Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
675 FXUS66 KPQR 280313 AAA AFDPQR Area Forecast Discussion UPDATED National Weather Service Portland OR 813 PM PDT Mon May 27 2024 ...EVENING UPDATE, for aviation weather... .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...Front offshore will slowly approach, with increasing mid and high clouds rest of tonight. Will likely see CIGS lower to mix of VFR and MVFR along the coast by 10Z, with occasional light rain or drizzle. Conditions along the coast will persist through Tue am, Front will push onshore towards early afternoon, with slow improvement to lower VFR in the afternoon. Farther inland, increasing mid/high clouds tonight with increasing onshore flow as well. Will see 3000 to 3500 ft CIGS developing after 10Z to 12Z, mainly as clouds form against the Cascades foothills and backbuild. Mix of upper MVFR and lower VFR will continue through at least mid afternoon, with areas of light rain increasing between 16Z and 19Z, mainly to north of Salem. .PDX AND APPROACHES...VFR through 10Z or 11Z, but will see MVFR deck around 3000 ft spreading across OPs area afterwards. Then, will have persistent MVFR through early afternoon, with light rain after 17Z. Not much change for afternoon, with light rain and MVFR. && .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