087 FXUS61 KBGM 241955 AFDBGM Area Forecast Discussion National Weather Service Binghamton NY 255 PM EST Thu Dec 24 2020 .SYNOPSIS... Rain becomes moderate to heavy this afternoon and continues 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 and a chance of freezing rain on Christmas may ice up roads. && .NEAR TERM /THROUGH FRIDAY NIGHT/... 12 pm update... Just minor changes again to the hourly temperatures and dewpoints. 930 am update... Rain is on schedule as it moves in this morning. Temperatures have risen into the 40s with some 50s already so upped hourly and max temperatures. Forecast is in good shape for the rest of today. 645 AM Update... Just adding an extra word to stress, that this is no ordinary Flood Watch; flooding is not just possible but indeed expected, and it will likely be quite significant for much of the area - both the initial flash flood/small stream phase late today into tonight, and the main stem river phase late tonight through Friday. Some of the signals that we look for in the models for flood potential, such as mean integrated water vapor transport, precipitable water, and southerly component of 850 mb winds; are either off-the-charts or nearly so for this time of year. This is a rare set up. This would be a problem even in the absence of snow cover, but unfortunately was also have plenty of water that will be released from deep snowpack in the warm moist and windy conditions later today through tonight - especially the Susquehanna and Upper Delaware basins. Anyone living in a flood- prone area should have a plan and know what to do in the event high water threatens. Previous discussion... High impact storm system is now encroaching the region, and unfortunately this will be a rather memorable one. The main reasons for the confidence in significant flooding are as follows. The amount of anticipated rainfall; a widespread 1.5 to 2.5 inches with higher terrain reaching 3 to 4 inches. Also, dewpoints that surge to mid 40s- lower 50s with stiff south- southeasterly winds, which will cause quick melting of the snowpack that exists for a large chunk of the area. The timing of heaviest rainfall appears likely to coincide with the rapid running off of snowmelt. Finally, a strong wave of low pressure forms along the front, hanging up its progress while prolonging the window of both melting and the moderate to heavy rainfall. A very amplified upper trough extends the length of 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 develops this morning, then becomes heavy at times this afternoon through tonight. Rain will linger a bit into Friday morning east of Interstate 81. Temperatures will also surge into upper 40s-mid 50s this afternoon through much of the night with dewpoints following similarly. This type of air mass, with winds forcing it into the snowpack, will cause snowmelt of most if not all of the heavy snowpack that exists across the area. It should be noted, that while snow depth has markedly decreased across the area since the big winter storm last week; it has really only settled and compressed. Most of the water content is still there; for example at NWS Binghamton the depth is now only 19 inches - now under half what it was - yet it contains slightly more than 3 inches of water. There is a lot of water ready to be released from the dense snowpack, especially for the Susquehanna Basin and Upper Delaware Basin. Confidence is now very high that precipitable water values will exceed an inch courtesy of a strong low level jet of 60-80 knots transporting moisture to very high levels for late December. 850mb winds and precipitable water levels get to a foreboding 5 standard deviations above normal. Surface winds will not be as strong as that due to low level stability, but the will still get quite stiff. Aloft however, some instability is now apparent; actually to the point that some embedded thunder could occur tonight in the heaviest waves of rain. Tonight, gusts will easily hit 25-35 mph for our central to eastern zones, and even 40-50 mph gusts for higher terrain of Poconos-Catskills. A Wind Advisory has thus been hoisted in coordination with WFOs Mount Holly, Upton, and Albany for Luzerne-Lackawanna-Wayne-Pike-Sullivan Counties. In addition to causing scattered power outages, this wind driving warm and moist air into the snowpack will only further enhance rapid melting. All told we are looking at a widespread 1.5-2.5 inches of rainfall, with localized maxima of 3-4 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. At least some flash flooding at this point is expected to 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. We