Area Forecast Discussion
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182 FXUS61 KPBZ 230748 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 348 AM EDT Fri Jun 23 2017 .SYNOPSIS...
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Downpours are possible through tonight with a tropical airmass in place. A cold front sweeping through early Saturday will return comfortable humidity levels by the weekend.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Rain showers associated with an upper-level shortwave are moving through northern WV and southwestern PA. These light to moderate showers will slide northeast this morning following the shortwave, bolstered by mid-level speed convergence and upper-level divergence. Despite any evidence of embedded thunderstorms, rain rates may increase with this activity in the pre-dawn hours as it lifts northward. This is due to an increasing 850mb jet that will continue to feed moisture into this already moisture-laden atmosphere. This rain will pre- condition much of the Flash Flood Watch area with around a half an inch of rain, before the heavier rain chances arrive later today. The already hard hit areas of Armstrong and Indiana Counties will also see this rain before daybreak. With many locations still impacted by the earlier flash flooding and many roads still closed, an Areal Flood Warning will carry through tonight. The hours just after daybreak will feature a break in the widespread rain as the first wave moves east. At this point the remnants of T.S. Cindy will be slowly moving through the lower Ohio Valley and the cold front descending through the southern Great Lakes. Showers and thunderstorms are expected Friday afternoon/evening everywhere as the converging systems begin to phase near the local forecast area. Strong ascent will meet a moisture-rich environment to the tune of 2" or greater PWAT values. Increasing 850mb flow into deep warm cloud depths nearing 13kft will be plenty sufficient to support tropical rain rates...causing very heavy rainfall at times. To prepare for this, the local WSR-88D has been modified to a tropical Z/R relationship. High surface dewpoints will continue to fuel modest instability with SB CAPE values progged in the 1000-1500 J/kg range. This, along with moderate shear through the mid-levels should be sufficient for thunderstorm development with a few strong to severe possible. Freezing levels will be lofty, so the primary threat will remain convective wind damage, but some veering in the lowest 200mb and relatively low LCLs reveals that an isolated tornado cannot be ruled out. The forecast area is currently blanketed by a marginal risk from SPC, and a small area of slight risk extending into our southern counties. Possibility of severe weather would likely expand if more clearing across our southern counties is achieved. With very high dewpoints in place and expansive cloud deck expected, temperatures won`t range wildly over the next 24 hours. Highs today will be near or a few degrees below average, but the muggy conditions will prevail.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
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Instability will lessen heading into the evening which will decrease the severe threat, however, another period of moderate rain still looks likely Pittsburgh along and ahead of the aforementioned cold front and under the right-rear quadrant of the jet. By Saturday morning precipitation should be east of the ridges as drier air moves in behind the front. Saturday should be noticeably more comfortable with highs in the upper 70s and lower humidity. Benign weather can be expected Saturday night with high pressure.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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Sfc high pressure will remain largely in control on Sunday, however, a shortwave passing to the north could result in scattered precipitation along and north of I-80 during the afternoon. A stronger shortwave will deepen the eastern CONUS longwave trough on Monday...maintaining slightly below normal temperatures and return chances of precipitation during the afternoon through Tuesday morning. Upper ridging will follow... bringing temperatures back to their normal location and returning dry weather.
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&& .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Most ports begin the 06z TAF period with VFR conditions, but these should deteriorate rather quickly. An upper-level shortwave trough will spread light to moderate rain north- northeast in the pre-dawn hours, lowering ceilings to low MVFR and perhaps IFR. Keeping most ports in low MVFR through daybreak, with gradual lifting through the afternoon. Restrictions will likely continue through Friday as the remnants of Tropical Storm Cindy merge with an approaching cold front. Showers, thunderstorms, and low ceilings will be common Friday afternoon through the evening. Wind will remain WSW through the period, with gusts expected Friday afternoon. .OUTLOOK... Widespread restrictions are likely through Saturday morning with the passage of a cold front. && .HYDROLOGY...
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A flash flood watch has been issued for areas along and south of interstate 70. In addition, Allegheny, Armstrong and Indiana counties have been included, due to very low flash flood guidance. Meteorologically values all point to high water issues with PWATS over two inches, warm cloud depths above 13KFT, and a strong low level jet. GEFS M-Climate values are near or at record levels for PWATS on return interval, and climate anomaly. Widespread 1.5" to 2.5" through tonight could lead to strong rises on the Monongahela, Cheat and Ohio Rivers and their tributaries. Slightly less is expected north of the Watch area, but the Allegheny basin could still see a widespread 1" to 2".
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&& .PBZ WATCHES/WARNINGS/ADVISORIES... MD...Flash Flood Watch through late tonight for MDZ001. OH...Flash Flood Watch through late tonight for OHZ057>059-068-069. PA...Flash Flood Watch through late tonight for PAZ021>023-029-031- 073>076. WV...Flash Flood Watch through late tonight for WVZ002>004-012-021- 509>514. && $$

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