Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 231722
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
122 PM EDT Sat Mar 23 2024

.SYNOPSIS...
Active period of weather today through tonight with heavy rain,
flooding, wintry weather, and marine hazards. Drying out Sunday
into early next week but rather windy along the coast,
especially Cape and Islands. Below normal temperatures will
prevail. The next chance of rain will be Wednesday and Thursday
as another frontal system approaches.

&&

.NEAR TERM /THROUGH TONIGHT/...

Forecast is largely on track. The main change with this update
was to continue to lower the forecast temperatures today by a
few more degrees, especially over southern areas. Observed
temperatures were 2-3 degrees behind the forecast and the latest
hi-res guidance indicates this trend continues through the day,
only reaching the low 50s in southeast MA. The winter weather
advisory is being extended into the early afternoon for northern
MA to account for lingering near/sub freezing temps.

Previous Discussion...

Expansive area of cooling cloud tops lifting north along the
mid Atlc and NE with widespread precip moving across SNE. Snow-
ice line is lifting north across far northern MA and will move
into southern NH/VT during the next 1-2 hours. Icing will be the
main concern this morning across interior MA where temps are in
the upper 20s to lower 30s with low level cold air lingering
through the morning and possibly into the early afternoon across
the higher elevations in far northern MA.

Strong signal for a period of very heavy rain moving into CT/RI
and portions of central and eastern MA later afternoon through
the evening. Very strong low level convergence develops along
the surface boundary ahead of low pres, with very favorable
coupled upper jet structure enhancing ascent. Potential for 2+
inches of rain in a 3 hour period will likely lead to
widespread street flooding and some river flooding.

Previous discussion...

* Heavy rain this afternoon through tonight may poor drainage,
  urban, small stream flooding... in addition to flooding on the
  larger rivers.

* Freezing rain potential for northern Connecticut and central
  and western Massachusetts through late morning.

* Period of gusty winds late Saturday night into Sunday morning for
  Rhode Island and southeast Massachusetts.

Surface low pressure system develops to the south, and will quickly
intensify throughout today, as it phases with northern shortwave. Do
expect the low pressure system to track from the Jersey coast to the
Cape Cod Canal this afternoon, before racing into the Gulf of Maine
this evening. There have been minor changes to the forecast, first
off 00Z guidance has come in colder, areas outside of Rhode Island
and southeast Massachusetts could struggle to warm above 40 degrees,
there will be a fairly sharp cutoff, mid 40s in Boston to mid 50s
near Taunton. Have lowered temperatures across most of Massachusetts
(outside of the southeast section) and north/central Connecticut.
It is here colder temperatures, at or just below freezing will
linger into the mid morning, thus expecting the threat for freezing
rain to expand into northeast Connecticut and southern Worcester
County. Have expanded the Winter Weather Advisory as a result. It is
worth noting, there could be minor ice accumulation for extreme
northwest Rhode Island, places like Burrillville where there is some
higher elevation. The freezing rain threat ends by noontime, the
only exception would be far northwest Franklin County in
Massachusetts where spotty freezing rain could linger as late as
2pm.

Elsewhere in northern Massachusetts, areas along and north or RT 2
will have minor snow/sleet accumulations of an inch or two before
turning over to rain, thus washing it away.

The heaviest rain still expected to arrive in western Connecticut
and Massachusetts around noon time, but may take until 4pm to reach
the I-95 corridor. Some good news, this is a progressive system,
exiting east of the coastal waters by midnight, with a few lingering
showers for Cape Cod and the Island through as late as 4am.

Rainfall amounts have not changed, we still expected widespread 2-3
inches, though the axis of the heaviest rain has shifted to the
west, with a maxima from Connecticut into Rhode Island. High-res
guidance support locally higher amounts as well. HREF QPF ensemble
probability-matched mean has a reasonable worst case scenario of
isolated amounts greater than 4 inches for central Connecticut! HREF
also highlights central Connecticut with the potential, an 80
percent chance, that one inch of rain could fall in as little as
three hour period. The potential is there for poor drainage, urban,
and small stream flooding in addition to flooding along the main
rivers. The Flood Watch remains in effect through early Sunday
morning. Areas that typically experience nuisance flooding will want
to take note and prepare accordingly.

A few of the area rivers we are going to need to monitor include the
Pawtuxet River at Cranston, where minor flooding is likely, greater
than an 80 percent chance, and 40 percent chance of moderate
flooding. The Wood River at Hope Valley, where minor flooding is
likely, greater than 95 percent, and a 35 percent chance of moderate
flooding. Both of these rivers could reach flood stage late tonight.
The larger river in Connecticut are forecast to remain below flood
stage, but we will have to keep an eye on the smaller streams,
especially those in Hartford County where there is a signal for a
localized 4"+ of rain. Eastern Massachusetts, the Taunton River at
Bridgewater has a greater than 65 percent chance of reaching minor
flood by early Monday morning, as this river is slow to respond from
the runoff.

Later this afternoon a strong low level jet develops as the surface
low passes over the waters south of Rhode Island, between 20z and
04z, the 925mb jet is 70-80 knots, with some guidance as strong as
85+ knots. Have opted to issue a Wind Advisory for areas of Rhode
Island, southeast Massachusetts and the islands. Timing, the core
winds are expected between 5pm and 11pm. This is where we have the
greatest chance of southerly winds gusts of 45 to 50 mph.

These strong winds also coincide with high tide for Narragansett
Bay, see the Tides/Coastal Flood section at the end of the AFD
for more information.

As the system exits the region late tonight there will be clearing
from west to east and wind shift to the north/northwest and remain
gusty between 30 and 40 mph. Temperatures fall into the 20s and low
30s.

&&

.SHORT TERM /SUNDAY/...
Sunday is much drier, with the surface low pressure system off shore
our region placed in a tight pressure gradient. Clouds linger over
the eastern waters, and Cape Cod, further west we trend towards more
in the way of sunshine. Afternoon highs are cooler than normal, in
the mid 30s to 40 degrees. Though it will feel much colder due to
the persistent north/northeast winds gusting between 25 and 35 mph.
The feel like temperature will be mostly in the 20s for Sunday
afternoon.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Highlights:

* Mostly dry with below normal temps Mon and Tue, but risk for some
  spotty light rain Cape/Islands and possibly eastern MA,
  especially Tue. Windy near the coast.

* Unsettled Wed into Fri. Low pressure may bring more rain sometime
  Thu into Fri

Monday and Tuesday...

Cutoff low will maintain a surface low pres well to the south
through Tue as high pres builds over the Maritimes. This will set up
a strong NE pressure gradient bringing windy conditions along the
coast Monday, especially Cape/Islands where some 40-50 mph gusts
possible. Winds should diminish somewhat by Tue as the low level
wind field weakens. Mainly dry conditions expected with below normal
temps, but area of deeper moisture rotating in from the east could
bring spotty light rain to Cape/Islands and possibly portions of
eastern MA Mon and especially Tue. There remains some uncertainty
with how far west this moisture gets so our PoPs are low, but better
chance for some wet weather will be on Tue. ECMWF and GGEM are most
aggressive with rain spreading west across much of SNE Tue. May need
to raise PoPs for Tue if this trend continues. Expect at least more
cloud cover across eastern New Eng Mon and Tue. Highs will range
from the lower 40s eastern MA to upper 40s CT valley.

Wednesday through Friday...

Longwave trough moves slowly east from the central CONUS with
shortwave energy and deeper moisture approaching from the west
possibly bringing showers Wed. Then a potentially potent southern
stream system will be developing and lifting NE from the SE CONUS.
Forecast confidence is low on how this system evolves and its
subsequent track but potential for another coastal low pres to bring
more rain sometime Thu into Fri. Details are uncertain.

&&

.AVIATION /17Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

18Z update...

This afternoon...High confidence in trends.

Rain will pick up in intensity after 20Z across CT then heavy
rain spreads NE into RI and portions of central and eastern MA
this evening. Southeast winds gradually increase throughout the
day ranging from 10 kt across the interior to 15 to 20 kt over
the coastal plain. A strong low level jet will likely increase
winds across Rhode Island and southeast MA between 21z and 03z,
gusting as high as 35 to 40 knots.

Tonight...High confidence in trends. Moderate in timing.

IFR/MVFR cigs improve to VFR returns as precip gradually ends
from west to east from 02Z to 09Z. Rain lingers a bit longer
over The Cape/Island terminals likely tapering off between 04
and 07Z with VFR returning. Winds strengthen after 00Z behind a
surface cold front. 15 to 25 knot sustained northwest winds
with gusts up to 35 knots.

Sunday...High confidence.

VFR. Strong winds with local gusts up to 40 kt.

Sunday night...High confidence.

VFR. Northeast winds 15 to 20kts, gusting to 30 kts just
offshore.

KBOS TAF...RA becoming heavy this afternoon with IFR
conditions. Rain ends late this evening around 04z. Gusty
southeast winds as well, nearing 30 knot gusts around 00z
through 02Z/03Z. Shift from southeast wind to northerly wind by
03z tomorrow with improving CIGS to MVFR.

KBDL TAF...Rain for the rest of today with IFR to LIFR
conditions. Rain becomes heavy this afternoon and through this
evening, ending after 00z. Light northeast wind will become
northerly after 00Z and gusting 20 to 25 knots, and improving
CIGS to MVFR.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday Night: VFR. Strong winds with local
gusts up to 40 kt.

Tuesday: Mainly VFR, with areas MVFR possible. Windy with gusts
up to 35 kt.

Tuesday Night: MVFR/IFR conditions possible. Windy with areas
of gusts up to 30 kt. Chance RA.

Wednesday: Mainly VFR, with local IFR possible. Chance RA.

&&

.MARINE...
Forecaster Confidence Levels:

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

* Gale Warning in effect for all waters Saturday and continues
  into Sunday.


Today: Conditions deteriorate rapidly tomorrow as a strong low
pressure moves over southern New England. Steady 20 knot
southeast winds persist with gusts up to 40 knots possible,
especially over the southern coastal waters. Seas gradually
increase from 4 feet early in the day to 6 to 9 feet by evening.
Nearshore eastern zones stay a bit lower with 4 to 6 foot seas.

Tonight: A cold front moves over the waters tomorrow night.
Winds become even stronger out of the north/northwest with 30
knot sustained winds for most of the marine zones. Wind gusts
from 30 to 40 knots. Seas increase to 10 to 13 feet over the
outer coastal waters, and 6 to 10 feet for the nearshore waters.

Sunday: Low pressure system to our north will continue northerly
gales with gusts to 40 knots. Seas on the outer waters are
greater than 12 feet, near shore waves are 6 to 10 feet.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Moderate risk for gale force winds with gusts up
to 45 kt. Rough seas up to 14 ft.

Monday through Monday Night: Moderate risk for gale force winds
with gusts up to 45 kt. Rough seas up to 14 ft. Slight chance
of rain.

Tuesday: Low risk for gale force winds with gusts up to 40 kt.
Rough seas up to 13 ft. Slight chance of rain.

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 11 ft. Slight chance of
rain.

Wednesday: Winds less than 25 kt. Areas of rough seas. Chance
of rain.

&&

&&

.TIDES/COASTAL FLOODING...
There is some concern for Narragansett Bay during the high tide this
evening. Strengthening low level jet will be lifting north along the
coast near the time of high tide but it is possible the core of the
jet stays just to the east. If it does remain to the east, surge
will likely be limited to 1.5-2 ft which will keep water levels
below flood. However, there is some risk for a 2.5 to 3 ft surge if
the low level jet axis and stronger winds impact RI. This would
bring Providence and Fall River (Mount Hope Bay) into minor flood.
Confidence is not high enough to issue an advisory so we will go
with a coastal flood statement. The worst case scenario is for
pockets of minor flooding.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through Sunday morning for CTZ002>004.
MA...Flood Watch through Sunday morning for MAZ005>007-009-011>024.
     Winter Weather Advisory until 3 PM EDT this afternoon for
     MAZ002>004-008>012-026.
     Wind Advisory from 5 PM this afternoon to 11 PM EDT this
     evening for MAZ017>024.
RI...Flood Watch through Sunday morning for RIZ001>008.
     Wind Advisory from 5 PM this afternoon to 11 PM EDT this
     evening for RIZ004>008.
MARINE...Gale Warning until 2 PM EDT Sunday for ANZ230>235-237-251.
     Gale Warning until 2 AM EDT Sunday for ANZ236.
     Gale Warning until 8 PM EDT Sunday for ANZ250-254>256.

&&

$$

SYNOPSIS...KJC/Dooley
NEAR TERM...BW/Dooley
SHORT TERM...Dooley
LONG TERM...KJC
AVIATION...KJC/BW
MARINE...KJC/Dooley
HYDROLOGY...
TIDES/COASTAL FLOODING...KJC


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