Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 100529 AFDPQR Area Forecast Discussion National Weather Service Portland OR 1027 PM PDT Tue Aug 9 2022 .SYNOPSIS...The upper low off the northern California coast will continue drifting slowly northward, but remains offshore. This will maintain scattered showers and evening thunderstorms, mainly well inland into tonight. But, will have stronger low level onshore flow, with clouds increasing tonight. Cooler Wednesday thanks to the marine clouds. Then, a return to dry and seasonal temperatures for the remainder of the week, with warmer weather to start next week. && .SHORT TERM (Tonight through Friday)...
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Key Messages... - Scattered showers and thunderstorms tonight. - Enhanced wildfire danger due to lightning across much of northwest Oregon tonight. - Cooler with onshore flow Wednesday. - DRY for Thursday to Monday, with seasonal temperatures. Somewhat active start to the day, as have seen convection over the Lane County lift northward before the activity weakened over southwest Washington in the early afternoon. Not much rain, but did see fair amount of lightning over the Cascades, with a few rumbles of thunder over the east side of Portland around lunchtime. Afternoon skies are partly to mostly sunny for inland areas, with temperatures into the middle to upper 70s. Stubborn upper low off the northern California coast still reluctantly drifting to the north. This has maintained somewhat persistent south to southeast flow across Oregon and Washington. Air mass remains unstable. While no significant show waves are in flow aloft, do seem to have minor perturbations that are the focus for the elevated convection. Next one will likely push across region later this afternoon into the early evening, with showers and thunderstorms developing over the Cascades. As this perturbation lifts north, will see 700 mb flow kink a bit, enough to support a few hours of southeasterly component. As such, any storms over the Cascades will drift west-northwest, with decent potential of moving away from the Cascades and over the east Willamette Valley. Will trend forecasts in this mindset. Most areas will see a few showers with less than a tenth of inch of rain (if that). But, a few lucky spots will get decent rain, perhaps a quarter to as much as a half inch of rain. Overall, flow stays southerly overnight. Bulk of showers and thunderstorms will lift farther to the north around/after midnight. BUt, models do hint at another upper perturbation that will move across the region later tonight. If this holds, could be enough to trigger more showers and perhaps a few thunderstorm again well after midnight. Latest models hinting that the higher potential will be along the coast (primarily north of Newport), and over the South Washington Cascades down to around Mount Hood. But, confidence not all that good. But, expect strengthening onshore flow tonight, with marine stratus surging through the coastal mountain gaps and into the interior tonight. Should see rather solid marine clouds overnight, with occasional drizzle along the coast. Could see some patchy drizzle farther inland as well, but will leave out of current forecast. Clouds will be slow to break up on Wednesday, with no major break up of clouds until mid afternoon. As such, will be cooler, with highs only in the 70s inland, and 60s along the coast. Back to uneventful weather pattern for Thursday, as high pressure builds over the inland Pac NW. Still have have the slow-moving upper low offshore. As such, will generally have mild southwest flow aloft. Should be partly to mostly sunny day on Thursday, with seasonal temperatures. /Rockey .LONG TERM...(Friday night through Tuesday)...Weather conditions should remain quite seasonable with dry and mild onshore flow. The latest deterministic models now show the upper low moving onshore into British Columbia sometime Friday night or Saturday. With surface high pressure building offshore, will likely maintain persistent near normal summer conditions over the weekend. Ensembles also show dry weather persisting well into next week, while a slight warming trend next week. Afternoon highs inland could be in the lower to middle 90s again come middle of next week. Still a ways out in time, and do have some issues in details amongst the models. So, will give a day or two to see how models resolve some differences.
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&& .AVIATION...
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06Z TAFs: A southwest marine push will result in a mixture of IFR and MVFR ceilings along the coast and result in MVFR ceilings push inland overnight into Wednesday morning. Inland ceilings should slowly lift and improve to VFR thresholds towards midday. In the meantime, an area of mid level convergence is driving clusters of elevated showers and thunderstorms to develop across western Oregon this evening. High resolution model guidance has struggled with the convection so far so confidence in details is quite low. Nonetheless, the forcing that appears to be driving the convection appears increasingly likely to stay more or less in place into Wednesday morning. Thus, there is a chance that multiple rounds of thunderstorms will move across at least a couple of terminals between now and 18z Wednesday. Expect brief heavy rain and IFR visibilities, hail up to 1 inch in diameter that could damage aircraft and frequent lightning with the strongest storms. For detailed Pac NW aviation weather information, go online to: https://weather.gov/zse KPDX AND APPROACHES...A southwest marine push will result in conditions deteriorating into MVFR thresholds towards 12-15z Wednesday. Expect conditions to then trend back into VFR thresholds towards 18-20z Wednesday. In the meantime, an area of mid level convergence is driving clusters of elevated showers and thunderstorms to develop across western Oregon this evening. High resolution model guidance has struggled with the convection so far so confidence in details is quite low. Nonetheless, the forcing that appears to be driving the convection appears increasingly likely to stay more or less in place into Wednesday morning. Thus, there is a chance that at least one or two rounds of showers or thunderstorms could at least briefly impact the terminal between 09-15z Wednesday. /Neuman
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&& .MARINE...Minimal changes to the current forecast. A marine stratus deck will remain over the waters for the rest of today through at least Wednesday morning, but the potential for fog with this stratus deck remains low. There is a chance of showers and a slight chance of thunderstorms tonight, mainly between midnight and 5 AM PDT Wednesday. Southerly winds are also expected to continue for the next several days as a closed surface low settles well off the Washington and north Oregon coast on Wednesday and remains nearly stationary through Friday. Southerly wind gusts should generally remain below 20 kt and small craft criteria is not expected to be met at this time. Significant wave heights should generally range between 4 and 6 feet tonight through Friday as a northwesterly swell with a dominant wave period around 8 to 9 seconds continues. -TK For information about upcoming marine zone changes, go online to: https://www.weather.gov/pqr/marinezone && .PQR WATCHES/WARNINGS/ADVISORIES...
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OR...Red Flag Warning until 6 AM PDT Wednesday for Central Oregon Cascade Foothills-East Slopes of the Central Oregon Coast Range-Mt. Hood National Forest West of Cascade Crest-North Oregon Cascade Foothills-North Oregon Coast Range- Willamette National Forest-Willamette Valley. WA...None. PZ...None.
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