Area Forecast Discussion
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000 FXUS61 KPBZ 231525 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 1125 AM EDT Fri Jun 23 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Warm rain process in full effect this morning. One area of rain is almost through with a break then another round this afternoon. Our radar is now in a tropical Z/R relationship. No changes to the forecast as its in good shape with radar trends. Previous discussion follows: Rain showers associated with an upper-level shortwave are moving through eastern OH 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. This rain will continue to pre-condition much of the Flash Flood Watch area with a quarter to half an inch of rain, before the heavier rain chances arrive later today. With many locations in Indiana and Armstrong Counties still impacted by yesterday`s flash flooding including many road closures, an Areal Flood Warning will be in effect through tonight. This morning will feature a break in the widespread rain as the initial 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. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... 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. && .LONG TERM /SUNDAY THROUGH THURSDAY/... 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. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... Tropical moisture will continue to stream northward ahead of an approaching cold front this afternoon. Interaction with the aforementioned front and digging shortwave will support periods of rain and an increasing chance for thunderstorms through the afternoon. Confidence in widespread MVFR and brief IFR visibility restrictions is high for the remainder of the day in periods of heavy rain, with the expectation that IFR cigs will lift over most terminals this afternoon. Some thunderstorms may be severe, with strengthening shear/jet support, which could result in strong/severe gusts at terminals this afternoon. Restrictions will likely continue tonight as the remnants of Cindy pass along the PA/WV border. Cigs may fall back to IFR after sunset with some fog possible with saturated conditions. But, improvement is expected by Saturday morning as surface ridging builds overhead. WSW wind will remain gusty through the afternoon and again tomorrow. .OUTLOOK... Upper trough may bring occasional restrictions Sun and Mon in afternoon showers/storms. && .HYDROLOGY... 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". && .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. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.