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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 122305

Area Forecast Discussion
National Weather Service Boston/Norton MA
705 PM EDT Thu Sep 12 2019

Clearing and drier air is moving south into Southern New
England. High pressure along with cooler and drier air will 
build into the region tonight and linger into Friday. A warm 
front may bring scattered showers/isolated thunder Saturday 
afternoon and night. Behind the front, dry and warm weather 
returns Sunday and Monday. High pressure then builds over the 
region for Tuesday through Thursday.



7 PM Update...

Clearing had reached the Mass Pike as of 7 PM. Current movement
would bring the clearing to the South Coast 10PM To Midnight.
Observed dew points in the 50s, but values upstream in Nrn NY
and Nrn New England show mid 40s. Expect further drying of our
dew points overnight. Current temps are within a degree or two
of forecast hourly values. 

This update adjusts the sky cover clearing line farther south
based on satellite observations. The remainder of the forecast
is on track, so no significant changes in other fields.

Previous discussion...

Cold front continues to be located across northern MA early this 
afternoon. Expecting this front to continue to progress 
southeastward through the remainder of the afternoon and into the 
early evening hrs, with any related showers being on the light side. 
Pretty strong drying noted across interior northern New England and 
seen in afternoon water vapor imagery. So while overcast skies will 
continue ahead of the front keeping temperatures for the rest of the 
day from climbing much, greater cloud clearing is likely behind the 
front Doesn't look like the front will clear coastal MA/RI and Cape 
Cod and the Islands until the mid-evening hrs, so expect overcast to 
linger the longest here. As surface ridging builds in from the 
north, looking for at least variable cloudiness through midnight 
with clearest conditions further north, but most areas will trend 
clear into the overnight. Though winds across the coast and adjacent 
waters will remain pretty robust around 10-15 kts, wind speeds 
should ease across the interior. With wet ground and good 
radiational cooling, this could set the stage for some patchy mist 
or fog into the overnight mainly along and west of Middlesex County. 

Sided lows tonight toward the colder end of guidance and weighted 
the cooler MOS a bit more heavily, with mid-upper 40s across 
the CT valley and Worcester Hills into eastern MA/RI (closer to 
low 50s in Boston). Lows out on the Cape and Islands being in 
the low- mid 50s with a shorter period of stronger radiational 



Into Friday, most models show a strong 1030+ mb high pressure center 
building into northern Maine and into the Gulf of Maine by early 
Friday night. Still have a fairly robust pressure gradient 
supporting breezy conditions toward southeast MA/Cape Cod and the 
islands through the daytime hours. Though mixing depth should be 
shallow and mixed layer winds do weaken through the day, eastern 
coastal MA should continue to see northeast gusts to 20-25 mph. Dry 
advection and ridging should lead to clear skies though a bit on the 
cool side given the northeast onshore winds. Highs mainly in the 
60s, though the CT Valley has the best chance at seeing highs around 

Friday night...

Should see winds shift from northeast to east and lighten as 
pressure gradient relaxes. Ridge aloft crests over the region early 
Friday night and then we start to see 500 mb heights falling into 
the evening/overnight ahead of the next cold front. Modest 
warm/moisture advection occurs especially late in the overnight with 
BUFKIT soundings showing a generous increase in cloud cover from 
west to east. 850 mb temperatures begin to recover to around the 
upper single digits to near +10C by 12z Saturday, but think enough 
radiational cooling occurs before the clouds arrive to support lows 
in the upper 40s to mid 50s.


Big Picture...

West-to-east flow across the Northern USA with shortwaves rippling 
through the flow. One prominent shortwave over the Great Lakes lifts 
north to James Bay and swings a cold front through New England 
Saturday. additional shortwaves moves over New England Sunday and 
Monday. A high amplitude ridge then builds over the Plains early 
week and moves across New England midweek.

Height contours are above normal through the weekend, then near 
normal early next week, then above normal again mid-week next. 
Surface pressure pattern may bring an easterly flow during Tuesday-
Wednesday and buffer any trend to warmer temps.

Forecast mass fields are quite similar across the Northern USA 
through Tuesday. The main differences arise over the Southeast USA 
Sunday through Thursday with the forecast behavior of a potential 
tropical system from the Bahamas. The Canadian and ECMWF move the 
east coast ridge offshore rather quickly for its high amplitude, 
while the GFS does not.  Ensemble tracks are less aggressive with 
north movement and seem reasonable.  The 12Z model runs of the 
Canadian and ECMWF now hold the storm and precipitation to our south 
through 12Z Friday. Forecast confidence is moderate-high through 
Tuesday, then trending to low by Thursday.

