Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 221123
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
723 AM EDT Wed Mar 22 2023

.SYNOPSIS...

Continued mild and dry today, but cooling sea breezes along the
eastern MA coast. A few showers possible tonight, especially 
across SE New England, then more showers Thursday as a cold 
front approaches. Drier and a bit cooler Friday behind the 
front. Active weather on Saturday into early Sunday with a 
wintry mix across the interior and higher terrain, while there 
is a cold rain elsewhere. Drier weather returns Sunday into 
early Monday. Unsettled once again late Monday into Tuesday.

&&

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

715 AM Update...

No major changes in the latest update. Did bump up the cloud
cover a bit from the previous forecast based on current mid/high
clouds in place. Otherwise the forecast is on track.

Previous discussion...

Weak cold front moving south across the region will reach the south 
coast by daybreak, then it returns northward as a warm front today 
with a wind shift to south. This front will result in a tricky temp 
forecast, especially across portions of eastern and NE MA where the 
front may get hung up today. While there will be mid/high clouds 
around, the low levels are dry so expect at least partial sunshine 
today which will help to boost temps well into the 50s, with lower 
60s in the CT valley. However, east winds may persist through the 
day across NE MA on the cool side of the boundary, holding temps in 
the upper 40s here, with lower 50s further south along the coast.

&&

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

Tonight...

The warm front lifts to the north tonight with modest S/SW flow 
across the region tonight which will result in a milder night with 
lows upper 30s northern MA and low/mid 40s elsewhere. An area of 
higher PWATs and deep moisture plume over the ocean advect northward 
into SNE tonight aided by a modest low level jet which may bring a 
few showers to SE New Eng tonight. Some of the guidance is also 
suggesting a second area of scattered showers moving into western 
New Eng toward daybreak assocd with modest lift in the left exit 
region of an approaching upper jet. Otherwise, increasing low level 
moisture may result in areas of stratus and patchy fog developing 
overnight. 

Thursday...

Northern stream shortwave passing well to the north will push a cold 
front southward through northern New Eng which will approach 
northern MA toward evening. Modest warm advection in the warm sector 
will result in scattered showers moving across the region, but not 
expecting a washout as the column does not fully saturate and 
forcing for ascent is not that strong. Rather mild low level temps 
will be advecting northward into region as 925 mb temps warm to 8-
10C from N to S by 00z Fri. Shallow mixing and cloud cover will 
limit full heating potential but still expect highs reaching the 
upper 50s to lower 60s, but cooler along the south coast. It will 
become breezy in the afternoon with pre-frontal gusts 20-30 mph 
developing as low level wind field increases.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...

Highlights...

* Showers tapering off Thu night into early Fri. Cannot completely 
  rule out a few rumbles of thunder along the south coast. 

* Dry weather returns for the rest of Fri.

* Unsettled Sat into early Sun. Wintry mix of sleet/snow across the 
  interior. Elsewhere will be a cold rain. Gusty winds at times.

* Dry once again on Sun before the unsettled weather returns at some 
  point early next week with wintry weather possible.

Thursday night through Friday...

A shortwave trough will initially be situated over northern New 
England/Upstate New York Thu night. The shortwave will dig offshore 
by early Fri AM. A ridge axis will build from the Mid Mississippi 
River Valley Fri AM into the central Great Lakes/OH Valley by Fri 
evening. A cold front will move through Thu night into the pre-dawn 
hours Fri AM. Behind the front, high pressure builds in from the the 
central Great Lakes/Ontario.

Cold front will be working its way through southern New England late 
on Thu into very early Fri. As the front is moving through will have 
rain showers, which will gradually taper off as the night 
progresses. Does appear that there is the potential for a few 
rumbles of thunder along the south coast. Will have roughly 50-100 
J/kg of MUCAPE available and K index values should be around 25-30 
as the front is moving through. 

Other concern is immediately along/behind the front we may have a 
period of gusty winds. At this point looks like W to WNW/NW low 
level jet at 850 hPa with speeds of 30-40+ kts. At this point does 
appear we will have a better opportunity to tap into these stronger 
gusts per GFS/NAM Bufkit soundings, but the NBM still appearing a 
bit too strong. Knocked wind speeds/gusts down a bit per GFS/NAM 
soundings. After the frontal passage will see the gusts/winds come 
down during the day Fri and Fri night as the high relaxes the 
pressure gradient.

Temperatures falling a bit closer to seasonable at 925 hPa temps 
fall to +/- 3 degrees Celsius. Given the NW flow should see some 
downsloping, so did nudge temps up a bit from the default NBM. Highs 
will generally be in the low/mid 50s outside of the higher terrain 
where readings will be in the 40s on Fri.

Saturday into early Sunday...

The ridge axis over the central Great Lakes late on Sat will build 
into New England by Sat AM and northeast of the region by late in 
the day. Behind the ridge axis a trough will lift from the Mid 
Mississippi River Valley into the central/eastern Great Lakes. The 
trough lifts into northern New England by early Sun. High pressure 
will be planted over Quebec on Sat and Newfoundland by late in the 
day. A low will deepen and become vertically stacked over the 
central Great Lakes late on Sat. The warm front associated with this 
system will lift into our region. A secondary low develops 
near/along the south coast late on Sat before moving into the Gulf 
of Maine by very early on Sun.

