Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 281035 AFDPQR Area Forecast Discussion National Weather Service Portland OR 330 AM PDT Thu Mar 28 2024 .SYNOPSIS... Post-frontal showers with a slight chance of thunderstorms continuing through Thursday evening as a low pressure system spins offshore. Conditions begin to improve on Friday, with high pressure bringing another period of warm and dry weather this weekend into early next week. && .SHORT TERM...
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Thursday morning through Saturday night...Between midnight-2am Thursday, radar and satellite observations depicted a line of thunderstorms training over the Newport area, with a few thunderstorms also drifting over Depoe Bay, Lincoln City and Siletz. Based on spotter reports, some of these thunderstorms were producing brief wind gusts of 20-30 mph as well as small pea size hail. Accumulating hail was reported in Newport around 12:15am, with hail covering sidewalks and roads. Anyone experiencing accumulating hail during/following heavier showers or thunderstorms should exercise caution while driving as roads will be slippery until the hail melts off. Farther inland, a line of showers was beginning to move back into the Portland/Vancouver metro, but with no observed lightning. The ongoing convective activity is occurring in the wake of yesterday`s cold frontal passage while a closed low pressure system spins around 400 miles offshore the WA coast (~992mb surface low). With a relatively cold airmass aloft/instability in place, SBCAPE values are expected to hover around 250 J/kg through Friday morning along and near the coast. This will help maintain thunderstorm chances, and the latest NBM backs this up well with a 20-30% chance of thunderstorms at the coast and a 15-25% chance in the Coast Range and Willapa Hills. Thunderstorm chances do spread into the Willamette Valley and Cascade foothills Thursday afternoon (20% chance), but any thunderstorms that do form inland should be isolated in coverage due to excess cloud cover limiting instability. If more sun breaks occur than expected, then thunder chances will increase a bit. For now, the latest HREF ensemble mean for cloud cover and current satellite imagery/trends suggest cloud cover will be stubborn to clear out for extended periods of time due to frequent rain showers and cloud cover associated with convective debris. In addition to showers and thunderstorms, Thursday afternoon will also feature breezy south winds with gusts up to 25-35 mph, strongest across the central Willamette Valley. Locally stronger gusts up to 45 mph will be possible for brief periods of time near the strongest showers or thunderstorms. Wind speeds of this magnitude are strong enough to down some tree branches here and there, but likely not strong enough to down entire trees (unless it`s a weak or dying tree that`s ready to fall). Would also like to mention that precipitation is falling in the form of snow in the Cascades with several inches of new snow expected at pass level. While snow amounts are not high enough to warrant headlines, anyone with travel plans over the passes should still be prepared for winter driving conditions as off-and-on snow showers linger over the area through Friday morning. Showers should finally decrease Friday afternoon or evening as offshore flow develops over the area. Models suggest light offshore flow will then continue through Saturday, bringing a period of dry weather with milder temperatures. This will be especially true on Saturday as the entire day will be dry with minimal cloud cover, allowing the strong late March sun to warm temps into the mid 60s. There is a 30-50% chance for high temps above 65F Saturday afternoon across the central/northern Willamette Valley from Salem to the Portland/Vancouver metro, but upper 60s seems to be the upper limit as the probability for highs at or above 70F is around 1-5%. Conditions look to remain dry Saturday night as low level onshore flow redevelops, albeit weak. It`s also worth mentioning that frost is likely in the Upper Hood River Valley, Coast Range, Willapa Hills and Cascade foothills Friday night and Saturday night as skies will be mostly clear, winds will be light, and temps will likely drop to around 32-35F. This setup is common with northerly flow aloft this time of year, which models and their ensembles are indeed showing. Anyone living in these zones with sensitive outdoor vegetation should consider bringing plants inside if possible, or taking other measures to protect your vegetation from frost. If you have a morning commute, it may be a good idea to give yourself a few extra minutes in case you need to scrape frost off your windshield before hitting the road. If you do see frost, be aware there is the potential for a few slick spots on sidewalks and/or roads. Will hold off on issuing Frost Advisories as we are not in the heart of growing season quite yet; we only issue Frost/freeze headlines after the start of April when growing season quickly picks up and impacts start to become more noticeable for the agricultural community. -TK .LONG TERM...Sunday through Wednesday