Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 200745 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 245 AM EST Tue Feb 20 2018 .SYNOPSIS... Unseasonably warm, near record setting temperatures are expected across the area through Wednesday along with periods of showers or light rain. A cold front will swing south through the area Wednesday evening with cooler temperatures in its wake for late week. An active weather pattern continues into the upcoming weekend with renewed chances for light rains and snows. && .NEAR TERM /THROUGH WEDNESDAY/... As of 245 AM EST Tuesday...The overall forecast picture remains largely unchanged over the next 42 hours as a dominant +590 dm subtropical ridge centered between Bermuda and the northern Bahamas controls the deep-layer synoptic flow across the eastern third of the nation. Deep southwesterly flow aloft will advect an unseasonably mild airmass into the region beginning today and persisting into early Wednesday afternoon. Numerous showers with periods of light rain will continue across central and northern counties today, lifting slowly north to the intl. border and northward tonight into Wednesday morning as a strong warm front lifts through the area. Back trajectory analysis at 500m suggests source air this afternoon originated across the northern Gulf Coast States yesterday, with tomorrow`s air wrapping around the ridge from the subtropical Atlantic near Grand Bahama Island. With mean 925 mb temperatures surging above +10C a period of extreme warmth by February standards will evolve during the period. Highs today should mainly top out in the 50s, with a few spot 60F readings in the southern St Lawrence/Champlain Valleys and a few upper 40s in far northeastern VT and near KMSS. Wednesday will be a record- setting day as 925 mb temperatures surge into the +12 to +15C range, pushing highs into the 60s to locally near 70F. If KBTV can surpass 62F (I currently have a high of 69F) it will be the second warmest February temperature on record, surpassed only by the 72F set just last year! I wonder what the statistical odds of that happening in the Queen City`s 134 year period of record? As the temperatures warm a surge of higher dewpoint air, (upper 40s to lower 50s) now poised off to our southwest across western NY/PA will advect northeastward into our area fostering a period of rapid snowmelt and robust river rises. By later Wednesday most, if not all snow cover below 2000 feet should be essentially erased leading to heightened concerns over ice breakup on area rivers. Given the broad consensus in the guidance the Flood Watch remains in place with this package (see hydro discussion below). By Wednesday afternoon the surface boundary will swing southward through the area as a cold front. The baroclinic zone with this feature is quite sharp, so hourly temperature drops could be quite noticeable within the hour of its passage (> 10F). Scattered to numerous showers will drop south into the bulk of the forecast area during this time, through trend more scattered in nature as one progresses toward early evening winds veer to northwesterly. && .SHORT TERM /WEDNESDAY NIGHT/... As of 500 PM EST Monday...Lots of interesting aspects to the forecast from Tuesday through Wednesday night. I`ll try to touch on all of them in this discussion. Very warm system impacting the region with a surface front stationary to our Northwest just across the International border. This will be the focus for several waves of energy and precipitation over the 48 hour forecast period. Record breaking maximum temperatures are forecast for Tuesday and Wednesday. Dewpoints will also be surging into the 50s. Temperatures and dewpoints result in thawing degree hours off the charts, we expect much of the snow to melt across the region. On top of this, a quarter of an inch to an inch and three quarters of rain is forecasted, highest in the Saint Lawrence valley and lowest in Southeastern Vermont. Losing much of our snowpack will release 3 to 7 inches of snow water equivalent into the river basins. The rainfall and snowmelt together have led to high confidence that we will have flooding across much of the area. The northern Adirondacks and Northern portion of the Vermont, as well as the Saint Lawrence valley have the highest chances for flooding. Also, any locations that still have ice jams in place from the January event will be especially susceptible to flooding. Temperatures on Tuesday, firmly in the warm sector, will surge into the 50s, dewpoints will be approaching the 50s as well. Unprecedented wet system with pwats surging to about 1.4 which is also near record breaking for Feb. Thawing degree hours have already started to accumulate, and will continue until sometime Wed night. Thawing degree hours reach 850-1200 which is huge, passing the 300 mark sometime Tuesday afternoon across our Western zones, and Tuesday night across the entire area. This is when I`d expect problems to start cropping up on the rivers as far as ice movement goes. Once again temperatures will not fall too far Tuesday night, lows in the upper 40s to lower 50s. Max temperatures on Wednesday will be even warmer, widespread 60s. Cold front will finally start moving and cross our area Wednesday night, by the time the cold front moves across the region, precipitation should have ended therefore not expecting much snow on the backside of this system. There may be a brief break in the precipitation Wednesday morning as the front briefly drifts even further north from the border. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 405 PM EST Monday...Cooler but still warmer than normal conditions will prevail through early next week. The region will also remain in an active pattern as fast zonal flow dominates aloft. High pressure will keep the weather dry Thursday and Thursday night. Then the first in a series of low pressure systems will bring a mix of rain and snow to the region Friday into Friday night, followed by another later Saturday into Saturday night. The third rain/snow system arrives Sunday into Monday. Although there are still differences in the tracks of each of these lows, overall it looks like temperatures will be cold enough for snow at night, while daytime highs will allow most areas to change over to rain. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... Through 06Z Wednesday...Latest Radar imagery shows rain filling in across much of the forecast area with downslope winds keeping KBTV VFR while the remaining TAF sites are a mix of IFR/MVFR. Conditions will worsen through the overnight hours with the New York TAF sites becoming solidly IFR through much of the daytime tomorrow while northern and central Vermont remain largely MFR. Heavier rain showers in the 14Z to 18Z time range may drop KBTV/KPBG/KMPV briefly down to IFR ceilings and/or vis. Much of the rainfall will move north into Canada by sunset and a gradual improvement in conditions is expected through the late afternoon and early evening hours but MVFR ceilings will likely persist. A strong low level jet will tail the warm front as it moves northward with most TAF sites seeing 35 to 40 kts of wind shear from surface to 2kft tomorrow afternoon and likely lingering through Wednesday morning. Outlook... Wednesday: Mainly VFR, with areas MVFR possible. Chance SHRA. Wednesday Night: VFR. Slight chance SHRA. Thursday: VFR. NO SIG WX. Thursday Night: VFR. NO SIG WX. Friday: VFR. Likely RA, Chance SN. Friday Night: Mainly MVFR, with areas VFR possible. Chance SN. Saturday: Mainly MVFR, with areas IFR possible. Chance RA, Chance SHRA, Chance SN. && .HYDROLOGY... As of 245 AM EST Tuesday...A 42-hour period of anomalously warm temperatures will affect the region through Wednesday afternoon. Periods of rainfall, modest winds and dewpoint temperatures climbing into the 40s to lower 50s will foster rapid snowmelt and ice break-up across the entire area. Also boosting confidence are extremely high thawing degree hour totals - averaging 900 to 1200 over the period. The model consensus shows rainfall totals averaging from 0.5 to 1.50 inches across far northern VT into the northern Adirondacks and St Lawrence Valley with the highest totals in the latter location. Lesser amounts generally under a half an inch are expected across central and southern VT. Taking this all into account, modest to substantial river rises look highly probable starting this afternoon and continuing into Wednesday, supported by NAEFS/SREF MMEFS hydrograph data. While widespread open water flooding is not expected, several rivers may approach minor flood including the Ausable R. at Au Sable Forks, NY, Missisquoi R. at North Troy, VT and the Winooski R. at Essex Junction, VT. More importantly, numerous ice jams remain in place from the substantial thaw this past January so the potential for localized high water and/or flooding near these features remain a real threat as ice break- up occurs. && .CLIMATE... Here are the current record high temperatures for February 20th and 21st: Record High Temperatures: ............Feb 20th....Feb 21st.... BTV.............58 (1981)...59 (1981)... MPV.............56 (1994)...57 (1953)... MSS.............63 (1994)...62 (1953)... St. Johnsbury...60 (1981)...62 (1981)... Record High-Minimum Temperatures: ............Feb 20th....Feb 21st.... BTV.............50 (1981)...49 (1981)... MPV.............47 (1981)...47 (1981)... MSS.............47 (1994)...41 (1981)... St. Johnsbury...40 (1981)...46 (1981)... && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Flood Watch through Wednesday evening for VTZ001>012-016>019. NY...Flood Watch through Wednesday evening for NYZ026>031-034-035- 087. && $$ SYNOPSIS...JMG NEAR TERM...JMG SHORT TERM...Neiles LONG TERM...Hastings AVIATION...Clay HYDROLOGY...TEAM BTV CLIMATE...TEAM BTV

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