Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 291129
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
729 AM EDT Fri May 29 2020

.SYNOPSIS...
Strong to severe thunderstorms are possible this afternoon and
evening ahead of an approaching cold front. A few storms will be
capable of strong, gusty winds and locally heavy downpours. An
additional cold front on Saturday will bring the return of near
normal to slightly below normal temperatures Sunday and through
the first half of next week. By mid week, temperatures will
return to mid 70s and bring a chance of showers to the North
Country.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 715 AM EDT Friday...A few showers managed to develop
across the Champlain Valley producing light sprinkles. It also
caused a sunrise surprise in the form a nice rainbow. Usually
those are supposed to happen after the storm, but we will take
it. The showers formed along a remnant boundary that has been
trekking slowly but surely east from the St. Lawrence County New
York, and based on satellite and radar, this boundary is
crossing east of Vermont now. With clearing skies, the question
is how warm we can get as we destabilize this afternoon.
Convection is already firing across Pennsylvania and New York.
While there is a weak warm layer around 700mb, suspect it will
not take much heating to get convection going. The forecast has
this covered, but did tweak sky cover given the mostly clear
conditions over northern New York.

Previous Discussion...
The Storm Prediction Center has much of Northern New York and
western Vermont in a slight risk category (Level 2 of 5) and
southeastern Vermont in a marginal risk category (Level 1 of 5).
The most likely threat will be strong, gusty winds.
Additionally, we`ll be monitoring the potential for training
cells that may result in localized minor flash flooding.

Currently, light shower activity has largely dissipated, but some
are shallow in depth and are not well seen by the radar.
Montpelier is currently reporting light rain. We turn our
attention to an old upper low along the Mississippi River
Valley that has now opened up and a northern stream shortwave
trough digging across the Great Lakes at the moment. We will
start to see heights fall as upper high weakens and shifts east.
Southwesterly flow ahead of the longwave trough over the
eastern US will continue to advect warm, moist air into our
region. Temperatures are currently in the upper 60s to mid 70s
with dewpoints in the lower to middle 60s as well.

A prefrontal trough will develop late morning/early afternoon. A
weak warm layer around 700 hPa will allow us to warm just a bit
before convection can pop, but around noon time, diurnal
heating should be plenty sufficient to generate 700-1500 J/kg of
CAPE. An 850mb low-level jet of around 35 knots develops to our
south, and we are positioned along the nose of this feature.
Weak synoptic forcing from an upper-level jet exists as well and
accelerating 500mb winds will result in the development of
40-50kts of 0-6km bulk shear. Along the nose of the LLJ will be
a helicity gradient that will become established along the
interior Adirondacks east into the southern half of Vermont.
Think this general area will have the greatest chance to see
transient bows, weak mesocyclones, and other signs of
intermittent organization. All severe threats will be possible,
but winds will be the most likely. Outside of that region,
dynamics should remain supportive enough that any discrete cells
will also need to be monitored closely for damaging downburst
winds and small hail. We also continue to watch the chance for
training convection, as we remain modestly unstable, even after
our airmass gets worked over. The upper jet becomes meridional
and accelerates overnight as the cold front comes across
overnight. The upper trough axis becomes negatively tilted, with
strong upper diffluence and divergence. The additional synoptic
forcing and CAPE values of around 500 J/kg will likely result
in one or two additional rounds of convection. The HREF
continues to highlight the potential for localized minor
flooding/flash flooding, with probabilities of 2" or higher
storm total precipitation higher than 30% across the Southern
Adirondacks and portions central Vermont. Antecedent dry
conditions should preclude significant impacts, but trends will
be watched closely.

Precipitation will begin to shift east around 2 AM on Saturday as
the vort max takes off north of our region with the wind shift piece
of the front coming through around dawn on Saturday. We then start
to trend drier and cooler with forecast highs on Saturday in the
upper 60s in the St. Lawrence warming into the upper 70s further
south and west near Springfield, VT. A secondary front will come
through later Saturday evening to reinforce the cooler air, and
could result in a few additional showers during the evening hours.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY/...
As of 331 AM EDT Friday...Strong shortwave and secondary cold front
rotate through early Saturday night with a few more showers and
sharply colder air. 925mb temps range from 3-10C and 850 mb temps
range from -5 to +3C thus highs in the 50s/L60s seem reasonable with
cold air advection still spilling into the area on Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 331 AM EDT Friday...No real changes from yesterday`s
discussion. We`re in cool trough that deepens across NH/ME into the
Gulf of Maine Sunday night-Monday with a chance of some mountain
sprinkles. This retreats with some building heights attached to
Southern Plains Ridge that attempts to build NNE but disturbances in
northern stream flow across Canada will likely dampen any heat ridge
for now. As the first weak disturbance moves through Tues ngt-
Wed...expect some rain showers with another disturbance directly on
it`s heels Wed ngt - Thursday.

QPF shouldn`t bee much but any little bit helps as we have been very
dry with record heat.

Temperatures are far from this past weeks heat, perhaps some
isolated frost early in the period for Mountain valleys if we stay
cloud free which is suspect. Highs starting in the L60s but ending
midweek close to 70 which is near seasonable. Lows starting in the
30s/L40s moderating into the 40s/50.

&&

.AVIATION /11Z FRIDAY THROUGH TUESDAY/...
Through 12Z Saturday...Currently VFR at all terminals, except
where a 3000ft agl deck has moved over KRUT. This deck should
quickly move east within the next hour. Scattered showers will
begin to develop after 16Z, with a mention of VCTS after 19Z
across all terminals. Storms could be strong and produce
localized heavy rain and gusty winds, which would reduce
vsby/cigs as well. There could also be the potential for
multiple rounds of showers and storms. Thunderstorm activity
should decline after 03Z. Showers will continue, though waning
in areal coverage, with many areas transitioning to VCSH around
06Z-09Z. Ceilings will also begin to fall to 2500-3500ft agl.
Steady south to southwest winds will continue throughout the
period around 8 to 15 knots with occasional gusts to 20 knots.

Outlook...

Saturday: VFR. Chance SHRA.
Saturday Night: Mainly VFR, with local MVFR possible. Chance
SHRA.
Sunday: VFR. NO SIG WX.
Sunday Night: VFR. Slight chance SHRA.
Monday: VFR. Slight chance SHRA.
Monday Night: VFR. NO SIG WX.
Tuesday: VFR. NO SIG WX.

&&

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

&&

$$
SYNOPSIS...Haynes
NEAR TERM...Haynes
SHORT TERM...SLW
LONG TERM...SLW
AVIATION...Haynes



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