Area Forecast Discussion
National Weather Service Boston/Norton MA
1042 AM EDT Tue Apr 7 2020

.SYNOPSIS...
High pressure remains in control Tuesday with morning clouds giving 
way to afternoon sunshine, spring-like temperatures and a modest NW 
breeze. Later Tuesday night, a warm front will spread a period of 
light rains across most of southern New England. A fast moving low 
will develop along that warm front and bring a period of rain 
through Wednesday. Another system moves in Thursday bringing more 
rain and scattered thunderstorms. Scattered shower activity is 
possible Thursday night through Friday. Drier weather for the 
weekend before our next wet weather system early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1030 AM Update: 

No significant changes needed to the going forecast at this
point, other than accounting for current observations. 

Early-day mid-level clouds have fully cleared, with most areas
currently seeing plentiful sun and west winds around 10-15 mph.
Biggest concern on the mesoscale is how far westward will the 
sea-breeze penetrate inland. Appears west flow will keep the sea
breeze and cooler onshore flow away from the Boston metro for 
much of the day, but may linger from an Ipswich to 
Hingham/Cohasset line. Thus, anticipating a rather mild early- 
April day with highs into the 60s for most (except the immediate
southern/eastern coasts and the Cape). 

With full sun, mixing of drier air as the PBL deepens to about 
700 mb will cause relative humidities to fall into the 20s 
percent in the interior, with west winds that may gust into the
20-25 mph range at times. SPS for elevated fire weather 
potential issued early this morning will take effect at 1 PM. 
See the Fire Weather Section for more details specific to the 
fire weather situation.

Previous Discussion...

Today ...

Another very nice day ahead with spring like temperatures and dry 
weather. This is courtesy of NW flow aloft. Short wave energy 
crossing the region this morning will result in mid and high clouds 
impacting the area. However as this jet energy exits mid/high clouds 
will move offshore later this morning and give way to mostly sunny 
conditions this afternoon across the entire region.

Seasonable temperatures aloft, about 0C at 850 mb. However model 
soundings indicate boundary layer extending up to about 750 mb this 
afternoon, courtesy of NW flow aloft. This will result in 
temperatures overachieving with highs in the low to mid 60s, even a 
few spots possible into the upper 60s given downslope with NW winds 
10-15 mph but gusting 20-25 mph at times. Despite a weak pressure 
gradient this deep boundary layer with modest NW winds thru the 
column will also preclude seabreeze development so just as warm into 
Boston and Providence as interior locations and probably all the way 
to the Cape Cod Canal. The outer Cape and Islands likely have a 
cooling SW wind so cooler conditions there. Elsewhere NW winds 
combined with boundary layer extending up to 750 mb will result in 
drier air aloft mixing to the surface, so expecting RH values to 
drop considerably this afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Tonight ...

Weak pressure gradient, mostly clear conditions and cooling boundary 
layer after sunset should promote temps to fall quickly during the 
evening hours. However second half of the night clouds will increase 
as short wave trough and associated surface low approach in the NW 
flow aloft. Rain then likely overspreads the area 06z-12z from west 
to east. Models have trended colder especially the 00z NAM with 
column potentially cold enough to support a period of wet snow 
around 12z across the high terrain of northern MA and possibly into 
northwest Middlesex county. Given how dry the antecedent airmass is 
across the region today into tonight, evaporational cooling maybe 
sufficient for a period of wet snow. NAM and other HREF members 
indicate potential period of wet snow. In addition there could be a 
period of heavier precip toward 12z in response to moderate to 
strong deformation axis forming as 850 mb low tracks across the 
area. Will have to watch how later model runs trend regarding 
thermal profiles and precip intensity. If heavier precip doesn't 
materialize or shifts farther south away from the high terrain, then 
any snow is less likely.

Wednesday ...

Periods of rain likely in the morning along with a risk of some wet 
snow across higher terrain of MA including Worcester per 
aforementioned above. Drying trend in the afternoon as frontal wave 
departs offshore. Although light cool NE flow likely precludes much 
if any afternoon sunshine, best chance late in the day. Thus mostly 
cloudy and cool with highs only 45-50, low 50s CT river valley.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
* Widespread rain Thursday with scattered thunderstorms 

* Lighter rain and high elevation snow on Friday with a few rumbles 
  of thunder possible

* Potential for minor coastal flooding Wednesday to Friday around 
  high tide across the east coast of MA and Nantucket.  See the 
  Tides and Coastal Flooding section below for more details.

* Temperatures running near to below normal through Saturday then a 
  warm up 

Details...

Wednesday night...

High pressure ridge briefly takes control Wednesday night keeping 
things dry and allowing for partial clearing skies early on. 
However, low level moisture fills back in toward midnight and 
beyond, so low level cloudcover will be on the increase. This should 
help keep temperatures in the upper 30s. High astronomical tides 
will contribute to a chance for some splashover or minor coastal 
flooding issues along the east coast for several tide cycles from 
Wednesday night through at least Thursday night. See the 
Tides/Coastal Flooding section below for details. 

Thursday...

