Area Forecast Discussion
Issued by NWS Burlington, VT

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621
FXUS61 KBTV 201917
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
317 PM EDT Tue Aug 20 2019

.SYNOPSIS...
Showers and thunderstorms return Wednesday afternoon with the
potential for some organized storms capable of gusty winds,
locally heavy rainfall, and frequent lightning. A cold front
will push through Thursday morning, followed by high pressure
building in and a return to cooler and drier weather for the end
of the week.

&&

.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
As of 310 PM EDT Tuesday...Period of quiet weather continues
tonight as high pressure departs the region to the east.
Light southerly winds will continue overnight, which will keep
overnight temperatures relatively warm despite the initial clear
skies. Clouds will spread over the region after midnight as a
warm front approaches the area. Overnight lows will be in the
upper 50s to mid 60s.

Interesting setup coming together for Wednesday with several
ingredients present for the development of some strong to severe
thunderstorms. However, as has been the case most of the summer
for the North Country, several factors also working against the
threat...so challenging forecast to weigh out both sides with
the result being a conditional severe threat for tomorrow. In
the upper levels, synoptic picture features an upper-level low
digging upstream over the northern Great Lakes Region during the
day Wednesday with southwesterly flow increasing over the BTV
CWA ahead of the system. Models continue to indicate a shortwave
will move through eastern NY and into western New England
during the early afternoon hours on Wednesday...which will be a
key player in deep convective initiation. Meanwhile, surface
temperatures and dewpoints will increase quickly on
strengthening southwesterly flow just ahead of the wave. A warm
front will lift through southern and central New England during
the morning hours, which will spread some cloud cover over much
of Vermont and into eastern New York...limiting development of
instability. Thus, greatest axis of instability is progged to
develop over the Saint Lawrence Valley (1500+ J/kg), with degree
of destabilization further east still in question. However if
Vermont is able to destabilize after morning cloud cover,
showers and thunderstorms are likely to develop in the early
afternoon ahead of the upper wave and along a prefrontal trough.

Severe threat:  Best deep layer shear will be removed to the
northwest closer to the heart of the upper trough, but still
looking at 25-35 kts 0-6 km shear in our forecast area...supportive
of organized strong to severe convection with organized
multicells being the primary convective mode. Forecast
hodographs indicating 0-3 km SRH values of over 250 m2/s2 over
southern Vermont...so the potential for rotating sustained
updrafts will be relatively high with any storms that develop in
southern into central Vermont. Low and mid-level shear and
helicity values decrease further north through the remainder of
the CWA. Lapse rates are not overly impressive, in fact latest
model runs have trended back on the amount of dry air available
in the mid- levels, instead indicating near moist adiabatic
lapse rates. This does work somewhat against the severe wind
threat, but overall after evaluating all the factors still
seeing enough signals for some storms with potentially damaging
gusty winds, so have included mention of this threats in the
forecast.

Rainfall: With regards to rainfall amounts, storms will be
capable of locally heavy rainfall given PW values > 1.75 inches
and deep warm cloud layers of upwards of 12k ft. However,
Corfidi vectors between 15 and 25 kts suggest storms will be
moving fast enough to not pose any widespread flooding threats.
Total QPF between Wednesday morning and Thursday morning is
generally in the quarter to two thirds inch range, with locally
amounts around an inch possible within any thunderstorms.

Storms and showers will dissipate Wednesday night as a cold
front moves through closer to dawn. Could still get a rumble of
thunder or two overnight, but main show will be over shortly
after sunset. Wednesday night lows will be in the mid to upper
60s.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
As of 320 AM EDT Tuesday...The main weather feature for Thursday will
be the passage of a cold front associated with a deepening low
pressure system across Quebec. Still some timing differences among
the models on when the front will pass with ECMWF slowest of the
solutions and GFS the fastest. Have settled on a blend between the
two, very similar to NAM timing with FROPA arriving between 12-18z
from west to east. Winds aloft will turn out of the west advecting
in lower dewpoints but better cold air advection does not arrive
until overnight Thursday into Friday with the passage of a secondary
cold front. Temperatures Thursday will still be ~6-10 degrees cooler
from Wednesday with highs in the mid 70s mountains and St Lawrence
Valley to around 80 elsewhere. As mentioned by previous forecaster
the passage of this front should be relatively uneventful with some
showers and possibly isolated embedded thunderstorm. Model progs
indicate very narrow band of moisture with this feature and weak
elevated instability and meager SB CAPE around 300-600 J/kg. Have
generally offered 30s-50s PoPs with highest chances across St.
Lawrence valley and International Border with lowest chances for our
southern counties.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 320 AM EDT Tuesday...No real changes made to the extended
forecast with upper level troughing remaining in place through
Friday/Saturday. Secondary cold front and reinforcing shot of cold
air arrives Friday with a slight chance of showers possible across
the northern mountains and Northeast Kingdom of Vermont. Partly
cloudy skies are expected with temperatures topping out in the mid
60s to around 70. Overnight lows will drop below normal into the mid
40s to mid 50s. Similar conditions are expected for Saturday under
mostly sunny skies. After this, upper level pattern becomes highly
amplified across the eastern US with high pressure and dry weather
expected trough midweek. Temperatures warm a couple degrees each day
with near normal highs in the mid 70s to around 80 to lows in the
low to upper 50s.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Through 18Z Wednesday...High pressure in control supplying VFR
flight conditions through 12Z. High and mid-level clouds will
overspread the region after 06Z as a warm front moves through,
which should limit fog potential overnight. Will see clouds
lowering and some vicinity showers develop after 12Z, with some
thunderstorms possible after 16Z. The best potential for
thunderstorms exists after 18Z however, so have left any mention
of thunderstorms out of this TAF package.

Outlook...

Wednesday: VFR. Likely SHRA, Chance TSRA.
Wednesday Night: Mainly VFR, with local MVFR possible. Chance
SHRA, Slight chance TSRA.
Thursday: Mainly VFR, with local MVFR possible. Chance SHRA.
Thursday Night: VFR. Slight chance SHRA.
Friday: VFR. Slight chance SHRA.
Friday Night: VFR. NO SIG WX.
Saturday: Mainly VFR, with local MVFR possible. NO SIG WX.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...None.
NY...None.

&&

$$
SYNOPSIS...Duell
NEAR TERM...Duell
SHORT TERM...LaRocca
LONG TERM...LaRocca
AVIATION...Duell



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