Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 032023
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
423 PM EDT Mon Aug 3 2020

.SYNOPSIS...
A warm and humid night is in store for the region. We expect
showers to start to move into western parts of southern New
England later tonight. Conditions tomorrow will diminish quickly
deteriorate in the afternoon. Strong locally damaging winds are
expected across the region. Heavy rainfall across the western
areas could result in some flooding. Minor coastal flooding
along the south facing coasts are also expected. Isaias will
race rapidly north, and rapidly improving weather will set in
for Wednesday. Quiet and seasonable weather will round out the
week, with warmer temperatures again for the weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
400 pm update...

no big changes from the previous forecasts for the overnight
hours. Looks to be a warm and humid one. That humid air should
result in the development of low clouds and fog across the Cape
and Islands. Other aspect to the forecast will be the
development of showers in far western areas later tonight. You
can see that precip creeping northward across the mid-Atlantic.
Blended in some of the hi-res models to tweak the timing and
evolution of hourly PoPs. Models do show some instability, so
included mention of thunder, but I think it will be
predominantly some tropical-like rain showers.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
400 PM update...

What I`ve got for a forecast is linked to the NHC forecast for
Isaias -- along with a lot of behind the scenes collaboration
with surrounding offices, NHC, WPC and SPC. Not a lot of time to
sneak in meteorology, but I did my best. With the 12z suite
coming even more in line with timing (though GFS is still on the
quicker side), that does gives some added confidence in some of
the details. Still looks like this will be a quick hitting
storm, probably 6 to 8 hours that any location deals with it.
Figured I`d just run through the various aspects and give some
insight into the concerns/questions/uncertainty that goes behind
the forecast.

1. Heavy rainfall/Flooding. See the hydro section at the bottom
of the AFD

2. Strong Winds. A big concern, and still a lot of uncertainty.
All the models for days have been showing Isaias beginning it`s
transition to extra-tropical as it reaches the area. One of the
aspects of this is that the wind fields move away from the core
of the storm and expand, especially further east. It`s
concerning to see a pretty big area of 55-60kt and higher winds
at the 925mb level sweep across almost the entire forecast area.
Lots of questions as to how much of that will mix down. Feel
confident that we`ll see 40-50 mph gusts being pretty common for
much of the area -- especially closer to the storm center.
However models also indicate the potential for narrow bands of
convection to develop on the eastern side as well. This would be
a mechanism to bring down some of those stronger winds. Thus
isolated 60 mph gusts are not out of the question. With fully
leafed trees and the winds from a south/southeast direction
which is more unusual -- we could see more downed trees than
otherwise expected. Of course if Isaias is a bit stronger than
currently forecast, that just means more wind.

3. Severe Weather/Tornado Threat. It`s the eastern side of the
storm again. Huge helicity values 300-600 m2/s2 develop late
afternoon/early evening. Bulk shear values are 50kt+. Plus some
of the hi-res convective allowing models show bands of
convection that suggest embedded small supercells. Pretty
classic of landfalling tropical systems. SPC has increased the
severe risk to "Slight" for tomorrow. Seems reasonable. Have a
feeling we`ll have several short lived and small, but still
damage causing tornadoes anytime from mid afternoon to late
evening.

4. Storm Surge/Coastal Flooding. All available guidance suggests
not a huge storm surge, which is good news. But with the timing
of the system most likely closely aligned with high tides
Tuesday evening around 9pm (and those high tides are already a
bit higher than normal due to the full moon), it won`t take much
to produce minor inundation along south facing coasts.
Narragansett Bay and Buzzards Bay have a bit more of our
attention as depending on the exact wind direction, we could see
a surge be pushed up to the top of the bay. Maybe resulting in a
little bit more flooding. At most we are talking 1 or maybe 2
feet of above ground inundation, at least as of right now. If
Isaias comes in a little slower or faster and is not quite
aligned with high tide, then it changes everything.

Phew. I think that about does it for now. Believe that this
won`t be an over the top storm, but it`s going to be a pain in
the backside for a lot of the region -- especially since it`s
been quite awhile since we`ve had a tropical system like this
one. Irene in 2011 is probably the closest recent analog.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

* Some lingering showers post Isaias on Wednesday but not a washout.
* Quiet weather for rest of the week with seasonable temperatures.
  Thursday may be the pick of the week.
* There may be possible showers along the south coast on Friday.
  Otherwise, typical mid summer pattern with scattered showers and
  thunderstorms developing in the afternoon.

The second half of the week is fairly quiet with high pressure
returning and leading to drier weather. Warm with temperatures in
the low and middle 80s and muggy on Wednesday with dewpoints near 70
degrees. Much more comfortable weather on Thursday and Friday with
dew point falling into the 50s! With high pressure in control,
rainfall chances will be lower than normal so these days may be the
pick of the week for outdoor activities.

There may be some lingering showers on Wednesday. K index and Total
Totals values suggest isolated thunderstorms cannot be ruled out.
This is consistent with SPC`s current Day 3 outlook depicting
general thunder for Eastern MA and RI. Of course, thunderstorm
chances will hinge on how quickly Isaias exit the region. But in
general, not expecting a washout, especially for our western zones.
With mixing up to 850mb and 850mb temperatures between +15 to +17C,
expect highs in the mid to upper 80s.

