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Area Forecast Discussion (AFD)

Issued by Boston, MA (BOX)

FXUS61 KBOX 170729

Area Forecast Discussion
National Weather Service Taunton MA
329 AM EDT Thu Aug 17 2017

Dry and pleasant conditions continue today. A weather system 
moving through the Great Lakes Region towards the end of the 
work week will bring showers and scattered thunderstorms to 
southern New England during Friday and Saturday. High pressure 
returns with fair seasonable weather Sunday through Tuesday. A 
cold front approaches from the west Tuesday night and Wednesday,
bringing our next chance of showers.



Forecast remains on track for the early morning hours. Weak 
wave moving through the northwest flow this morning has trigger 
some strato cu per latest IR imagery. Dewpoints continue to drop
with a few locations across northeast MA in the 40s. Surface 
temps are radiating out with some locations in the mid to upper
50s. Overall a very pleasant night across southern New England.


Transition day for southern New England. Surface high pressure 
crests overhead today resulting in light and variable winds. 
Thus sea breezes will develop along both coastlines. High temps 
will be a bit cooler today compared to yesterday thanks to the 
cool morning start and very little airmass change. Temps will be
in the upper 70s to low 80s. Overall another pleasant summer 
day across the region. 

BEACH Forecast:  Hurricane GERT is well east of the region. However 
there is still some lingering swell and thus anticipate a moderate 
risk for rip currents. Have contemplated about issuing a rip current 
statement out but decided to hold off as both waves, swells and the 
period will continue to drop through the day. Thus making on the 
morning hours more of a risk per the Surf matrix. Swimmers are urged 
to still use caution if heading into the surf today. 


Quiet weather for the first half of the night. High pressure will 
move offshore as winds turn more to the south. Dewpoints will begin 
to rise through the overnight hours ahead of approaching warm front. 
Clouds will be on the increase within the WAA pattern limiting 
radiational cooling. Therefore temps will remain in the low to mid 
60s. While most of the night will be dry, will have to watch for the 
development of a few showers moving into the region from overrunning 
in the higher theta-e. Best area will be across CT and western MA 
during the early morning hours on Friday.  



Friday and Friday Night...

Overview: Mid-level ridge axis will slide through the region on 
Friday as a potent upper level trough moves into the Great Lakes. 
Upper level disturbance moving through the flow will push a warm 
front into the area on Friday, keeping southern New England in the 
warm sector into early Saturday morning when a cold front approaches 
from the west.  

Details: Warm, moist humid airmass will advect into the region early 
Friday. This will result in dewpoints rising into the low 70s with 
PWAT above 2 inches, which is 2 STD above normal. Quite anomalous 
for this time of year. Passing warm front will some precipitation in 
the morning. However, the focus is on later in the day when diurnal 
heating takes place, and elevated instability begins to move into 
the region. This combined with increasing 850mb LLJ could result in 
more widespread precipitation as well as a few thunderstorms. This 
could make for a messy evening commute. 

The potential for strong or severe storms will be limited as SB CAPE 
values are quite low with poor lapse rates. However, shear values 
are quiet strong and with the 70+ dewpoints, could help compensate 
the lack of instability. LI values and showalters do drop below 0 
indicating some elevated instability so will keep the mention for a 
few thunderstorms. Main concern with any convection will be tropical 
downpours which could result in high rainfall rates. This is also 
suggested by WPC day 2 excessive rainfall outlook. Contemplated the 
idea of some sort of flood watch, but the surface and low level flow 
is moving so not expecting training cells. For now will hold off and 
let day crew get another look. 

Thermal axis will peak over the region Friday night as upper level 
trough begins to approach. Still anticipate ongoing showers and 
isolated thunder as the airmass remains unchanged. However upper 
level trough will begin to approach so the first half of the night 
may be busy with ongoing precipitation. Conditions will slowly 
improve from west to east through the early morning hours, however 
the approaching cold front appears to stall either over or west of 
the region. This may be the focus for precip on Saturday. For now, 
anticipate ongoing precip and fog through the night with conditions 
slowly improving from west to east by the morning hours. 


Big Picture...

Subtropical high remains in control across the Southern USA while a 
general zonal flow continues across Canada and the Northern USA. The 
high actually builds over the Western USA early next week, 
supporting a deepening of the wave pattern during next week. One 
shortwave moves through New England over the weekend. A second 
shortwave moves across from the Gulf of Alaska, forms a closed low 
over northern Canada, and then digs over Quebec midweek.  

Contour heights are normal to a little higher than normal through 
early next week.  As the Quebec closed low deepens, heights over New 
England dip below normal midweek next.

Model mass and thermal fields from the 00Z suite are in general 
agreement through Wednesday, building forecast confidence. 


Saturday... Moderate confidence.

