Area Forecast Discussion
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000 FXUS61 KPBZ 231741 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 141 PM EDT Mon Oct 23 2017 .SYNOPSIS... A cold front will return rain to the area through tonight. Low pressure will maintain shower chances and cool temperatures through mid week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Update to depict current light rain across the Pittsburgh metro area. Rain will spread nwd ahead of an advancing cold front and will increase in coverage through the day. Previous discussion follows... Complex system will bringing rain and wind to the area today. Deepening 500mb low will move from the Lower Mississippi valley north-northeastward toward the Ohio Valley by this afternoon. Strong shortwave energy will rotate around the upper low as it moves toward the region. At the surface, low pressure will develop on the northern flank of the cutoff system this afternoon, and its accompanying surface front, will stretch from the Upper Midwest low pressure, southward, all the way into the Gulf of Mexico. As the entire system deepens, wind fields throughout the atmosphere will strengthen, further enhancing the system. Strong winds aloft will pull moisture northward from the Gulf, while low-level winds, on the eastern side of the system, will tap into Atlantic moisture. This process will enhance rainfall coverage and intensity ahead of the boundary as it swings eastward. The main rain-shield will begin to overspread my western counties early this afternoon, with full coverage of the region expected by early evening. Scattered showers could develop ahead of the main area of rain under height falls and low-level moisture convergence, but this should remain light. Rain could become heavy at times, directly ahead the surface front, as the increasing large scale ascension and low-level convergence interact with the substantial atmospheric moisture. Ballpark model timing would place the risk for heavier rain over Ohio/Northern WV late this afternoon and evening. Tightening surface pressure gradient this afternoon, and lowering mixing heights, will increase surface winds today and promote stronger gusts, with the highest over the eastern ridges. Models still not in real good agreement on gust strength in the lower elevations, however occasional gusts to 40mph are possible in the ridges. Temperatures will remain warm today, with readings 5 to 10 degrees above normal. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/... 500mb low will interact with strong northern stream energy diving southward from the northern Plains into the Midwest tonight. This phasing will further deepen the surface low, which by this time, will be moving rapidly northward over the Western Great Lakes. The upper level phasing and strong wind field aloft will increase the forward speed of the surface front. This will mean the threat for heavy rain continues just ahead of the boundary, but it will not last long as it streaks east- northeastward. The heaviest rain should exit the region shortly before midnight. By late tonight the northern energy will take over, as it absorbs what is left of the southern low. As this process ensues, a large dry slot will develop ahead of the main upper low, decreasing rainfall coverage into Tuesday. The area will remain in the dry slot through Tuesday morning, until a second surface front/shortwave trough rolls through Tuesday afternoon. The risk for scattered showers will return and this will be the initial push of the much colder air. A second dry slot develops Tuesday night, so would expect most of the region to remain dry. However, the strong cold air advection will be under way. The main trough axis looks to slowly cross the region on Wednesday, continuing the cold air advection and returning the risk for scattered showers. High temperatures on Wednesday will be 10 to 15 degrees below normal. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... The chance for showers will come to an end late Wednesday night as the upper low moves east. Thursday and Friday should remain cool and dry. While the ECMWF and GFS have general agreement of low pressure dragging a cold front to the area for the weekend, the models have shown little consistency in the speed of the front and when any precipitation would arrive. Have stuck close to the Superblend for this portion of the forecast. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... VFR conditions will drop to MVFR during the afternoon and evening hours as a cold front moves through the region. While wind gusts were expected to have developed by 18Z, since they have not, decided to start the TAFs with a period of low level wind shear as both speed and directional shear are present. Along the cold front itself, isolated IFR conditions will be possible along with gusts up to 30 kt. The bulk of the rain should have moved east by the early morning hours on Tuesday, and low ceilings will remain. Some gradual improvement back to VFR should happen by the afternoon. .Outlook... Periodic restrictions will continue into early Thursday with the region under upper troughing. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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