Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
916 FXUS66 KPQR 131707 AFDPQR Area Forecast Discussion...Aviation Update National Weather Service Portland OR 1007 AM PDT Sat Jul 13 2024 .SYNOPSIS...Above normal temperatures will persist for inland portions of SW Washington and NW Oregon through next weekend, though onshore flow should keep triple-digit heat out of the forecast area. Flow aloft will turn more southerly Tuesday, potentially carrying moisture and instability northward from California. This will lead to about a 5-10% chance of thunderstorms Tuesday afternoon into Tuesday night, with the best chance being over the Cascades Tuesday afternoon and evening. && .SHORT TERM...Today through Monday...Strong upper level high pressure has taken residence over the Four Corners region while a weak upper level low persists off the California coast. Stable W-SW flow aloft persists over the Pac NW with high 500 mb heights in the upper 580s dam range, which remains about 5-10 dam above normal. Thermal low pressure remains stronger east of the Cascades versus west, which will allow onshore flow to persist through the weekend. The result is a shallower-than-normal marine layer which will keep the coast and coastal valleys cool, but not so much the interior. With 850 mb temps in the +17 to +19 deg C range, this means inland valleys can expect highs around 90 degrees today and Sunday. The air mass cools slightly Monday, with the 00z EC ensembles showing 850 mb temps in the +15 to +18 deg C range. The difference will probably be hardly noticeable, but a few more valley locations may stay in the 80s Monday afternoon, and a slightly deeper marine layer may lead to a few morning clouds pressing inland into the Portland metro area Monday morning. Although the air mass is presently too dry and stable for thunderstorms, there are signs that may start to change Sunday night into Monday. Various models open up and eject the upper low presently off the California coast northward, potentially advecting elevated moisture and instability northward by Tuesday. At this point the NBM still suggests moisture and instability will be insufficient for thunder, giving less than a 5% chance anytime in the forecast period. Suspect chances are in fact a little higher than this, based on deterministic 00z GFS/GEM - this may show up in future cycles of the NBM. Given that this is still 3-4 days out, will allow the dry NBM forecast to ride for the time being - but the evolution of that upper low/trough will be important to watch over the coming days, as our recent hot/dry weather has dried out vegetation across our area... making it more susceptible to ignition by any lightning strikes that would occur. Weagle .LONG TERM...Tuesday through Friday...As mentioned in the short term discussion, all eyes will be on the upper low/trough lifting northward from the California coastal waters Tuesday. Flow aloft turns more southerly, adding moisture and instability aloft, but NBM suggests this will be insufficient for thunderstorms in our CWA, giving only about a 5% chance as the highest probability. In reality, would put the probability higher than that based on pattern recognition, with the best chance (10-15%) being Tue night/Wed morning. Given the low confidence, we decided not to fight NBM and to let its dry forecast to ride for the time being. If the upper low/trough ejects north-northeastward Tue/Tue night as the 00z deterministic GFS/GEM suggest, this would probably be followed by a deeper marine layer and somewhat cooler temps inland Wednesday and/or Thursday. Latest ensembles suggest a fairly quick recovery in 500 mb heights behind the upper trough, so it still appears temperatures will remain above mid-July climatological averages for the foreseeable future. Another note about Tuesday - this time regarding temperatures. Tuesday has the potential to be the hottest day this week with 00z GEFS/EC ensemble mean 850 mb temps generally in the +20 to +22 deg C range. This could support mid 90s with full sunshine and no marine influence, but that may not be the case on Tuesday due to the shortwave energy advecting toward us from the south at that time. The southerly flow aloft could easily advect debris clouds (or smoke) from Monday`s convection and fires, which could keep temps a little cooler. That said, any relief in temperatures would probably come with an increase in humidity, so Tuesday has the potential to be an uncomfortably very warm and humid day across the forecast area. Weagle && .AVIATION...Surface high continues to impact the region, with marine stratus currently in the process of lifting. Mixed MVFR/IFR conditions in place at coastal terminals, expected to lift by early afternoon, 20z Sat at the latest. Coast terminals return to IFR by 02z Sunday, with a 20% chance of LIFR during that time. Inland terminals will just remain VFR throughout the TAF period. Winds will also pick up in the afternoon, with winds around 10-15 kts, gusting to 20 kts. Residual smoke and smog across the general area continue to obscure distant topography, but visibilities should continue to remain well above 6 SM. PDX AND APPROACHES...VFR conditions through the TAF period. Northwesterly gusts, with winds up to 10 kt at times during Saturday afternoon. Generally few/no concerns expected. /JLiu && .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. 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 next Tuesday and Wednesday. In the meantime, seas will primarily be a hybrid of fresh swell/wind-driven waves through the weekend. /Neuman && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ252- 253. Small Craft Advisory until 5 AM PDT Sunday for PZZ271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland