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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
937 PM EST Mon Dec 9 2019

Synopsis...

A milder airmass will continue to surge northward across the region
and will be accompanied by showers into Tuesday. The rain will be
heavy at times. A cold front moves through Tuesday evening, ushering
in much colder air that will likely change rain to snow and linger
into Wednesday with accumulations likely. Large high pressure will
bring cold and dry conditions Thursday and Friday. Another system
will impact the region Saturday with potentially periods of heavy
rain, possibly beginning as freezing rain over northwest
Massachusetts. A trend toward drier and cooler weather is scheduled
for Sunday.

&&

Near term /until 6 am Tuesday morning/...

* rain diminishes in intensity and areal coverage from west to
east after midnight, but it remains unseasonably mild

935 PM update...

Widespread rain impacting the region this evening, but the
heaviest rain was across Rhode Island/southeast Massachusetts on the nose of a strong
southerly low level jet of 60 to 80 knots. Some nuisance poor drainage
street flooding likely, but no significant flooding issues
expected. Southerly 30 to 40 mph wind gusts are mixing down
along the southeast New England coast, but inversion should be
enough to keep things mainly below Wind Advisory criteria except
for perhaps a few spot locations.

The shortwave and core of the low level jet moves offshore after midnight.
Therefore, expect the rain to diminish in areal coverage and
intensity after midnight from west to east. In fact, most
locations should be dry by daybreak with just a few spot
showers. Temperatures will remain unseasonably mild as we remain
in the warm sector and in southwest flow. Temps should hold in
the 50s through daybreak in most locales, with even a few
locales near 60 across Rhode Island/southeast Massachusetts. Patchy fog will remain possible
especially over the rapidly melting snowpack across the
interior. However, enough wind in the boundary layer should
prevent widespread dense fog.

&&

Short term /6 am Tuesday morning through 6 PM Tuesday/...
a cold front is anticipated to move across southern New England
during the afternoon hours. This front will provide an
additional focus for rainfall. Expecting another round of
showers to develop from mid morning into the afternoon hours.
Low level convergence will not be ideal. Precipitable water will
also remain above normal, but not exceptionally above normal.
Overall, looks like an efficient rainfall situation. There
simply should not be a lot of rainfall.

Our region will spend most of the day in the warm sector.
Despite the clouds, Max temperatures generally in the 50s are
expected. A few locations could make a run at 60 degrees. Some
locations across the higher terrain of the Worcester Hills and
east slopes of the Berkshires should remain closer to around 50
degrees.

&&

Long term /Tuesday night through Monday/...
highlights...

* rain changing to snow Tue night/Wed with accumulating snow likely

* colder than normal Thu into Fri but dry except possible ocean
effect snow showers Outer Cape cod

* another weather system Sat with heavy rain & potential flooding

Details...

Tuesday night and Wednesday ...

*** accumulating snow possible Wed morning commute ***

Cold front pushes offshore with shallow cold air overspreading the
region and becoming deeper as the night progresses. Strong short
wave approaching the Great Lakes maintains southwest flow aloft
across New England, thus warm air overrunning cold air at the
surface setting the stage for another precip event. All guidance
indicates the column is cold enough to support all snow especially
after midnight. The question then becomes how much qpf will overrun
the boundary into the cold air across southern new england?

Both ensemble data sets from the ec and GFS including deterministic
guidance from the NAM/href/GFS/ec suggest heaviest qpf will be
confined to the South Coast of MA/RI/CT. Ensembles offering 0.25 to
0.33 in this area and obviously deterministic guidance higher with
12z NAM showing a stripe of 0.75+ inches from Cape Cod to mvy. 12z
href ends 12z Wed but also has some members supporting higher qpf
06z-12z Wed. In fact the 12z ecens has over 30 of its 50 members
supporting at least 2 inches of snow for much CT/Rhode Island and MA!

At this model time range (42+ hrs) typical model error for a frontal
boundary is probably in the +/- 50-100 miles range. Thus this
heavier qpf band could shift and verify offshore but also could
shift inland to perhaps hfd-orh-bos. So there remains a range of
possibilities. Tracing back the short wave that will enter the Great
Lakes early Wed that will back the mid level flow across southern
New England and impact our forecast, this feature traces back to the
Arctic Circle this afternoon. Obviously this is a data sparse area
so expecting models to initialize this feature differently/better
once it reaches lower latitudes with more data. Typically these
Arctic short waves verify stronger/more robust given poor model
initialization at higher latitudes in data sparse regions. In
addition this approaching Arctic short wave will be accompanied by
an anomalous upper level jet streak across northern New England into
southern Quebec Tue ngt/Wed with speeds of 180+ kts! Thus a lot of
jet dynamics to watch unfold.

So at this early stage our preliminary snowfall forecast could range
as little as a coating to an inch (our 10% percentile snow forecast)
to possibly 2-4 inches (50% percentile snow forecast) if Arctic
short wave verifies stronger with mid level flow backing more to the
south-southwest and with longer duration yielding higher qpf. Also can't
completely dismiss the more extreme NAM/href solution with 4+
inches, albeit a low prob but within the envelope of solutions.
Nevertheless will need to watch this portion of the forecast given
its potential impact to the Wed morning commute.

