NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Dec 15, 8:20 AM EST, 1 hour 40 minutes earlier.
Area forecast discussion, Dec 15, 8:20 AM EST, 1 hour 40 minutes before
Area Forecast Discussion
National Weather Service Taunton MA
820 AM EST Thu Dec 15 2016
.SYNOPSIS...
A cold front moves across the area early this morning, accompanied
by scattered snow showers. Colder air overspreads the region behind
the departing front. An arctic front barrels through the region this
evening with dangerously cold temperatures and wind chills along
with the potential for damaging winds. Dry weather Friday. Warm-moist
air moving up from the south will bring a period of accumulating
snow Friday night and early Saturday. This transitions to rain by
Saturday night and Sunday with the possibility of a brief period
of freezing rain in-between. A quick shot of cooler air early next
week is replaced with more seasonable conditions beneath high
pressure initially followed by a series of milder weather systems
into late week.
&&
.NEAR TERM /THROUGH TONIGHT/...
820 am update...
***Dangerous snow squalls moving across northern CT and will move
into RI and portions of southeast MA***
Fast moving arctic cold front bringing a line of dangerous snow
squalls across northern CT and these will move into Rhode Island and
portions of southeast MA through 10 am. Uncertainty on how well
the activity will hold together, but brief white-out conditions,
strong wind gusts and roads quickly becoming snow and ice covered.
This has already occurred across portions of northern CT and this same
may happen in the next hour or so in RI and perhaps
portions of southeast MA. Special Weather Statement in effect.
Low pressure in Quebec is swinging a cold front into Southern New
England. Combined radars show a band of snow showers moving across
New York. The snow band has narrowed overnight as it moved east
and may break apart as it enters our area. Time of arrival in the
CT Valley and vicinity would be about 5-6 am, Worcester about 6-7
am, and to Boston-Providence 7-8 am. These brief snow showers,
where they survive, could leave a coating on the ground during
the morning rush hour.
Cold advection behind the front will generate deep mixing, up to
at least 700 mb. Winds in this layer will be 35-40 knots this
afternoon, and we expect an increasing amount of this wind to mix
down to the surface in gusts.
Temps aloft will cool by about 5C through the day, starting the
day favoring max sfc temps near freezing. But by mid afternoon
those cooler temps aloft will support max temps in the 20s. Expect
most temperatures to max out during the morning and then fall
during the afternoon.
Temps aloft -8C to -10C will move over 9-11C water temps. Thus
air-water differential will be near 20C, very favorable for ocean
effect snow showers to form. Southwest winds ahead of the cold
front will favor a fetch grazing the South Coast and Islands with
ocean effect snow. Behind the front the winds will become more West-
Northwest and favor keeping the snow bands offshore.
&&
.SHORT TERM /FRIDAY/...
Tonight...
Upper trough and its cold pool...the coldest of the cold air
aloft...will swing through. This will push the arctic cold front
through our area. Impressive cold air. Temps at 850 mb will fall
to -25C...if this were during the daytime that would support max
temps in the lower teens! This airmass at night should also
impress...observed temps upstream early this morning are in the
single numbers with a few below zero. Expect Southern New England
to experience something similar.
Low pressure in the Maritimes will deepen, increasing the pressure
gradient across New England. That along with cold advection
should maintain the strong winds much of the night. Winds aloft in
the mixed layer are expected to increase to around 50 knots, some
or all of which should mix to the surface. With winds at or near
50 knots expected, we will convert the High Wind Watch into a High
Wind Warning.
Cold air...strong wind...yep, that's right. How about wind chills?
Based on the temps and wind, we expect wind chills to reach -15F
or lower across the northern part of MA and in nearby parts of
northern CT and northern RI. Forecast wind chill values remain
above -24F in most places, with the possible exception of extreme
western Franklin County. We have converted the existing Wind Chill
Watch into a Wind Chill Advisory, and have added Nrn CT and part
of Providence Co RI, as well as areas along the I-95 corridor in
Eastern MA.
