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Area forecast discussion 
National Weather Service Taunton Massachusetts 
647 PM EST Wed Dec 13 2017 


Synopsis... 


* update on present winds in the near-term ... 


A fast moving low pressure will track S of New England late 
tonight and early Thursday bringing a period of accumulating 
snow to the South Coast and especially the islands. Cold 
persists into the weekend with a chance of snow showers Friday 
night into Saturday. Blustery and cold for Sunday but the 
beginning of a milder trend Monday ahead of which there's the 
possibility of a mixed wintry precipitation event. Quiet and dry 
with an Ebb and flow pattern during the week up till late 
Thursday into Friday during which time there's the possibility 
of another mixed wintry precipitation event. 


&& 


Near term /until 6 am Thursday morning/... 


7 PM update... 


Light snow associated with a clipper low fast approaching. From 
the Storm Prediction Center mesoanalysis, modest low-level frontogenetical push ahead 
of the storm yielding some decent output as discerned via local 
WSR-88D radars. Associated deep moist convergence on the backside 
with elements over-running ahead along the leading warm front, 
beneath strong ascent per main vortmax and parent h2 jet streak 
aiding with divergence aloft (h85-2 differential divergence). 
Decent warm air advection, can even see a nose of steep mid-level 
lapse rates sneaking into the Ohio River valley. 


But downstream, the cold, dry airmass in place, low surface dew- 
points holding firm, keeping both the surface low and better 
moisture fetch well S offshore. Limitations on what can be lofted 
north of the low and as to the potential of that being lofted in 
reaching the ground. Nevertheless, do expect snow to move in 
beginning around 4a, the height around 7a (9z-12z). Reductions 
to visibility with light accumulations, the greatest of which 
around 1-2 inches confined to the S-coast and across the islands, 
the islands likely seeing the most. 


Potency of the system will need to be monitored closely as we go 
through the evening into the overnight. Have adjusted the forecast 
bringing pops up to highly likely. Echoing the prior forecaster, 
increasing clouds, snow settling in, at first appearing on radar 
to be overspreading the region, but likely initially not reaching 
the ground until a little later, closer to daybreak. 


As to winds ... holding onto the Wind Advisory. Still seeing peak 
wind gusts this hour that are actually at their strongest of the 
day in some spots, close if not exceeding 45 mph (40 kt) criteria. 
Vertical wind profilers from our radar and tdwr in Boston has 
h925 winds out of the west in excess of 50 mph (45-50 kts observed). 
Upstream winds are diminishing, so will continue to evaluate as 
to whether headlines need to be dropped or extended. 


Previous discussion... 


Cold air advection continues across southern New England with Storm Prediction Center mesoanalysis 
indicating -19c air at 850 mb upstream over northern portions of New York 
and Vermont. Thus modified Arctic air continues pour across the area on 
blustery west-northwest winds up to 45 mph, courtesy of 975 mb low over New 
Brunswick. This will result in very cold conditions this evening 
along with bitterly cold wind chills. 


Fairly robust mid level trough currently over the Great Lakes races 
eastward overnight with increasing clouds and light snow/flurries 
developing toward daybreak especially across CT, Rhode Island and south of the 
Massachusetts Pike in western part of the state. Initially airmass is very dry 
so while radar may look impressive late tonight probably a few hours 
of virga before snow reaches the ground toward daybreak Thu. 


&& 


Short term /6 am Thursday morning through Thursday night/... 


*** accumulating snow likely immediate South Coast and islands 
Thursday morning *** 


4 PM update... 


Thursday... 


Mid level trough moves across the region from west to east during 
the morning. Good synoptic scale lift associated with this system as 
seen on q-vector maps. This results in surface low jumping from near 
Cleveland at 06z to redeveloping south of Long Island 12z Thu and 
tracking east along the 40n parallel during the day. Good model 
agreement that modest Omega (15 -ubar/s) occurs in the dgz (snow 
growth layer of -10c to -20c) 12z-15z Thu along the immediate South 
Coast and islands. Thus should see at least modest size flakes 
(dendrites) which will help reduce vsby and yield an efficient snow 
system with 15-1 snow ratios. In addition, models suggest some 
instability in this layer too in the form of -epv. So the 
combination of modest synoptic scale lift, good fgen in snow growth 
layer may result in this system over performing. However limiting 
factors for more snow is very dry airmass on the front end of this 
system. Will likely lose a few hours of qpf to virga as blyr slowly 
saturates. Furthermore, system is very progress which leaves just a 
1-3 hr window for moderate snowfall. Thus have leaned toward a 1-3 
inch snowfall for immediate South Coast and islands with impact to 
the morning commute from westerly to Newport to New Bedford to Cape 
Cod. Given cold antecedent conditions snow will immediately stick to 
all surfaces. Will have to see how 00z models trend as these systems 
sometime trend northward. This would result in accumulating snow 
possibly as for north as Hartford to Providence to Plymouth. 
Nonetheless there will be a very sharp cutoff on the northern edge 
of the snow shield. 


