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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
702 am EST Fri Dec 6 2019

Synopsis...
clouds will increase ahead of a fast-moving clipper system that will
push through with periods of light snow this afternoon and early
evening. Best chance of seeing light accumulations is along and
north of the Massachusetts Turnpike. The east slopes of the Berkshires could
see upwards of 2 inches. Colder, drier air moves in Friday night and
Saturday as high pressure brings dry weather for the weekend. The
high moves offshore Sunday, starting a return flow of milder and
moist air for the first half of the week. This will bring off
and on rain for Monday and Tuesday. A cold front moves through
Tuesday night, followed by a return to cooler temperatures for
Wednesday and Thursday.

&&

Near term /until 6 PM this evening/...

7 am update...

Mid and high clouds increasing over southern New England, while
lower clouds remain to our west in New York/PA, roughly on a roc-bfd
line. Expect increasing sky cover this morning with the western
New York/PA precip reaching US in the afternoon. No significant
changes to the grids.

Previous discussion...

***a clipper system is expected to bring coating to an inch of snow
especially along & north of the Massachusetts Turnpike late this afternoon into
early evening. Locally 2-3 inches are possible in the east slopes of
the Berkshires.***

Satellite shows clouds mostly clearing out early this morning, but
it will be short lived. Looking upstream we see the deck of mid
level clouds making its way toward southern New England. These will
fill in from west to east by sunrise in western Massachusetts/CT and eastern Massachusetts
by 9-10 am.

These clouds precede a quick hitting clipper type system that will
strength over the Great Lakes in the next several hours before
crossing directly over southern New England with its surface Arctic
front from 21-03z (4-10 pm). This system brings our next shot of
snow and rain. At the upper levels we have good jet dynamics in play
beneath the rrq of a modest upper jet which will help increase
synoptic lift while in the lower levels speed convergence on the
front of a 50 kt low level jet will enhance precip formation. The very
progressive nature of the storm should limit snowfall to generally
late morning through early evening, with accumulations of only a
coating to 2 inches for most, mainly along and north of the Massachusetts Pike,
and perhaps some spots of 2-3 inches in the highest elevations of
the Berkshires. Nearly all guidance is in favor of this solution,
with the latest eps giving a 0% chance of >3" in southern New
England, and the 00z href mean guidance trending down toward the
synoptic models. Given the amounts expected, we will continue to
message this with a Special Weather Statement highlighting the
impact that it may have to the evening commute. Those in
eastern/central Massachusetts will continue to be impacted by snow showers
during the Friday evening rush hour which could cause some
additional travel hazards. Winds will also become breezy as the
pressure gradient tightens south of the low, mainly along the South
Coast and over the southern waters where mixing is better; winds
gust 25-30 mph.

&&

Short term /6 PM this evening through 6 PM Saturday/...
snow and rain showers should be tapering off in extreme eastern Massachusetts
by 8-10 PM as the low center makes a hasty exit. Winds slowly but
steadily decrease as a sprawling surface high moves in and the
gradient relaxes. With this we also see Post frontal northwest winds begin
to bring in much cooler air, though the breezy winds will likely
limit much contribution from radiational cooling. Even so,
temperatures dip into the teens to low 20s; warmer on the South
Coast.

Saturday colder and drier air funnels into the region as high
pressure builds in, though cyclonic flow lingers aloft. Subsidence
under this high will keep things dry and skies mostly clear, though
there is still some question as to whether we'll see more diurnal
clouds than are currently forecast given that tendency under
cyclonic flow with cold air advection and daytime heating. Overall a very nice, if
cold, day with high temperatures in the upper 20s (higher
elevations) to low 30s.

&&

Long term /Saturday night through Thursday/...
big picture...

Zonal flow over the weekend, but this changes early next week as a
digging upper trough sweeps south from the Canadian prairies into
the USA plains and Great Lakes. This digging trough turns the flow
over the eastern USA out of the south and southwest, drawing milder
air up over US as well as increasing moisture. The trough axis then
moves across our area Wednesday with drier air moving in.

Normal 500-mb heights in this area in early December are in the
upper 540s and 550s. Forecast heights moderate to the higher/warmer
side Sunday through Tuesday. These values then lower again Tuesday
night through Thursday as the upper trough moves overhead. Expect a
milder trend for early next week, and a return to colder temps
midweek.

Mass fields and thermal fields in the models are similar through
Monday but then show some model-to-model differences midweek. This
means moderate to high confidence through the early week, then
diminishing confidence midweek.

Daily concerns...

Saturday night and Sunday...

High pressure builds over the region with sufficient subsidence to
bring clearing skies and light wind. This will allow for radiational
cooling Saturday night, with dew points in the upper single numbers
and teens, that should allow temperatures to fall to similar values
overnight...possibly around 20 near the coast.

The high pressure slides offshore Sunday, making room for milder air
to move north. The milder air will move earliest at the higher
levels, which may bring some mid and high clouds Sunday afternoon.
Overall a fair day. Temps at 925 mb and 950 mb were used to
represent the lingering cold air near the surface. Those values will
support Max sfc temps in the 30s to low 40s.

