463
FXUS61 KBTV 080749
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
349 AM EDT Mon Jul 8 2024

.SYNOPSIS...
Surface high pressure will provide the North Country and Vermont
with dry weather again today, and 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 Hurricane Beryl.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 348 AM EDT Monday...We`ll enjoy another day with warm
temperatures and somewhat comfortable humidity again today as
surface high pressure crests over the region. Morning fog will
rapidly lift into fair weather cumulus through the midday to
afternoon hours, and 925mb temps just 1-2C warmer than yesterday
will support highs once again in the mid- 80s to low 90s. As it
occurred the past 2 days, an isolated shower can`t be ruled out,
likely developing along the lake breeze over the Adirondack Coast
this afternoon. Confidence is low on this occurring so will
highlight just some 5-10% PoPs.

For tonight, mid/high clouds will shift into the region after
midnight which should limit the fog threat, and any evening showers
will dissipate. Lows will be a little milder than the previous night
in the 60s to around 70. Finally for Tuesday, the picture is murky
on the potential for convective development as there`s little
forcing available, but some weak embedded shortwave energy aloft
interacting with rising humidity, dewpoints in the mid/upper 60s,
and some weak surface instability may spark some isolated to widely
scattered storms. The main threat from any of these would be heavy
rain as PWATs look to rise towards 1.5" again. Highs will be similar
to Monday in the mid-80s to low 90s, and combined with the increased
humidity some areas could flirt with heat advisory criteria. Right
now there doesn`t appear to be enough coverage to warrant headlines,
but the experimental heat risk does highlight a moderate risk for
parts of southern Vermont, mainly due to consecutive days of heat.

&&

.SHORT TERM /TUESDAY NIGHT/...
As of 348 AM EDT Monday...This is the calm before the storm,
i.e. the increased flooding risk associated with the enhanced
moisture plume from Tropical Cyclone Beryl. Models have trended
drier for overnight Tuesday across our region. WPC has our area
in the Marginal (level 1 of 4) category for their Day 2
Excessive Rainfall Outlook but any isolated flash flooding risk
will be tied to convection. It will be a muggy night with dew
points remaining mainly in the 60s to near 70 in the Champlain
Valley. Overnight lows should generally be in the mid 60s to low
70s. For the discussion from Wednesday and beyond, including
the entirety of the heavy rain and flooding threat, refer to the
long term discussion.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 348 AM EDT Monday...BLUF: The potential for a significant
hydrologic event continues to increase for mid to late this week. We
remain concerned about the potential for scattered to perhaps
numerous flash flooding given the risk for rounds of heavy rain
falling on saturated grounds. At this time, the most probable time
frame for flash flooding threat is from Wednesday into Thursday time
frame. Unfortunately, it coincides with the 1 year anniversary of
the July 9-11, 2023 Vermont floods. To be clear, there are important
differences between July 2023 and July 2024. Last year`s flooding
was not tropical in nature. But there are also many similarities.
According to the Northeast Regional Climate Center data, much of our
forecast area with the exception of Windsor county saw 125-200
percent of normal monthly rainfall in June. And July has been wet so
far. Grounds are saturated and dry stretches have been hard to come
by.

The ICON model, which has pretty much nailed the track of Tropical
Cyclone Beryl so far, has the heftiest rainfall over northern NY and
northern VT, which corresponds to the stronger low pressure system
over the Great Lakes and a comparatively strong ridge over the
Atlantic Ocean. While the highest PWATs remain just to our south, it
does look like the best dynamics for rounds of moderate to heavy
rainfall is increasingly likely over our CWA, particularly the
northern two-thirds. So that would include most of northern NY, as
well as central and northern VT, roughly from I-89 corridor and
point north. A word of caution is that past performance is not
indicative of future results. Warm season convection remains rather
challenging to pin down, especially outside of the Convection-
Allowing Models time frame. If we were to wager a preliminary
rainfall forecast, expect widespread 1 to 2 inches of rain, with a
more focused swath of 2 to 4 inches to fall in the 48 hours between
Wednesday morning and Friday morning. Remember, that is not taking
into account convection. There are increasing indications that we
could be seeing scattered if not numerous 4 to 6 inches totals,
where convection or training of the moderate to heavy rainfall
occurs, with isolated 6 to 8 inches amounts. Those numbers, if they
do materialize, would be comparable to the flooding event we
experienced almost exactly a year ago.

