BOX issues Area Forecast Discussion (AFD) at Aug 3, 9:55 AM EDT583
FXUS61 KBOX 031355
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
955 AM EDT Sat Aug 3 2024
.SYNOPSIS...
A period of mainly dry weather for southern New England today,
before scattered showers and thunderstorms develop late in the
day. Another round of showers and thunderstorms is expected
Sunday. The stronger storms this weekend will contain torrential
downpours and gusty winds. It remains very warm and humid,
especially Saturday. Dry for much of Monday, but a cold front
will bring scattered showers and thunderstorms during the
afternoon. A few stronger storms cannot be ruled out. Cooler
after Monday, but the pattern remains active for much of the
upcoming week with a frontal boundary stalled nearby or
overhead.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10 AM update...
* Another hot and humid day with heat indices 95-104
* Mainly dry weather, then risk of storms after 4 pm
Weak short wave moving across northern New England early this
morning and triggering scattered showers and thunderstorms along the
MA/NH border northward. This activity will exit NE into Maine with
weak short wave ridging, associated subsidence and some column
drying develops in its wake across SNE later this morning and
afternoon afternoon. Thus, a period of dry weather today along with
morning clouds giving way to sunshine. Still a very warm airmass
over the region with temps aloft +23C at 925 mb and +18C at 850 mb.
These temps aloft combined with mixing to around 850 mb, supports
highs from the upper 80s across the high terrain, to low 90s across
most of the region, with low to mid 80s along south coastal beaches
of MA and RI. These temps combined with tropical dew pts in the low
to mid 70s, will yield dangerous heat indices from 95 to 104 across
much of the region, including Boston, New Bedford, Fall River,
Providence and Hartford. Thus, heat advisory remains posted. SW
winds 10-15 mph, with gusts up to 20 mph over southeast MA will
provide some minor relief.
Latest SPC mesoanalysis shows moderate instability is building
with CAPEs already near 1500-2000 J/kg. However, forcing for
ascent does arrive until after 20z across CT into western-
central MA. This instability combined with steep low level lapse
rates up to 9C/KM, coupled with PWATs climbing to 2 inches and
dew pts in the low to mid 70s, supports a low risk of wet
microburst. Latest Hi-res guidance and SPC outlook indicate
greatest risk will be across western CT into southwestern MA
after 20z/4PM. The remainder of the region will remain mainly
dry, with convection likely holding off until 00z/8pm and
thereafter.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
415 AM update...
* Another round of scattered showers/thunderstorms Sunday, with a
few strong to severe storms possible
* Not as hot Sunday, but remaining very humid
Saturday night...
Short wave energy continues to advect across the region. This
increases deep layer shear up to 30 kt and combined with high dew
pts yielding some instability, scattered showers and isolated
thunderstorms are expected to traverses the entire region. A few
strong storms are possible this evening up to about 03z. Environment
favors wet microburst storms. Remaining warm and humid tonight,
with lows in the upper 60s to lower 70s, mid 70s for the urban areas
of Boston. Given high dew pt airmass continues, patchy fog is
possible.
Sunday...
Short wave trough moves across the region, yielding deep layer shear
up to 30 kts along with moderate instability with CAPEs up to 1500
j/kg. Hence, modest shear and CAPE profiles will support the risk
for a few strong to severe storms, with potentially damaging winds.
This is supported by CSU machine learning wind probs. Tropical
airmass remains in place with PWATs up to 2 inches and dew pts in
the 70s, supporting localized flash flood threat.
Otherwise, very warm and humid conditions continue. Not as hot as
previous days, with highs in the low to mid 80s. However, with dew
pts in the 70s, it will remain very uncomfortable with heat indices
in the low 90s. Southwest winds 10-15 mph with gusts up to 25 mph
across Cape Cod and the Islands.
&&
.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
415 AM update...
Highlights
* Dry for much of Monday, but cannot rule out hit or miss
showers/storms during the afternoon. A few strong storms are
possible. Oppressive heat and humidity.
* Unsettled on Tue as a frontal boundary slides in.
* Forecast confidence drops significantly mid to late in the week as
the front may still be nearby/overhead. Thus, could still be
unsettled and will need to keep an eye on a potential tropical
cyclone traveling up the east coast.
Monday...
Beginning of a pattern change. Initially will have a shortwave over
southern New England late on Sun. The shortwave lifts offshore by by
early on Mon. Flow turns more cyclonic and quasi zonal for the rest
of Mon. A shortwave digs into southern Quebec by late in the day.
High pressure nudges in early on Mon, but a cold front will approach
from the NW late in the day.
Could have a few lingering showers/storms before sunrise across
portions of the south coast before sunrise. Overall expect dry
weather to prevail for much of the day. Though will have increasing
chances for showers and thunderstorms during the afternoon and
heading into the evening. Will need to keep an eye on this period as
there are signals that we could have some strong to perhaps severe
thunderstorms. Ahead of the front will have a few hundred to roughly
2000 J/kg of MUCAPE and deep layer shear in the 0-6 km layer
increasing to 30-40 kts. The big questions are we are quite removed
from the upper forcing (and questions on timing the cold front) and
given the S/WSW flow will have some downsloping. This results in
slightly drier dewpoints and warmer temperatures. Mid level lapse
rates are around 6 degrees Celsius and low level lapse rates are
nearly adiabatic. Based on the environment think that a few
strong/severe storms cannot be ruled out. This is supported by
latest CSU ML guidance, which shows a 15-30 percent probs of
strong/damaging winds and low probs (5-15 percent) for severe hail.