are anticipating that many river forecast points in the Susquehanna and Delaware basins will surge above flood stage and a few could hit major levels. Temperatures late tonight through Friday will fall from west to east behind the front changing lingering rain to snow. However, with shallow cold air undercutting still relatively mild air aloft for a time, freezing rain/sleet could even occur for a brief window during the changeover. So in addition to wet roadways possibly freezing over on Christmas Day as temperatures plummet, we may actually get some light icing via freezing rain as well especially in higher terrain. This threat for hazardous icing over of remaining wet roads could reach Steuben-Yates counties prior dawn Christmas morning; then extending to about the I-81 corridor by late morning-early afternoon; and finally east of I-81 in afternoon-early evening. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... 300 pm update... The main concern on Saturday will be lake effect snow as winds shift more westerly and the band sags into northern Oneida county. Dry air intruding into the boundary layer will weaken the snow band as it moves south so only a few inches of snow is expected in the Camden and Boonville. A few flurries will be able to make it as far south as the Finger Lakes as well. High pressure building in cuts off the threat of snow showers late Saturday into Sunday. SW flow aloft brings in warmer air for Sunday with highs getting back above freezing. An area of low pressure develops near the Great Lakes with clouds moving back in on Sunday. PoPs increase Sunday night as the low moves into the St Lawrence River valley with us in the warm sector. 850 temperatures are currently expected to be right around 0C and surface temperatures around freezing so a wintery mix is expected for any precipitation that moves in by Monday morning. && .LONG TERM /MONDAY THROUGH WEDNESDAY/... 300 am update... A weak ridge of high pressure at the surface will be tracking east through the mid Atlantic/Northeast Sunday night as the next storm system prepares to cut through the Great Lakes Monday/Mon night into srn Quebec. A trailing cold front will sweep across NY/PA late Monday into early Tuesday with a mix of rain and/or snow Monday afternoon as temperatures warm into the mid to upper 30s...changing to all snow by Monday night as temperatures drop back into the teens overnight. Behind this cold upper trough another blast of Canadian air will drop 850mb temperatures into the -14 to -17 deg C range and trigger another round of lake effect snow southeast of Lake Ontario. At this time the set up for LES looks very favorable, but the window of opportunity looks relatively small. Will need to keep an eye on this Tuesday time frame for the potential of significant accumulating lake snows. It appears the lake snow will taper off quickly late Tuesday night as the ridge builds in from the west and temperatures warm back slightly above freezing going into Wednesday. An upper level ridge axis will move in from the west by the middle of the week and supply enough large scale suppression, combined with the drying air mass, to keep weather conditions quiet. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... Conditions at all sites will fall to IFR vsbys and fuel alternate MVFR cigs by 20z. Conditions worsen to IFR cigs in heavy rain tonight. Some improvement around 15z behind a strong cold front as cigs return to MVFR. The weather will be a mix of rain and snow showers, and fog then. Low level wind shear has set up as 2k foot winds are south at 50 kts. This will continue until around 09z with the front. Surface winds are southeast to south at 10 to 15 kts with gusts into the 20s except ELM. ELM will remain around 10 kts with no gusts. Starting around 09z winds shift to the southwest and west at 5 to 10 kts behind the cold front. Outlook... Friday afternoon...Restrictions persist as strong system gradually lifts out of the area. Precipitation will change to all snow before ending. Friday night 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. Monday night through Tuesday...Restrictions possible in snow. && .BGM WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch through Friday morning for PAZ038>040-043-044-047- 048-072. Wind Advisory from 6 PM this evening to 7 AM EST Friday for PAZ040-044-047-048-072. NY...Flood Watch through Friday morning for NYZ009-022>025-036-037- 044>046-055>057-062. Wind Advisory from 6 PM this evening to 7 AM EST Friday for NYZ062. && $$ SYNOPSIS...MDP/TAC NEAR TERM...MDP/TAC SHORT TERM...AJG LONG TERM...BJT AVIATION...TAC