Area Forecast Discussion
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000 FXUS61 KPBZ 111032 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 532 AM EST Sun Dec 11 2016 .SYNOPSIS... Snow will overspread the area through Sunday with many areas changing to rain or a wintry mix by the evening. Much colder air arrives late in the week. && .NEAR TERM /THROUGH MONDAY/...
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Light snow continues over the area this morning with reports of just over an inch around Zanesville having fallen as of 5 AM. Luckily enough, the warm front should be passing Zanesville within the next couple of hours, so the snow totals there look to be on target. Elsewhere, wet bulb effects have overcome modest surface warm advection as of this hour and been manifest as largely a degree or so of surface temperature falls amongst the observations that have started to snow in the past couple of hours. The multi- model consensus indicates decent moisture transport should increase through the next several hours above the boundary layer in the 285-295K layer with condensation pressure deficits crashing through the floor largely CWA-wide and especially north of I-68. As such, not only will snow develop, but it should become progressively more efficient as the early morning progresses. Snow to liquid ratios should quickly increase to 13-15 to 1 given a very impressive depth of saturation in the dendritic growth layer north of the advancing warm front. This should lead to several hours with deep layer isentropic ascent and strong ice crystal growth potential. While QPF from all models is fairly modest, efficient growth should allow most areas in the CWA north of I-70 to manage at least an inch through the morning. However, the cutoff in precipitation should be pretty stark south of I-68, so those areas may see little/no snowfall with the warm front. As the warm front rides northward during the daytime hours today, dry air in the boundary layer will accompany the northward migration of the pre-frontal isentropic ascent. Thus, most areas will simply stop snowing behind the warm front with a seemingly lower chance of an immediate changeover to rain compared to what the forecast implied this time yesterday. Once in the warm sector, PoPs were thus reduced decidedly generally south of Pittsburgh this afternoon as temperatures warm toward/above freezing in these areas. Warm frontal northward progression largely stalls across the northern periphery of the CWA tonight, however warm air aloft looks to be marginally sufficient to melt some/all hydrometeors for a period across the northern zones. The mention of freezing rain/sleet was thus maintained in the forecast. However, there is some trend toward suggesting the surface itself may challenge or rise above freezing toward Monday morning even there. Thus, accumulation expectations were tempered during this time frame, particularly over Armstrong and Indiana County. As the cold front begins to sweep eastward tonight, much stronger northward moisture transport moves over the area. This will bring showers back into the warm sector of the system and allow for rain and snow to again traverse the entirety of the CWA ahead of the cold front. Most areas from Pittsburgh southward stand little/no chance of accumulation due to warm boundary layer temperatures even if snow does fall. A different problem arises for the ridges, however. In the trapped valleys of the higher terrain, SE surface flow looks to persist in the pre-cold frontal environment tonight. This should hold surface cold air into especially Garrett County until the cold frontal passage and a turn of surface flow to the SW by 4 AM. Until that time, incoming pre-frontal showers will likely result in a period of freezing rain in Garrett County as well as the ridges of Fayette and Westmoreland County. A freezing rain advisory was thus issued for these areas for this. Even these areas, however, will transition to mostly plain rain by Monday morning as SW surface flow is efficient at eroding surface cold air even in Garrett County. Westerly flow follows the frontal passage on Monday with drier air in the column likely to shut off showers everywhere except the ridges quickly. Without much post-frontal cold advection and deep layer westerly flow, lake effect showers should not affect our area. Fries
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/... The cold front will be well to our east Monday night. Any remaining cold advection showers that exist will largely be confined to the ridges and north of I-80. These shouldn`t linger, however, as the post-front flow is westerly which is unfavorable for lake-effect or upslope production. Mid and upper-level moisture evacuates, leaving only saturation below about 900mb. This could leave patchy drizzle in addition to whatever light showers remain. Tuesday will begin dry as high pressure briefly settles into the region. By Tuesday evening, a relatively dry cold front will sweep through, bringing only a chance of snow, mainly north of Pittsburgh. This cold front will effectively open the door to the a cold continental air mass to stream in from the west-northwest. Snow showers will again ramp up downwind of the Great Lakes aggregate moisture plume Wednesday into Wednesday night as the primary upper trough axis sweeps through, bottoming out the mid- level temperatures. Temperatures will begin near averages, and then steadily fall through mid-week to well-below averages. && .LONG TERM /THURSDAY THROUGH SATURDAY/... The persistent upper trough and re-enforcement of cold air will continue the potential for lake-effect snow showers and below average temperatures through Friday. Thereafter, operational model guidance and ensembles continue to show a digging trough through the four corners region of the southwest CONUS. This will aid in building heights across our region, shutting down the lake- effect snow, and leading to a general warming trend. The digging trough will then eject from the Rockies and impact our area over next weekend. Utilized SuperBlend for the increase of likely PoPs Saturday into Sunday. Will continue to carry a relatively vague rain/snow mix through the warming transition, but will need to monitor the potential for icing as this system nears. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... Current VFR conditions will begin to deteriorate this morning as widespread light snow commences along the warm front. Reduction in visibilities and ceilings will spread northward, engulfing the area by about 12z. The southern edge will quickly lift northward later this morning, meaning southern ports of KMGW, and to a lesser extent KLBE/KHLG will see improvement by mid morning. Along the north, the snow will reside for most of the day, keeping low MVFR or IFR restrictions in through the period. Between these areas, some improvement from MVFR will be seen, but to what extent remains in question. Cold front approaches late Sunday bringing at least a mix of rain and snow. There is at least some potential for freezing rain along the leading edge of the rain/snow mix, but confidence is too low for TAF inclusion at this time. Best chances for freezing rain would be for KDUJ/KFKL 03-09Z Monday. .OUTLOOK... Restrictions are likely to continue through Monday, returning again Wednesday, with crossing areas of low pressure. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...Freezing Rain Advisory from 10 PM this evening to 4 AM EST Monday FOR MDZ001. OH...None. PA...Winter Weather Advisory until 4 AM EST Monday FOR PAZ007>009- 013>016-022-023. Freezing Rain Advisory from 10 PM this evening to 4 AM EST Monday FOR PAZ074-076. WV...None. && $$

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