textproduct: Portland

SYNOPSIS

High pressure will result in dry and warm conditions across the area today. A significant pattern change begins on Tuesday with a return of cooler temperatures, rain, and breezy southerly winds through the end of the week. Wednesday and Thursday are expected to be the wettest days with also a 10-20% chance for thunderstorms in the afternoon. Conditions trend slightly drier over the weekend, but there remains chances for lingering showers.

LONG TERM...Wednesday through Sunday

Wednesday into Thursday continues to be far more impactful in terms of precipitation as the majority of ensemble members continue to show a second, much deeper upper-level trough dropping down out of the Gulf of Alaska and into the Pacific Northwest. The associated surface front with this more robust system swings through on Wednesday, bringing widespread rain, cooler temperatures, and breezier winds from the south. In addition, there will be a 10-20% chance for thunderstorms. Chances for exceeding 1 inch of rain in 48 hours between 5 AM Wednesday to 5 AM Friday are high (70-90%) across northwest Oregon and southwest Washington, with the highest chances along the coast, Coast Range, and Cascades. Chances for exceeding 2 inches during this same timeframe are around 35-50% for these higher terrain features, and 5-15% along the I-5 corridor. It should be noted that the primary cause for the uncertainty is the exact track of this low. While most ensembles show the center of the low coming on shore south of the Olympic Peninsula and then moving eastward, some solutions do have this low slowing down and hugging the OR/WA coast before pushing inland near the mouth of the Columbia River Bar. A more southerly track could result in precipitation totals being slightly less than currently forecasted.

Thursday afternoon to Saturday, the upper-level trough remains over the Pacific Northwest and we transition into a post-frontal environment with lingering showers over the CWA. Given that there will be cooler air aloft, this will favor increasing atmospheric instability and thus chances for thunderstorms. There will be another 10-20% chance for thunderstorms on Thursday following the initial frontal passage. Any passing thunderstorms may produce lightning, heavy rain, erratic winds, and small hail. Overall, while the upcoming rain will not quite yet be a "fire season ender," it will definitely slow down fire season and help put a damper on ongoing wildfires. /42-10

AVIATION...Through 00z Tuesday

Prevailing VFR conditions expected across NW OR and SW WA through the majority of the TAF period. Light offshore (north to northeast) winds overnight will limit chances for MVFR cigs to 20-30% along the coast and in the Willamette Valley. Winds will generally remain light less than 6 kts through Monday morning. East winds through the central Columbia River Gorge will locally increase to 12-15 kts with gusts up to 20-25 kts through the day Monday. An approaching weather system will slow shift winds back offshore (west to northwest), first along the coast around 18z Mon with northwest winds increasing to 10-14 kts through 03z Tue around and north of KAST. Winds then shift west to northwest inland after 20z Mon to 00z Tue. This onshore shift will also allow for the return of marine stratus to the coast Monday evening with a 60-80% chance of MVFR/IFR cigs returning south of KTMK between 02-06z Tue.

KPDX AND APPROACHES...VFR conditions expected throughout the period, with low chances (10%) of MVFR cigs after 09z Mon. Northwest winds less than 6 kts, turning west after 21-23z. -03

AVIATION...Through 12Z Tuesday

VFR conditions expected across the airspace for the majority of the TAF period. Winds will generally remain light less than 6 kts through around 19Z Monday. Afterwards, expected northerly winds less than 10 kt for inland locations with the coast seeing northerly gusts approaching 20 kt from 20Z Monday through 03Z Tuesday. East winds through the central Columbia River Gorge will locally increase with gusts up to 25 kts from 20Z Monday through 03Z Tuesday but not expecting any impacts to any terminals.

An approaching weather system will slowly bring about a southerly wind shift across the airspace. Look for this wind shift to start around 00Z Tuesday near K6S2 and spread northward along the coast as well as for inland locations. This wind shift will enable MVFR/IFR marine stratus to impact coastal locations starting with KONP around 00Z Tuesday and KAST around 05Z Tuesday. Expect low-end VFR/high-end MVFR conditions also impact inland locations starting around 10Z Tuesday for KEUG and afterwards for all other inland locations.

KPDX AND APPROACHES...VFR conditions expected throughout the TAF period. Northwest winds less than 10 kts. Expect a southerly wind shift around 09Z Tuesday, which will support low-end VFR/high-end MVFR conditions impacting the terminal towards the end of the TAF period. /42

MARINE

Weak ridging over the region will maintain north/northwest winds through tonight. Thermal troughing along the coast will result in strengthening near-shore winds this afternoon with gusts to 25 kt within 10-20 NM north of Cape Falcon and including the Columbia River Bar. Will maintain the current Small Craft Advisory from 2 PM to 8 PM PDT this afternoon and evening. Largely wind- driven seas will persist at 4-6 ft during this period.

On Tuesday, winds become southerly as the start of a widespread pattern change is expected. A series of robust troughs, diving out of the Gulf of Alaska and subsequently moving eastward over the Pac NW will bring about increasing winds and building seas across all waters. By Tuesday evening/Wednesday morning, a robust frontal system will move across all forecast zones. This frontal system will result in southerly wind gusts up to 40 kt as well as seas building towards 10-12 ft across all waters. Therefore, have issued a Gale Watch for all waters as well as the Columbia River Bar starting Tuesday evening through at least Wednesday evening. There is around a 20% chance for gusts up to 50 kt through the Gale Watch time frame, with highest probability within the near- shore waters. Also, there is a less than 10% chance at this time for significant wave heights to reach 15 ft. Behind this system, winds and seas are expected to ease below 15 kt and 8 ft for the remainder of the week and into the weekend. /42-36