Daily Concerns...


Weak shortwave crossing Canada drives a front through Southern New 
England late Saturday afternoon and early at night. Right entrance 
region of the supporting jet will be in position over our area to 
promote lift as will any low level convergence along/ahead of the 
front. PW values are forecast at 1.75 inches. Stability parameters 
are mediocre, but not so low as to preclude a thunderstorm or two. 
Prime time would be from around 00Z to 08Z. 

Early morning dew points will be mid 40s to low 50s, and with light 
wind that should allow morning lows in the mid 40s to mid 50s. 
Mixing to 925 mb during the day will tap 13-15C temps, equiv to 5-7C 
at 850 mb, so expect max sfc temps in the upper 60s to mid 70s. 
Winds in the mixed layer will be around 20 kt, so we bumped wind 
gust guidance up a few knots to cover 20-24 kt. 


High pressure builds over the region. Leftover showers from the 
overnight may affect RI and SE Mass during the morning, but will be 
moving off to the southeast by afternoon. 

Even with shortwaves moving through, moisture remains a problem. On 
Sunday the moisture is above 700 mb with dry air below. On Monday 
the moisture is below 700 mb with dry air above. Can't rule out a 
shower during this time, especially on Monday, but limited in 
extent. Mixing will tap temperatures aloft of 8-10C, so max sfc 
temps will be in the mid 70s to near 80.

Tuesday through Thursday...

High pressure builds over the region surface and aloft. This brings 
light winds to Southern New England and suggests a light east flow 
depending on where the ridge axis sets up. Easterly winds would 
buffer the sfc layer temperatures, with values several degrees lower 
than what the deeper layer would indicate.


Forecaster Confidence Levels...

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

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


Lingering 2000 foot cigs along the South Coast and Islands, but
satellite shows clearing and lifting cloud bases trending 
south. Should reach the South Coast possibly about 10 PM, 
certainly by midnight. 

Radiational cooling overnight as high pressure builds in. This
may generate local fog, especially in the usual fog-prone 
spots. Where fog occurs expect IFR/LIFR conditions, most likely
after 2 AM.

VFR all terminals. NE winds continue at 10-15 kt gusting to 
around 25 kt along E coastal terminals, closer to 20 kt gusts at
Boston and winds mainly under 10 kts from the NE across the 

Friday night...
VFR. Increasing sky cover after midnight. 

KBOS Terminal...
High confidence in TAF trends.  Winds should stay northeast 
around 10-12 kts.

KBDL Terminal...
High confidence in TAF and trends. Improvement to VFR before

Outlook /Saturday through Tuesday/...High confidence.

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance

Saturday Night: MVFR/IFR conditions possible. Chance SHRA,
slight chance TSRA, patchy BR.

Sunday through Sunday Night: VFR. 

Monday: VFR. Breezy. 

Monday Night through Tuesday: VFR.


Forecaster Confidence Levels...

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

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

Small craft advisories continue in effect for all waters. 


NE sustained winds will continue, though gusts will diminish on
the eastern waters overnight but seas will remain around 5 ft. 
Gusts up to 25-30 kt will continue on the southern open waters 
as well as the nearshore sounds. Seas will increase up to 6 ft. 
Visibility restrictions in patchy fog and leftover showers this 
evening, then should improve as the showers move offshore.


NE winds gust up to 20-30 kt, highest on the southern waters
through midday, then will gradually diminish during the
afternoon. Small Crafts will continue through most of the day.
Good visibility.

Outlook /Saturday through Tuesday/...Moderate confidence.

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

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, slight chance of thunderstorms. 

Sunday: Winds less than 25 kt. Chance of rain showers, slight
chance of thunderstorms. 

Sunday Night: Winds less than 25 kt. 

Monday through Tuesday: Winds less than 25 kt. Seas locally
approaching 5 ft.


MARINE...Small Craft Advisory until 2 PM EDT Friday for ANZ232>234-237.
     Small Craft Advisory until 9 PM EDT this evening for ANZ236.
     Small Craft Advisory until 5 PM EDT Friday for ANZ235.
     Small Craft Advisory until 6 PM EDT Friday for ANZ250.
     Small Craft Advisory until midnight EDT Friday night for 


SHORT TERM...Loconto