Area Forecast Discussion
National Weather Service Boston/Norton MA
752 PM EDT Sat Apr 10 2021

.SYNOPSIS...
A backdoor cool front later tonight will spread overcast 
conditions and cooler onshore flow across the region into 
Sunday. Meanwhile, a frontal system passing to our west will 
also bring a period of light rain for the interior late Sunday. 
Early next week features temperatures that are near average for 
mid-April though it will feel colder given the recent warm 
weather. Showers return midweek but will remain unorganized with
little appreciable precipitation. Becoming more settled late 
week along with slightly warmer temperatures.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
750 PM Update...

Since the last update we've had to tweak the onset of the low
level clouds moving in from the Islands towards southern Rhode
Island, southeast Massachusetts, and Cape Cod. Places like
Nantucket, Martha's Vineyard, and Block Island have been
reporting visibility less than a mile and at times less than
1/4th of a mile. We decided to issue an SPS for areas of dense
fog for southern and central Rhode Island, southeast
Massachusetts, Cape Cod, and the Islands. Because we weren't
confident in how widespread the fog will become we decided to
hold off on issuing an advisory. The overnight shift can
reassess if there will be a need for one. Otherwise it's a 
fairly quiet evening with a lonely shower that pulsed up in 
southern New Hampshire and has moved southeast across the border
of Essex and Middlesex Counties... the shower should move out 
into Mass Bay around 0030z. 

All that being said our forecast remains on track and you can
read the previous forecast discussion below. 

-------------------------------------------------------

Weak surface high pressure is currently in control across
Southern New England, allowing for a beautiful end to the day. 
But changes are on the way and a look at mesoscale obs shows a
surface frontal boundary draped across from the St. Lawrence 
Valley through downeast Maine with temperatures well into the 
70s across southwest Maine and in the 50s north of the boundary 
in northern Maine. This is associated with a lobe of H5
shortwave energy that will rotate south through New Brunswick
this evening through the overnight hours. As it does so, it will
swing push the aforementioned boundary southwest as a backdoor
cold front. Latest high-res guidance shows the front doesn't
arrive in Northeast MA till close to daybreak tomorrow. So
this will allow plenty of time for southerly flow to continue,
at least for the first half of tonight. 

So expect areas of low clouds and locally dense fog to make further 
headway inland with light winds and dew points in the 50s. With 
initially clear skies to start the night and surface high pressure 
in place, dew point depressions should collapse rather quickly. Then 
sometime after midnight, the winds shift to the north and then 
northeast for Central/Eastern MA and parts of northern RI as a 
backdoor cold front approaches from the northeast. This is 
associated with a H5 low that slides southeast across Downeast Maine 
into the Canadian Maritimes. There is some uncertainty with the 
timing of the cold front, but latest high-res guidance suggests that 
it should approach Southern New England from the northeast right 
around daybreak, and then push through the remainder of our region 
from east to west during the day. So while overnight lows are mostly 
in the 50s across the region, people in Eastern MA including the 
Cape and Islands will likely see lows in the mid to upper 40s. 
Typical lows this time of the year are in the upper 30s, so still 
above average.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
A backdoor cold front will sweep from east to west during the day. 
Dew points in the 50s will be replaced by 40s and highs should be 10 
to 15 degrees cooler than on Saturday with plenty of cloud cover and 
a cool northeast flow. Typical highs for the second week of April 
are in the mid 50s so while a shock to the system given our 
prolonged stretch of anomalous warmth (away from the immediate 
coast), the forecast highs are right around seasonal average. 

As an aforementioned H5 low slides east off downeast Maine and 
meanders in the Canadian Maritimes, another H5 closed low will track 
eastwards across the Midwest and stalls in the vicinity of Northern 
Ohio/Western Lake Erie. This is probably associated with the omega 
block that is forecast to develop between the two upper lows. This 
has implications on the northeast extent of meaningful precipitation 
associated with an occluded surface front. The 12z suite of 
mesoscale guidance places the axis of steadier rainfall to the 
southwest of Southern New England. So in coordination with WPC, we 
have trended the QPF amounts lower, generally between a few 
hundredths of an inch to perhaps two tenths of an inch towards 
Northern CT and Western MA. In other words, this looks to be a 
missed opportunity for reducing our rainfall deficit and our recent 
dry pattern continues. 

As for timing of the precipitation, the best chance for
steadier but overall light rain looks to be evening into the
overnight hours. So other than a few scattered showers, most of
the day looks to be dry but cooler with the northeast flow and
increasing cloud cover.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Highlights...

* After a period of above average temperatures, next week will feel 
  noticeably "cooler" as the daily maximum temperature returns more 
  seasonal values. 

* Unsettled conditions will lead to periods of light rain during the 
  first half of the work week.

* Latter half of the week looks to remain dry with temperatures 
  slightly above average. 

Monday and Monday Night...

A nearly vertically stacked low off the southeast coast of Nova 
Scotia will provide an easterly to northeasterly flow for New 
England. Latest satellite observations tell us the ocean 
temperatures are between 44F and 48F. With this on shore flow, areas 
across eastern MA, Cape Cod, and the Islands will be some of the 
cooler locations on Monday - near 50F. Some modification over land 
will allow for temperatures to climb by a few more degrees and top 
out in the mid-50s across the CT River Valley.

