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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 260141

Area Forecast Discussion
National Weather Service Boston/Norton MA
941 PM EDT Tue Jun 25 2019

A frontal boundary will bring a period of showers early in the
night, followed a drying trend later tonight. Summer like 
warmth is expected Wednesday through Friday with a few showers 
and thunderstorms at times, but should be mainly dry. A cold 
front sweeps south through Southern New England Saturday 
bringing another chance of showers and thunderstorms. Cooler 
weather then moves in Sunday, followed by warming Tuesday.


940 PM update...

Last of the showers moving through east coastal MA and
Cape/Islands this evening. PWAT axis across the Cape and will 
be moving east overnight with drying in the column. The showers
will gradually exit the Cape and ACK around or just after 
midnight. Meanwhile isolated showers and embedded thunder have 
developed in western MA and far western CT. Environment is 
somewhat unstable in western New Eng with CAPEs 500-1000 J/kg so
can't rule out an isolated shower/t-storm through midnight, 
otherwise mainly dry overnight. Weak front will move east across
SNE with wind shift to light N/NW but not enough to bring drier
air to low levels near the coast and eastern New Eng where low 
level moisture will persist. As a result, low clouds will hang 
tough in eastern half New Eng with partial clearing developing 
in the CT valley

Other concern overnight is the potential for patchy dense fog 
near the south coast where heaviest rainfall was observed. 
Patchy fog possible inland as well just thinking more widespread
along/near the coast. Given moist airmass temps only bottom out
60-65 for overnight mins.


Wednesday ... 

Low pres SE of New Eng will gradually pull away, but the northerly 
flow along the coast and lingering low level moisutre may keep areas 
of lower clouds persisting through the morning across eastern MA. 
Thereafter subtle height rises over the region should provide some 
subsidence and column drying as evident by PWATs falling to about an 
inch along with K indices really dropping off. Thus partly to mostly 
sunny conditions and warm with mixing to 850 mb and temps at this 
level around +14C, supporting highs in the mid to upper 80s. It will 
feel warm with dew pts in the 60s. Afternoon seabreezes will take 
the edge off the heat along and near the coast. 

Late in the day short wave trough moves across northern NY state. 
Convection from this feature may hold together and spill into 
western MA/CT late in the day before dissipating per 12z CAMs. 

Wednesday night ...

Other than possible early evening convection weakening across 
western MA/CT, column begins to dry supporting quiet weather 
overnight. Remaining warm and humid with dew pts in the 60s. Patchy 
fog possible as well.


Big Picture...

Zonal flow over the Northeast USA through Friday. Broad upper ridge 
over the Southern USA builds north through the Plains late in the 
week, then shifts east early next week. Large closed low lingers 
over the Pacific Coast through Friday, then ejects northeast into 
Canada over the weekend. This ejecting low kicks another low away 
from Hudsons Bay. The Hudsons Bay upper low sweeps southeast to New 
England by Sunday and to our east Monday. The remnants of the 
Pacific upper low moving southeast out of Ontario Tuesday.

Contour heights from 500 mb will be above normal through Saturday. 
Heights then lower to the low side of normal Sunday-Monday as the 
Hudsons Bay upper low sweeps across New England. Heights return to 
seasonable levels Tuesday-Wednesday. Expect above normal 
temperatures Thursday to Saturday, cooler Sunday-Monday, then 
warming again Tuesday-Wednesday.

Forecast confidence is moderate-high. The various model mass fields 
are similar through Sunday morning. They agree on the general 
pattern after that through Wednesday, but with differences in 
handling the departing upper low Sunday and the following shortwave 
over Ontario Tuesday.



Two shortwaves move through the flow during this period. The GFS 
brings them through Thursday night and Friday night while the ECMWF 
shows just one passage on Thursday night. Moisture is limited to the 
700-850 mb layer with plenty of dry air above. This would suggest 
shallow clouds. The supporting upper jets with the shortwaves mostly 
aim their favorable dynamics north of the Mass border, directing 
most cloud development there. PW values top out 1.0 to 1.25 inches 
Thursday, drop back, then resurge to 1.5 inches Friday night. 

