Area Forecast Discussion
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000 FXUS61 KPBZ 271540 AAB AFDPBZ Area Forecast Discussion...UPDATED National Weather Service Pittsburgh PA 1140 AM EDT Thu Jul 27 2017 .SYNOPSIS... Showers and thunderstorms return to the forecast today, with rain chances continuing into Saturday. Heavy rainfall is possible, especially southeast of Pittsburgh. Dry and seasonable weather develops by Sunday. && .NEAR TERM /THROUGH TONIGHT/...
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A low-amplitude shortwave trough associated with ongoing morning convection in swrn Ohio will move ewd today across the Ohio Valley. Quality low-level moisture and decent lapse rates through 500mb are present across the CWA, but, until recently, the region has been veiled by ci. In the last hour, pockets of insolation have become more prevalent. With the potential for ample insolation ahead of ongoing convection, there is little reason to expect the storm potential to diminish through the day ahead of the shortwave trough. PoPs have been focused highest especially across the srn third of the forecast area, where warm frontogenesis is expected to occur in association with the approaching wave, with a sharp gradient nwd from the boundary. It is entirely possible that very little convection occurs in the far-nrn zones, if the hi-res models are to be believed. But, given the ambient conditions and potential for heating, the forecast will carry a slight chance of precipitation in this area. Conditions will favor storms capable of moderate to locally- heavy rain given the considerable instability below the freezing level of 13.1 kft and its attendant warm-rain processes. The potential for wet updrafts and modest shear suggest the possibility of downbursts from any vigorous storms, but it is unclear if storms will achieve such vigor given the weaker shear and warm air aloft above 500mb. As the afternoon shortwave trough exits ewd and diurnal heating wanes, a relative decrease in precipitation is expected this evening. But a more-potent upper low will dig sewd overnight from the Great Lakes region, leading to a sfc cyclone, sharpening frontal boundary and increase in warm air advection into the frontal zone. Showers and storms capable of very heavy rain are expected to begin in earnest late tonight especially to the south and east of Pittsburgh in the Mon Basin.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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The upper low will continue to dig sewd on Fri and strengthen through this timeframe while embarking on its slow trip across wrn PA Friday night en route to the Delmarva. A surface low will also strengthen off the Mid-Atlantic coast in response to its approach. The departure of the low continues to trend slower on the models as its strength increases and becomes better resolved, with impacts continuing to linger into Sat at least as vorticity maxima pivot around the parent low. Strong 850-700 mb frontogenesis across southern PA may couple with moist easterly flow to support heavy rain. Nearly- saturated low levels and a deep warm-cloud layer also support the heavy rain potential. Localized rain amounts in excess of two inches are possible, which could promote flooding. A flood watch of some sort will be considered on this shift, as the geographic details become clearer. Likely PoPs continue across the region through Fri evening as the upper low crosses, with a slow taper toward the southeast during the night. Little to no severe risk exists during this period. The main concern likely will be heavy to excessive rainfall, especially southeast of Pittsburgh. In response to the slower model trends with the system`s departure, the decrease in PoPs was slowed through Sat evening, with all precipitation exiting the area late Sat night.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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Great Lakes high pressure should provide mostly-dry weather during this period. The trend with the mid-week front appears slower, and have stuck with a dry forecast for Wednesday for now. Temperatures will be fairly seasonable overall.
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&& .AVIATION /15Z THURSDAY THROUGH MONDAY/... After patchy morning MVFR fog dissipates, mainly VFR conditions are expected today. Shower and thunderstorm chances are expected to increase this afternoon, mainly S of I 70 with the approach of a cold front. Maintained VCTS mentions for ZZV and MGW, with some TEMPO restrictions at ZZV where tstm occurrence is more probable. Precip should diminish this evening with exiting shortwave support, though increasing low level moisture should result in MVFR cigs for OH ports overnight. .OUTLOOK... Restrictions are likely Fri and Sat with slow moving low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$ Update...Kramar

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