Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 100015
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
708 PM EST Fri Dec 9 2016

.SYNOPSIS...
Mainly dry, but unseasonably cold temperatures are on tap for our
region through mid afternoon Sunday.  Low pressure will bring
accumulating snow to much of the region Sunday night and Monday
morning, which will likely change to rain on the coastal plain and
perhaps a period of ice across the interior depending on the
exact track. Another wave of low pressure may bring rain and/or
snow to the region Wednesday, but that is uncertain. Regardless, a
shot of arctic air appears to be in the cards to close out next
week.

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

7 pm update...

NW flow continues but beginning to relax as surface high pressure
builds in from the W coincident with weak ridging aloft and drier
air. Cold air advection slows with H85 temperatures bottoming out
around -16C by Saturday morning. Mainly clear skies except for a
few clouds sliding over the Berks and perhaps some ocean effect
clouds brushing the outer-Cape at times. Low temps by daybreak will
mainly be in the teens to around 20.  Tack on the winds, looking
at wind chill values down into the single digits.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...

Saturday into Saturday night...

One more piece of mid-level energy through the prevailing cyclonic
flow across the region. Some low to mid level decks and perhaps
some flurry activity over the W portions of CT and MA. Otherwise
thereafter a ridge of high pressure builds into the region yielding
an increasing presence of surface high pressure. NW winds becoming
light and variable late Saturday into Saturday night as conditions
turn mostly clear. Highs barely getting above freezing, more than
likely feeling like the teens and 20s given the NW breeze. Lows
back down into the teens, possibly single digits, considering the
light and variable flow, mostly clear conditions. Decent signs of
radiational cooling and thus leaning with the coldest MOS guidance.
Wicked cold.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Highlights Sunday night and Monday...

* Accumulating snow likely Sunday night/Mon am for most of SNE

* Significant impact to Monday am rush hour possible especially
  northwest of a Boston to Providence line

* Snow likely changes to rain across coastal plain and perhaps
  to a period of ice across the interior sometime Monday morning

* Early Snowfall Estimates may shift higher or lower:
- South Coast/Cape and Islands coating to 1"
- Boston to Providence corridor 1-3"
- Interior southern New England 3-6"

Details Sunday and Monday...

The main concern with this forecast package will be the first
appreciable snow event for portions of southern New England.  Large
high pressure in control should keep dry weather in place through
mid afternoon Sunday.  As the high moves off to the east, warm
advection over the cold dome should allow light snow to overpsread
much of the region from west to east late Sunday afternoon and early
evening.  A very cold antecedent airmass with 925T between -6 and
-8c, should allow initially for a period of light snow even on the
south coast and Cape Cod.  The forcing looks rather weak late Sun
afternoon into most of Sunday evening, but moisture/weak lift may
allow for up to an inch of snow accumulation.

The main event will be late Sunday night into Monday morning.  Low
pressure will be lifting towards the Great Lakes, inducing a
stronger level jet and enhancing the baroclinicity.  At the same
time, warmer air will be advancing northward in the 850 to 950 mb
layer.  The strength of the initial low pressure headed up towards
the Great Lakes, secondary re-development off the, coast and exact
positioning of the high across eastern Canada will determine snow
amounts and ptypes which will discuss more below. The bulk of the
precipitation should be over by early Monday afternoon, as a dry
slot overspreads the region with just a few left over showers
possible.

Snow amounts and Ptype/Uncertainty...

As mentioned in the previous paragraph, there are a lot of moving
parts to this forecast. The ECMWF/GGEM are weakest with the primary
low and hence the coldest, while the GFS and especially the NAM are
about 10 mb stronger.  This results in a stronger low level jet
allowing the 850 to 950 mb levels warm quicker.

We blended the guidance at this point given the uncertainty and came
up with a coating to 1 inch south coast/Cape/Islands, 1 to 3 inches
on the Boston to Providence corridor, and 3 to 6 inches across much
of the interior.  Keep in mind we are still 60+ hours out from the
event.  If the models trend colder, snow amounts will have to be
increased with higher amounts into the Boston to Providence
corridor.  A milder trend would result in snowfall totals a few
inches less, and also bring an increased risk for a period of light
freezing rain across portions of interior southern New England.  We
tried to depict this in our most likely snowfall forecast, along
with our reasonable worst/best case scenarios.  Regardless of
amounts a significant impact to the Monday morning rush hour,
especially to the northwest of the Boston to Providence corridor
where Ptype is less in question at the time.

Tuesday through Friday...

The majority of this forecast was focused on the impending winter
weather event Sunday night/Monday morning.  Appears seasonable and
mainly dry weather follows Tuesday.  Tremendous uncertainty Wed into
Wed night, whether or not another wave of low pressure tracks close
enough to bring us more rain and/or snow.  Regardless of what
happens, a shot of very cold to arctic air follow Thu/Fri.  The
extent of just how cold it gets remains to be seen, but the 12z
ECMWF is the coldest and would indicate the potential for near
record cold by the end of next week.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

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

VFR conditions persist through Saturday night. Northwest wind
gusts of 20 to 25 knots at times into Saturday in the typical
vulnerable locations.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday...High confidence in VFR conditions though at least mid
afternoon.

Sunday night and Monday...Moderate to high confidence in an extended
period of MVFR-IFR conditions with localized LIFR conditions. Snow
develops and changes to rain on the coast and perhaps a period of
ice across the interior. The worst of the conditions will be late
Sunday night into Monday morning.

Tuesday...Moderate confidence.  VFR conditions probably dominate.

Wednesday...Low confidence.  Uncertain if a wave of low pressure
tracks close enough to bring us another round of rain/snow or we
remain dry.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

Converting remaining Gale Warnings to small craft headlines this
evening, as wind gusts will continue to gradually diminish. Still
may see a few gusts approach 35 knots over the outer-waters until
midnight, but overall strong small craft headlines expected.
Small craft wind gusts probably persist into Saturday, but should
gradually diminish and come to an end from west to east Saturday
evening as high pressure builds into the region.

Outlook /Sunday through Wednesday/...

Sunday...High confidence.  Winds/seas remain below small craft
advisory levels with high pressure overhead.  Rain and snow develops
Sunday evening and may reduce VSBYS for mariners.

Monday and Monday night...Moderate to high confidence.  Small craft
wind gusts/seas develop as secondary low pressure develops.  Winds
shift from south to the west by Monday night behind the low pressure
system.  VSBYS reduced Monday morning mainly in rain and fog patches.

Tuesday...Moderate to high confidence.  Small craft northwest wind
gusts continue with enough cold advection continuing over the
relatively mild ocean.

Wednesday...Low confidence.  Winds/seas highly uncertain depending
on track and potential strength of perhaps another low pressure
system.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EST Saturday for ANZ231>235-
     237.
     Small Craft Advisory until 4 AM EST Saturday for ANZ230.
     Small Craft Advisory until 1 AM EST Saturday for ANZ236.
     Small Craft Advisory until 11 PM EST Saturday for ANZ250-254.
     Small Craft Advisory until 11 AM EST Saturday for ANZ251.
     Small Craft Advisory until 7 PM EST Saturday for ANZ255-256.

&&

$$
SYNOPSIS...Sipprell/Frank
NEAR TERM...Frank/Sipprell
SHORT TERM...Sipprell
LONG TERM...Frank
AVIATION...Sipprell/Frank
MARINE...Sipprell/Frank


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