Lexington Weather

Lexington, Massachusetts, USA

70°F
8/17/2022 6:58pm 
  • Lexington Conditions: Overcast
  • Temperature: 69.6°F / 20.9°CColder 0.6°F than last hour.
  • Dew Point: 57.4°FDecreased 1.8°Fsince last hour.
  • Relative Humidity: 65%Decreased 3.0% since last hour.
  • Wind: Calm, 10-min avg: Calm, gust: 3 mph
  • Barometer: 29.85 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 120253
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1053 PM EDT Thu Aug 11 2022

.SYNOPSIS...
Mainly dry, seasonable weather expected Friday with a low chance
for showers across Cape Cod, southeast MA, and RI. Shower
chances linger into Saturday morning across this region, but 
our first shot of drier, cooler fall like air comes Saturday 
morning, as lows drop into the 50s across much of interior 
southern New England! Broad low may brush southeast New England
early on Saturday, but overall high pressure builds in keeping 
the weather dry and quiet. Unsettled for much of the upcoming 
week with the potential for a multiple day rain event. Cooler 
than normal temperatures anticipated.

&&

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

1045 PM Update...

Monitoring a strong thunderstorm capable of producing small hail
and locally heavy downpour across Hartford into Tolland county 
in CT. Made some adjustments to the PoPs for the next few hours
based on radar trends. Forcing is weak but there is around 20 kt
of effective bulk shear across CT and RI to help organize
updrafts. Also noted around 700-900 J/kg of downdraft CAPE so
there is a low probability of gusty winds from the downpours or
thunderstorm. 

Additionally, trimmed the areal and temporal coverage of the
fog across southeast MA based on obs and near-term trends. 

PREVIOUS DISCUSSION...

745 PM Update...

Have added slight chances of precip across SW MA/CT to account
for the showers and perhaps an isolated thunderstorm or two
bubbling up across eastern NY/far western MA. Leaned toward the
latest ARW, which is doing well based on the current situation.
Otherwise the forecast remains on track at this point, but will
need to monitor the stratus/fog pushing into the Cape/Islands
and perhaps eastern MA. Some guidance suggests it could be dense
at times especially across the Cape/Islands, but think this 
will be short lived as flow shifts and becomes more NW/N late.

Previous discussion...

Rain showers have all but come to an end across Cape Cod and 
Nantucket as clearing continues to push east. Fog and stratus 
is expected to redevelop over Cape Cod and the Islands given 
high SSTs, low dewpoint depressions, at or below 1F in some 
cases, and light winds. Fog is anticipated to be limited to the 
Cape, Islands, and immediate southeast coastal plains as dry air
will begin to filter into interior southern New England this 
evening and overnight. As of this writing, daytime mixing has 
allowed dewpoints to drop into the 50s across portions of 
interior southern New England, in areas such as Fitchburg and 
Orange. While a brief surge in dewpoints is expected this 
evening along an approaching cold front from the northwest, 
conditions will remain sub-optimal for fog formation away from 
the coast. 

The aforementioned cold front is currently located over upstate
New York, but will begin to push into our region around sunset.
CAMs are in agreement that this front will bring the possibility
of a spot shower or weak thunderstorm to our area overnight. The
12Z CAMs present two likely scenarios with both the FV3 and NAM
3K showing the development of a more potent line of showers 
across the north shore of Massachusetts, and the HRRR draping a 
line of showers across Connecticut and central MA. Embedded 
thunder cannot be ruled out in the strongest cells with elevated
CAPE values approaching 1000 J/KG. Overall, not expecting a 
widespread convective event this evening, but cannot rule out 
the potential. 

Winds will be variable again overnight, but will ultimately take a 
northerly stance by sunrise. With high astro-tides in the mix and a 
high tide expected around midnight, it is possible that some minor 
splashover may be experienced in our most coastal flood prone 
locations. This evening's tide looks to be our only shot for 
splashover/minor flooding in the foreseeable future.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
A rather "fall-like" 500mb trough begins to dig into our region on 
Friday behind Thursday evening's cold front. While conditions look 
to stay mostly dry during the day on Friday, the trough will begin 
to invert, which coupled with a few weak bubbles of low pressure to 
our east may bring unsettled, showery weather to parts of our 
region. Cape Cod and the Islands again have the best chance for 
showers on Friday due to their proximity to the best available 
moisture and forcing, but shower chances may expand into southeast 
MA and RI during the day Friday. Even so, we are not expecting a 
washout, with just the chance for scattered showers. Cloudiness will 
accompany the chance for showers across eastern MA and Rhode Island, 
but much of western MA and central CT will experience much sunnier 
skies; similar to what we have experienced the last few days. 