Friday...
The upper trough and its cold air aloft start moving out on
Friday. Surface high pressure builds in, providing sunshine and
diminishing wind. Much more shallow mixing with a max height near
925 mb. Temps at that level climb through the day, but are
equivalent to an 850 temp of -25C early afternoon. So expect max
surface temps in the teens and lower 20s. This and the lighter
wind will mean much more manageable wind chill.
Overall a cold and dry day. Looking at increasing sky cover late
in the day as the high pressure shifts to our east.
&&
.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
*/ Highlights...
- Wintry weather forecast beginning late Friday night into Saturday
- Plowable snow, solid advisory level for N / W / Central portions
of Southern New England
- Wet weather with breezy S winds late Saturday night into Sunday
- Shot of cold air into Monday, moderating warmer into next week
*/ Overview...
The EPO / WPO shift is on. Ridging over the NW Pacific
deamplifying, the flow becoming more zonal across the CONUS lends
the usherance of milder air off the Pacific yielding a moderating,
warming trend over the lower 48 with cold, Arctic air becoming
locked up over NE Canada as the NAO / AO trends strongly positive.
A return to a more La Nina pattern setup, energy off the Pacific
troughs to the lee of the Rockies but is ultimately stretched N/E
through the progressive down- stream pattern, with stronger
ridging E over the Atlantic and the polar low situated across NE
Canada / Greenland. Subsequently at the surface, areas of low
pressure are for the most part inside-runners. Perhaps some
initial wintry precipitation types, otherwise a more mild,
seasonable pattern with wet-weather outcomes. Amazing that we may
see another above-average holiday season, a third in a row if that
turns out to be the case. With lesser weighting towards the 15.0z
NAM, a consensus of forecast guidance along with ensemble and
probabilistic guidance is preferred with this update. Highlights
in the daily discussion below.
*/ Daily Discussion...
Friday night into Saturday...
Beginning around midnight Saturday and continuing through much of
the day confident advisory-level snows but can not rule out warning-
level for the W MA / NW CT high terrain that undergo a transition S
to N mainly to rain with a potential wintry mix for N / W / Central
interior portions of S New England.
Signals of an overrunning setup against the lingering Arctic airmass.
Fairly apparent we will see a good slug of precipitation given low-
level convergence and deep layer forcing of a moisture-laden airmass
with precipitable waters exceeding 1-inch (+2 standard deviations).
Along a leading warm front and push of milder air, some uncertainty
on where heaviest precipitation develops especially with secondary
low development associated with regions of best deep-layer forcing.
15.0z NAM appears to be the most robust and furthest N with higher
outcomes of around 0.8-0.9 with all other guidance S. This having
implications on snow accumulations. Will go with a consensus of
forecast guidance and lean away from heavily weighing on the NAM.
But there are other factors to consider: 1) As the prior forecaster
noted, the snow growth region initially is near-surface and will
gradually lift with the low to mid level warming. Snow-to-liquid
ratios will alter during this transition with warm air intrusion,
initially high around 18:1 dropping down to around 8:1. So timing of
precipitation along with depth / magnitude of cold air is essential
in determining accumulation rates which are likely to vary with
time. 2) depth and orientation of the ageostrophic wind, how
effectively it aides in maintaining the longevity of sub-freezing
temperatures associated with the lingering Arctic airmass. Again it
would appear per model wet-bulb analysis that the NAM is stronger
with the ageostrophic flow and subsequently maintains sub-freezing
temperatures slightly longer. 3) Drier air within the snow-growth
region beginning around 18z Saturday that erodes the presence of ice
aloft. In hand with the longevity of sub-freezing temperatures at
the surface, there is the possibility of freezing rain should
precipitation be ongoing, of which is expected to be light as
strongest forcing and slug of heavier outcomes pushes offshore.
Based on a consensus of ensemble, probabilistic, and deterministic
guidance, expect onset of snow beginning around midnight Saturday.
