textproduct: Portland

SYNOPSIS

A strong frontal system begins to move across the Pacific Northwest today, bringing widespread rain, Cascade snow, and breezy south to southwest winds. Accumulating snow along the Cascade passes will create hazardous travel conditions through Thursday morning, especially along Santiam and Willamette Passes where snowfall rates may exceed 1+ inch per hour at times. Showers decrease Thursday afternoon. High pressure builds Friday into the weekend, returning dry weather, increasing sunshine, and a warming trend. Warm and dry conditions may continue into early next week.

DISCUSSION...Now through Monday

As of early this morning, a robust Gulf of Alaska system is moving inland. Steady rain is spreading across northwest Oregon and southwest Washington, with widespread precipitation expected through the day as the warm front lifts north and the trailing cold front approaches.

Southerly to southwesterly winds will increase through the morning, particularly along the coast and higher terrain. Gusts of 20 to 30 mph are likely across the interior valleys, with gusts of 30 to 40 mph possible along the coast and exposed ridgelines. While the probability of advisory-level wind gusts remains relatively low (5-15% chance), localized stronger winds cannot be ruled out during frontal passage.

Impacts over the Cascades will be significant. Winter Storm Warnings go into effect at 5 AM Wed for the Marion, Linn, and Lane County Cascades and remain in effect through 5 PM Thursday. Winter Weather Advisories also take effect at 5 AM for the north Oregon and south Washington Cascades, where lower but still impactful snow amounts are expected. Snow levels will lower to 2000-3000 feet by this evening as colder air filters in behind the cold front. High confidence exists for substantial accumulations along Santiam and Willamette Passes, where probabilities for 1+ foot of snow this morning through Thursday morning are around 70-80%. Ensemble guidance also indicates a 30-60% chance of hourly heavy snowfall rates of 1+ inch, with the highest likelihood late this afternoon into early Thursday morning as snow levels bottom out towards 2000 feet. As for Highway 26 near Government Camp, there is a 50-60% chance of 6+ inches of snow this morning through Thursday morning, and 15-25% chance of hourly heavy snowfall rates of 1+ inch this afternoon into early Thursday morning. Travel through the Cascade passes will become hazardous, particularly during periods of heavier snowfall. If you have plans to travel through the Cascades, make sure to pack an emergency supply kit and refer to ODOT/WSDOT for the latest road conditions and restrictions.

West of the Cascades, rainfall totals through Thursday afternoon are expected to be highest along the coast and across the southern Willamette Valley, where probabilities for 1+ inch from roughly 40-90% depending on location (40-60% across the southern Willamette Valley, 70-90% for the coast south of Astoria, 50-60% at Astoria, and 10-30% from Albany northward). A 10-20% chance of thunderstorms exists this afternoon and evening as colder air aloft increases instability behind the front. For the interior valley, model soundings suggest CAPE increasing to around 100-300 J/kg between 1-7 PM today, corresponding well with the timing of the thunderstorm chances. Any thunderstorms could produce brief heavy downpours, small hail, lightning, and locally gusty winds.

Showers persist Thursday as the upper trough gradually shifts east, though coverage and intensity decrease through the afternoon. Thursday night into early Friday, clearing skies and diminishing winds may allow temperatures to fall into the low to mid 30s across interior valleys. Current guidance suggests a 10-20% chance of freezing Friday morning temperatures across much of the Willamette Valley. There are higher chances of freezing Friday morning temperatures at some locations, such as a 40-50% chance from Corvallis to Eugene. The Upper Hood River Valley has a bigger exception, with a 20-60% of freezing morning temperatures from Odell to Parkdale on Thursday and Friday morning. With the growing season underway, frost development may become a concern and future frost and freeze headlines could be needed if confidence increases.

