Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 142243
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
643 PM EDT Sat Oct 14 2017

.SYNOPSIS...

Showers, drizzle and fog will occur tonight ahead of approaching
warm front from the S. Very warm and humid conditions will occur
on Sunday. A sharp cold front will sweep the region Monday behind
which it will be breezy as temperatures drop down close to freezing
overnight into Tuesday morning. Gradual warm-up through the week,
possibly into the weekend, overall dry and quiet.

&&

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

7 pm update...

Given the forecast light winds overnight and soupy airmass associated
with a warm frontal boundary lifting back N, and after interrogating
mesoscale and synoptic conditions along with latest near-term high-
res guidance (i.e., HRRR TLE / NARRE and NCAR ensembles), have gone
ahead with the issuance of a DENSE FOG ADVISORY where confidence
is highest concerning impacts of a quarter mile or less. Well
aware that the region is actually experiencing some clearing and
the sunset is gleaming through, in some instances resulting in
the observance of rainbows. However, hardly mixed out from the
damp and dismal conditions of today. As the boundary layer cools
overnight and winds are light, would expect the development of
dense fog, especially those areas in the advisory. Followed up
with the SPECIAL WEATHER STATEMENT for inland and E-coastal areas
noting the uncertainty of the impact of dense fog, whether it`ll
get down to a quarter mile or less for such regions.

Fair amount of low level moisture and weak ascent acting on the
boundary layer. Interrogating 0-1 km lapse rates, an inversion
remains in place for the Cape and Islands while there`s signal
of some isothermal mixing through the CT River Valley towards
Worcester MA into Sunday morning. Keep the areas of fog and
drizzle especially for SE New England given S/SW low-level flow
of higher theta-E air, increasing surface dewpoints, raising the
risk of dense fog. Lows into the 50s and 60s.

&&

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

Sunday...

Showers and drizzle will slowly come to an end after daybreak on
Sunday as surface warm front push through the area. Behind the
front, 850 mb temps warm to 17C and 925 mb LLJ strengthens to near
30-40 kts. If clouds break during the afternoon, and mixing
increases up to 850 mb, then temperatures could warm into the low
80s. Aside from warm temperatures, we could see gusts to near 25 MPH
or higher if true mixing occurs. Highest gusts will be across the
eastern half of the region, which is closer to the strengthen LLJ.
Sunday will continue to trend dry, but still cannot rule out a few
isolated showers developing across the south coast.

Sunday night...

Strengthening low pressure system over the Great Lakes will move
eastwards towards northern New England by Monday morning. This
surface low will drag a cold front through upstate NY and southern
New England during the overnight hours. The main story however is
the secondary shortwave rounding the trough late Sunday pushing this
front through the region during the predawn hours of Monday morning.

Ahead of this front, cannot rule out a few isolated showers across
the south coast with continue low level moisture and dewpoints near
the mid to upper 60s. Overnight lows will remain warm as WAA
continues to stream into the region. Some showers will develop along
the front, but mid-levels are still on the dry side so do not expect
much in the way of heavy rainfall.

After midnight, the cold front will begin to approach the region
resulting in a surge of southerly winds with gusts near 25 MPH. The
front appears to quickly push through around 2-6 AM. Stout CAA
behind the front will result in a quick drop in temperatures and a
surge of wind. Temperatures could from 60F to 45F within a 3 hour
span. BUKFIT soundings show good mixing behind the front with 850 mb
LLJ increasing to 50-55 kts. This could result in a quick 35-40 MPH
gusts with some higher gusts across the higher elevations right
behind the front. This could result in down tree and wires during
the overnight hours. There is the potential for wind headlines but
will let later shifts take another look.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...

*/ Highlights...

 - Sharp cold front Monday, turning cold
 - Gradual warm-up through the week, possibly into the weekend
 - Perhaps a pattern change into late October, more active, cooler

*/ Overview...

Struggles with the synoptic pattern interpretation for the rest of
October. Presently the preferred region of deeper vortex development
lies across the state of Alaska. Energy earlier this week has become
displaced and is now shooting E over S Canada / N CONUS, amplifying
the mid-level flow, creating a see-saw pattern of airmasses through
the week into the following weekend over the NE CONUS via trailing
influences with a shot of cooler air Monday through as late as Friday
with a potentially considerable warm-up thereafter through the weekend
(dependence upon the influence of an offshore sub-tropical low). This
as a deeper vortex re-emerges over Alaska shearing and cutting off
downstream energy over the SW CONUS.

Hereafter the synoptic set-up is a bit tricky to diagnose. A dynamic
shift is plausible as indicated by the change in atmospheric tele-
connections. Perhaps the near-tropopause polar vortex shifting away
from Alaska towards N Asia, or rather the warmer tropical pineapple
connection into the W CONUS as indicated via a rather good consensus
forecast of the MJO going from a moderate to strong phase 4 through
phase 5 into phase 6 indicating tropical activity along the ITCZ is
shifting gradually W. Subsequently we could see a reversion in the
mid-latitude pattern over the E CONUS from one of preferred ridging
to that of deeper troughing towards late October.

