Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
652 FXUS66 KPQR 142216 AFDPQR Area Forecast Discussion National Weather Service Portland OR 316 PM PDT Sun Jul 14 2024 .SYNOPSIS...Above normal temperatures will persist for inland portions of SW Washington and NW Oregon through next weekend, though onshore flow should help to guard against more extreme temperatures. Flow aloft will turn more southerly Tuesday, potentially carrying moisture and instability northward from California. This will lead to about a 10-20% chance of thunderstorms Tuesday afternoon into Tuesday night, with the best chance being east of Interstate 5. && .SHORT TERM...Now through Tuesday Night...The upper level pattern has changed little over the past 24 hours as strong high pressure remains anchored over the Four Corners region while a cutoff low continues to sit off the California coast. Warm and stable west-southwest flow remains in place over the Pacific Northwest to the north of these features, yielding another hot and sunny day with marine stratus limited to the coastal waters and the immediate coastline. Temperatures well into the 80s as of mid afternoon remain on track to peak in the low 90s across the Portland metro area and closer to the mid 90s from Salem to Eugene later this afternoon. Models continue to depict a slight deepening of the marine layer tonight, resulting in probabilistic guidance showing a 50-60 percent chance for stratus to make it up the Lower Columbia River Valley and into Portland and the northern Willamette Valley Monday morning. This will have implications on the high temperature for Monday and represents the best chance for highs to top out below 90 within the coming week. That said, latest guidance still gives around a 75 percent chance to hit 90 at Salem and over 90 percent at Eugene where marine relief is less likely to occur. Largest forecast concerns arise on Tuesday as the upper level pattern amplifies and results in a pivot to more southerly flow over our area. First, the stout ridge building over the western CONUS will send 500 mb heights upward and 850 mb temps into the 23-25C range, coinciding with highs potentially in the mid to upper 90s in the interior valleys on Tuesday. The probability to reach triple digits has crept upward to around 30 percent in the Portland metro and 35-45 percent through the rest of the Willamette Valley. HeatRisk continues to show Portland , the Gorge, and the Hood River Valley back in the Major category as a result, which may eventually warrant Heat Advisories for the areas on Tuesday. The complicating factor with southerly flow remains the potential for additional high level cloud cover, but even if temperatures are a few degrees cooler than currently forecast the relief would likely be minimal as the influx in moisture would result in slightly more humid conditions than we are accustomed to in the Pacific Northwest. The other primary forecast challenge will concern the aforementioned cutoff low near the California coast as models continue to depict it becoming dislodged by the amplification of the pattern and moving northward as a negatively tilted shortwave trough Tuesday into Tuesday night. This will bring concern for lightning strikes as mid level moisture and instability are advected northward into our area, yielding surface based convection Tuesday afternoon transitioning to more of an elevated nocturnal threat overnight Tuesday into Wednesday morning. Ensemble based probabilistic guidance is still somewhat underplaying the thunder threat as it tends to struggle with scenarios, but still has shown signs of catching on to what the deterministic runs have been signaling, with the chance for thunderstorms increasing to 10-20 percent across the Oregon ans south Washington Casacdes Tuesday evening through Tuesday night. Will continue to monitor how this threat evolves over the coming forecast cycles, but any lightning will obviously present heightened fire weather concerns given the recent prolonged stretch of hot and dry weather. /CB .LONG TERM...Wednesday through Sunday...Guidance has backed off of previously advertised mid to upper 90s for Wednesday as areas south of Salem, as well as the coastal valleys, have a better chance of seeing substantial onshore flow Tuesday night into Wednesday as the upper disturbance will likely deepen the marine layer, potentially even pulling stratus inland behind it. If the upper disturbance indeed lifts through western Oregon Tuesday night, Wednesday, could be cooler than the lower 90s we are generally showing for the interior lowlands Wednesday afternoon. For those hoping for an end to the unusually warm weather this July, looking at the long term forecast may be a bit of a disappointment as GFS/EC ensembles show good agreement on 500 mb heights climbing back toward 590 dam Friday and Saturday. If this occurs, it would again compress the marine layer to the point where it can only effectively cool the coast and coast range, and low clouds would likely be limited to the coast. So, expect inland valleys to see highs remaining around 90 degrees on average through next weekend, likely with mild to warm nights in between. Weagle/CB && .AVIATION...Marine stratus has been more persistent than the last few days, with continued MVFR at the northern coast, and IFR at southern terminals (KONP). KAST will continue to see around a 60% chance of MVFR marine stratus hanging around throughout the rest of daytime hours, lowering to IFR again tonight by 02-03z Mon. KONP will most likely also remain IFR throughout the day, though raised ceilings to MVFR during the afternoon is possible. Expect all coastal terminals to drop to LIFR at times overnight Sunday night, with 80%+ confidence. Winds remain fairly similar to yesterday, with NW flow slightly increasing Sunday afternoon. Winds at the coast can expect gusts up to 20 kt, while inland terminals will see sustained winds up to 10 kt. All inland terminals remain VFR with clear skies throughout the period. PDX AND APPROACHES...Clear skies VFR throughout the TAF period. Sustained winds up to 10 kt Sunday afternoon (22z Sun - 06z Mon), but otherwise mild conditions expected. /JL && .MARINE...High pressure will maintain northerly winds across the waters for much of the next week. Winds will generally be strongest off the central coast of Oregon and lightest off the mouth of the Columbia River each day. Winds will typically peak each day in the late afternoon and evening hours, with gusts at times just barely meeting Small Craft Advisory criteria at 20-25 kt. Probabilities of 20 kt winds or greater do decrease substantially (to less than 20%) across the waters off even the central coast of Oregon towards Tuesday and Wednesday. In the meantime, seas will primarily be a hybrid of fresh swell/wind-driven waves through the weekend. -JH && .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