Lexington Weather

Lexington, Massachusetts, USA

36°F
3/18/2024 11:46pm 
  • Lexington Conditions: Clear
  • Temperature: 35.8°F / 2.1°CColder 0.2°F than last hour.
  • Dew Point: 24.1°FIncreased 0.7°Fsince last hour.
  • Relative Humidity: 62%Increased 2.0% since last hour.
  • Wind: Calm, 10-min avg: Calm, gust: 1 mph
  • Barometer: 29.55 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 182326
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
726 PM EDT Mon Mar 18 2024

.SYNOPSIS...
Clear and cool tonight with blustery west winds. Gusty west
winds again tomorrow with mainly dry conditions. Rain and snow 
shower chances overnight Wednesday as a moisture starved 
shortwave swings through. Anomolously dry conditions return 
Thursday and Friday before a potential precipitation maker this 
weekend; though the existence and the track of the low remains 
shrouded in uncertainty.

&&

.NEAR TERM /THROUGH TUESDAY/...

7 PM Update...

No substantial changes made to the forecast with this update.
Some minor changes so sky cover and the temperature trend after
sunset were made to bring the forecast more in line with current
observations. As mentioned by the previous forecaster, we are
headed towards a seasonably chilly night with temps either side
of freezing region wide. 

Previous Update...

The core of the 500mb cold pool works in overhead with temperatures 
of -35 aloft. Strong CAA along with 925mb winds around 20-25 knots 
will continue to promote mixing overnight. Expecting the gusty winds 
from this afternoon to drop off, however, sustained winds will 
likely stay around 10-15 mph. Despite the non-ideal radiational 
cooling conditions, strong cold air advection will bring temps down 
below freezing for most, and mid to upper 20s across the high 
terrain in western MA.

&&

.SHORT TERM /TUESDAY NIGHT/...
Tomorrow.

The core of the cold pool moves quickly offshore bringing 500mb
temps back towards -28C. With the mid to upper level warming, 
700mb - 500mb lapse rates have become very meager around 4 to 6
C/km. This combined with a mid-level ridge building in should 
limit shower chances in the afternoon with peak diurnal heating.
Still expecting diurnal cumulus to form again tomorrow afternoon.
With the cold start and continued cold air advection, 
temperatures will only top the low to mid 40s. Winds turn gusty 
again tomorrow afternoon with westerly winds and a well mixed 
boundary layer. Peak wind gusts of 25-30mph is possible in the 
afternoon.

Tomorrow night.

Winds begin to turn southerly and diminish overnight as the pressure 
gradient briefly relaxes ahead of a surface low north of the Great 
Lakes. Skies look mostly cloudy due to mid/high level clouds 
out ahead of the surface low. Overnight lows look to drop near 
or below freezing again for most of the region with mid to 
upper 20s in the high terrain.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
* Light rain and snow showers Wednesday night associated with 
  shortwave digging into the region

* Dry Thursday and Friday

* Increasing chances for more substantial precipitation this 
  weekend, though uncertainty in track of low pressure remains high

Persistent amplified trough across the western CONUS and broad but 
shallow trough across the east continues to dominate the synoptic 
pattern through the the period. Deep digging shortwave and 
associated low pressure dives into southern New England  late 
Wednesday bringing the chance for rain and snow showers overnight 
into Thursday. Tranquil but seasonable weather to end  the work week 
before shower chances increase next weekend. There is considerable 
uncertainty in the forecast beyond day 3,  with all global guidance 
providing different solutions, as  discussed in more detail below. 

Wednesday through Friday...

Previously mentioned shortwave digs into southern New England 
increasing rain and snow chances late Wednesday evening into the 
overnight hours. While the shortwave appears at first to pack a 
punch, there will be limited moisture available to the forcing that 
will result in not much more than scattered showers/snow showers 
during the Wednesday night period as dewpoints drop from about a 
third of an inch to less than one quarter of an inch as flow shifts 
from the southwest to the west/west northwest overnight. 

Strong CAA will allow temperatures to drop below zero overnight into 
Thursday morning across the interior, which will allow for some 
novelty snowflakes that may yield a few tenths of an inch of 
accumulation across the terrain. Overall, ensembles hint at less 
than a tenth of an inch of QPF associated with this shortwave, so 
regardless of if any snow may accumulate, it will have little to no 
impact on the morning commute. 

