Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 150345
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1145 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/...

1130 PM Update...

Dense fog has moved into portions of RI and SE Mass, mainly S of
Boston, with the lowest conditions from Plymouth to Providence S
and E at 03Z. With the light/variable or calm winds and high
dewpoints, expect conditions to continue to lower across RI/SE
Mass so have extended the Dense Fog Advisory to include all of
RI and SE Mass from Norfolk county S and E through 9 AM. Will
continue to monitor conditions further up the E coast and into
NE interior and central Mass to see if the advisory needs to be
extended further N and W.

Remainder of forecast in pretty good shape, but have updated to
bring conditions current and incorporated into the overnight
trends.

Previous Discussion...

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 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 /04Z 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.

Overnight...
Areas of IFR-VLIFR mainly from S of Boston to Providence and
S/E overnight. Expect 1/4SM FG across most areas, especially
from KGHG-KPVD-KWST and S/E through 12Z-13Z. Low risk of -DZ
along the immediate SE coast. Elsewhere, may see areas of IFR
conditions into interior NE Mass and across the CT Valley into
NE CT.

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/...

Overnight...Moderate confidence.
Areas of dense fog across the near shore waters, and likely
further offshore. May see patchy drizzle as well. Light S-SW
winds. Seas remain at around 5 ft on the southern outer waters.

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 tonight could be rather close the record
high minimum 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 until 9 AM EDT Sunday for MAZ013-016>024.
RI...Dense Fog Advisory until 9 AM EDT Sunday for RIZ001>008.
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...Sipprell/EVT
SHORT TERM...Dunten
LONG TERM...Sipprell
AVIATION...Dunten/Sipprell/EVT
MARINE...Dunten/Sipprell/EVT
CLIMATE...Staff



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