Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 172321
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
721 PM EDT Thu Aug 17 2017

.SYNOPSIS...
Out ahead of a deep low pressure center lifting into Southern
Canada, rain is to be expected, possibly heavy at times along
with the threat of localized flooding, for Friday into Friday
night. A low risk of strong to severe thunderstorms mainly over
S/W portions of MA and CT. High pressure builds back in for
Saturday, returning with fair and seasonable weather Sunday
through Tuesday. A cold front approaches from the west Tuesday
night and Wednesday, bringing our next chance of showers.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
7PM update...
Other than high CI already spilling over S New England ahead of
an approaching warm front, a generally pleasant evening out
there. This CI is the first indications of moisture loading
which will gradually filter downward through the column into the
morning, which will slow the onset of the initial round of SHRA
until mainly a few hours after midnight. Already noting a band
traveling across W NY/PA. Timing was slowed slightly with this
update, especially with respect to TS potential as the initial
overruning should be stable, with the better instability burst
not arriving until during the daylight hours. Mins were also
adjusted slightly, with cooler values now featured mainly across
E MA and RI where building cloud cover will be slowest.
Otherwise, forecast on track at this time.

Previous discussion follows...

Quiet weather into this evening, clouds lowering and thickening
overnight into early morning Friday. High pressure shifting S/E,
winds beginning to increase out of the S with increasing surface
dewpoints. Dry and clear conditions likely to prevail longest
over the N/E resulting in the opportunity of radiational cooling
and lows getting down into the low 60s, while all other locations
drop down around the mid to upper 60s as showers encroach. Chance
to likely PoPs into N/W portions of MA and CT by morning as tropical
moisture begins to over-run out ahead of the lifting warm frontal
boundary. More on that below.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday into Friday night...

Main concern of a heavy rain / flooding threat, with a lower risk of
strong to severe thunderstorms. An atypical event for mid-August, on
the order of +1-2 standard deviations above average, with anomalous
S flow of tropical moisture with precipitable waters in excess of 2-
inches into a deep area of low pressure over SE Canada with surface
dewpoints rising up into the low 70s. Will focus on 1.) a potential
heavy rain threat associated with a low-level jet impinging upon a
lifting warm front during the morning into afternoon period, and
possibly again overnight lingering into Saturday morning, and 2.)
afternoon-evening thunderstorms from the S/W, some of which may be
strong to severe, at a minimum containing heavy rain threats.

1.) Potential heavy rain threat.

Synoptically low-mid level forcing is apparent given convergence of
a S H925-85 jet rearward of a lifting warm frontal boundary beneath
ascent associated with lead shortwave energy, though the morphology
and strength differs among the forecast guidance. Absent is upper-
level support given lack of height falls, neighboring anticyclonic
motions with the immediate mid-upper level ridge E, as well as the
H3 jet axis N/W. But can`t ignore the anomalously moist environment
and weak elevated instability being acted upon. K-indices exceeding
40 indicative of a potential heavy rain threat.

Potential renewed heavy rain across E/SE New England during the
overnight period with the warm front lingering SW-NE just offshore
out ahead of a H5-7 westerly convergent dry-punch. Continued low-
level confluence of tropical air brought N by H925-85 SW flow.
Along with elevated instability, main threat of heavy rain with
embedded thunder that lingers into Saturday morning ahead of a
slow moving cold front and the aforementioned mid-level dry punch.

Prefer high-res guidance which is in good agreement with the NCAR
ensemble signaling significant probabilities of an inch per hour
rain rates with storm total rainfall amounts in excess of 3 inches
over areas of S New England, especially over the S-coast W to E on
Friday. Heaviest rain S/E to which CIPS Analogs agrees.

Overall, despite a slight risk of excessive rainfall with tropical
downpours exceeding flash flood guidance per WPC, given uncertainty
of outcomes plus differences in forecast solutions, will forego head-
lines with this forecast package. Leaning towards the potential of
an inch or more of rainfall in a short period of time that would
lead to urban, poor-drainage flooding.

Likely PoPs. Will go with a heavy rain mention with a lesser threat
of thunder given the low-level convergent forcing and warm-moist
column up to around 14 kft coinciding with the tropical airmass.
Warm-rain processes snailed. Likely low clouds and visibility
issues associated through the day and overnight.

2.) The risk of strong to severe thunderstorms.