Did bump up our temps and lowered our dewpoints due to the
downsloping anticipated. Should result in another day of oppressive
heat with highs solidly in the 80s with some low 90 degree readings
across the Merrimack and CT River Valley. Should see dewpoints in
the 60s for most other than the immediate south coast where they are
in the lower 70s. The end result are heat indices ranging from 85-95
degrees.
Tuesday...
Cyclonic quasi-zonal flow in place through this timeframe. Fast
moving shortwave sliding across the Great Lakes region on Tue. Some
uncertainty on the exact timing this wave slides into and through
our region. The cold front previously mentioned will slowly slide
into and perhaps through much of southern New England.
Main change in the latest update was to increase precipitation
chances from the default NBM through this timeframe, but especially
during the afternoon hours. Should have a southwesterly (15-30 kt)
low level jet impinging on the region as the front is sliding in.
This will likely bring rain showers for much of southern New
England, but the best shot will be along the immediate south coast.
The big question will be where the frontal boundary stalls/sets up
as at this point we appear to be on the cool side of the front, so
there is not much instability to work with. Should have PWATs of
1.75 to 2+ inches, which is around 1-2 STD above model climo per
NAEFS/EPS. Still have decent warm cloud layer depths of 3-4 km, so
warm rain processes may be efficient. Think the stalled out front in
combination with the moisture and LLJ could result in some training
storms, so flooding potentially a risk. This matches up well with
WPC latest Day 3/4 ERO.
With the cold front finally through much of southern New England
will see a cooler airmass in place. NBM generally looked reasonable
so have kept the temps generally in the 70s.
Mid to late in the week...
Considerable uncertainty during this timeframe with a front stalled
nearby or over us. This may interact with the tropical system
lifting from the Southeastern US. There are still significant
differences with the exact timing and track of the tropical system,
along with little run-to-run consistency. Due to this have stuck
with the NBM. Could have a heavy rain risk, especially if front
remains overhead and we tap into the tropical moisture. The EPS is
showing PWATs of 1-1.5 STD above model climo whereas the NAEFS is
more muted through this period. Something we will need to monitor as
we get closer to this timeframe.
&&
.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Today...Moderate confidence.
IFR/LIFR across much of the region in low clouds and fog,
except MVFR over northeast MA including KBOS. Terminals will
gradually improve to VFR by this afternoon. Though may still
have IFR/LIFR ceilings lingering nearby ACK. South coast
terminals take the longest to achieve this. Dry weather much of
the day, before storms begin to fire after 20-22Z across CT
into western/central MA. Thus, included a PROB30 for
thunder at BDL/BAF and ORH. SW winds 10-15 kt, gusts up to 20
kt southeast MA.
Tonight...Moderate confidence.
Scattered showers and thunderstorm until about 03z with a focus
across CT into western-central MA. Then mainly dry after 03z.
VFR/MVFR at sunset transitions to IFR/MVFR overnight. SW winds
10-15 kt.
Sunday...Moderate confidence.
IFR/MVFR in the morning lifts to MVFR/VFR in the afternoon.
Scattered showers and thunderstorms develop in the afternoon. A
few of the stronger storms may produce gusty winds and
torrential downpours. SW winds 10-15 kt, gusting up to 20 kt
over southeast MA.
KBOS TAF...High confidence in trends, lower confidence in
exact timing.
Mainly dry weather today. No seabreeze, SW winds 10-15 kt.
Could see some showers/storms tonight, but have held off on
thunder mention for now as am not confident on thunder coverage
across eastern MA.
KBDL TAF...High confidence in trends.
MVFR to start, but will improve to VFR 15-18Z. Mainly dry
weather but increasing chance of storms 20z-02z. Thus, have
inserted a PROB30 of -TSRA during that time. Gusty winds and
torrential downpours with any strong storm.
Outlook /Sunday Night through Wednesday/...
Sunday Night: VFR. Breezy. Slight chance SHRA.
Monday: VFR. Breezy. Chance SHRA, slight chance TSRA.
Monday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA, slight chance TSRA.
Tuesday: Mainly MVFR, with areas VFR possible. Chance SHRA.
Tuesday Night: MVFR/IFR conditions possible. SHRA likely.
Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA,
isolated TSRA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
415 AM update...
Today through Sunday...
Persist SW winds 10-15 kt, with winds increasing up to 20-25 kt
Sunday afternoon. This combined with long SW fetch, SCA remains
posted for building seas and increasing winds tonight and Sunday.
Late night/early morning fog expected, but improves by late morning
into the afternoon each day. Scattered thunderstorms late Sat, Sat
evening, then again Sunday afternoon/early evening.
Outlook /Sunday Night through Wednesday/...
Sunday Night: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers.
Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.
Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers, isolated thunderstorms.
Tuesday Night through Wednesday: Winds less than 25 kt. Areas
of seas approaching 5 ft. Rain showers likely, isolated
thunderstorms.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 7 PM EDT this evening for CTZ002>004.
MA...Heat Advisory until 7 PM EDT this evening for MAZ005>007-
011>021.
RI...Heat Advisory until 7 PM EDT this evening for RIZ001>004.
MARINE...Small Craft Advisory from 6 PM this evening to 8 PM EDT Sunday
for ANZ254>256.
&&
$$
SYNOPSIS...Nocera/BL
NEAR TERM...Nocera/BL/KP
SHORT TERM...Nocera
LONG TERM...BL
AVIATION...Nocera/BL
MARINE...Nocera/BL
Source:
BOX issues Area Forecast Discussion (AFD) at Aug 3, 9:55 AM EDT----------------
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