Area Forecast Discussion
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000 FXUS61 KPBZ 220045 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 845 PM EDT Fri Jul 21 2017 .SYNOPSIS... Yet another muggy night tonight keeping those air conditioners running. Saturday could be active with more than one round of storms moving through the region. Sunday, pop up storms forecast, along with very warm and humid weather. Cooler and less humid early next week. && .NEAR TERM /THROUGH SATURDAY/...
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Some lower dewpoints have managed to sneak into the northern CWA this evening, where values have fallen off into the 60s, which has actually been enough to minimize instability. Farther south, while dewpoints are higher, earlier convection and convective debris have managed to suppress temperatures enough such that instability is fairly measure. In between however, there exists a chasm between two easily-identifiable boundaries on radar imagery where low level convergence is maximized, and thus convective initiation may be fostered. While we are getting well beyond the convective maximum of the diurnal cycle, ample low level instability combined with low level convergence may be enough to spark off a shower or storm down the heart of the CWA for the next few hours. With abundant low level moisture in place, dying cumulus, and only modest incoming upper level convective blow off, fog development should be fairly efficient overnight. Areas of fog were added to the grids ahead of the next incoming wave, which should arrive by morning, at which time fog will likely mix our rapidly. Fries All signs are pointing to an active Saturday with more than one round of severe storms. MCS development over the northern plains this afternoon will be moving into western Ohio around daybreak. There should be enough destabilization through the morning once any morning fog burns off. H7-H5 lapse rates approach 6.5ckm-1 while MUcapes push 2.0kjkg-1. Wind shear is impressive as well with nearly 40 knots of shear. All of this yields to a damaging wind threat with a line of storms from late morning in eastern Ohio to early afternoon in western Pennsylvania and northern West Virginia. Atmosphere keeps its tropical characteristics with PWATS around two inches and warm cloud depths nearing 13KFT. This will lead to heavy downpours with any storm. the concern for flash flooding is not high with the initial line of convection, however given there may be around round of weather /albeit maybe not severe/ in the evening, if repeated areas get hit then high water is a concern. Daytime highs are tricky with precipitation forecast to occur during peak heating. It would not be surprising to see some places stay in the lower 80s if convection moves through midday and residual cloud cover holds until mid and late afternoon. Overall temperatures were a blend of the bias corrected all and raw blend.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... Line of showers and storms could be ongoing through locations mainly south of I-70 in the evening. With dewpoints still around 70F atmosphere remains uncapped through the overnight hours. This combined with another embedded wave traversing in the mean flow and residual outflow boundaries will warrant the need to keep pops in the forecast for the overnight. A severe weather threat remains a concern with the round of weather coming through in the evening given decent upper level support and atmosphere favorable for upstream support. Sunday confidence is lower in organized convection moving through the region however we still remain in northwest flow with disturbances riding through. BUFR soundings show main threat being damaging winds given surface delta theta e values nearing -30 around 10-15K feet supporting wet microbursts. Still hard to pinpoint an exact time but at this juncture will go with afternoon and evening, Thermodynamic instability is impressive over far sw PA, northern WV and eastern OH, however shear is not with 20 kts of mean flow. Locations north US 422 may be spared Sunday per less instability. Flash flooding remains a threat, but as stated earlier 20 kts of mean flow will be keep storms moving, we just have to watch out for areas that get hit Saturday so training does not become an issue. Activity wanes Sunday night with the loss of daytime heating, but again expect a muggy night with temps well into the 60s and lower 70s. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Highlights: - Lower humidity - Active weather regime end of next week returns Mid level trough sweeps through Monday afternoon so this will be the last round of showers / storms before a welcome break occurs. Given time of occurrence and associated steeper lapse rates with the cooler air aloft, there is a low risk of severe weather during peak heating hours. H5 heights build Tuesday into Wednesday offering a reprieve from daily chance of storms. After Tuesday being the coolest day of the week, temperatures ramp back up towards normal values in the lower 80s. A two standard deviation below normal trough establishes across eastern Canada Wednesday night through the balance of the week. This will return passing shortwave troughs permitting the return of precipitation under northwest flow. Much the case for this week, thunderstorm complexes could traverse the region given we are on the southern periphery of the trough, however confidence not high enough to warrant anything greater than climo pops. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... Generally VFR this evening, although a few storms may pop along the old, lingering frontal boundary that should avoid most terminals. Tonight, due to lingering moisture and recent rainfall, especially south of PIT, expect areas of fog to develop with MVFR/IFR restrictions. A few terminals may even get down to LIFR, but with clouds expected to increase during the predawn hours, did not feel confident enough to go that low. Fog may lift a little earlier than normal if mixing can get going ahead of the next shortwave. After 13Z or so, one or two round of storms are possible. Handled with VCTS for much of the daylight hours for now, pending improved timing in future issuances. Gusty winds and hail will be possible with any storm. .OUTLOOK... Periodic restrictions are expected through the weekend in morning fog and daily rounds of thunderstorms. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.