Old Res Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 210836
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
436 AM EDT Wed Apr 21 2021

.SYNOPSIS...

A strong cold front will bring a period of showers and scattered 
thunderstorms this afternoon and early evening. Some storms will be 
capable of producing locally damaging wind gusts. Windy and much 
cooler weather follows behind the front late tonight into Thursday, 
then a warming trend Friday and Saturday. A coastal storm will 
likely bring rain and coastal wind Sunday followed by blustery and 
drier conditions Monday. Mild and dry weather expected into midweek 
as high pressure builds into the region. 

&&

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

Highlights

* Slight risk for severe thunderstorms across western MA and much of 
  CT. Expecting a line of storms to move in from the west bringing a 
  risk of damaging wind gusts during the afternoon into the early 
  evening. Line anticipated to weaken as it traverses eastward. 

Shortwave trough located over the central/eastern Great Lakes early 
today. The trough will lift northeastward across the eastern Great 
Lakes into northern New England by the evening. Southern New England 
will remain within the warm sector for much of the day until a cold 
front moves through. The surface low associated with the front will 
lift from PA this morning northeastward through western MA/CT into 
northern New England.

Main concern for southern New England today is the risk of strong to 
severe thunderstorms. The risk is maximized across western MA/CT as 
highlighted by the SPC Day 1 Slight risk, but still not completely 
out of the question across areas further east. Expecting the 
low/front to provide the lift necessary for thunderstorm initiation. 
Ahead of the low will see southerly flow advect in upper 40 to low 
50 degree dew points. Bulk shear values in the 0-6 km layer will be 
on the order of 40-60+ kts ahead of and along the forcing. 
Orientation of the shear to the forcing is mostly parallel, so 
anticipating convective mode to be linear. Instability values are on 
the order of a few hundred J/kg of MLCAPE with little MLCIN. The 
limited instability should be compensated for by the strong forcing 
for thunderstorm development today. Low level lapse rates are on the 
order of 6-8 degrees Celsius per km with mid level lapse rates 
falling in the 6-7.5 degree range. 

Given this setup and forecast soundings am thinking the main risk 
will be damaging wind gusts. Cannot completely rule out some hail, 
but there just isn't a lot of CAPE within the hail growth zone. 
Lastly, SPC indicates there is a low risk of a tornado, but am not 
sold on this given the 0-1 km SRH generally remains out ahead of the 
forcing and does not overlap. If the forcing and SRH can overlap a 
bit then the risk is there especially with the low LCL heights and 0-
3 km CAPE over 100 J/kg in place across western MA and CT. CAMs show 
a line developing near/just west of the Capital District in NY and 
progressing eastward. Have kept the gusty wind mention in the 
forecast to highlight this as there is a 40-50 kt 925 hPa southerly 
jet moving across the area today. Have heavily leaned toward this 
guidance in the forecast for PoPs, QPF, wind speeds and gusts today. 
Does appear that based on the timing of the front moving in that the 
instability will be waning, which should limit how far east the 
severe risk extends and that the convective line weakens as it 
progresses eastward. 

Should see some fairly good boundary layer mixing out ahead of the 
system moving in, which given the strong low level jet will bring 
another day of gusty winds. Have gone with the 75th percentile of 
guidance for high temperatures today given some uncertainty on how 
quickly mid to high clouds move in. The result is highs in the 60s 
for much of the region with some 70 degree readings in eastern MA. 
Could see a few scattered showers this morning, but the highest risk 
for any precipitation will be this afternoon into the evening.

&&

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

Highlights

* Any lingering showers/storms come to an end as the cold front
  exits this evening. Gusty WNW to NW winds expected in wake of
  the front with much colder temperatures.

* Dry, but strong W to WNW winds across southern New England on
  Thursday. Not out of the question a few spots may need a Wind
  Advisory. 

Expecting a closed off circulation to develop at 500 hPa as it
lifts into northern New England from the eastern Great
Lakes/Ontario/Quebec. This feature will lift northeastward
toward New Brunswick on Thursday, but southern New England will
remain under cyclonic flow. At the surface will remain pinched
between the exiting system and high pressure building over the
Southeastern US. 

Tonight...

