textproduct: Portland

SYNOPSIS

Lingering rain showers will dissipate Sunday night as an upper level low centered over central OR shifts eastward. Trending drier Monday through Wednesday with temperatures gradually warming each day with the development of low-level offshore flow. Confidence has increased dry weather will continue through Thursday as well, with temperatures remaining seasonably warm. The forecast becomes more uncertain late Friday into Saturday, 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...Now through Saturday night

Radar, satellite, and surface weather observations from early to mid Sunday afternoon showed rain showers lingering over northwest Oregon and in portions of southwest Washington to the east of the I-5 corridor. This precipitation is occurring on the backside of an upper level low centered over central Oregon. This low will continue moving eastward Sunday evening and Sunday night, which will bring an end to any lingering showers. The latest suite of hi-res model guidance suggests showers will end across the entire area by 10-11 PM Sunday, except in the far south WA/north OR Cascades and Columbia River Gorge where a few showers may linger beyond 11 PM into the overnight hours (15-30% chance).

Come Monday, all locations should be mainly dry as surface high pressure strengthens over the area. Admittedly, there is a 1-10% chance for a stray shower on Monday with the passage of a weak upper level shortwave trough, however any showers that do develop would be very light and short-lived with limited vertical growth. Most locations will be dry the entire day, with stray showers having the highest chance of occurring over the south WA/north OR coast and Willapa Hills. With skies becoming sunnier Monday afternoon, temperatures should 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 low-level offshore flow develops. 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 1-10% chance of reaching 80 degrees or warmer, specifically on Wednesday when offshore winds have the potential to last into the early afternoon. Chances for highs in the 80s are closer to 30% for inland coastal communities such as Astoria and Tillamook.

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 pushing into the Pacific Northwest from British Columbia. Confidence is high that temperatures will trend cooler on Friday compared to earlier in the week, but 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. Will note the 12Z/September 6 iteration of the LREF has trended drier on Friday with 80-90% of the total ensemble space from the ENS/GEFS/GEPS combined showing dry weather and around 75% showing dry weather on Saturday. The NBM is consistent with this and suggests only a 10-20% chance of rain late Friday into Saturday. This means dry weather with highs somewhere in the 70s is currently the most likely outcome, however anyone with outdoor plans should monitor the forecast for updates. -23

AVIATION...Through 18Z Monday

A low pressure system overhead will shift east of the Cascades after 00z Monday with increasing northerly flow aloft today. Steady rain across the Willamette Valley will gradually dissipate becoming more showery through this afternoon. Areas within the heaviest rainfall are experiencing reduced VIS and lowered CIGs to low MVFR and brief periods of IFR. Expect conditions to generally improve to widespread VFR by 20-21z today. Confidence is high that marine stratus will push onto the coast by 02-03z with low MVFR to IFR CIGs likely. There are also increasing chances for MVFR stratus to develop after 12-14z Monday across inland terminals. Expect generally light and variable winds at most terminals through tomorrow morning.

KPDX AND APPROACHES...Predominately VFR conditions expected through at least 12z Monday. Light rain expected to become more scattered this afternoon as showers gradually dissipate through 04z Monday. There is around a 40-50% chance for MVFR stratus to develop between 12-18z Monday. Light and variable winds expected through the period, except SW winds to around 6 kt possible this afternoon. /DH

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 possibly up to 25 kt (80% chance of wind gusts exceeding 21 kt by Tuesday evening). 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. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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