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000 
FXUS61 KBOX 222353
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
753 PM EDT Mon May 22 2017

.SYNOPSIS...
Showers will cross through southern New England this evening, with 
the most numerous showers affecting RI, northern CT, and MA south of 
the Mass Pike. A weak but moisture rich area of low pressure will 
likely keep showers in the vicinity of RI and southeast MA overnight 
and even into Tuesday morning on the Cape and Islands. Low pressure
will pass south of New England, bringing scattered showers 
Tuesday night into Wednesday. Weak high pressure brings a brief 
period of dry but seasonable conditions. Another stronger low 
moves northeast out of the Ohio Valley, bringing more showers 
Thursday and Friday. The low shifts to the Gulf of Maine 
Saturday, so will see mainly dry conditions. Another weather 
system may approach late Sunday or Memorial Day with another 
period of showers possible.

&&

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

725 PM Update...
Two areas of rain seen on latest NE regional 88D radar imagery,
one across N central Mass and the second larger one mainly near
and S of the Mass Pike, with the steadiest rainfall from about 
KGHG-KIJD-KBDR. Heavier rainfall will move onshore along the S 
coast of RI and Mass over the next couple of hours, but the rain
is steadily moving NE, so precip may taper off to spotty light 
showers or patchy drizzle from W-E. Latest HRRR and RAP13 POP 
model forecast suggest precip will taper off across central and 
western areas by around 04Z-06Z, with rain lingering across E 
Mass mainly S of Boston through the remainder of the night. A 
weakening cold front will push across the region tonight, while 
low pressure slowly pushes E-NE toward Nantucket.

Will see back edge of the clouds moving E into W Mass/N Central
CT by around 06Z or so, then slowly shift E as the weak cold
front passes. It may actually stall for a time near or just off
the Mass E coast by Tuesday morning. 

Have updated near term forecast based on radar and observation
trends, which has slowed down the precip progression a bit
overnight into Tue morning. Also updated T/Td and cloud cover 
to bring current and incorporated into overnight trends. 

Previous Discussion...

Precipitable Water values increase to the E of the front, 
placing southeast MA in the vicinity of 1.5+ inch PWATs late 
tonight into Tuesday morning. In addition, models are showing at
least a modest low level jet affecting southeast MA, especially
the Cape and Islands. NAM is the most robust with the LLJ 
peaking at 40-45 knots. Also K indices in the low 30s support 
the chance for thunder in the vicinity of RI and southeast MA as
well. So in this area rain chances continue right thru 
daybreak. Cape and Islands could see an inch or so of rainfall. 

To the north and west of RI/southeast MA, while precip chances will 
be diminishing overnight, low level moisture trapped under a 
developing subsidence inversion will likely allow for areas of fog 
and possibly low clouds. 

Temps continue to look tricky thanks to the risk for clearing in the 
far NW interior. Some cooling there could allow temps to drop back 
into the 40s while areas further E remain nearly steady in the low 
to mid 50s.

&&

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

Tuesday...
Away from southeast MA, expect improving conditions with 
decreasing clouds and a north wind, primarily under the 
influence of weak high pressure. However models are showing the 
potential for the offshore low pressure to cross southeast of 
Nantucket during Tuesday, and showers associated with it may 
linger in portions of southeast MA during the day. With high 
PWATs near the Cape/Islands in the morning, could see a 
continuation of moderate or brief heavy rainfall early in the 
day. 

Forecast high temps reflect the anticipated difference in 
conditions along the Cape/Islands, and elsewhere across the 
region. Highs 70 to 75 should be common for much of the area, 
with somewhat cooler temps along east coastal MA and south 
coastal MA/RI where sea breezes would keep temps a bit lower. 
Across the Cape and Islands, thinking highs will be in the upper
50s to mid 60s, warmest near the Canal. 

