Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 081813
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
113 PM EST Sun Dec 8 2019

.SYNOPSIS...
Strong high pressure over southern New England this morning will
shift off the coast during the afternoon, which will start a
return flow of increasingly milder and more moist air. Scattered
showers late tonight will turn into a soaking rain Monday into
Tuesday. Strong southwest winds are expected in southeastern 
New England and all coastal waters late Monday afternoon and 
Monday night. A cold front moves through Tuesday night, and 
there might be some snow Tuesday night into Wednesday although 
confidence is very low at this point. Dry weather returns late 
Wednesday with colder temperatures for Thursday. Another coastal
weather system may bring a mix of precipitation Friday or Saturday.

&&

.NEAR TERM /UNTIL 5 PM THIS AFTERNOON/...

115 PM update ...

Temperatures finally at or above freezing across most of
MA/RI/CT, the only exception is the high terrain where temps are
in the mid to upper 20s. Winds have shifting to the SSW, helping
to modify the cold airmass over the region. Fortunately sunshine
and SSW winds remaining light taking some of the edge off the
unseasonably cold temps. Keep in mind normal high temps for 12/8
are 40-45. Previous forecast verifying nicely so no major
changes with this update. 

Earlier discussion below.

=================================================================

IR satellite shows some thin cirrus overhead, but generally
clear skies. Thicker mid and upper clouds are found over far
Western NY moving east. If these clouds hold together they may
reach Western MA/CT around 2 PM, BOS-PVD 4-5 PM. Light winds
become southerly this afternoon as the high pressure moves
offshore. Overall a fair weather day. And although we have
started rather chilly in some areas... subzero in parts of
Western and Central MA... the sun and developing south wind will
help temps recover into the 30s. Have adjusted hourly temps and
dew points, but no significant changes to the forecast.

&&

.SHORT TERM /5 PM THIS AFTERNOON THROUGH 6 PM MONDAY/...

340 AM update...

Tonight...There is increasing southwestly flow aloft, with a 
925 mb low level jet max of 50 knots noted from southeast NH to 
eastern ME late. Low level warm advection pattern will cause 
temperatures to slowly rise overnight. Low levels saturate 
toward daybreak Monday. Low cloudiness develops in CT and 
spreads northeastward overnight. SREF shows high probabilities 
of ceilings lower than 1000 ft. With increasing moisture in the 
low levels but still relatively dry air aloft most of the night,
this spells more of a chance of some drizzle. Actual showers 
may develop by 3 or 4 AM. Temperatures in northwest MA could 
still be only in the mid 30s, so cannot rule out a few flakes of
snow mixed in with any rain showers there, but it would be very
short-lived. Temperatures should be in the 40s across RI and 
southeast MA by daybreak Monday.

Monday...

** Rain and Wind **

Warm advection on strong south to southwest winds will lead to a
rain, with embbeded heavier showers, across the entire region on
Monday. Expecting 1 to 1.75 inches of rain before Monday night,
when additional rain will occur (see below). A very strong low
level jet will be working its way from the NJ coast to Long
Island during the morning, reaching southern RI and the southeast
MA coast very late in the afternoon. The NAM has been the
strongest, with 80-90 kt at 2000 ft whereas other models are
more like 60-70 kt. The big question is how much will mix down
to the surface. This is tough because temperatures will soar to
the mid and upper 50s in southeast MA and southern RI. That, 
combined with forecast lapse rates of 6C/km in the 925-700 mb 
layer, would make it likely that at least half would reach the 
surface. That would translate to wind gusts of 40-50 mph by 4 or
5 PM on Cape Cod and the Islands. Since the bulk of the winds 
would be Monday night, have held off on a Wind Advisory at this 
point, but either a Wind Advisory or High Wind Watches or 
Warnings may be issued later today. We have already issued Gale 
Warnings for all coastal waters...mariners take note.

Coincident with the arrival of the aforementioned low level jet,
an isolated rumble of thunder cannot be ruled out along the
south coast toward evening.
 
Even in northwest MA, temperatures will rise into the lower 50s,
which will melt much of the remaining snow. Per discussions
yesterday, there is 1 to 2 inches of water equivalent in the
snowpack, which, when added to the falling rain, could lead to
rises in river levels, but we are not expecting river flooding
at this time. There will, however, likely be some urban and poor
drainage flooding.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...

Big Picture...

Broad upper trough digs over the Plains and Great Lakes Monday night 
and Tuesday while an upper ridge sits over the Western Atlantic. 
This creates a southwest flow aloft across the Eastern USA and 
brings milder air into Southern New England. The upper trough moves 
east midweek, crossing New England Thursday. Less agreement on the 
pattern for Friday-Saturday, but possibly a trough digs over the 
Plains, then sweeps east to New England over next weekend.

Above normal 500-mb heights Tuesday fall below normal Thursday-
Friday before increasing again on Saturday. So the deep layer is 
expected to be mild Tuesday, then falling below normal late week 
with signs of a recovery by Saturday.

Mass fields and thermal fields in the models are similar through 
midweek but then show some model-to-model differences late in the 
week. This means moderate confidence through Tuesday, then 
diminishing to low confidence by Friday and Saturday. 

Daily Concerns...

Monday night-Tuesday...

Two features of note Monday night. One is a strong southerly low-
level jet moving up the coast over Southern New England. This 
provides an inflow of moisture with PW values between 1 and 1.25 
inches as well as low level forcing and lift. The second is a strong 
150-kt upper jet moving east from the Plains and eventually curving 
up over Southern New England. The left exit region of this jet moves 
over us Monday evening and night, also generating lift. Temperatures 
will be above freezing through the lowest 10 thousand feet, so high 
confidence the precip type will be rain/showers. 

