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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 151100

Area Forecast Discussion
National Weather Service Boston/Norton MA
700 AM EDT Sun Sep 15 2019

Morning clouds and fog gives way to a dry warm sunny afternoon 
behind a departing cold front. A secondary trailing cold front 
may result in a few showers tonight into Monday. Otherwise, an 
extended period of dry weather is expected beginning Tuesday and
likely continuing into next weekend as Canadian high pressure 
crests across the region. The high will initially be accompanied
by unseasonably cool weather Tuesday into Thursday morning. A 
gradual warmup is expected with temperatures possibly rebounding
into the 80s Friday and Saturday.



7 AM update ...

Light winds, clearing skies and dew pts in the upper 50s to mid
60s has resulted in radiational fog across portions of northern
CT/northern MA and into parts of RI. Thicker cloud cover over
southeast MA has precluded fog from forming. Thus have issued a
short fused dense fog advisory until 9 AM. West winds just 
above the surface will mix down and begin to erode fog and give 
way to increasing sunshine within the next hour or two. Dew pts
will fall into the 50s as west winds increase however it will be
a warm day with 850 mb temps around +12C thru the day. These
temps aloft combined with west winds will support highs of
75-80. Clouds will take longer to clear the Cape and Islands,
probably not until this afternoon. Otherwise previous forecast
remains on track. 



Tonight and Sunday...

The nearly zonal H5 flow across the region will allow moisture
from approaching low pres out of NY state to move in. 00Z
guidance giving a strong signal of this low as it shifts E-SE
overnight into Sunday. With this system coming from moisture
starved inland areas, may not see much precip. PWATs may rise up
to 1.3 to 1.4 inches by Monday afternoon across southern areas,
but not expecting much in the way of QPF. May see around 0.1
inches, maybe a bit higher across western areas closer to the
passing low. Expect the precip to push S of the Mass Pike during
the afternoon, but should weaken as it shifts toward the S 
coast late in the day. 

Temps should fall back to the 50s to around 60 tonight, mildest
along the S coast, with highs on Monday in the upper 60s to 
around 70.



Updated 340 AM ...


* An extended period of dry weather is likely Tuesday thru Saturday
* Fall-like Tue/Wed with highs only in the 60s and lows in the 40s 
* Summer-like warmth possible Fri and Sat with highs in the 80s

Precipitation ...

Looks like an extended period of dry weather as high amplitude 
pattern evolves with deep closed mid level low over the northwest 
Atlantic along with upstream 588 dam ridge from the MS Valley into 
the Great Lakes. This setup supports dry northwest flow into New 
England. Thus dry weather should prevail Tue thru Sat per ensembles 
and deterministic guidance. Definitely a pleasant stretch of weather. 

Temperatures ...

Very pleasant fall-like weather this period featuring mild days and 
cool nights. Cooler air begins to advect into the region Monday 
night into Tue as trailing short wave energy dives into the base of 
Maritime trough. EC ensembles have core of cool air centered over 
eastern MA Wed into Thu morning with 850 mb temps down to only +3C! 
This will translate to highs only in the 60s Tue and Wed despite 
plenty of sunshine. Normal high this time of year is 70-75. Lows 
will be mainly in the 40s, low 50s in the urban areas. As 588 dam 
ridge over the OH Valley builds eastward into New England expect a 
noticeable warmup with summer-like warmth possible Fri and Sat as 
highs likely climb into the 80s with EC ensembles offering +15C at 
850 mb. Given 1025+ mb Canadian high pressure over the region much 
of next week, weak pgradient/light winds and dry airmass will 
support radiational cooling. Thus will lean toward the cooler MOS 
nighttime temps. Light pgradient will also support afternoon 

Tropical Storm Humberto ...

Good model agreement that Humberto will track away from the east 
coast next week and then possibly impact Bermuda around Thursday. 
Thereafter the ECMWF continues its theme of anomalous western 
Atlantic trough capturing Humberto and lifting the tropical cyclone 
northward across eastern Georges Bank and towards Nova Scotia 
Fri/Sat. While this track is still southeast of New England it's 
close enough to watch closely. At the very least this track would 
yield large swells for MA/RI waters and with temps likely warming 
into the 80s Fri/Sat, rough surf and strong rip currents could be a 
concern for ocean exposed beaches. The ECMWF solution is a low 
probability but will have to be watched given other EC ensemble 
members support the operational run.


Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

11z Update ... 

Areas of IFR/LIFR in fog across CT and interior MA including
Cape Cod and the Islands will lift to MVFR/VFR as the morning 
progresses. Otherwise VFR with light west winds.


Clouds increase from NW-SE during the night as another weak 
system approaches. Mainly VFR conditions, but may see local MVFR
in any showers or patchy fog. MVFR-IFR VSBYS through around 05Z
in patchy fog across Cape Cod and the islands. Scattered 
showers may push into into N central and NW Mass after 06Z with 
the next system. Light W-NW or calm winds. 

Mainly VFR conditions. May see brief, local MVFR in any 
showers, but should weaken as they move S during the
afternoon. Light northerly winds early, becoming NW 5-10 kt
during the afternoon. 

KBOS Terminal...High confidence in TAF trends.

KBDL Terminal...High confidence in TAF trends. 

Outlook /Monday Night through Thursday/...High confidence.



Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

Winds to diminish after frontal passage this morning, though 
will stall along the S coast or across the coastal waters by 
this evening. Expect W winds up to about 10 kt on the eastern 
waters, but light W-SW winds on the southern waters. Mainly good
visibility, but low clouds/fog may linger across the waters 
near the mid and outer Cape with visibility restrictions. Seas 4
ft or less.

Dry conditions across the waters tonight. W wind 10 kt or less
shift to N toward daybreak. Visibility restrictions linger 
early tonight across the southern waters to the waters E of Cape
Cod, but should improve by 05Z-06Z. May see local visibility
restrictions as scattered showers move into the eastern waters
by daybreak. Seas 3 ft or less. 

On Monday, area of showers moves across the region rather
quickly, weakening as they push off the S coast late in the day.
May see local visibility restrictions in the showers. N winds at
10 kt or less become light/variable during the afternoon. Seas 3
ft or less.

Outlook /Monday Night through Thursday/...Moderate confidence.

Monday Night: Winds less than 25 kt. 

Tuesday through Thursday: Winds less than 25 kt. Areas of seas
approaching 5 ft.


CT...Dense Fog Advisory until 9 AM EDT this morning for CTZ002>004.
MA...Dense Fog Advisory until 9 AM EDT this morning for MAZ002>016-
RI...Dense Fog Advisory until 9 AM EDT this morning for RIZ001-003-


LONG TERM...Nocera