Tuesday night...
Models showing next chance for rain with another wave of low 
pressure. It appears the low center will pass well southeast of 
Nantucket, but cloudiness may stretch into interior southern New
England. Have primarily chance pops for portions of northern 
CT, RI, and southeast MA Remainder of the region especially 
along and north of the MA pike likely remains dry. However there
may still be some changes to the pops as we get closer to 
Tuesday night, depending on if this low tracks closer to our 
area or looks to stay offshore.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

*  Scattered showers Wed morning, then drying out 
*  Next area of showers late Wed night, continuing through Fri
*  May see another weather system approach late this weekend or 
   Memorial Day with more showers possible

Details...

Wednesday...

12Z model suite continues to push low pressure further off the S 
coast, but northern edge of precip shield looks to bring scattered 
showers mainly across south coastal areas, with a few spotty showers 
possible further inland through about midday Wednesday. As the low 
pushes further offshore, precip will taper off, though a few might 
linger across the E slopes of the Berkshires into Wed night. Some 
sun may break through Wed afternoon across portions of the interior, 
but a persistent onshore wind will keep clouds lingering near the 
coast. High temps will range from the 60-65 degree range along E 
coastal areas to the mid-upper 70s across the CT valley. 

Thursday and Friday...

H5 cutoff low pres and associated long wave trough will shift NE 
during this timeframe. The low rotates across the region during 
Friday. Another area of precip will work up the eastern seaboard 
Thu, bringing another batch of rainfall. Big question will be 
whether another surface low develops off the mid Atlantic coast and 
shifts NE late Thu/Thu night, which could enhance the precip. Have 
noted some elevated instability with moderate lapse rates, but with 
stable conditions in the lower levels, do not expect convection at 
this point but something to keep an eye on. The precip shield should 
start to lift NE Fri night.

Forecasted QPF amounts from 0.7 to 1.25 inches possible from Thu 
afternoon through Fri. Will also see steady easterly winds, which 
will keep cool and damp conditions in place, along with patchy fog 
developing each night. 

Holiday Weekend...

Have lower confidence during this timeframe due to model solution 
spread in bringing H5 short wave eastward out of the Great Lakes, 
with a possible weak low and front developing across the mid 
Atlantic states. 

At this point, the low pushes eastward by Saturday, so will see some 
drying conditions though can not completely rule out a few spotty 
showers Sat afternoon across the interior. Big question will be 
possible development and movement of low pressure from the mid 
Atlantic states eastward for late this weekend into Memorial Day. 
Have carried chance POPs for both Sun and Mon, though temperatures 
look seasonal.

&&

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

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

Short Term /through Tuesday Night/...

730 PM Update...

This evening...Moderate confidence. 
MVFR-IFR CIGS, lowest from Central-E Mass southward. Cigs 
dropping to IFR between 22Z and 03Z across much of the area, 
though these lower cigs may hold off across Cape/Islands and 
vicinity until later tonight. Winds SE-E, with some gusts up to 
20 kt across Cape Cod and the islands by around midnight.

Overnight...Moderate confidence.
IFR-MVFR with low CIGS/fog/SHRA mainly S and E of a line from 
KHFD-KORH-KLWM. Improving conditions possible N and W of this 
line during the early morning hours. Otherwise a damp night. 
Some S-SW wind gusts up to 20-25 kt around Cape/Islands through
06Z. Otherwise, winds shift to the NW by sunrise Tue.

Tue...Moderate confidence.
Improvement to VFR most locales during the morning. IFR conditions 
in SHRA may linger during Tue across Cape/Islands. Sea breezes 
possible on coastlines. 

KBOS TAF...High confidence in TAF trends, lower confidence in exact 
timing for the lower categories this afternoon and evening.
Sct showers in KBOS vicinity this afternoon, with showers becoming 
more likely for this evening.

KBDL TAF...High confidence in TAF trends, lower confidence in exact 
timing for the lower categories. Showers in vicinity of KBDL this 
afternoon (mainly to the S), becoming more numerous towards/after 
21Z. Additional showers probable this evening.

Tue night...Moderate Confidence. VFR except MVFR possible south 
coast in risk of showers.