Looking ahead to Friday through the weekend, ensemble guidance supports rebuilding upper-level ridging over the Pacific Northwest. This will favor dry conditions, increasing sunshine, and a steady warming trend. High temperatures are expected to climb into the upper 60s by the weekend, with reasonable chances of reaching 70 degrees on Saturday through Monday (40-70% on Saturday, greater than 90% on Sunday, and 65-85% on Monday). Chances decrease sharply on Tuesday, with a 15-35% chance of afternoon temperatures over 70 degrees. While uncertainty remains, at this time, expect a Minor HeatRisk within the Willamette Valley this weekend and into Monday, where these temperatures would primarily affect those who are extremely sensitive to heat and without cooling/hydration. While ensemble clusters have come into moderate to high agreement regarding ridging to persist through Monday, uncertainty remains beyond Monday. At this time, ensemble clusters suggest about a 50/50 chance of ridging keeping conditions relatively dry or troughing bringing rain to the region. ~12

AVIATION

A robust Pacific frontal system pushes inland, bringing widespread light rain with it. Cigs continue to lower towards MVFR for many terminals, with probabilities for MVFR cigs steadily increasing towards 60-80% across the area by 18Z Wed. Expect most terminals to deteriorate to MVFR by 13-15Z Wed. Heavier rain will arrive at coast terminals after 12Z Wed and inland terminals after 15Z Wed. Around 14-17Z Wed, southerly winds will increase and become gusty, especially along the coast. Expect coastal gusts 25 to 30 kt and inland gusts 20 to 25 kt to persist through at least 00-06Z Thu, with KONP potentially remaining gusty until 12Z Thu.

Note there is also a 10-20% chance of thunderstorms at all terminals Wed afternoon and evening. Any thunderstorms that form will have the potential to produce brief periods of small hail under the size of peas, gusty and erratic outflow wind gusts as high as 35 kt and a few flashes of lightning. Stronger showers will also bring a risk of small hail and gusty outflow winds, albeit without the lightning risk. Probabilities for thunderstorms at any given terminal are not high enough to warrant a PROB30 group.

KPDX AND APPROACHES...A robust Pacific frontal system pushes inland, bringing widespread light rain. As the front moves through, current chances of MVFR cigs around 30-40% will increase to 60-80% by 15Z Wed. Current steady and light rain will become heavier at times after 15Z Wed. Southeasterly to southerly winds currently 4-8 kt will increase to around 10-14 kt with gusts up to 20-25 kt by 17-19Z Wed. However, brief outflow wind gusts up to 35 kt will be possible near stronger showers or thunderstorms this afternoon and evening, corresponding with a 10-20% chance of thunderstorms. Stronger showers or thunderstorms will also bring the potential for small hail, most likely under the size of peas. MVFR conds will likely return back to VFR by 00Z Thu and wind gusts likely ending by 03-06Z Thu. ~12

MARINE

Southerly winds increase this morning as a front moves through the waters. A Gale Warning for southerly winds gusting up to 35 to 40 kt remains in effect until 11 AM today. Expect winds to abruptly shift westerly behind the frontal passage this morning, between 8 to 11 AM. Wind waves build through today in response to the increasing southerlies, creating steep and choppy seas, with significant wave heights building to around 11 to 13 ft. Seas will likely fall back to around 9 to 10 ft by this afternoon.

A surface low pressure system moves over the waters late this evening and westerly winds ease overnight into Thursday morning. An increasing west swell will also push into the waters tonight into Thursday, building seas to around 11 to 13 ft with a period of 12 seconds. This will likely maintain Small Craft to marginally hazardous seas conditions through Thursday. Marine conditions then ease beginning Friday and into the weekend as high pressure rebuilds and northerly winds return to the coastal waters. Expect seas around 5 to 6 ft Friday and through the weekend. ~12/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 PM PDT Thursday for ORZ126.

Winter Storm Warning until 5 PM PDT Thursday for ORZ127-128.

WA...Winter Weather Advisory until 5 PM PDT Thursday for WAZ211.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ210.

Gale Warning until 11 AM PDT this morning for PZZ251>253-271>273.