Cold front moves through and will bring an end to any
showers/storms during the evening. Still have leaned on the CAM
guidance as there if fairly good agreement. Expecting strong
cold air advection in wake of the front with 925 hPa air of -3
to -5 degree Celsius air moving in with winds of 30-40 kts. It
will not be too difficult to mix this down, so have gone with
the 95th percentile of guidance for wind gusts. Should see
speeds of 25-35 kts with the higher gusts after midnight. Much
colder with low temperatures bottoming out in the 20s across the
higher terrain the low to mid 30s elsewhere.

Thursday...

Strong W/WNW winds across southern New England. The pressure
gradient remains quite tight with the exiting system and high
pressure well to the southwest. Given the setup went with the 
95th percentile of guidance as BUFKIT soundings show a well
mixed boundary layer. Gusts are in the 35-40 kt range for much
of southern New England, so anticipate a few spots will need a 
Wind Advisory in future updates. Have held off for now as most 
locations are borderline advisory criteria at this point in 
time. 

Given the strong mixing with WNW flow. Increased high
temperatures to the 75th percentile of guidance to account for
some downsloping. However, with the cold airmass in place most
locations will struggle to get into the mid 40s. Do have some
upper 40 degree readings across the CT River Valley and eastern
MA. On top of this have also lower dew points/RH values and this
may lead to fire weather concerns for portions of the CWA. See
the Fire Weather section below for more details on this.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...

Highlights

* Gradual warming trend Friday into Saturday
* Rain likely Sunday along with gusty winds near the coast
* Blustery and mainly dry weather Monday

Thursday night into Saturday... 

Blustery and cold conditions Thu night as strong pres gradient 
remains in place. Lows mostly in the mid 30s but close to 30 over 
higher terrain with wind chills dropping into the 20s. 

Still blustery on Fri with deep and well mixed boundary layer up to 
800 mb supporting 25-35 mph west wind gusts but with moderating 
temps under abundant sunshine. Highs will recover well into the 50s 
to near 60 in the coastal plain and interior valleys. Then further 
moderation on Saturday with less wind as high pres builds to the 
south. 925 mb temps around 10C will support highs well into the 60s 
to near 70 degrees, but cooler along the south coast. Sunshine will 
give way to increasing afternoon clouds on Sat ahead of next system. 

Saturday night into Sunday night...

Good agreement among deterministic and ensemble guidance of a 
coastal low impacting the region late Sat night into Sun evening. As 
expected at this time range, there is spread in the guidance with 
the low track but enough clustering near or south of SNE to suggest 
a period of heavy rain is possible. Details on timing and location 
of heaviest rain still have to be worked out. There is no 
antecedent cold air ahead of this system or during the event so 
ptype will be all rain. Gusty winds will also be a concern along the 
coast as SE low level jet ahead of the storm lifts across the 
region. 

The risk for coastal flooding will depend on timing of the low level 
jet and highest storm surge with respect to the high tide. The 2 
tide cycles of note are the Sunday morning and Sunday evening high 
tide. The Sun evening high tide is the higher of the 2 tide cycles. 
Latest guidance suggests the surge and seas may not be built up 
enough for much impact during the Sun morning high tide, while winds 
may have shifted to NW before the Sun evening high tide. However, 
timing of the wind and surge is uncertain at this time range and 
will be important to the coastal flooding risk. 

Monday and Tuesday...

Trend will be for improving conditions Mon with blustery NW winds. 
However, there is uncertainty with how quickly the closed low exits 
New Eng which will impact temps and shower threat for Mon. A slower 
solution would keep threat of showers around on Mon, especially in 
the east, along with cooler temps Mon and Tue. If closed low exits 
further to the east, warmer temps to the west will spill into 
interior SNE on Tue.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels.

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

Through 12Z...High confidence

VFR with winds shifting to the S/SE. May become gusty across
portions of the south coast toward daybreak. Could see some
15-20 kt gusts. Cannot rule out a few showers spreading into
western MA/CT with cloud bases lowering to 5-10 kft. 

Today...High confidence in trends moderate in timing.