Main concern during this period is the wintry mix anticipated across 
the higher terrain of the interior. Will note that 
deterministic/ensemble guidance has trended with less snowfall 
accumulation. This is largely a result of more a mixed bag of precip 
versus the colder solutions advertised yesterday. Have stuck with 
the NBM for now as it appears quite reasonable at this point in time 
for temps. Should start out as a mix of sleet/snow across the 
interior on Sat before gradually changing over to rain/sleet and 
potentially all rain other than the highest elevations. Will really 
struggle to accumulate much on Sat other than in locations where 
models prog 2M temps below freezing. This also a result of the late 
March sun angle and light precip rates. Thinking the best shot for 
accumulation comes late on Sat into early Sun as the secondary low 
is lifting across SE MA. This will also coincide with any wintry mix 
changing back over snow across the interior. Winds shift to the W, 
which will advect 0 to -2 degree Celsius air at 925 hPa across the 
higher terrain.

Given there is still some uncertainty in the thermal profiles will 
really need to hone in on exact Ptypes to get idea on impacts 
expected. Will note that GEFS/EPS guidance have lowered 
probabilities of 24 hr 10:1 SLR snow accumulations AOA 3 inches into 
roughly the 10-40 percent range. The GEPS has also retreated with 
these probabilities as well, likely a result of the stronger surge 
of warmer air aloft and more mixed ptypes. Risk is highest along 
roughly the Route 2 corridor. Not completely out of the question 
Winter Weather Advisories may be needed, but at the moment impacts 
appearing less likely. Elsewhere will have a cold rain.

Also anticipating gusty winds as the system moves through. Should 
see 925 hPa jet of roughly 30-40+ kts out of the S/SE initially and 
shifting to the W late on Sat into early Sun as the secondary low 
lifts into the Gulf of Maine. Thinking that the wind risk is greater 
late Sat/into early Sun as we get into the W flow. May be difficult 
to mix down the SE/S winds given the warmer air advecting in and 
inversion in place per GFS Bufkit soundings. Have stuck with the NBM 
for now, but may need to be lowered a bit in future updates 
especially during the day Sat. 

Sunday into early Monday...

A ridge axis builds into the central/eastern Great Lakes from the 
Mississippi River Valley on Sun. High pressure will nudge into 
southern New England behind the departing system on Sun into early 
Mon.

This will result in another period of dry and quiet weather Sunday 
into early Monday with more seasonable temperatures.

Late Monday through Tuesday...

Next window for potentially impactful winter weather. Lots of 
uncertainty with timing, track and intensity of this system as 
guidance is all over the place. At this point have just stuck with 
the NBM, which features chances of precipitation. Depending on the 
exact track/intensity of a low moving through offshore there could 
be accumulating snow. Ensembles featuring low probabilities (10-30 
percent chance) of 10:1 SLR 24 hr snow accums of 1" and 3" per the 
GEPS/GEFS and EPS guidance.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels:

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

Today...High confidence. 

VFR with light and variable winds to start. Will gradually shift
to the E/SE later this morning and S approaching 10 kts this
afternoon. The only exception is across NE MA where E/SE winds
persist. Mid/high clouds increasing as the day progresses.

Tonight...Moderate confidence. 

VFR to start, but cigs lowering to MVFR overnight with pockets 
of IFR possible. Areal extent of lower cigs is uncertain. 
Scattered showers developing across eastern New Eng. Winds out
of the S at 5-10 kts.

Thursday...Moderate confidence. 

Areas of MVFR/IFR in the morning expected to improve to MVFR/VFR
in the afternoon. Scattered rain showers. S winds around 5-10
kts in the morning shifting to the SSW/SW and increasing to
10-15 kts with 20-25 kt gusts during the afternoon.

KBOS Terminal...High confidence in TAF. VFR into this evening,
then potential for MVFR cigs overnight. Could perhaps see some
localized IFR toward the Thu AM push.

KBDL Terminal...High confidence in TAF. VFR into this evening,
then potential for MVFR cigs overnight.

Outlook /Thursday Night through Sunday/... 

Thursday Night: Mainly VFR, with local MVFR possible. Windy 
with gusts up to 35 kt. Chance SHRA.

Friday: VFR. Windy with local gusts up to 30 kt. 

Friday Night: VFR. Breezy. Chance FZRA.

Saturday: Mainly VFR, with local IFR possible. Windy with gusts
up to 35 kt. RA, chance PL, chance FZRA.

Saturday Night: MVFR/IFR conditions possible. Windy with gusts
up to 35 kt. RA, FZRA, chance SN.

Sunday: VFR. Windy with gusts up to 30 kt. Slight chance RA.

&&

.MARINE...

Forecaster Confidence Levels:

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

Today through Thursday...High Confidence.

Winds turn southerly today less than 20 kt across the south coastal 
waters, with mainly easterly winds over NE MA waters. S/SW winds 10-
20 kt tonight, increasing Thu with 25-30 kt gusts developing during 
the afternoon over the south coastal waters with building seas as a 
low level jet approaches from the south. Scattered showers tonight 
into Thu. 

Outlook /Thursday Night through Sunday/... 

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of rain showers,
isolated thunderstorms. 

Friday: Winds less than 25 kt. Local rough seas. 

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

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Seas up to 5 ft. Rain. 

Saturday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 8 ft. Chance of rain. 

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 9 ft. Slight chance of rain. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...KJC/BL
NEAR TERM...KJC/BL
SHORT TERM...KJC
LONG TERM...BL
AVIATION...KJC/BL
MARINE...KJC/BL