Models hold the the cold front over western and central sections of 
Southern New England while moisture fields race east and drier air 
moves in during the afternoon. PW fields show 1.5 inch values move 
east of BOS-PVD by Saturday afternoon. CAPE fields show 1000-1500 
J/Kg east of a ASH-ORH-IJD line in the afternoon. Winds aloft are 
marginally significant, with 25-30 knots at 850 mb and 35-40 knots 
at 500 mb. Stability parameters are mixed, with Totals in the mid 
40s, except upper 40s ECMWF, and LI values subzero. 

Expect the main area of showers/heavy downpours to move east of 
Massachusetts during the morning, but with lingering scattered 
showers/thunder possible in the afternoon until the cold front moves 

With the flow becoming more southwest, expect deeper mixing than on 
Friday...and so a warmer airmass. Temps of 15-17C at 850 mb would 
support mid-upper 80s. Depending on the degree of clearing, temps 
could be a little lower than this. Expect max temps in the 80s. 
Surface dew points in the southerly flow will linger in the upper 
60s and low 70s, so expect a warm and very humid day.

Saturday night...

Cold front moves offshore, and surface winds become northwest. 
Expect any showers to taper off. Skies will either clear or 
partially clear overnight. Dew points will linger in the 60s, with 
min temps close to that level.


Upper shortwave and the coldest of the cold mid-level temps will be 
over New England. The more significant cold temps as well as cloud-
level moisture will be over NH-VT. Cross-sections show Southern New 
England will have a moist layer based at 850 mb, with drier air 
closer to the surface. Expect diurnal clouds, probably a bit more 
than guidance is suggesting, but otherwise a fair day with less 
noticeable humidity.  

Surface high pressure builds in with light variable wind Sunday 
night. With dew points in the upper 50s and around 60, expect min 
temps around this same range. Guidance temps were nudged a couple of 
degrees colder.


High pressure builds over the region Monday and shifts offshore 
south and east by Tuesday. Cross sections show dry air in place at 
most levels through Tuesday evening, so expect dry weather both 
days. Increasing mid and high clouds Tuesday ahead of the next 
weather system. Temps aloft 14-15C Monday and 15-17C Tuesday, 
supporting low-mid 80s Monday and mid to upper 80s Tuesday.

Tuesday night-Wednesday...

Closed low digs over Quebec and the Northeast USA with shortwave 
driving a cold front into/through New England. Expect 
showers/scatterd tstms with the front Tuesday night/Wednesday.


Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Before 00z...High confidence. VFR except some very patchy 
ground fog early AM mainly at typically prone terminals.

Today...High confidence. VFR. Winds shift lightly W during the 
day except sea breezes along coast.

Tonight...High confidence. VFR to start with deteriorating conditions
after 09z as widespread rain approaches. Winds will turn more
southerly overnight. Patchy ground fog is possible at typically
prone terminals

Tomorrow into Friday night...Moderate confidence in trends, 
lower confidence on timing. Conditions will fall to MVFR/IFR 
through the day as rain moves from west to east and could be 
heavy at times restricting vsbys. Iso -TSRA is also possible on 
Friday. Southerly winds with gusts near 15 kts by the afternoon.
Low clouds and precip will continue into the overnight hours.
Slow improvements out west after 10z. 

KBOS Terminal...High confidence in TAF. Sea breeze development
expected for Thu.

KBDL Terminal...High confidence in TAF.
Outlook /Saturday through Monday/...

Saturday... Moderate confidence.

MVFR/IFR cigs and vsbys in showers and fog Saturday morning will 
improve to predominant VFR during the day. Continued MVFR 
ub scattered showers/thunder through afternoon especially in 
RI/Eastern Mass. Southerly winds during the day will shift from the 
west/northwest Saturday night after a cold front moves through.

Areas of IFR cigs/vsbys in fog and low clouds later Saturday night.

Sunday and Monday...Moderate-High confidence.

VFR. Areas of IFR in early morning fog/low clouds, then improving 
to VFR by each mid morning and continuing the remainder of each day.


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.

Seas and swell have begun to subside early this morning across
the southern waters. Thus have dropped the SCA. Otherwise, mainly quiet 
boating weather continued into tonight once seas subside. 

Approaching weather system on Friday will reduce vsbys in heavy
rain. Waves will also build as southwesterly flow increases with
gusts near 15 to 20 kts by Friday evening. SCA may be needed but
confidence is low.

Outlook /Saturday through Monday/...

Saturday...Moderate confidence.

Lingering potential for scattered showers/tstms through the day, 
diminishing west to east Saturday night. Diminishing wind through 
the day. Some gusts 20-25 knots in the morning.  Seas near 5 feet on 
the outer waters and on RI Sound, but trending lower later in the 

Sunday and Monday...Moderate-High confidence.

High pressure builds over the waters. Winds remain 20 knots or less, 
and seas 4 feet or less. Leftover showers early, but these move off 
to the east during the morning.




NEAR TERM...Dunten