Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 281955
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
355 PM EDT Sun May 28 2017

.SYNOPSIS...

Isolated spot showers mainly across central and northern
Massachusetts will come to an end early this evening. Otherwise,
dry weather is expected tonight, but a fast moving low pressure
system will bring a period of widespread rain and unseasonably
cool temperatures to the region on Memorial Day. Upper low and
cold pool linger during the mid and late week over the Great
Lakes and Northeast USA while surface high pressure builds in. A
couple of weak cold fronts are possible during this time
bringing showers and possibly thunderstorms. Otherwise fair
weather with near seasonable temperatures.

&&

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

350 pm update...

Just enough diurnal heating coupled with low level moisture has
allowed for a few spot showers to popup this afternoon, mainly
across central and northern MA. Areal coverage will be quite
limited and expect these brief isolated showers to come to an
end early this evening with the loss of daytime heating.

Otherwise, dry and tranquil weather in store for the region
tonight with just an abundance of clouds. Low temps should
bottom out between 50 and 55 in most locales.

&&

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

***Period of widespread rain overspreads the region from west to
 east through early afternoon on Memorial Day with unseasonably
 cool temps***

Monday /Memorial Day/...

A shortwave approaches the region on west to southwest flow
aloft, which increases the forcing for ascent. This coupled with
a modest ESE low level jet and Pwats 1+ standard deviations
above normal, will allow rain to overspread the region. Timing
still uncertain given model differences, but a period of
widespread rain should overspread the region Monday morning
through early afternoon from west to east. The rain should
taper off to scattered light showers/drizzle late Mon
afternoon/early evening. Elevated instability parameters look
marginal for thunder, but there is a low risk for an isolated
t-storm or two in our southern zones mainly along the south
coast.

It also will be an unseasonably cool Memorial Day as high
pressure over the Canadian Maritimes will result in cool easterly
surface winds, coupled with clouds and rain. High temps will
only be in the 50s to near 60 and it will be breezy on the
Cape/Islands.

Monday night...

Shortwave and deeper moisture will have exited the region by
early Monday evening. However, moist northeast low level flow will
probably allow scattered light showers/drizzle and fog to
persist into the evening across eastern New England. While most
of the scattered light showers/drizzle should come to an end by
midnight, areas of low clouds and patchy fog may persist longer as
low level moisture may remain trapped below the inversion. Low
temps will mainly be in the upper 40s to the lower 50s.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...

Big Picture...

Longwave scale maintains a ridge west/trough east pattern for much
of the long term period.  Shortwave scale shows a closed low slowly
moving from the Great Lakes into Eastern Canada, maintaining a
cyclonic flow over the Northeast USA and especially New England
through Saturday. Several shortwaves move through this flow. One
additional shortwave dives south from the Canadian Arctic and over
the Northeast USA Sunday as the closed low moves off.

Mass fields are similar among the long-range models through
Thursday, but differ in handling the Sunday shortwave. Thermal
fields are similar through Thursday. This brings moderate-high
confidence in the overall synoptic pattern through Thursday. Low
confidence in events for next weekend.

Details...

Tuesday through Friday...

Upper low and associated cold pool remain to our west Tuesday but
move over us Wednesday through Friday. Multiple shortwaves move
through the flow Tuesday through Friday, and exact timing at this
stage is difficult. Already some model timing differences from the
12Z suite yesterday. At this time, the best chance of shortwave
passages along with associated surface fronts/troughs would be
Wednesday and Friday. Convection firing over Upstate NY and PA
Tuesday could work into our area later Tuesday.

Precipitable water values are highest Tuesday, then diminish
Wednesday and Thursday. PW values return in surges Friday afternoon
and Sunday afternoon. The cold pool will aid in destabilizing the
airmass each day, with expected low level heating on Wednesday
bringing the greatest destabilization. Convective parameters show
increasing instability on Tuesday, greater instability Wednesday,
stable air Thursday, and returning instability Friday.  This
suggests scattered showers and a few thunderstorms Tuesday,
Wednesday, and Friday.

We note that SPC shows a marginal risk of severe wx just to our west
on their Day 3 outlook for Tuesday. Conditions do seem more
favorable to our west and less favorable over our area. But Western
Mass and adjacent CT are close to the outlooked area and will need
to be monitored as Tuesday gets closer. It is possible a squall line
or bow could develop to our west and then move into the adjacent
areas late in the day. In such a case, the concern would be for
straight-line winds late in the day.

