Area Forecast Discussion
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000 FXUS61 KPBZ 242243 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 643 PM EDT Wed May 24 2017 .SYNOPSIS...
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An active weather pattern will persist through the weekend. Warmer air and higher humidity are expected to return for the Memorial Day weekend.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Minor changes to refine forecast coverage of showers and adjust hourly temperatures this evening. Otherwise the ongoing forecast is on track. Previous discussion follows.. Temperature in the low-mid 60s in s-central PA, where rain limited diurnal temperature increase, will climb slowly for the remainder of the day as cloud cover thickens in advance of an upper low approaching from the Plains. Elsewhere, temperature in the mid-upper 60s will persist through early evening. A shortwave trough in advance of the upper low will lead to widespread rain late this afternoon through tonight as abundant moisture aloft is advected nwd. Given the current presentation on radar upstream and the 18Z ILN sounding, model forecast soundings depicting modest elevated instability overnight do not seem far-fetched. Therefore, embedded convective elements may generate moderate rainfall overnight, and the current forecast increases QPF a bit over the previous forecast for tonight and includes chances for thunderstorms during the late afternoon and evening for at least the southern part of the forecast area. With the expected widespread rain and cloud cover, temperature will remain rather steady tonight once the wet bulb effect sets in. Uniform minima in the mid-50s will be common in most areas, with slightly-cooler overnight temps in the mountains. One other concern is the possibility of a mountain wave as sely low-level flow strengthens beneath a strong temperature inversion. Hi-res models do depict increasing wind speed in the ridges in the 0000-0600 UTC timeframe, often indicative of a mountain wave. Pattern recognition in the soundings suggests there may be a brief window for such a wave, but confidence is too low to warrant a wind headline. The evening shift will monitor this possibility.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/... Widespread morning rain will subside early in the day as a closed upper low deepens in the Ohio Valley and a dry slot asserts itself. As the moist boundary layer in the wake of the morning rain has a brief window to heat, cold air will be advected aloft, leading to steepening mid-level lapse rates and increasing instability despite only a minimal change in surface temperature. Convection-allowing models suggest that showers and storms will be generated quickly in this increasingly-unstable airmass, with a cluster of storms expected to fill in the dry slot by mid- afternoon Thu. Instability is not expected to be sufficient for organized severe weather, but moderate rain rates will be possible in any storms. By Friday, the upper low will have shifted toward New England with northwesterly flow in control and drier air arriving. This respite will be rather short-lived, however, as the H5 ridge axis will shift through the area by Saturday with deep swly flow becoming established. This shift will allow increasingly- humid and warmer air to arrive for the weekend. However, it will also bring an increasing chance for showers and thunderstorms. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... The pattern going into next week looks to continue to amplify as an upper low again drops into the Mississippi Valley with a downstream ridge popping up rapidly along the eastern seaboard. The Ohio Valley will be stuck between these two areas, with broad southwesterly flow remaining in place and abundant moisture streaming toward the area. As a result, chances for convection will remain in the forecast with temperature near normal. Of interest in the long term is that synoptic models are offering hints at a series of MCSs developing in the Plains and moving ewd toward the Ohio Valley along the nrn edge of the instability axis. These MCSs could impact the region Saturday and Sunday during a busy holiday weekend, so anyone with outdoor plans should monitor forecasts toward the end of the week. && .AVIATION /22Z WEDNESDAY THROUGH MONDAY/... Limited rain showers moving though the area early this afternoon. Mostly VFR conditions with isolated MVFR conditions as low-level moisture increases with a crossing shortwave. VCSH in the TAFs with sct shower coverage this afternoon. Improvement back to VFR all TAFs is expected by late afternoon before deterioration back to MVFR in overnight hours as approaching low pressure arrives. Increasing SE winds are expected today with an increasing pressure gradient, with a few 20-25kt gusts especially for the higher-terrain ports. .Outlook... Areas of non-VFR through Thursday in showers and possible thunderstorms and again for the weekend. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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