Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 222321
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
721 PM EDT Tue Aug 22 2017

.SYNOPSIS...
A cold front will bring the potential for scattered showers and
thunderstorms late tonight, which may persist into Wednesday
morning along the coastal plain. High pressure brings dry
weather with very comfortable humidity levels Thursday through
early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
*  Severe Thunderstorm Watch remains in effect for western MA
   until midnight

7PM update...
Convection across NY/PA attempting to form QLCS within a
corridor of enhanced shear along a 45-50 kt SSW-NNE oriented LLJ
and pre-frontal trof. Overall, trend has actually been for this
convection to wane somewhat as it approaches, likely given it is
entering an area where moisture is lacking (over us here in S
New England) as K-indices remain between 26-28. However, the
moisture plume is advecting E with the LLJ and could allow for
the maintenance of at least some MU CAPE values near 1000j/kg
during the late overnight hours. POPs were timed using a blend
of WRF and HRRR data which are currently handling the situation
relatively well from a timing standpoint.

The lackluster mid lvl lapse rates will remain, and this could
prove a convective mitigation factor to consider as the storms
approach, but given the ongoing Severe Watch and very strong
forcing/LLJ, even in a more stable environment storms could
retain a risk for strong winds.

Previous discussion follows...

Tricky forecast tonight. The high heat and humidity resulted in
very high CAPE values for our region. Most unstable CAPE values
were between 2,500-3,000 J/kg, with mixed layer CAPE values
between 1,500-2,000 J/kg. Despite this, our region remains
fairly capped this afternoon due to poor mid level lapse rates
and low shear.

The complication in the forecast tonight will be how quickly the
CAPE values drop after sunset, and how quickly stronger shear
arrives. It will be delicate balance. At this time, thinking
strong convection may arrive east of the Berkshires between 8
and 10 PM, then weaken as it moves east from there. Helicity
values in the 0-1km AGL layer range between 150-300 m2/s2. This
would mean at least a very low risk for a tornado as these
storms move across our region overnight.

The other risks from any strong thunderstorms tonight will be
gusty winds and locally heavy rainfall. Thinking any flooding
risk tonight will be very localized as storms should be moving
rather quickly. Training storms also appear to be unlikely.

Well above normal temperatures continue tonight.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
The threat for scattered showers and thunderstorms may persist
into Wednesday morning across the coastal plain, and perhaps
even into early afternoon across the Cape and islands. Much
drier air will arrive behind a cold front, which should move
offshore by late afternoon. Near normal temperatures expected
through this period.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Overview and model preferences...
22.12Z model guidance suite continues the trend of a mainly dry
forecast through early next week. Synoptically, there remains
good agreement, and this is in spite of a fairly amplified
pattern across the N CONUS and Canada, something long range
guidance typically struggles with. Reinforcement of an Alaskan
vortex is the catalyst for building a highly amplified ridge
across central Canada, and although not a true block, leads to
enough downstream trof digging to reinforce the trof upstream of
New England. In spite of the cyclonic curvature, flow aloft is
confluent predominantly, shifting the mean jet over New England
rather than in the typical ageostrophic lift region of the mean
jet. This coincides with an area of subsidence in development of
reasonably strong high pres which will generally define the
sensible wx across New England through early next week. Thermal
profiles are supported by drier/cooler flow within the upstream
trof out of central Canada, supporting the previous
forecaster`s thinking that a break from the summer heat/humidity
can be expected.

Details...

Thu...
Front shifts further offshore allowing deeper column drying and
cold advection aloft. Dewpoints drop into the 50s and H85 temps
drop to +10C to +12C, which should keep highs mainly slightly
below normal, or in the mid-upper 70s with a few spots around
80. Subsidence maintains a dry forecast outside of CU
development.

Fri...
Vigorous shortwave will rotate through during the daytime Fri.
Lapse rates are reasonably steep given the cold advection
leading the wave aloft, nearing 6.0C/km across much of the
region. A slight spike in moisture possible as well as PWATs
shift closer to the 1.00 inch mark according to some soundings.
Although some convective feedback QPF is noted, but likely not
well progged, will feature diurnally timed increase in POPs to
at least slight chance during the afternoon and early evening
hours Fri with this wave passage. Shower, or even a very
isolated thunderstorm not out of the question. Otherwise, highs
once again slightly below normal, mainly in the mid-upper 70s.

This weekend...
High pres crests over the region through the period as the base
of the longwave trof also shifts W-E across New England.
Soundings continue to be mostly dry, suggesting the risk for
even any showers remains quite low, especially given the lack of
strong forcing. Will continue to feature a mainly dry forecast
with NIL POPs. Highs in the 70s predominantly, with comfortable
overnights dipping into the 50s thanks to dewpoints in the upper
40s and 50s as well.

Early next week...
Low lvl ridging is reinforced by the influence of the stronger
synoptic ridge shifting E in New England as the longwave moves E
of New England. In fact, already noting that high pres will
approach 1030+ hPa. Dry weather looks likely to continue, along
with comfortable dewpoints.

&&

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

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

Short Term /through Wednesday Night/...

Tonight...Moderate confidence.
SCT SHRA/TSRA should move east of the Berkshires between 01Z-
04Z. Storms are expected to weaken as they shift east with time,
but a few stronger storms cannot be ruled out. This line weakens
for a bit early in the morning, but then another round of
showers and storms could impact mainly southeastern MA after
10Z. Mix of mainly VFR with occasional MVFR/IFR in
showers/storms, low confidence in IFR/LIFR stratus mainly Cape
Cod and the Islands. Winds gusting 20-30 kt at times, with LLWS
given 40-45 kt at around 2kft.

Wednesday...High confidence in trends, but lower confidence in
overall timing. SCT SHRA/TSRA may persist near and especially
southeast MA into early Wednesday afternoon. Improvement to VFR
conditions expected from west to east, but timing somewhat
uncertain.

Wednesday Night...High Confidence. VFR.

KBOS Terminal...High confidence in TAF trends. Moderate
confidence in timing.

KBDL Terminal...High confidence in TAF trends. Moderate
confidence in timing.

Outlook /Thursday through Sunday/...

Mainly VFR. Mainly W-NW flow with sea breezes by day.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Strong southwest low level jet moves across the waters tonight
into Wednesday morning. Gusty southwest winds ahead of an
approaching cold front. Some risk for thunderstorms, but showers
more likely. Will continue Small Craft Advisory headlines, but
did tweak times.

Winds and seas diminish Wednesday afternoon, with more tranquil
boating conditions starting Wednesday night.

Outlook /Thursday through Sunday/...

Generally quiet boating weather. NW winds dominate, with gusts
peaking only around 15 kt at any given time. Seas/waves remain
below 4 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ231>234-
     250.
     Small Craft Advisory until 2 AM EDT Wednesday for ANZ230-236.
     Small Craft Advisory until 2 PM EDT Wednesday for ANZ235-237.
     Small Craft Advisory until 8 AM EDT Wednesday for ANZ251.
     Small Craft Advisory until 4 PM EDT Wednesday for ANZ254>256.

&&

$$
SYNOPSIS...Belk/Doody
NEAR TERM...Belk/Doody
SHORT TERM...Belk
LONG TERM...Doody
AVIATION...Belk/Doody
MARINE...Belk/Doody


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