Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 262326
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
726 PM EDT Sun Jun 26 2016

.SYNOPSIS...
High pressure will begin to give way overnight to an approaching
frontal system as we head into the workweek. A chance for showers
and thunderstorms is in the forecast for Monday and Tuesday as the
front moves through. Tuesday`s storms could produce heavy rainfall,
gusty winds, and small hail. A chance for showers will linger into
Wednesday, then the week finishes up with high pressure and a dry
forecast.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 637 PM EDT Sunday...Going forecast in real good shape and
no real changes needed at this time. See previous discussion
below.

Center of the surface high pressure has shifted east, with southerly
low level flow. 500mb ridge lagging behind a bit with the ridge
axis over the area through this evening. Heights begin to fall
later tonight in advance of the next low pressure system. Closed
500 mb low moving east across southern Ontario will open up into a
positively tilted trough through Monday as it continues on eastward.
At the surface an initial prefrontal trough will approach the St.
Lawrence Valley in the pre-dawn hours. POPs increase ahead of the
trough after midnight and spread east through the morning. Best
chances will be Champlain Valley westward. Better moisture/instability/shear
remains further west along actual cold front however, and only general
thunder expected with this first wave.

20-30 kts of south wind around 850 mb tonight and during the day
Monday, however an inversion should keep the stronger winds from
mixing down for much of the time period. Surface gradient winds in
the low teens with gusts in the upper teens to near 20 during the
day Monday.

Radiational cooling we`ve seen the past several nights not a
player tonight as pressure gradient keeps a warm southerly breeze
blowing and lows generally in the 60s. A little cooler on Monday
with lower heights, and 850 mb temps in the mid teens supporting
surface max temps in the 80s.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 314 PM EDT Sunday...A very active weather day is anticipated on
Tuesday with widespread showers and thunderstorms. The trends
continue to support strong to possibly severe storms from the
northern Adirondacks into all of VT...with a slower frontal
arrival and greater surface heating resulting in higher
instability values. Primary threat would be one inch hail and
scattered damaging winds...along with localized very heavy
rainfall rates which could cause isolated flash flooding.

Strong mid/upper level trof will be across the eastern Great Lakes
by 18z Tuesday...with potent 5h short wave energy ejecting from the
Ohio Valley into the NE CONUS along a surface cold front draped
across our forecast area. GFS/NAM both show a favorable 250mb jet
couplet from right rear quad of 80 knot jet over southern Canada and
nose of left front of approaching jet over Ohio Valley arriving
around 18z Tuesday. This jet energy and approaching height falls
with trof will produce a strong upper level divergent signature
across our region...helping to promote deep layer lift.

Still some disagreement on amount of surface heating and associated
instability...but trends have been slower with frontal arrival and
greater with instability...especially dacks eastward. NAM surface
based CAPE values are between 1400 and 1800 j/kg with GFS 800 to
1200 j/kg...mainly across central/eastern VT. Have noted surface
temps in the lower 80s off the NAM and only lower 70s on the GFS
resulting in the instability difference. Based on progged 850mb
temps between 11C and 13C...and limited 850 to 500mb moisture across
our region in the morning...thinking temps will warm quickly into
the mid 70s to lower 80s on Tuesday...with surface dewpoints well
into the 60s. Noting lower 70s dwpts already pooling ahead of the
boundary across the Ohio Valley today and these will make a run at
us by Tuesday. These higher temps and dwpts will support CAPE values
around 1500 j/kg with pockets near 2000 j/kg by 18z Tuesday...which
combined with deep layer (0 to 6km) shear of 40 to 45 knots will
support organized convection. Based on progged instability and shear
parameters thinking mini supercells will be possible...evolving into
multicell clusters during the afternoon hours. The primary threat
will be damaging winds and one hail...especially from the dacks
eastward between noon and 6 pm on Tuesday.

The orientation of approaching boundary and favorable upper level
jet position...and pw values pooling between 1.50 and 1.75
inches...thinking localized heavy downpours will be
likely...especially as storms evolve into multicell clusters. The
intense rainfall rates associated with deep convection will need
to be watched closely for creating an increased threat for flash
flooding...especially if cells backbuild or train over mountains
terrain. Sounding vector analysis shows the potential for
backbuilding cells...as mid/upper level flow is paralleling the
low level boundary.

Given the increased severe weather threat and potential for
localized heavy downpours we will update HWO and place small
hail...gusty winds...and heavy downpours in zones with this package.
Our thinking is that if models are faster with front and show less
instability on Monday we can remove wording or if trends continue we
can add additional enhancement. Still plenty of time for shear and
instability parameters to change...but Tuesday should be monitored
closely.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 314 PM EDT Sunday...Front finally sweeps across our region overnight
on Tuesday into Weds with some lingering moisture and weak
instability noted near the international border. Thinking this
instability along with some short wave energy from 5h trof will
produce scattered showers on Weds...especially northern sections.
Will continue to mention chance pops...NAM is slower and would
suggest greater instability with a small threat for small
hail/gusty winds. My current thinking is convection on Tuesday and
associated outflow boundaries will help to forward the progression
of boundary into NH by Weds. Cooler and drier weather associated
with high pres ridge for Thursday.

A warm southwest flow develops ahead of our next front and
associated energy on Friday. Latest GFS trends show axis of
instability developing across our cwa by 00z Sat with CAPES around
800 j/kg and deep layer shear between 25 and 35 knots. Given
uncertainty on timing of system and amount of instability and
shear...will mention chance for showers and storms at this time.
A few lingering showers are possible on Saturday...before drier
and cooler air arrives by Sunday. Overall temps will be near
normal on Thursday...jump to above normal ahead of boundary on
Friday...before falling back close to normal over the weekend.

&&

.AVIATION /00Z Monday THROUGH Friday/...
Through 00Z Tuesday...VFR throughout most the TAF forecast
period. Mostly clear skies give way to mid/high clouds moving in
west to east after midnight associated with next trough of low
pressure. Chance for showers along the trough after 09z St.
Lawrence Valley & KMSS, moving east through the morning across the
rest of the region for the remainder of the period. Thunderstorms
are also possible but confidence is low at this time and have
chosen to not include the TAFs just yet. MVFR ceilings are
possible during this time for KMSS. Light South winds across the
region overnight pick up again Monday morning through 18z at 10-20
knots as pressure gradient tightens ahead of the trough.

Outlook 18Z Monday through Friday...

18Z Monday through 00Z Wednesday...VFR deteriorating to
MVFR/brief IFR as a trough Monday and cold front Tuesday brings
showers and thunderstorms. Best chances for stronger storms on
Tuesday.

00z Wednesday to 00z Thursday...Mainly VFR with morning IFR in fog
mainly KMPV/KSLK. Lingering scattered showers over the higher
terrain affecting KSLK KMPV with MVFR vis.

00z Thursday through Friday...VFR with nightly occurrence of
IFR/LIFR radiation fog at MPV and SLK.

&&

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

&&

$$
SYNOPSIS...Hanson
NEAR TERM...Evenson/Hanson
SHORT TERM...Taber
LONG TERM...Taber
AVIATION...MV/Hanson


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