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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 142156
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
456 PM EST Wed Nov 14 2018

.SYNOPSIS...
Cold high pressure builds across northern New England tonight
yielding much colder than normal temperatures for southern New
England.  The high provides dry weather most of Thursday however
snow begins to overspread CT and RI around or shortly after sunset,
as low pressure tracks up the eastern seaboard. As this low tracks
along the south coast of New England Thursday night and Friday, snow
changes to rain along and near the coast with snow to ice inland.
Behind the storm dry weather follows Friday night into Saturday. A
weak system may bring scattered snow showers Sunday night into
Monday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...

4 PM update...

Two decks of clouds to consider this afternoon/evening. The
lower clouds are linked to lake-effect which has been persistent
since early this morning. However, with core of +1035mb high
pres about to crest early overnight, these should be cutoff
within the next few hours. The upper lvl CI, on the periphery of
a modest jet, will likely persist, given little change in the
mid and upper lvl pattern.

Even with the CI, the high pres crest will yield decent
decoupling, in spite of the breezy conditions through the
afternoon. Mixed afternoon dwpts mainly in the single digits
suggest plenty of room for inversion development overnight.
Doubtful that the CI will limit radiational cooling, so leaned
most heavily on the colder guidance, defined mainly bet the
latest MAV/MET guidance.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/...

445 PM update...

*** Winter Storm Watch for Snow & Ice Inland Thu night ***
*** Winter Weather Advisory for a wintry mix Thu night ***
*** Wind Advisory - strong winds Thu night/Fri Cape/Islands ***

Synoptic Overview...

Anomalous cold airmass over New England tonight into Thu will set
the stage for a winter storm as a loaded southern stream system over
the Gulf states induces cyclogenesis along the eastern seaboard.
Models in fairly good agreement on the large scale features
including track of mid level low and surface circulation along with
trajectory of the dry slot. As usually the case with these winter
storms there are model differences in thermal fields which is
typical at this time range.

Details...

Thursday...

Mid level ridging in concert with 1034 mb high over the region
provides dry weather Thu. While it will be cold with highs only in
the 30s, much less wind than today which will make tomorrow more
tolerable. However moisture from southern stream system approaches
from the southwest late in the day with snow possible overspreading
the CT and RI between 4-7 pm. The column along with surface temps
are cold enough to support snow across all of CT-RI. Strong 850 mb
jet approaching the region with lift focused in the snow growth
region may yield a descent front end dump of snow. HREF indicating
30-50% probability for hourly snowfall rates approaching 1" toward
00z across CT and RI. Thus could see some impact to the late day
commute from HFD-PVD. Elsewhere snow likely holds off until after 7
pm.

Ptype/Snowfall potential...

Anomalous cold airmass over the region this evening with dew pts in
the single digits!  This 1030+ mb high over Maine and New Brunswick
combined with pres falls approaching from the southwest will likely
induce a coastal front to setup over eastern MA, shifting winds from
E to NNE during Thu evening. This will maintain low level cold air
and support all snow for at least a few hours Thu evening. However
as the low level jet approaches, onshore flow will increase and the
coastal front will likely retreat inland first to the RT-128
corridor then I-495 corridor. This will result in the rain-snow line
moving onshore into eastern MA. Complicating the forecast is also
warm air aloft surging northward with mid levels becoming isothermal
suggesting snow mixing with sleet. However the forcing for ascent is
fairly robust during the evening (00z-06z) which may be sufficient
to cool this layer along with heavy snow melting in this layer
providing additional cooling. NAM soundings hold onto this snow
potential until about midnight or so for the Greater Boston area. If
this verifies looking at potential 2-4" before a change over to
sleet and finally rain. However if sleet mixes in sooner along with
coastal front moving inland quicker, a coating to 2 inches is more
likely for Boston area. Just too early to say which scenario will
play out. Given the wintry mix of precipitation expected a Winter
Weather Advisory has been issued.

