, the morning peak in the 850-hPa jet is not optimal for really strong winds during the afternoon. Still expecting (90 percent confidence) widespread breezy to windy westerly to northwesterly winds, but confidence in reaching advisory-level wind gusts (45 mph or greater) is too low to issue any wind headlines. From a probabilistic standpoint, the NBM has not exhibited much day-to-day change, and still generally paints low-medium (30-70 percent) chances of reaching advisory criteria through the Cascade gaps and across wind-prone portions of the Columbia Basin and surrounding lowlands.

Looking ahead, there is significant uncertainty in the evolution of the 500-hPa pattern Wednesday through the remainder of the week and into the weekend as ensemble guidance struggles to resolve the both the track of the offshore cut-off low in the Pacific as well as an additional trough/low. 12Z runs keep the closed low in the vicinity of the OR/CA border or northern/central California Wednesday through Friday. The trend towards a split flow pattern has resulted in an upward trend in forecast temperatures and would keep above-normal temperatures in place for the duration of the week. A slight chance to chance (15-40 percent) of showers is forecast Thursday and Friday along the Cascades of central Oregon and across the Blue Mountains, but thunderstorm chances are still low (less than 15 percent). By the weekend, all ensemble clusters show some flavor of a trough or closed low returning to the Pacific Northwest, though there is a large amount of spread in scenarios. This does place the majority of the forecast area under slight chance to chance (15-50 percent) PoPs, lowest for the lower Columbia Basin and central Oregon, highest for the mountains. 86

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

VFR conditions will prevail through Monday. Winds will generally be 10 kts or less through Monday afternoon, then at DLS, RDM and BDN, winds will gust to around 20 kts before decreasing during the evening. All other sites will remain 10 kts or less.

Preliminary Point Temps/PoPs

PDT 75 47 64 34 / 0 0 0 0 ALW 75 50 63 37 / 0 0 0 0 PSC 80 48 67 35 / 0 0 0 0 YKM 77 41 65 34 / 0 0 0 0 HRI 79 49 67 35 / 0 0 0 0 ELN 72 38 55 32 / 0 10 0 0 RDM 75 36 67 30 / 0 0 0 0 LGD 74 43 62 32 / 10 0 0 0 GCD 74 41 65 32 / 20 20 0 0 DLS 77 47 63 37 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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