Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 250739 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 339 AM EDT Fri May 25 2018 .SYNOPSIS...
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Summerlike weather arrives today as high pressure brings warm temperatures to the area. A cold front will approach from the north tonight, and linger across the region over the weekend. As a result, expect an increase in cloud cover, cooler temperatures and chance of showers and thunderstorms through the period. A few of the storms could produce locally heavy downpours.
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&& .NEAR TERM /THROUGH SATURDAY/...
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As of 339 AM EDT Friday...Warm temperatures remain in the cards for today as a westerly surge of continental air has arrived overnight on the nose of robust 50+ kt mid-level jet (Whiteface Summit winds of 50G59kt as of 330 am). High temperatures should top out in the upper 70s to lower 80s area wide with perhaps a few mid- 80s in favored downslope areas of the Champlain and CT River Valleys. There will also be a broad increase in columnar moisture by this afternoon with model- averaged PWAT values increasing to near 1.5 inches by early evening. On the nose of this higher moisture variable mid- level cloud cover should advect across central and northern counties through the day and especially by early evening. Only limited dynamical support for lift exists however, so outside a very brief light shower or sprinkle in these northern areas mainly dry weather is expected through late afternoon. With the boundary layer deepening into the 850-825 mb level this afternoon gusty winds also look like a certainty with west to southwesterly peak gusts topping out in the 20-30 mph range in the noon to 500 pm time frame. By this evening a more concerted push of moisture and lower end mid level instability advect into the region in advance of a backdoor cold front dropping south/southwestward from southern Quebec. I`ve largely ignored the swaths of excessive QPF offered by some of the 00Z hi-res NAM/WRF output given only marginal lifting mechanisms, though weak bundles of shortwave energy traversing the area in the west to northwesterly flow aloft should be sufficient to produce scattered showers across the northern tier of counties accordingly. An isolated storm also can`t be ruled out but coverage of storms should be on the low side. Mainly dry weather continues across southern counties. Low temperatures a bias-corrected consensus blend of guidance with values generally ranging from the mid 50s to lower 60s in milder valley locales. The start of an active weekend of weather then begins on Saturday as a fairly sharp backdoor cold front is forced south/southwest into the area by expansive high pressure bridging east across central Quebec. Moisture pooling to the south of the boundary along with CAPE values from 250-750 J/Kg should be sufficent to generate numerous showers and a band of embedded thunderstorms by afternoon. Some variability exists within this morning`s models on the exact zone of convective development, though a consensus solution would suggest areas from the Adirondacks east through central and southern VT have the highest probabilities where brief heavier downpours will be possible. The thunder threat across the northern tier isn`t non- zero either, but our current forecast of a morning frontal passage suggests an earlier arrival of a cooler, more stable airmass under increasingly cloudy skies. Maximum temperatures will also support this timing with another day of upper 70s/lower 80s across the south, and upper 60s to lower 70s across the northern tier.
