Lexington, Massachusetts, USA

2/24/2018 2:59pm 
  • Lexington Conditions: Overcast
  • Temperature: 51.4°F / 10.8°C 
  • Dew Point: 37.6°FDecreased 0.9°Fsince last hour.
  • Humidity: 59%Decreased 2.0% since last hour.
  • Wind: WNW 2 mph, gust: 3 mph
  • Barometer: 30.08 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in
Winter Weather Advisory WINTER WEATHER ADVISORY - Middlesex County

Area Forecast Discussion
for Boston, MA

FXUS61 KBOX 241735

Area Forecast Discussion
National Weather Service Taunton MA
1235 PM EST Sat Feb 24 2018

Unseasonably mild today with a period of showers possible south
of the Mass Pike late today and early this evening. A frontal
system will bring more precipitation late tonight and Sunday 
with rain mixed with sleet at times in the coastal plain and 
snow sleet and freezing changing to rain in the interior. Near-
seasonable, quiet and dry for the first half of next week, 
attention turns towards late week and an anticipated stormy 


10 am update...
Very pleasant Feb conditions this morning. Temps already running
in the upper 40s and low 50s with plenty of sunshine out of
terrain-waves near the Berkshires and lingering stratus across
MVY/ACK, which should be clearing in the next hour or so.
Increased highs a bit as a result as timing of the clouds
associated with a weak leading wave moves in this afternoon
being slower than previously forecast. Many locations make the
50s, or even upper 50s.

Otherwise, narrow corridor of SHRA later this afternoon and
evening, mainly S of the Mass Pike. Not too much with this
initial round as the moisture in the column remains somewhat 
low until tonight.


*** Mixed precipitation event for interior late tonight and
 Sunday with some snow and ice accumulations ***

A few showers likely impacting south coast early evening, 
otherwise dry weather through midnight before main area of 
overrunning precip ahead of the warm front begins to overspread
the region from the SW to NE late tonight into Sunday. 

Ptype will be biggest challenge with this event. Initially, the
column is warm but all the guidance is indicating a good cold 
air damming signature developing overnight into Sunday as high 
pres builds over the Maritimes. In fact, notable cooling in the 
925-950 mb layer 06-15z with temps dropping to -2 to -4C before 
warmer air moves in from the south. Warming temps above this 
layer will set up a sleet and freezing rain scenario in the 
interior where low level cold air will be difficult to dislodge
given warm front remaining to the south and a sfc wave tracking
along the boundary to lock in the cold air near the surface.
Some snow is also likely late tonight and early Sunday, north 
of the Pike and especially near NH border where cold air just 
deep enough as precip moves in. In the coastal plain, mainly 
rain is expected, but given cold air below 850 mb, some sleet 
will likely mix in with the rain late tonight and Sunday morning
even along the south coast.

Expect 1-3 inches snow/sleet accum in the interior north of the
Mass Pike with highest amounts near NH border. If warmer air 
aloft is delayed a few hours it is possible a few locations
could approach 4-5 inches but this is a low probability. We are
concerned with several hours of freezing rain and icy 
conditions across portions of the interior during Sunday, 
especially over the Berkshires, Worcester hills and portions of 
the CT valley. Ice accretion up to one tenth of an inch is 
possible with up to 0.25" over the Berkshires. Expect temps to 
gradually rise above freezing Sun afternoon in the interior as 
precip intensity diminishes. 

Winter weather advisories will be issued for all of central and
western MA and northern CT. Some freezing rain is possible in 
NW RI and interior NE MA but confidence not yet high enough for 
an advisory in this area. Across NE MA may be dealing with more 
of a snow/sleet to rain scenario.


*/ Highlights...

 - Quiet, dry, near-seasonable early week period
 - Stormy pattern emerging late week for the beginning of March 

*/ Overview...

Blocky N Atlantic pattern, associated +3 standard deviation H5 high 
retrograding W into SE Canada, parent 1040+ surface high, pressing 
the thermal wind axis S through which N/S stream impulses merge, dug 
through a prevailing W CONUS H5 trof before ejecting and stretching 
E. Energy piling up, cutting off for a time given the block, an area 
of favorable storm development emerges off the SE Canadian coast, 
evolving S/W towards the NE CONUS per retrograding high, E ejecting 
energy. Airmass juxtaposition key as to outcomes towards the end of 
the week after a quiet start. Only confidence is persistence of NE 
winds of significant duration during high astro-tides, concern with 
respect to coastal flooding. Lower confidence otherwise, consensus 
forecast guidance heavily weighted with ensemble means. Hit targets 
of opportunity below. 

