Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 232303

Area Forecast Discussion
National Weather Service Taunton MA
700 PM EDT Sun Oct 23 2016


A brief burst of moderate to heavy rain expected between  2 and 8 am
Monday morning mainly near and especially south of the MA Turnpike.
Otherwise, winds become blustery out of the W/NW beneath a cool, dry
airmass which continues though midweek. A period of rain is likely
Thursday into Friday as low pressure crosses the region. Seasonable
temperatures are expected for next weekend.



7 pm update...

***Brief burst of moderate to heavy rain tracks from west to east
between 2 and 8 am mainly near and especially south of the MA

Previous forecast pretty much on track, but opted to up pops to
categorical well after midnight to the south of the MA Turnpike.
Interesting scenario as dynamical shortwave induces a brief period
of 30 to 40 microbars per second of lift across the southern half of
our region.  This coupled with steep mid level lapse rates/-EPV
signature may result in a brief period of heavy rain and perhaps
even a rumble of thunder.  The biggest fly in the ointment is that
these type of scenarios can often be located in the wrong place, so
certainly could end up a bit further north/south than expected.

The timing of this rain looks to enter our western zones after 2 am
and then exit the southeast New England coast by 8 or 9 am.  The
rain should only last a few hours in a given location, but again may
come down briefly heavy with even a rumble of thunder not out of the
question.  Across northern MA and especially near the NH border,
it may end up totally dry or perhaps just a few brief showers.

Previous Discussion Below:

Perhaps an under-estimated overnight disturbance? Clipper-like low
sweeping S of New England with attendant vortmax through the overall
broadly cyclonic open wave trough. Along the parent warm front, tight
thermal packing especially along the 290-300K isentropic surfaces
in tandem with the nose of a h925-85 low level jet. Subsequent
strong low to mid level isentropic and frontogenetical lift.
Accompanying continental polar airmass while latching into a
higher tongue of theta-E offshore, there`s a decent shot of
moisture with precipitable waters up to 1 inch. Strong forcing and
dynamics apparent beneath decent venting aloft with a measure of
elevated instability within the mid-levels, consensus of guidance
forecasting strong omega values.

Expecting a W to E band of moderate rainfall with amounts around
2 to 3 tenths. Can not rule out some isolated thunder and will
insert wording into the forecast mainly over the waters S of New
England. Difficulty is nailing down the N/S gradient of expected
outcomes of which there could be multiple depending on the W-E
level of frontogenetical/isentropic forcing from which it emerges.
Mesoscale outcomes to which forecast guidance is struggling with

A couple of camps with the RAP/HRRR a bit N than all other guidance.
Latest EC has nudged slightly N, GFS has remained persistent. This
would be more worrisome if it was colder and this was snow. Systems
like these have a big bust potential. Consensus of forecast guidance
places high chance to likely PoPs S along and S of the Mass Pike to
the S coast and adjacent waters. Can not rule out a N-S wobble in
the forecasts as we get closer to the actual event.

Winds taper and turn more S in advance of the system. Looking at
milder conditions overnight with lows ranging around the upper 30s
to low 40s.


Monday into Monday night...

Cooler, dry and breezy weather. Strong cyclonic flow aloft pushing
an impressive H5 polar airmass S with temperatures of -30C. H5-7 mid-
level lapse rates becoming unstable. This within a atmospheric column
relatively dry. But can not rule out some isolated shower activity
especially off the Great Lakes. Attendant mid-level forcing with a
vortmax rounding the cyclonic flow. But with downsloping along the
280-290K isentropic surfaces within the low levels, feel any
shower activity is mainly confined N/W of our region and any
impacts, if any, will be across the high terrain.

Scattered cloud decks during the day with deep layer mixing up to
H85 where temperatures around 0 to -2C with cold air advection
ongoing. Will see the mix down of faster momentum and drier air so
expect very low dewpoints in the 20s with NW wind gusts of 20 to 30
mph. Highs around the mid to upper 50s with abundant sunshine but
feeling cooler given the breezy winds. Winds taper slightly during
the overnight hours. Lows around the mid 30s with cold air advection
still dominating. Given the winds, will not go with any frost in
the forecast. More concerned about freezing temperatures. But given
how marginal overnight lows will be, at or above freezing, in the
areas of Southern New England that the growing season is still
considered, will pass on headlines at this time and mention in the
hazardous weather outlook.




  * Unseasonably chilly but mainly dry Tue/Wed other than a few
    ocean effect rain showers across Plymouth County/Cape/Islands

  * Lows should drop well down into the 20s Wed night/Thu am

  * Period of rain likely sometime Thu into Fri


Tuesday and Wednesday...

Strong 1035+ MB High pressure north of Lake Michigan builds eastward
into Quebec. At the same time, low pressure over the Canadian
Maritimes will slowly shift east. The result will be a north to
northwest flow of unseasonably chilly but mainly dry weather into
the region Tue/Wed. 850T are expected to drop to between -6C and
-8C. In addition, the delivery of this airmass will be from more
of a north northwest direction allowing for less modification /
downslope than the one this weekend. Therefore, expect highs
Tue/Wed to be generally in the 40s but it will feel colder on Tue
with gusty winds. Low temps Tue night and Wed night will be below
freezing in most locations, other than downtown Boston and
portions of southeast New England coast where ocean effect clouds
are expected. Wed night should be the coldest night when many
locales drop well down into the 20s as winds diminish.

While mainly dry weather is expected, the setup is favorable for a
few ocean effect rain showers from Plymouth county and onto the
Cape/Islands Tue night/Wed. Surface winds will turn more northerly
as strong Canadian High pressure builds into eastern Canada. Given
ocean water temps still near 60F, the 850mb/sea surface differential
should exceed 20C. Although 850 mb temps will be around -5C, the
boundary layer will be way too warm to support anything more than
a few ocean effect rain showers.

Thursday and Friday...

Upper trough quickly lifts east of the region as low pressure from
the midwest lifts northeast towards the Great Lakes. This will
increase the forcing for ascent and allow for a period of rain
sometime Thursday into Friday. Specific timing remains uncertain
this far out, but latest guidance suggests highest risk will be
late Thu into early Fri. The antecedent airmass ahead of this
system is quite cold/dry for late October standards. Therefore, it
is not out of the question for a very brief period of snow and/or
sleet at the onset across the high terrain along the east slopes
of the Berks and perhaps even the northern Worcester Hills on Thu.
Even if this were to occur for a short time, it would have little
impact with a quick change to rain. Lastly, there are some signs
that a secondary low will try to develop near the south coast. If
this occurs, there might be some enhancement to the rain across
portions of our region but it is just too early to say.

Saturday and Sunday...

Low confidence in this time range. A cold front may bring a few
brief showers, but guidance suggests that much of the time may end
up dry with temperatures not too far from normal readings.


Forecaster Confidence Levels.

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

Short Term /through Monday Night/...

Tonight...Moderate to high confidence. Winds gusts continue to
diminish and mainly VFR CIGS through 6z despite an increase in
clouds.  Between 6z and 12z, brief MVFR conditions possible in a
period of rain and reduced VSBYS with the main impacts south of the
MA Turnpike. Very brief IFR conditions are certainly possible in
heavy rain.

Monday into Monday Night...High confidence.
Low-end VFR / MVFR quickly clearing S/E. Increasing W/NW winds with
gusts 20 to 25 kts, up to 30 kts along the coast and perhaps across
the high terrain. FEW to SCT cigs 4-5 kft agl.

KBOS TAF...Moderate to high confidence in TAF.  Brief MVFR
conditions possible between 8z and 12z in rain, but the higher risk
will be south of the terminal.

KBDL TAF...Moderate to high confidence in TAF.  Brief MVFR
conditions possible between 6z and 10z in rain.

Outlook /Tuesday through Friday/...

Tuesday...High confidence in VFR conditions. Northwest winds may
gust to around 30 knots.

Tuesday night and Wednesday...High confidence in mainly VFR
conditions. A period of MVFR CIGS are possible in ocean effect
clouds and a few rain showers across Plymouth county and onto the

Thursday and Friday...Moderate confidence. A period of MVFR-IFR
conditions expected in rain along with some lower clouds and fog
patches. Timing uncertain but greatest risk right now appears to
be late Thu into early Fri.


Forecaster Confidence Levels.

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

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

4 pm update...


W gales persist into the evening hours, concluding shortly before
midnight as winds turn out of the S ahead of an area of rain and
possible visibility restrictions, maybe even a clap of thunder
over the S waters, from shortly after midnight into early monday
morning. Gales will be converted to small crafts as winds
diminish. Waves slowly subside as well.

Monday into Monday night...

Behind the disturbed weather clearing out Monday morning, W/NW
winds increase to near gale force over the waters out through
Tuesday morning. Waves remaining 5 to 7 feet.

Outlook /Tuesday through Friday/...

Tuesday...High confidence. Northwest winds should gust to 30 knots
over most waters in the strong cold advection pattern given the
relatively mild ocean. There is even a low risk for brief winds up
to Gale Force. Seas build to between 5 and 6 feet across our
outer- waters.

Wednesday...Moderate to high confidence. Pressure gradient weakens
some, but still expect northerly wind gusts up to 25 knots given
good mixing over the relatively mild ocean.

Thursday and Friday...Low to moderate confidence. Winds turn to
the southeast Thu ahead of an advancing low pressure system and
probably more southerly by Thu night. Behind the low pressure
system, should see more of a westerly component to the winds
develop on Friday. Probably will see a period of small craft wind
gusts and seas late Thu into Fri, but still a lot of uncertainty
this far out especially with timing.


MARINE...Gale Warning until 11 PM EDT this evening for ANZ231>235-
237-     250-251-254>256.
     Small Craft Advisory until 2 AM EDT Monday for ANZ230-236.


NEAR TERM...Frank/Sipprell
SHORT TERM...Sipprell
MARINE...Sipprell/Frank is the U.S. government's official web portal to all federal, state and local government web resources and services.