The Friday cold front stalls south of New England Friday night/early
Saturday.

Saturday-Sunday...

High pressure brings a brief break in unsettled weather Saturday.
Upper low sweeping south from the Canadian Arctic approaches
our area on Sunday. This in turn generates a wave along the
stalled cold front. The rain shield with this wave moves across
our area on Sunday. As noted above, low confidence in this
solution due to run- to-run variations.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...

Tonight...Low confidence. We kept things mainly VFR, but some
of the guidance brings in a lot of IFR to even LIFR conditions.
Certainly possible, but just did not have enough confidence to
go that pessimistic given easterly flow and best moisture to our
southwest. Later shifts will have to monitor trends closely
this evening as confidence is rather low.

Monday and Monday night...Moderate to high confidence. MVFR to
IFR conditions overspread the region through the morning and
into the early afternoon in a period of widespread rain. There
also is a low risk for an embedded t-storm or two, mainly near
the south coast on Mon. Easterly wind gusts around 20 knots are
expected on the coast, with up to 25 knots across portions of
the Cape/Islands.

The rain will taper off to light showers/drizzle from west to
east late Monday afternoon/early evening. However, MVFR- IFR
conditions may persist for much of Mon night with the lowest
conditions most likely on the coast.

KBOS TAF...Moderate confidence in TAF.

KBDL TAF...Moderate confidence in TAF.

Outlook /Tuesday through Thursday/...Moderate confidence.

Generally VFR. Potential for brief IFR in early morning fog.
Potential for MVFR cigs/vsbys in afternoon/evening showers and
scattered t-storms. Best chance for the latter will be in
Western/Central Mass and Northern Connecticut. Least chance for
showers/tstms will be Thursday.

South winds Tuesday, becoming southwest Wednesday, west Thursday,
and southwest Friday. All winds should be less than 20 knots.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...

Tonight...High confidence. The ridge of high pressure will
continue to move east of the waters. However, pressure gradient
will remain weak enough to keep winds/seas below small craft
advisory thresholds for most of the overnight hours.

Monday...Moderate to high confidence. High pressure building
across the Canadian Maritimes coupled with a wave of low
pressure passing to our south will generate easterly wind gusts
around 25 knots for many waters on Monday. Therefore, small
craft headlines posted for many of our waters. Easterly fetch
will also build seas between 3 and 6 feet across our open
waters. The strongest wind gusts/seas will be across our
southern waters. Rain and patchy fog is also expected to develop
and an isolated t-storm or two can not be ruled out Monday
across our southern waters.

Monday night...Moderate to high confidence. Winds and seas will
begin to diminish later Monday evening and especially after
midnight as disturbance moves away from the region and pressure
gradient weakens. Areas of fog may persist into early Tue am
reducing vsbys for mariners.

Outlook /Tuesday through Friday/...Moderate confidence.

Tuesday through Friday...

Winds less than 25 knots and seas less than 5 feet through the
period. Scattered afternoon showers possible. Water temps may weaken
most thunderstorms that approach the coastline, but one or two
storms could briefly affect the waters.

&&

.TIDES/COASTAL FLOODING...

Latest observations continue to show a storm surge just over a
half foot along the eastern MA coast. Another high astro tide of
12.0 feet occurs just after 2 am in Boston. While that is a bit
lower than the last two nights, there will be a little more of
an onshore component and probably a storm surge of a half foot
or even a bit more. While no significant problems are expected,
very minor nuisance coastal flooding is possible along the most
vulnerable shore roads along the eastern MA coast. Therefore,
have opted to go with another coastal flood statement.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 9 AM to 8 PM EDT Monday for
     ANZ231>235-237.
     Small Craft Advisory from 11 AM Monday to 2 AM EDT Tuesday for
     ANZ250.
     Small Craft Advisory from 6 AM Monday to 2 AM EDT Tuesday for
     ANZ254>256.

&&

$$
SYNOPSIS...Frank/WTB
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...WTB
AVIATION...Frank/WTB
MARINE...Frank/WTB
TIDES/COASTAL FLOODING...Frank


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