Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 221654
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1254 PM EDT Sun Mar 22 2026

.WHAT HAS CHANGED...

Made adjustments to the timing of steadier rain into this
afternoon, with only intermittent showers anticipated before
then in most areas. Still monitoring for the possibility of 
scattered snow showers Monday afternoon to evening in northeast 
and eastern MA, which could impact the Monday afternoon commute 
if they are heavy enough.

&&

.KEY MESSAGES...

- Overcast with rain/snow mix north of the Mass Pike early this
  morning. Best chance for widespread rain is this afternoon 
  and into tonight. Rain could mix with/end as sleet pellets 
  and/or wet snowflakes overnight to early Monday as cold front 
  moves offshore.

- Overcast, chilly and raw Monday as bulk of overnight
  precipitation moves offshore. However scattered snow showers
  (some briefly heavy) are possible mainly eastern and northeast
  MA Monday afternoon to evening. 

- Splashover or very minor (shallow-depth) coastal flooding
  possible early Monday morning on the eastern MA coast.

- Dry and seasonable Tuesday becoming unsettled again after 
  mid-week with a few rounds of showers. No significant 
  precipitation expected.

&&

.DISCUSSION...

KEY MESSAGE 1...Overcast with rain/snow mix north of the Mass 
Pike early this morning. Best chance for widespread rain is this
afternoon and into tonight. Rain could mix with/end as sleet 
pellets and/or wet snowflakes overnight to early Monday as cold 
front moves offshore.

Overcast skies elsewhere as we await the frontal system's surface 
cold front, which will bring the best chance at widespread rainfall 
today as temperatures warm into the 40s to lower 50s. Latest 
guidance doesn't bring its leading edge into northwestern portions 
of Southern New England until the early afternoon; wouldn't rule out 
isolated or widely scattered showers prior to then, but thinking 
most of the morning hours ends up generally dry. This cold front 
will be moving into rising PWAT values given WSWly advection of a 
warm and moist airmass, with PWATs of around 1 inch. The front 
should end up moving more slowly southward through the afternoon to 
early evening, and there could be steadier rain or even some brief 
downpours. So all told, the better chance for rain in most areas is 
mainly focused during the afternoon. 

For the evening, the cold front will continue to sag southward. At 
the same time, a secondary surface low will try to form along the 
slowing frontal zone and lead to anafrontally-enhanced 
precipitation, especially south of the Mass Pike. Northerly flow 
will eventually bring cooling sfc temperatures, however per NAM/GFS 
thermal profiles, it looks as though a layer of subfreezing temps 
between 950-925 mb advects southward overnight before sfc temps
drop to near freezing. This means steady rain could become 
mixed with sleet pellets during the overnight hours as this
shallow cold air surges south, but no accumulation or impacts 
are expected, other than it leading to a frankly miserable 
period to be outside overnight. Further north, rain/sleet 
pellets could end as a brief period of wet snow but here too, 
nothing necessarily impactful. 

KEY MESSAGE 2...Overcast, chilly and raw Monday as bulk of 
overnight precipitation moves offshore. However scattered snow 
showers (some briefly heavy) are possible mainly eastern and 
northeast MA Monday afternoon to evening. 

Residual rain/snow showers from the overnight should be pulling 
offshore Monday late morning. However shallow moisture is trapped 
underneath a dry layer of air aloft, with enhanced NNE winds along 
the eastern coast. The net result is a pretty dreary, chilly and raw 
day with lots of overcast. Leaned on cooler side of guidance with 
highs only in the 30s, with optimism for spot 40 degree readings in 
the CT Valley but it will feel pretty chilly and raw with northerly 
winds.

However a number of convection allowing models and even the coarser 
GFS are showing a band of scattered snow showers in eastern/east-
central MA northward into NH as soon as the mid-afternoon hours,
aligning along a leftover surface inverted trough. These snow 
showers could be infused by the passage of a shortwave trough 
aloft, and models also show some shallow instability associated 
within these narrow bands. Late-season snow accumulation 
scenarios which tend to be time-of-day and snow-rate driven. And
it's possible some greasy coatings of snow on pavement could 
occur in eastern and northeast MA, and possibly impact the 
Monday afternoon to evening commute. This would hinge on if we 
can get snow showers to occur in brief bursts of higher 
intensity. This activity should move offshore before midnight. 

KEY MESSAGE 3...Splashover or very minor (shallow-depth) 
coastal flooding possible early Monday morning on the eastern MA
coast.

As the cold front sags southward, it brings with it enhanced NE 
winds. We're also headed into a period of higher astronomical tides, 
with Boston Harbor's high tide around 3 AM Monday morning is 11.04 
ft MLLW. As per P-ETSS and Stevens Institute datasets, the northeast 
winds seem progged to generate a storm surge of around 1 to 1.2 ft. 
The timing of the windshift to NE is important here, as a more-
delayed windshift after the high tide would lead to little if any 
coastal flooding risk at all. Most probabilistic forecast 
hydrographs show total water levels in the near-flood stage. A 
coastal flood statement could address this potential. Storm surge 
values should be on the higher end of the above range for the Monday 
afternoon high tide, although that astro tide is quite a bit lower 
(9.63 ft MLLW) and not of concern.

