362 
FXUS61 KBOX 041406
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1006 AM EDT Thu Nov 4 2021

.SYNOPSIS...
High pressure in control brings mostly dry and seasonably cool
weather. Low pressure well offshore on Thursday night into 
Friday may clip the south coast with showers. The weekend 
continues to look dry with a bit of a warmup. Another low 
pressure system may pass southeast of the area Sunday night and 
Monday. A third low may approach from the Great Lakes on 
Wednesday. Cool temperatures over the weekend followed by a 
moderating trend the first half of next week.

&&

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

10 AM Update...

Current forecast looks on track. There is still a bit of valley
fog lingering in the CT River Valley near the VT/NH border,
which will continue to gradually lift/dissipate during late this
morning. Also some ocean effect clouds were developing in the 
vicinity of Cape Cod Bay. Otherwise mostly sunny skies 
prevailed. Still expecting an increase in cloud cover this 
afternoon, from south to north.

Early Morning Discussion Follows...

Following a chilly start with widespread sub-freezing overnight lows 
across the interior, a rather cool day is in store with a mix of sun 
and clouds. With 850mb temperatures around -5C and mixing to that 
level, expect highs to top out in the mid 40s to low 50s. For 
reference, normal highs for this time of the year are in the mid 
50s. Mid to high level clouds will also filter in from the south 
associated with a weak disturbance in the Carolinas that will move 
offshore and track to the northeast.

&&

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

The low pressure center of the aforementioned disturbance looks to 
pass southeast of the 70W/40N benchmark. So any showers are expected 
to remain mainly offshore but may clip the south coast. Bufkit 
soundings show some saturation in the low levels overnight Thursday 
into early Friday. So have slight chance to chance PoPs for Cape Cod 
and the islands in this time frame. But given the lack of deep 
moisture, precipitation will be in the order of hundredths of an 
inch. Otherwise dry and seasonably chilly weather is expected. There 
will also be quite a temperature spread across the region, with 
upper 20s to low 30s across the interior and low to mid 40s along 
the south coast into Cape Cod and the islands. 

Friday...

Positively tilted H5 trough swings across New England, so any 
showers along the south coast should quickly come to an end by 
Friday morning. A seasonably cool and dry day is expected. With 
mixing up to 850mb and 850mb temps a degree or two milder than on 
Thursday, used the MOS guidance to yield highs in the upper 40s to 
low 50s. Could also see some 20-25 mph gusts especially near the 
south coast and Cape Cod as some of the 20-25 kt 925mb winds get 
mixed down to the surface.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Big Picture...

Upper flow flattens to zonal Saturday. Northern stream shortwave 
moves through on Sunday while southern stream shortwave sweeps out 
to sea from the Carolinas Sunday night and Monday. GFS and GGEM show 
a shortwave move through the northern stream on Wednesday. At the 
surface, high pressure and subsidence dominate. Weather system from 
the Carolinas passes well offshore Sunday night and Monday.  GFS and 
GGEM show Midwest low pressure moving across New England on 
Wednesday.

Height contours at 500-mb, an analog for the deep-layer temperature, 
are forecast near normal over the weekend and then trend above 
normal early and midweek next. Normal max temps are in the mid 50s 
and normal min temps in the upper 30s and low 40s. 

Confidence is moderate, except lower for next Wednesday. 

Details...

High pressure and subsidence remains in control for most of the 
period. This favors dry weather.

There are two concerns for precipitation. Weather system on 
Sunday night and Monday should remain offshore, but there remains a 
low chance that the track curves closer to the shore and brings a 
few showers to the South Coast. For now the forecast will 
handle it as a non-event. 

The second concern comes on Day 7 or thereabouts. Low pressure moves 
east through the Great Lakes.  The GFS and GGEM suggest light 
precipitation...from passage of a warm front in the morning and then 
from passage of a cold front late in the day. The ECMWF is slower 
with the low and turns it northeast into Canada, keeping most of the 
precipitation to our north. But it also hints at light warm frontal 
precip, a trace to a couple of hundredths, Wednesday morning.  At 
this time we favor a dry forecast, but as this happens on Day 
7...or later...it will need to be monitored for changes.

Small changes were made to min temps on Friday night and Saturday 
night. Light wind and mainly clear skies will favor radiational 
cooling, so the min temp forecast for each night was lowered 2-3F.

The mixed layer lowers to 925-mb most of the period. Temperatures at 
this level start around 850-mb equivalents of minus 4 to minus 6 
Saturday, then trend to zero to minus 2 Monday through Wednesday. 
This suggests max sfc temps of 47 to 54 Saturday trending to 55-61 
Monday through Wednesday.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
Low - less than 30 percent. 
Medium - 30 to 60 percent. 
High - greater than 60 percent.

14Z Update.

Today: High Confidence. Mainly VFR, but we can not rule out 
some brief marginal MVFR ceilings along the eastern MA coast at
times. Light and variable winds becoming ENE at 5-10 knots 
along the coast.

Tonight: High Confidence. VFR except for MVFR ceilings with
VCSH for the Cape terminals. Light and variable winds away from
the immediate coast, but terminals along the south coast could 
see northeast winds with gusts 15-20 kt after 06z. 

Friday: High Confidence. MVFR to low VFR ceilings possible for
Cape terminals prior to 16z before improving to VFR for all
terminals. North winds up to 10 kt for the interior terminals
but northeast winds with 15-20 kt gusts possible for the Cape
terminals, diminishing after 18-20z. 

KBOS Terminal...High confidence in TAF. 

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night through Saturday Night: VFR. 

Sunday through Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels.

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

Today through Friday...

High pressure will bring tranquil conditions across the coastal 
waters. Light east/northeast winds with seas ranging from 1-2 
feet nearshore and 2-4 feet for the outer waters.

Outlook /Friday Night through Monday/...

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

Sunday: Winds less than 25 kt. 

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

Monday: Low risk for Small Craft Advisory winds with gusts up
to 20 kt. Areas of seas approaching 5 ft.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides are at their monthly highs with Boston Harbor at 
12.1 ft MLLW on Saturday early afternoon and 12+ ft MLLW on Sunday. 
Flood stage at Boston is 12.5 ft, so it only take one-half foot to 
generate minor coastal flooding. An east swell of 4-5 feet
builds off Cape Cod and Islands Friday night, then subsides
Saturday afternoon. Will monitor to see if it can reach farther
up the coast.

Later...an offshore low passes south of the Islands Sunday and 
Monday. Some remaining uncertainty as to how close it tracks to
the coast along with its intensity. A 4-5 foot southeast swell 
builds off the coast Sunday night and Monday. Tides then come 
down slowly next week. Depending on the timing of this offshore
low, Monday could be in play for potential minor coastal 
flooding. Just too early at this time range to elaborate much 
more.

&&

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

&&

$$
SYNOPSIS...WTB/Chai
NEAR TERM...Chai/NMB
SHORT TERM...Chai
LONG TERM...WTB
AVIATION...WTB/Chai
MARINE...WTB/Chai
TIDES/COASTAL FLOODING...Staff