708
FXUS61 KBTV 081436
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1036 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 1034 AM EDT Monday...No changes for the 1030 am update
with going forecast in good shape. Some fair weather cumulus is
forming and temperatures are on track to reach the upper 80s to
around 90 this afternoon. Previous discussion follows.

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 /15Z MONDAY THROUGH FRIDAY/...
Through 12Z Tuesday...After morning fog lifts from KMPV, VFR
will prevail across the region through the period with FEW-SCT
fair weather cumulus developing through the afternoon, then
trending to SKC-FEW after sunset. Light winds this morning will
trend SSW at 4-8kts from midday through the afternoon, with the
exception being KPBG where the lake breeze keeps the direction
southeasterly. Winds return to light and variable tonight, with
lingering mid level clouds around which should help to inhibit
widespread fog formation.

Outlook...

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

&&

.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/Neiles
SHORT TERM...Chai
LONG TERM...Chai
AVIATION...Lahiff
HYDROLOGY...Haynes