Given trough is very progressive expecting sunshine to develop 
during the afternoon. Still colder than normal with highs only in 
the upper 20s and lower 30s. Not nearly as windy as today but still 
a chilly day. 


Thursday night... 


Becoming blustery during the evening behind departing low. However 
winds drop off late as weak high pres builds in. This will allow for 
another cold night including the South Coast given a fresh snow 
cover. Lows in the teens most areas with single digits well inland. 


&& 


Long term /Friday through Wednesday/... 


*/ highlights... 


- light snow showers possible Friday night into Saturday 
- cold and blustery Saturday night into Sunday 
- possible mixed precipitation Sunday night through Monday night 
- perhaps quiet, cold until Thursday night into Friday 
with another mixed precipitation event 


*/ overview... 


Interpreting through late December. Considerable forecast warming in 
the stratosphere, polar low / night-time jet displaced to the other 
side of the north hemisphere subsequent of noteworthy anomalous ridging 
across northern North America. Beneath which warm air continues to surge 
into the Arctic from the northwest Pacific (with perhaps mjo contributions) 
and the northwest Atlantic displacing colder air S where it becomes bowled 
up across NE Asia / NE North America. Ensemble means continue to signal 
an active weather pattern with an amplified 500 mb ridge-trof pattern 
over the conus, anomalous 850 mb T warming over the North Pacific with cold 
anomalies continually reloaded over central/east North America, however a 
trend towards higher heights / surface mean sea level pressure with 
stratospheric cross-polar flow shifting into northwest Europe. 


Making sense of it all, northwest Pacific disturbances translating east across 
the Continental U.S. Seemingly look to flatten the 500 mb pattern, lock colder air 
north for a time, the NE Continental U.S. Falling into the transitional zone of air- 
masses before colder air reloads and surges S again. An Ebb and flow 
setup however not barred from north shots of colder air, the lack of S- 
stream dominance. Yet a trend of higher heights as signaled from the 
ensemble means, cold shots may not be as deep, storm development may 
not be so pronounced long-term. Perhaps La Nina signals are starting 
to emerge across North America with a lesser pronounced 500 mb trof pattern 
over central/east North America downstream of Stout ridging / warmth over 
the northwest Pacific. 


Tough to get down on the details and specifics, taking a Broad View 
approach with the discussion above. Will hit on any threats/impacts 
in the details below. With perhaps a flatter pattern evolving and 
the lack of downstream traffic over the North Atlantic, the move into a 
zone of transition between the airmasses as noted above, as forecast 
guidance suggests, could end up in a regime of quick-moving systems 
with mixed precipitation type outcomes that later deepen downstream 
and yield that reloading shot of colder air, yet that colder air not 
as deep, perhaps simply swiping the NE Continental U.S. More to the north. 


*/ Discussion... 


Friday through Saturday... 


Cold, blustery at times, chance of snow showers Friday night into 
Saturday. A nod to ensemble means for 24-hour rainfall probabilities 
in excess of 0.01 inches as north-stream clipper energy translates to a 
dominant S-stream disturbance across the offshore baroclinic front. 
Yet leaning very light outcomes. Lack of ascent within moist snow 
growth regions parent with the positively-tilted 500 mb trof undergoing 
neutral transition along the surface transition boundary. It isn't 
till better mid-level curvature and ejecting vortmax above the S- 
stream can get things going, in this instance further downstream. 
Leaning snow accumulations along the north/west slopes of high terrain 
given orographic support, and along the southeast coast as energy / ascent 
begins to come together before ejecting out to sea. Highest snow 
accumulations possibly over an inch for S/southeast coast, especially over 
the islands and Outer Cape. Visibility impacts, perhaps some slick 
Road conditions. Behind the disturbance, winds kick up, colder air 
drives back in, wind chills drop back down into the single digits 
Sunday morning, lows in the teens. 


Sunday into Monday... 


Potential mixed precipitation event. Ascent overall light, seemingly 
quasi-parallel flow along the lifting warm front. Above a cold air 
damming signature with an indication of northerly ageostrophic flow, 
the surface high to the north however a weaker low over the Great Lakes, 
could see a period of freezing rain / drizzle centered around late 
Sunday night into early Monday morning, especially in the interior 
sheltered valleys where cold will be more difficult to dislodge. A 
nod again to ensemble mean for 24-hour rainfall probabilities in 
excess of 0.01 inches which is roughly 70 percent or greater. Again 
light outcomes but enough to potentially make for hazardous travel 
and could yield winter weather advisories for a trace of ice. Some 
question as to how quickly the warm front will lift north and how long 
threats / impacts may linger. Could hold through Monday evening. A 
low confidence forecast given poor handling via operational guidance 
on individual upstream waves. 


