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  Friday July 12, 2024

 

NWS Area Forecast Discussion



308
FXUS61 KBTV 091701
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
101 PM EDT Tue Jul 9 2024

.SYNOPSIS...
A shortwave passing through aloft will increase the chance for
scattered showers, thunderstorms, and heavy rain today. Thereafter,
our attention turns to an increasing potential for heavy rain and
flash flooding tomorrow night into Thursday associated with the
remnants of Tropical Cyclone Beryl.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 100 PM EDT Tuesday...A few thunderstorms have now managed
to fire. Given the mid-level dry air, but plenty of surface
heating and instability, they are quickly going up and then back
down. We should see them expand in coverage as a weak trough
shifts east. Only minimal changes here and there were needed
with the rest on track. Have a great day! Previous discussion
below:

Previous discussion...Tuesday will be another hot day with
925mb temperatures peaking at around 25-26 degrees C in some
areas. This will result in surface high temperatures in the mid
80s to lower 90s F, with the core of heat in the Champlain and
Connecticut River valleys. In addition to the pure heat, it will
be quite muggy with dew points reaching into the lower 70s for
the aforementioned valleys and 60s elsewhere. This will result
in heat indices climbing into the upper 80s to upper 90s. The
most intense apparent temperatures will be located in eastern
Essex County, New York, western Addison and western Rutland
counties of Vermont, and we have issued a Heat Advisory for
these three zones from 3 PM to 6 PM today. Those working or
recreating outdoors during these hours may want to consider
taking extra breaks from the heat and being extra vigilant about
good hydration.

Upper level shortwave energy and deep tropical moisture passing
through the forecast area and the significantly warm air mass today
will encourage the development of some scattered showers and
thunderstorms this afternoon and evening. Modeled anomalous
precipitable water values in the 1.75-1.90" inch range will
allow any thunderstorm to potentially be heavy. Most of the rain
will fall east of the Adirondacks, and this area is under the
WPC`s marginal level of excessive rainfall. This means there is
at least a 5% chance of flash flooding. These storms continue to
look very scattered in coverage, so we are not expecting
widespread issues. This will, however, also be a primer for
Wednesday`s more widespread rainfall, which will be directly
associated with Tropical Cyclone Beryl`s remnants and tropical
air mass. More on that in the short term discussion.

Due to strong heating, high dew points, 40-50 knot flow at the 500mb
level, modest CAPE, and up to 50 knots of 0-6km wind shear, there is
also the potential for some of the storms, particularly in southern
and eastern zones, to produce strong winds approaching 50 knots.
However, the threat for severe weather is of lower likelihood and
concern than localized flash flooding. Tonight will be another mild
night with lows in the mid-60s to lower 70s as clouds increase ahead
of the remnants of Tropical Cyclone Beryl. Chances of precipitation
will increase towards sunrise from west to east.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
As of 453 AM EDT Tuesday...Those looking for more clarity in the
forecast for expected impacts from post Tropical Cyclone Beryl won`t
be happy as there still remains considerable differences amongst the
latest NWP guidance on where the heaviest rain axis will set up. The
large scale synoptic setup remains unchanged with Beryl still
expected to phase with a digging northern stream trough Tuesday
night before slowing it`s northward movement over the Great Lakes
Wednesday into Thursday due to blocking high pressure across eastern
Ontario and Quebec. A warm front lifting through the Northeast
Wednesday continues to be the focus for convective development
driven by an seasonally strong upper level jet which combined with
tropical PWATs rising to near 2.5" will enhance convective rainfall
rates and increase the risk for flash flooding over the forecast
area. Where the heavy rain sets up remains very much uncertain with
2 different model camps in the 00Z guidance. The ECMWF/GFS/NAM
continue to focus the heaviest rain over northern zones along the
Canadian border while the CAM consensus (HREF) probability match
mean QPF is much further south from the Tug Hill eastward through
the southern Adirondacks and southern Vermont. Not wanting to
discount either of these camps, our official forecast of regionally
averaged QPF features 1-3", but a swath of 2-4" with isolated
amounts up to 5" remains very likely as the CAMs do highlight
pockets of hourly rainfall rates up to 2".