VFR to start with gusty southerly winds developing by mid
morning. Will gradually see MVFR and borderline IFR as showers
and thunderstorms move through generally in the afternoon to
early evening hours. As any storms move through could see
conditions fall locally to LIFR. Have only mentioned thunder in
the BAF/BDL TAF at this point in time as instability will be
waning and should see the line weaken as it progresses 
eastward. As the cold front moves through will see winds shift 
to the SW/W. Gusts on the order of 25-30 kts throughout the day.


Tonight...High confidence.

Any showers come to an end during the evening and should see any
lingering MVFR conditions improving to VFR. Winds shift to the
WNW/NW and will be strong at 10-15 kts. Gusts of 25-30 kts
during the evening increasing to 30-35 kts late.

Thursday...High confidence.

VFR with strong WNW/NW winds. Speeds of 15-25 kts and gusts of
30-40 kts. Could see some cloud bases of 4-6 kft during the
afternoon.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF. Moderate confidence on the 
exact timing of thunderstorms moving through this afternoon and 
how strong gusts will get as storms move through. 

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Strong winds with local gusts up to 40 kt.

Friday: VFR. Windy with gusts up to 35 kt. 

Friday Night through Saturday: VFR. Breezy. 

Saturday Night: Mainly VFR, with areas MVFR possible. Breezy.
Chance RA.

Sunday: Mainly MVFR, with areas IFR possible. Windy with local
gusts up to 30 kt. RA likely.

&&

.MARINE...

Have extended the Gale Warning into Thursday night as Gale Force
gusts persist. 

Today...High confidence in much of the forecast, but uncertain
on the thunderstorm risk as the line should weaken as it moves
toward the waters.

Southerly winds increasing to 15-20 kts by this afternoon with
gusts of 20-30 kts. Not out of the question there are a few
thunderstorms late in the day as a cold front approaches. 

Tonight...High confidence

Strong cold front crosses the waters during the evening. Expect
any showers/storms to end during the evening. Winds shifting to
the W/WNW once the front passes. W/WNW wind speeds increasing to
around 30 kts after midnight with gale force gusts of 35-40 
kts. Seas increasing to 6-10 ft.

Thursday...High confidence

Strong westerly winds persist across the waters throughout the
day with speeds between 25-30 kts. Should see gale force gusts
around 40 kts. Seas between 6-10 ft.

Outlook /Thursday Night through Sunday/...

Thursday Night: Strong winds with gusts up to 40 kt. Rough seas
up to 11 ft. 

Friday: Strong winds with gusts up to 35 kt. Areas of rough
seas. 

Friday Night: Winds less than 25 kt. Local rough seas. 

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Chance of rain. 

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas. Rain likely. 

&&

.FIRE WEATHER...

Strong W winds are anticipated on Thursday in wake of the system
exiting the region. The strong mixing will bring dry conditions
for much of the CWA with minimum relative humidities between
25-40 percent. Should see gusts around 40-45 mph at times this
in combination with the relative humidities may bring elevated
fire weather concerns. This will be impacted by the
precipitation that falls across southern New England today. Have
reached out to the fire weather partners to determine if there
will be need for an SPS. Right now based on the forecast am more
concerned across eastern portions of the CWA, but will need to
address this in later updates.

The pressure gradient remains tight with dry weather on Friday.
Minimum relative humidities range between 25-40 percent once
again with gusts around 30 mph at times. This will likely be
another day where there could be potential elevated fire weather
concerns. Stay tuned for later updates.

&&

.TIDES/COASTAL FLOODING...

Tides will be astronomically higher Sunday and especially 
Monday. Depending on the location of the low pressure, there 
could be a couple of tidal cycles impacted, first with southeast
to east flow, then with north or north-northwest flow. Right 
now, minor coastal flooding appears possible, but over the next 
few days we will be watching closely for any possibility of 
pockets of moderate coastal flooding.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 4 AM Thursday to 2 AM EDT Friday for 
     ANZ231>235-237-250-251.
     Gale Warning from 6 AM Thursday to 2 AM EDT Friday for ANZ230-
     236.
     Gale Warning from 2 AM Thursday to 2 AM EDT Friday for 
     ANZ254>256.

&&

$$
SYNOPSIS...KJC
NEAR TERM...BL
SHORT TERM...BL
LONG TERM...KJC
AVIATION...KJC/BL
MARINE...KJC/BL
FIRE WEATHER...
TIDES/COASTAL FLOODING...