Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 291046
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
646 AM EDT Fri May 29 2020

.SYNOPSIS...
High pressure remains well offshore with a warm, moist south to
southwest flow of air over the region. A cold front will move
through Friday night and Saturday, bringing a chance of showers
and a few thunderstorms. A dry and cooler air mass moves in
behind the front on Sunday and lingers for early next week.
Another weather system may bring scattered showers midweek.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

645 am Update...

Cloudy skies over most of the region, with surface obs showing
most ceilings below 1000 feet. The highest ceilings were in the
Merrimack Valley. Several reports of fog along the south coast,
with reports of dense fog on the RI coast. With low vsbys in
this zone, and based on the events of the past couple of
mornings, will be extending the Dense Fog Advisory until 10 AM.

Radar shows light showers moving northeast from the ocean into
RI and SE MA. Could continue to be scattered showers for a few
more hours, so the forecast will continue to mention scattered
showers along the South Coast.

Otherwise, no changes to the forecast. Thinning of clouds is
expected, especially in northern MA where cloud bases are higher
and satellite is showing some thin spots in the cloud cover.

Previous discussion...

* Warm and muggy with a mix of sun and clouds north of the MA Pike
  and mostly cloudy conditions and scattered showers near the South
  Coast.

A vertically stacked upper level trough will be advancing eastward
from the Great Lakes. It will become amplified as the closed off
upper low over AR/MO gets absorbed by the northern stream 500mb jet.
A 120kt jet streak will stretch from Western NY into Quebec, putting
Eastern PA into the Hudson Valley in the favorable right entrance
region. At the surface, a cold front currently over MI will slowly
move east, providing frontogenetic forcing needed for the large
scale ascent mechanism. As a 590 Dm upper level ride continues to
move offshore, near-surface southwest flow will allow for rich
subtropical moisture to be advected up the Eastern seaboard, with
dew points reaching near 70 degrees. PWATs will also rise to about 2
inches, which is above the 90th percentile for CHH SPC observed
sounding climatology. The best instability on Friday afternoon looks
to be located between the I-81 and I-87 corridor from Syracuse to
Albany, where upwards of 1500 J/kg of SBCAPE should reside. This is
also where SPC has their Day 1 slight severe risk with winds and
hail being the primary severe threat. So despite the anomalously
high PWATs, not expecting anything more than a few showers during
the daytime due to a lack of large scale ascent forcing.

As for temperatures, expect areas north of the MA Pike to reach
into the 80s, with northeast MA possibly reaching the mid 80s
with 925mb temperatures between +18C to +20C. 00z HREF guidance
shows that while clouds hang tight near the MA and RI South
Coast, there will be enough breaks in the clouds further north
and inland for a good amount of sunshine. Bufkit soundings show
mixing up to 900mb away from the immediate south coast. With a
30 to 35 kt jet at 925mb, it will be quite breezy tomorrow,
especially over Eastern MA where southwest winds could gust 20
to 25 mph at times. Also along the south coast, expect scattered
showers at times to go with the overcast conditions.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...

* Threat of isolated strong thunderstorms producing heavy downpours
  and gusty winds over Western MA/CT, especially along the East
  Slopes of the Berkshires.

By early this evening, there looks to be also a swath of 1000
J/kg of SBCAPE over Western MA and CT along with 30 to 35 kt of
effective bulk shear. However, large scale ascent will be
lacking as models have come into agreement with the cold front
not arriving in Western MA and CT until late tonight. This
makes sense given the amplified upper level pattern as explained
above. And since this is past daytime heating, it appears
likely that the storms would weaken by the time they cross the
Berkshires. However, a couple of strong to possibly even
marginal severe storms cannot be ruled out, especially if
discrete cells were to form out ahead of the main frontal line
of showers. This is why SPC has far Western MA and CT in a
marginal severe risk.

Overall, a mild and muggy night is in store with the cold front
still off to our west and storms weakening as they cross over the
Berkshires. Given PWATs of up to 2 inches, brief localized heavy
downpours and gusty winds can be expected, especially over Western
MA and CT. Fog is also expected to redevelop along the south coast
with scattered showers. Expect lows in the 60s.

Saturday...

* Isolated thunderstorms possible across Eastern MA and RI in the
  afternoon hours.
* Much more refreshing following the cold front with dew points
  falling into the 50s.

Cold front crosses the area with 850mb temperatures falling from
+16C early Saturday morning to +10C on Saturday evening. As
temperatures cool aloft, Bufkit soundings show mixing up to
800- 850mb for most locations. 1000 to 850mb lapse rates also
steepen to 9C/km. The winds at 850mb are about 20 kts, so expect
west winds 10 to 15 mph at the surface with gusts up to 20 to
25 mph possible. With southerly flow ahead of the cold front,
temperatures could reach the low 80s across Eastern MA and RI.
Models also suggest an area of MLCAPE of about 500 J/kg across
this area. With K index of around 30C and Total Totals of around
45C, thunderstorms, possibly strong are possible along the cold
front. Behind the cold front, a much more refreshing air mass
will infiltrate the area, with dew points falling into the 50s.
Skies will clear quickly behind the frontal passage. With
abundant sunshine and the high sun angle for late May, Western
MA and CT are still expected to reach the mid to upper 70s with
a few locations in the CT River Valley touching 80.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Big Picture...

