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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 200000
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
700 PM EST Mon Nov 19 2018

.SYNOPSIS...
Developing storm center tonight through Tuesday night will 
bring accumulating snows mainly along and N of the MA-Turnpike, 
impacting the Tuesday AM commute for some. Otherwise an all rain
or rain/snow mix. Colder weather follows for Wednesday into
Friday, with the possibility of some snow squalls Wednesday.

&&

.NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
7PM Update...
Main change with this forecast update is to speed to speed the
timing of the POPs, initial overruning precip is moving across 
the area N of a BDL-ORH-BVY line with mixed RA and some SN in 
the high terrain along the Berkshires. Watching trends suggest 
this first round may actually dissipate in response to continued
deepening of the low pres upstream as it approaches. HRRR seems
to suggest this as well, so POPs will actually decrease a bit 
initially until the stronger lift associated with the developing
frontal wave moves in during the early AM hours. Otherwise, 
based on latest mesoscale guidance, see no need for significant 
changes to snowfall amounts and headlines as they stand.

Previous discussion follows...

Break in the action. Between the waves, quiet conditions prevailing.
But precipitation moving back in during the evening hours, becoming
widespread towards midnight. Overall light to start, and we've got
some warmer air to overcome out ahead. Anticipating a start to
snow for the high terrain whereas as lower elevations around the
Berkshires and Worcester Hills it'll start off as a rain / snow
mix prior to the lower column cooling and snow prevailing. N winds
will be driving through this forecast period, no longer light and
variable, and likely wet-bulbing will prevail. Most of the impacts
along and N of the MA-pike, can't rule out some travel hazard
conditions developing especially in the way of reduced visibility.

&&

.SHORT TERM /MIDNIGHT TONIGHT THROUGH TUESDAY NIGHT/...
Tonight through Tuesday...

Strengthening storm system across New England bringing accumulating 
around 3-inch snows mostly N of the MA Turnpike, especially the high 
terrain. Tuesday AM travel impacts with reduced visibility and snow 
on area roadways for those areas noted. 

Synoptically. Initial low sweeps New England with maturation further 
downstream. Mainly open wave, not so much closed H925-85, tracking 
over interior New England around the MA / VT / NH border, low level 
convergent focus of the anti-cyclonic warm-moist conveyor belt out 
ahead of a digging, sharpening mid-level trof axis and accompanying 
vortmax energy. Initiated low to mid level thermal packing as well 
as deformation / frontogenesis beneath mid-level ascent and initial 
diffluence aloft, majority low-level forcing up thru the dendritic 
growth zone, H6-7, so not impressively deep. Omega values through 
the column weak, especially through the dendritic growth zone which 
happens to exhibit some fairly steep lapse rates. Overall, a decent 
front-end thump leading out ahead of the surface to H85 low. The 
storm becomes better organized through Tuesday as it exits into the 
Gulf of Maine having swept across the Cape Cod Canal around 1 pm.

Challenges. Thermal fields / thickness tell the story. Right on the 
freezing cusp with deeper Arctic air absence N, hardly a N/NE high 
over Canada. While N isallobaric flow, it'll only drain S surface 
temperatures ranging around 30 with dewpoints a tad lower such that 
wet-bulbing can only do so much. Low-level column nearly isothermal 
up through H8, warmer air intruding as the H925-85 flow flutters 
along the MA / NH / VT border, possibly some drier air working in 
through the dendritic growth zone, while fluttering around freezing 
as the event evolves into the daylight hours, dealing with forecast 
headaches concerning precipitation types and snowfall accumulations 
as well as snowfall intensity. Speaking of, as model forecast mass 
fields look pretty similar, small-scale focusing forcing mechanisms 
vary as discerned via model forecast plume diagrams. Considerable 
spread in respect to liquid-equivalent precipitation and snowfall, 
the latter ranging from sub-advisory to warning levels in particular 
locations. Relates back to snowfall intensity, some low probability 
of 1"/hr noted in high-res guidance, and thermal field alignment as 
to where the rain / snow cut-off line will be at any one particular 
time. Forcing majority is, however, below the dendritic growth zone 
with this synoptic setup. Of last note, model forecasts assume that 
everything which falls accumulates on the ground which we know is 
not always the case. So it's definitely not easy but we're going to 
do our best as we always do. 

