textproduct: Pendleton

KEY MESSAGES

- Areas of dense fog and freezing fog today.

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

- 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 splitting trough over the forecast area, with the main circulation centers tracking over northern California and the Canadian Rockies. Nighttime microphysics imagery reveals middle and high cloud cover across far eastern Oregon and Washington, while much of central to eastern Oregon and southeast Washington are under a blanket of low stratus. Elsewhere, mostly clear skies are noted over south-central Washington. Radar reveals isolated showers over the Columbia Plateau, mainly near and north of Walla Walla. At the surface, most valley locations are nearly saturated at around 90-100% RH, and additional fog formation remains a concern for early morning. Have issued a Dense Fog Advisory for the northern and southern Blue Mountains where low stratus is intersecting the terrain and creating near-zero visibility in places; this low visibility is affecting routes over the Blue Mountains including but not limited to Interstate 84, Highway 395, and Highway 204. Elsewhere, patchy to areas of dense fog and dense freezing fog are present in central Oregon; have issued a Special Weather Statement for this fog, though if it becomes more widespread, an advisory may be needed.

Ensemble guidance indicates the trough will exit to the east this evening, and an amplified upper-level ridge will build in from the Pacific through Monday. 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. The majority of solutions (~80%) take the low too far south across southern Oregon or northern California to produce significant precipitation for our forecast area, and mostly dry conditions (PoPs of 20% or less) are forecast to continue until late Wednesday when ensemble guidance hints at the return of some more active weather in the form of a shortwave trough. By Friday and Saturday, ensemble clusters are showing medium potential (roughly 50% chance) for a return to a wet pattern. NBM probabilities indicate a low-medium (20-60%) chance for at least 0.50" of precipitation across the Cascades and northern Blue Mountains during the Friday through Saturday period; highest chances are for the Washington Cascades. That said, the alternate scenario is for an upper-level ridge to be overhead with precipitation displaced to the northwest of our forecast area.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Fog and low stratus remain the primary drivers of aviation concerns through the period. Low stratus at BDN/RDM is forecast to keep at least IFR conditions in place through morning, while fog and/or low stratus at PDT/ALW/PSC will present at least periodic MVFR or lower CIGs/VSBYs. Showers in the vicinity of ALW are expected to taper off through morning. Clearing skies are forecast by this evening for all sites, though there is some potential for low stratus to remain through the day (30% confidence or less). Winds of 12 kts or less are anticipated.

Preliminary Point Temps/PoPs

PDT 48 31 57 36 / 20 0 0 0 ALW 49 35 56 39 / 20 0 0 0 PSC 54 30 56 34 / 10 0 0 0 YKM 58 29 55 37 / 0 0 0 0 HRI 53 31 56 34 / 10 0 0 0 ELN 55 25 52 30 / 0 0 0 0 RDM 49 21 57 28 / 10 0 0 0 LGD 48 29 56 35 / 30 0 0 0 GCD 48 30 58 35 / 10 0 0 0 DLS 57 33 57 40 / 0 0 0 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...Dense Fog Advisory until noon PST today for ORZ502-503.