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WHAT HAS CHANGED

* Impactful winter weather is now likely for all of central Pennsylvania. Confidence continues to increase with time. * Winter Storm Warnings have been issued for the Laurel Highlands and the Lower Susquehanna Valley where confidence is highest on significant snowfall amounts. * A new Winter Storm Watch has been issued for portions of central Pennsylvania where snow is likely but confidence is lower on total amounts.

KEY MESSAGES

1) Heavy snowfall over the Lower Susq Valley and Western Poconos is increasingly likely on Sunday and Monday.

2) A high degree of uncertainty remains with expected snowfall over the rest of central PA.

3) Ice jam risk continues into the weekend, but no significant flooding is expected.

DISCUSSION

KEY MESSAGE 1: Heavy snowfall over the Lower Susq Valley and Western Poconos is increasingly likely on Sunday and Monday.

Confidence continues to increase in an area of low pressure developing off the east coast on Sunday as a potent upper trough digs southeast from the Great Lakes region and amplifies across the Central Appalachians and Mid Atlantic Coast. The latest 12Z guidance continues to come into agreement on the position and strength of the coastal low along the New Jersey coast. This deep center of low pressure will bring ample moisture and cold into the region for winter storm conditions Sunday night into Monday.

This system will bring heavy snowfall into portions of the Lower Susquehanna Valley. The WPC Winter Storm Outlook continues to tick higher, with much of the eastern half of the forecast area now seeing a 50 to 80 percent chance of seeing snowfall exceed warning criteria. We have issued a Winter Storm Warning for portions of the Lower Susquehanna Valley where we currently have the highest confidence in seeing 6 to 10 inches of snowfall.

A prolonged period of upslope snow is also anticipated in the Laurel Highlands as flow behind the surface low becomes northwesterly. Most guidance continues to show a 48 hour period from early Sunday morning through early Tuesday morning where snow should fall, with 8 to 12 inches of snow expected along the ridges of Cambria and Somerset Counties. As a result, we have issued a Winter Storm Warning for those two counties as well. The enhanced wind gusts on the back side of this low pressure system could allow for gusts up to 40 mph in the Laurels, and brief periods of blizzard like conditions are not out of the question.

KEY MESSAGE 2: A high degree of uncertainty remains with expected snowfall over the rest of central PA.

Significant uncertainty still remains regarding snowfall over the rest of Central PA, with the trickiest part of the forecast stems from the potential for quasi-stationary north/south band of heavy snow (lasting several hours or more) along the axis of an inverted llvl trough and leading edge of the lower/deeper DGZ that will be situated from the Scent Mtns to somewhere between KUNV/KSEG and KIPT later Sunday afternoon/Sunday night.

A couple of things to keep in mind regarding this snowfall forecast: * There is still some uncertainty regarding the eventual track of the low. A more westerly track will favor heavier snowfall over parts of Central PA while an easterly track may shift the heaviest snow to the east of our area. * As the low quickly deepens off the east coast, expect areas of enhanced low to mid level frontogenesis and UVVEL to develop in the western periphery of the system. Much of this banding will likely set up east of the CWA, but could still develop across parts of our central and eastern zones. * Multiple models continue to show a snow band associated with an area of low level convergence within an inverted trough extending northwestward from the coastal low. While this type of band is more likely to set up somewhere in central PA, it will likely be narrow with a widely varying snowfall totals on either side of the band.

KEY MESSAGE 3: Ice jam risk continues into the weekend, but no significant flooding expected.

Cooler air moves in this evening as winds become westerly, allowing for a few snow showers over the north and west.

The ice jam threat continues into the weekend as daytime highs in the 30s and 40s will support additional ice breakup. The threat for significant flooding is low as we are not anticipating any significant rainfall. Flooding would likely be localized and minor.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

Widespread VFR conditions outside of BFD where the low-level cloud deck has continued to allow for MVFR conditions. Current satellite is showing some erosion of the low-level deck; however, an approaching area of low-to-mid level clouds are expected to gradually swing over BFD and could allow for a continuation of MVFR ceilings based on a combination of HREF and GLAMP model guidance. This low-to-mid level deck will continue pushing eastward throughout the area this evening and into the overnight hours.

Rapid and widespread deterioration is expected overnight and into Sunday morning (between 06Z-14Z Sunday) with snowfall expected to gradually overspread the area in this timeframe based on all model guidance. The biggest uncertainty will actually remain at BFD, where recent HREF model guidance is tending to hold mentions slightly back; however, given the atmospheric set-up have leaned slightly towards NBM/GLAMP model guidance for onset timing of snow at BFD. Snow will remain light throughout much of the 00Z TAF period; however, IFR conditions are being progged by the bulk of model guidance, and see no reason to stray from this at this juncture. Locally heavier snowfall will be possible across southern terminals (JST/AOO/MDT/LNS) towards the end of the TAF package with enhanced forcing, thus some potential for LIFR conditions between 20Z Sun and 00Z Mon with low-to-moderate (30-50%) confidence. The bulk of restrictions will be based on visibility based on recent GLAMP/NBM model guidance; however, will highlight that low ceilings (between 500-1000ft AGL) are also expected across much of central Pennsylvania as snowfall begins across central Pennsylvania.

Winds will remain light throughout a majority of the TAF package; however, a slight increase is progged by recent GLAMP/NBM model guidance, thus have started to increase winds between 18Z Sun and 00Z Mon with high (~70-80%) confidence this cycle. Best signals for gusty winds come after 00Z Monday, thus will need to continue to monitor trends in upcoming model guidance for gusty winds.

Outlook...

Mon...Widespread IFR restrictions in snow. Gusty winds late Monday.

Tue...MVFR-IFR restrictions possible in snow showers at BFD/JST, largely VFR elsewhere.

Wed-Thu...Scattered rain and snow showers, restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES

Winter Storm Watch from 5 AM EST Sunday through Monday afternoon for PAZ017>019-025>028-034>036-042-049>053. Winter Storm Warning from 1 AM Sunday to 1 AM EST Tuesday for PAZ024-033. Winter Storm Warning from 5 AM Sunday to 1 PM EST Monday for PAZ056>059-063>066.