textproduct: Portland

SYNOPSIS

Lingering rain showers will dissipate by sunrise Monday morning as an upper level low centered over central OR continues moving east of the Cascades. Trending drier Monday through Thursday with temperatures gradually warming and peaking on Wednesday with the development of low-level offshore flow. The forecast becomes more uncertain late Friday into Sunday, with the potential for weather conditions to range anywhere from cool and showery to warm and dry depending on the evolution of the large scale weather pattern.

DISCUSSION...Monday morning through Sunday

As of early Monday morning, the upper closed low pressure system that has been producing days of rain and thunderstorms has finally moved east of the Cascades. A few lingering showers over the Cascades should dissipate by sunrise, and a weak shortwave on the back side of the low moving through the area today could produce a few stray showers this afternoon over the south WA/north OR cost and Willapa Hills. Otherwise, dry conditions are expected today with clearing skies by early afternoon as daytime heating burns off any stratus that forms in the morning hours. Daytime temperatures are expected to warm into the low to mid 70s across the interior lowlands and mid to upper 60s along the coast.

Confidence is high for dry weather and warmer temperatures Tuesday through Thursday as high pressure over the Four Corners region pushes northwest into Oregon. A surface thermal trough pushing north from northern CA will allow for weak low- level offshore flow to develop. Most inland valley locations will see high temperatures in the upper 70s to mid 80s each day, warmest on Wednesday. The coast will remain a bit cooler with highs most likely in the low to mid 70s. That said, even the coast has a 1-15% chance of reaching 80 degrees or warmer, specifically on Wednesday when offshore winds have the potential to last into the early afternoon.

Although confidence has increased for dry and warm weather on Thursday, confidence remains relatively lower late Friday into Saturday. This is due to model differences regarding the exact timing, location, and amplitude of an upper level trough that will attempt to push into the Pacific Northwest from British Columbia. Confidence is higher on Friday, then decreases more as time goes on. There is high confidence that temperatures will trend cooler on Friday compared to earlier in the week, but it is not clear how much cooler as the full range of outcomes for high temperatures is anywhere from the lower 60s to upper 70s. The outcome will be highly dependent on how deep the trough becomes and if it tracks into Washington and Oregon or further to the east towards the Northern Plains, which will also determine whether or not rain will return to the area. If rain does return, highs in the 60s would be expected. If rain does not return, highs would likely top out somewhere in the 70s, assuming enough sun breaks occur. The latest LREF run indicates about 25-35% of ensemble solutions suggest the trough will take a more eastern push towards the Northern Plan with 15-20% of the solutions suggesting the trough will deepen offshore over the eastern Pacific, both of which are more drier solutions. The other 65-85% of solutions indicate the trough deepening over the PacNW, the wetter solution. This is a bit flipped from the 12Z Sept 6th run, indicating how much uncertainty still remains in the forecast. By Sunday, these solutions are even more divergent. However, one thing I will note is that remnant moisture from Hurricane Marie is no longer expected to be injected into this trough, meaning the overall amount of precipitation expected through the weekend is low. The high end forecast over 48 hours ending 5 AM Monday is around 0.15-0.5 inch across most of the area, locally up to 0.75 inch over the Cascades. The current deterministic forecast suggests less than 0.1 inch, mainly over terrain. -23/03

AVIATION...Through 12Z Tuesday

With the low pressure system now moving east out of the airspace, rain has ended at all terminals. Still, MVFR CIGs continue to impact most terminals as of 05Z Mon and may continue to do so overnight as there is high confidence that wet surface conditions from recent rains has allowed for the formation of stratus. Probabilistic guidance suggests a 20-60% chance of MVFR CIGs between 06-18Z Mon at inland terminals, with the highest chances between 12-16Z Mon. As for coastal terminals, there is a 60-80% chance of MVFR CIGs until 18-20Z Mon, along with a 20-40% chance of IFR and 10-30% chance of LIFR conditions at KONP. Otherwise, expect generally light and variable winds at most terminals through the TAF period, with the exception of northwest winds around 8 kt at coastal terminals during the afternoon.

KPDX AND APPROACHES...Current MVFR CIGs likely to continue into the morning hours as guidance suggests current 20-30% chances of MVFR CIGs to increase to 50-60% between 12-18Z Mon. Expect VFR conditions to return by 18-20Z Mon. Light and variable winds expected through the period, becoming northwest after 20Z Mon. -12/03

MARINE

High pressure offshore will maintain predominately northerly winds across the coastal waters through the week. A weak front will bring more variable winds early Monday. A strengthening pressure gradient on Tuesday will bring stronger northerlies with gusts up to 25 kt as there is an 80% chance of wind gusts exceeding 21 kt by Tuesday evening. As a result, Small Craft Advisories have been issued from 8 AM to 11 PM Tuesday for all the waters. Breezy winds continue through Thursday, but should remain 20 kt or less. Seas at around 3 to 4 ft with a dominant northwest swell at 8-9 seconds will gradually build to around 6 ft late Tuesday. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM to 11 PM PDT Tuesday for PZZ210- 251>253-271-272. Small Craft Advisory from 8 AM to 11 PM PDT Tuesday for PZZ273.