246
FXUS61 KBTV 071919
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
319 PM EDT Sun Jul 7 2024

.SYNOPSIS...
Few impacts are expected over the next 48 hours, with temperatures
remaining near 90 for most locations. Tuesday afternoon will bring a
threat of thunderstorms, with the potential for locally heavy
rainfall. The bigger story is Wednesday and Thursday, where considerable
uncertainty exists. There is a potential for flooding as a warm
front taps into the moisture associated with Tropical Storm
Beryl.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 308 PM EDT Sunday...A widespread fair weather cumulus field
will linger through the evening hours, with some chance for
precipitation to fall mainly along the international border. The
clouds will dissipate in the evening when the daytime heating shuts
off. Overnight, skies will clear and the winds will become light and
variable. We will see some fog formation along low lying areas.
Monday will be another warm day, with temperatures reaching into the
90s in some locations. No heat headlines are anticipated, but the
experimental Heat Risk product shows widespread moderate impacts for
Vermont.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 308 PM EDT Sunday...On Tuesday, another warm and humid day is
anticipated. An upper trough will enter a pocket of channeled flow
between an upper low in the Labrador Sea and a strengthening Bermuda
High. This will act to stretch the feature apart. There`s plenty of
shear present on the order of 45 to 55 knots due to the faster flow
aloft. In the low levels, there is some 850mb divergence and the
upper jet is not very favorably set up at this point, though. So
thunderstorm activity will mainly be driven by micro to mesoscale
features and propagate from there with only a minor amount of
frontal forcing available across southern and eastern Vermont mid-
afternoon before scooting east. Conditions start out relatively dry
followed by the increasing moisture and instability towards evening
although the front comes through earlier in the afternoon. So the
timing of features does not appear to align. But if anything can get
going in the evening, PWATs are expected to surge up to 2 inches. So
any storm will be capable of locally heavy rainfall for which a
marginal risk Excessive Rainfall Outlook (Level 1 of 4) remains and
and perhaps gusty winds due to strong winds aloft.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 308 PM EDT Sunday...Attention turns to Wednesday and tracking
the remnant moisture of Tropical Storm Beryl (at time of writing).
Before reaching our area, the tropical cyclone will have undergone a
couple interactions with two northern stream shortwaves, and the
track of moisture will also be influenced from the placement of that
stretched out frontal boundary that will have crossed through on
Tuesday. So there are some track complexities to consider, along
with some model differences with the speed this all takes place.
However, there is potential that these features line up in such a
way that we could receive a lot of rain between Wednesday evening
into Thursday. The timing on last year`s flood anniversary is
unfortunate, but like severe weather, July is climatologically a
common month for dealing with Flash Flood concerns. The 90th
percentile (or a 10% chance) based on the trained NBM data yields
about 2 to 3 inches of rain, and with any convective element would
likely include locally higher amounts. However, given the
uncertainty in exact track, there is a similar 10% chance of
receiving a tenth to quarter inch. Little has changed with the
picture of climatological statistics, PWATs running 2-2.25" and even
2.5" possible, which is at least 3 standard deviations above normal.
The GFS rainfall forecast exceeds 90 percent of its model
climatology and the extreme forecast index from the ECMWF is on the
upper end, but not nearly so stark as last year`s flood event (for
which the ECMWF EFI will have in its reforecast system). There is a
large amount of model spread, and so users are encouraged to
monitor, but average NHC low tracks in the 4 to 5 day time range are
150-200 miles. The forecast for this impacting our region has only
begun to emerge in the last 24 hours, and the axis of heavier rain
has been in flux with no clear trend at this time. More will be
known tomorrow.

Forecast models diverge even further beyond Friday. The pattern
looks warm and humid, but the chances for precipitation will depend
on the moisture plume around a Bermuda High that models are quick to
weaken. This seems unlikely given the strength of it Thursday into
Friday where the 500hPa heights are above 600 dam. So would think we
might be more along the ring of fire than not.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
VFR conditions will continue at all TAF sites for the rest of the
day. Widespread light SW winds will transition to light and variable
overnight. These light winds will allow for fog development for the
favored areas such as SLK, MPV, and EFK.

Outlook...

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

&&

.HYDROLOGY...
The potential for heavy rainfall on Wednesday evening into Thursday
will drive the potential for flash flooding. There could be two
phases to this with initially more widespread rainfall Wednesday
night and early Thursday, and then scattered to numerous
precipitation Thursday afternoon with locally heavy rainfall. The
forecast range for precipitation is currently high, and multiple run
model trends are not showing much consistency in placement. In
short, confidence is high that somewhere will get very heavy
rainfall amounts with totals in excess of 3 inches. Exactly where
that will be will need some more time.

For now, early forecast estimates between Wednesday evening and
Thursday indicate averaged totals across the region of roughly 1 to
2 inches. The 90th percentile (or 10 percent of model forecast
probability) suggests 3 inches, and we are not yet within the range
of high resolution guidance that can depict heavy rainfall processes
better. On the other end, the reasonable best case scenario suggests
about a tenth to quarter inch of rain. This is mainly due to track
uncertainty impacting rainfall totals.

At this time, ensemble forecasts of rivers do not indicate much of a
river response. However, streamflows are currently running on the
90th percentile of observations, mainly in central Vermont. So we
may have to monitor the evolution of river forecasts as well.
Additionally, if rainfall in excess of 3 inches is realized, we may
need to consider low level, farm field and road type flooding. The
Day 4 Excessive Rainfall Outlook covering this event indicates a
slight chance (15 to 40 percent chance of rain exceeding Flash Flood
Guidance).

&&

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

&&

$$
SYNOPSIS...Langbauer
NEAR TERM...Langbauer
SHORT TERM...Haynes
LONG TERM...Haynes
AVIATION...Langbauer
HYDROLOGY...Haynes