textproduct: Portland

SYNOPSIS

Dry and cooling temperatures will occur through the work week before weak storm systems return chances for beneficial rain over the weekend and early next week.

DISCUSSION...Now through Tuesday

Ensembles are in good agreement shortwave ridging will give way to a weak shortwave trough traversing the region Thursday. Some moisture and weak instability will result in a 20-30% chance for light showers across the Cascades Thursday afternoon and evening.

A weak front, currently well offshore, is currently associated with the shortwave trough, but models are in good agreement it will dissipate before reaching our forecast area, and thus have few impacts aside from bringing an increase in cloud cover and cooler temperatures to the region. On that note, models generally agree skies will clear Thursday and Friday nights away from the Cascades in the Willamette Valley, which will allow temperatures to drop to some of the coldest values so far this year. It appears increasingly likely we will need a Freeze Watch for at least the southern Willamette Valley and Coast Range Valleys with a more expansive threat of a freeze on Friday night per NBM probabilities.

The next shortwave trough or cutoff low is forecast to drop towards the Pacific Northwest over the weekend into early next week. There remains quite a bit of uncertainty around whether or not this storm system will cutoff, clip the region and then hang well offshore for 24-48 hours before pushing inland, hover over or close enough to the region to bring a couple bouts of rain over 48-72 hours, or if the entire storm system will remain an open shortwave trough that slides southeastward across the Rockies and keeps the area largely dry.

AVIATION...Through 00Z Thursday

High pressure over the region will lead to predominantly light winds and VFR conditions through 00z Friday. The main exception to this will be between 09-18z Thursday when clear skies, light winds and low level moisture will lead to pockets of fog and low clouds developing at several taf sites. The highest probabilities for IFR conditions developing are at KEUG and KHIO between 09-18z Thursday where there is a 30-40% chance for these conditions occurring at any given hour during that timeframe. There is around a 20% chance of IFR conditions developing at any given hour at our remaining taf sites during this time period. In addition, there is a signal that marine clouds along the coast may result in a period of IFR conditions along the central coast of Oregon this evening, primarily between 01-04z Thursday.

KPDX AND APPROACHES...High pressure over the region will lead to light winds and predominantly VFR conditions through 18z Thursday. The main exception to this will be in the 12-18z Thursday time period when there is a 20-30% chance for MVFR to IFR conditions developing at any given hour.

MARINE

North winds will develop across the waters this evening and persist through at least Saturday. The winds strengthen tonight and Thursday as high pressure builds in the wake of a weak front with high confidence in small craft conditions. The persistent winds will increase seas. The seas are expected to become very steep Thursday night and Friday prompting a Hazardous Seas Warning.

A stronger low will approach the waters Friday. Model ensembles have been trending towards the low remaining further offshore this weekend. There is uncertainty on when and if the low will move closer to shore and a diminishing threat of gale conditions this weekend. ~11

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 11 AM Thursday to 8 AM PDT Friday for PZZ210. Small Craft Advisory from 5 AM to 8 PM PDT Thursday for PZZ251- 271-272. Hazardous Seas Warning from 8 PM Thursday to 8 AM PDT Friday for PZZ251>253-271>273. Small Craft Advisory from 11 AM to 8 PM PDT Thursday for PZZ252-253-273.