Low level south to southwest winds reach 60 kt within a couple of 
thousand feet of the surface, with highest values just offshore. 
Lapse rates will be near moist adiabatic in a shower-filled warm 
sector. Expect strong wind gusts to be brought to the surface, with 
gusts of 35 to 45 kt. Values similar to this are forecast at 1000 mb.
Thus expect strong gusty winds Monday night and Tuesday.

Guidance also suggests the precipitation comes in two segments: one 
Monday night followed by a break Tuesday morning and then a second 
segment of rain/showers late Tuesday and Tuesday night. Total 
rainfall during this time will be roughly 1 to 2 inches.

Temperatures in the warm sector are indicated by 12C temps at 950 
mb. If we have a lapse rate close to isothermal at night, expect 
nightime min temps in the lows to mid 50s. A similar temperature
aloft with a dry adiabatic lapse to the surface would suggest 
max temps around 60 during the day Tuesday. Will show upper 50s
and around 60 in the coastal plain, and mid 50s in
Western/Central MA 

Tuesday night...

Cold front sweeps through Southern New England Tuesday evening and 
early night. After it moves through, indications are that it turns 
parallel to the upper flow just as we move under the right entrance 
region of the upper jet. This should keep the front nearby
overnight and maintain support for precipitation much of the 
night. With colder air moving in aloft, this may mean a 
changeover to snow with potential of a couple of inches. Cross 
sections show the deep column of moisture dries out by 12Z 
Wednesday, suggesting a diminishing chance of snow by sunrise.

Wednesday-Thursday...

Upper trough and associated cold pool move across New England on 
Wednesday. This should push any lingering precipitation offshore. 
Cold pool instability may generate some clouds and scattered 
rain/snow showers during Wednesday, with best potential in Western 
and Central MA and areas north of the MA border. 

High pressure then builds in with cold and dry weather Thursday. 

Friday-Saturday...

GFS shows another coastal system moving up from the south Friday, 
spreading precipitation into Southern New England with continues 
into Saturday.  The ECMWF is slower with the coastal system, moving 
precipitation ashore on Saturday. GFS temperatures are borderline, 
with potential for both rain and snow. The ECMWF temperature show 
predominently rain. Expect a mix of precipitation on one or both
of the days, but with low confidence.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

1815z ...

Thru Midnight ... VFR, dry weather and increasing SSW winds.

After midnight ... VFR lowering to MVFR in scattered showers
toward morning. 

Monday ... MVFR with embedded IFR especially inland. Periods of
rain, moderate to heavy at times. LLWS 020/19045kt. Surface 
south at 20-25 kt, lighter winds elsewhere. Areas of fog will 
restrict vsbys in the MVFR/IFR category. 

Monday night ... A mix of MVFR/IFR in periods of rain, moderate
to heavy at times. Areas of fog too. Rain possibly tapering off
after 06z. LLWS 020/19050kt

KBOS Terminal...High confidence in TAF trends, lower confidence
on exact timing details.

KBDL Terminal...High confidence in TAF trends, lower confidence
on exact timing details.

Outlook /Tuesday through Thursday/...

Tuesday: Mainly MVFR, with areas VFR possible. Windy with gusts
up to 30 kt. SHRA likely.

Tuesday Night: MVFR/IFR conditions possible. Windy with local
gusts up to 30 kt. SHRA likely, chance SHSN.

Wednesday: Mainly VFR, with areas IFR possible. Breezy. Chance
SHSN.

Wednesday Night through Thursday: VFR. Windy with local gusts
up to 30 kt.

&&

.MARINE...
Short Term /through Monday/...High confidence.

340 AM update:

** Gale Warnings now in effect for all coastal waters **

Today...Strong high pressure over southern New England moves 
offshore this afternoon. This shifts winds to a SSW direction. 
Dry weather and good vsby continue.

Tonight...A low level jet over mainly northern New England will
cause increasing wind gusts over the outer waters off Cape Ann
and over Massachusetts Bay tonight. Winds there could gust over
30 kt. Seas will build to around 6 ft in those waters. Scattered
showers developing late, mainly in the southern waters.

Monday...Rough conditions. SW gales expected, with gusts of
35-45 kt all waters...but highest over the southeastern outer 
waters by late in the afternoon. Will need to watch for
potential upgrades to Storm conditions. Widespread rain showers
and the potential for an isolated thunderstorm toward evening
over the southern coastal waters. Areas of fog. Visibilities
reduced to 1 to 3 nm.

Outlook /Monday Night through Thursday/...

Monday Night: Moderate risk for gale force winds with areas of
gusts up to 45 kt. Rough seas up to 15 ft. Rain showers likely.
Local visibility 1 to 3 nm.

Tuesday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 14 ft. Rain showers likely. 

Tuesday Night: Moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Areas of rough seas. Rain showers
likely. 

Wednesday: Winds less than 25 kt. Local rough seas. Chance of
rain showers. Local visibility 1 to 3 nm.

Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Seas up to 5 ft. Local visibility 1 to
3 nm.

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 1 PM Monday to 6 AM EST Tuesday for 
     ANZ231>235-237-250-251-254>256.
     Gale Warning from 4 PM Monday to 6 AM EST Tuesday for ANZ230-
     236.

&&

$$
SYNOPSIS...WTB/GAF
NEAR TERM...WTB/Nocera
SHORT TERM...GAF
LONG TERM...WTB
AVIATION...WTB/Nocera
MARINE...WTB/GAF