textproduct: Portland

SYNOPSIS

A frontal system moving through today will bring widespread rain, Cascade snow, and breezy southerly to southwesterly 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 and persists through Monday, returning dry weather, increasing sunshine, and warmer temperatures. Precipitation chances return on Tuesday as a weak upper level trough approaches the region.

DISCUSSION...Today through Tuesday

Radar imagery and surface observations as of early Wednesday afternoon depicts widespread rain across northwest Oregon and southwest Washington as a cold front swings through the region. Webcams are currently showing some light snow along the Cascade passes, but snow levels will gradually fall this afternoon and evening as cooler air filters in behind the front. Winter Storm Warnings remain in effect through 5 PM Thursday across the Marion, Linn, and Lane County Cascades, and Winter Weather Advisories remain in effect for the north Oregon and south Washington Cascades. Precipitation will turn more showery and convective behind the front, potentially leading to heavy showers. Based on REFS ensemble guidance, there is a 60-80% chance for snowfall rates of 1 inch or greater along Santiam and Willamette Pass, with the highest chances occurring between 2-7 PM Wednesday and 2-8 AM Thursday. For Highway 26 at Government Camp, there is a 40-60% chance for snowfall rates exceeding 1 inch per hour, with the highest chances occurring between 12-7 PM Wednesday and 3-6 AM Thursday. 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.

Atmospheric instability will also increase as air aloft cools down behind the front, with sounding profiles and REFS guidance suggesting CAPE values peaking around 100-200 J/kg. This will result in a 15-35% chance for thunderstorms today across northwest Oregon and southwest Washington. The highest chances (25-35%) will be along the coast between 5-11 PM this evening. Any thunderstorms may produce lightning, heavy downpours which can reduce visibility, erratic winds, and small hail. Other than thunderstorm chances, minimal impacts expected from rain and wind west of the Cascades. Southerly to southwesterly winds will be breezy with the frontal passage with gusts up to 20-30 mph for interior valleys and stronger gusts of 30-40 mph for higher terrain and along the coast, which observations have already been showing.

Rain showers and Cascade snow showers persist through Thursday, but coverage and intensity gradually decrease through the afternoon as this system exits eastward. Thursday night into Friday morning, clearing skies and calm winds will lead to efficient radiational cooling and temperatures potentially dropping into the low to mid 30s across interior valleys. This would support frost development over vegetation and metal surfaces. Given that April 1st marks the start of growing season, we may need to issue Frost/Freeze headlines during this time. Chances for Friday morning lows dropping below 36 degrees is around 40-60% across most areas along the I-5 corridor and 70-90% across the Upper Hood River Valley. For dropping below 32 degrees, chances are around 20-40% for the southern Willamette Valley and 40-60% for the Upper Hood River Valley above 1000 feet.

Friday through Monday, ensemble guidance remains in agreement that upper level ridging will build over the Pacific Northwest, returning dry weather, sunny skies, and temperatures warming at least to the mid to upper 60s by the start of the weekend. There is high confidence now (80-90% chance) that temperatures exceed 70 degrees across the Willamette Valley and Clark County lowlands Sunday and Monday. For this time of year, average high temperatures are in the mid to upper 50s, so this warm-up would be around 10-15 degrees above average. By Tuesday, the majority of ensemble members suggest a weak upper level trough approaching the Pacific Northwest, returning slightly cooler temperatures and chances (20-40%) for light precipitation. -10

AVIATION

Current observations continue to show a mixture of VFR and MVFR spread across the airspace. MVFR conditions are still expected to slowly become the dominant flight category through 00Z Thursday as a cold front moves through the airspace overnight. This frontal passage will bring widespread showers and gusty southerly winds. There is a slight chance (15-30%) of thunderstorms across the majority of the airspace through 15Z Thursday, with the highest chance for thunderstorms through 06Z Thursday. Any thunderstorms that form will have the potential to produce brief periods of small hail, gusty and erratic outflow wind gusts as high as 35 kt, lightning and reduced flight conditions. Will maintain a PROB30 for TSRA for KONP and KEUG through 06Z Thursday for KONP and through 03Z for KEUG. Other locations are currently not high enough to warrant a PROB30 group, but we will continue to monitor current conditions and update as needed. Also, stronger showers will also bring a risk of small hail, gusty outflow winds as well as the potential for brief periods of IFR/LIFR conditions.

Along the coast, the front has mostly pushed eastward and is resulting westerly winds with gusts up to 25 kt through around 12Z Thursday. Southerly winds are also expected inland with gusts up to 25 kt trough 03Z Thursday, with southerly winds less than 10 kt expected around 06Z Thursday. Behind the front, expect the mixture of VFR and MVFR conditions to persist with MVFR expected to be the predominant flight category.

KPDX AND APPROACHES...VFR conditions are expected to degrade towards MVFR as a cold frontal system moves across the airspace through around 06Z Thursday. This front will bring a 15-20% chance for thunderstorms through 06Z Thursday. Any thunderstorms that form will have the potential to produce brief periods of small hail, gusty and erratic outflow wind gusts as high as 35 kt, lightning and reduced flight conditions. Also, expect southerly winds with gusts up to 25 kt through 03Z Wednesday. Behind the front, expect the mixture of VFR and MVFR conditions to persist with MVFR expected to be the predominant flight category. /42

MARINE

A cold front continues to slowly push onshore through the late afternoon. Expect southerly winds to become more westerly as well as subside through the evening hours. Seas will remain elevated as a robust westerly swell moves into the region this evening. This will result in seas around 9 to 12 ft through late tonight and peaking around 10 to 15 ft by early Thursday morning. Seas then subside through Thursday evening towards 9 to 11 ft and finally falling below 10 ft by Thursday night. As a result have issued and extended Hazardous Seas Warnings for all offshore waters through at least Thursday evening. The Columbia River Bar (PZZ210) will maintain a Small Craft Advisory as seas remain elevated thanks in part to the westerly swell and the Strong Ebb cycle.

As the latter part of the week approaches, conditions start to ease as high pressure rebuilds and northerly winds return to the coastal waters. Expect seas around 5 to 7 ft Friday and through the weekend. /42

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 Thursday for PZZ210.

Hazardous Seas Warning until 5 PM PDT Thursday for PZZ251>253- 271>273.