Area Forecast Discussion
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241 FXUS61 KPBZ 291236 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 836 AM EDT Thu Jun 29 2017 .SYNOPSIS... Temperatures will continue to warm into the weekend, and humidity levels will also increase. This will return the risk for showers and storms, mainly Friday and Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Morning update to adjust sky and temperatures to match current trends. The cool and dry morning will give way to a quick warm up and increasing dewpoints. A shortwave trough will stream through the southern Great Lakes this morning, reaching NW PA by this afternoon. This will flatten out the current ridge and could generate some showers and thunderstorms across the northern 1/3 of the forecast area. Model solutions vary widely in terms of southern extent of showers/thunderstorms this afternoon, or whether they`ll exist at all. Current RAP analysis seems very reasonable with skinny SB CAPE values nearing 1000J/kg along the north, given current surface dewpoint forecast. This could develop in spite of warm mid-level temperatures. Model difference with temperatures aloft become important this afternoon. The NAM wants to cutoff the warming at 700mb this afternoon, while the GFS does not. Thus, the GFS shows the cap holding through the day, while the NAM dissolves it. So will find a compromise between the models and focus on the area of shortwave induced ascent. With strong low-level lapse rates and increasing surface instability, will include chance PoPs over areas north of PIT this afternoon. Speed shear does increase this afternoon, especially over northern counties, so its not out of the question that updrafts could intensify and allow for convection to become more organized. Another point to keep in mind, if any convection does intensify, there will also be outflow boundaries. These boundaries would lead to further development, which could expand the risk for storms. This will need to be watched as the day progresses. Expect to see gusty south-southwesterly winds develop late this morning and through the afternoon due to a strengthening wind profile aloft and rising mixing heights. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... Any convection will dissipate tonight with sunset and the absence of large scale ascent. Weak riding aloft is expected as well. Models are trying to squeeze out some QPF tonight, likely due to the passing shortwave trough and brief lowering of 500mb heights, but at this point, not sure if that is realistic. Temperatures aloft begin to warm again with the passage of the wave and deep moisture is not available. For now will keep most of the tonight period dry, but this will need to be reevaluated in future model runs. Friday looks similar to today, but with even greater warming aloft and rising heights in the morning. This would mean the atmosphere would again be capped. Surface instability will be increasing due to a rise in surface dewpoints, but again, this is dependent on which model hits those numbers correctly. If anything, Friday looks like more popup summer time convection, so will just go with low chance PoPs for the entire area. Temps Friday will be above normal and humidity levels will be much more noticeable. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... The Midwestern upper trough is progged to advance E Saturday driving a cold front across the area. Dry weather is expected to return Sun/Mon as surface ridging builds underneath the upper trough. Another trough is progged to advance E from the Midwest and Plains regions Tue, though with uncertainty in timing and location limited POPs to the chance category for now on the 4th of July. && .AVIATION /13Z THURSDAY THROUGH MONDAY/... Mainly VFR conditions are expected through the TAF period. Mid/high clouds should continue streaming E from weakening thunderstorms to the W. CU rule/model soundings show SCT to BKN CU by late morning, though bases should still be VFR. A shortwave trough is progged to move across Srn Ontario/NW PA by afternoon with sct showers/tstms expected. Mentioned a VCTS for FKL/DUJ, though capping warmth aloft should limit tstm potential elsewhere. A tightening pressure gradient and mixing should also result in gusty SW winds today. .OUTLOOK... Restriction chances will increase Fri and Sat with the approach and passage of a cold front. && .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.