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&& .SHORT TERM /SATURDAY NIGHT/... As of 338 PM EDT Thursday...Cold front continues to sag slowly southward during the day on Saturday and weakens with time. Nevertheless it will be a focus for convection Saturday afternoon as instability develops along and south of the boundary and precipitable water values increase. Best chance for showers and possibly some thunderstorms will be across northern New York and the central and southern sections of Vermont. Saturday night the front washes out and the flow turns more west and southwest. Thus any lingering showers should move back east and northeast before weakening as it encounters more stable air...which is in response to a developing maritime flow as high pressure builds down and creates a more easterly component to the low level flow. Should not see much in the way of precipitation overnight...but would expect plenty of clouds and maybe some fog. Lows Saturday night should be in the upper 40s to upper 50s. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 338 PM EDT Thursday...Complex and dynamic upper level and surface pattern prevails across our forecast on Sunday into Monday with backdoor front draped over our cwa. This front will result in a sharp west to east thermal/instability axis and make for a challenging pops forecast for Sunday into Monday. NAM/GFS/ECMWF show best combination of instability, moisture, and lift/convergence across northern NY/SLV therefore have mention likely pops with thunder in this region and just chc of showers cpv/central and eastern VT zones. Have noted pws btwn 1.50 to 1.75", CAPE values of 1000 to 1500 j/kg, and good se to sw convergence across the slv/western dacks where boundary becomes stationary. While the shear is limited with 0 to 6 km values of 25 to 30 knots, thinking localized very heavy rainfall is possible across the SLV/Western Dacks on Sunday aftn/evening, given the cape/pw profiles, along with vector analysis showing some potential for training/back building storms. Sharp qpf/pop gradient anticipated with much drier/stable maritime airmass from CPV eastward. Progged 850/925mb temps with boundary overhead support highs lower 80s west to upper 60s to mid 70s east...with 75f at BTV. On Monday...surface boundary dissipates across our cwa as surface to 925mb flow becomes southerly and west/northwest flow aloft develops. This southerly winds will help to advect some additional moisture/instability across our central/western cwa on Monday aftn. This will interact with s/w energy dropping down in the flow aloft with an approaching cold front to produce additional chances for showers/storms. Best chc for storms will be along and west of the cpv...where cape profiles approach 1000 j/kg. Progged 850mb temps near 14c support highs in the mid/upper 70s with maybe a few lower 80s possible in the warmer valleys. The overall threat for stronger/severe storms looks limited do to the lack of shear/instability, but if more sfc heating and moisture can develop a few stronger storms are possible on Monday afternoon. As front clears the area overnight Monday Night into early Tuesday, clouds will linger but taper off relatively quickly as dry air moves in after 12z. Expect clear sunny skies by the afternoon on Tuesday with temperatures in the mid to upper 70s. Pressure gradient remains relatively strong, and with good daytime mixing, could have some breezy conditions during the day Tuesday. Surface high pressure builds in by Tuesday night, thus expect clear and calm conditions overnight. Good radiational cooling is likely so have low temperatures at or slightly below normal (normal this time of year is around 48) with mid 30s in the Adirondacks and mid to upper 40s in the Champlain valley and points east. High pressure and dry air remain in place through Wednesday before our next system approaches from the west on Thursday. This system is relatively complex with timing of upstream shortwave and the phasing of the potential tropical system over the southeast US playing an important role in it`s develop. Models overall agree on some phasing of these systems, but diverge on the timing and placement of the heaviest moisture, with GFS indicating higher moisture and rain chances towards the end of Thursday, and ECMWF holding off until early Friday morning with heaviest precip over mid Atlantic. At this time have gone with Superblend of PoPs between 20-40 to indicate uncertainty but possibility of unsettled weather for the end of the period. Temperatures through this time will likely be above normal with highs in the upper 70s to around 80 and lows in the upper 50s to low 60s. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Through 06Z Saturday...VFR thru the period. SKC through 12Z with westerly LLWS in the 35-40kt range at most terminals other than KBTV where south winds will trend gusty into the 15-20 kt range. After 12Z south/southwest winds from 10-15 kts and gusts from 18-28 kts expected at all terminals. SCT/BKN mid level cigs in the 070-120 AGL range to push east across the area in the 15-22Z time frame, though no precipitation other than an isolated sprinkle is expected with these clouds. After 00Z south to southwest flow from 8-12 kts continues, though less gusty. SCT/BKN cigs in the 050-080 AGL range expected with an increasing threat of -shras toward the end of the forecast cycle, especially at northern terminals of KMSS/KPBG and KBTV. Outlook... Saturday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. Saturday Night: Mainly VFR, with areas MVFR possible. Chance SHRA. Sunday: Mainly VFR, with local MVFR possible. Chance SHRA, Likely TSRA. Sunday Night: Mainly VFR, with local IFR possible. Chance SHRA. Memorial Day: Mainly MVFR, with areas VFR possible. Chance SHRA, Chance TSRA. Monday Night: Mainly VFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Tuesday: Mainly VFR, with local MVFR possible. Slight chance SHRA. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...JMG NEAR TERM...JMG SHORT TERM...Evenson LONG TERM...LaRocca/Taber AVIATION...JMG

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