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Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
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501 FXUS66 KPQR 160028 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 527 PM PDT Thu Aug 15 2024 Updated short term forecast for Thursday evening. .SYNOPSIS...An upper level low will most likely bring a widespread soaking rain to northwest OR and southwest WA late Saturday through Saturday night as showers and thunderstorms move into the area. Rain rates look to be heaviest over the Cascades, posing a threat of debris flows over/near recently burned areas associated with ongoing wildfires as hourly rain rates may approach 0.75 inches/hr. There is also a marginal risk of severe thunderstorms over the Cascades and foothills Saturday into Saturday night, as isolated damaging wind gusts up to 60 mph will be possible with any discrete cells that are able to form. Cool and cloudy weather then continues into the middle of next week an onshore flow maintains the pattern in place. && .UPDATE...Thursday evening...Radar and surface weather observations from 520 PM Thursday depicted isolated light rain showers developing in the Salem, McMinnville, Forest Grove, Hillsboro and Beaverton area. NBM PoPs are too low at <10%, so the forecast was adjusted to manually increase PoPs to 15-30% in the aforementioned areas. Also included this chance for the rest of the Portland metro and the south WA/north OR Cascade foothills as these light showers slowly shift east-southeastward. The latest iteration of the HRRR backs this up well, as simulated reflectivity guidance is showing these showers moving over parts of the Portland metro between 6-8pm Thursday evening, approaching the Columbia River Gorge and Cascade foothills after 8pm. It appears showery activity will diminish completely by 4-5am Friday before chances for showers and thunderstorms increase significantly late Saturday. This is discussed below in the short term discussion. -TK && .SHORT TERM...Now through Saturday Night...The short term forecast is highlighted by increasing chances for a widespread soaking rain across all of western WA/OR (50-80% chance) as widespread showers and scattered thunderstorms bring a threat of heavy rain and isolated wind damage. These showers and storms are associated with an upper level low that models and their ensembles show lingering over the coastal waters off the south Oregon coast. This will result in southeasterly flow aloft across southwest WA and northwest OR, helping to advect moisture into the area and increase instability. Ample QG forcing for ascent is evident as well as CVA and WAA increases ahead of an incoming trough axis and associated vort max, resulting in notable Q-vector convergence. The threat for thunderstorms is currently highest in the Cascades where instability will be maximized according to the latest model guidance (CAPE values rising to 500-1500 J/kg according to the GFS/EURO/NAM). Strong wind shear is also evident with impressive 0-6km bulk shear values between 40-55 knots. Admittedly, there is some uncertain regarding the exact timing of this system and which areas will undergo enough daytime heating for these higher CAPE values to materialize. Nevertheless, the environment is favorable enough to support at least a 20-30% chance of thunderstorms from the Coast Range to the Cascades, with the best chance being over the Cascades. In fact, the Storm Prediction Center (SPC) has introduced a marginal risk for severe thunderstorms over the Cascades/foothills, mainly for potential isolated convective wind gusts up to 60 mph with any discrete cells that are able to form before cells congeal into an MCS. Regardless of the storm mode, heavy rain will be a threat given high PWAT values over 1", fairly deep warm cloud layer depths over 7kft, and somewhat slow storm motion with cloud layer winds <20 knots. The UWWRF 1km and the NAM Nest are both showing rain rates up to 0.75 in/hr, which is not quite high enough for debris flow concerns over old burn areas, but is high enough to warrant concern for our new 2024 burn areas associated with ongoing active wildfires. As such, a Flash Flood Watch has been issued for the south WA Cascades and Oregon Cascades from Lane County northward to cover this threat as this will pose a safety concern for crews working on these fires. While confidence is high somewhere in the Cascades will be hit hard from thunderstorms, confidence is low in regards to the exact location and exact timing. Nevertheless, confidence has reached the 50% threshold needed to issue a Watch, and zones may be added or removed from the Watch as confidence increases. For now, anyone near or within the watch area should ensure they have a timely way to receive NWS warnings in case a Flash Flood Warning is issued. While the bulk of convective activity should shift northward into northwestern WA Saturday night, forecast soundings show lingering low and mid level moisture / weak instability over northwest OR and especially over southwest WA. This will likely result in at least some continued showery activity, however convection will become more shallow and chances for thunder will decrease to 15-20% or less. Despite convection becoming more shallow Saturday night, brief heavy downpours will still be possible. Showers and storms should finally begin tapering off Sunday morning, which is discussed below in the long term discussion. -TK && .LONG TERM...Sunday through Wednesday...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. Otherwise suspect the Pacific Northwest will remain under the influence of broad upper level troughing centered offshore through the middle of next week, which is well agreed upon by ensemble guidance. As such, expect a continuation of seasonably cool and cloudy weather through the end of the period with daily highs remaining in the mid to upper 70s. The proximity to the upper level trough will yield continued chances for sporadic shower activity, but no significant rainfall or thunderstorm potential is signaled beyond Sunday morning. /CB && .AVIATION...Marine stratus has mostly cleared out of all areas, with KONP being the last holdout. 40-50% chance of southern coastal terminals remaining MVFR or lower throughout the day, dropping back to IFR by 3z Fri. For other coastal terminals, MVFR conditions redevelop around 3z Fri. Inland terminals should generally remain VFR until 12-18z Fri, when the northern Willamette Valley sees marine stratus pushing up the Columbia and causing high-end MVFR cigs during that time period. 60% confidence in MVFR cigs between 12-18z Fri. Winds at all terminals are generally W/NW and under 8 kts throughout the entire period. PDX AND APPROACHES...VFR expected until 12z Fri, with a 60% chance of MVFR cigs between 12z-18z Fri due to marine stratus pushing up the Columbia. Northwest winds under 8 kt throughout the period. /JLiu && .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