Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 150703 AFDCTP Area Forecast Discussion National Weather Service State College PA 203 AM EST Sat Dec 15 2018 .SYNOPSIS... A slow moving storm system over the Gulf Coast states will track northeast to the Mid Atlantic region this weekend, bringing periods of rain tonight through Sunday. Cooler and drier conditions are expected Monday and Tuesday with lake effect snow showers and flurries over the west and north. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Visibilities across the southeast have continued to dip below 1SM at times this evening so extended patchy fog wording through the overnight hours. Rain continues to advance to the north. KCCX radar is undergoing ongoing maintenance for an issue that developed earlier this evening. We hope to have radar operation back to normal as soon as possible. Earlier discussion below. Northern edge of large baroclinic leaf associated with sprawling storm system over the lower Mississippi River Valley is overspreading the southern third of PA this afternoon. Areas of rain have lifted north of the Mason Dixon line and will increase in coverage through late afternoon. Steady rain not expected to arrive until evening over my southern tier, but will then spread across the southern third of central PA through late tonight. Sharp deep layer moisture gradient across central and northern PA means the northern edge of measurable rainfall will be the most problematic part of this forecast tonight through much of Saturday as surging and waning upper level forcing nudge or retract that line northward and southward. A compromise in solutions brings the steady rain to the central PA mountains close to State College late tonight through the pre dawn hours Saturday before being sheared off to the east as the first of several upper level waves moves off. Latest NAM12 actually shows drying extending into the southern tier for several hours from Saturday morning through Saturday afternoon, which means that a good part of Saturday could be just cloudy with patchy drizzle or spotty light rain showers. Much wetter GFS supports higher POPS but European supports the idea of a break between rainfall tonight into Saturday morning and another higher likelihood of measurable rainfall late Saturday afternoon and night. Have tried to reflect this in POP/wx grids overall. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... The southern stream closed low will slowly lift NE on Saturday, focusing best chances of rainfall along and south of the Mason Dixon line for most of the day as dry high pressure tracks across southeast Ontario into Southern Quebec Saturday afternoon and Saturday night. Next best chance of steady precipitation over central and southern PA arrives late Saturday afternoon and continues Saturday night. As noted above, much of Saturday may end up being overcast and drizzly across central and southern areas, while northern areas may even see some breaks as drier air entrains from the north under the influence of aforementioned SFC ridge over srn Ontario. PW gradient is extremely tight, with sub 0.25" across my northern tier ranging to over 1 inch over the southern tier Sat aftn and evening. Determining the northern extent of measurable rainfall during this time is the forecast challenge, and will continue to be the focus tonight and Saturday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... As a southern stream upper low begins to open up and move through the Mid-Atlantic for the latter part of the weekend, wet weather will continue. The best jet support and moisture inflow for steadier rain still looks to occur late Saturday night and early Sunday, particularly over south-central PA. Although rainfall of 0.75-1.5" may occur on a localized basis, such rainfall amounts will likely be spread out over a long enough period of time (generally 18-24 hours) to preclude any significant hydro issues. Across north-central PA, the two main issues later this weekend are how far north steadier precipitation gets, and also how much low-level cold air is able to bleed down into the region. At this time, we think that enough light rain and boundary layer cold advection will each prevail for potentially some light freezing rain late Saturday night into early Sunday morning. Thus, we continue to highlight this possibility in our Hazardous Weather Outlook. Sunday afternoon, steadier precipitation should taper off from west to east, as the aforementioned upper system and associated surface cyclone push well off the east coast. It remains questionable whether cold air on the back side of the low deepens quickly enough for a period of wet snow later Sunday over parts of Central PA, before precipitation tapers off/ends. Even if this were to occur, it would very likely be short- lived. Monday-Tuesday, it still appears that a drier, colder northwest flow regime will settle in. The best potential for lake enhanced snow showers in the mountains appears to be Monday into Monday night. Relatively shallow moisture should keep accumulations on the light side. As ridging briefly pushes in mid-week, Wednesday looks dry and seasonable. Late in the week, still plenty of uncertainty, as model divergence is large with respect to how short-wave energy progressing towards/through the eastern CONUS behaves. For now, we`ll indicate at least somewhat higher chances for light rain or snow for Central PA. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... Rain will continue across the southern airspace today with widespread MVFR/IFR. High pressure drifting eastward across southern Ontario into the St. Lawrence Valley will maintain dry wx over the northern airspace with MVFR to VFR conditions. This keeps continuity from the previous thinking that rain would be more suppressed and confined mainly to the southern 1/3 of the airspace later today. Precipitation will gradually spread northward later tonight into Sunday with an icy mix possible over the the northern tier. Expect precipitation to taper off as a rain/snow mix Sunday afternoon. .Outlook... Sun...Light rain/snow and MVFR/IFR cigs. Mon...MVFR cigs/snow showers north/west airspace. Gusty winds. Tue-Wed....No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...DeVoir/Martin NEAR TERM...Martin/Tyburski SHORT TERM...DeVoir/Martin LONG TERM... AVIATION...Steinbugl

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