Tropical Storm and Flash Flooding in Massachusetts, July 9, 2021

Area Forecast Discussion
National Weather Service Boston/Norton MA
738 AM EDT Fri Jul 9 2021

.SYNOPSIS...

Tropical Storm Elsa moves up the coast this morning, crossing
Rhode Island and southeast Massachusetts, then across the Gulf
of Maine into the Maritimes. Heavy rainfall from Elsa will
result in the potential for flash flooding, especially in the
Boston to Providence corridor. Unsettled weather in the form of
scattered showers/thunderstorms will linger through the weekend
and into next week, although there will be periods of dry
weather as well. Seasonable temperatures trending upward by mid
week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

7 AM Update...

No changes in the latest update. Forecast remains on track.

430 AM Discussion...

* Tropical storm force gusts expected much of RI and eastern MA
  with power outages possible

* Widespread rainfall of 2-4 inches; locally higher totals up to 6
  inches with flash flooding possible.

* Low risk of an isolated tornado this morning along the south
  coast of MA/RI

Latest observations and radar imagery indicate Tropical Storm
Elsa was just off the coast of Atlantic City, NJ at 08z/4am.
Mesonet site along the NJ coast indicating wind gusts up to 71
mph at Beach Haven, NJ! Model guidance continues to indicate
Elsa will slowly intensify (via height and pressure falls) as it
lifts up the coast this morning. This is more of a function
from non-tropical processes, such as the upper level jet streak
over southeast Canada, enhancing synoptic scale lift.
Also, trailing short wave energy across the Great Lakes is
aiding Elsa to take on a negative tilt. Given this
intensification, Elsa wind field will increase and expand as the
system transitions from Tropical to Post Tropical as it moves
across RI/southeast MA later this morning. This appears to be
verifying already on satellite and radar imagery, with
impressive convective band just to the north of the center.

Wind Threat...

Core of its wind field moves onshore to RI and southeast MA
between 12z-16z, lingering until 18z across Cape Cod. Therefore
this will be the period of strongest wind (greater than 34 kt)
and risk of wind damage/power outages. Given upward trend in
convective bands and rotating cells within these bands,
increasing concern for isolated tornado threat along the coast
(within 20 miles or so of the coastline) of RI and southern MA
including Cape Cod 12z-16z. In addition, intense rain bands will
enhance wind threat with stronger winds aloft mixing down to
the surface.

Flash Flood Threat...

Flash flood threat remains nearly unchanged, with just a slight
SE shift of heaviest rainfall/flood threat into the Boston to
Providence corridor, 12z-16z but lingering into Cape Cod thru
18z. This coincides with frontal boundary over RI and southeast
MA, enhancing low level convergence. Storm total of 2-4 inches
will be common with isolated amounts up to 6 inches possible
across RI and eastern MA.

As Elsa transitions to Post Tropical later today, system
accelerates northeast and exits eastern MA 18Z-21Z, with a wind
shift to the WNW and strong subsidence ending the rain along
with some breaks of sunshine possible. Highs in the 70s to near
80 in the CT River Valley.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...

*** A few strong thunderstorms possible this evening in CT ***

430 AM update...

Tonight...

Trailing short wave energy currently entering the Great Lakes
approaches CT this evening and south of the MA Pike. Could be
enough CAPE and shear for a few strong storms especially across
CT this evening, before weakening later this evening into RI and
eastern MA. Otherwise, a drying trend and seasonably warm with
lows in the 60s and moderately humid with dew pts in the 60s.
Thus, areas of fog possible.

Saturday...

Frontal boundary lingers along/near the south coast. Mid level
flow remains cyclonic and appears to be sufficient low level
moisture/instability to pop isolated or scattered thunderstorms
in the afternoon across RI and eastern MA, where greatest
instability and moisture is located. Not a washout with many
hours of dry weather, but may have to dodge a few afternoon
storms across RI and eastern MA. Seasonably warm with highs in
the upper 70s to lower 80s, and it will feel like summer with
dew pts 65-70. Also Beachgoers be alert for leftover rough surf
and strong rips along south facing ocean beaches, as it will
take time for wave energy from Elsa to completely erode.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

430 AM update...

Highlights

* Unsettled with scattered showers and storms most days. Best chance
  for widespread precipitation is late Sunday into Monday.

* Seasonable temperatures into early next week. Temperatures
  trending upward for mid to late week.

Ridge axis builds into the eastern Great Lakes on Sunday, but will
have a cutoff low situated over the Mid to Upper Mississippi River
Valley. Heights will continue to rise into early next week, but have
some uncertainty with how quickly the cutoff lifts out. Should see
several shortwaves eject into the region from the cutoff into early
next week. Less certain how things evolve mid to late in the week
given the differences with respect to the cutoff lifting out. Should
see increasing southwesterly flow, which will bring warmer air into
southern New England.

Saturday night through Monday...

Confidence in the forecast higher during this timeframe given there
is pretty good model agreement. Ridge axis builds into the region on
Sunday into Monday. The first shortwave lifts into southern New
England on Sunday and slowly lifts into northern New England by late
Monday. Will have a frontal boundary south of the region that
becomes a warm front and begins lifting northward on Sunday. The
front may still remain nearby or south of the region on Monday.

Have kept slight chance to chances of precipitation on Sunday with
the ridge building overhead and shortwave energy lifting in.
Thinking there will be better opportunities late on Sunday through
Monday with the frontal boundary closer to southern New England.

Will have better upper level support being in the equatorward
entrance region late on Sunday into early Monday where have chances
to likely precip. Given the prolonged southwesterly flow will have a
surge of 1.5-2.25 inch PWATS late on Sunday into Monday. Given warm
cloud layer depths are between 3.5-4.5 km could have some heavy rain
with any showers/storms. Nearly identical temperature wise on Sunday
and Monday with 925 hPa temperatures between 16-18 Celsius. Highs
around seasonable with readings in the mid 70s to the low 80s.

Tuesday through Thursday...

Daily chances of isolated to scattered showers and thunderstorms
across the region during this window. Should see the warm frontal
boundary lift through on Tuesday and should be within the warm
sector for Wednesday/Thursday. Don't appear to be any washouts in
this timeframe, but will have increasing instability. Deep layer
shear appears to be limited, so not anticipating an organized
convective threat at this point in time. High temperatures in the
80s for Wednesday and Thursday with 925 hPa temps between 20-25
Celsius.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...

Forecaster Confidence Levels.

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

*** Strongest winds & heaviest rain from Elsa 12Z to 16Z, except
 12Z to 18Z Cape Cod ***

Today...High confidence in flight categories, but moderate in
wind speed and direction.

Condition deteriorating as Tropical Storm Elsa moves into the
region. This will bring IFR to LIFR conditions this morning. Its
low level wind core (SSE winds) moves across eastern CT to RI
and eastern MA between 12Z to 18Z. This coincides with the
period of strongest winds and heaviest rain. Elsa moves rapidly
NE and exits into the Gulf of ME 18Z-21Z, with a wind shift to
the WNW. Behind Elsa will see improving ceilings and
visibilities with a drying trend. Could have some showers/storms
moving into western MA/CT late in the afternoon.

Tonight...Moderate confidence.

Scattered showers and storms sliding into the region. Should see
this activity diminishing as it moves eastward into RI and
eastern MA. VFR but MVFR/IFR along the coast in areas of stratus
and fog, with uncertainty how far inland these conditions will
track.

Saturday...Moderate confidence.

VFR but MVFR over RI and southeast MA in scattered afternoon
showers and thunderstorms. Uncertainty on how far inland
shower/thunderstorm threat along with lower cigs/vsbys. Winds
light and variable.

KBOS TAF...High confidence in TAF. Strongest wind and heaviest
rain 14Z to 20Z then improving with the wind shift to the WNW
18Z to 21Z.

KBDL TAF...High confidence in TAF. Heaviest rain until 20Z.
Strongest winds from 16Z to 20Z. Improving conditions 19Z to 22Z
with the wind shift to the WNW. Low risk for -TSRA 21Z to 03Z.

Outlook /Saturday Night through Tuesday/...

Saturday: Mainly VFR, with local IFR possible. Isolated SHRA,
isolated TSRA.

Saturday Night: VFR.

Sunday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Sunday Night: Mainly VFR, with areas MVFR possible. SHRA
likely.

Monday: Mainly MVFR, with areas VFR possible. SHRA likely,
isolated TSRA.

Monday Night: Mainly MVFR, with areas VFR possible. Slight
chance SHRA, isolated TSRA.

Tuesday: Mainly MVFR, with local VFR possible. Chance SHRA,
isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

*** Tropical Storm Elsa moves across the waters this morning ***

Today...

Elsa tracks across RI and southeast MA this morning before
exiting into the Gulf of ME this afternoon. SE winds 34+ kt with
winds shifting to the NW later this afternoon. Low risk of
waterspouts this morning near the coast. Torrential rain limits
vsby.

Tonight...

Elsa moving thru the Maritimes with wind and seas relaxing
across SNE waters. Leftover rough seas persist. Patchy dense fog
may impact vsby.

Saturday...

Light and variable winds but with leftover boundary/front just
offshore, scattered showers/thunderstorms possible during the
afternoon especially Cape Cod waters westward.

Outlook /Saturday Night through Tuesday/...

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Isolated rain showers, isolated thunderstorms, patchy
fog. Local visibility 1 to 3 nm.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers, isolated
thunderstorms.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers.

Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.
Rain showers likely, isolated thunderstorms.

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, isolated thunderstorms.

Tuesday: Winds less than 25 kt. Slight chance of rain showers,
isolated thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flash Flood Watch until 4 PM EDT this afternoon for CTZ002>004.
MA...Tropical Storm Warning for MAZ007-013>024.
     Flash Flood Watch until 4 PM EDT this afternoon for MAZ002>021-
     026.
RI...Tropical Storm Warning for RIZ001>008.
     Flash Flood Watch until 4 PM EDT this afternoon for RIZ001>007.
MARINE...Tropical Storm Warning for ANZ230>237-250-251-254>256.
&&

SYNOPSIS...Nocera/BL
NEAR TERM...Nocera/BL
SHORT TERM...Nocera
LONG TERM...BL
AVIATION...Nocera/BL
MARINE...Nocera/BL