Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 270229
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1029 PM EDT Tue May 26 2020

.SYNOPSIS...
High pressure will move offshore tonight and will provide
summerlike warmth and mugginess away from the coast for the 
rest of the work week. We end the week with showers and an 
isolated chance for a thunderstorm or two on Friday as a cold
front crosses the region. High pressure will bring drier and 
cooler weather for the second half of the weekend into early 
next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

1010 PM Update...

* Expanded Dense Fog Advisory to include southern half of RI and
 southern Bristol county in MA. 

Previous forecast is largely on track. With a light southerly
flow and a warm, moist air mass being advected over relatively
cold ocean waters, the main concern tonight is how far north the
fog bank is going to get. Based on the latest metar obs and
nighttime microphysics satellite imagery, the fog bank is making
its way up Narragansett Bay and is just south of Providence.
So have decided to expand the Dense Fog Advisory to include Kent
and Washington County in RI and the southern half of Bristol
County in MA. May have to expand the Dense Fog Advisory further
north but will monitor trends in the next few hours. Another
area of interest is Eastern MA coast where the marine stratus is
hugging or just off the coast. With a predominant southerly
flow, it will be hard for the fog to get into Boston like last
night but Cape Ann would be most likely to get into the action
given its geography. 

The other main change was to bring the dew points up to match
the current obs. 

PREVIOUS DISCUSSION...

7 PM Update: Forecast is on track.

* Dense Fog Develops on Cape Cod and the Islands Tonight

Satellite loop shows low clouds heading inland onto South Coast,
Cape Cod, and Islands this evening. High-res guidance slowly
advances this cloud deck inland tonight, mainly as far north as
Hartford, Providence, and Plymouth though it could make it into
Boston as well for a time. 

Webcams indicate fog is still lurking offshore south of the
islands, but expect it to become more widespread near South
Coast this evening due to low temp/dewpoint spreads and light
S/SW winds. Cape Cod and Islands are typically favored for dense
fog in this pattern so we are continuing the Dense Fog Advisory
for tonight. It's possible dense fog also makes it onto south
coastal RI and MA but still not confident enough to expand the
advisory just yet.

Lows in 50s and lower 60s look good.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
* Very Warm & A Bit Humid Away From The South Coast Wednesday

Wednesday...

The anomalous upper level ridge with height fields around 590 DM 
will remain in control of our weather. 850T are bit milder than 
today around +15C to +16C. This coupled with southwest flow should 
yield afternoon high temps in the upper 80s to near 90 away from 
cooling marine influence of the south coast. Unlike today, there 
will not be a sea breeze in Boston so they will get in on the summer-
like warmth as well. It will be a bit muggy too with 60+ dewpoints, 
but southwest wind gusts of 20 to 25 mph during the afternoon which 
will help a bit. Dry weather will persist with the upper level ridge 
in control.

Wednesday night...

Model cross sections indicate low clouds developing from south to 
north Wednesday night. They will likely be more widespread, covering 
most if not all of the region by daybreak. We may even see some 
drizzle develop with some fog possible too especially near the south 
coast. Overnight lows will only drop into the lower to middle 60s in 
most locations with some 55 to 60 degree readings near the south 
coast.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Thursday...

Morning fog likely to start off across coastal Rhode Island 
through the south shore of Cape Cod. One of the biggest 
questions for Thursday will be the extent to the cloud over. 
Models show a decent swath of moisture between 925 and 850 mb...
especially across western MA/CT. PWATS range between 1.5 and 
2.0 inches across the Mid Atlantic and up into southern New 
York. With some weak forcing an isolated shower isn't out of 
the question. At 18z the 850mb temperatures climb to 14 degrees 
(C), indicating afternoon highs could reach the mid-80s. Those 
along the coast will have cooler afternoon highs in the upper 
60s to low 70s. If clouds don't erode those temperatures will be
difficult to produce, but, with a predominant southwesterly 
flow we should be capable of mixing them out. 

Friday...

A cold front will spark scattered showers and possibly an isolated 
thunderstorm. PWATS range from 1.5 to 2.0 inches across the Mid 
Atlantic to Southern New England. Thunderstorms may develop NW of 
our area where better instability is present and could move into 
western Massachusetts during the mid-afternoon with locally heavy 
downpours. 

Saturday...

Temperatures still remain rather warm on Saturday with 850 mb 
temperatures between 8 and 10 degrees Celsius. A few morning 
showers could linger depending on the speed the front moving 
offshore. The greatest chance of seeing a shower will be in 
eastern Massachusetts, the Cape, and Islands. Clouds clear from 
west to east late in the day as drier and cooler air moves in 
from Quebec. 

Sunday/Monday...

An area of high pressure moves out of from the Great Lakes Region 
Sunday into Monday with an upper-level through moves across New 
England and eastern Quebec. The 500 mb temperatures above Southern 
New England are -12 degrees Celsius! That will result in cooler
than normal temperatures in the middle and upper 60s.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels...

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

00z TAF Update...Moderate confidence.

CIGS steadily lower to IFR across much of CT, RI, and SE MA
tonight as well as near Berkshires, but conditions remain VFR
elsewhere. S/SW winds prevail as sea breezes subside. 

IFR CIGS improve to VFR Wed morning as low clouds retreat 
offshore, with process being the slowest across the Cape and 
Islands. Otherwise, VFR conditions expected with SW wind gusts 
into the lower 20 knots during the afternoon across the 
southeast New England coast. 

Low clouds return Wed night from south to north. Appears this 
time the lower clouds will overspread most if not all of the 
region. So expect mainly lower end MVFR to IFR conditions. LIFR 
conditions with fog may impact the south coast, Cape, and 
Islands.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday: Mainly VFR, with local IFR possible. Slight chance
SHRA.

Thursday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SHRA, chance TSRA.

Friday through Friday Night: Mainly VFR, with local IFR
possible. Breezy. Chance SHRA, chance TSRA.

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

Saturday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Sunday: VFR. Breezy.

&&

.MARINE...
Tonight through Wednesday night...High confidence. S to SW winds 
will continue through Wed night, but winds/seas should generally 
remain below SCA thresholds. Some choppy seas may develop Wednesday 
afternoon though with some lower 20 knot wind gusts near shore.
The main issue will be for areas of fog that will be dense 
especially during the overnight to mid morning hours.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Slight chance of rain showers.

Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers. 

Friday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers, chance of thunderstorms. 

Friday Night: Winds less than 25 kt. Areas of rough seas.
Thunderstorms likely, chance of rain showers. 

Saturday: Winds less than 25 kt. Local rough seas. Chance of
rain showers, chance of thunderstorms. 

Saturday Night: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 8 AM EDT Wednesday for MAZ020>024.
RI...Dense Fog Advisory until 8 AM EDT Wednesday for RIZ003>008.
MARINE...None.

&&

$$
SYNOPSIS...Frank/Gaucher
NEAR TERM...Chai/JWD
SHORT TERM...Frank
LONG TERM...Gaucher
AVIATION...Frank/Gaucher/JWD
MARINE...Frank/Gaucher