Fog will dissipate across southeast and Coastal MA after sunrise as 
we begin to mix out. Given mostly sunny to partly cloudy skies 
across our western zones, daytime mixing will again allow dewpoints 
to drop into the 50s west of the Worcester area. This drop in 
dewpoints will be aided by much drier 850mb air rushing into the 
region behind the departing cold front. Some guidance, namely the 
ECMWF, actually mixes high dewpoints out all the way to the coastal 
plain , opening the door for dews in the upper 40s late Friday 
afternoon and evening! While the ECMWF presents the most aggressive 
solution, other guidance, like the GFS, Canadian, and NAM, are all 
in line with a substantial drop in humidity by Friday afternoon; in 
general widespread dewpoints in the 50s can be expected, especially 
west of I-495.

Hi-res guidance, along with the ECWMF, keeps these low dewpoints 
with us overnight Friday, which opens the door for some of the 
chilliest overnight lows we have seen in weeks! Widespread sub 60F 
lows will be observed Saturday morning across interior southern New 
England, but clear skies, relatively light winds, and nice radiative 
cooling across western MA may drop temperatures into the upper 40s 
across the highest elevations of Franklin and Worcester 
counties! 

Cape Cod looks to stay immune to these almost September like lows as 
cloud cover and moisture from the inverted trough set-up keep shower 
chances alive overnight Friday into Saturday. Overnight low 
temperatures across the Cape and coastal plain will remain above 
60F. Shower chances look to be limited the this region.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights

* Could see spotty showers across eastern areas especially early 
  Sat, but overall should be dry and quiet through the weekend. 

* Unsettled for much of the upcoming week as the pattern becomes 
  more fall-like. Chances of rain especially in the Tue-Thu 
  timeframe. Temps near to below seasonable.

Saturday through Sunday...

Trough/cutoff over New England initially lifts into northern New 
England/Nova Scotia by late Saturday. Behind the trough a shortwave 
ridge builds into the eastern Great Lakes late on Saturday and over 
New England on Sunday. Will have a broad low offshore lifting toward 
Nova Scotia on Saturday as high pressure builds in. The high pushes 
offshore on Sunday, but will still nudge into southern New England.

Expecting much of this period to be dry and quiet weather wise 
across southern New England. Do think there is the opportunity for 
some spotty showers across the Cape/Islands and southeast MA due to 
the broad low offshore especially during the AM. Expect this 
activity to push offshore as the low lifts further off. Even the 
current PoPs could be a bit overdone given the NW flow aloft drying 
out the mid levels, but slight chance to chance does not seem 
unreasonable at this point. 

Will see cooler than seasonable temps across the region on Saturday. 
Expecting NW cold air advection to keep temps in the roughly 8-10 
degree Celsius range. Not completely out of the question we are a 
bit cloudier than currently forecast as well given the cutoff 
overhead combined with diurnal heating, but have not adjusted from 
the NBM at this point. May need to in future updates. The result 
will be cooler than normal highs with readings in the mid/upper 70s 
for most. Anticipate there will be some 80 degree readings over the 
CT River Valley. 

Drier weather in store for Sunday, but warmer as the high shifts 
offshore and flow becomes more W/SW due to the position of the high. 
This will result in 850 hPa temps warming to roughly 10-13 degrees 
Celsius. Temps will be more seasonable for this time of year with 
highs in the upper 70s to mid 80s. 

Monday through Thursday...

Unsettled through much of this period. A trough digs into the 
eastern Great Lakes/New England on Monday. This feature then cuts 
off and remains nearby/overhead for much of the week. 

At this point appears that early on Monday will start off relatively 
dry, but will have increasing chances of rain as the day progresses 
due to low pressure developing to the south of us. At this point 
think that the activity over our region will be more spotty on 
Monday precip wise and should not be a washout especially as the 
best upper forcing should be to the west of us. The NBM seems 
reasonable at this point with increasing chances of precip and temps 
in the mid 70s to low 80s. Do suspect this may need to be lowered 
due to persistent onshore flow, but will have to get a better handle 
on the sky cover in place.

