269 FXUS61 KBGM 240607 AFDBGM Area Forecast Discussion National Weather Service Binghamton NY 107 AM EST Thu Dec 24 2020 .SYNOPSIS... Rain develops this morning, becoming heavy at times this afternoon through tonight as a strong storm system brings much warmer temperatures and significant snow melt. Flooding will be a big concern for much of the region. Winds also get quite gusty, especially higher elevations of the Poconos and Catskills. Falling temperatures on Christmas may ice up roads. && .NEAR TERM /THROUGH TODAY/... 730 PM update... A quiet evening before the storm. Temperatures are holding between 34 and 39 degrees in most places, continuing a slow melt of the existing snow pack. Temperatures will hold steady for most of the night, then begin to rise between 1 AM and 4 AM. Rain will arrive in earnest across our western forecast area between 8 AM and 10 AM Thursday. Expect 0.10" to 0.20" of rain accumulation by midday. Previous discussion... Changes to the forecast include adding Oneida and Madison to the flood watch. Expected rainfall amounts have increased. Parts of these counties got the heavy snow from last week. Otherwise more certainty that flooding will happen late Thursday into Friday. Rainfall amounts have increased again as the models slow the timing of the rain exit with the strong cold front. By Thursday morning, a very amplified upper trough will be located over the Mississippi Valley, connecting a conveyer belt of anomalously high moisture levels coming up from the Gulf of Mexico, right across the Appalachians to the Northeast. Rain will start to develop in the morning, but then becomes heavy at times Thursday afternoon through Thursday night. Temperatures will also surge into the 50s with dewpoints following similarly as south-southeasterly winds increase across our region. This will cause snowmelt of most of the heavy snowpack that exists across the area. A strong low level jet of 60-70 knots will transport moisture to boost precipitable water values to around or slightly in excess of an inch, which is very high for late December. Surface winds will not be as strong as that due to low level stability, but the will still get quite stiff. Late Thursday into Thursday night, gusts could easily hit 30-40 mph in higher terrain portions of the area; and even 45 mph or so for the Poconos- Catskills. In addition to potentially causing scattered power outages, this wind driving warm and moist air into the snowpack will further enhance rapid melting. Seeing how this is a late second to third period borderline wind advisory will hold off on the issuance. Models have skewed just a few hours later with the eventual cold frontal passage of this system. That may not sound like much, but unfortunately this widens the window for significant snowmelt from high dewpoints driven by steady south-southeast winds, as well as prolonging rainfall a bit more. The reason for this delay, is a wave of deepening low pressure that forms in the southern Appalachians late Thursday, as the amplified upper trough to the west cuts off an upper low. This will hang up the front for a time, until the surface low passes directly through our region late Thursday night. Only after that, will the now negatively-tilting cold front go through the region, early Christmas morning. All told we are looking at a widespread 2 to 3 inches of rainfall, with localized maxima around 4 inches in highest terrain, especially Poconos-Catskills where topographic upslope enhancement will occur. This coinciding with most of the snowpack melting, and rapidly, points to a significant flood threat beyond just ponding of water. Flash flooding could occur in urban/low lying areas, especially where snow is blocking drains. Meanwhile, small streams and creeks will quickly respond. Rivers will take more time and will continue to rise through Friday. Many rivers in the Susquehanna and Delaware basins will get above flood stage and could hit major levels. Temperatures late Thursday night fall from west to east behind the front changing rain to snow. By 7 AM only Steuben County is below freezing. An inch or two of snow is possible there. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... 330 PM Update... Frontal rain will lift out Friday morning, to be replaced by snow showers but with light amounts. The bigger problem will any wet roadways freezing over, along with those light amounts of snow on top, to create potentially hazardous travel. Timing of the cold front looks to be Christmas morning, traversing quickly from west to east. By sunset on Christmas, temperatures will be in the lower 20s to near 30, and falling rapidly. Cold air continues to advect into the region with 850 mb temps down to around -13C. With Great Lake temperatures still in the 40s, some lake effect snow is likely going to develop, with Steuben getting some snow from Erie and then the NY Thruway corridor getting some snow off of Ontario. A more defined band of lake effect may develop off of Ontario, and slight shifts in the wind may bring that band into the southern Tug Hill in northern Oneida. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... 330 PM Update... An 850 mb ridge builds in Sunday and brings an end to the lake effect snow on Sunday with warming temperatures. Upper level ridge builds in late Sunday into early Monday with dry weather expected along with warmer temperatures. Highs on Monday will climb into the 30s to near 40. Conditions become more active late Monday as a cold front approaches from the west and ushers in a chance of rain and snow...changing to all snow by Monday night into Tuesday as another cold blast of air arrives, with possibly some lake effect snow through the day Tuesday. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... An incoming strong storm system is already resulting in widespread southerly low level wind shear, and will cause restrictions and rain to develop south to north between 10-16Z. Conditions further deteriorate for this afternoon through evening with moderate to heavy rain, on top of existing snowpack for most terminals, with resulting fog and low ceilings. There is some uncertainty as to whether terminals with thicker snowpack, especially KBGM, could get worse restrictions than already forecast - perhaps even to near airport minimums. Surface winds will generally be southeasterly 10-15 knots with gusts increasing to 22-30 knots. Low level wind shear meanwhile will reach 40-55 knots within the lowest 2 kft agl, with significantly stronger winds just above that. Outlook... Late Thursday night through Friday morning...Low level wind shear abates predawn Friday morning, but restrictions persist as strong system gradually lifts out of the area. Rain may briefly turn to snow before ending. Friday afternoon through Saturday night...Restrictions possible in scattered lake effect snow showers, especially KSYR-KRME. Sunday through Sunday night...Mainly VFR. Monday...Restrictions possible with a chance of mixed rain-snow. && .BGM WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch from noon EST today through Friday morning for PAZ038>040-043-044-047-048-072. NY...Flood Watch from noon EST today through Friday morning for NYZ009-022>025-036-037-044>046-055>057-062. && $$ SYNOPSIS...MDP NEAR TERM...DJP/TAC SHORT TERM...AJG/MDP LONG TERM...AJG/BJT AVIATION...MDP