Two areas of focus for thunderstorm development where both shear and
instability warrant the MARGINAL RISK of severe weather advertised
in the Day 2 outlook from the Storm Prediction Center: Upstate NY
and PA, and around the Delmarva Peninsula.

The expectation is that within these two regions of focus shower and
thunderstorm activity will develop during the afternoon hours and
advect into S/W portions of New England late. As prior forecaster
mentioned, there is marginal instability, however strong shear which
can potentially compensate. There is, subsequently, a risk for strong
to severe thunderstorms, along with heavy rain given the tropical
environment. Though a lot of this will rely on how quickly the morning
warm front lifts N/E and whether the S/W zones can destabilize with
partial clearing. That remains unclear at this time.

From the NCAR ensembles, lightning activity level encroaches from
the W, diminishing E towards evening seemingly with lack of daytime
heating and, more likely, a renewed focus of the low-level jet along
the warm front S/E. Indications of a better convective environment
further S/W of Worcester, with helicity noted in the 0-1 / 0-3 km
layer given the turning hodograph profile. K-indices get up to 40 as
lapse rates are conditionally unstable throughout the column, 5-6
C/km. Overall a high shear, weak instability event. Not overly
confident given activity from the W will be entering a region of
more anticyclonic flow within the mid-upper levels S/E, and it is
quite possible that activity around the Delmarva Peninsula robs the
environment N. Support given via hazard guidance from CIPS analogs.

Will go with likely PoPs for both showers and thunderstorms over
W/SW MA and CT towards the later half of the day, diminishing into
evening. Keep with heavy rain but won`t mention gusty winds.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights...

* Showers linger early Saturday.
* Dry with progressively warming conditions Sunday into Tuesday.
* Wet conditions possible again mid week.

Overview and model preferences...
Longwave trof will rotate through the region and gradually lift
and open as ridging is forced eastward by a cutoff moving into
N Canada. This will lead to a transition, from more unsettled,
wet conditions Sat, to a somewhat prolonged drier pattern by
early next week. This is partially thanks to reinforcement of
subtropical ridging which will help to build the ridging
embedded within the mean jet across the N CONUS. It`s not until
the secondary cutoff mentioned above phases with a Greenland
vortex, allowing a transition back to more unsettled and wet
conditions as the trof deepens and taps some of the subtropical
moisture. With the 17.12Z model guidance update, there continues
to be relatively good agreement in the synoptic mass/thermal
fields such that overall forecast confidence continues to rise
through mid next week.

Details...

Saturday...
Although there continues to be good agreement that a cold front
crossing the region will slow as it begins to parallel the flow
aloft, soundings support dry air moving into the column from
the W faster than the front moves. Although modest instability
on the order of 500-1000j/kg of SB CAPE lingers in the morning,
it will be race between the forcing provided by the front and
dry air capping convective potential from the W. The dry air
does maintain the CAPE but due to steepening lapse rates.
SHRA/TSRA risk will actually drop through the day in spite of
the slowed front as K- indices drop quite rapidly in response to
the dry punch. POPs will gradually drop from W-E through the
daylight hours. With some clearing of clouds from W-E mixing
should allow temps to rebound somewhat, back into the low-mid
80s especially where enough sunshine is noted. Humid as dwpts
will remain elevated in the upper 60s to low 70s.

Saturday night and Sunday...
Improvement from W-E through the overnight hours with some
clearing. Dwpts will be dropping but remain in the 60s overnight
such that low-mid 60s is the likely floor for Sat night min
temps even as clearing occurs and pres gradient slackens.
Pleasant on Sun as dwpts mix down into the 50s across much of
the region. Coolest H85 temps during this period, mainly +12C to
+13C, although with some W down sloping component to the wind.
Therefore, expecting highs mainly near to even slightly above
normal, low-mid 80s.

Mon and Tue...
Mainly dry conditions thanks to 1020hPa high pres sliding over
the region through the period. Should be generally good eclipse
weather across S New England save for a little bit typical
diurnal CU on Mon. Warming H85 temps, from as low as +14C on Mon
to up to +17C on Tue suggest gradual warming each day. Highs
Mon in the mid-upper 80s, warming to the predominantly upper 80s
by Tue. Overnight coolest Sun night, in the upper 50s and low
60s, then mainly in the 60s (mainly mid 60s per afternoon dwpts)
Mon night. Although return flow and slight cooling aloft does
suggest better destabilization on Tue with some CAPE already
noted on soundings, best forcing is well upstream in NY/PA and
the great lakes region, so will continue to highlight a dry
forecast Tue.