*/ Discussion...

Thursday into the weekend...

While considerable ensemble member / deterministic guidance spread 
there's cohesive signal of a Great Lakes warm occlusion transferring 
energy offshore beneath a more favorable region of storm development 
evolving towards the NE CONUS. Occluded front transitioning to an 
inverted trof, energy pinwheeling round a closed, stacking low well 
offshore. Questions surround frontal boundary magnitude buffeting 
the pressing N high, how far N/E outcomes slide into New England. 
NCEP noting poor run-to-run continuity with ejecting waves, EC, even 
the 24.0z, worse than GFS which appears to be locking in. Warm core 
low, absent Arctic air, anticipate precip-type issues, rain / snow, 
lean to occluded front / inverted trof hanging up into the NE CONUS 
as energy transfers, low rolls up offshore. Only confidence is the 
persistence of NE winds during high astro-tides, concern of coastal 
flooding. Only chance PoPs given spread.


Forecaster Confidence Levels...

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

Short Term /through Sunday/...

Through 06Z tonight...High confidence.
Mainly VFR, although brief MVFR vsbys are possible in -SHRA at
terminals S of the Mass Pike late this afternoon/evening. Winds
NW will shift around to the E-NE late this evening.

After 06Z through Sunday...Moderate confidence.
Precipitation shield begins to build SW-NE mainly between 08Z
and 12Z. Initially, mix of SN/PL possible, with gradual
transition to PL/FZRA after 10Z though 16Z from S-N, with all RA
expected eventually outside of the highest terrain. Timing may
be off a a bit in TAF, and this will be refined through the
overnight updates. Mainly IFR/LIFR as the precipitation gets
going, both due to low CIGS and vsbys.

KBOS Terminal...Moderate confidence, mainly due timing
uncertainty. VFR into early AM, then gradual reduction to IFR as
precipitation moves over the terminal. Will likely see some PL
mixing in with the RA for the first 2-4 hrs before a change to
just rain until the precipitation ends by late Sun afternoon.
Wind gusts during the afternoon tomorrow 20-25 kt out of the E.

KBDL Terminal...Moderate confidence, mainly due timing
uncertainty. Precip may start early Sun AM as mix of SN/PL then
transition to period of PL/FZRA before becoming all RA. Timing 
of this in TAF may change. Mainly IFR once precipitation begins.
If Winds remain predominantly N, PL/FZRA lingers longer.

Outlook /Sunday Night through Wednesday/...Moderate confidence.

Sunday Night: Mainly VFR, with local IFR possible. Slight
chance RA.

Monday through Wednesday: VFR.


Forecaster Confidence Levels...

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

Short Term /through Sunday/...Moderate confidence.

Today...Diminishing winds becoming northerly late today. Seas
below SCA. 

Tonight and Sunday...Increasing easterly winds late tonight and
especially Sunday as low level jet moves across the waters.
Given cold air above the boundary layer, mixing will likely be
sufficient for a period of gale force gusts, especially over 
the south coastal waters during Sun. Otherwise, expect SCA 
conditions with gusts to 25-30 kt. Gale watches issued where 
highest confidence of gales.

Outlook /Sunday Night through Wednesday/...

Sunday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 10 ft. Rain. Visibility 1 nm or less.

Sunday Night: Winds less than 25 kt. Areas of rough seas.
Slight chance of rain. 

Monday: Winds less than 25 kt. Seas up to 5 ft. 

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

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

Tuesday Night: Winds less than 25 kt. 

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


CT...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for 
MA...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for 
MARINE...Small Craft Advisory from 3 AM to 4 PM EST Sunday for 
     Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ230.
     Small Craft Advisory from 3 AM to 7 PM EST Sunday for ANZ231.
     Small Craft Advisory from 6 AM to 1 PM EST Sunday for ANZ236.
     Gale Watch from Sunday morning through Sunday afternoon for 
     Small Craft Advisory from 6 AM to 7 PM EST Sunday for ANZ250-
     Gale Watch from Sunday morning through Sunday afternoon for 


LONG TERM...Sipprell