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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
582 
FXUS61 KBOX 160806
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
406 AM EDT Thu Aug 16 2018

.SYNOPSIS...
More heat and humidity today, then increasing risk of showers and 
thunderstorms with locally heavy rain Friday afternoon into Saturday 
as a cold front approaches. This front moves through later Saturday, 
followed by mainly dry and cooler weather Sunday into Tuesday as 
high pressure builds in from the north. The humidity will return by 
Wednesday leading to the risk of more showers and thunderstorms.

&&

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

Dry conditions in place across the region early this morning as
a weak front sits across central NH/VT. However, the summery 
feel remains as dewpoints remain in the mid 60s to around 70 
regionwide with light W-SW or calm winds. Shower activity was 
confined to northern VT/NH into portions of ME at 07Z. 

Through the pre-dawn hours, may see some shallow patchy fog 
develop through around 10Z to 12Z, mainly across the interior 
valleys and across the E slopes of the Berkshires with low T/Td 
spreads and light/variable or calm winds. 

Looking at the GOES-East RGB Nighttime Microphysics composite 
imagery, may see the pre-frontal trough trying to slip across 
central Mass with little fanfare other than some lower clouds 
across parts of the Berkshires. 

The main cold front was lying across N VT to interior downeast 
ME at 06Z, but short range models continue to push this front 
slowly but steadily S today. Most members of the short range 
model suite suggest that, as this front pushes S, a bubble low 
may form around the Route 128/495 beltways of E Mass around 
midday or early this afternoon. The weak convergence zone (sea 
breeze front) may be enhanced a bit around midday or during the 
afternoon, which may kick off a few showers or even an isolated 
thunderstorm. Models have been signaling this signature for the 
last couple of days, but not a guarantee this will occur. 
Something to monitor, especially for potential brief delays at 
Logan Airport.

The dewpoints will lower just a tad across the Route 2 area of N 
Mass this afternoon as the cold front passes, though the readings 
will remain in the lower 60s. Elsewhere, it will be muggy with 
dewpoints from 65 to 70, highest along the S coast. As for temps 
today, expect them to top off in the mid 80s to around 90, hottest 
across the lower CT valley and interior E Mass. It will be a little 
cooler along the immediate coast, Cape Cod and the islands.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...

Tonight...The cold front will slowly push into central Mass 
this evening, then stall through the remainder of the night. 
With low instability to start off early tonight, will be mainly 
dry except for a few showers that may push across western areas.
Then, as low pressure moves across the Great Lakes, winds shift
to S-SE. May see leading edge of precip try to move into 
western areas after 06Z or so. Have mentioned a chance for 
showers, maybe an isolated thunderstorm. 

Friday...The stalled front will lift back N early Friday as the
low continues to trek E-NE out of the eastern Great Lakes. 
Noting a swath of high PWATs (around 2 inches), about 2 to 3 SD 
above normal, will push across the region on the SW flow at the 
surface and aloft. Instability will also increase as a pre- 
frontal trough may approach during the afternoon. With the 
moisture laden airmass, if any convection does develop, could 
see heavy downpours as well. 

With more clouds around, along with the threat for precip, 
temps will be a tad cooler than today. Highs will be mainly in 
the 80s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Showers/t-storms likely Fri night into Saturday
* Locally heavy rainfall possible along with the potential for
  a few strong to severe storms Fri evening and again Saturday 
* Mild to warm days and cool nights Sun/Mon/Tue
* More humid with increasing risk of showers/t-storms Tue night/Wed

Friday night into Saturday...

Warm front may be in the vicinity of northern MA Fri evening before 
lifting north. Warm sector airmass will be firmly entrenched across 
SNE with PWATs exceeding 2 inches and dewpoints in the 70s. 
Scattered showers and t-storms will likely be ongoing in the 
evening, especially interior, and expect convective activity to 
persist through the night and move east across SNE as mid level 
trough approaches and instability is maintained. Models indicate a 
strengthening low level jet Fri evening which will increase low level 
shear and helicity so a few strong to severe storms possible with 
potential for rotation. Given high dewpoints in the 70s and low LCLs 
this will have to be watched closely. In addition, heavy rainfall 
and localized flash flooding will be a threat with high PWAT airmass 
and +2SD low level jet. 

On Saturday, there is still some uncertainty with timing of fropa 
but consensus of model guidance favors slower timing with cold front 
moving south across SNE during the afternoon. This will keep 
tropical airmass across the region with threat of showers/t-storms 
continuing. Deep layer shear is marginal but enough instability for 
a few strong to marginally severe storms with heavy rainfall threat 
as well. Some of the guidance doesn't push the front to the south 
coast until Sat evening. Drying trend possible across northern MA in 
the afternoon depending on exact timing of fropa. If front is 
delayed, temps will reach well into the 80s, especially south of the 
Pike. Some showers may linger near the south coast in the evening, 
otherwise, cooler and drier conditions will filter into the region 
from north to south Sat night with gusty NE winds developing, 
especially along eastern MA coast.

Sunday into Monday...

Uncertainty with location of frontal boundary south of New Eng leads 
to a lower confidence forecast for Sun/Mon. ECMWF trended further 
north with boundary and surface wave track closer to New Eng which 
keeps it unsettled Sun into Mon. GFS and ensembles are further south 
and drier and have support from UKMET so we leaned toward drier 
solution. Cooler and drier airmass overspreads SNE Sunday and Monday 
as high pres builds to the north. Looks mainly dry during this 
period although will have to watch moisture to the south Sun into 
Mon. Coolest day will be Sunday, especially SE coastal MA where 
temps may struggle to reach 70 with gusty NE winds, but ranging 
through the 70s elsewhere. Clouds may linger through the day Sunday 
near the south coast. Gradual airmass moderation Mon.

Tuesday into Wednesday...

Model differences with timing and amplitude of next upstream trough, 
but it appears humidity and risk of showers/t-storms return by Tue 
night or Wed. It is possible showers could impact region as early as 
Tue, but more likely Tue night into Wed.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday/...

Short Term /through Friday/...

Through 12Z...High confidence. VFR. Local MVFR conditions in 
patchy fog through sunrise, mainly across the inland valleys and
E slopes of the Berkshires. Otherwise, dry conditions.

Today...High confidence. VFR. Mainly dry conditions. Any patchy
fog with local MVFR VSBYS western areas through around 13Z. 
Isolated showers/thunderstorms may develop across E Mass around 
midday into the afternoon.

Tonight...Moderate to high confidence. Mainly VFR. May see 
patchy MVFR CIGS across N central and E Mass. Scattered 
showers/isolated thunderstorms may approach western areas after 
06Z with local MVFR conditions. 

Friday...Moderate confidence. Showers with scattered 
thunderstorms through most of the day across N central and 
western areas, with low risk for showers/thunderstorms further E
during the day. MVFR conditions at times. Locally heavy 
rainfall possible. 

KBOS Terminal...Weak sea breeze may impact the terminal at 
times this afternoon. Low risk of brief showers that may reduce 
visibility around midday into the afternoon. Moderate 
confidence.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: MVFR/IFR conditions possible. Breezy. Chance
SHRA, chance TSRA, patchy FG.

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

Saturday Night: Mainly VFR, with local MVFR possible. Breezy. 

Sunday through Sunday Night: VFR. Breezy. 

Monday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Today...A weak front will stall across the eastern waters by 
around midday, which may cause winds to shift from light W to 
easterly during the afternoon. W-SW winds at 10-15 kt with some 
gusts up to 20 kt on the southern waters. Seas 4 ft or less. 
Small Crafts on the southern outer waters have ended. Generally 
good visibility. 

Tonight and Friday...With the front stalled across the waters, 
expect light winds, mainly E-NE on the eastern waters and light 
southerly on the southern waters. Seas 3 ft or less. Good 
visibility.

Outlook /Friday Night through Monday/...

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, chance of thunderstorms, patchy
fog. 

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

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers, isolated thunderstorms. 

Sunday through Monday: Winds less than 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this 
     evening for MAZ020-022>024.
RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this 
     evening for RIZ006>008.
MARINE...None.

&&

$$
SYNOPSIS...KJC/EVT
NEAR TERM...EVT
SHORT TERM...EVT
LONG TERM...KJC
AVIATION...KJC/EVT
MARINE...KJC/EVT