Wed...
Models continue to be in relatively good agreement on Wed as longwave
trof begins digging and deepening process near the Great Lakes.
The sfc reflection is a low pres wave moving across S Ontario
and Quebec, dragging a cold front through New England. This
whole process slows as the synoptic pattern continues to
amplify, so with K-values reaching over 30 and PWATS increasing
to between 1.75- 2.00 inches on soundings, looking for a period
of wet weather by Wed. Modest destabilization on soundings
within this plume of moisture. Early CAPE values are nearing the
1000j/kg mark. However, given the front will approach early,
this may limit destabilization somewhat. Will feature a risk for
thunderstorms given the possibility, heavy rain possible given
the relatively good agreement for high PWATS.

Thu and Fri...
Lower confidence in sensible wx details. Although synoptic
agreement that longwave trof will remain in place, and likely
just upstream, New England will remain on the cyclonically
sheared side of the trof while dry air associated with high pres
slides in from the W. Diurnally driven SHRA possible in this
scenario, but it is dependent on how dry the column ultimately
becomes.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Rest of today...High confidence.
VFR. Light winds S/W winds with an immediate sea-breeze along
the E coast.

Tonight...High confidence.
VFR dominates most of the of the overnight hours, with MVFR/IFR
CIGS not arriving until mainly after 10Z across the west. Winds
generally light, shifting mainly S.

Friday into Friday night...Moderate confidence.
MVFR-IFR CIGs prevailing especially 12Z-16Z from W-E. -RA/RA
with embedded +RA with lower risk TSRA accompanies the dropping
categories., holding across E terminals late and overnight as
SCT RA/+RA/TSRA develop into S/W portions of MA and CT during
the afternoon, dissipating into evening. Strong S winds around
10-15 kts with gusts up to 25 kts, strongest over S/SE New
England. Potential LLWS overnight with 40-45 kt SW jet across
S/SE New England. All conditions improving as winds turn W into
early morning Saturday.

KBOS Terminal...Moderate confidence. VFR most of the overnight
hours with a gradual transition through the morning to MVFR/IFR
along with a band of SHRA and occasional +SHRA moving in mid
morning. Low risk for occasional TSRA through the day.

KBDL Terminal...Morning SHRA with IFR possible especially after
10Z. Some improvement is possible into the later-half of Friday
prior to RA/+RA/TSRA encroaching from the S/W. May dissipate
before reaching the terminal. Low confidence concerning timing
and outcomes with 18-22z TAF issuance.

Outlook /Saturday through Tuesday/...

Saturday... Moderate confidence.
MVFR/IFR conditions will linger in a mix of low clouds with a risk
for showers/thunderstorms early, but this will diminish through the
day such that VFR dominates by afternoon.  NW winds gust 20-25 kt
mainly Cape/Islands.  Some IFR conditions in fog Sat night.

Sunday through Tuesday...High confidence.

VFR. Areas of IFR in early morning fog/low clouds, then improving
to VFR by each mid morning and continuing the remainder of each day.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

High pressure and light winds will lead to good boating weather
through this evening. But S winds will be on the increase over-
night, gusting up to 25 kts across the S/SE waters Friday into
Friday night. SMALL CRAFT ADVISORY issued accordingly as winds
will result in heightened seas up around 5 feet.

Heavy rain with embedded thunderstorms sweeping W to E across
the waters Friday into Friday night. Will likely be some
visibility restrictions.

Outlook /Saturday through Tuesday/...

Saturday into Saturday night...Moderate confidence.

Lingering potential for scattered showers/tstms through the day,
diminishing west to east Saturday night. Diminishing wind through
the day. Some gusts 20-25 knots in the morning.  Seas near 5 feet on
the outer waters and on RI Sound, but trending lower later in the
day. Small crafts likely early, but these will likely be dropped
later.

Sunday through Tuesday...High confidence.

Generally quiet boating weather with high pressure.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 5 PM Friday to 6 AM EDT Saturday for
     ANZ231>234.
     Small Craft Advisory from 5 PM Friday to 2 AM EDT Saturday for
     ANZ236.
     Small Craft Advisory from 2 PM Friday to 6 AM EDT Saturday for
     ANZ235-237-255-256.
     Small Craft Advisory from 8 PM Friday to 6 AM EDT Saturday for
     ANZ250-254.

&&

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



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