Dry conditions work in for Thursday and Friday as PWATs drop to 
around -1.5 sigma of normal and PWATs continue to drop to around 
0.1" by Friday. Thus, expecting a generally sunny period Thursday 
into early Friday ahead of our next potential precip maker. 
Seasonable to seasonably chilly temperatures expected. With very low 
dewpoints, potentially dropping below 0F by Friday, renewed fire 
weather concerns will develop on Thursday as gusty west winds 
redevelop; concerns will be less Friday as high pressure allows 
winds to go near calm for the better part of the daylight hours. 

Saturday and Sunday...

There remains pretty considerable uncertainty between deterministic 
guidance regarding precipitation this weekend, with the 12Z GFS 
indicating a pretty robust inland runner will impact SNE on 
Saturday. The ECWMF and Canadian provide two completely different 
scenarios, with the ECMWF indicating scattered showers, and the CMC 
diagnosing a south of the benchmark low that will initiate showers 
across eastern MA and CT. 

For now, we continue to rely on the ensembles that paint a broad 
picture of rain chances Saturday and Saturday night with support of 
between a quarter and an inch of rain over the period. Depending on 
the existence and track of the low, there is a possibility of some 
mixed precipitation across the high terrain, but not placing much 
weight in that potential at this time as we continue to try to hone 
in on the potential for a storm.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
Forecast Confidence Levels:

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


Tonight...High Confidence. 

VFR. Gusty winds drop off across the interior but remain
sustained WNW at 5 to 15 knots. Gusty winds pick up over the
Cape and the Islands overnight, with gusts to 20-25kt possible.

Tuesday...High Confidence.

VFR. WNW 10 to 15 knots with gusts of 20 to 25 knots. Another
round of mid level CU with bases of 5-8 kft.

Tuesday Night...High Confidence

VFR. Decreasing winds turning south

KBOS TAF...High confidence in TAF. 

VFR with gusty WNW winds

KBDL TAF...High confidence in TAF.  

VFR with gusty west winds

Outlook /Wednesday through Saturday/...

Wednesday: VFR. Breezy. Chance RA.

Wednesday Night: Mainly VFR, with local MVFR possible. Windy
with gusts up to 35 kt. Slight chance RA, slight chance SN.

Thursday: VFR. Strong winds with gusts up to 40 kt. 

Thursday Night: VFR. Strong winds with local gusts up to 40 kt.

Friday: VFR. Breezy. 

Friday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SN.

Saturday: Mainly VFR, with local IFR possible. Windy with gusts
up to 30 kt. Chance RA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight: Moderate Confidence

Marginal small craft criteria gusts overnight for both the 
inner and outer waters. Seas remain 2-4 feet.

Tomorrow: Moderate Confidence

Continued marginal small craft conditions with gusts near 25
knots esspically for the southern waters. Seas increase to 4-6
feet. 

Tomorrow night: High Confidence

Winds turn SW overnight, but remain gusty below 25 knots. Seas
of 3-5 feet. 


Outlook /Wednesday through Saturday/...

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

Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Local rough seas. Slight chance of
rain. 

Thursday: Moderate risk for gale force winds with gusts up to
35 kt. Areas of rough seas. 

Thursday Night: Low risk for gale force winds with gusts up to
40 kt. Areas of rough seas. 

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

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft. Chance of
rain, chance of snow. 

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Chance of rain. 

&&

.FIRE WEATHER...
Tuesday.

Minimum afternoon relative humidity values will drop into the 
20 to 30 percent range Tuesday. West to northwest wind gusts of
20 to 25 mph with a few gusts up to 30 mph are anticipated by 
late morning/early afternoon Tuesday. In coordination with our 
fire weather partners, we have held off on issuing another SPS 
for elevated fire weather concerns, but may be reassessed 
overnight.

Thursday...

A cold front that will move through on Wednesday...will also 
set the stage for the potential of elevated fire weather 
concerns again on Thursday. Minimum relative humidity values of
20 to 40 percent are expected with WNW wind gusts of 30 to 40
mph.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 AM to 11 AM EDT Tuesday for 
     ANZ231>235-237-251.
     Small Craft Advisory from 2 AM to 2 PM EDT Tuesday for ANZ250.
     Small Craft Advisory from 2 AM Tuesday to 8 AM EDT Wednesday 
     for ANZ254>256.

&&

$$

SYNOPSIS...KP/KS
NEAR TERM...KP
SHORT TERM...KP
LONG TERM...KS
AVIATION...KP/KS
MARINE...KP/KS
FIRE WEATHER...KP/KS
      

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