Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
513 FXUS66 KPQR 071019 AFDPQR Area Forecast Discussion National Weather Service Portland OR 319 AM PDT Fri Jun 7 2024 .SYNOPSIS...High pressure will maintain above-normal temps and mostly dry conditions through early next week. Today will be the warmest day of the week. A weak shortwave trough Saturday will bring increasing mid to high level cloud cover and a 15-30% chance for showers in the Cascades. The Lane County Cascades have a slight chance for thunderstorms Saturday afternoon/night. Onshore flow will keep temps moderated along the coast. && .SHORT TERM...Today through Sunday...A high pressure ridge over the Pacific Northwest will reach it`s maximum amplitude today, leading to dry weather, sunshine, and the warmest temperatures of the week. Afternoon highs today are forecast in the mid to upper 80s for interior valleys, which are about 15 degrees above normal for this time of year. NBM probabilities of exceeding 90 degrees has fallen to around 15-30% for the Portland Metro to Upper Hood River Valley. Meanwhile, onshore flow will help keep temps moderated along the coast, with highs in the upper 60s to low 70s. Keep in mind that even though the air will be warm tomorrow, rivers and lakes will still be very cold! Use extra precaution if swimming in local bodies of water, as cold water can lead to hypothermia or cold water shock. Overnight, there will be decent relief from the warm temps as Saturday morning lows are forecast in the 50s across the area. The aforementioned ridge will also begin to progress eastward as a weak upper-level shortwave trough approaches the region. This shortwave will bring more moist, southwesterly flow aloft as well as increasing mid to high level cloud cover Saturday. Saturday high temps for inland valleys will still be warm and around 10 degrees above normal, but likely a few degrees cooler than the previous day due to increased cloud cover. Precipitation with this shortwave trough is also unlikely since our pre-existing air mass will dry. However, an exception would be the Cascades, where surface heating and upslope flow could support a 15-30% chance for showers Saturday afternoon-night. HREF does show MUCAPE values of 250-500 J/kg Saturday afternoon over the Cascades. And looking at SREF and HiResW-ARW soundings, the instability is mainly above 10,000 ft. Combine that with increased mid-level moisture, we could see an elevated thunderstorm or two pop up along the Cascade crest. However, with the westerly component to the wind aloft, any showers or thunderstorms that do develop on Saturday would likely get pushed into central and eastern Oregon. Went ahead and kept the 15-20% chance of thunderstorms given by NBM for the Lane County Cascades. The slight cooling trend continues on Sunday as the upper-level shortwave trough moves over us and exits the Pacific Northwest. We`ll have onshore flow which will help with cooling temps a bit. However, temperatures are still forecast above-normal for inland valleys, topping out in the upper 70s to low 80s. NBM suggests a 40-50% chance that inland valleys exceed 80 degrees on Sunday. Overall, pleasant summer-like conditions through the weekend. -Alviz && .LONG TERM...Sunday Night through Thursday...To summarize, the weather pattern in the early part of the extended forecast favors a continuation of above-normal temperatures through Tuesday. However, uncertainty comes mid-week with how an upper trough near the Gulf of Alaska will influence the weather in the Pacific Northwest: Based on WPC cluster analyses, a low amplitude ridge will re- build behind the shortwave trough Sunday night into Monday. The clusters are also in agreement of above-normal 500 mb heights through Tuesday. This will maintain warm temperatures going into early next week. The latest NBM suggests a 40-50% chance of inland valleys exceeding 80 degrees Monday and Tuesday. However, all of the clusters show the ridge beginning to flatten on Tuesday as an upper trough near the Gulf of Alaska progresses into British Columbia. By Wednesday, ensembles are showing more zonal flow, which would lead to further cooling of temperatures. More uncertainty comes on Thursday with the upper level pattern. About 50% of ensemble members show the upper trough dipping southward toward the Pacific Northwest, increasing southwesterly flow aloft. If this pans out, then we could see even cooler temperatures and potential for precipitation. Meanwhile, the other 50% of members show either zonal flow or a building ridge. If these scenarios pan out, then we would maintain relatively warmer and drier conditions. -Alviz && .AVIATION...High pressure persists through 12Z Saturday leading to VFR CIGs and VIS. Onshore flow through the day with a northerly wind in the afternoon. Misty conditions along the coast until after sunrise around 15Z. Warm conditions are expected today, especially from KPDX south to KEUG within the Willamette Valley, and around K4S2. It is possible that thermals will be present along runways. Winds will be northwesterly up to 3000 ft AGL up to 20 kt. PDX AND APPROACHES...VFR with no concerns. Increased winds in the afternoon. Thermal eddies possible in the afternoon along runways due to elevated heating. -Muessle && .MARINE...High pressure remains through Saturday morning. Northwesterly flow will dominate with gusty conditions up to 25 kt. Small craft advisory has been extended from Cape Falcon southward through the evening as gusts will remain around 25 kt. In the northern waters, those winds and seas will ease sooner. Some models are suggesting a brief period of gusts to 25 kt in the southern portions of PZZ271/251 from 1400-1900. Given that they are going to be localized and isolated, have not continued the advisory for the northern waters. Seas will be around 7-9 ft at 13 seconds until Saturday when they ease to less than 7 ft. This will be short-lived though because a weak cold front arrives on Sunday causing winds to increase again from the west. The westerly winds will combine with the westerly background swell causing significant wave heights to rise near 10 ft. -Muessle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 8 AM PDT this morning for PZZ210. Small Craft Advisory until 6 AM PDT early this morning for PZZ251-271. Small Craft Advisory until 11 PM PDT this evening for PZZ252-253- 272-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland