767
FXUS61 KBTV 082329
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
729 PM EDT Mon Jul 8 2024

.SYNOPSIS...
Surface high pressure will provide the North Country and
Vermont with dry weather through tonight. A weak disturbance
passing through aloft will increase the chance for isolated
showers, thunderstorms, and heavy rain on Tuesday. Thereafter,
our attention turns to an increasing potential for heavy rain
and flash flooding Wednesday Night into Thursday associated with
the remnants of Tropical Storm Beryl.

&&

.NEAR TERM /THROUGH TUESDAY NIGHT/...
As of 642 PM EDT Monday...No significant changes needed with
this update. Clouds have dissipated over much of the region,
though more persistent cloud cover still lingers over portions
of the northern/eastern Adirondacks down into the southern
Champlain Valley. Temperatures remain in the upper 70s to upper
80s at this hour, but they should continue to gradually fall
this evening as the sun sets. After some initially clear skies,
mid/high clouds will increase later tonight, limiting fog
potential. The forecast has this all covered, so just made some
minor tweaks to temperatures/dewpoints/cloud cover to match the
latest trends.

Previous discussion...Our weather will remain dry tonight
into Tuesday morning with high pressure remaining in control of
our weather. A few isolated showers are possible this evening
along a lake breeze. Mid and high clouds will move into the
region overnight, and should limit fog development, though some
fog will be possible in far eastern valleys of Vermont towards
early Tuesday morning. Low temperatures will be quite warm, only
dipping into the lower 60s to around 70. Tuesday will be another
muggy day with temperatures rising into the mid 80s to lower 90s
even. Increasing moisture on southerly flow will push dewpoints
into the lower to mid 70s in some locations. In the Champlain
valley heat indices will get very close to advisory criteria for
a brief time. Some upper level shortwave energy passing overhead
on Tuesday should be enough forcing for some showers and
isolated thunderstorms to develop. Heavy rain will be possible
with any thunderstorm that develops. Lows on Tuesday night will
once again be very warm, only dropping into the lower 60s to
lower 70s. Will have increasing chances for showers towards the
morning hours on Wednesday ahead of next approaching system, but
much of the area will remain dry overnight.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
As of 337 PM EDT Monday...Over the course of Wednesday, a warm
frontal boundary will intensify as what will be the remnant
circulation and moisture of Beryl lift north. An unseasonably strong
upper jet, up to the 99th percentile, will result in an ageostrophic
circulation at the lower levels that will help maintain a tighter
thermal gradient from Tuesday`s frontal boundary. This will set the
stage for our region to be under threat due to moderate to locally
torrential rain from the strong warm front and convective
enhancement. Often, warm fronts come with an inversion or weak
instability in the mid-levels as warm advection drives poor lapse
rates. This does not seem to be present, and we could achieve modest
instability up to 500 to 1250 J/kg of CAPE while forcing for ascent
begins to develop in association with the jet streak and
strengthening warm front. Convective allow models are starting to
come in, and they are producing some very heavy rainfall as has been
generally anticipated. The latest NAM3 and FV3 are supporting of a
streak of rainfall rates approaching 1.5 to 2.5 inches in the 3 or 6
hour time frame, which would be sufficient to exceed Flash Flood
Guidance. And they are not yet in the time range to resolve the
additional precipitation that will likely occur overnight that
hasn`t come in range yet. As such, regionally averaged 1 to 2 inches
still appears on track, and there remains the likelihood for a swath
of 3 to 4 inches of rain. Embedded within that higher swath a
localized spot 5 inch total is possible, but on the extreme end of
things. Forecast guidance has generally shifted to the north in line
with the remnant circulation tracking closer to our region as
opposed to our south. Additional impacts and notes in the Hydro
section are offered below. No wind impacts expected.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 337 PM EDT Monday...The upper low that will absorb what is left
of Beryl will continue to rotate north of Lake Ontario Thursday into
Friday. However, a dry slot will advect over Vermont and northern
New York. Nevertheless, there is an occluded boundary that will
shift east across the area, and we will like along the periphery of
a rich, tropical moisture feed. Some showers and thunderstorms are
likely to redevelop Thursday afternoon along that moisture gradient,
but the areal extent of showers and storms may be limited due to
that mid-level dry slot where what appears will be a strong capping
inversion will be present. Activity could be shallow as a results.
So the ingredients aren`t lining up and we may be able to lower the
additional flash flood potential Thursday afternoon.