Usherance of milder air shortly after sunrise, will begin to see the
transition to rain working N. However into afternoon, sub-freezing
air lingering as drier air works in aloft as heavier precipitation
pushes offshore, there is the possibility of a period of freezing
rain for interior N / W / Central portions of S New England.
Considering all signals, could see upwards of a 6-hour period for
spots, especially within sheltered interior valleys, especially
should winds funnel out of the N. But precipitation outcomes that
are forecast to be light, would expect a glaze of accretion.
Saturday night into Sunday night...
With an inside-runner low across the Great Lakes region, warm-moist
air surges N across the NE CONUS within the low-levels advected with
SW flow of 50 to 80 mph at H925. The surface warm front slower in
its advance to the N lending to a shallow surface inversion, there
is some question as to how well winds will transfer to the surface.
Once again that warm-moist air is highly anomalous with pwats above
1-inch which again is +2 standard deviations for this time of year.
Expect a lull in precipitation during the evening hours with the
brief intrusion of drier air around the mid-levels, at this point
mainly in the form of rain for all of S New England. Looking at a
line of rain sweeping the region ahead of the cold front parent to
the inside-runner low during the early morning into afternoon hours
with rainfall amounts on the order of around half an inch, perhaps
locally higher given the dynamics and low-level jet convergence of
strong SW winds.
Speaking of winds, there are some signals to indicate gusty winds at
the surface but most of this appears driven by the gradient wind.
Can not rule out gusts as high as 40 to 50 mph for SE New England
especially Cape Cod and the Islands. Given the magnitude of the
inversion forecast within BUFKIT profiles have little confidence
that we'll see the stronger, decoupled flow aloft from reaching the
surface, not even via precipitation drag.
A non-diurnal temperature trend, just looking at SREF plumes it
would appear that lows for Saturday are realized right at sundown,
the start of the evening period.
Topping out early Sunday, cold air advection proceeds. Perhaps quick
enough to transition rain on the backside of the cold front to snow
but thinking it mainly dries out. Echo concerns via the previous
forecaster that the brunt of the cold air with any residual water
along area roadways, likely to be dealing with some black ice come
the Monday morning commute.
Next week...
A roughly 1040 high early on shifting E allowing for a moderating
airmass across the region, tending milder. Monitoring an area of low
pressure center S possibly skirting S New England Tuesday night into
Wednesday. Brief period of dry, mild weather prior to another inside-
runner low for the late week period. With onset, a mix of wintry
weather outcomes is possible. A consensus of forecast guidance is
preferred.
&&
.AVIATION /13Z THURSDAY 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.
12z update...
Today...
Mainly VFR with SCT-BKN cigs. SCT SHSN possible throughout today
which could lead to TEMPO IFR. Bigger story is increasing winds as
they turn out of the NW. Initially 20 to 30 kts, picking up to 35
to 45 kts into the afternoon.
Tonight...
VFR. Ocean effect snow showers off the coasts will generate
MVFR/IFR cigs/vsbys, but with a NW flow we would expect most of
this to remain offshore. NW winds will increase through midnight
with gusts of 50-55 knots possible, perhaps 60 knots on Cape Cod
and Islands.
Friday...
VFR. NW winds will diminish as high pressure builds over the
region. Morning gusts may linger 25-35 knots,especially along the
coast. All wind should diminish below 20 knots in the afternoon.
Lingering ocean effect snow showers offshore with MVFR cigs, but
this should diminish in the afternoon.
KBOS TAF...High confidence in TAF.
KBDL TAF...High confidence in TAF.
Outlook /Friday Night through Monday/...Moderate confidence.
Friday night into Saturday...
Lowering to LIFR beginning 6z Saturday. Initial onset of IFR-LIFR SN
with visibilities down to half a mile. Transition to RA with a brief
period of FZRA in-between especially for N / W / Central interior S
New England terminals. Snow accumulations expected with a general 1-
2 over SE MA / S RI increasing N/W to 4-6 over NW MA. A glaze of
icing possible for N/W/Central MA.
