Scientific Forecaster Discussion
000 FXUS61 KPBZ 251905 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 305 PM EDT Thu May 25 2017 .SYNOPSIS... Passing low pressure will provide rain chances into Friday morning. Approaching low pressure will keep precipitation chances in the forecast for the holiday weekend, with Sunday projected to be wetter than Memorial Day. && .NEAR TERM /THROUGH FRIDAY/... Vertically stacked low will swing across the region tonight. More organized moisture and lift has moved north of the region, but steep low-level lapse rates have started to produce popup showers especially close to the low. Expect this trend to continue through the afternoon and into the evening, with likely PoPs appropriate. with one or two thunderstorms possible in the generally low instability. Region is still scraped along the ridges by a marginal risk of severe thunderstorms according to SPC. Think that this threat is minimal given the marginal instability and expected weakening shear profiles. PoPs will slowly decrease through the night with the loss of instability, although the presence of the low and a shot of lift will keep showers going. An overnight low level jet may produce some 25-30 MPH gusts along the ridges overnight, with lesser values elsewhere. The ongoing mixing will keep overnight temperatures a few degrees above climatology. Showers should pull east with the low by Friday afternoon, giving our region a brief dry interlude, although some clouds should linger. Warmer temperatures are expected with a bit more sun, with near-normal values in southeast Ohio but still below- normal to the east. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... PoPs will increase again starting Friday night in increasing isentropic lift ahead of an advancing warm front. Model agreement on precipitation placement appears to be improving, but intensity is still a bit in question. Will continue with previous idea of a north/south gradient in rain chances, highest close to the front, with better thunder chances there as well. The front will likely become quasi-stationary across West Virginia. Passage of a mid-level wave will keep precip chances going through Saturday, before tapering a bit Saturday evening. At this time, it still appears that the best severe risk will lie to our south, in better instability and shear south of the stalled boundary, but this will need to be monitored for any model variances. Approaching surface low pressure will push a cold front towards the region for Sunday, with an attendant increase in showers and thunderstorms. Kept likely PoPs going. Severe risk will likely be mitigated by the presence of morning clouds/showers limiting instability. Temperatures will rise to near or even slightly above normal levels by the end of this period. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... A fairly broad Great Lakes/northeast U.S. trough will be in place through this period. Shortwaves rotating through the trough will keep periodic shower chances in the forecast each day, with a somewhat diurnal pattern to the rise and fall of the numerical values. Near to slightly below normal temperatures can be expected in this pattern. A transition to a more zonal regime may begin by the end of the period, with lessening rain chances. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... Upper low continues to sit and spin across eastern Ohio. A wave of showers has moved through the forecast area this morning, and will rotate through the area once again this evening into the early morning hours. Some scattered showers will remain in between the waves, and cannot rule out a chance of an isolated thunderstorm, although confidence/coverage is low. In between the waves, many locations from Pittsburgh to the south and east should have at least a few hours of VFR ceilings, although ZZV/BVI/FKL/DUJ are expected to remain close enough to the bands of steadier rainfall for ceilings to remain IFR/MVFR. All terminals will drop to IFR, if not LIFR ceilings overnight in steadier rain, and low clouds will continue well into Friday. .Outlook... Showers continue through much of the period with episodic restrictions. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None.