Lexington Weather

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 251817
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
217 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.

&&

.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 6 PM WEDNESDAY/...
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 /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights...

* Weak frontal systems will bring scattered showers/isolated 
  thunderstorms Wed night through Thu night
* Dry conditions with warm temperatures on Friday
* Another round of scattered showers/spotty thunderstorms late  
  Friday night into the upcoming weekend with near or slightly 
  below normal temperatures

Overview...

Nearly zonal mid level steering flow will continue across the 
northern tier states through the end of the work week, with 
potential for spotty showers and thunderstorms with passing 
frontal systems. Medium range models signaling a digging cutoff 
H5 low moving out of Hudson Bay into central Quebec then 
shifting across northern New England around Saturday night into 
Sunday, which could bring another round of scattered showers and
thunderstorms along with cooler than normal temperatures for 
late June into early July. 

Noting mid level heights generally above seasonal normals for 
most of this period. However, with the passing cutoff H5 low 
pushing across the region next weekend, lowered heights will 
bring temperatures down close to normal or as much as 5 degrees 
below normal. 

Details... 

Wednesday night through Friday...

Several fast moving, mid level short waves will pass across the
northeast during this timeframe. Noting one wave, with its 
associated cold front, pushing across NE and central Mass into N
central CT Wed evening. PW values during this timeframe running
up to 1.4 inches or so, along with marginal instability. Total 
totals forecast around 50, TQ values in the upper teens and K 
indices around 30. So, have mentioned isolated to slight chance 
for thunderstorms across the interior. As this front shifts E, 
it pushes into more stable air so the chance for 
showers/thunderstorms decreases toward daybreak Thu. 

Current models suggest a brief break in the action Thu morning 
through midday, then another short wave along a slow moving 
front may bring scattered showers/isolated thunder across E Mass
around midday or Thu afternoon, which should shift offshore but
could bring some leftover showers or a rumble of thunder into 
Thu night across SE coastal Mass to Cape Cod and the islands. 
This should weaken as it shifts offshore into more stable 
conditions.

A patch of dry air may cross the region on Friday. Noting very 
good mixing from H85 down to H95 on W-NW winds. Will see temps 
rise to the mid-upper 80s away from the immediate coast, and may
even see a few spots eclipse 90 degrees. 

Friday night through Sunday...

Cutoff mid level low pushes SE out of Hudson Bay and central 
Canada, crossing the region Sunday. May see scattered precip 
move in late Friday night, along with the threat for isolated 
thunderstorms. A surface reflection of the cutoff system should 
cross late Sat night or Sun. Could see some developing strong 
instability across the region midday Sat into Sat evening, with 
CAPEs up to 900 J/kg, K indices in the lower 30s and TQ values 
near 20. So, have mentioned scattered thunder as well.

Showers will linger into Sunday as the mid level low and 
surface front pushes across. Looks like cooler air works in with
lowering mid level heights. Expecting temps to only reach to 
70s.

Sunday night-Monday...

As the H5 low pushes SE off the coast Sunday night, should see 
showers head off the E coast around or after midnight. Winds 
shift to NW and may become gusty across Cape Cod and the S 
coast. Overnight lows will range from 55 to 60, close to 
seasonal levels. 

For now, looks like mainly dry conditions return on Monday, 
though some question whether weak short waves will wrap S 
around to departing cutoff mid level low. Will be cool 
especially along E coastal areas with onshore winds as the mid 
level systems pushes S of Nova Scotia. Have carried a mainly 
dry forecast.

&&

.AVIATION /18Z 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/...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 /Wednesday Night through Saturday/...Moderate confidence.

Wednesday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, isolated TSRA, patchy BR.

Thursday: Mainly VFR, with local MVFR possible. 

Thursday Night through Friday: VFR. 

Friday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA, isolated TSRA.

Saturday: Mainly VFR, with areas MVFR 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/...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 /Wednesday Night through Saturday/...Moderate confidence.

Wednesday Night: Winds less than 25 kt. Patchy fog. Visibility
1 to 3 nm.

Thursday: Winds less than 25 kt. Local visibility 1 to 3 nm.

Thursday Night: Winds less than 25 kt. Slight chance of rain
showers, patchy fog. Local visibility 1 to 3 nm.

Friday: Winds less than 25 kt. 

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

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

&&

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

&&

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