Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
916 FXUS66 KPQR 161738 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 1037 AM PDT Tue Jul 16 2024 Updated Aviation and Marine Discussion .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...A complicated forecast for the airspace starting at 18Z Tuesday and going through at least 00Z Thursday. A low pressure system swinging northward from California will result in increasing instability and moisture aloft. This is resulting in a 20%-30% probability for SHRA/TSRA developing in SW Oregon and moving northward through 00Z Thursday. Given that the convection is aloft, flight conditions will likely remain VFR with the potential for gusty and erratic winds near any thunderstorms. Timing will be a challenge, but current guidance has PROB30 TSRA starting for areas around KEUG starting around 00Z Wednesday, pushing northward to KSLE around 06Z Wednesday and reaching KPDX around 09Z Wednesday. Note that the TSRA potential will extend from the Coast to the Cascades. 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 aforementioned low will also deepen the marine layer and result in a robust SW surge of marine air pushing into inland areas. Expect IFR/MVFR conditions along with -DZ (55%-65% probability), which will spread northward along the coast behind the disturbance. MVFR stratus appears likely (45%-55% probability) to reach KEUG around 09Z Wednesday, with around a 20% probability of the MVFR conditions pushing into KSLE by 15Z Wednesday. PDX AND APPROACHES...Predominately VFR flight conditions. Increasing chance of high-based PROB30 TSRA starting around 09Z Wednesday. Elevated nature of the SHRA/TSRA should keep VFR flight conditions, but winds near any SHRA/TSRA will be gusty and erratic. /42 && .MARINE...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. Weagle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Heat Advisory until 8 AM PDT Wednesday for ORZ111-112-120-122. WA...Heat Advisory until 8 AM PDT Wednesday for WAZ206-207-209-210. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland