Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
478 FXUS66 KPQR 272226 AFDPQR Area Forecast Discussion National Weather Service Portland OR 326 PM PDT Mon May 27 2024 .SYNOPSIS...Weak cold front moves over the region on Tuesday and lingers through Wednesday. Precipitation will be light with the highest accumulations along the terrain. Conditions will become showery late Thursday through Wednesday. High pressure tries to form on Thursday-Friday which will bring another round of springtime weather. && .SHORT TERM...Now through Friday...A long wave trough extends southward over the region with persistent onshore flow. A low level jet is forming with 500 mb winds around 55 kt, and 850 winds closer to 45-50 kt on Tuesday. This jet streak is collocated with the most enhanced time frame of the front. Overall, looking at a cooler and wetter pattern. High confidence in the temperature forecast with below normal highs, and cooler low temperatures. Cloudy skies though will act like a blanket and stop the radiational cooling thus overnight temperatures will be warmer than the previous day. Wednesday will see minimal changes other than being post frontal with lingering showers. Snow levels will be right around 4500-5000 ft so some light snowfall is possible over the volcanoes. By Thursday, we will begin the quick transition into a ridging pattern. The trough that brought the cooler temperatures will move over the northern Rockies and high pressure builds over the Pacific. Flow will become more northerly during this time so could see slightly breezier conditions along the beaches and through the Willamette Valley. Friday will see this high pressure intensify and the northerly winds increase even more. The main ridge axis sits well offshore though so will not see any strong easterly or southerly winds. Temperatures will warm though. The NBM is showing about a 10 degree spread between the 25-75th percentile - though is trending towards the 25th percentile in the deterministic forecast. If Thursday ends up being clearer, then Friday has a strong chance of warming more than currently forecast. Because of that, have increased temperatures a bit, especially within the Willamette Valley. -Muessle .LONG TERM...Friday Night through Sunday...Zonal flow takes over on Saturday with neutral onshore winds. Will be a benign weather day with no notable features. Some of the ensembles are attempting to bring in a shortwave trough (around 31% of the members and specifically within the Canadian). This would bring rainfall to the region, but would not be high amounts. Because there is background westerly flow that`s zonal, I am trending towards a slightly wetter pattern that could be a bit more drizzly. As we near Sunday, the pattern is a mess of ridging, troughs, cold air intrusion, warming from the south and it feels like a mixed bag. This overall unrealized scenario persists through early next week as well. This can further be seen in some of the probabilities for temperatures, rainfall totals, etc. In some cases, there is nearly a 20 degree spread in the high temperature forecast. -Muessle && .AVIATION...High pressure will maintain light NW flow across the area this evening and overnight. VFR conditions will give way to MVFR conditions around 6Z along the coast (70-80%) and around 8-9Z across the Willamette Valley (60-90%). There is also a chance (50-60%) for IFR CIGs along the coast to develop but confidence is slightly lower. A weak cold front will pass through the area tomorrow, bringing light rain to the coast before noon and during the afternoon for the interior valleys. PDX AND APPROACHES...Light NW winds around 5 knots will become light and variable overnight. SW winds will pick up to between 5-10 knots tomorrow with the passage of a cold front. VFR cigs will be maintained into the overnight with an MVFR cloud deck developing later tonight. -Batz && .MARINE...A mostly quiet pattern is expected through at least the middle of the week. Surface high pressure will maintain light, onshore winds less than 10 knots. A quick moving trough will support a weak cold front that will pass through the area tonight but won`t bring substantial impacts, just a wind shift from the NW to the SW tomorrow. Seas will remain 4-6 feet through a least Friday. A late week system may bring borderline Small Craft Advisory conditions to portions of the forecast area but confidence is not high at this time. -Batz && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland