Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
253 FXUS66 KPQR 161010 AFDPQR Area Forecast Discussion National Weather Service Portland OR 300 AM PDT Mon Sep 16 2024 .SYNOPSIS...High pressure will bring plenty of sunshine and the warmest temps of the week on Monday. Rapid change to cool and wet weather on Tuesday as a frontal system brings widespread rain to the region. Mainly dry Wednesday through Friday with onshore flow and slightly below average temps. && .SHORT TERM...Now through Tuesday night...The short term forecast is highlighted by a quick transition from warm and dry weather to cool and wet weather. High pressure will remain over the area through Monday, bringing mostly sunny conditions and warm afternoon temps in the mid to upper 70s, except upper 60s at the coast. While most of the day should be sunny, cannot rule out some shallow patchy fog in the lowlands during the early morning hours. NBM probabilities for high temps of 80 degrees or warmer are around 15-25% across the Willamette Valley, Portland/Vancouver metro and Columbia River Gorge, except 40% in the Salem area. Monday will be a great day for outdoor activities. Conditions will quickly change to cool and wet on Tuesday as a stronger frontal system spreads widespread stratiform rain over western WA/OR. The HREF mean suggests rain will most likely begin at the coast during between 7-10am before spreading inland between 1-3pm, which aligns well with NBM PoPs and simulated reflectivity guidance from the latest suite of hi-res model guidance. The earliest possible start time according to the HREF is 3-6am for the coast and 9-11am inland. The latest possible start time is noon-2pm at the coast and around 5pm inland. Regardless of the exact start time, expect stratiform rain to transition to post-frontal showers Tuesday evening or Tuesday night. How much rain will fall Tuesday through Tuesday night? Likely over 0.25 inch for all lowland locations west of the Cascade crest (80-90% chance). Higher rain amounts are likely for the coast, mountains and southern Willamette Valley; these areas have a 75-90% chance for rain amounts over 0.5 inches and a 40-60% for more than one inch. -TK .LONG TERM...Wednesday through Sunday...Ensemble guidance suggests that the upper level trough associated with Tuesday`s rain will exit southward toward California on Wednesday. Expect some post-frontal showers early Wednesday morning, however showers will quickly diminish by the afternoon as weak high pressure re- builds. Model soundings also suggest decreasing clouds Wednesday afternoon, allowing temps to rebound into the upper 60s and lower 70s. Expect similar temps Thursday through Saturday. Most ensemble guidance depicts weak shortwave ridging Thursday into Friday, resulting in dry conditions. The forecast becomes more uncertain over the weekend. This is highlighted well by the latest ensemble clustering analysis as roughly half of the total ensemble space from the GEFS/ENS/GEPS suggests an upper level trough will reside over the Pacific Northwest while the other half depicts ridging. If troughing occurs, cooler and wetter weather would return. If ridging occurs, warmer and drier weather would prevail. Given this uncertainty have maintained NBM PoPs which show a 20-40% chance of rain going into Saturday and 15-20% chance on Sunday. The NBM 1D Viewer highlights uncertainty in the temperature forecast over the weekend as the 10th-90th percentile for high temps range from the lower 60s to mid 70s on Saturday and upper 50s to lower 80s on Sunday. With no clear regime favored over the other, have also left the deterministic NBM for the official temperature forecast, which suggests high temps in the low to mid 70s inland and upper 60s for the coast. -TK/Alviz && .AVIATION...Overall expect VFR conditions through the TAF period. With mostly clear skies patchy fog or low stratus chances increase. HREF shows 30-50% chance for MVFR cigs south of KS12 to KEUG from 12-16Z Mon. Current satellite images show clouds already forming just south of the KEUG area, so HREF looks reasonable. Forecast confidence a bit lower north of KS12, but overall VFR looks to prevail with about a 20% chance for MVFR cigs between KPDX and KSLE. At the coast, satellite shows patchy fog developing in coastal valleys. Already KTMK in LIFR in fog. KONP should see a few hours 12-15Z of IFR as valley stratus flows to the coast. PDX APPROACHES...General VFR through the next 24 hours although a layer of FEW-SCT at 1.5kft may materialize right around sunrise with only a 20% chance to see MVFR cigs or lower 14-16z early morning period. Winds may become breezy in the late afternoon, but gusts not expected to exceed 15 kt. /mh -Schuldt && .MARINE...High pressure over the outer waters continue today with northerly pressure gradients bringing gusty north wind gusting 20 to 25 kt. With the persistent northerly winds seas will be somewhat choppy, especially over the outer water where winds are stronger. Overall, seas stay in 6 to 9 ft through Monday, with highest seas over the outer waters. Later Tuesday into Wednesday we will see yet another pattern shift as a strengthening low pressure system drops down from British Columbia. By time it reaches the waters, it will elongate/weaken. Models still vary some on the timing, strength, and position of this system which will have an impact on the forecast. So will have to see how things shape up as get closer in time to Tuesday. /mh && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 11 AM PDT this morning for PZZ252-253. Small Craft Advisory until 11 PM PDT this evening for PZZ271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland