Flash Flooding in Connecticut, August 18, 2024

Area Forecast Discussion
National Weather Service Boston/Norton MA
432 PM EDT Sun Aug 18 2024

.SYNOPSIS...
Dangerous rip current continue due to distant Ernesto into
Monday. Low clouds will be slow to erode today due to persistent
onshore flow. A slow moving frontal system moving in from the
Great Lakes will bring several opportunities for showers and
thunderstorms through Monday night. Localized downpours are the
main risk, but severe weather is possible later Monday afternoon
and early evening. Drier weather for the middle portion of the
week with below normal temperatures. Warming trend for Friday
into next weekend, but with continued generally dry weather.

&&

.NEAR TERM /UNTIL 11 PM THIS EVENING/...
430 PM Update:

Moist airmass in place over Southern New England, with residual
ridge of high pressure from northeast Nova Scotia helping to
maintain overcast but generally dry weather for most of the central
and eastern two thirds of Southern New England. On the western
periphery of the surface ridge over western MA and Hartford County
in CT is a persistent stream of moderate to at times heavy rains
with the heaviest rains over southern Litchfield and adjacent
Fairfield Counties. This is ahead of a slow-moving frontal system
that is very slowly circulating north of Lake Ontario; this feature
will be the main driver of our weather tonight and into Monday.

Ongoing area of moderate rains in western MA/CT (with heavy rains in
southern parts of Hartford County) will continue to slowly progress
north and east, and should weaken as it does so through late this
evening, as it moves into the cooler maritime airmass entrenched
over RI and eastern MA. Elsewhere in Southern New England, continued
overcast should expand in coverage along with areas of mist, but at
least through late evening the remainder of Southern New England
should be largely dry.

&&

.SHORT TERM /11 PM THIS EVENING THROUGH MONDAY NIGHT/...
430 PM Update:

We head into a more active weather pattern overnight tonight and
continuing into the early part of Monday evening, as the frontal
system north of Lake Ontario lurches towards Southern New
England.

Latest guidance indicates two periods of more active weather
that will need to be monitored in this period of time:

(1) Warm Frontal Showers/Embedded Thunder Overnight to Mid
Morning Monday - Most of Southern New England.

Weak warm front, which is focusing rounds of scattered to
numerous thunderstorms over central NY/PA as of this afternoon,
is expected to pivot ENE through the Hudson Valley and into and
through Southern New England overnight tonight into mid-Monday
morning. Localized downpours are expected associated with this
band of warm-frontal rain with embedded t-storms therein, and is
likely to coincide with the morning rush hour commute in
eastern MA and RI. It is a lower-probability outcome, but we do
have to watch for possible rotation with any t-storms which
develop along this warm frontal band during the overnight into
the Monday morning timeframe. Besides low lifted condensation
levels, southeast surface winds in conjunction with a 25-30 kt
950-925 mb low level jet lead to forecast curved low-level
hodographs, especially in eastern CT, central and southern RI
and into southeast MA. The significant mitigating factor is the
lack of instability, even in the lowest 3- km of atmosphere. So
while the main threat is from heavier downpours, we can't
discount the risk for brief rotation that could lead to
waterspouts and/or a short-lived tornado. We expect these warm
frontal rains/embedded storms to exit offshore by mid-morning
Monday.

(2) Scattered Strong/Locally Severe Storms Late Monday Afternoon
to Early Evening - Mainly North and West of I-95.

In the wake of the morning warm front, much of Southern New
England becomes entrenched in a narrow but very humid warm
sector. Ongoing stratus should begin to disperse during the
afternoon, which will allow for the atmosphere to destabilize at
least to some extent. Focus will then turn to western New
England, where an approaching cold front should interact with a
destabilizing airmass (CAPE values 1000-1500 J/kg in central and
western Southern New England) and foster scattered thunderstorms
to re-develop and quickly morph into a line or line segments.
Current thinking for these storms is later in the day (earliest
start after 4 PM, peaking during the 5-10 PM timeframe). A few
of these storms could produce isolated instances of straight
line wind damage/wet microbursts as they move into CT and
western and central MA. With eastern New England having less
time to really destabilize, storms should weaken upon approach
the I-95/I-495 corridor. Opted for gusty wind enhanced wording
with this round of thunderstorms.

Activity should really pipe down by late evening with
intermittent rain showers expected for the 2nd half of the
night, as the cold front moves eastward and flips winds to a
northerly direction.