Expect mainly dry weather Thursday, but with diurnal clouds 
developing, either cumulus or alto-cu. SBCAPES are forecast around 
500 mb along the NH border. Total-totals show a similar favoring of 
VT/NH. Scattered or widely scattered afternoon showers should form, 
with placement favoring northern Mass. Could be isolated thunder as 
well. Mixing will reach between 800-850 mb, which will tap equiv 850 
temps of 14-16C. This supports max sfc temps in the 80s.

Friday should be dry during the day, but expect scattered showers 
with the Friday night shortwave. Most of these should develop in 
Upstate NY in the afternoon and move into our area Friday evening or 
Friday night. The SREF shows a 50 pct chance of CAPE over 1000 in CT 
and Western MA Friday evening, so we will mention some thunder as 
well. Temps aloft support 80s again, possibly around 90 in some 


Upper low from Hudsons Bay drives southeast, aided by a 100-kt upper 
jet. This drives a cold front southeast across New England Saturday 
afternoon and night. The cold front provides a source of low-level 
convergence, the upper jet provides dynamic support, and daytime 
heating generates instability. CAPE of 1000-1500 J/Kg and totals 
near 50 are forecast. So we expect showers/thunder to develop. 
Mixing is not expected to go as deep as previous days, but should 
still reach 850 mb, where temps will be around 15C. So max temps 
will again be well into the 80s.


Surface cold front moves south of New England, while the upper 
low/cold pool/cold pool moisture move south across Southern New 
England Sunday. With the colder air moving in aloft, expect to 
around 850 mb and possibly near 800 mb. The cold pool moisture 
should be enough to generate clouds, and stability parameters 
support showers and scattered thunder. Temperatures at 850 mb will 
be 10-11C, supporting max sfc temps 75 to 80.


The upper low pulls east Monday, taking much of the cloud cover and 
showers. Another cold front may dive out of Canada Tuesday night, 
bringing another chance of showers.


Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...Moderate confidence. 

Tonight... High confidence on trends, lower on exact details.

Any heavy showers will diminish as they move east. This will
leave an area of low ceilings over Eastern Mass and RI, while
Northern CT/Western MA will have slow lifting/clearing. The
lingering humidity will lead to lowering vsbys in fog. This will
be especially true along the South Coast where heavier rain took
place during the day...areas of dense fog may form in those

Wednesday ... high confidence.

Patchy MVFR/IFR cigs/vsbys in the morning across portions of 
east coastal MA, otherwise VFR. Chance of a late day 
shower/t-storm across western MA/CT.

Wednesday night ... 

Scattered showers and T-storms over eastern NY at 00z may hold
together and enter western-central portions of MA/CT but slowly
weaken as well. 

KBOS Terminal...heavy showers and embedded thunder over
southeast CT at 18z may stay mainly south of Logan but will be a
close call through 21z/22z

KBDL Terminal... High confidence in trends. Lower confidence in 
specific details and timing.

Outlook /Thursday through Sunday/...Moderate confidence.

Thursday through Friday: VFR. 

Friday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA, isolated TSRA.

Saturday: Mainly VFR, with areas MVFR possible. Chance SHRA,
chance TSRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance
SHRA, slight chance TSRA.

Sunday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA, chance TSRA.


Forecaster Confidence Levels...

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

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

Tonight ... S-SE winds becoming light north toward morning. 
Vsby reduced in scattered showers and areas of dense fog. 

Wednesday ... light and variable winds with weak low pressure 
meandering east of Nantucket. Areas of morning fog may reduce vsby 
but improving trend for the afternoon.

Wednesday night ... Weak low east of Nantucket exits east with light 
and variable winds over the waters of MA/RI. 

Outlook /Thursday through Sunday/...Moderate confidence.

Thursday through Thursday Night: Winds less than 25 kt. Local
visibility 1 to 3 nm. Seas less than 5 ft.

Friday: Winds less than 25 kt. Seas less than 5 ft.

Friday Night: Winds less than 25 kt. Seas less than 5 ft. Slight
chance of rain showers. 

Saturday: Winds less than 25 kt. Seas less than 5 ft. Slight 
chance of rain showers, slight chance of thunderstorms. 

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

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