Lexington Weather

Lexington, Massachusetts, USA

71°F
6/25/2019 4:04pm 
  • Lexington Conditions: Light rain, Mist, Overcast
  • Temperature: 70.9°F / 21.6°C 
  • Dew Point: 69.4°F 
  • Relative Humidity: 95
  • Wind: W 3 mph, 10-min avg: 2 mph, gust: 8 mph
  • Barometer: 29.83 in Steady
  • Visibility: 6 miles
  • Rain Today: 0.07 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 251944
AFDBOX

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

.SYNOPSIS...
An approaching cold front and wave of low pressure will bring a
period of widespread showers and a few embedded thunderstorms 
with locally heavy rainfall this afternoon, heaviest over
eastern CT into RI and eastern MA. 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 and Monday, followed by warming Tuesday.

&&

.NEAR TERM /UNTIL 8 PM THIS EVENING/...
*** Locally heavy rainfall this afternoon along with a low risk
 of localized urban and poor drainage flooding, with greatest
 risk over RI and eastern MA ***

215 PM update ...

No change from previous forecast. Current thinking based on
latest satellite and radar trends along with Hi Res guidance is
for heavy showers with embedded thunder to impact eastern CT/RI
and eastern MA this afternoon and possibly impacting the late
day commute with localized street/highway flooding. Earlier
discussion below.

===============================================================

Warm front with anomalous moisture plume (PWATs up to 2.1 inches
from Long Island southward per SPC mesoanalysis) approaching
southern New England from the southwest. Two areas of heavy rain
at the moment, first area is over northwest MA. This region will
lift north into VT/NH as low to mid level jet advects northeast. 

Greater area of interest is rain with embedded convection over
Long Island southward across the ocean. Unfortunately none of
the model qpf or simulated radar reflectivity fields capturing
this area very well. Some of the 00z CAMs model doing a better
job but differ on simulations thru this afternoon with some
outcomes increasing convection while others weakening.

However model mass fields all agree on low level jet fueling
this convection will strengthen this afternoon as it traverses
northeast across RI and eastern MA. This is being verified on
latest GOES16 satellite imagery with cloud tops cooling and
lightning activity picking up. Thus our concern for heavy rain
and flood threat continues across RI into eastern MA this
afternoon. Previous forecast captures these trends nicely so no
major changes with this update.

&&

.SHORT TERM /8 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
Tonight... 
Models are indicating a wave developing on the front this 
evening and tracking south of New Eng which will likely allow 
showers to persist well into the evening across SE New Eng and 
Cape/Islands. Otherwise, improving conditions tonight from west 
to east as deep moisture and PWAT plume move to the east. Areas 
of low clouds and patchy fog expected, especially in eastern New
Eng. Lows will be mostly in the 60s.

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. Otherwise, partly to mostly sunny skies 
expected in the interior with increasing sun during the 
afternoon in the east. Models are generating marginal 
instability late in the day in the west with CAPEs 500-1000
J/kg. A few showers or an isolated t-storm is possible late in 
the day across western and central MA as another shortwave 
approaches from the west. 

850 mb temps 15-16C will translate to highs into the upper 80s
in the CT valley and 80-85 across the rest of SNE away from the
coast. Cooler 925 mb temps noted in eastern MA which should  
keep temps in the 70s in east coastal MA as sea breezes develop.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...

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.

Concerns... 

Thursday-Friday...

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 
spots.

Saturday...

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.

Sunday...

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.

Monday-Tuesday...

The uppper 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. 

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
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. 

1815z update ...

Thru 00z ... high confidence on trends, lower on exact details.

Showers with embedded thunder and heavy rain over southeast CT
will hold together and may intensify as it moves across RI and
southeast MA thru 21z. Lowest cigs/vsbys will accompany this
activity. 

After 00z ... high confidence on trends, lower on exact details.

Any heavy showers with embedded thunder at 00z should be
confined to Cape Cod and the Islands. A second but smaller area
maybe over CT but both areas will be trending
downward/weakening. Then next area of interest will be areas of
dense fog in IFR/LIFR along the south coast. 

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.

&&

.MARINE...
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. 

Winds and seas below SCA thresholds through the period. 
However, southerly gusts may reach 20+ kts this afternoon and 
evening as a low level jet moves across the waters. Light 
northerly winds Wed becoming onshore in the afternoon as sea 
breezes develop. Vsbys reduced this afternoon and evening in 
showers and fog. Isolated t-storms possible. Patchy fog may 
linger into Wed morning across eastern and SE waters. 

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. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...WTB/KJC/Nocera
NEAR TERM...Nocera
SHORT TERM...KJC
LONG TERM...WTB
AVIATION...WTB/Nocera
MARINE...WTB/KJC