Decisions. Despite uncertainties and challenges noted, a consensus 
weighted forecast does not seem like a bad idea. Gave greater weight 
towards the HREF blended within. With probabilities noted, and the 
19.12z forecast suite slightly jogging S, focus is around the MA / 
NH / VT border. Coating N of the MA-pike, with 1-3" lower elevations 
while 3-6" higher, 500 feet and above especially. Potentially near 
warning-level snows up around the N MA towns of Ashburnham, Ashby, 
and Townsend. Keep with present N/W MA WINTER WEATHER ADVISORIES.
The storm center sweeping NE across the Cape Cod Canal important
with respect to blustery N/NE winds out ahead. 

Impacts. Expect hazardous travel over N/W MA especially the higher 
terrain of Worcester Hills and Berkshires. Route 2, western-half of 
MA I-90, N MA I-95, I-190 out of Worcester, and the NW beltway of 
495 all have the risk of accumulating snows on roadways. Reduced 
visibility as well especially within any moderate to heavy snow as 
near 1"/hr snowfall rates are possible. Thinking a very low risk of 
any mixed wintry precip-types mainly in the form of freezing rain.  

Tuesday night...

Turning colder and drier. The storm continuing to deepen into the
Gulf of ME, blustery NW winds follow. Some lingering snow showers
possible, especially for the high terrain. Otherwise the main storm
is the driving cold along with breezy conditions that'll make it
feel 5 to 10 degrees colder than the forecast nighttime lows around
the low to mid 20s, possibly upper teens for the high terrain.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
4 AM update...

Highlights...

* Arctic front late Wed with snow squalls possible
* Near record cold & bitterly cold wind chills Wed night/Thanksgiving
* Moderating temps and dry Friday into Saturday
* Wintry weather possible Sunday

Overview and model preferences...
The longwave trof which has been a persistent player in the 
daily sensible wx across the northeast finally gets a break 
later this period, but not before one last push of cold air. Two
mid lvl cutoffs Fujiwhara near Hudson's Bay with one and its 
attendant vort-max sliding close to New England Wed into Thu. 
The surface reflection is an arctic cold frontal passage Wed. 
This ushers in a very anomalous airmass with H85 temps nearly 
-20C below seasonal normal values which will be with us for 
Thursday into Saturday. Upper lvl ridge finally shifts E, 
shifting the trof offshore by the weekend. However, it too 
flattens as a second wave, linked more closely to the southern 
stream per latest guidance moves in late weekend into early next
week. Given the guidance has been in fairly good agreement with
this arctic influenced pattern a consensus blend will be used 
except for temperatures, which will feature the coldest of the 
guidance Wed-Fri.

Details...

Wed...
Arctic cold frontal passage linked to acute, cyclonically- 
curved shortwave rotating through the base of the longwave trof 
mentioned above. The main issue will be the risk for snow 
squalls. Soundings indicate a sfc-H7 which is both moist and 
with potentially higher than dry-adiabatic lapse rates. With 
this implied instability, cold air aloft and strong dynamic 
cooling in play, could very well see brief bursts of heavy snow 
impacting commuters. LLJ at the top of this well mixed layer is 
around 40 kt, which could mix down some of this momentum to 
accompany the snow. Hence the risk for squalls, this will be 
something to watch given the typically busy travel day. 

Thu...
Core of cold air working E but as nearly 1040mb high pres and 
deepening low pres in the Maritimes combine to yield strong 
pres rises note very breezy 20-30 kt winds at times as well. 
Ambient mixed temps will struggle to break out of the teens and 
low 20s, but with winds, expecting chill values int the single 
digits to near 0F. With pres gradient weakening late Thu night, 
some radiational cooling should allow ambient 2m mins to drop 
into the single digits as well. Ocean effect SN for the outer 
arm of the Cape particularly thanks to +15C SST-H85 delta-T 
values.

Fri and Sat...
Core of high pres slides across New England with the 
continuation of dry wx. Temps moderate but remain well below 
normal on Fri, with highs only making it into the upper 20s to 
low 30s. Overnight mins still cold given good setup for 
radiational cooling once again. Highs moderate further Sat, 
reaching the low-mid 40s thanks to increase in mid lvl temps.

Sun into Mon...
Will be monitoring low pres development in convection near the 
Gulf of Mexico, linked to a S stream shortwave. The track of 
this feature is still uncertain given wide model spread and the 
features not well sampled at this time. This will dictate 
whether its an inland passage (warmer solution) or offshore 
track (colder, but also a risk for a miss). Exact p-type details
will be worked out with time.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High Confidence.

Tonight and Tuesday...
Mainly VFR vsby with mixed MVFR/VFR CIGS through about 04Z 
south of a line from BDL-ORH-BVY. Areas N will mix MVFR/IFR in
light RA/SN through this time. After 04Z, trend will be toward
mainly IFR all sites except immediate S coast due to lowered
CIGS. VSBYS MVFR/IFR in SN mainly along and N of the Mass Pike
with RA to the S. This continues into Tue with some improvement
after 18Z, slowly from W-E. Winds N-NE.

