Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 260540
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
140 AM EDT Mon Sep 26 2016

.SYNOPSIS...
High pressure will center over New England tonight and allow for
efficient radiational cooling. A low pressure area and associated
front will bring showers to the area late Monday night into Tuesday
morning. Considerable forecast uncertainty exists for the rest of
the week. An easterly flow may persist for the rest of the week
with a chance of showers or areas of drizzle from time to time,
especially near the south coast.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
10PM update...
Adjusted overnight mins/temps toward MET which was better handling
the beginning stages of radiational cooling late this evening.
This does not impact current Frost headlines so no need for
adjustment there. Only impact is that frost may be just slightly
more widespread than previously thought. Otherwise, CI may try to
ruin the radiational cooling but not enough to trend toward these
cooler temperatures.  Forecast on track otherwise.

Previous discussion...
Surface high pressure crests over the area tonight and sets the
stage for efficient radiational cooling under clear skies and with
light winds. using minimum temperatures of 35F or lower as a
guide, have expanded the Frost Advisory some from the initial
issuance this morning to include far NW Middlesex County, Norfolk
County, eastern Hampden and southern Worcester Counties. Feel
reasonably confident about Norfolk County. Some of the other areas
unlikely to be validated with ASOS data but think rural areas of
eastern Hampden and southern Worcester County have a fairly good
chance at temperatures near or a little below 35F for a few hours
with virtually no mixing. Anticipate that we may experience patchy
frost across other portions of more rural Middlesex County and our
northern CT zones but probably not extensive enough to warrant an
advisory. That can be reevaluated, however, with the next update.
Low temperatures will make it to the 30s most areas outside the
major urban areas and some immediate coastal areas. NW winds
should diminish quickly in vicinity of sunset.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
Monday should start out mostly clear with high clouds streaming in
from the west during mid to late afternoon. High temperatures look
to be similar to today`s values. A light gradient will set the
stage for sea breezes along both east and south facing coasts on
Monday.

Short wave energy rotating about a long wave closed low approaching
the NW Great Lakes will likely bring a baggy surface cold front or
occlusion toward the area early Tue AM. Several factors gleaned
from a consensus of numerical prediction models favor a high
likelihood of showers with this system including divergence aloft,
a near coupling of a strengthening low level jet and the right
entrance region to the upper jet and precipitable waters over 1.5
inches. Anticipate showers moving into the CT River Valley soon
after 06z Mon and reaching the MA east coast around 09Z Mon.
Believe the moisture pooling and favorable dynamics could set the
stage for some heavy showers, albeit progressive. A band or two of
heavy showers could have some impact on the start of the Tuesday
morning rush hour. Expect temperatures to remain nearly stationary
or rise Mon night in response to cloud cover and warm advection.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
HIGHLIGHTS...

* Widespread showers Tue, especially coastal plain. Improving
  conditions west
* Unsettled weather possible through end of week but low confidence
* Temperatures near or slightly below normal

Overview...

Rather complex high amplitude pattern across the CONUS and Canada in
the extended period will lend itself to low predictability. All
revolves around the evolution of a cutoff low as it digs south from
the Great Lakes with strong upstream and downstream in place. GFS
has trended toward Ecmwf and there is agreement among the
guidance that cutoff will set up to the SW of New Eng but exact
location of the cutoff along with area of confluence to the north
is uncertain. And spread increases significantly next weekend.
Additionally, there is considerable spread with location/track of
sfc lows to the south which will impact sensible weather. Given
the complexity of the pattern it is certainly a low confidence
forecast as predictability of this pattern at extended time ranges
is quite low.

Tuesday...

Cold front will be moving across SNE Tue morning. Strong signal for
a band of widespread showers along and ahead of the front. Good
coupling between low level jet and right entrance region of upper
jet will result in deep forcing for ascent acting on deep moisture
axis with anomalous PWAT plume around 1.75". All suggests widespread
showers and can`t rule out a few t-storms near the coast where
elevated instability parameters are favorable. Still some
uncertainty with timing of the front and showers but focus Tue
morning appears to be across the coastal plain with improving
conditions in western New England. In fact, expect developing
sunshine in the interior during the afternoon. Front expected to get
hung up near the coast as flow aloft becomes parallel to the front
and this may keep showers going near the south coast and
Cape/Islands much of the day. It will be somewhat humid with
dewpoints climbing into the low/mid 60s and breezy near the coast in
the morning as low level jet swings through with gusts to 30 mph
possible.

