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  Friday July 12, 2024

 

NWS Area Forecast Discussion



626
FXUS61 KBTV 101445
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1045 AM EDT Wed Jul 10 2024

.SYNOPSIS...
A warm front will lift north as moisture from the remnants of Beryl
move into the region today. Multiple rounds of showers and storms
will begin this morning, which will be heaviest this afternoon into
the overnight hours. Scattered to numerous instances of flash
flooding will be possible due to locally heavy rainfall rates. A few
strong to locally severe storms may also take place near the
Adirondacks and in the southern half of Vermont. Precipitation will
become isolated to scattered tomorrow, with a general drying trend.
Temperatures will remain warm and humid.

&&

.NEAR TERM /THROUGH THURSDAY NIGHT/...
As of 1044 AM EDT Wednesday...Heavier rain is shifting in the
St. Lawrence Valley shifting up a well defined trough axis.
Bumped up rain across that region, and took rainfall amounts
just a hair north while trying to preserve heavy rainers moving
into Rutland County. Tightened today`s thermal gradient some,
but the rest is on track.

Previous discussion...A warm front will lift northward through
the forecast area today as Post-Tropical Cyclone Beryl approaches
from the southwest. Tropical moisture will accompany Beryl with
anomalous precipitable water values up to 2.30 inches anticipated.
The warm front will gradually destabilize the region`s atmosphere
throughout the day today, and rain should arrive from west to east.
The heaviest rain appears to arrive in two bursts, one this
afternoon/evening along the frontal boundary, then another tonight
as a triple point develops off the low pressure center over the
forecast area. The warm front will be the focal point of convection
for the first round ahead of Beryl as it becomes the convergence of
areas of highest low level instability and best upper level forcing
this evening.

There is at least a 40% chance of excessive rainfall triggering
flash flooding today and tonight for most of our forecast area, and
HREF`s Grand Ensemble is showing an impressive swath of 60-80%
chances of 2 inches or higher of rainfall through the Adirondacks
and central Vermont today and tonight. HREF ensemble mean is showing
a widespread 1.5-3.0 inches within 48 hours now through Thursday
evening. Our own forecast for rainfall totals is currently 1.20-3.40
inches of rain, with most spots falling around 2 or 3 inches, which
is not greatly changed since yesterday.

There is also the potential for some higher amounts in heaviest
showers or thunderstorms, but it`s hard to say exactly where that
may occur. CAMs and global models continue to disagree on where a
swath of heaviest rain will set up. Globals mostly place heaviest
rain across the St. Lawrence Valley and north of the international
border, while CAMs drape it directly across our forecast area from
the Adirondacks to central and northern Vermont. One exception,
however, is the Canadian global model, which seems to agree with the
CAMs about heavy rain driving directly through our area. Recent
runs of the CAMs have also fluctuated slightly, including one
HRRR run inching the area of heavy rain slightly northward.

In addition to flash flooding, river flooding is also possible, but
it will also depend on where the heaviest rain falls. Currently, the
East Branch Ausable River appears to be the most likely to reach
Minor Flooding, though the Missisquoi River and Otter Creek are
also candidates to reach at least Action Stage. River rises are
expected to occur overnight tonight, peaking during the day
tomorrow. Highs today and tomorrow will be largely limited by
rainfall and clouds, so we`re expecting temperatures to reach
into the upper 70s to mid-80s for most. Lows will be mild in the
mid-60s to lower 70s.

One other impact we are monitoring is the potential for severe
weather with this system due in part to a significant amount of
wind shear. Most likely area for severe thunderstorms will be
southern Vermont, and greatest threat is damaging wind gusts.
The environment is also favorable for tornadoes, however, our
greatest threat remains flooding here in northern New York and
Vermont, with tornadoes and even damaging winds being much less
likely at the moment.

&&

.SHORT TERM /FRIDAY/...
As of 306 AM EDT Wednesday...Deep tropical moisture and dynamic
synoptic-scale support for precipitation associated with the
remnants of TC Beryl will exit east through the evening hours on
Thursday. As such, scattered shower activity will wane and
taper off from southwest to northeast through the night leaving
largely dry conditions by sunrise Friday as surface high
pressure bridges eastward. Lows to remain mild in the 60s to
locally around 70 in favored warmer spots of the Champlain
Valley. Then partly sunny with occl periods of more persistent
cloud cover on Friday with mainly dry weather expected as dry
air aloft creates a mid-level synoptic inversion, trapping
lingering mid-level moisture. Highs to run seasonably warm from
the upper 70s to mid 80s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 306 AM EDT Wednesday...Quiet weather largely continues
through the weekend with weak surface high pressure draped
across the region. Mean 18Z 925 hPa temperatures warm once
again, so afternoon highs on both Saturday and Sunday should
range solidly in the 80s to locally around 90 in the Champlain
and lower CT River Valleys.

