textproduct: Portland

SYNOPSIS

A warming trend persists into Tuesday as high pressure re-builds overhead, albeit temporarily. Precipitation chances then return this evening as a trough approaches the West Coast. Can't rule out a period of high-based thunderstorms this evening into Wednesday morning (15-25% chance) as well. After lingering showers decrease Thursday, another low from the Gulf of Alaska maintains precipitation chances along with temperatures trending slightly cooler for the latter part of the week and into the weekend.

DISCUSSION...Today through Monday

Mostly sunny/clear conditions today as an upper level ridge of high pressure remains over the region. For most locations daytime, high temperatures today will likely be the warmest of the upcoming week, reaching into the low to upper 60s along the coast, low 70s to low 80s the Cascades, and upper 70s to mid 80s for the inland valleys/lower Cascade valleys. The past few model runs continue to relatively cool the area off today. Thus decreasing the moderate Heat Risk threat across the Portland/Metro and I-5 corridor. The latest NBM has lowered the probabilities for temperatures to exceed 90 degrees, giving the Portland metro only a 5-10% change to do so while the rest of the inland valleys are less than 5%. So, at least heat related impacts should remain a bit more muted than previously anticipated as a result.

As the evening approaches, a shortwave trough will take aim at the Pac NW coast line. This approaching trough will result in an increase in overall instability through late tonight/early Wednesday morning. The pattern as a whole is resulting in a 15-25% chance of thunderstorms of the nocturnal variety, which does result in a challenging forecast. To add more of a challenge, any thunderstorms that do develop, will likely not be surface based. The primary question for today is going to be where are the thunderstorms likely to start? CAMs (Convective Allowing Models) do show a favorable pool of CAPE (Convective Available Potential Energy) along the Central and Southern Oregon Cascades as well as the Southern Washington Cascades, with values around 300-1800 J/kg. Now, it should be noted that this spread does include the NAM Nest, which is known to run rather "Hot" here in the Pac NW. The more conservative CAMs are showing CAPE values in the 300-500 J/kg range, with most CAMs also showing around 20-75 J/kg of CIN (Convective Inhibition). Overall, conditions are favorable for thunderstorm development starting late this afternoon through late tonight/early Wednesday morning, with a likely starting location along the Cascades, with a general storm motion of south to north.

Wednesday morning/early afternoon, the shortwave trough will be east of the area which will usher in a period of more stratiform rainfall. The added cloud cover and cooler airmass on Wednesday afternoon pushes temperatures into upper 50s to mid 60s for much of the area. Looking towards the latter part of the week, a more zonal flow pattern looks to emerge, with perturbations within the upper level flow. This looks to maintain a cooling trend for the region through at least Friday. However, looking towards the weekend and into the start of next week, most ensemble members continue to show another trough dipping down from the Gulf of Alaska by the end of the week around Friday/Saturday. This will maintain chances for precipitation across the area and bring lower 500 mb heights, resulting in temperatures cooling down to or even below seasonal normals. /42-99

AVIATION

High pressure will maintain predominately VFR under mostly clear skies through around 00Z Wednesday, except for the central Oregon coast. Southwest flow aloft increases as the upper level ridge shifts east and an upper level trough over the Pacific approaches the region. A southerly wind reversal along the coast will push marine stratus into KONP around 12Z Tuesday as chances for IFR conditions increase to around 80-90% and chances for LIFR are around 60-70% between 12Z-16Z Tuesday. Stratus may reach as far north as KAST with around a 15-25% chance for MVFR conditions between 12Z-18Z Tuesday. There is also around a 5-10% chance that stratus pushes into the southern Willamette Valley after 12Z Tuesday. Light northwest winds turn southwesterly on Tuesday.

There is a slight chance (15-25%) of showers and thunderstorms after 03Z Wednesday for all inland TAF locations. Thunderstorm chances decrease after 09Z Wednesday, but the chance for showers spread across the airspace. Showers and a mixture of VFR/MVFR conditions persist through the remainder of the TAF period.

KPDX AND APPROACHES...VFR expected through Tuesday with increasing high level clouds. Northwest winds become light overnight, becoming southwesterly around 00Z Wednesday. There is a slight chance (15-25%) of showers and possible thunderstorms after from 03Z-09Z Wednesday. Showers and a mixture of VFR/MVFR conditions persist through the remainder of the TAF period. /42

MARINE

Relatively benign conditions expected through the week with predominately westerly winds, mainly under 10 kts. An upper level trough and associated front moves through the waters on Wednesday, which bring a 40-55% chance of occasional small craft gusts up to 25 kt across all waters. Will continue to monitor the situation as there remains a fair amount of uncertainty. Winds remain westerly but decrease once again Wednesday night. Seas generally below 8 ft around 10-12 seconds through the majority of week, but could see seas close to 10 ft by Friday/Saturday. /42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.