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Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
121 FXUS66 KPQR 161109 AFDPQR Area Forecast Discussion National Weather Service Portland OR 409 AM PDT Fri Aug 16 2024 .SYNOPSIS...One more day of benign onshore flow today with highs mainly in the upper 70s. A strong upper low developing offshore will then bring widespread wetting rain to the area Saturday and Saturday night, with concerns for localized flash flooding near ongoing wildfire burn areas. There is also a chance of thunderstorms across the area on Saturday, a few of which could produce severe wind gusts from the Cascades west to the I-5 corridor. Showers taper off on Sunday, yielding a return of mild onshore flow through much of next week. && .SHORT TERM...Now through Sunday Night...Satellite imagery shows broken stratus filling in across the inland valleys of northwest Oregon and southwest Washington early Friday morning as the region remains under the influence of stable onshore flow. In the upper levels, a trough beginning to dig south from the British Columbia coast along 130W will feature prominently in the potential for widespread wetting rain and scattered thunderstorms across the area this weekend. In the meantime, expect one more day of benign weather as morning clouds give way to mostly sunny skies this afternoon with temperatures near or slightly below seasonal norms in the upper 70s to low 80s in the inland valleys. The primary short term forecast concern remains rain and thunderstorm potential Saturday and Saturday night. The upper trough will continue to deepen as it digs southward over the offshore waters today, with models depicting an unseasonably strong upper low centered approximately 300-400 miles off of the southern Oregon Coast by Saturday morning. This will place our area in south to southeasterly flow aloft, helping to advect rich mid level moisture and enhanced instability into the region while a 90 kt jet rounding the base of the trough provides strong forcing for ascent. These factors will create a favorable environment for thunderstorms Saturday afternoon and evening as a shortwave trough swings northward across the area. One notable trend in the guidance over the past 12-24 hours has been to delay the arrival of this feature, which could hold off the onset of precipitation and thunderstorm potential farther north towards Salem and Portland until late afternoon or early evening. The trade off of this delay is that this will allow temperatures to warm into the low 80s and contribute to increased instability, thus enhancing the thunderstorm potential. This is reflected in a slight uptick in thunderstorm probabilities to 20-30 across the Willamette Valley and Coast Range and 30-40 percent along the Cascades. WPC continues to highlight the Cascades for a marginal risk (5-15 percent chance) of severe thunderstorms, but has also extended this area slightly farther westward to include the I-5 corridor. The primary severe threat from these storms would be potential for a few outflow gusts of 60 mph as the strong wind field aloft mixes down to the surface. Shifting gears to heavy rain potential, the combination of strong forcing and ample moisture characterized by model precipitable water values as high as 1.25-1.40", or the 90-95th percentile of mid August climatology, suggests any showers or thunderstorms will be capable of producing heavy downpours, with hi-res guidance continuing to depict maximum hourly rainfall rates around 0.75". While this remains below debris flow thresholds for older burn areas, it does continue to present a threat for newer burned areas associated with ongoing wildfires. The Flash Flood Watch therefore remains in effect along the Cascades to capture this threat. Given the aforementioned delay in timing of the best forcing, some hi-res guidance is beginning to suggest the greatest potential for these rainfall rates may ultimately be across northern portions of the Oregon Cascades into southern Washington, but given the lingering uncertainty have opted to leave the watch in place across all Cascade zones at this time. In terms of a wetting rain of a quarter inch, guidance continues to depict a 50-70 percent chance area wide through Saturday night, with higher end outcomes remaining closer to 0.75-1.00" for most locations. Rain will be starting to taper off by Sunday morning as most of the energy associated with the upper trough shifts north of the region, but certainly could see a few stray showers linger into the day on Sunday and bring minimal additional rainfall amounts to the area, especially over southwest WA. Temperatures will continue to hover right around or slightly below seasonal norms, with Sunday afternoon highs in the upper 70s to around 80. /CB .LONG TERM...Monday through Thursday...Little change in the long term forecast as ensemble guidance continues to agree on upper level troughing over the NE Pacific through much of next week. This will keep western Oregon and Washington in a stable onshore flow regime through the period, yielding seasonably mild temperatures with morning cloudiness and afternoon highs generally in the upper 70s. The proximity to the upper level trough will also yield continued chances for sporadic shower activity, but no significant rainfall or thunderstorm potential is expected beyond this weekend. /CB && .AVIATION...Currently low stratus clouds along the coast, lower Columbia River Valley, and Cascade foothills as of 10Z Friday. General onshore flow continues across the area as we maintain westerly to southwesterly flow aloft and an onshore surface pressure gradient. Expect MVFR CIGs along the north OR coast (KAST area) and LIFR/IFR CIGs at KTMK and southward (KONP) between 10-19Z Friday. Specifically, there is a 20-40% chance of LIFR conditions for KTMK and KONP, with chances around 40-60% of lowering to at least IFR. KAST has a 40-60% chance of MVFR CIGs. Inland will see VFR conditions through most of the TAF period, with the exception of a 30-50% chance of MVFR CIGs until 18-19z Friday. Will improve to VFR thereafter. Otherwise, winds will be out of the northwest and remain under 10 kt. PDX AND APPROACHES...Predominately VFR thresholds through the TAF period, with a 20-40% chance of MVFR cigs between 13-17z Friday. Expect improving CIGs with increasing cloud breaks towards clear skies by Friday afternoon. /Sala && .MARINE...Expect benign conditions over the waters through early next week. Winds remain N/NW through Saturday before turning SW as surface low pressure develops. Winds are expected to remain at 10 kts or less with gusts up to 15 kts; wind-driven seas of 2 of 4 feet. -Batz && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland