Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 210549 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 1249 AM EST Wed Feb 21 2018 .SYNOPSIS... Record breaking warm temperatures expected overnight into Wednesday which will cause significant snow melt and sharp rises on local streams and rivers. Areas of ice jam and open river flooding is possible overnight into Wednesday with scattered rain showers. A cold front will quickly drop temperatures back into the 20s and 30s by Wednesday night with drier conditions expected on Thursday. && .NEAR TERM /UNTIL 11 AM THIS MORNING/...
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As of 1238 AM EST Wednesday...Current forecast remains on track with temperatures steadily climbing across the North Country. Valley locations within the Green Mountains remain decoupled and fog has developed with increasing moisture while the temperature have been a tad slower to respond. Previous Discussion...Forecast remains on track late this evening. The main concern continues to be hydro and potential for ice jam and open water flooding tonight into Weds from snow melt caused by record warm temperatures. Record high of 59F was set at BTV around 10PM. Meanwhile, still 35F at MVL and VSF with colder air locked in the narrower valleys east of the Green Mtns. Have updated temperatures, and also added patchy fog areawide based on localized reports (in areas of light winds over melting snowpack and 2-m dewpoints near 50F). All the convergence has lifted north of the intl border, and with anticyclonically curved flow in the mid-upr troposphere, not expecting much in the way of additional precipitation overnight...just a chance for a few spot showers, mainly across the St. Lawrence Valley. Water vapor shows 2 to 3 std above normal height ridge across the eastern conus with pw values between 1.0 and 1.4, which is 4 to 5 std above normal. In addition, southwest 925mb to 850mb jet of 30 to 40 knots is helping advect very warm temps into our region with 925/850mb values btwn 9-14c. This has already produced temperatures of 57f at Whiteface base and 50F atop Mt Mansfield, which has caused some melting of snow pack, per web cams. Expect additional significant snow melt overnight into Weds as temps remain steady at mid/upper elevations in the upper 40s to mid/upper 50s. In the valleys expect a wide range in temps overnight with l/m 50s cpv/western dacks to upper 30s deeper valleys of central/eastern VT, including the NEK. Meanwhile...a sfc warm front is located near the international border, which will serve as a focal for additional rain showers this evening...before best 1000 to 500mb rh and 5h vorticity in the southwest flow aloft pushes north of our cwa. Additional rainfall amounts will be light and generally between 0.10 and 0.20 from MSS to PBG to CDA. Highest pops will be across the northern tier...with just schc over Rutland/Windsor Counties. The combination of very warm temps and high sfc dwpts in the mid 40s to lower 50s advecting over cold snow pack...expect areas of fog to develop, especially eastern/central VT. Winds/mixing should limit fog development over the cpv and parts of the SLV/dacks overnight. Weds...very warm day expected as 925mb temps reach 13-14c across central/southern VT by 18z Weds...supporting record breaking warm temps of mid 60s to lower 70s. Elsewhere...the approaching sfc cold front and associated ribbon of mid level moisture and sharp thermal gradient highs start in the mid 50s to mid 60s...but drop back into the upper 30s and mid 40s by evening over northern NY. NAM/GFS continue to show ribbon of 850 to 500mb moisture...good sfc convergence...and some 5h energy to help enhance a 1 to 3 hour window of rain showers on Weds aftn. Additional qpf with boundary will range from 0.05 to 0.20...in the heavier convective elements. Temps drop 15 to 25 degrees in several hours behind the boundary on Weds evening.
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&& .SHORT TERM /11 AM THIS MORNING THROUGH THURSDAY NIGHT/... As of 351 PM EST Tuesday...Forecast area is under modest low level cold air advection on brisk northwest winds of 10 to 20 knots. By 06z...925mb to 850mb temps range btwn -5c to -8c with lows generally in the upper teens to mid 20s. The cwa will be between building 1044mb high pres to the north and leftover moisture/energy along the boundary across mid Atlantic/SNE. Have continued to mention schc/chc pops south for Thurs Morning with thermal profiles supporting a mix of rain/snow. Any qpf/snow would be light...as best moisture/dynamics still to our south closer to the boundary. Highs near normal for Thursday with mid 20s to mid 30s. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 437 PM EST Tuesday...A deep subtropical high over Florida will continue to push warmer air northwards and lead to seasonably warm temperatures through the weekend. Expect highs in the upper 30s to 40s with lows overnight in the low 30s. We will still be in a fast flow regime with a shortwave moving in late Friday and a stronger system moving in late over the weekend. Both systems will follow a similar track up over the Great Lakes going west and north of the North Country. This means we`ll be in the warm sector with rain and higher winds. The winds should be stronger on Sunday as we see some downsloping developing. Anticipate 20-25kts with gusts of 25-35kts along the western slopes of the Greens and the northern slopes of the Adirondacks. Based on trends in guidance it does appear that pattern change heading into next week but that remains to be seen how it plays out for the North Country. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
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Through 06Z Thursday...Conditions at KMSS are finally improving as all signs point to winds shifting to the SW and finally mixing out the low stratus and fog that has been observed for the greater portion of the past 24 hours. Otherwise, VFR and MVFR conditions prevail across the TAF sites low-level wind shear at all sites except KBTV as they continue to hold onto a 12 to 15 kt wind. Frontal Passage will be observed this afternoon with a brief period of IFR conditions expected across KSLK and KMSS ahead of frontal passage with brief MVFR conditions elsewhere. Following the frontal passage, VFR conditions will prevail and the low-level wind shear will come to an end. Outlook... Thursday: VFR. Slight chance SHSN. Thursday Night: VFR. Chance SHSN. Friday: Mainly VFR, with local MVFR possible. Chance SHRA, Chance SHSN. Friday Night: Mainly MVFR, with areas IFR possible. Chance SHSN, Chance SHRA. Saturday: Mainly MVFR, with local IFR possible. Slight chance SHRA, Slight chance SHSN. Saturday Night: Mainly VFR, with local MVFR possible. Slight chance SHSN. Sunday: Mainly MVFR and IFR, with local VFR possible. Likely SHRA, Likely SHSN.
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&& .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...
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VT...Flood Watch through this evening for VTZ001>012-016>019. NY...Flood Watch through this evening for NYZ026>031-034-035-087.
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&& $$ SYNOPSIS...Taber NEAR TERM...Clay SHORT TERM...Taber LONG TERM...Deal AVIATION...Clay HYDROLOGY...TEAM BTV CLIMATE...TEAM BTV

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