For RI and southeast MA brief period of snow, heavy at times from 7
pm to 10 pm then snow changing over to sleet and finally all rain.
Snow accumulations of 1-2" possible with up to 3" possible northern
RI. A winter weather has also been issued for this area.

For CT and especially western - central MA 2-4" of snow is possible
before dry slot overspreads the area, changing snow and sleet to
freezing rain around or shortly after midnight. Concern here is for
an extended period of freezing rain/drizzle into Fri morning as low
level cold air will be difficult to scour out with pres falls
tracking up the eastern seaboard. This will support good northerly
drainage flow down the CT river valley supporting freezing
rain/drizzle from about midnight into Fri morning. So given the
potential for 2-4" of snow followed by significant icing decided to
go with a Winter Storm Watch to highlight the potential for
significant winter weather. This is for all of northern CT into
western-central MA Thu evening into Fri morning.

Other contributing factors for the potential for a period of heavy
snow is strong forcing for ascent in the snow growth region (-10C to
-18C) and isothermal layers aloft.

Strong Wind Potential...

We hoisted a wind advisory for Cape Cod and the islands Thu night
into Fri as low level jet moves across this area. Model soundings
suggest high probability for 40+ kt ENE gusts.


&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...

Highlights...

*  Cooler than normal conditions persist.
*  Weak low pressure may bring scattered rain/snow showers late
   Sun into Mon

Overview and model preferences...
Zonal flow, mainly at the base of a persistent longwave trof,
develops in the wake of late week storm and attendant shortwave.
Given this is really on the elongated base of this trof, there
is a continued conduit for Canadian CP air to filter into the
region through the weekend, in spite of the slight SW-NE
orientation of the mean jet aloft. The next feature to watch, is
a lobe of arctic energy rotating around a cutoff low pres near
James Bay. This should enact at least modest cyclogenesis along
a stalled frontal boundary along or just S of S New England by
late Sun into Mon. Whether the inland low pres, tied closely to
the vort-max to the N is able to efficiently transfer energy to
coastal low later Mon. Given there is some uncertainty, thermal
profiles also retain uncertainty. These will be better resolved
with time. Otherwise, with vort-max shifting E into the middle
of next week, a return of drier but chilly weather is
anticipated, given the longwave trof remains in play. Models are
showing enough agreement with the 14.12Z guidance update,
enough that a consensus blend can be used.

Details...

Fri night into Sat...
Blustery W-NW flow continues following the low pres shifting
into the Canadian Maritimes. Strong cold advection coincident
with this flow. Lows mainly in the 20s with colder wind chills
Fri night give way to highs mainly in the mid to upper 40s Sat.

Sun...
Weak W-E oriented front settles near S New England, but the
area remains entrenched beneath cold airmass and influence of
lingering inverted ridging. Therefore, will favor mainly dry
conditions through the daylight hours with temps remaining in
the low-mid 40s.

Sun night into Mon...
Inland low pres development, with potential coastal low
development possible. At this time, airmass looks to cool enough
in the low lvls Sun night to support predominantly SN across
the region except mainly the immediate S coast where onshore
component will draw across SSTs still around +10C. Whether the
low pres develops further offshore Mon will determine whether
colder air gets locked in. At this time, consensus blend
supports a mainly wintry event and will continue with this.
Limiting factor will be QPF, as the origins of this low pres
across the CONUS. Therefore, looking at mainly an advisory lvl
system if that.

Into mid next week...
High pres, associated with an arctic airmass will settle across
the region into mid week. The pattern remains under influence
of the longwave trof. So, expect the dry and cooler than normal
conditions to persist.

&&

.AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday/...High confidence.

18z update...

Tonight...

VFR and dry weather with gusty NW winds diminishing with sunset.

Thursday...

VFR to start but MVFR 21z-00z across CT-RI and south coast of MA
as snow overspreads this region. Snowfall hourly rates may
approach 1 inch per hour toward 00z from BDL-PVD and points
southward.

Thursday night...