Have still stuck with the NBM in the Tuesday-Thursday timeframe. It 
is here where we will have the best shots for potentially 
significant beneficial rains. Expecting the cutoff to slide closer 
or over southern New England on Tuesday into Wednesday before 
potentially lifting out on Thursday. There is some question with how 
quickly this feature lifts out, which at this point is where most of 
the uncertainty lies especially later in this period. Think that the 
greatest chance for widespread rains is on Tuesday and Wednesday as 
we will have PWATs rising to roughly 1-2 inches with the highest 
values across eastern areas. Warm cloud layers depths appear to be 
in roughly the 3-4 km range and given the forcing there could 
potentially be some heavy downpours at times. Instability appears 
limited and honestly looks more like a stratiform rain pattern, so 
have kept out thunder at this point. Temperatures generally in the 
70s on Tue/Wed, but may become more seasonable by Thu.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels:

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

Tonight...Moderate confidence.

Expecting most terminals to remain VFR tonight. The exception is
the Cape/Islands and perhaps the immediate eastern MA coast.
Expecting MVFR to LIFR stratus/fog to spread in due to onshore
flow. Should see visby fall to roughly 1-2 SM other than ACK
where it may go down to 1/2 SM to 1/4 SM at times. Was not
confident enough at this point to bring down to 1/4SM, but may
be necessary at ACK. Will note that BOS appears to be on the
edge of this MVFR/IFR stratus deck. Have hinted at this point,
but keep VFR for now. Should see this risk go away at BOS as
winds flip to the W/NW between 5-6Z. 

More immediately will need to keep an eye on the shower activity
spreading into SW MA and CT. Leaned toward ARW which slides this
activity through BAF/BDL. Not out of the question there could be
a rumble of thunder or two, but anticipate the activity to
diminish as instability wanes. Winds will remain rather light 
and variable overnight, but will trend NW to N by sunrise. 

Friday...High confidence; moderate confidence for Cape and
Island terminals.

Mainly VFR, with areas of MVFR lingering across the Cape and 
Islands for much of the day. Think the best shot of breaking out
will be late AM into the afternoon. Cannot completely rule out a
spot shower across RI/SE MA. Winds N to NE at 5-10 kts.

Friday night...High confidence.

VFR with increasing chances for showers across RI and southeast
MA. Winds out of the N/NNE at 5-10 kts. Could see a few gusts of
15-20 kts across the Cape/Islands toward daybreak.

KBOS...High confidence in trends, moderate in exact timing. Main
concern is the MVFR and borderline IFR stratus leaking in
between 00-06Z. Once winds shift to the W/NW should see this
threat go away. Have hinted at the possibility for now, but keep
conditions VFR.

KBDL... Moderate confidence. May see a spot shower and perhaps a
rumble of thunder between 01-04Z. Have as VCSH for now, but may
need to up to VCTS. Uncertain due to the waning instability.
Afterwards confidence high with VFR conditions and light N/NW
winds.

Outlook /Saturday through Tuesday/...

Saturday through Sunday Night: VFR. 

Monday: VFR. Slight chance RA.

Monday Night: VFR. Chance RA.

Tuesday: Mainly MVFR, with areas VFR possible. Chance RA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight into Friday...High confidence.

Winds and seas will remain below SCA criteria during this period. 
Low pressure will track across the far southern waters today, 
bringing light and variable winds along with periods of rain showers 
during the day into the early evening hours. Localized fog is 
also possible overnight tonight into Friday morning especially 
across the southern waters into Cape Cod and the Islands. 

Friday night...High confidence.

Increasing N to NNE winds. Should see speeds of 10-15 kts late
with gusts up to 20 kts. Seas 2-4 ft. Will have shots of
scattered rain mainly across the south coast.

Outlook /Saturday through Tuesday/... 

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

Saturday Night through Sunday Night: Winds less than 25 kt. 

Monday: Winds less than 25 kt. Slight chance of rain. 

Monday Night: Winds less than 25 kt. Chance of rain. 

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain.

&&

.TIDES/COASTAL FLOODING...
A Coastal Flood Advisory in place from 01Z to 07Z this evening 
due to high astro tides and developing northerly flow. Winds are
expected to remain light overnight, but splashover and 
inundation of up to one half foot is possible in our most 
coastal flood prone areas such as Morrissey Boulevard and 
Nantucket Harbor. Tonight's high tide is expected to be the only
high tide of concern for the foreseeable future.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Coastal Flood Advisory until 3 AM EDT Friday for MAZ007-015-
     016-019-022-024.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...BL/KS
NEAR TERM...BL/Chai/KS
SHORT TERM...KS
LONG TERM...BL
AVIATION...BL/KS
MARINE...BL/KS
TIDES/COASTAL FLOODING...
      

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