textproduct: Pendleton

KEY MESSAGES

- Minor river flooding in south-central Washington through Monday

- Elevated river levels in south-central Washington continue through this week after flooding subsides Monday

- Active weather returns early Tuesday

DISCUSSION

Today, dry conditions accompanied by near-seasonal temperatures are ongoing beneath zonal flow aloft. Very dry air aloft (see 12Z soundings at SLE, UIL, and BOI) has partially mixed to the surface across portions of the Blue and Cascade mountains, but the lack of significant wind should limit fire weather concerns.

Ensemble NWP guidance shows a robust plume of moisture (PWATs 150-250 percent of normal) arriving from the Pacific by late Monday. Precipitation chances increase by Monday night as a strong offshore surface low tracks northeast towards British Columbia. Its attendant frontal system will move inland Tuesday into Wednesday, with perhaps another cold frontal boundary in its wake later Wednesday into Wednesday night as a secondary shortwave moves overhead.

While some light snowfall is forecast late Monday night into early Tuesday morning for the Washington Cascade passes, strong warm advection should push snow levels above the passes by late Tuesday morning (90 percent confidence) and limit winter weather concerns. Snow levels will drop again Tuesday night into Wednesday with the passage of a decaying cold front, though probabilities of snowfall are quite low. For example, the NBM shows a modest 30-70 percent chance of reaching or exceeding 2 inches of snowfall between early Wednesday morning and early Thursday morning for the Washington Cascade passes, with even lower 10-30 percent chances for the Oregon Cascade passes.

The early-week system will also usher in breezy to windy conditions across the forecast area. However, isentropic analysis of latest deterministic guidance does not suggest the advertised 35-55 kt 850-mb jet will mix uniformly to the surface across the majority of our lowlands Tuesday into Wednesday. Ridges and especially wind-prone locations will likely (60-80 percent confidence) see advisory-level gusts (45 mph or greater), but confidence is still low-medium (20-50 percent) in advisory-level gusts for our population centers.

Looking ahead, per analysis of ensemble clusters, there is still significant uncertainty in details of the 500-mb height pattern. That said, a building upper-level ridge is favored (60-80 percent of ensemble members), and mostly dry weather is forecast later Thursday through Saturday. 86

AVIATION /00Z TAFS THROUGH 00Z TUESDAY

VFR conditions prevail for all sites with winds less than 12 kts. Feaster/97

HYDROLOGY

Minor river flooding from snowmelt and previous rainfall is ongoing for the Yakima and Naches rivers in south-central Washington. These rivers have crested for most locations in the Yakima and Kittitas valleys. However, downstream locations towards the lower Columbia Basin have yet to crest, and the Yakima River at Kiona is forecast to crest Monday morning. The only change to headlines this afternoon was to extend the flood warning for the Yakima River at Easton into Monday morning; previously, the warning was set to end this afternoon.

Looking ahead, several forecast points along the Yakima and Naches rivers, including those at Easton, Cliffdell, Naches, and perhaps Umtanum, are anticipated to remain above action stage through the week according to the latest forecasts from the Northwest River Forecast Center in Portland.

Preliminary Point Temps/PoPs

PDT 35 60 43 63 / 0 0 10 70 ALW 38 60 47 61 / 0 0 10 80 PSC 36 64 46 62 / 0 0 10 70 YKM 34 61 42 61 / 0 0 10 70 HRI 35 63 44 63 / 0 0 10 60 ELN 30 57 38 53 / 0 0 10 80 RDM 30 64 40 63 / 0 0 0 30 LGD 35 64 43 61 / 0 0 10 70 GCD 36 65 42 67 / 0 0 10 50 DLS 38 63 44 60 / 0 0 10 80

PDT WATCHES/WARNINGS/ADVISORIES

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