MVFR to start but then lowering to IFR/LIFR after 03z. Snow
heavy at times 00z-03z but then changing over to rain south
coast. Snow and sleet changes over to freezing rain inland with
all rain in the coastal plain. Strong NE winds develop south
coast after 06z.

Friday...

IFR/LIFR in the morning with rain showers except freezing rain
possible interior. Strong NE winds early Cape Cod. Improving to
MVFR- VFR during the afternoon with wind shift to the west.

KBOS Terminal...High confidence in TAF thru 00z Fri. Snow may
hold off until after 00z Fri then briefly heavy until about
03z/04z. Coating upwards to 2 inches possible. Low prob of 3 to
4 inches but coating to 2 inches more likely as of now.

KBDL Terminal...High confidence in TAF thru 18z Thu. Snow
arrives Thu 21z-00z with hourly snowfall rates approaching 1
inch per hour possible toward 00z and thereafter until 03z
before changing to sleet and freezing rain.

Outlook /Friday Night through Monday/...

Friday Night through Saturday Night: VFR. Breezy.

Sunday: Mainly VFR, with local IFR possible late. Slight chance
SN late.

Sunday Night: Mainly IFR, with areas MVFR possible. Chance SN,
chance RA.

Monday: MVFR/IFR conditions possible. Breezy. Chance RA, chance
SN.

&&

.MARINE...

*** NE Gales Likely Thu Night/Fri morning ***

Tonight and Thursday...light winds with 1030+ mb high over
Maine.

Thursday night...NE gales with vsby reduced less than a mile
initially in heavy snow then over to rain as low pressure tracks
along the south coast.

Friday...low pres near Cape Cod exits with improving weather
during the afternoon.

Outlook /Friday Night through Monday/...

Friday Night: Moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Rough seas up to 13 ft.

Saturday: Winds less than 25 kt. Areas of rough seas.

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

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

Sunday Night: Winds less than 25 kt. Chance of rain.

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

&&

.HYDROLOGY...
Renewed river flooding is possible across RI and eastern MA with
0.50 to 1.0 inches of rainfall possible Thu night into Fri after
a few inches of snow first. Will have to watch this closely.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides are very low (below 9 ft) in Boston. Thus
with this upcoming storm worse case scenario would be isolated
minor flooding possible.

&&

.CLIMATE...
Record low temps for Nov 15

BOS  17 in 1933
PVD  18 in 1905
BDL  17 in 1905 and 1933
ORH  14 in 1933

Record low max temps for Nov 15

BOS  30 in 1933
PVD  30 in 1933
BDL  30 in 1933
ORH  30 in 1972 and 1996

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Watch from Thursday afternoon through Friday
     morning for CTZ002>004.
MA...Wind Advisory from 1 AM to 10 AM EST Friday for MAZ022>024.
     Winter Weather Advisory from 4 PM Thursday to 1 AM EST Friday
     for MAZ017>021.
     Winter Weather Advisory from 7 PM Thursday to 1 AM EST Friday
     for MAZ007-013>016.
     Winter Storm Watch from Thursday evening through Friday
     morning for MAZ002>006-008>012-026.
RI...Wind Advisory from 1 AM to 10 AM EST Friday for RIZ008.
     Winter Weather Advisory from 4 PM Thursday to 1 AM EST Friday
     for RIZ001>007.
MARINE...Gale Warning from 10 PM Thursday to 4 PM EST Friday for
     ANZ232>235-237-254>256.
     Gale Warning from 7 AM to 4 PM EST Friday for ANZ230.
     Gale Warning from 1 AM to 4 PM EST Friday for ANZ231-250-251.
     Small Craft Advisory until 7 PM EST this evening for ANZ236.

&&

$$
SYNOPSIS...Nocera/Doody
NEAR TERM...Doody
SHORT TERM...Nocera
LONG TERM...Doody
AVIATION...Nocera/Doody
MARINE...Nocera/Doody
HYDROLOGY...
TIDES/COASTAL FLOODING...
CLIMATE...