Thursday will be the most active weather day of the mid-late week 
period as a strong storm system passes over New England, 
strengthening as it does. At the synoptic scale we see a sprawling 
upper level low and sfc reflection move into the Great Lakes region 
early Thursday morning, spawning a secondary triple point low over 
or near southern New England. This system bring plenty of moisture, 
very good dynamics, and some elevated instability to boot. Ahead of 
the low a very strong 45-55kt 850 mb LLJ moves overhead (2-3 
standard deviations stronger than average) pulling in a plume of 
moisture on southerly flow. PWATs approach 1" while dewpoints may 
get into the low 50s. There remains some discrepancy on exact low 
placement and extent of moisture advection among the guidance but 
the potential is good for a very moist and dynamic system. A robust 
vortmax, strong LLJ, and approaching cold front will lead to 
widespread heavy rain; up to a half inch is likely, with locally 
higher amounts before mid/upper level dry air surges in behind the 
cold front. The front moves offshore sometime in the evening or 
early overnight hours. I've added a chance of thunderstorms on 
Thursday as well, with such a dynamic system, some instability, and 
ample shear. It starts with several hundred J/kg CAPE in western 
MA/CT late morning shifting east through the afternoon. Mid level 
lapse rates are favorable, near 7C/kg while the strong LLJ 
contributes to 0-6 km bulk shear values >70 kts. 

Friday...

Guidance has begun to come into better agreement as to how quickly 
the low then lifts out into ME overnight and into Canada Friday. 
Meanwhile the upper trough remains overhead keeping cyclonic flow in 
place as wraparound moisture and a strong westerly LLJ brings a 
resurgence of lighter rain showers and even high elevation snow on 
Friday. Best chance for showers and even a few rumbles of thunder 
comes during the afternoon as diurnal heating and the cold pool 
aloft contribute to some marginal instability, though the shear is 
very much lacking by then. High temperatures reach into the upper 
40s to low 50s. 

Saturday through Monday...

The weekend looks drier as a ridge of high pressure makes its way 
overhead. Going with a blend of guidance we can expect dry weather 
through the weekend before our next storm potential comes early next 
week. Temperatures top out near normal on Saturday, in the low 50s, 
but another warm up is ahead. Southerly flow out ahead of the early 
week trough will bring a few days of more mild temps beginning 
Sunday approaching 60, and into the low 60s by Monday.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

15z TAF Update:

Today: High confidence.

VFR. WNW to W winds 8-12 kt with gusts up to 20 kt at times. 
This will likely preclude seabreeze development except over the 
outer Cape and Islands. Better chance of SE winds near the coast 
late in the day (mainly after 21z, towards/into the afternoon 
push).

Tonight ... High confidence but some uncertainty on
precipitation type toward 12z Wednesday.

VFR to start the evening but then lowering to MVFR and possibly
IFR 06z-12z from west to east as rain overspreads the region.
Rain may mix with or change to snow briefly toward 12z Wed
across northern MA. Winds becoming NE late. 

Wednesday ... high confidence on trends lower on exact timing.

A mix of MVFR/IFR in morning rain possibly wet snow over
northern MA. Mainly MVFR in the afternoon along with a drying.
Modest NE winds.  

KBOS Terminal...High confidence in the TAF. WNW winds likely
persist with gusts up to 20 kt into the afternoon, keeping 
seabreeze just offshore. 

KBDL Terminal...High confidence in the TAF.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA.

Thursday: Mainly VFR, with local IFR possible. Windy with gusts
up to 30 kt. SHRA, chance TSRA.

Thursday Night: VFR. Breezy. 

Friday: VFR. Breezy. Chance SHRA.

Friday Night through Saturday: VFR. Breezy.

&&

.MARINE...
Today ... high confidence.

Weak high pressure provides light winds except near shore where WNW 
winds 10-15kt gusts up to 20 kt. Dry weather and good vsby.

Tonight ... high confidence but lower on exact details.

Weak fast moving low pressure exits the Great Lakes and moves across 
southern New England, accompanied by periods of rain toward morning.

Wed ... high confidence but lower on exact details. 

Low pressure over the area in the morning exits offshore in the 
afternoon. Despite weak NE winds a drying trend and improving vsbys 
excepted in the afternoon. 

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt. 

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of rough seas. Chance of rain showers, slight
chance of thunderstorms. 

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 9 ft. 

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Rough seas up to 10 ft. Slight chance of rain
showers. 

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. 

Saturday: Winds less than 25 kt. Local rough seas.

&&

.FIRE WEATHER...
Min Relative humidities are expected to fall to 20 to 30 percent 
this afternoon away from the coastline. This combined with WNW winds 
10-15 mph with gusts up to 20 mph along with dry weather will yield 
an elevated fire weather concern. Thus will continue the Special 
Weather Statement that was issued last evening.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides are on the rise (across multiple tidal cycles) 
this week with the upcoming Supermoon.  Stevens Institute indicates 
both tide cycles on Wed (midday and midnight) across eastern MA may 
experience minor coastal flooding. Then Thu with southerly flow the 
south coast could become the focus.

&&

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

&&