Area Forecast Discussion
Issued by NWS

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KPBZ 280927 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 527 AM EDT Fri Jul 28 2017 .SYNOPSIS... Showers and thunderstorms will increase in coverage today, and continue through tonight and into Saturday. Heavy rainfall is possible, especially southeast of Pittsburgh. Dry and seasonable weather develops by Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --
Tweaked grids over the next few hours for current radar/hi-res model trends. Already seeing shower coverage increase over West Virginia as lift increases due to increasing upper divergence/jet support. This forcing will increase from south to north in our area this morning. Overall forecast appears on track. Previous discussion... Water vapor imagery shows an upper low over the UP of Michigan. Models continue to drop this into Ohio during the day. Also, an upper jet will nose into the lower Ohio Valley, and increasing left exit region divergence from this will promote increased lift, especially this afternoon. This will allow showers and eventually thunderstorms to bloom over the area, with the greatest concentration southeast of Pittsburgh in the area of greatest lift. It is also this afternoon that heavy rain begins to become a greater concern, although initially higher totals will be spotty due to the convective nature of the precipitation. Still, a general inch or even a bit more may be seen in the southeast counties by 00Z, with amounts decreasing to the north and west. While the upper low will promote cooling aloft which will aid instability, think that any severe risk will be limited to areas south of Pittsburgh that can manage to heat sufficiently. Cannot rule out a strong wind gust or two, but rainfall amounts will eventually be the bigger concern.
-- End Changed Discussion --
&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Interesting developments during this period that are more often seen during the winter season. As the upper low drops across western PA tonight and toward the Delmarva area Saturday, the system takes on a more neutral tilt with time. Very strong U-component anomalies, around 4 standard deviations on the NAEFS, will promote moist onshore flow into the ridges. There is also a clear trowal signal over PA late tonight and into Saturday in the 700-500 mb layer. This all points to an unusually strong summertime synoptic setup that could result in excessive rainfall over parts of the area, particularly the ridges. Have continued with the highest PoPs in this region, tapering off to the northwest. The system will only slowly pull away with time, with a few showers lingering into Saturday night southeast of Pittsburgh, before finally ending overnight. Targeting the axis of heaviest rainfall remains a bit problematic. The NAM keeps higher QPF run totals mostly east of the CWA, but this seems a bit optimistic given the strength of forcing over parts of the area. The GFS has a heavy axis from Indiana down to Garrett counties, with the ECMWF focused a bit more into northern West Virginia. Given the onshore flow and placement of best lift/forcing, going to lean away from the NAM and more towards the other models. The grids have 2 inch plus storm totals from southern Indiana County to down along and east of the Big 3 ridges. Amounts taper to the west in the mean, but localized higher totals could still result in flooding issues. Elected to stretch the Flash Flood Watch until 2 PM Saturday along the ridges for the extended threat of heavy rain. To the west, the watch will still expire at 1 AM Saturday for now. However, as the event unfolds, a shift in the axis of heavy rain may require further adjustments to the watch. Temperatures will be below normal due to the presence of the upper low. && .LONG TERM /SUNDAY THROUGH THURSDAY/... After this heavy rain event, a welcome dry period will follow from Sunday through at least Tuesday, as high pressure from the Great Lakes dominates. Upper troughing will help keep temperatures a bit below normal initially, but a warmup to seasonal level is expected by midweek as 500 mb heights start to rise. A frontal boundary is still projected to approach in the Wednesday/Thursday period, but timing it remains problematic. Kept the idea of low chance PoPs for now. && .AVIATION /10Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
Low-level cloud bank evident on experimental GOES-16 Nighttime Microphysics channel south of Pittsburgh is continuing to hold, mostly south of HLG. This will gradually ooze northward as the surface low begins to deepen. Elsewhere, bouncing ceilings and variable sky conditions has led to patchy fog development. The deepening system will gradually lead to an increase in rain showers through Friday, with the visibility restrictions in the heaviest activity. This will mostly be confined to areas south and east of Pittsburgh, but cannot be ruled out areawide. Depending on developing instability this afternoon, some thunderstorms could develop and impact terminals, as well. With the surface low nearly overhead, winds will remain generally light and variable through TAF period. .OUTLOOK... Restrictions will continue through Sat with slow-moving low pressure.
-- End Changed Discussion --
&& .PBZ WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
MD...Flash Flood Watch through Saturday afternoon for MDZ001. OH...None. PA...Flash Flood Watch through Saturday afternoon for PAZ023- 073>076. Flash Flood Watch through late tonight for PAZ014>016-020>022- 029-031. WV...Flash Flood Watch through Saturday afternoon for WVZ510>514. Flash Flood Watch through late tonight for WVZ001>004-012-021- 509.
-- End Changed Discussion --
&& $$

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.