textproduct: Portland

SYNOPSIS

After pleasant weather to start the day Thursday, an incoming low pressure system will bring an abrupt transition to cool, rainy, and windy conditions beginning late Thursday afternoon at the coast and late Thursday evening inland. Remaining cool and windy on Friday with periods of rain. After widespread soaking rain comes to an end late Friday, high pressure will bring a quick return to dry and quiet fall-like weather Friday night through the weekend. Chances for rain increase slightly early next week, mainly over southwest Washington and on the coast.

LONG TERM...Saturday through Wednesday night

The long term forecast is highlighted by quieter fall-like weather with slightly cooler than normal temperatures and mainly dry conditions. On Sunday, WPC's cluster analysis for 500 mb heights and anomalies shows excellent agreement between the Canadian, GFS, and EURO ensembles with a weak trough moving over the Pacific Northwest late Saturday into Sunday. Despite the incoming trough, moisture will be lacking, and guidance remains in good agreement for dry weather Saturday through Sunday. The NBM backs this up, as probabilities for precipitation are near 0%. However, this trough will bring slightly cooler than average temperatures for late September with highs in the 60s across the lowlands.

As the trough moves eastward early next week, it appears weak shortwave ridging will attempt to develop on the backside of the trough, bringing near normal upper level heights to northwest Oregon and southwest Washington. In response, forecast temperature increase a few degrees. A glancing shortwave trough behind the shortwave ridge will bring low chances for light rain for some areas on Tuesday, mainly over southwest WA and along the coast (15-35% chance).

It is also worth noting that chances for frost remain in the forecast over the Upper Hood River Valley Saturday morning through Monday morning as temperatures likely dip into the 30s, especially near Parkdale and Dee. -23

AVIATION...Through 12Z Friday

West to southwest flow aloft will turn more southerly Thursday afternoon as a low pressure system approaches the region. High clouds continue to increase over the area this morning with predominately VFR conditions expected through Thursday afternoon. However, there is a low (15-25%) chance for MVFR conditions or lower in the southern Willamette Valley between 12-17Z Thursday with potential patchy fog/low stratus development.

Conditions will become rainy and windy as a frontal boundary ahead of the incoming low moves inland late today, with rain likely arriving at the coast by approximately 00Z Friday and at inland terminals by 06-07Z Friday. South winds will increase substantially, likely reaching 30-35 kt at KAST or stronger by 10Z Friday and upwards of 40 kt at KONP. Gusts up to 25-30 kt are likely over inland terminals by the end of the 12Z TAF cycle. Winds increase a bit more after 12Z Friday with scattered showers expected through the day Friday. There is also a 15-20% chance of isolated thunderstorms FRiday afternoon at all terminals. Stronger showers or storms will have the potential to produce gusty and erratic outflow winds upwards of 35-40 kt. Low level wind shear will also be on the increased after 12Z Friday. -23

KPDX AND APPROACHES...VFR conditions with high clouds will continue through this afternoon. An incoming front will bring rain and increasing south winds by 06-07Z Friday, with rain then continuing through Friday morning as wind gusts increase to 25-30 kt. Heavier showers and potential thunderstorms Friday afternoon could produce outflow wind gusts as high as 35-40 kt. Low level wind shear will also be on the increased after 12Z Friday. -23

MARINE

A strong low pressure system will move over the coastal waters late Thursday into Friday, bringing rapidly increasing winds and seas along with widespread rain. Guidance has come into better agreement regarding the strength and track of the low, suggesting the low will come in stronger than what was favored 12-24+ hours ago. Confidence is now high for widespread gale force wind gusts over 34 kt across all of the coastal waters, including the Columbia River Bar, as probabilities for gale force wind gusts have increased to 80-95%. In fact, wind gusts will likely peak as high as 40-45 kt along the south side of the low (60-80% chance). Brief and isolated storm force wind gusts upwards of 50 kt cannot be completely ruled out near the low center on the south side (30-50% chance). The highest probabilities for isolated storm force wind gusts are over the central waters between Cape Falcon and Cape Foulweather.

Given the uptick in forecast wind speeds, the Gale Watch has been upgraded to a high-end Gale Warning for widespread wind gusts up to 40-45 kt and isolated gusts as high as 50 kt. Winds will initially start out southerly Thursday night, then become southwesterly early Friday morning, then westerly by mid to late Friday morning.

With the rapidly increasing winds Thursday evening/night, wave heights will increase quickly with seas becoming steep and hazardous. Seas will most likely peak somewhere between 14 and 17 ft Friday morning, beginning around sunrise. There is a 10% chance seas could peak as high as 20 ft where the belt of strongest winds occur. Seas over the main channel of the Columbia River Bar are expected to peak close to 15 ft. Given seas will be mainly wind driven, wave heights will fall rapidly Friday evening/night as winds decrease. By sunrise on Saturday, seas should be closer to 6 to 7 ft. Northerly winds increase over the weekend, strongest Saturday afternoon and evening when wind gusts should increase to 20-25 kt. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning from 11 PM this evening to 2 PM PDT Friday for PZZ210-251-252-271-272.

Gale Warning from 8 PM this evening to 11 AM PDT Friday for PZZ253-273.