Too early to say but at least for the foreseeable future expect
seasonable conditions and generally quiet weather. Will hit the
specifics in the details below.

*/ Discussion...

Monday into Monday night...

Sweeping sharp cold front early. Chance to likely PoPs. Pressure
rises behind the front along with cold air advection allow for
deeper boundary layer mixing, likely gusty NW winds immediately in
wake at least to 30 mph, possibly as high as 40 mph. Could be set
with a non-diurnal trend in temperatures. Highs just prior to cold
frontal passage with temperatures steady or falling with passage
during the day. Dropping out overnight with continued cold air
advection, lows falling into the 30s, requiring frost / freeze head-
lines, more likely across N/W MA and CT with light NW winds. Else-
where, breezy winds would prevent frost development however not so
much freezing conditions.

Tuesday through Wednesday...

Gradual warm-up. Westerly interior winds as high pressure builds S/E
of our region. Temperatures slowly climb day by day. Dry. Quiet.

Thursday into Friday...

Dry cold frontal passage. A subtle change in the airmass with winds
becoming more W/NW instead of SW. However the warm-up continues.
Temperatures rebounding to seasonable values for mid to late October
in the 60s and 70s for highs.

Weekend into early next week...

Some question as to the magnitude and position of high pressure up
against a sub-tropical low offshore. From the consensus of the model
guidance the warm-up continues with a westerly flow, however with a
shift to the W we could be under more E flow associated with the off-
shore low. Will keep with a seasonable pattern in the 60s and 70s
with westerly winds.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...

Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...Moderate confidence.

Tonight...
Lowering IFR-VLIFR. Greatest risk of 1/4SM FG over S-coast MA,
Cape and Islands. DENSE FOG ADVISORY in effect. May need to
extend. S/E terminals at greatest threat of areas of FG and DZ.

Sunday...
Gradual improvement S/E during the day. IFR-LIFR may hold long
over S/SE coast, perhaps all day. VFR elsewhere, increasing S
winds with gusts near 25 kts.

Sunday night...
IFR-LIFR CIGs along the S coast. Lesser VSBY impact with continued
S/SW breezy winds. Improving NW to SE as winds shift W/NW, immediately
gusty, potentially up to 40 kts, becoming VFR. SCT -SHRA associated
with the wind shift.

KBOS TAF...Moderate confidence in TAF. Going to have to watch closely
during the overnight into morning push for IFR-LIFR VSBYs. Will hold
to 1SM BR with this latest issuance given a more S rather than E flow.


KBDL TAF...Moderate confidence in TAF. Terminal may remain out of
IFR-LIFR VSBYs, however can not rule out IFR-LIFR CIGs and the
possibility of MVFR BR and DZ.

Outlook /Monday through Thursday/...Moderate confidence.

Monday...

Initially MVFR / low-end VFR mix with -SHRA, clearing and lifting
with SCT low-end VFR becoming SKC. Initial SW winds reverting NW and
immediately breezy with gusts up around 30 kts possible.

Tuesday through Thursday...

VFR. W winds. Quiet.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...


Tonight...Moderate confidence.
Southerly flow will low visibility in fog, drizzle and scattered
showers. Seas around 5 feet for the outer waters. DENSE FOG
ADVISORY in effect for the Cape, Islands, and S-coastal MA.

Sunday...Moderate confidence.
Passing warm front will increase southerly winds to 25-30 kts by
the afternoon. Seas will being to strengthen, SCA will continue
for all waters.

Sunday night...Moderate confidence.
Increasing southerly LLJ will aid in wind gusts to near 30 kts.
Stout cold front will approach the waters switching the winds to
a more westerly direction. Low prob for gale force winds for
just an hour or two behind the front.

Outlook /Monday through Thursday/...Moderate confidence.

Sharp cold front sweeping the waters early Monday. Initial SW winds
will turn immediately NW with passage and become blustery. Can`t
rule out near-gale force gusts immediately behind the front, then
diminishing. Seas already 5 to 7 feet at the start of the forecast
period then diminishing through Tuesday. Quiet boating weather under
high pressure the rest of the period.

&&

.CLIMATE...

Min temperatures Saturday night could be rather close the
record maximum low temperatures for our four long term climate
sites.

October 15 Record maximum low temperatures

Boston      64/1954
Worcester   64/1954
Hartford    63/2014
Providence  63/2014

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...Dense Fog Advisory from midnight tonight to 9 AM EDT Sunday
     for MAZ019>024.
RI...None.
MARINE...Small Craft Advisory from 1 PM Sunday to 9 AM EDT Monday for
     ANZ231>234.
     Small Craft Advisory from 2 PM Sunday to 9 AM EDT Monday for
     ANZ230-236-251.
     Small Craft Advisory from 11 AM Sunday to 9 AM EDT Monday for
     ANZ235-237-250.
     Small Craft Advisory until 9 AM EDT Monday for ANZ254>256.

&&

$$

SYNOPSIS...Dunten/Sipprell
NEAR TERM...Dunten/Sipprell
SHORT TERM...Dunten
LONG TERM...Sipprell
AVIATION...Dunten/Sipprell
MARINE...Dunten/Sipprell
CLIMATE...WFO BOX Staff


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