As we head into Friday, what will be Beryl`s remnant circulation
lifts out of the region. The Bermuda High appears likely to
strengthen beyond 600 dam at 500hPa. This will start to edge a
moisture plume back towards the region. However, long range models
disagree on how far west this plume shifts, and how quickly it may
break down as a southern stream trough rounds the periphery of the
Bermuda High with a northern stream trough also nearby. What remains
will be warm and humid weather, though.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Through 18Z Monday...VFR conditions should mostly prevail through
this period. Any fog development tonight will be much less
widespread than last night, though it cannot be completely ruled out
at MPV and SLK. Winds will be light or terrain driven tonight,
generally changing to light and southwesterly during the day
tomorrow. A high cloud deck will develop during the day and slowly
lower and thicken, but conditions should remain VFR at all terminals
through 00Z.

Outlook...

Tuesday Night: VFR. Chance SHRA, Slight chance TSRA.
Wednesday: Mainly VFR, with areas MVFR possible. Chance SHRA,
Slight chance TSRA.
Wednesday Night: Mainly MVFR, with local IFR possible. Definite
SHRA, Slight chance TSRA.
Thursday: MVFR/IFR conditions possible. Likely SHRA, Slight
chance TSRA.
Thursday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA, Slight chance TSRA.
Friday: Mainly VFR, with local IFR possible. Chance SHRA, Slight
chance TSRA.
Friday Night: VFR. Chance SHRA, Slight chance TSRA.
Saturday: VFR. Chance SHRA, Slight chance TSRA.

&&

.HYDROLOGY...
As of 337 PM EDT Monday...Confidence is moderate to high on
heavy rainfall Wednesday afternoon into early Thursday morning
will drive the potential for flash flooding. Late Wednesday
morning, scattered showers and thunderstorms will develop,
followed by a period of moderate to heavy rainfall during the
overnight hours. The heaviest rain will lift northeast after
Thursday morning. Some scattered showers may redevelop, but
ingredients for heavier rain appear less likely to persist, and
so the Thursday afternoon potential has decreased. The axis
where the highest rainfall totals are likely remains uncertain,
but the spread continues to decrease.

For Wednesday afternoon into Thursday morning, increasing the
potential for flash flooding is relatively high soil moisture
content across the Adirondacks and across parts of northern and
Central Vermont. The experimental SPoRT model indicates soil
moisture values in the 80th to 90th percentile. Both low resolution
and high resolution guidance are depicting the potential for
accumulations of 1.50 to 2.50 inches in 6 hours, which would be
enough to exceed Flash Flood Guidance. As such, a Slight Risk (Level
2 of 4) Excessive Rainfall Outlook has been issued for our region.
At this time, the probabilities of exceeding 2 inches is greatest
across Upstate New York and the northern half of Vermont. Unlike
last year`s flood event, this will not be a long duration scenario.
The heaviest rainfall is likely to fall within a 6 to 12 hour window
late Wednesday afternoon into Wednesday night. With this, rainfall
totals are forecast to be 1 to 2 inches and this would likely
involve an embedded swath where values of 3 to 4 inches could take
place. Within that 3 to 4 inch swath, there could be a few spots
that approach 5 inches. If values in excess of 3 inches occur, then
there could be additional flooding across low lying plains and roads
in addition to flash flood concerns. The most recent forecast has
been shifted to the north. This forecast will continue to evolve as
we become more certain on the track and development of convection.
Users are encouraged to prepare for scattered flash flooding, and
consider the possibility of a swath where embedded numerous flooding
could take place.

On Thursday, runoff from the rain will begin to bring river levels
up. At this time, river levels are above normal, mainly in north-
central Vermont. Although the latest NAEFS only depicts the Mad
River at Moretown reaching minor flood stage, the reasonable worst
case scenario, or 10th percentile, would also support the potential
at the Winooski River at Essex Junction (ESSV1), Otter Creek at
Center Rutland (CENV1), and East Branch Ausable River at Ausable
Forks (ASFN6). At this time, it appears that redevelopment of
convection could be limited Thursday afternoon, but we will continue
to monitor the position of dry, stable air that could limit this
potential.

&&

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

&&

$$
SYNOPSIS...Neiles
NEAR TERM...Hastings/Neiles
SHORT TERM...Haynes
LONG TERM...Haynes
AVIATION...Myskowski
HYDROLOGY...Haynes