In addition to the flash flood threat is the potential for strong to
severe thunderstorms. The threat is conditional to the position of
the warm front Wednesday afternoon which consensus supports draped
through central portions of the forecast area, leaving
central/southern Vermont as the potential zone for convective
development. North of the front 0-3km shear is off the charts
nearing 60kts along the Canadian border which would hamper updraft
development, but in the warm sector it`s closer to 30-40kts and
coincides with MUBCAPE`s upwards of 1500 J/kg. Adding to the threat
is strong rotational shear/helicity with soundings indicating big
hodographs and 0-1km SRH of 200-300 m2/s2. This supports a non-zero
threat for tornados though the risk is low and more likely we could
see some strongly rotating mesos capable of producing damaging winds.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
As of 453 AM EDT Tuesday...Heading into Thursday there is strong
consensus among global models that the aforementioned warm front
lifts north of the Canadian border and is replaced by a strong mid-
level dry slot from early Thursday morning through the afternoon.
This should allow for rain to taper to isolated to scattered
showers, but also opens the door for additional convective
development as well as a moderate heat risk as 925mb temps support
highs across VT in the mid/upper 80s and combined with dewpoints
around 70 we`ll see heat indices up into the low/mid 90s. It won`t
be until Thursday night into Friday that the system lifts far enough
north to drag a cold front through the region, lowering PWATs below
1.5" and dewpoints come back down to more reasonable vales in the
upper 50s to low 60s.

Friday into the weekend guidance continues to favor dry conditions,
though the Bermuda high appears to strengthen again and some
increased moisture work back into the region supporting the chance
for showers. There`s not much consensus on this so we`ll offer a dry
forecast for now until Monday when a weak northern stream trough
potentially brings a chance for showers.

&&

.AVIATION /17Z TUESDAY THROUGH SUNDAY/...
Through 12Z Wednesday...VFR conditions are expected to prevail
for much of the next 24 hours as fog burns off quickly at MPV.
Winds this morning will be generally out of the southwest, with
some gusts possible 18Z-22Z Tuesday about 15-20 knots. There
will likely be some scattered showers and thunderstorms out
across the forecast area this afternoon and evening, which will
have the potential to be heavy and restrict visibilities. After
00Z Wednesday, winds will become much more calm and variable,
and the potential for patchy fog returns to the forecast area
overnight Tuesday night.

Outlook...

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

&&

.HYDROLOGY...
As of 453 AM EDT Tuesday...Confidence continue to be moderate
to high on the potential for heavy rainfall Wednesday afternoon
into early Thursday morning increasing the risk for flash
flooding. Uncertainty still exists in where the axis of heaviest
rainfall will be but periods of scattered to numerous showers
and thunderstorms are likely through the day with rainfall rates
within the strongest cells upwards of 2" per hour.

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 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, which
would be enough to exceed Flash Flood Guidance. In coordination with
neighboring offices and WPC, we`ve maintained a Slight Risk (Level 2
of 4) Excessive Rainfall Outlook for our region, mainly due to the
uncertainty in where the heaviest rains will occur, but if greater
consensus is found in the next forecast cycle we could see a portion
of our area upgraded to a moderate risk.

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, widespread rainfall totals are forecast to be 1 to 3 inches
and this would likely involve an embedded swath where values of 2 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. 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, and a reasonable worst case scenario would support
the potential for minor flooding at the Mad River at Moretown,
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.

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...Heat Advisory until 6 PM EDT this evening for VTZ009-011.
NY...Heat Advisory until 6 PM EDT this evening for NYZ035.

&&

$$
SYNOPSIS...Storm
NEAR TERM...Haynes/Storm
SHORT TERM...Lahiff
LONG TERM...Lahiff
AVIATION...Storm
HYDROLOGY...Haynes



 
 
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