textproduct: Portland

SYNOPSIS

Scattered light showers continue at times today. However, our focus remains on a stronger frontal system arriving Tuesday evening through early Thursday morning. This system will bring widespread precipitation to the region and a period of heavy Cascade snowfall. As a result have upgraded the Winter Storm Watch to a Winter Storm Warning from late Tuesday into early Thursday above 2500ft. It should be noted that there is around a 5% chance for wet, heavy snow to be observed along with some accumulation for elevations above 1000 ft during the times mentioned above. Looking beyond, while showers in the wake of the front decrease, cooler overnight temperatures quickly follow with increasing frost concerns. Then, looking towards the end of the week and into the weekend a warming and drying trend briefly returns.

LONG TERM...Thursday through Sunday

By Thursday, the upper level low and associated front have left the CWA, but will leave the region cool and moist conditions. Thursday into Friday looks to have overnight lows in the mid 20s to mid 30s. This means that there is the potential for frost development or even freezing temperatures to impact the valley floor. Currently, the Portland/Vancouver Metro there is a 10-40% chance of overnight lows at or below 36 degrees F. From the Portland/Vancouver Metro areas southward towards Salem and Eugene, OR, expect a 45-85% chance of overnight lows at or below 36 degrees F. For locations north of Vancouver, expect a 50-75% chance of overnight lows at or below 36 degrees F. Additionally, there is a 80-95% chance of at least freezing temperatures in the Upper Hood River Valley for Wednesday night into Thursday morning. Similar conditions may develop again Thursday night into Friday morning, though with slightly lower probabilities (only 10-30% lower).

While frost potential starts for most areas Thursday morning, the Upper Hood River Valley will have frost potential sooner and longer, with a 45-85% chance of frost Wednesday morning, 90-99% Thursday and Friday morning, and 65-95% Saturday morning. Chances of frost plummet to 10-35% by Sunday morning for the Upper Hood River Valley.

Looking towards the weekend, temperatures gradually warm, reducing frost concerns. Ensembles show a shortwave ridge of high pressure moving quickly overhead on Saturday, but being quickly followed by another low pressure system on Sunday originating from the Gulf of Alaska. Now, beyond Sunday and into the start of next week, WPC Cluster Guidance is favoring this low holding just off the OR/WA coast or moving over the inland Pac NW, which means that cooler and moister conditions are favored for the start of next week. /42

AVIATION

Northwest flow aloft with diminishing showers today. Predominately VFR conditions expected through tonight, with only a 10-20% chance of MVFR occurring with passing showers through this evening. Westerly surface winds around 10 kt or less continue today. A cold front will approach the region from the northwest Tuesday morning, bringing increasing chances for rain and lower ceilings to the area on Tuesday. Chances for MVFR conditions begin to increase again after 12z Tue at the coast while winds are expected to become more southerly ahead of the front.

KPDX AND APPROACHES...Predominately VFR conditions expected through at least 18z Tuesday. Showers will continue to diminish this afternoon. Northwest winds around 4-6 kt expected to become southerly by 08z Tuesday, with south winds increasing to around 10-15 kt later Tue morning. /DH

MARINE

Relatively calm conditions are expected through this evening as westerly winds persist at around 10 kt or less with seas of around 5 to 7 ft. Southwesterly winds begin to increase later tonight into Tuesday as a strong cold front approaches the waters. Expect southwest winds to gust up to 30 kt across all coastal waters on Tuesday as the front slowly moves south. Winds are expected to be strongest Tuesday afternoon into early Tue evening. During this time, there will be around a 3-6 hour period when gusts could (50% chance) exceed 34 kt, producing marginal Gale Force wind gusts. Have maintained the strongly worded Small Craft Advisory beginning early Tue morning for the northern waters and extending across all waters, including the Columbia River Bar, Tue morning through Tue night. The increased wind waves are expected to build seas to around 10 to 12 ft with a period of around 8-10 seconds. Seas remain steep and choppy through Tue night as well.

Winds are expected to abruptly shift to the northwest behind the frontal passage later Tuesday through Tuesday night as the cold front moves south across the coastal waters. Breezy northwest winds continue through Wednesday with gusts increasing back up to 20-25 kt as an area of low pressure drops along the Canadian coast toward the Puget Sound. This will also bring a 15-25% chance of thunderstorms over the waters through Wed evening. Seas are expected to persist at around around 8 to 10 ft on Wednesday, building slightly to around 9 to 11 ft Wed night as a northwest swell moves through the waters. Will likely need to extend the Small Craft Advisory into Thu morning. High pressure then builds over the waters later this week as northerly winds return and seas gradually subside to around 4 to 6 ft Friday into Saturday. There is potential for another weather system to impact the waters later this weekend. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Storm Warning from 5 PM Tuesday to 5 AM PDT Thursday for ORZ126>128.

WA...Winter Storm Warning from 5 PM Tuesday to 5 AM PDT Thursday for WAZ211.

PZ...Small Craft Advisory from 8 AM Tuesday to 5 AM PDT Wednesday for PZZ210-252-253-272-273.

Small Craft Advisory from 5 AM Tuesday to 5 AM PDT Wednesday for PZZ251-271.