Tuesday night...

Winds shifting NW as conditions improve towards VFR, the 18z
storm center lifting out into the Gulf of ME. Gusts up to around
25 kts possible especially over the high terrain and along
coastline terminals. 

KBOS Terminal...
Moderate confidence, mainly due to uncertainty in timing of
lower categories/precipitation. VFR conditions to gradually give
way to MVFR then IFR with lowered CIGS then vsbys as a mix of
RA/SN develops late tonight. Mainly RA into tomorrow but a brief
change back to SN is possible late tomorrow afternoon before the
precip ceases entirely.  

KBDL Terminal...
VFR gives way to mixed MVFR then IFR within the next 3-4 hours.
RASN mix right through the Tuesday morning push, becoming all 
rain towards midday. Mostly IFR, improving MVFR to VFR late 
Tuesday into Tuesday evening. N winds throughout shifting NW 
late. Somewhat breezy, especially late with potential gusts up 
to 20 kts. 

Outlook /Wednesday through Saturday/...

Wednesday: Mainly VFR, with local IFR possible. Windy with
gusts to 35 kt. Isolated SHSN.

Wednesday Night through Thanksgiving Day: VFR. Windy with gusts
to 35 kt. 

Thursday Night: VFR. Windy with areas gusts to 30 kt. 

Friday: VFR. 

Friday Night: VFR. Breezy. 

Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA or
SHSN.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High confidence. 

Storm center passing over the Cape Cod Canal around 18z Tuesday.
Will see increasing E/NE winds out ahead especially over the E
waters, S/SW for the S waters. Behind the storm center, winds
turning NW, remaining blustery. Throughout, gusts up around 25
kts possible. Can't rule out near gale force gusts. SMALL CRAFT
HEADLINES in effect for the later-half of Tuesday going into 
Tuesday night as seas build up around 5 to 7 feet on the outer 
waters. 

Outlook /Wednesday through Saturday/...

Wednesday: Low risk for gale force winds with gusts up to
35 kt. Seas up to 5 ft. Isolated snow showers. Local visibility
1 to 3 nm.

Wednesday Night: Moderate risk for gale force winds with gusts 
up to 40 kt. Rough seas up to 10 ft. Isolated snow showers. 
Areas of visibility 1 to 3 nm.

Thanksgiving Day: Strong winds with gusts up to 35 kt. Rough
seas up to 10 ft. Isolated snow showers. Areas of visibility
1 to 3 nm.

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Local rough seas. Local visibility 1 to
3 nm.

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

Friday Night: Winds less than 25 kt. 

Saturday: Winds less than 25 kt. Slight chance of rain showers.

&&

.CLIMATE...
With the potential of the coldest air of the season moving into
the region around Thanksgiving, here are the record temperatures
for November 22nd and the holiday of Thanksgiving.

November 22nd 
Location / Record Low / Record Minimum High

Boston..........9 (1879) / 24 (1880)
Hartford.......14 (1969) / 27 (1978)
Providence.....16 (1987) / 30 (2008)
Worcester......11 (1987) / 24 (2008)

Thanksgiving
Location / Record Low / Record Minimum High

Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901)
Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989)
Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996)
Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory from 10 PM this evening to 5 PM EST 
     Tuesday for MAZ002>004-008-009-026.
RI...None.
MARINE...Gale Watch from Wednesday evening through Thursday morning for 
     ANZ231>234-251.
     Small Craft Advisory from 4 PM Tuesday to 4 AM EST Wednesday 
     for ANZ232>234.
     Small Craft Advisory from 1 PM Tuesday to 1 AM EST Wednesday 
     for ANZ231-251.
     Gale Watch from Wednesday afternoon through Thursday morning 
     for ANZ235-237.
     Small Craft Advisory from 4 PM Tuesday to 6 AM EST Wednesday 
     for ANZ235-237.
     Gale Watch from Wednesday afternoon through Thursday evening 
     for ANZ250-254>256.
     Small Craft Advisory from 10 AM Tuesday to 6 AM EST Wednesday 
     for ANZ250.
     Small Craft Advisory from 1 PM Tuesday to 6 AM EST Wednesday 
     for ANZ254>256.

&&

$$
SYNOPSIS...Doody/Sipprell
NEAR TERM...Doody/Sipprell
SHORT TERM...Sipprell
LONG TERM...Doody
AVIATION...Doody/Sipprell
MARINE...Doody/Sipprell
CLIMATE...