Area Forecast Discussion
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000 FXUS61 KPBZ 180113 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 913 PM EDT Wed May 17 2017 .SYNOPSIS... Dry and warm weather will continue through Thursday morning. An approaching front brings shower and thunderstorm chances Thursday afternoon and into Friday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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As the upper ridge gives way to more broad southwesterly flow aloft this evening, convective blow off and debris clouds will continue to episodically overspread the area. This will mean a mostly cloudy overnight will be in the cards, though it may clear out in interludes between cloud islands. That said, fairly consistent mixing will be likely as a pressure gradient driven southwesterly flow will be unlikely to allow the surface to decouple. Combining this with clouds moving overhead will yield temperatures well above normal overnight. Fries
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/... The upper low across the central Plains today will lift northeastward into the western Great Lakes by Thursday and then shear off across southern Canada thereafter. For us, the main weather-maker during this period will be a frontal boundary which sags into the region starting late Thursday night. With the best synoptic support staying to the north and west with the shortwave, it still appears that precipitation associated with the boundary will be fairly scattered, and thus will continue to top out with high chance PoPs for the passage. Also, there will not be a strong push to the boundary given the shearing wave. The models still differ a bit on how far south the boundary gets before stalling, but for now will continue to expect it to be to our south at 12Z Saturday. Regarding severe risk tomorrow...the main issue will be the timing of the trigger. Still expect some capping to be present around midday, which should be alleviated somewhat later on by cooling aloft. Model soundings do have 1000-1500 J/kg of CAPE during the afternoon and around 40 knots of shear along with decent low and mid-level lapse rates, suggesting the risk of a few severe storms. However, given the early capping, it may take the approaching frontal boundary to kick off convection to any meaningful degree. Given the late timing of its approach, the front will have to work with whatever instability remains across the northwest during the late afternoon/evening. The new SPC Day 2 severe outlook maintains a slight risk across the far northwest corner of the CWA, with marginal risk northwest of a ZZV-PIT-DUJ line. Think that this is a reasonable depiction. Above-normal temperatures will continue through the period, although Friday will be noticeably cooler and less humid. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Upper ridging makes a comeback over the middle Ohio Valley on Saturday, with the ridge axis pulling east by evening. Still some question on if and how far the front lifts back northward during the day. Going to maintain some low PoPs across the area, but much, if not most, of the area could remain dry during the day. The ridge continues east on Sunday, allowing for better moisture to arrive in southwest flow ahead of the next front. Model timing may be a touch faster than previous, with Sunday afternoon and night looking like the period with the most widespread rainfall. Frontal passage on Monday will lead to dry weather and seasonable temperatures into Tuesday. Another system may quickly follow for Wednesday. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... VFR is expected through the TAF period. An approaching low level jet should result in an overnight/early morning LLWS potential for ports across wrn PA and OH. Gusty SW winds are expected after sunrise with mixing. Capping warmth aloft should preclude TS most of the day Thu, though a weak pre frontal wave and cap erosion could result in a few aftn TS. Included a brief VCTS mention for this, though the best chance of any precip is expected with a cold front after the TAF period. .Outlook... Restrictions are possible Thu night/erly Fri with a cold front, and again late Sat thru Mon with another front. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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