Highs on Monday should reach into the mid to upper 70s, with
lower 80s in the CT Valley. With a significant air mass change
coming in for Monday evening, lows should drop into the mid 50s
to lower-mid 60s with a substantial drop in humidity level.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Key Points:

* Dry with unseasonably cool temperatures

* Warming trend into the weekend

Upper level closed low and cold pool aloft stalls over SNE through
Thursday keeping temperatures unseasonably cool for this time of
year. High temperatures through Thursday look to struggle to top the
mid 70s, with lows dropping into the low 50s and even some upper
40s. Expecting mainly dry conditions through the end of the week
with Wednesday being the one day where there may be some isolated
low topped showers. Otherwise, mid level warm layer and dry air
aloft should keep show activity at bay even under cyclonic flow.

Upper level closed low looks to exits the region to the northeast by
Friday with a shortwave ridge building in behind. Surface flow
begins to turn more WSW Friday through the weekend ushering more
seasonable temperatures in the low to mid 80s. Surface high pressure
underneath the upper level ridge should suppress any shower activity
leaving the region dry for the weekend.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Forecast Confidence Levels:

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

18Z TAF Update:

Rest of the Afternoon: Moderate confidence.

Generally MVFR bases in stratus, with periods of mist. SHRA are
more concentrated from ORH westward and that should continue to
be the case; visbys in those in the 2-6 SM range with the
lowest visbys for BAF and BDL. SE winds 4-8 kt.

Tonight through Monday Morning: High confidence in trends, but
lower to moderate on timing.

Categories deteriorate to IFR-LIFR levels with stratus expanding
northward through 05z Mon along with 2-6 SM mist. An
approaching warm front will bring a period of SHRA with possible
embedded TS northeastward from southern NY/southern CT for the
overnight through Monday morning. See TAFs for timing which was
indicated with 2SM SHRA, but that timing could slip a bit.
While this threat looks greatest during the overnight to early
Monday period for BAF-BDL, greater impacts to the Monday
morning push for eastern terminals associated with this band of
SHRA/TS. Categories are still expected to be in the IFR- LIFR
range until we get to Mon aftn, so once categories drop to these
levels, they are not expected to improve much at all until the
aftn. SE winds 5-10 kt should shift to S/SSW upon passage of
warm front.

Monday Afternoon: Moderate confidence.

Expecting to see some breaks in clouds/improvement in flight
categories toward MVFR levels. Couldn't rule out a rogue SHRA or
TS during the afternoon but think better chances for +SHRA/TSRA
arrive late Monday afternoon (after 22z?) for the western
airports associated with the approach of a cold front. These
storms may become strong and produce gusty winds, frequent
lightning and heavy downpours reducing visby. SW winds 5-10 kt.

Monday Night: High confidence.

SCT TSRA, some strong to start, should advance eastward off the
Berkshires/CT Valley and weaken with eastward extent between
01-03z Tue. MVFR/IFR to start but should see rapid improvement
to VFR upon passage of cold front, but may take until later
Mon night or early Tue before we see VFR categories for eastern
airports.

KBOS TAF...High confidence in trends, moderate in exact timing.

IFR to start with decreasing -SHRA. Gradual improvement to MVFR
this afternoon, though it is becoming less likely that cloud
bases rise into VFR range. Ceilings deteriorate to IFR tonight
before another round of showers (some heavy at times?) overnight
into early Monday morning. SE winds 4-8 kt.

KBDL TAF...High confidence in trends, moderate in exact timing.

MVFR ceilings, with 3-6 SM -SHRA (TEMPO 2 SM visby SHRA thru
17z). Lighter SHRA for the afternoon to early evening before
another round of showers (some heavy at times?) overnight. Light
winds.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with local IFR possible. Chance SHRA.

Tuesday Night through Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.

Through Monday Night...High confidence.

Expecting persistent SE to S flow at 5-10 kts through the vast
majority of the period. Wind gusts remain generally below 20 kts
as well. Seas remain elevated due to the powerful swell
associated with Ernesto. This keeps 5-7 ft seas across the outer
waters through Monday. Small Craft Advisory continues through
Monday night across the outer coastal waters. This is also the
case for the nearshore waters across coastal Rhode Island where
3-5 ft seas linger through Monday. Rain showers along with a few
thunderstorms could spread into the waters tonight into Monday.

Outlook /Tuesday through Friday/...

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers, isolated thunderstorms.

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Wednesday through Friday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents through Monday evening for MAZ007-
     022>024.
RI...High Risk for Rip Currents through Monday evening for RIZ006>008.
MARINE...Small Craft Advisory until 8 PM EDT Monday for ANZ235-237.
     Small Craft Advisory until 8 AM EDT Tuesday for ANZ250-254>256.

&&

SYNOPSIS...Loconto/KP
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...KP
AVIATION...Loconto/KP
MARINE...Loconto/KP