Area Forecast Discussion
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000 FXUS61 KPBZ 230907 AFDPBZ Area Forecast Discussion National Weather Service Pittsburgh PA 407 AM EST Mon Jan 23 2017 .SYNOPSIS...
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Low pressure will spread rain over most areas, with some snow as well in some locations this afternoon into Tuesday. The next cold front arrives Wednesday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Still an average to below average confidence in the forecast. Models are fairly similar with the track of the upper low over the next 24 hours, taking the center across the Carolinas and off of the Delmarva by 12Z Tuesday. Categorical PoPs are still appropriate with the main deformation band crossing during the day providing plenty of ascent, with ample moisture in the form of 0.75-0.9 PWAT values. Main questions center on precip type and snow amounts. Still expect a mix with snow this afternoon and tonight, with a full changeover possible tonight north of Pittsburgh and along the ridges. The timing of that change and resulting snow amounts will be tricky. NAM is still the coldest in the column this afternoon and tonight, quite likely due to an enhanced dynamic cooling effect as compared to the GFS, which continues to show this to a lesser degree. A scenario more like the NAM could result in a quicker changeover and higher snow accumulations than currently depicted, and could in theory require a snow advisory issuance in the far northeast counties and down the ridges. However, even if dynamic cooling results in a burst of snow this afternoon/evening, the warm boundary layer and ground may help to cut down any accumulations. By the time the boundary layer cools tonight, precipitation will be on a downward trend as deformation lifts north and a more showery regime takes hold in developing N/NW flow. For now, will continue with relatively low accumulations along the ridges, topping out in the 1-2 inch range. Will continue the HWO mention due to the uncertainty, and cannot rule out an eventual snow advisory issuance. Will also monitor winds, as there are indications in model soundings that mountain wave activity could lead to enhanced downslope gusts west of the ridges, particularly in Westmoreland and Fayette counties. For now, thinking ridgetop inversion may not be strong enough to allow this to occur, but will continue to watch. Temperatures will likely slowly fall through the day as colder air wraps in. Took a model blend for low temps tonight.
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&& .SHORT TERM /TUESDAY THROUGH THURSDAY/...
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The vertically-stacked low pressure system will advance up the East Coast Tuesday, loosening it`s grip on the Mid-Atlantic and Upper Ohio Valley. As a result, deep moisture will erode eastward, leading to a general decrease in shower activity through midday Tuesday. Any lingering showers that do remain will be light, transitioning from snow or a rain/snow back to all liquid as temperatures rise. We`ll see a brief lull in precipitation Tuesday afternoon through at least early Wednesday with a transient high pressure. The next low will move through the western Great Lakes Wednesday evening, draping a relatively dry cold front through the region. Will keep only chance PoPs along the front as the low nears. The better chance of precipitation areawide will come Wednesday night stretching into Thursday with the passing upper trough axis, as cooler air pours into our mid-levels. Steepening lapse rates and injection of Great Lake aggregate moisture will allow for rain showers to eventually transition to at least a rain/snow mix. Temperatures will remain well-above average in through the middle part of the week.
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&& .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
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A conglomeration of upper lows will lead to broad troughing of the upper-level flow through at least the weekend. This will bring the return to more seasonal conditions. Several impulses passing through the mean flow, will result in periodic snow chances. Some accumulation potential exists through the weekend, especially north and into the high elevations. While the Great Lakes are still generally ice-free, surface temperatures of Lake Erie, in particular, have dipped into the mid 30s. This will help limit any lake enhancement as colder air arrives.
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&& .AVIATION /09Z MONDAY THROUGH FRIDAY/... Flight conditions will continue to deteriorate through the night through MVFR, to IFR areawide for most of Monday. Upper level low pressure tracking to our southeast will spread a band of rain over the area for much of the day. Snow may begin to mix in north of Pittsburgh and along the ridges during the late afternoon/evening hours. Easterly winds will increase with time, and gust to between 25 and 30 knots during the day. .OUTLOOK... Restrictions are likely through Tuesday as low pressure moves off the coast, and again late Wed through Fri with a crossing cold front and subsequent upper troughing. && .PBZ WATCHES/WARNINGS/ADVISORIES... MD...None. OH...None. PA...None. WV...None. && $$

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