KEY MESSAGE 4... Dry and seasonable Tuesday becoming unsettled again 
after mid-week with a few rounds of showers. No significant 
precipitation expected. 

Conditions will continue to dry out Tuesday morning as surface high 
pressure builds into southern New England. This will yield a brief 
period of fair weather feature sunshine and light NW winds. High 
temperatures will range in the 40s, which is right around normal for 
late March. 

These conditions will be short-lived as the upper level pattern 
transitions back to an active/unsettled regime by mid-week. Ensemble 
guidance shows general agreement on quasi-zonal flow aloft, bringing 
a series of shortwave troughs through the region from Wednesday PM 
through at least late week. This setup will support a few rounds of 
showers; however, subtle discrepancies in the timing of individual 
waves among model guidance have resulted in uncertainty regarding 
exactly when each round of showers will occur. Generally, we will 
see a round of showers sometime Wed night-Thu followed by another 
round sometime in the Thu night-Fri timeframe.
 
Currently, guidance does not highlight a strong signal for 
significant precipitation with any of these systems. A plume of above-
normal moisture advects into the region Wednesday night ahead of the 
initial wave, supporting scattered to widespread showers. 
Temperatures lean near to slightly above normal in the mid to late-
week period. Overnight lows drop into the upper 20s and low 30s 
across the interior higher terrain, and mid to upper 30s elsewhere. 

If a round of showers were to move through during the overnight 
hours, particularly Thu Night-Fri, can't rule out the lower 
probability scenario of snow mixing in/light wet snow across 
portions of northern MA. Models are varied with the strength of the 
WAA Thu night, which is why the mixed precip is lower probability 
right now. If it did occur, it would likely be low impact.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...

Forecaster Confidence Levels...

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

18z TAF Update...Moderate confidence.

Lower end MVFR-IFR conditions will dominate tonight in periods
of rain south of Route 2...while north of that region the rain
will be more intermittent. Rain will begin to transition to 
snow/sleet between 8z and 12z Monday in areas away from the 
south coast to the northern edge of the steady precipitation shield near
route 2. This should allow for a period of IFR to even localized
LIFR conditions. A coating to 2" of snow is possible in the 
area of 20 miles north or south of I-90. Much of the 
accumulation will tend to be on grassy surfaces...but a slushy 
accumulation may occur on runways briefly if intensity is enough
to overcome the warm ground temps. S-SW winds of 8-15 knots
early this evening shift to the N by midnight with gusts of 
20-25 knots developing on the coastal plain toward daybreak. 

Conditions may improve to mainly MVFR levels by Mon
afternoon...perhaps even marginal VFR conditions for a time
across the interior. However...scattered snow showers and a few
snow squalls are expected to develop later Monday into Monday
night. The activity is possible anywhere...but will likely be 
most widespread across eastern MA especially Monday evening. 
IFR to LIFR conditions are expected in heavier snow showers and 
squalls and some minor accumulations are possible. Greatest risk
will be across eastern MA Mon night. Any lingering lower 
cigs/vsbys across the eastern MA coast should come to an end 
toward daybreak Tue. 

KBOS Terminal...Moderate confidence in TAF. Greatest concern for
a period of snow/sleet is after 8z/9z until about 15z.
However...scattered snow showers and a few snow squalls will be
in the vicinity of the terminal later Monday and especially
Monday night. 

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday through Thursday/...

Tuesday through Tuesday Night: VFR. 

Wednesday: VFR. Breezy. 

Wednesday Night: VFR. Slight chance RA.

Thursday: Mainly VFR, with local IFR possible. Breezy. Chance
RA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Small craft advisories have been posted on all waters for tonight 
into Monday. 

SE to S winds will increase today into the 15-20 kt range, and could 
at times gust to near small craft advisory levels along/ahead of a 
cold front that will be moving southward today. This will also bring 
with it reduced visibilities from a band of steady light to moderate 
rain. The period of greatest concern though is an abrupt windshift 
as the cold front move southward to northerly, with winds increasing 
to around 25-30 kt in gusts. Onset of these northeast winds to begin 
soonest in the northeast MA waters, then build steadily southward 
tonight and into Monday. SCA level gusts are expected to continue 
into at least Monday afternoon, but could linger into Monday evening 
and overnight. Seas will build and become rough as the northerlies 
pick up, in the 4-8 ft range. 

Outlook /Monday Night through Thursday/...

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

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

Tuesday Night through Wednesday: Winds less than 25 kt. Seas up
to 5 ft. 

Wednesday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain. 

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM this evening to 8 PM EDT Monday 
     for ANZ230>237-250-251-254>256.

&&

$$

DISCUSSION...Loconto/Mensch
AVIATION...Frank
MARINE...Loconto/Mensch