Broad upper trough digs from Canada over the Northeast USA over the
weekend.  At the same time an upper ridge builds over the Plains,
and a second broad trough digs along the Pacific Coast. The
Northeast trough brings minus 30C temps at 500 mb over Quebec and
minus 26C to 28C temps over Southern New England Monday. The upper
ridge remains over the Plains, providing a flow from the Pacific NW
across Canada and then across the Northeast USA. A shortwave ejects
from the Pacific trough and follows this path, reaching New England
Wed night or Thursday.

Average heights at 500-mb at the end of May are around 570 Dm
or in the low 570s. Forecast values fall below average Sunday
and well-below average Monday. Values then return to near normal
Tuesday night through Thursday. Expect cooler than normal
temperatures early next week, followed by a return to near
normal values midweek.

Mass fields are similar through Monday, then diverge in two spots.
The first is within the trough crossing the Gulf of Maine Monday
night...the second is in the shortwave ejecting across Canada on
Monday. Forecast confidence is on the high side of moderate through
Monday, then trends to the low side of moderate midweek.

Daily Concerns...

Saturday night...

Cold front moves offshore early Saturday night, with northwest winds
bringing cooler drier air over the region. Dew points fall out of
the 60s into the upper 40s and 50s, supporting min temps in the 50s

Sunday-Monday...

Surface high pressure builds east from the Plains Sunday, while
upper trough and cold pool dig southeast from Canada. The result is
dry weather Sunday with cooler temperatures. By Monday there is
additional cold advection aloft, which along with daytime heating
should be enough to destabilize the airmass. Moisture increases
between 850 mb and 700 mb, and with the destabilizing should be
enough to generate diurnal clouds and scattered showers Monday
afternoon. The trough and cold pool then move off to the east Monday
night.

The mixed layer both days reaches above 800 mb, possibly to 750 mb.
Temperatures in the mixed layer support max sfc temps in the mid 60s
to low 70s Sunday (based on 5-7C), and in the 60s Monday (based on 2-
4C).

Tuesday-Wednesday-Thursday...

Midwest high pressure brings fair weather Tuesday, although some
clouds may linger from the departing upper trough/cold pool. The
Pacific shortwave then rides the flow east and southeast, bringing
clouds/showers to Srn New England midweek. The 00z models show
differences in timing, although not as broad as the 12z models. Both
models bring showers in at different times Tuesday night and
continue them into Wednesday. A second period of showers arrives
Wednesday night and Thursday. With the lower confidence during this
period, the forecast will broad-brush the timing uncertainty with
chance pops for showers from Tuesday night through Thursday.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Today...Moderate confidence. IFR most places with dense fog
along the South Coast. Ceilings are a little higher in the
Merrimack Valley and vicinity. Expect conditions to improve to
VFR in Northern MA, while IFR may linger along the South Coast.
Could also be scattered showers in RI/Southeast MA. Southwest
winds gusting to 20-25 kt.

Tonight...Moderate confidence. MVFR becoming local IFR with
scattered showers. Best chance of isolated TSRA in western MA and
northern CT, where locally heavy downpours and gusts to 40 kt
possible. Areas of fog and LIFR conditions along the South Coast. SW
winds 10-15 kts with gusts of 20-25 kts.

Saturday...Moderate to high confidence on trends, lower on exact
timing. IFR/LIFR conditions trending VFR in Western terminals and
MVFR/low VFR in Eastern terminals following cold frontal passage.
Isolated TSRA and localized IFR conditions possible in Eastern
terminals during the afternoon hours. SW winds turning W at 5 to 10
kt except up to 15 kts near the coast.

KBOS Terminal...Moderate to high confidence in TAF.

KBDL Terminal...Moderate to high confidence in TAF.


Outlook /Saturday Night through Tuesday/...

Saturday Night through Sunday Night: VFR. Breezy.

Monday: Mainly VFR, with areas MVFR possible in SHRA. Chance
SHRA.

Monday Night through Tuesday: VFR.

&&

.MARINE...
4 AM Update...

Today through Saturday...High confidence. Areas of dense fog are
possible through mid morning over the southern waters. SW winds will
gust up to 25 kt and seas build to 5 to 7 ft over the outer waters
where Small Craft Advisories are in place. Areas of dense fog return
for the southern waters tonight, with a slight chance of showers. A
cold front crosses the waters Saturday afternoon, with the
possibility of a thunderstorm. SW winds turning W at 10 to 15 kt.


Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Sunday Night: Winds less than 25 kt. Seas 3 feet or less.

Monday through Monday Night: Winds less than 25 kt. Slight
chance of rain showers. Seas 3 feet or less.

Tuesday: Winds less than 25 kt. Seas 4 feet or less.

&&

.CLIMATE...
Humidity levels have become noticeable, with most observed dew
points in the mid 60s. Forecast dew point values through
Saturday go as high as near 70. How significant is this?

Average dew point values Today and Sat (period since 1936)

ORH 48
BOS & BDL 50
PVD 51

Daily highest dew point 5/29
BOS 68
PVD 69
BDL 72
ORH 73

Daily highest dew point 5/30
BOS 68
PVD 70
BDL & ORH 71

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 10 AM EDT this morning for MAZ020>024.
RI...Dense Fog Advisory until 10 AM EDT this morning for RIZ006>008.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ231>235-
     237-250.
     Small Craft Advisory until 6 AM EDT Saturday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Chai
NEAR TERM...WTB/Chai
SHORT TERM...Chai
LONG TERM...WTB
AVIATION...WTB/Chai
MARINE...WTB/Chai
CLIMATE...



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