Showers that began on Sunday will come to an end during the first 
part of the day as drier air aloft moves into place. This drier air 
should help to thin out the cloud cover across southern New England, 
but it doesn't appear at this point we will completely clear out. 
BUFKIT soundings for Monday night into Tuesday morning have the 
minimum overnight in the upper 30s to low 40s. This might seem 
"cold" since the last week overnight lows had been in the upper 40s 
and lower 50s. But, normal overnight lows are actually upper 30s to 
40F. 

Tuesday and Wednesday...

Weakening area of low pressure over the Great Lakes Region will 
shift towards the Mid Atlantic Region. Recent model runs show the 
weakening low becoming only slightly more organized as it reaches 
the coast and producing widely scattered showers across southern New 
England. Earliest  thoughts on precip accumulation are in the 
neighborhood of a few hundredths of an inch. PWAT values are in the 
neighborhood of 0.50" and 0.75" - this might not sound light a lot, 
but it's 100 to 120 percent of normal for mid-April. Afternoon 
maximum temperatures reach the low-50s at the coast and mid-50s. 

Tuesday night into Wednesday the surface low moves east into the 
North Atlantic and high pressure moves in from the northwest. A few 
unorganized showers could linger for the first half of Wednesday. 
Wind direction remains out of the east and northeast keeping coastal 
areas cool and likely socked in with clouds. Areas like the CT River 
Valley could warm up as easterly winds downslope from the Worcester 
Hills. BUFKIT soundings show there could be breezy winds gusting to 
20 knots Wednesday as well. There is a possibility that temperatures 
west of the CT river could over perform on Wednesday as a result. 

Thursday and Friday...

Latest 12z guidance shows high pressure building in for the latter 
half of the work week - giving us increasing heights and drying out 
the low and mid-levels and should lead to a health mix of sunshine 
and clouds. Thursday is likely to be the "warmest" day of the week 
with highs away from the water reaching the low-60s... possibly 
reaching the mid-60s across the CT River Valley... coastal 
communities once again remain in the low to middle 50s. 

Next Weekend... 

The ECMWF paints a picture of a cold front moving across the 
northeast Saturday afternoon/evening - followed by a cooler and dry 
Sunday. The GFS holds off on the arrival of the front until Sunday 
afternoon. Since this part of the forecast is still a week away we 
won't elaborate any further.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

20z UPDATE...

Tonight: Moderate confidence. 

VFR conditions continue, although HYA and ACK are already 
seeing IFR and LIFR ceilings at this time. Main concern this 
evening into the overnight hours would be the northward push of 
low stratus and locally dense fog. The main question is the 
northward extent of IFR to LIFR visibility. While IFR to LIFR 
ceilings should be widespread across the entire region after 
06z, it looks like LIFR vsby may stay just south of BOS. This 
will need to be monitored as more high-res guidance comes in 
later overnight. Winds are generally around 10 kt but will 
switch from the southwest to northerly after 06z and then 
northeast by 10 to 12z. 

Sunday: Moderate confidence. 

MVFR to LIFR conditions from stratus and mist/fog, with
intervals of scattered light rain mainly after 18z. Northeast 
winds around 10 kt, with occasional gusts 15 to 20 kt. 

Sunday night: Moderate confidence. 

MVFR to IFR conditions with occasional light rain but gradual 
improvement to VFR for the Eastern MA terminals after 06z. 
Northeast winds 10 kt with gusts 15 to 20 kt, higher along the 
coast. 

KBOS TAF...Moderate confidence in TAF. 

KBDL TAF...Moderate confidence in TAF. 

Outlook /Monday through Thursday/...

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

Monday Night: Mainly VFR, with local MVFR possible. Breezy. 

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

Tuesday Night: MVFR/IFR conditions possible. Chance SHRA,
patchy BR.

Wednesday: Mainly VFR, with local IFR possible. Chance SHRA.

Wednesday Night: VFR. Slight chance SHRA.

Thursday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
* Small Craft Advisory for southern outer waters late Sunday
  into Monday, mainly for 5 ft seas. 

Tonight, initial SW winds to become N/NE around 10-15 kt on the
northern/eastern waters, and SE around 10 kt on the southern 
waters. Seas remain in the 2 to 3 ft range. 

NE to E winds around 10-15 kt, gusts in the low 20s kt range will 
gradually overspread the rest of the waters into Sunday. Seas also 
building into the 2 to 4 ft range, though may near 5 ft on the 
southeastern offshore waters late in the day. This is why SCAs
have been hoisted for the southern and southeastern outer waters
late Sunday into Monday.  

Outlook /Monday through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Slight chance of rain showers. 

Monday Night: Winds less than 25 kt. Seas up to 5 ft. 

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers. 

Tuesday Night through Wednesday: Winds less than 25 kt. Areas
of rough seas. Chance of rain showers. 

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 5 PM Sunday to 8 PM EDT Monday for 
     ANZ254>256.

&&