Tuesday Onward... 


Leaning a flatter, progressive flow regime with transitioning air- 
masses. With any additional waves will be looking for potential 
mixed precipitation events with the Ebb and flow of 2m temperatures. 
Weaker energy ejecting east and deepening, expecting behind each system 
the return of colder conditions ushered by breezy north/west winds. Lack of 
confidence in the forecast beyond 96 hours (which does include the 
Sunday into Monday timeframe). In leaning with ensemble means, after 
the early week disturbance, our next event may not occur until late 
Thursday into Friday, an initial cold blustery pattern transitioning 
warmer, more seasonable in-between. 


&& 


Aviation /00z Thursday through Monday/... 


Forecaster confidence levels... 
low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent. 


Short term /through Thursday night/...high confidence. 


Tonight... 
VFR. Blustery west winds, gusts 25-35 kts, subsiding towards morn. 
Increasing cigs, lowering as -sn spreads SW-NE across terminals 
9-12z, MVFR- IFR visibility impacts expected as lighter west winds prevail. 


Thursday... 
baf-orh-bos MVFR 3sm -sn beginning around 9-12z, through roughly 
15z then tapering. S of the line, 2sm -sn again 9-12z, IFR yet 
MVFR vsbys. Lowest conditions over S-coast and islands with more 
appreciable accumulations around 1-2 inches subsequent of -sn/snow 
and vsbys down to 1/2sm and IFR cigs around 800 ft agl. Continued 
west winds, -sn/snow activity tapering 15-18z, becoming VFR and dry. 


Thursday night... 
VFR, dry, west winds becoming gusty again, especially along the S/southeast 
coast. 


Kbos terminal... 
anticipating flurries to overspread the terminal around 9z then 
transitioning towards -sn with MVFR vsbys. Brief period 12-15z 
and then improving thereafter. Light accumulations possible on 
area runways but mainly a dusting. A dry snow, easily blown 
around by jet turbines. 


Kbdl terminal... 
flurries into the terminal airspace around 9z then becoming more 
light, potentially moderate, snow reducing vsbys to 2sm / IFR. 
Light accumulation on runways possible up to an inch, but a 
light and fluffy snow easily blown around by jet turbines and 
expected west flow around 10 kts. Improvement after 15z, snow 
tapering as vsbys improve. 


Outlook /Friday through Monday/... 


Friday: VFR. 


Friday night: mainly VFR, with areas MVFR possible. Breezy. 
Chance shsn. 


Saturday: VFR. Windy with gusts to 30 kt. 


Saturday night: VFR. Windy with areas gusts to 30 kt. 


Sunday: mainly VFR, with local MVFR possible. Slight chance snow. 


Sunday night: mainly VFR, with areas MVFR possible. Chance sn, 
chance ra, slight chance freezing rain. 


Monday: mainly VFR, with local MVFR possible. Breezy. Chance 
ra, chance sn, slight chance freezing rain. 


&& 


Marine... 


Forecaster confidence levels... 


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


Short term /through Thursday night/...high confidence. 


Tonight... 


West-northwest gales this evening then diminishing toward midnight as 975 low 
over New Brunswick lifts farther north into Canada. 


Thursday... 


Weak low moves east across 40n south of New England with a period of 
morning snow lowering vsbys especially near South Coast of MA/RI. 
Low races out to sea with improving conditions during the afternoon 
and winds becoming northwest. 


Thursday night... 


Modest northwest winds in the evening becoming light late as weak high pres 
builds in from the west. Dry weather and good vsby. 


Outlook /Friday through Monday/... 


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


Friday night: low risk for Small Craft Advisory winds with 
local gusts up to 30 kt. Areas of seas approaching 5 ft. Chance 
of rain showers, chance of snow showers. 


Saturday: low risk for gale force winds with gusts up to 35 kt. 
Areas of rough seas. Slight chance of rain showers, slight 
chance of snow showers. 


Saturday night: low risk for gale force winds with gusts up to 
35 kt. Local rough seas. 


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


Sunday night through monday: winds less than 25 kt. Chance of 
rain. 


&& 


Box watches/warnings/advisories... 


CT...none. 
Massachusetts...Wind Advisory until 10 PM EST this evening for maz007-019- 
022>024. 
Wind Advisory until 8 PM EST this evening for maz002>006- 
008>018-020-021-026. 
Rhode Island...Wind Advisory until 10 PM EST this evening for riz008. 
Wind Advisory until 8 PM EST this evening for riz001>007. 
Marine...Gale Warning until 1 am EST Thursday for anz230>235-237-251. 
Gale Warning until 9 PM EST this evening for anz236. 
Gale Warning until 3 am EST Thursday for anz250-254>256. 


&& 


$$ 


Synopsis...nocera/sipprell 



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