NWS Forecast Discussion - Boston Area

Forecast Discussion for BOX NWS Office
FXUS61 KBOX 161149

Area Forecast Discussion
National Weather Service Boston/Norton MA
649 AM EST Sat Feb 16 2019

A cold front over the outer Cape and Nantucket early this morning
exits offshore before sunrise followed by drier and seasonably cool
weather for this weekend. Weak low pressure tracks south of New
England Sunday night into Monday and will likely be accompanied by a
period of light snow and possibly some minor accumulations. High
pressure and drier colder weather follows later Monday into Tuesday.
Another low pressure approaches the region late Wednesday into
Thursday and will likely be accompanied by snow, possibly changing
to a wintry mix inland and possibly changing to rain along the
coast. Dry weather likely returns to close out the work week.



7 AM update ...

Post frontal airmass overspreading the region with gusty NW
winds up to 20-25 mph. Modest cold air advection with upstream
850 temps down to -12C over western NY and advecting toward New
England. So not much of a temp rise today with highs mainly
35-40, right on track for this time of year. Also expecting more
sun than clouds this afternoon. Winds drop off this afternoon as
well. Overall a fairly pleasant mid Feb day by New England
standards. Previous forecast captures these details nicely so no
major changes with this update.


High pressure ridge at the surface should remain in control of
our weather through much of this period, hence dry weather
should prevail. Some timing differences with the approach of a
low pressure moving into the central Appalachians late Sunday.
16/00Z ECMWF is quite a bit faster than the operational GFS, but
compares well with the NAM. The big question will be how quickly
the low level dry air can be overcome to get precipitation to
the ground.

Radiational cooling tonight will set the stage for well below
normal temperatures, which will continue as below normal
temperatures Sunday. Thinking we will be cold enough where any
precipitation should fall as snow late Sunday afternoon.



4 AM update...


* Light snow with minor accumulations likely Sun ngt/Monday
* Dry cold weather Tuesday
* Another chance of snow or a wintry mix late Wed into Thu

Sunday Night and Monday ...

Very strong 200 kt subtropical jet from Texas to New England ejects
Pacific moisture into our region this period. However confluent flow
across New England shunts deeper moisture/PWAT plume to the south.
Also system begins to weaken as it encounters anticyclonic flow
riding up and over the southeast ridge. Furthermore, confluent flow
over New England and associated subsidence will also deamplify wave
as it approaches New England. This is apparent at 850 mb as a closed
wave becomes open and positively tilted, decreasing thermal
advection aloft. This is also reflected in model POPs only ranging
from about 45 percent in northeast MA to 65 percent south coast.
Given this synoptic setup the lower qpf from the ECMWF seems much
more reasonable than the wetter GFS and NAM. Thus will lean toward
the more conservative ECMWF and its previous runs and forecast just
a coating to 2 inches possible. Thus not a major snow event but
perhaps just enough to decorate the landscape a bit. Given this
event is still 48+ hrs out in time, the wetter solutions are
possible but unlikely and will have to just watch how this system
evolves with time.

Tuesday ...

Colder and drier weather overspreads the region in response to 1040
mb high advecting eastward from the Great Lakes. 850 mb temps from
both the 00z EC and 00z GFS are only -10C to -12C across the region.
These cold temps aloft combined with tight pres gradient yields
blustery and cold with highs only in the upper 20s to lower 30s but
gusty WNW winds provide wind chills in the teens and 20s.

Wednesday and Thursday ...

Wednesday likely starts out dry and cold with 1030+ mb high pres
over the region. Then next Pacific system approaches the region late
Wed into Thu. Dilemma here is southeast ridge amplifies with
increasing heights up the eastern seaboard. Thus Pacific short wave
approaching from the southwest will deamplify as it encounters
anticyclonic flow riding up and over east coast ridge. Thus another
weakening system moving across New England and prefer the lower qpf
from the ECMWF than the GFS here. Thermal profiles support snow
initially but with mid level low tracking well northwest of New
England mid level warming will invade southern New England with snow
possibly changing over to a wintry mix inland and possibly rain
along the coast. By no means a big qpf event but could be enough
wintry precip to impact one of the commutes.


Dry and cold weather likely returns behind departing system.


Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

12z update...

VFR and dry weather. Main issue will be some gusty NW winds this
morning up to 20-25 kt then diminishing this afternoon. Previous
discussion below.


Today...VFR. Diminishing W to NW winds.

Tonight...VFR. Light N winds.

Sunday...VFR. Light N winds becoming light SE to S.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Washingtons Birthday: MVFR/IFR conditions
possible. Chance SN.

Monday Night: Mainly VFR, with local IFR possible. Slight
chance SN.

Tuesday through Tuesday Night: VFR.

Wednesday: Mainly VFR, with areas IFR possible. Chance SN.


Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

A cold front moves across the waters early this morning. winds
will shift W to NW behind this front and gradually diminish.
Seas will take longer to subside, especially across the outer
coastal waters. Small Craft Advisories will continue across most
of the coastal waters into this afternoon, then only across the
outer coastal waters tonight. Rather tranquil boating conditions
expected Sunday.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Winds less than 25 kt. Chance of snow. Visibility
1 to 3 nm.

Washingtons Birthday: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of snow, chance of rain. Visibility
1 to 3 nm.

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of snow. Areas of visibility 1 to 3 nm.

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

Tuesday Night: Winds less than 25 kt.

Wednesday: Winds less than 25 kt. Slight chance of snow. Areas
of visibility 1 to 3 nm.


High astronomical tides next week with several tides above 12 ft
in Boston. Thus won`t take much onshore flow to produce minor
coastal flooding. Greatest risk comes late Wed into Thu as
surface wave tracks south of New England providing onshore flow.


MARINE...Small Craft Advisory until 5 PM EST this afternoon for ANZ235-
     Small Craft Advisory until midnight EST tonight for ANZ250-254-
     Small Craft Advisory until 7 AM EST this morning for ANZ251.
     Small Craft Advisory until 10 PM EST this evening for ANZ256.


NEAR TERM...Belk/Nocera
LONG TERM...Nocera

NWS BOX Office Area Forecast Discussion