Tuesday night...

There is uncertainty with how far south drying gets as front stalls
south of New Eng. GFS and Ecmwf show drying making it to the
south coast by evening. However, as upstream trof/upper low digs
south from the Gt Lakes flow aloft over New Eng backs and this may
bring moisture back north and keep threat of showers along the
south coast. ECMWF and UKMET have showers near the coast and this
is how we will play it.

Wednesday through Sunday...Low Confidence.

As cutoff low digs south of the Gt Lakes backing upper flow will
bring back deeper moisture into SNE, but the extent of moisture is
unknown as moisture will be battling upper confluence and high
pressure to the north. Also, location and track of low pres to the
south will be dependent on exact positioning of the cutoff low.
Potential for a period of unsettled weather with showers from time
to time, but it could end of dry for a good portion of the period if
there is greater influence from upper confluence and high pres to
the north. Lots of uncertainty so we have just low pops in the
forecast. With persistent NE flow likely, temps expected to be near
or a bit below normal.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels.

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

Short Term /through Tuesday/...

Through today...High confidence.
VFR. Winds generally light, although shifting toward the SW
through the day.  Sea breezed expected on coasts.

Tonight and Tuesday...Moderate confidence, mainly due to timing.
VFR to start the overnight. Gradually expect lowering to MVFR/IFR
from W-E especially 06Z through 12Z, first due to lowering CIGS,
then due to mix of RA and BR with lower vsbys. Expect this to
gradually lift from W-E 12Z-20Z, although these lower categories
may linger longer across the S Coast/Cape/Islands into Tue
evening.

KBOS TAF...High confidence in TAF through 06Z Tue, including sea
breezes.

KBDL TAF...High confidence in TAF through 06Z Tue.

Outlook /Tuesday Night through Friday/...

Tuesday...Moderate confidence. Area of showers with MVFR/IFR likely
moving across SNE with main focus in the coastal plain. Improving
conditions to VFR in the west. South wind gusts to 25-30 kt possible
in the morning near the south coast.

Wednesday through Friday...Low confidence. Areas of MVFR with
showers possible at times, but VFR conditions if it is dry.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Tuesday/...High confidence.

Plan to let Small Craft Advisories play out as originally construed.
Main issue moving into this evening will be residual rough seas as a
combination of wind wave opposing a swell from Karl. The wind
forcing should end by this evening, but the swell will linger into
tonight across the outer waters.

S to SW winds will likely frequently gust to/above 25 KT late Mon
night and probably result in a new round of Small Craft
Advisories. Areas of fog also likely late Mon night into early
Tue AM.

Outlook /Tuesday Night through Friday/...

Tuesday...High confidence. Low level jet ahead of cold front will
result in a period of pre-frontal southerly gusts to 25-30 kt in the
morning, then diminishing winds int he afternoon. Vsbys reduced in
showers and fog. Low risk for a few t-storms across the south
coastal waters.

Wednesday through Friday...High confidence in an extended period of
E/NE winds with speeds mostly below SCA although occasional gusts to
25 kt possible. Vsbys may be reduced at times in showers but this is
low confidence.

&&

.CLIMATE...
Record low temperatures...

September 26th (last set in...)

Boston ....... 34 (1879)
Hartford ..... 33 (1978)
Providence ... 37 (1967)
Worcester .... 28 (1925)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Frost Advisory until 8 AM EDT this morning for MAZ002>004-
     008>013-017-018-026.
RI...None.
MARINE...Small Craft Advisory until 3 AM EDT early this morning for
     ANZ250-254-255.

&&

$$
SYNOPSIS...KJC/Thompson
NEAR TERM...Doody/Thompson
SHORT TERM...Thompson
LONG TERM...KJC
AVIATION...KJC/Doody
MARINE...KJC/Thompson
CLIMATE...


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