Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
372 FXUS66 KPQR 210949 AFDPQR Area Forecast Discussion National Weather Service Portland OR 245 AM PDT Sat Sep 21 2024 .SYNOPSIS...High pressure offshore will maintain pleasant days and cool nights through MON. A weak front will push across the region later SUN into early MON, but little if any rain expected. Afterwards, high pressure rebuilds over inland Pac NW, bringing much warmer temperatures with offshore flow for TUE. Then, back to more typical late September temperatures for WED through FRI as onshore flow strengthens. && .SHORT TERM...(Today through Monday)... Light offshore flow this morning across the region, giving clear skies and cool temperatures. Air mass relatively dry, so only patchy fog or low clouds expected into early this am, mainly along the coast and inland from Portland northward. Otherwise, not much change today, as high pressure maintains mostly sunny skies and mild temperatures with 60s along the coast, and 70s inland. Onshore flow will increase a tad for Sunday, as a weakening front approaches. So, will see increasing mid/high clouds through the day. Lower clouds will gradually increasing along the coast during the afternoon, with those clouds spreading farther inland Sunday night. Overall, little chance in temperatures along the coast. But, a few deg warmer on Sunday as compared to Saturday, given slight increase in the 850 mb temperatures. With that, highs on Sunday across the interior mostly in the mid to upper 70s. Now, a weakening front will push across the region later Sunday night. But, not much in way of precipitation expected. Will maintain small (20%) chance of showers for the coast and Willapa Hills, with any precipitation being quite light (less than 0.05 inch). But, will have to watch, as may have to push a similar chance on the east across the Cowlitz Valley to the south Washington Cascades for Sunday night into early Monday. Otherwise, gradually clearing on Monday, with light onshore flow. Does appear that temperatures will be similar to Sunday, with inland highs 75 to 80 for most areas, though could reach into the lower 80s closer to Corvallis and Eugene. This due to the low/mid level flow becoming more northerly in the afternoon, and bit warmer 850 mb temperatures. /Rockey .LONG TERM...(Tuesday through Friday)... No changes as overall trends look good. Prior discussion as follows: Next week begins with models in good agreement on an upper level ridge building over the Pacific Northwest, producing warm and dry conditions through the first half of the week.. Tuesday will be the hottest day next week as the ridge axis shifts inland and an upper trough digs over the Rockies, bringing about a short lived period of modest offshore flow. Temperature guidance has nudged slightly upward but remains in excellent agreement, producing temperatures mainly in the upper 80s across the interior valleys with little spread noted within the forecast envelope. To further demonstrate the relatively high degree of confidence in the temperatures, NBM probabilistic guidance still only indicates around a 10 percent chance to surpass 90 degrees in the Portland area and around a 25 percent chance from Salem to Eugene, even as NBM mean temperature guidance resides in the upper 80s. Offshore flow will also lead to a warm day on the coast on Tuesday, with most coastal communities likely to warm into the mid to upper 70s and probabilistic guidance indicating a roughly 10-30 percent chance to reach 80 degrees depending upon the location. Regarding the potential for east winds as flow turns offshore on Tuesday, expectations remain tempered by a few factors. The synoptic pattern is sub-optimal for a stronger wind event as guidance shifts the ridge axis inland relatively quickly and has trended farther east with the digging trough. As a result, forecast offshore pressure gradients remain rather weak (0 to -1 TTD-DLS/-4 to -5 OTH-GEG). As such, ensemble members are mostly clustered around maximum wind gusts in the 25 mph range through the Gorge and over higher Cascade slopes, with the a few of the higher end members maxing out around 35-40 mph in those locations. Will certainly still be worth keeping an eye on for those in the fire weather community, but the bulk of the guidance at this time points to a lower end and short lived offshore flow event. The forecast becomes more uncertain for Wednesday and Thursday. This uncertainty is clearly evident via a cluster analysis as roughly half of the GEFS/ENS/GEPS ensemble space favors weak ridging to relatively stronger ridging, while the other half favors troughing or zonal flow. This means there is roughly a 50% chance conditions will become cooler and wetter and a 50% chance conditions will remain warm and dry. This uncertainty is also highlighted well by the model spread of high temps; the NBM 10th-90th percentile ranges from 70 to 90 degrees on Wednesday and mid 60s to the mid 80s on Thursday. Since neither outcome is favored over the other at this time, have maintained the deterministic NBM for the official forecast. -CB/TK && .AVIATION...High pressure through the next 24 hours will lead to widespread VFR CIGs and VIS under clear skies. There is around a 10-15% chance of MVFR CIGs along the coast after 10Z. Northerly winds through the Willamette Valley would traditionally increases chances for reduced visibility and fog formation but with the lower levels of the atmosphere being dry, the probability is low. From KPDX southward in the lowest elevations there is around a 15-25% chance of CIGs less than 1000 ft AGL. Given the low probability have omitted from the TAFs, but cannot rule it out completely. If the stratus does form, it will erode by 18Z Saturday. A weak front will move in after 10Z Sunday but will take on the form of dense stratus versus precipitation. The bulk of the MVFR CIGs will move in after the end of the forecast time frame. PDX APPROACHES...VFR with no concerns. Still remains a 15% chance for MVFR CIGs this morning between 12-17Z Saturday but would be patchy and would back-build off the Cascades. -Muessle && .MARINE...High pressure lingers over the region which will maintain persistent conditions. Seas are around 6-8 ft at 11-13 seconds and will do so for the next several days. Winds are a bit more tricky as they are flirting with marginal Small Craft Advisory levels. Based on current observations have issued a Small Craft Advisory for the south-central outer waters which will be in place through the afternoon. Conditions are supported further by the afternoon northerly wind surge which will increase chances for gusty winds up to 25 kt in the central outer waters. Will continue to watch conditions through the day for improvement. On Sunday through Monday, a weak front will move in but it will have minimal impact on the marine environment. Clouds will move in but winds and seas are not expected to respond. -Muessle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 9 PM PDT this evening for PZZ210. Small Craft Advisory from 2 PM this afternoon to 10 PM PDT this evening for PZZ272. Small Craft Advisory until 10 PM PDT this evening for PZZ273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland