Area Forecast Discussion

KEY MESSAGES

- Near- to below-normal temperatures into next week.

- Periodic showers today, then more widespread precipitation this evening into Sunday morning, mainly for eastern Oregon and far southeast Washington.

- Low chance (30% or less, except for freeze-prone areas) of a season-ending freeze for the eastern Columbia River Gorge Monday.

DISCUSSION

Water vapor imagery shows a large, quasi-stationary low- pressure system located just offshore of the Pacific Northwest, centered in the vicinity of 45.2N, 129.5W. Middle and high cloud cover is present across much of central to eastern Oregon and southeast Washington, while mostly clear skies are noted over north-central Oregon and south-central Washington. Radar reveals isolated showers over the Columbia Plateau, mainly along a line from the Bend metro to the Tri-Cities metro. At the surface, most valley locations are nearly saturated at around 95-100% RH, and fog formation remains a concern for early morning. That said, low stratus and any widespread fog currently remains north and east of our forecast area of responsibility.

Over the weekend, the low is expected (99% confidence) to transition to an open-wave trough and move inland. While chances of precipitation have continued to trend downward, they are still medium-high (30-70%) across the majority of central/eastern Oregon and southeast Washington this evening into Sunday. The best chance of significant precipitation will be over northeast Oregon and southeast Washington where the NBM places a 20-50% chance (locally higher in eastern Wallowa County) of 0.25" through Sunday.

Ensemble solutions have coming to a concensus with regard to when the trough will exit to the east and upper-level ridging will build in from the Pacific; specifically, they suggest the trough will slide east late Sunday afternoon into Sunday night, and an amplified upper-level ridge will be overhead by Monday morning. Given clearing skies and dry conditions, this presents the best chance, albeit low at 30% or less, of a season-ending freeze for the eastern Columbia River Gorge in the next week. Freeze-prone portions of the Gorge do have somewhat higher chances of freezing (locally in excess of 50%), but at this time confidence remains too low to issue any watches.

Uncertainty in the 500-hPa pattern then grows Tuesday through the remainder of the week. Ensemble clusters show a range of solutions regarding a compact closed low Tuesday into Wednesday. Most members (~55%) take it into the Pacific Northwest across northern California or southern Oregon; the remaining members keep the low offshore or delay its inland transit. The majority of solutions (~80%) take the low too far south to produce significant precipitation for our forecast area, and mostly dry conditions (PoPs of 20% or less) are forecast to continue until Thursday when ensemble guidance hints at the return of some more active weather. Even then, chances of precipitation remain low (40% or less, highest for the Cascades) until Friday when a more robust low-pressure system is advertised.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

A range of conditions are forecast over the next 24 hours. Aside from high confidence (80-100%) in low stratus at DLS, confidence was too low (30% or less) to include any mention of sub-VFR conditions this morning. Later today into Sunday morning, a low-pressure system will move onshore, facilitating additional chances of rain (generally 10-40%, highest at ALW). Moreover, chances of sub-VFR CIGs increase significantly overnight into Sunday morning, peaking at 80% at RDM/BDN and 20-40% elsewhere. Winds of 12 kts or less are forecast through the period.

Preliminary Point Temps/PoPs

PDT 50 35 51 31 / 10 20 20 0 ALW 51 38 51 34 / 10 30 30 0 PSC 54 35 57 31 / 0 10 10 0 YKM 54 32 60 30 / 0 0 0 0 HRI 51 33 55 30 / 0 10 20 0 ELN 50 27 56 24 / 0 0 0 0 RDM 48 26 48 20 / 10 50 0 0 LGD 56 36 49 29 / 20 40 30 0 GCD 58 35 47 29 / 10 40 30 0 DLS 55 36 60 34 / 0 30 10 0

PDT WATCHES / WARNINGS / ADVISORIES

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