In coordination with the WPC and neighboring offices, we have
continued with the Slight risk (level 2 of 4) Excessive Rainfall
Outlook for Day 3 to 4 (Wednesday 8 AM to Friday 8 AM). A reminder
that Slight risk is 15-40 percent probability of rainfall exceeding
flash flood guidance within 25 miles of a point. PWATs, which are
already AOA 90th percentile, will be enhanced even further past
maximum of SPC sounding climatology due to the moisture from
Tropical Cyclone Beryl. PWATs of 2.3 to 2.5 inches are 3 to 4
standard deviations above normal. With slow-moving surface
boundaries and generally a lack of atmospheric forcing to push out
the tropical air mass, rounds of heavy rainfall and embedded
thunderstorms will greatly the risk of flash flooding given the
grounds are becoming saturated. The greatest sources of uncertainty
right now are the areal placement of the richest PWAT plumes and
timing of convection. It is entirely plausible that our region still
escapes this week relatively unscathed. After all, warm season
convection remains extremely challenging to forecast. But it is
important to be prepared for the reasonable worst case scenario,
which is for scattered to numerous flash flooding if we do get
repeated rounds of heavy rainfall and storms to tap into the
abnormally moist atmosphere. Please stay up to date with the latest
forecast. For more details on the flooding threat, refer to the
hydrology section of the AFD.

As we head into Friday and next weekend, a cold front should push
the richest PWAT plume (200+ percent of normal) into southern New
England and the mid Atlantic states which will allow for slightly
drier albeit still seasonably moist air mass to potentially give the
North Country a break from the rainfall. Some of the more
pessimistic guidance shows potential for the cold front to lift back
north as a warm front and bring the risk for more heavy rainfall
into the North Country next weekend. So removed the heavy rain
wording from the forecast from Friday onwards but remember that if
we do get a break from the heavy rainfall, we might still be dealing
with main stem river flooding, especially further downstream. But
that is a low probability scenario at this time so generally went
with a blend of guidance for this portion of the forecast.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
Through 06Z Tuesday...IFR fog will continue to develop at KMPV
and KSLK over the next few hours with TEMPO fog likely at KEFK
from 07-10Z. After 12Z, VFR will prevail through the period with
FEW-SCT fair weather cumulus developing through the afternoon,
then trending to SKC-FEW after sunset. Light winds overnight
will trend SSW at 4-8kts from midday through the afternoon, with
the exception being KPBG where the lake breeze keeps the
direction southeasterly.

Outlook...

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

&&

.HYDROLOGY...
As of 348 AM EDT Sunday...Moisture from the remnants of Tropical
Cyclone Beryl could enhance rainfall locally and lead to increased
risk for flash flooding across the North Country Wednesday into
Thursday given recent antecedent rainfall and saturated grounds.
Much of northern NY and northern two-thirds of VT saw 125 to 200
percent of normal rainfall last month, with widespread 6 to 10
inches of monthly rainfall. Mid-range model guidance is coming into
better consensus on rounds of moderate to heavy rainfall impacting
these aforementioned areas. While there remains a good amount of
uncertainty with regards to the heavy rain placement and potential
convection outside of the Convection-Allowing Models time range, the
synoptic scale dynamics are increasingly favoring a reasonable worse
case scenario. With an unseasonably strong vertically stacked low
pressure system over the Great Lakes as well as a strong upper level
ridge centered southeast of the 70W/40N benchmark, the synoptics
favor the axis of heavy rainfall over northern NY and
central/northern VT in the Wednesday into Thursday time frame. With
an air mass that has 2 to 2.5 inches PWATs or 3 to 4 standard
deviations above normal, expect 1 to 2 inches widespread rainfall,
with a more focused swath of 2 to 4 inches to fall in the 48 hours
between Wednesday morning and Friday morning. Remember, that is not
taking into account convection. There are increasing indications
that we could be seeing scattered if not numerous 4 to 6 inches
totals, where convection or training of the moderate to heavy
rainfall occurs, with isolated 6 to 8 inches amounts. Those numbers,
if they do materialize, would be comparable to the flooding event we
experienced almost exactly a year ago. All things considered, there
is increasing risk for scattered if not numerous flash flooding to
occur mid to late week.

In coordination with the WPC and neighboring offices, we have
continued with the Slight risk (level 2 of 4) Excessive Rainfall
Outlook for Day 3 to 4 (Wednesday 8 AM to Friday 8 AM). Terminology
wise, Slight risk is 15-40 percent probability of rainfall exceeding
flash flood guidance within 25 miles of a point. As for potential
river flooding, the NAEFS probability of exceedance is currently
only showing 10 percent probability of exceeding minor flood stage
at Winooski River at Essex Junction (ESSV1), Otter Creek at Center
Rutland (CENV1) and Mad River at Moretown (MOOV1) as well as a 10
percent probability of exceeding moderate flood stage at East Branch
Ausable River at Ausable Forks (ASFN6). In other words, it would
take a 90th percentile rainfall scenario to result in the
aforementioned minor to moderate river flooding. As discussed above,
the 90th percentile rainfall scenario, or the reasonable worst case
scenario is becoming increasingly probable. There would likely be
two peaks in the river stages, one during overnight Wednesday into
early Thursday for the flashier river basins in the upper reaches
(e.g. ASFN6 or MOOV1) and a second crest on Friday after the steady
rain abates for the downstream mainstem rivers (e.g. ESSV1). Be sure
to stay up to date with the latest forecast.

&&

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

&&

$$
SYNOPSIS...Lahiff
NEAR TERM...Lahiff
SHORT TERM...Chai
LONG TERM...Chai
AVIATION...Lahiff
HYDROLOGY...Haynes