Outlook /Wednesday through Saturday/... 

Moderate confidence through the period.

Wednesday...Mainly VFR. May see periods of MVFR-IFR VSBYS in patchy 
fog Wed night along the S coast, Cape Cod and the islands. MVFR-IFR 
CIGS after midnight moving from S-N in low stratus and scattered 
showers. 

Thursday...MVFR to local IFR conditions in showers. Areas of fog 
developing Thu night with IFR VSBYS. 

Friday...Mainly IFR VSBYS in patchy early morning fog, improving to 
VFR. MVFR CIGS linger in low clouds and scattered showers, ending 
from S-N late Fri and Fri night.

Saturday...Patchy early morning fog along the coast and across N 
Central Mass, improving by mid-late morning. Otherwise VFR. May see 
isold showers across portions of interior central and N Mass around 
midday or early afternoon.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

Tonight...Moderate confidence. During this evening, showers will 
become more numerous, combining with stronger southerly winds 
reaching around 25 kt this evening/tonight. While seas should 
generally remain at or below 5 ft, continuing a small craft advisory 
for these winds thru tonight until the winds shift to the W-NW 
tomorrow morning. These will mainly be for the open waters, rather 
than the sheltered bays/sounds. Fog/showers will reduce visibility 
mainly late this afternoon through the overnight.

Tomorrow...High confidence.
Gradual improvement across the waters expected, though showers may 
linger thru the day in the vicinity of Cape/Islands and east. Winds 
and seas will remain below small craft advisory thresholds. Winds 
may gust to 25 kt early on the eastern outer coastal waters. 

Tuesday night...Moderate confidence. 

Weak wave of low pres tracks just south of New England Tue night and 
Wed morning. Winds/seas expected to remain below SCA thresholds for 
this period. 

Outlook /Wednesday through Saturday/... 

Wednesday...Moderate confidence. Low pres passes across or just S of 
the southern waters. E-NE winds 15-20 kt on the outer waters with 
seas up to 5-6 ft. Visibility restrictions in patchy fog Wed night. 

Thursday...Moderate confidence. Persistent E-NE wind gusting up to 
20 kt on the outer waters with seas up to 5-6 ft. Winds shift to SE-
S Thu night and diminish but seas remain at or above 5 ft on the 
outer waters and the southern near shore waters. Visibility 
restrictions in morning and nighttime fog and areas of showers.

Friday and Saturday...Low to moderate confidence. Winds shift to NW 
Fri afternoon/evening. Gusts up to 25 kt on the southern outer 
waters during the afternoon/evening. Seas remain at or above 5 ft, 
highest on the southern outer waters. Visibility restrictions in 
early morning and nighttime patchy fog. Scattered showers early
Fri.

&&

.TIDES/COASTAL FLOODING...
High astronomical tides will occur over several tide cycles along 
eastern Massachusetts coast from Thursday into Memorial Day, with 
tides around 12 feet in Boston and 4 feet on Nantucket.

At the very least, minor nuisance flooding will occur in the most 
vulnerable locations such as Morrissey Blvd in Boston. Any surge on 
top of these tides would lead to more widespread, but minor, 
coastal flooding, including on Nantucket. Right now, this looks 
to be a possibility Thursday into Friday due to expected onshore
winds. Model surge guidance (ESTOFS) shows a potential 0.6 ft 
surge, which would give a storm tide near 13 feet Thursday 
night in Boston and just under 5 ft on Nantucket.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Small Craft Advisory until 8 AM EDT Tuesday for MAZ022-024.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ231-232.
     Small Craft Advisory until 4 AM EDT Tuesday for ANZ233-234.
     Small Craft Advisory until 4 AM EDT Tuesday for ANZ235-237-256.
     Small Craft Advisory until 8 AM EDT Tuesday for ANZ250-254-255.

&&

$$
SYNOPSIS...EVT/NMB
NEAR TERM...EVT/NMB
SHORT TERM...NMB

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