Increasing south to southwest flow Sunday night will bring
increasing clouds. The flow itself will be strong with speeds
increasing to 40-50 kt overnight. Not sure how much of this will
reach the surface in gusts, but suspect the warm advection will work
against any mixing.

The 00z ggem brings light precipitation into our area late at night,
while the 00z GFS and European model (ecmwf) keep precipitation just south of US.
Guidance is more aggressive with pop values. The forecast will trim
values below 15 pct for most of the night based on the dry air
forecast in the nucleation zone for Sunday night, as seen in
BUFKIT soundings.

With the warm advection in place, expect min temps in the early
night followed by warming temps overnight. Temperatures will be
above freezing all areas after midnight, so any late night precip
should be as rain.

Monday-Tuesday...

Mild south-southwest flow will continue to bring warmer air and
increased moisture. Precipitable water values will climb over 1 inch and relative humidity fields
show a deep column of moisture over US for Monday and Tuesday. Low
level jet at 50-60 kt will support low level convergence and lift
while also supplying the moisture. The GFS and European model (ecmwf) show a 150-kt
upper level jet approaching on Monday. The GFS shows srn New England
in the left exit region for at least a part of the time while the
European model (ecmwf) is a tad farther north. All of this points to good lift and
good moisture both days, and a deep layer of above-freezing temps
supports Max sfc temps mainly in the 50s each day. Will forecast
showers both days.

Another concern will be winds. A low level southerly jet of 50-60 kt
will linger overhead through the period and may bring gusts of 30 to
40 mph down to the surface each day. Gridded guidance is much more
conservative. The forecast will increase winds from the grid
guidance closer to 25 kt/30 mph, and it may need to go higher in
later forecasts.

The upper trough pushes a cold front through our area Tuesday night,
with showers diminishing after passage.

Wednesday-Thursday...

Colder drier air moves in Wednesday. Cold advection will encourage
mixing 35-45 mph winds to the surface in gusts. High pressure builds
in by Thursday with fewer clouds and lighter winds. Mixing to 850 mb
will tap -10c air Wednesday and -18c air Thursday. Will go with Max
temps 35-45 Wednesday and 20s to mid 30s Thursday.

&&

Aviation /12z Friday through Tuesday/...
forecaster confidence levels...

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

Short term /through Saturday/...high confidence.

Today...VFR initially, but cigs will lower to MVFR along with
brief periods of light to moderate snow 18z-00z. Expecting
mainly rain across the cape and islands and a period of a
rain/snow mix at pvd. Potential for brief periods of IFR across
northern MA, including bos, with any heavier snow bands moving
through. Precipitation ends from west to east Friday evening.

Tonight...MVFR north of the Mass Pike, VFR to the south. Trend
to VFR all areas between 00z and 06z with clearing skies after
midnight.

Saturday...VFR. Light northwest winds.

Kbos terminal...high confidence in taf. Moderate confidence
after 18z.

Kbdl terminal...high confidence in taf.

Outlook /Saturday night through Tuesday/...

Saturday night: mainly VFR. Breezy. Patchy br.

Sunday: VFR. Breezy.

Sunday night: mainly VFR. Windy with local gusts up to 30 kt.
Patchy br.

Monday: MVFR/IFR conditions possible. Windy with local gusts up
to 30 kt. Chance rain showers.

Monday night: mainly MVFR, with local IFR possible. Windy with
gusts up to 30 kt. Rain showers likely.

Tuesday: mainly MVFR, with local IFR possible. Windy with local
gusts up to 30 kt. Rain showers likely.

&&

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

Today ... northwest winds decrease and become SW through the morning
and early afternoon before increasing again tonight. Winds on
the southern waters gust 25-35 kts this evening into tonight as
a clipper low moves across CT/Rhode Island and southeast Massachusetts today. Vsby
will lower to 1 mile or less in snow. Seas 3-4 ft.

Tonight... gusty SW winds 25-35 kts decreasing through the
night. Seas 8-10 ft on southern waters.

Saturday... clear skies and light winds. Seas 3-5 ft.

Outlook /Saturday night through Tuesday/...

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

Sunday: winds less than 25 kt. Seas less than 4 feet.

Sunday night: low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Areas of seas approaching 5 ft.

Monday: low risk for gale force winds with gusts up to 35 kt.
Local rough seas building to 5 to 10 feet. Chance of rain
showers.

Monday night: moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 13 ft. Rain showers likely.

Tuesday: low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 12 ft. Rain showers likely.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 7 am EST Saturday for anz232>235-
237.
Small Craft Advisory until midnight EST tonight for anz236.
Small Craft Advisory until 10 am EST Saturday for anz250.
Small Craft Advisory until noon EST Saturday for anz254.
Gale Warning from 5 PM this afternoon to midnight EST tonight
for anz255-256.

&&

$$
Synopsis...wtb/bw
near term...wtb/bw

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