Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
782 FXUS66 KPQR 170346 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 846 PM PDT Mon Sep 16 2024 .SYNOPSIS...Cool and wet weather on Tuesday as a frontal system brings widespread light rain to the region. Mainly dry Wednesday through Friday with onshore flow and slightly below average temps. Rain potential this weekend, but confidence is low in the overall pattern. && .SHORT TERM...Now through Wednesday Night...The high pressure ridge that brought sunny and warm weather to the region will shift rather abruptly on Tuesday as the next upper level trough digs south over the NE Pacific and sends a frontal boundary through the area, bringing the next round of rain. Still expect rain to arrive along the coast around mid morning and then push inland to the Willamette Valley by early afternoon, lingering through Tuesday evening before tapering off from northwest to southeast during the overnight hours. There has been a notable downward trend in precipitation amounts with the 12z model runs as more hi-res model guidance enters the picture, and this seem to make sense from a conceptual standpoint with the trough remaining offshore of the Oregon Coast on Tuesday and evolving into another closed low over northern California by Wednesday morning. As such, have lowered amounts east of the Coast Range to around a tenth or two from Portland down to Salem. The probability for a true wetting rain of quarter inch has fallen to around 20 percent in these area, with the best chances (50-60%) found in the south valley around Eugene and also in orographically favored parts of the Cascades. Probabilities remain higher along the Coast, where most areas are still expected to see roughly a half to three quarters of an inch of rain through Tuesday evening. A few showers may linger across the area into Wednesday morning as the low resides over northern California, but expect this activity to diminish by the afternoon as a positively tilted ridge noses into the region from the Pacific. This will yield seasonable conditions with temperatures in the low to mid 70s from Wednesday into the latter half of the week. -CB .LONG TERM...Thursday through Monday...Thursday and Friday will feature benign, pleasant September weather as the ridge axis shifts slowly southward across the region, opening the door to increased onshore flow as we head into the end of the week. Expect inland high temperatures generally in the low to mid 70s each day. Forecast confidence remains low headed into the weekend as ensemble clusters remain fairly evenly split between ridging or troughing developing over the region by Sunday into early next week. Meanwhile, most deterministic solutions depict a somewhat zonal flow pattern through next Monday. This uncertainty is reflected by the continuing high spread between the 25th and 75th percentile NBM temperature guidance, which places highs for next Sunday and MOnday anywhere between the mid 60s and upper 70s across the interior lowlands. As such, continues to stick closely to NBM mean which depicts seasonable temperatures and broad brushed chance to slight chance PoPs across the area this weekend into early next week. -CB && .AVIATION...Northerly flow aloft tonight as high pressure ridge moves across the region. Widespread VFR this evening, except at the coast where marine stratus is developing with low-end MVFR CIGs around 1000-1500 ft. Guidance suggests there is a 40-60% chance that conditions deteriorate to IFR through Tuesday morning, especially at KONP. A southerly wind reversal begins around 12Z Tuesday ahead of an approaching low pressure system, with stratus likely pushing into the southern Willamette Valley. Stratus is expected to become more widespread inland between 12-14Z Tuesday, with chances for MVFR CIGs around 40-60%. Light rain is likely to begin at the coast around 16z Tuesday morning, and gradually spreading inland by 20-22z Tuesday afternoon. PDX APPROACHES...VFR with variable high clouds expected through at least 12z Tuesday. Then, stratus begins building across the area with chances for MVFR CIGs increasing to 40-60% after 14z Tue morning. Chances for light rain increases through Tuesday afternoon. Northwest winds become light and variable after 08Z Tuesday. -DH && .MARINE...High pressure over the outer waters continue today with northerly pressure gradients bringing gusty north wind gusting 20 to 25 kt. With the persistent northerly winds, seas will be somewhat choppy, especially over the outer waters where winds are stronger. Winds are expected to decrease into tonight, therefore will end the Small Craft Advisory at 11PM tonight. Seas will remain between 5 to 8 ft today and tomorrow. Late tomorrow into Wednesday, we will see yet another pattern shift as a strengthening weather system drops down from British Columbia. By the time it reaches the waters, it will elongate and weaken. Considering the expected speed of this weakening system as it will move south along the West Coast, impacts look to be minimal. This storm will elevate seas up to 8 ft and winds up to 20 kt. -JH && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland