Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 180758

Area Forecast Discussion
National Weather Service Burlington VT
358 AM EDT Sun Aug 18 2019

Clouds with patchy fog this morning will give away to partly
sunny skies by mid morning with temperatures warming into the
80s by this afternoon. Once again a chance of showers and
thunderstorms will persist today, but areal coverage and
intensity will be less than on Saturday. Additional showers and
embedded storms are possible overnight into Monday, before a
mainly dry and warm day is expected on Tuesday. Much cooler and
drier air arrives toward the end of this upcoming week.


As of 323 AM EDT Sunday...Water vapor shows deep mid/upper level
dry layer acrs our cwa, however plenty of moisture trapped below
shallow subsidence inversion has resulted in areas of stratus
and fog across our cwa this morning. Expect these low clouds to
slowly burn off this morning, which may slow the rate of temp
rise for several hours.

Meanwhile, water vapor continues to show a complex mid/upper
level pattern across the Ohio Valley into NE CONUS, with
developing ridge aloft, while several pieces of short wave
energy tries to undercut the ridge. First piece of energy is
crntly over the central Great Lakes approaching western NY with
area of rain/embedded storms. This should slide by to our west
this morning/early aftn, but trailing vort lobe may produce a
few showers across our western cwa. Meanwhile, stronger short
wave energy is expected to lift from central NY into
central/southern VT btwn 00z and 06z this evening, with another
round of showers/storms. Soundings show instability parameters
are elevated with MU CAPE values of 800 to 1200 J/kg, mainly
across our southern CWA, while deep layer shear is in the 25 to
35 knot range. Thinking elevated convection is likely with
localized heavy rainfall possible, but given stable layer below
850mb, probably of winds mixing toward the sfc is minimal. Have
highest pops 40 to 60% toward 00z this evening across Essex
County NY into Southern VT, expanding northeast toward
central/eastern VT through 03z, before shifting east of our cwa
by 06z. Rainfall generally 0.10 to 0.25, but localized up to an
inch possible in the heavier convective elements. Temps will be
tricky today, as clouds will result in slow warming thru mid
morning, but expect breaks in the overcast to develop and temps
should warm into the upper 70s to mid 80s most locations. This
is supported by 925mb temps btwn 19c and 21c. Similar to
previous couple of nights, would not be surprised areas of
fog/br develop aft 06z, given saturated bl profiles and some
clearing behind departing short wave energy. Temps generally in
the 60s overnight with maybe a few values near 70f in the cpv
with some mugginess to the air mass.

Monday...Plenty of uncertainty with regards to thunderstorm
potential and timing of short wave energy and amount of
instability. ECMWF is later with arrival of short wave energy
near the international border and is most aggressive with cape
values approaching 2000 j/kg with favorable shear parameters
from approaching 850 to 700mb jet couplet. Meanwhile, nam/nam
nest is faster with s/w energy and passage of weak sfc
reflection, resulting in less instability and moisture profiles
and minimal threat for aftn/evening convection. Given
uncertainty in timing and potential magnitude will continue to
mention low chc pops for now from west to east across our cwa.
Progged 925mb temps warm another degree or two with values 20 to
22c, supporting highs mid/upper 80s cpv/lower ct river valley to
upper 70s/lower 80s mountain towns. Have noted in the high
resolution nam soundings, better mixing develops in the aftn,
which helps to mix drier air aloft toward the sfc and bl dwpts
drop into the low/mid 60s. This would help warm temps, while
mixing out the higher humidity values.


As of 355 AM EDT Sunday...Tuesday should be a really nice day.
Building heights aloft with subsidence and drying over the area
should lead to mostly sunny skies and low relative humidities.
Temperatures warm nicely into the low to mid 80s with no
precipitation expected. A weak shortwave will pass to the south and
then east of the area Tuesday night so have kept with a chance for
rain after midnight for southern portions of Windsor county. Lows
overnight Tuesday will be slightly above normal in upper 50s to
low/mid 60s.


As of 355 AM EDT Sunday...The weather turns more active for
Wednesday with deepening cyclonic flow across the area ahead of
an approaching upper-level trough. Tricky forecast for
convective potential on Wednesday however as not all ingredients
align for a shoo-in kind of day. In classic North Country
fashion no cold front can come in all at once so we`ll be
dealing the prefrontal trough ahead of the main vort and
attendant cold front. Depending on which model you look at
moisture progs ahead of this front are completely different.
This has real implications on how the convective threat will
evolve for several reasons...Lack of moisture ahead of the
deepening vort will keep areal coverage of storms limited but
will minimize extent of cloudiness increasing available
instability. On the contrary however, more available moisture
will sustain more widespread convection but would likely dampen
instability. If we step away from the moisture potential,
thermodynamic variables are certainly in place to aid in the
develop of organized storms should they get going. Temperatures
Wednesday should warm nicely into the low to mid 80s with CAPE
values generally between 800-1000 J/kg. 0-6km bulk shear values,
while not off the chart, are near 35 kts for northern tier and
30 kt across our southern areas with the nose of 35 kt 700mb jet
advecting into the region. This time frame is currently outside
the scope of CAM models so the next couple forecast cycles
should be able to get a better idea of the evolution and threat
of storms for Wednesday.

After this front moves through broad upper level trough remains with
several shortwave impulses pivoting around main vort bringing
chances for showers Thursday and Friday, mainly across our northern
zones. Temperatures behind the front on Thursday will be in the low
to upper 70s with a reinforcing shot of cold air dipping
temperatures on Friday into the upper 60s to low 70s. Overnight lows
Thursday and Friday will be chilly in the mid 40s to mid 50s. High
pressure remains in control through the weekend with a return to
more seasonable temperatures in the mid 70s to around 80.


Through 06Z Monday...Challenging early morning aviation forecast
with regards to low clouds and fog/br development. Crnt
conditions vary from lifr at slk/pbg to ifr for cigs at mpv to
vfr at btv/mss/rut. Thinking fog is most likely to prevail at
slk/mpv from 06z thru 12z or so, given saturated bl conditions
and crnt obs. Meanwhile, thinking intervals of ifr/lifr is
likely at pbg/mss and rut, given the recent rainfall and very
small temp/dwpt spread. At btv a period of mvfr with fog deck
nearby is possible around sunrise, but will continue to monitor
for potential clouds and associated impacts at all sites.
Otherwise, any ifr should lift by 12z with mainly vfr conditions
from 12z through 00z Monday, with some threat for showers/storms
aft 21z.


Monday: VFR. Chance SHRA, Chance TSRA.
Monday Night: VFR. Slight chance SHRA.
Tuesday: VFR. NO SIG WX.
Tuesday Night: VFR. Chance SHRA.
Wednesday: VFR. Chance SHRA, Chance TSRA.
Wednesday Night: VFR. Chance SHRA, Slight chance TSRA.
Thursday: Mainly VFR, with local IFR possible. Chance SHRA.




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