Thereafter, sensible weather conditions become somewhat more
active as 500 hPa heights lower slightly under mean zonal flow
and a series of weak ripples/shortwave troughs traverse the
region. Mean blended solutions typically have some difficulty
in assessing the timing of these quick moving features, so until
more clarity can be gained generally chance PoPs from 30-40%
will be offered from Monday onward for a few/scattered showers
and storms. Temperatures to remain seasonably warm with highs in
the 80s to around 90 on Monday, cooling slightly into the upper
70s to mid 80s by the middle of next week due to the shower
threat and higher probabilities of variable cloud cover.

&&

.AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
Through 12Z Thursday...Patchy fog is clearing at MPV, leading
to a brief period of likely VFR conditions for all sites through
about 16-18Z Wednesday. During this time, rain showers are
expected to arrive from west to east, but will likely start
relatively light with VFR cigs. Around 16-18Z is when we are
anticipating heavier rain and lower ceilings associated with
Post-Tropical Cyclone Beryl, producing MVFR to IFR conditions
for the remainder of the 24 hour TAF period. Winds currently
light and variable will become northeasterly to southeasterly by
the end of the TAF period. Some sites could have brief wind
gusts 15-25 knots, and MSS is the most likely spot for this.
LLWS is expected at most sites around 00Z onward.

Outlook...

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

&&

.HYDROLOGY...
As of 400 AM EDT Wednesday...Confidence is high regarding heavy
rainfall. Although wobbles and shifts in model solutions
continue to take place, the overall certainty of higher
rainfall rates indicates a 40-70 percent chance for rainfall
rates to exceed flash flood guidance, resulting in a Moderate
Risk Outlook (Level 3 of 4) in the Excessive Rainfall Outlook
for most of our forecast area.

Over the past 40 days portions of the Adirondacks and
central/northern Vermont have seen rainfall in excess of 8", some
areas over 10" which ranks 150-200% above normal. Streamflows in
these areas have responded and are currently running 70-85% above
normal though river stages are relatively low but still above
normal. These antecedent conditions, in addition to the locally
heavy rain that has occurred in central Vermont will only
increase the threat for flash flooding with both low and high
resolution guidance depicting the potential for rainfall of 1.5
to 2.5 inches in 6 hours.

Unlike last year`s flood event, this will not be a long duration
scenario. Additionally, parts of the Adirondacks are at a
greater risk for flash flooding than last year`s July event.
The heaviest rainfall is likely to fall within a 9 to 15 hour
window today into tonight. With this, widespread rainfall
totals are forecast to be 2 to 3 inches and this would likely
involve an embedded swath where values of 3 to 5 inches. Within
that 3 to 5 inch swath, there could be a few spots that approach
6 inches. Where values in excess of 3 inches occur, additional
flooding across low lying plains and roads in addition to flash
flood concerns. Widespread washouts and water entering homes and
businesses could also take place at the upper bounds.

On Thursday, runoff from the rain will cause sharp river rises.
At this time, the East Branch of the Ausable at Ausable Forks
is forecast at minor flood stage about Thursday morning, which
can have impacts in Keene, New York and sections of road near
Route 9N in and around Marcy Field. A few of the higher range
ensemble forecasts continue to indicate a low probability of
reaching flood stage at the Winooski at Essex Junction, the
Missisquoi River at North Troy, Otter Creek at Center Rutland,
and the Mad River at Moretown. 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.

For details on specific area rivers and lakes, including observed
and forecast river stages and lake elevations, please visit the
Advanced Hydrologic Prediction Service /AHPS/ graphs on our
website.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...Flood Watch through Thursday morning for VTZ001>011-016>021.
NY...Flood Watch through Thursday morning for NYZ026>031-034-035-
     087.

&&

$$
SYNOPSIS...Storm
NEAR TERM...Haynes/Storm
SHORT TERM...JMG
LONG TERM...JMG
AVIATION...Storm
HYDROLOGY...Team BTV



 
 
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