night...The long term forecast is highlighted by mainly dry weather with mild temperatures. The EPS/GEFS/CMC ensemble mean all show northerly flow at 500mb transitioning to northeasterly flow Sunday into Monday ahead of a southwest-northeast oriented ridge axis over the coastal waters. The ridge axis is currently set to move inland Monday/Monday night before shifting east of the Cascades Tuesday. This ridge will bring a period of dry and warmer weather to the area, however there is some uncertainty regarding exactly how warm it will be, especially on Tuesday. This is when model spread ranges from the upper 50s to mid 70s for high temps over the Willamette Valley and interior lowlands of southwest WA. The most likely outcome will be highs somewhere in the 60s Sunday through Tuesday, except upper 50s to around 60F at the coast. Monday and/or Tuesday should be the warmest days of the week, and this is when the NBM shows a 20-50% chance for high temps of 70F or warmer (highest probabilities on Tuesday). The more noticeable model spread for high temps on Tuesday makes sense given the scenario at hand, as the next Pacific frontal system will be approaching the region at that time. There is uncertainty regarding the exact timing of this system and the cloud cover that will come with it. If model solutions showing a faster progression of this frontal system verify, then cloud cover would begin to increase through the day on Tuesday and limit high temps quite a bit. If models showing a slower progression verify, then most if not all of the day will be sunny and highs would likely rise above 70F over the interior lowlands given the relatively mild temps to start the day. By Wednesday, it appears very likely we will cool down as light rain returns with the aforementioned frontal system. -TK
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&& .AVIATION...
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The majority of terminals currently are experiencing VFR conditions early this morning although conditions will likely fluctuate between MVFR and VFR cigs through the day. These fluctuations will be tied to the prevalence of shower activity which is expected to increase late this morning into the afternoon(after ~17-19z) at all inland sites, thus increasing the likelihood for sub-3kft CIGS. While chances(20-25%) for isolated thunderstorms remain pinned to the coast this morning, they`ll again bleed inland this afternoon - something to keep in mind. Confidence in thunderstorm timing is too low to warrant adding it to the latest TAFs. Winds will also become gustier later this morning into the afternoon with intermittent gusts of to 20-30 knots possible at all terminals. * Note: The observations out of KONP are generally limited to wind and pressure at this time. Periodically cloud cover, vis, and present wx are reported but are unreliable. PDX AND APPROACHES...Mainly VFR conditions continue through the period although waves of showers will briefly drop ceilings to MVFR criteria. Chances(40-60%) for reaching MVFR criteria peak late this morning and this afternoon. Intermittent gusts continue around 22-28 knots as well. /Schuldt
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&& .MARINE...
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Expect active and unsettled conditions the next couple of days before a more summer-like pattern shifts overhead this weekend into early next week leading to a return of calmer winds and seas. Current satellite observations early this morning depict the center of a broad area of low pressure approximately 300 nautical miles off the south Washington coast and as the day progresses this feature will slowly meandering eastward before stalling offshore. The movement of the aforementioned low pressure system likely leads to an uptick in southwesterly winds and convective showers with guidance heavily favoring(70-90% chance) of Gale Force wind gusts (33-40 kt) mixing down to the surface through this evening. We’ll also see a large west-southwesterly swell push into the coastal waters today as well with seas increasing into the 15-20ft range - highest Cape Falcon southward. Seas will most likely peak sometime during the late afternoon and early evening hours before falling back down into 8-12ft range on Friday. From this point late Friday through the weekend model guidance is in agreement on a positively tilted ridge of high pressure moving across the Pacific Northwest, thus facilitating a more summer-like northerly wind pattern across the coastal waters. Looking ahead, model hint at the return of a more active weather pattern around the middle of next week, although confidence is low on the exact timing and marine impacts at this time. /Schuldt
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&& .PQR WATCHES/WARNINGS/ADVISORIES...
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OR...None. WA...None. PZ...Hazardous Seas Warning until 5 AM PDT Friday for PZZ210. Gale Warning until 8 PM PDT this evening for PZZ251>253-271>273. Hazardous Seas Warning from 8 PM this evening to 11 AM PDT Friday for PZZ251>253-271>273.
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