Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
704 FXUS66 KPQR 161038 AFDPQR Area Forecast Discussion National Weather Service Portland OR 338 AM PDT Tue Jul 16 2024 .SYNOPSIS...Rather hot today with highs approaching triple digits for inland portions of SW Washington and NW Oregon. Slight chance of thunderstorms for the central Oregon Cascades this afternoon. Then, upper level disturbance brings increasing chances for elevated convection later this evening and tonight from the Cascades to the coastal waters. Above average temperatures expected to continue through the weekend while onshore flow persists at the coast. && .SHORT TERM...Today through Thursday...Hot and dry today, followed by a chance for elevated convection tonight. An upper level cutoff low has developed off the northern California coast this morning and is expected to lift northeastward today toward the Oregon coast. More southerly flow aloft ahead of the incoming shortwave is still expected to draw in very warm 850 mb temperatures peaking around 23- 25C late this afternoon into NW Oregon and SW Washington. Expect this to translate to highs around 95-100 degrees across the interior valleys, including the Columbia River Gorge and Hood River Valley. As the upper level disturbance approaches the region this evening, high pressure building along the southern Oregon coast will drive a strong onshore push of marine air at the surface. Expect a southerly wind reversal to begin to cool temperatures in the southern Willamette Valley by late afternoon/early evening, gradually expanding northward overnight. With the warmer air lingering across the Portland/Vancouver Metro to Hood River, will maintain the Heat Advisory. Along with the heat, the approaching shortwave will bring potential for thunderstorms, initially surface based in the central Oregon Cascades, eventually evolving into elevated nocturnal convection later this evening and overnight. The upper level shortwave will take on a negative tilt which is a favorable pattern for enhancing lift, while mid level moisture and instability are advected northward. Any thunderstorms that do develop will have the potential to produce prolific lightning, presenting a threat for heightened fire weather concerns given the recent stretch of hot and dry weather. Currently have capped thunderstorm chances at around 20%, though trends are certainly indicating the actual probability may be higher than that, as models sometimes struggle with elevated convection. Temperatures are once again expected to warm to around 85-90 degrees for Wednesday and Thursday across inland valleys as the ridge builds back in. Onshore flow will maintain mild temperatures for the coast. Upper level troughing will likely deepen the marine layer late Thursday. /DH .LONG TERM...Friday through Monday...Cluster analysis indicates the ensemble guidance showing good agreement of the upper level ridge amplifying and shifting back west for Friday and Saturday. This will likely cause temperatures to bump back up a few degrees with highs in the 90-95 degree range. NBM guidance suggests a slight cooling trend into early next week as upper level troughing begins to nudge into the Pacific NW. Something to watch will be the potential for another cut-off low and shortwave trough to slide across the area again this weekend, which could produce another round of thunderstorms, but confidence remains low at this time. /DH && .AVIATION...IFR/LIFR marine stratus continues to cling to the coast this morning, affecting mostly just the coastal terminals. The marine layer has become shallower over the past 24 hours, returning to a depth of only 1000-1500 ft. This means it is unlikely to reach any inland terminals this morning, and hi-res guidance looks a little too high with the chances in showing a 20% chance of IFR reaching KPDX and a 30% chance for KTTD. So, most likely scenario is that inland terminals remain VFR through this afternoon. Coastal terminals should clear out shortly after sunrise as weak low pressure off the northern CA coast begins to pull easterly offshore flow as it moves northward along the coast. The forecast becomes quite a bit more complicated after about 23z as instability and moisture aloft become involved with the low pressure moving up the coast. There is a 20-30% chance of SHRA/TSRA developing over SW Oregon and moving north into our forecast area beginning around 17/00z. Any SHRA/TSRA will move into Lane County around 00z then progress northward across KSLE and KPDX metro area overnight. This convection will be high-based so conds should remain VFR for the most part, though there will be gusty and erratic outflow winds with any TSRA. If a line of showers forms, it will likely come with a sharp wind shift from NE winds ahead of it to S-SW winds behind it, which will persist but ease for the remainder of the night. To further complicate things, the disturbance triggering the TS threat will also deepen the marine layer and initiate a robust SW surge of marine air into inland areas. Mixed IFR/MVFR conds will and areas of -DZ will spread northward along the coast behind the disturbance overnight. MVFR stratus appears likely to reach KEUG sometime overnight, but high-res guidance is more stingy with the stratus farther to the north - perhaps too stingy. PDX AND APPROACHES...VFR through this evening, with only a 10% chance of low MVFR stratus affecting KPDX around sunrise. Increasing chance of high-based TSRA after 17/03z, with best chance focusing around 17/09z. High-based nature should keep cigs/vsbys VFR in showers, but winds near any showers will be gusty and erratic. Weagle && .MARINE...North to northeast winds are slowly easing over the coastal waters, enough to allow the SCA for winds to expire for the inner waters. Upper level disturbance over the N CA waters will lift northward today and tonight, increasing the chance of thunderstorms over the waters overnight. This is the type of system that has the ability to produce prolific lightning and gusty/erratic outflow winds, so these thunderstorms could be more hazardous than the weaker springtime storms that are more typical for the coastal waters. Mariners planning on being out on the waters tonight should pay close attention to the skies to the south, and listen to NOAA weather radio for any Special Marine Warnings. These showers/thunderstorms appear most likely to move into our southern waters PZZ253/273 around 5-6 PM then gradually progress northward through the coastal waters (and NW Oregon) overnight. N-NE winds today will shift suddenly to a gusty S-SW wind near the line of showers/thunderstorms. This will cause confused seas for a bit this evening, but fortunately combined seas should only be 5-6 ft at the time. S-SW winds may briefly gust 20-25 kt, but then slowly ease overnight as the disturbance passes. South winds will persist into Wed, but should only be 10-15 kt, easing from there as high pressure returns Wed night and Thu. Thermal low pressure re-strengthens over SW Oregon Friday, likely bringing a return of gusty N winds to the Oregon and southern WA coastal waters. One last thing - with the marine layer only about 1000 ft deep, marine stratus/fog may be locally dense today. Webcams along the coast suggest the dense fog isn`t widespread enough to warrant a Dense Fog Advisory, but patches of dense fog reducing visibility to just a few hundred yards are certainly possible. Weagle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory from 11 AM this morning to 8 AM PDT Wednesday for ORZ111-112-120-122. WA...Heat Advisory from 11 AM this morning to 8 AM PDT Wednesday for WAZ206-207-209-210. PZ...Small Craft Advisory until 9 AM PDT this morning for PZZ272-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland