Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 161102
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
702 AM EDT Fri Apr 16 2021

.SYNOPSIS...
A late season snowstorm will continue today across the higher 
terrain of the Berkshires and Worcester Hills; while rain and
snow along with some minor accumulations are possible in some
lower elevations. A few showers may linger on Saturday, but
mainly dry weather is expected Saturday night through Monday. 
Turning mild and dry for Tuesday before a cold front brings our 
next chance for more widespread rain around Wednesday.

&&

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

7 am update...

Heavy rain and high elevation heavy, wet snow continue this
morning. Regional radar shows the swath of heaviest
precipitation over eastern MA and RI at 7 am rotating
cyclonically around the low centered just off Nantucket, but
this will lift north through the morning as the dry slot invades
from the south. This means a quicker respite from the rain for 
far eastern MA than central/western region. 

Previous discussion...

Highlights...

* Significant Snowstorm Continues in the high terrain of the
  Berks/Worcester Hills today & some power outages are possible

* A small slushy accumulation possible in some lower elevations

Details...

A classic setup continues today for a mid April snowstorm across
the high terrain of southern New England. It is quite difficult
to get significant snowfall this time of year unless you have a
very dynamical system, which is exactly what we have in this
case. However, this will continue to be a difficult forecast as
thermal profiles are very marginal. This means that a subtle
shift in track, precipitation intensity will make a huge
difference in heavy wet snow versus plain rain. We will break
things down a bit more below.

The latest guidance has quite the dynamic situation especially
this morning. Intensifying closed 700 mb low moving across 
southern New England coupled with deep moisture plume supports 
heavy precipitation and banding. We think the heaviest
precipitation will be roughly from 6 am through 2 pm in 
response to those intensifying mid level centers. Precipitation
will continue though after that time.

Still uncertainty involving where the strongest banding sets up
and how far northwest the mid level dry slot penetrates. Given 
extremely marginal thermal profiles and the fact that it is the 
middle of April, a subtle change will make a huge difference 
between rain and heavy wet snow. We still think the main area 
of snow accumulations will be above 400-500 feet with 
significant accumulations expected near and above 1000 feet. 
Greatest concern remains in the Worcester Hills and east slopes 
of the Berkshires; where 6+ inches of 32-33 wet snow will result
in the potential for some power outages. Localized 9-10 inch 
amounts are possible in the highest of these elevations.

In areas below 400-500 feet, odds favor this being mainly a rain
event with some wet snow flakes mixed in at times. However, it
is possible that precipitation comes down heavy enough for a
time this morning to result in a quick inch or two of slush. 
This would mainly be west of the I-495/128 belt. East of that 
region and into the Boston to Providence corridor, the surface 
low and mid level dryslot appear to track too far inland. So in 
this region, just expect rain with perhaps a few mangled 
snowflakes during times of heavier precipitation.

So in a nutshell...we expect the bulk of the heavy snow and
perhaps some power outages to be in the Worcester Hills and the
Berks. We will continue Winter Storm Warnings for this region;
where some power outages will be possible. Snow accumulations
are also possible in the northeast CT Hills and high terrain of
northwest RI, mainly this morning. Across some of the lower 
elevations of the interior; minor slushy accumulations are 
possible but precipitation type maybe more in the form of rain 
outside heavier intensity.

Lastly, we did hoist a Wind Advisory for coastal essex County.
This is where the nose of an easterly 925 mb jet, where a period
of 40 to 50 mph gusts are expected later this morning and
afternoon. Across the rest of our coastline, expect wind gusts
on the order of 30-40 mph.

&&

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

Vertically stacked only low slowly move off the coast & will 
allow rain and snow showers to continue tonight; particularly 
across our eastern zones. However, the intensity will not be 
nearly as strong as what we are expecting today. This coupled 
with temperatures mainly above freezing will make for any 
additional minor snow accumulations limited to the very highest
terrain of the Berks/Worcester Hills.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Saturday and Sunday...

The weekend features periods of clouds and sun with some hit and 
miss showers, though not a washout at all. The low to our east very 
slowly moves away thanks to the blocking pattern in the mid levels, 
and the column doesn't truly dry out until around Tuesday. In the 
mean time, flow remains rather zonal to slightly cyclonic with a 
meager cold pool aloft, helping to keep clouds and periodic rain 
chances around. Still, lacking any large scale forcing for ascent, 
most places will be dry at any given time. Temperatures moderate a 
bit each day as warm advection from the west begins. Highs will be 
in the upper 40s Saturday except for the Connecticut River Valley 
which reaches the low 50s. Warmer still on Sunday on milder 
northwest flow, with highs in the mid to upper 50s. 

Monday and Tuesday...

Largely dry and warmer on Monday, but a weak shortwave swings 
through the flow bringing the chance for scattered showers during 
the day. The better moisture and forcing remains far to our south, 
so not expecting a washout by any means. Notably, warm advection 
leads to highs back in the 50s on Monday and 60s on Tuesday! 
Tuesday's warmer than average temperatures will be the result not 
only of a warmer airmass (850 mb temps +5C) but full sunshine under 
clear skies. This is thanks to a brief ridge of high pressure. With 
the warm temps, more moisture surges north. Dewpoints rise back into 
the upper 40s to near 50, so we may deal with some fog along the 
south coast overnight Tuesday into Wednesday. 

Wednesday and Thursday...

Unsettled weather returns by mid week thanks to an incoming 
shortwave trough and surface cold front. Too soon to for details 
like timing and placement of the heaviest rain, but more widespread 
precipitation appears likely around Wednesday or Thursday.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

12Z Update...

Today...Moderate Confidence. IFR to LIFR conditions dominate
today in widespread rain and snow; which will be heavy at times
especially this morning. Ptype will mainly be snow across
interior southern New England above 500 feet in elevation; at
least when the precipitation is coming down heavy. Below 500
feet towards the Boston to Providence corridor; Ptype will 
mainly be rain but a few wet snowflakes may mix in when the 
precipitation is heaviest. E wind gusts of 25 to 35 knots along 
early today, but across the Cape/Islands they will shift to the 
SW for a time later today.

Tonight...Moderate Confidence. MVFR-IFR conditions persist. Rain
and snow showers will linger tonight; particularly across the 
eastern half of the region. NE winds shifting to the NW at 10 to
15 knots with some gusts near 25 knots possible on the coast.

KBOS TAF...Moderate Confidence in TAF.

KBDL TAF...Moderate Confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SHRA.

Saturday Night through Sunday: Mainly VFR, with local MVFR
possible. 

Sunday Night: VFR. 

Monday: VFR. Slight chance SHRA.

Monday Night: VFR. 

Tuesday: VFR. Breezy.

&&

.MARINE...
Today...High Confidence. Intensifying low pressure will result
in strong SCA wind gusts across western waters. However, strong
Easterly low level jet will result in 30 to 40 knot wind gusts 
across our eastern waters and perhaps brief gusts up to 45 knots
in our far northern waters. Meanwhile, should see the winds
shift to the southwest temporarily across our southern waters 
this afternoon as the surface low moves temporarily inland
across southeast MA. Seas will build to between 8 and 15 feet 
with the highest of those waves across our northeast outer- 
waters.

Tonight...High Confidence. Winds shift to the northwest with
gusts of 25 to 35 knots as the low pressure system slowly shifts
offshore. Seas will still be in the 5 to 10 foot range across
our outer-waters.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Chance of rain
showers. 

Saturday Night: Winds less than 25 kt. Areas of rough seas. 

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

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

Monday through Monday Night: Winds less than 25 kt. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until 4 PM EDT this afternoon for MAZ007.
     Winter Storm Warning until 8 PM EDT this evening for MAZ002-
     004-008-026.
     Winter Weather Advisory until 8 PM EDT this evening for MAZ003-
     009-010.
RI...None.
MARINE...Gale Warning until 2 PM EDT this afternoon for ANZ231-232-250-
     251-254-255.
     Small Craft Advisory until 8 PM EDT this evening for 
     ANZ233>237-256.
     Gale Warning until 5 PM EDT this afternoon for ANZ230.

&&

$$
SYNOPSIS...Frank/BW
NEAR TERM...Frank/BW
SHORT TERM...Frank
LONG TERM...BW
AVIATION...Frank/BW
MARINE...Frank/BW