Saturday night into Sunday...
IFR continuing with RA. Breezy S winds. Possible LLWS impacts as
winds 2 kft agl will be around 50 to 60 kts out of the SW. Will seem
improvement late in the day with S/SW flow shifting out of the W/NW.
Monday into Tuesday...
VFR. NW winds initially blustery, diminishing with time.
&&
.MARINE...
Forecaster Confidence Levels.
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Friday/...High confidence.
7 am update...
Today and Tonight...
A cold front crosses the waters this morning ahead of which rain /
snow showers are possible. There is the possibility of reduced
visibility especially over the near-shore waters adjacent to the
Cape and Islands. Will see increasing W/SW winds ahead of the
front initially around 20 to 25 kts increasing 30 to 35 kts with
passage. Ocean effect snows possible as seas build 5 to 10 feet.
An arctic front moves through this evening, followed by extremely
cold air. This will allow Northwest winds with gusts 50-60 knots
at times during the night. Exposed waters will continue showing 5
to 12 foot seas during this time.
In light of these conditions, the Storm Watch has been converted
to a Storm Warning.
Friday...
High pressure builds in from the W. Expect winds to diminish
through the day with morning gales giving way to small craft
advisory conditions in the afternoon and lighter winds by sunset.
The diminishing winds will allow seas to subside through the day.
Outlook /Friday Night through Monday/...
Friday night through Sunday...
Winds turning S will increase during the day Saturday and into
Sunday ahead of a sweeping cold front. Strongest winds out ahead of
the cold front with gales forecast. With cold frontal passage on
Sunday, winds turn out of the NW and begin to diminish. Periods of
rain, some moderate to heavy at times with visibility restrictions
on the waters ahead of and along the cold front.
Monday into Tuesday...Moderate confidence.
NW winds diminishing. Seas allowed to subside.
&&
.TIDES/COASTAL FLOODING...
Astronomical high tide at Boston Thursday at midday is near 12
ft. Given developing offshore wind and pressure rises should
work against minor coastal flooding. However the expected levels
are close to the Tuesday levels which caused some minor flooding,
so we can't rule it out for one more day.
&&
.CLIMATE...
The early storm force winds will give way to Gales, then finally
small craft advisory level winds through the daylight hours on
Fri as they veer from NW to S-SW by evening. In fact,
Lowest Min Temps December 16
BOS 1/1883
PVD 3/1910
BDL -4/1917
ORH -2/1932
Lowest Max Temps December 16
BOS 18/1951
PVD 15/1910
BDL 15/1917
ORH 12/1968
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...High Wind Warning from 5 PM this afternoon to 4 AM EST Friday
for CTZ002>004.
Wind Chill Advisory from midnight tonight to 9 AM EST Friday
for CTZ002>004.
MA...High Wind Warning from 5 PM this afternoon to 5 AM EST Friday
for MAZ022>024.
High Wind Warning from 5 PM this afternoon to 4 AM EST Friday
for MAZ002>021-026.
Wind Chill Advisory from midnight tonight to 9 AM EST Friday
for MAZ007-013>017.
Wind Chill Advisory from 10 PM this evening to 11 AM EST
Friday for MAZ002>006-008>012-026.
RI...High Wind Warning from 5 PM this afternoon to 5 AM EST Friday
for RIZ008.
High Wind Warning from 5 PM this afternoon to 4 AM EST Friday
for RIZ001>007.
Wind Chill Advisory from midnight tonight to 9 AM EST Friday
for RIZ001.
MARINE...Storm Warning from 5 PM this afternoon to 5 AM EST Friday for
ANZ231>235-237-250-251-254>256.
Storm Warning from 7 PM this evening to 4 AM EST Friday for
ANZ230-236.
&&
SYNOPSIS...WTB/Sipprell
NEAR TERM...Frank/WTB/Sipprell
SHORT TERM...WTB
LONG TERM...Sipprell
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell
TIDES/COASTAL FLOODING...
CLIMATE...