Thursday looks to feature rather strong westerly winds, which will
help keep the sea breeze at bay and allow for downsloping effect. A
cold frontal passage will also usher in a cooler air mass, with
850mb temperatures falling to +12 to +14C. With mixing up to that
level, afternoon highs only top out in the low to mid 80s with upper
70s possible in the high terrain. On Friday, pressure gradient
weakens, which will allow for sea breeze development by late
morning, keeping coastal areas including Boston cooler than inland
locations. For most areas away from the coast, with deep mixing up
to 800 mb, dew points in the afternoon would easily drop into the
low to mid 50s for many locations away from the coast. So have
leaned towards the MOS guidance to lower the dew points. Thursday
evening looks to a good radiational cooling night, so have also
leaned towards the MOS guidance for widespread upper 50s and low
60s.

Then a disturbance riding along a stalled front to our south may
bring some showers to the south coast but confidence is low this far
out. But we are more confident in a warming trend beginning Friday
afternoon with 60s dew points making a return and temperatures
returning to slightly above seasonal normal. For reference, the
normal highs for early August are in the low 80s. Still comfortable
than of late for Saturday and most of Sunday with dew points in the
low 60s but mid 60s dew points return by late Sunday into Monday. So
enjoy before the more oppressive humidity makes a return! With a
return to mid summer pattern, the weekend will be accompanied by a
chance of afternoon thunderstorms but by no means a washout.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

18Z update...

Rest of Today...High confidence

VFR with WSW to W winds. Winds become gusty during the
afternoon.

Tonight...Moderate confidence

VFR initially. IFR expected to develop across the Cape and
Islands due to low ceilings and fog. Potential for scattered
showers to develop after 06z or so across far western MA and
northern CT. Kept to mainly VCSH in the TAFs.

Tuesday...Moderate confidence

Expecting ceilings to lower throughout the day, with MVFR
becoming common at most TAF sites. Chances for showers or
thunderstorms increase, especially western sections (eg: BDL,
BAF). Isaias will be rapidly approaching in the 18z-00z
timeframe, and expecting the winds to quickly ramp up during
that period.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: VFR. Breezy. Chance SHRA.

Wednesday Night through Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, patchy BR.

Saturday: VFR. Slight chance SHRA.

&&

.MARINE...
As of 345 pm...

Have raised Tropical Storm Warnings for all coastal waters since
the onset of dangerous conditions are within about 24 hours. The
primary time period for the strongest winds and highest seas
will be later Tuesday and the first half of Tuesday night. Seas
will take some time to diminish on Wednesday.

Outlook /Wednesday through Saturday/...

Wednesday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers.

Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

Thursday Night through Friday: Winds less than 25 kt. Slight
chance of rain showers.

Friday Night: Winds less than 25 kt. Chance of rain showers,
slight chance of thunderstorms. Local visibility 1 to 3 nm.

Saturday: Winds less than 25 kt. Chance of rain showers, slight
chance of thunderstorms.

&&

.HYDROLOGY...
As of 345 PM...

The exact track of Isaias will make a difference regarding how much
rainfall occurs in which portions of southern New England.
Precipitable water with Isaias ranges from 2 to 2.5 inches as it
makes its closest approach to/through our area, tied into a deep
plume of Atlantic moisture.

At this time and based on the foreast track, the highest rainfall
with respect to the NWS Boston forecast area is over western MA and
north central CT. In particular, a track along or west of the
Berkshires would produce a SE/SSE low level upslope flow along the
east slopes of the Berkshires and also the higher terrain in western
Hartford County. Locally higher rainfall totals are possible in
upslope areas. So have issued a Flash Flood Watch from midday
Tuesday through Tuesday night for western MA and Hartford County CT.

The main threats in the Flash Flood Watch area are small stream
flooding and the potential for significant urban and poor drainage
flooding. However, if we manage to get enough intense rain in a
broader watershed, it is possible that we could see a larger
tributary to the Connecticut River go into flood as well. Antecedent
conditions are quite dry with portions of the region in drought,
this should hinder more serious flooding.

Further east in our forecast area, rainfall amounts are forecast to
drop off, and while we may see some sharp rises on some small
streams in the Worcester Hills, the flood threat should be
diminished. Thus holding off on an eastward expansion of the FFA at
this time.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Tropical Storm Warning for CTZ002>004.
     Flash Flood Watch from Tuesday afternoon through Wednesday
     morning for CTZ002.
MA...Tropical Storm Warning for MAZ002>024-026.
     Flash Flood Watch from Tuesday afternoon through Wednesday
     morning for MAZ002-003-008>011.
RI...Tropical Storm Warning for RIZ001>008.
MARINE...Tropical Storm Warning for ANZ230>237-250-251-254>256.
&&

$$
SYNOPSIS...Nash
NEAR TERM...Nash
SHORT TERM...Nash
LONG TERM...Chai
AVIATION...Nash
MARINE...Nash
HYDROLOGY...NMB


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