textproduct: Portland

UPDATE

Observations show the freezing level remains around 2500 ft. Mixed rain/snow showers are expected to continue through today down to 1500 ft elevation, but these showers are unlikely to produce accumulating snow.

SYNOPSIS

The back edge of a cold frontal boundary which has spread ample precipitatiion and mountain snowfall across the Pacific Northwest quickly progresses through the region this morning followed by a showery airmass and a 15-30% chance for thunderstorms this afternoon. A Winter Storm Warning remains in effect for the Cascades through tonight. Concerns shift to cooler overnight temperatures and possible frost/freeze conditions to round out the week. After a brief stint of dry and warmer weather Saturday, precipitation chances return by early next week although forecast confidence Sunday onward is low.

LONG TERM...Friday through Tuesday

Looking towards the weekend confidence is high among both deterministic and ensemble guidance in a ridge of high pressure continuing to build aloft allowing temperatures to gradually warm and reduce frost concerns. However the pattern remains rather progressive as models show the ridge quickly followed by another low pressure system on Sunday originating from the Gulf of Alaska. WPC Cluster Guidance does favor this low dropping almost straight southward across the eastern Pacific favoring this low holding just off the OR/WA coast. How far this low holds off the coast will heavily modulate conditions locally. Should it stay more to our west, warmer conditions and broad southerly flow would be favored, That said, any moisture riding south to north within this flow pattern could spell the return of convection. Thus forecast confidence Sunday onward into the middle of next week is low. -99/42

AVIATION

Post frontal showers are expected to persist through the day with increasing chances for convection in the afternoon after 20Z Wed. Highest probabilities lie along the Cascade foothills and to the north with around a 25-35% chance inland within the Willamette Valley. Thunder is challenging here as often it may be one storm that pops up, while other times it can be a wide swath. In this case, because the flow is northwesterly (not as favorable) behind the previous system, would trend towards a more isolated thunderstorm scenario. Overnight, showers will slowly dissipate and clear. Dropping temperatures mean that there is a slight chance for frost formation over exposed surfaces.

KPDX AND APPROACHES...Combination of MVFR and VFR conditions with showers. Around a 30% chance of thunderstorms between 20Z Wed through 06Z Thu. Thunderstorms will trend towards an isolated scenario. Frost potential overnight if skies remain clear. -27

MARINE

A cold frontal boundary is quickly progressing across the coastal waters early this morning with winds switching northwesterly in its wake. Expect gusts between 20-30 knots the remainder of the day as an area of low pressure drops along the Canadian coast toward the region. This feature will also bring a 15-25% chance of thunderstorms over the waters into the evening hours. Seas are expected to persist at around around 9 to 10 ft, building slightly to around 10 to 12 ft tonight as a fresh northwest swell moves through the waters. The Small Craft Advisory remains in place through Thursday morning for both the inner and outer waters, including the Columbia River Bar. High pressure then builds over the waters Thursday into the weekend as northerly winds return and seas gradually subside to around 3 to 6 ft Friday into Saturday. It appears there's a 40-60% chance for north winds to gust above 21 knots into Small Craft Criteria on Saturday. Looking ahead there is potential for another weather system to impact the waters later this weekend, possibly increasing winds and seas. -99/DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 9 AM PDT Thursday for ORZ104>110- 114>118-123>125. Freeze Warning from 1 AM to 9 AM PDT Thursday for ORZ121. Freeze Watch from late Thursday night through Friday morning for ORZ121-124-125. Winter Storm Warning until 5 AM PDT Thursday for ORZ126>128. WA...Frost Advisory from 1 AM to 9 AM PDT Thursday for WAZ203>205- 208. Winter Storm Warning until 5 AM PDT Thursday for WAZ211. PZ...Small Craft Advisory until 5 AM PDT Thursday for PZZ210. Small Craft Advisory until 5 PM PDT Thursday for PZZ251>253- 271>273.