Wednesday night through Friday...

As you would expect pretty cold airmass overspreads the region Wed
night into Thu behind departing Arctic short wave. By 12z Thu 925 mb
temps fall to about -13c over southern New England and 850 temps
down to -20c upstream over northern Vermont/NH! However mainly dry
weather prevails as 1040+ mb Arctic high builds into the region.
Gusty northwest winds will provide frigid wind chills Wed night but by Thu
afternoon gradient relaxes so diminishing winds later Thu into Fri.
However before this occurs low level winds turn to the north-northwest and may
yield ocean effect snow showers for the Outer Cape and Nantucket
Thu/Thu evening. Otherwise dry but cold weather prevails. Given the
magnitude of the cold air (-1 to -2 std) will derive temps from a
50/50 blend of the coldest guidance and superblend.

Next weekend ...

*** heavy rain & flood potential ***

Another potent system appears to impact the region Saturday with
ensembles showing precipitable water anomalies of +1 to +2 Standard. Thus heavy rain
threat and given heavy rain from today and tomorrow along with snow
melt contribution, increased risk for river and urban flooding
Saturday. Shallow cold air may linger across the CT River Valley of
western Massachusetts and may result in freezing rain/wintry mix at the onset
Friday night.

Ensembles suggest pattern remains progressive so some improvement
possible Sunday with a drying trend and cooler weather.

&&

Aviation /03z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Tuesday/...high confidence.

00z update ...

No major changes from previous tafs. Now that warm air has
overspread the region, cigs and vsbys have improved somewhat and
expect IFR/MVFR to dominate the terminals. Thus no longer
expecting LIFR overnight. Earlier discussion below.

==================================================================



Mainly MVFR ceilings/VFR visibilities well inland with local VFR conditions
across north mass. Farther S, mix of VFR-MVFR conditions at most
sites, except IFR-LIFR across portions north CT into S central mass.
Expect lowest conditions across north central/northwest mass with deep
snowpack in place.

IFR-LIFR conditions will continue early tonight, then visibilities may
slowly improve as SW winds will increase overnight. Ceilings will
remain low.

Scattered -shra linger across region Tue with MVFR-IFR ceilings and
areas of MVFR-IFR visibilities mainly along S coast.

Low level wind shear continues due to light winds at surface and 2kft winds
19040kt. Later today and tonight, focus of low level wind shear shifts into Rhode Island
and eastern Massachusetts as southerly low level jet increases to 50-60kt
at 2kft.

Kbos terminal...high confidence in taf trends. Moderate
confidence in timing.

Kbdl terminal...high confidence in taf trends. Moderate
confidence in timing.

Outlook /Tuesday night through Saturday/...

Tuesday night: MVFR/IFR conditions possible. Breezy. Pl likely,
chance ra, chance sn, freezing rain likely.

Wednesday: mainly VFR, with local IFR possible. Chance snow.

Wednesday night: VFR. Windy with local gusts up to 35 kt.

Thursday: VFR. Windy with local gusts up to 30 kt.

Thursday night: VFR.

Friday: MVFR/IFR conditions possible. Breezy. Chance shsn,
chance rain showers.

Friday night: mainly IFR, with areas MVFR possible. Breezy.
Rain showers likely, shsn likely, freezing rain likely.

Saturday: mainly MVFR, with areas IFR possible. Breezy. Rain showers
likely.

&&

Marine...
short term /through Tuesday/...high confidence.

** Gale warnings for most of waters through Tue morning **

Increasing S-SW winds into tonight. Gale force gusts this
afternoon and tonight, with peak gusts of 35-40 kt expected.
Gusty winds remain in place Tuesday. Expecting gale warnings to
be gradually replaced by small craft advisories Tuesday morning
as conditions warrant.

Steadier showers continue into this evening. Patches of dense
fog possible mainly on south coastal waters. More in way of
scattered showers Tuesday morning before next round of steadier
rainfall arrives later in the day. Expect poor visibility around
1 nm in the heavier showers.

Outlook /Tuesday night through Saturday/...

Tuesday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Rain likely, sleet
likely.

Wednesday: winds less than 25 kt. Local rough seas. Chance of
snow, rain likely, sleet likely. Local visibility 1 nm or less.

Wednesday night: moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas.

Thursday: low risk for Small Craft Advisory winds with local
gusts up to 30 kt. Local rough seas.

Thursday night: winds less than 25 kt. Areas of seas
approaching 5 ft.

Friday: winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers. Local visibility 1 to 3 nm.

Friday night: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Rain showers likely.

Saturday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft. Rain showers likely.

&&

Climate...
record highs for Tuesday 12/10. Best chance to tie or break
record would likely be at Boston.

Bos ... 64/1907
pvd ... 67/1946
bdl ... 67/1946
orh ... 63/1946

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Gale Warning until 9 am EST Tuesday for anz231>235-237-250-251-
254>256.
Small Craft Advisory until 1 PM EST Tuesday for anz230-236.

&&

$$
Synopsis...Belk/nocera
near term...Frank
short term...Belk

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