NWS Technical Forecast Discussion

550
FXUS61 KBOX 230216
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1016 PM EDT Fri Mar 22 2019

.SYNOPSIS...
Scattered showers will change to a period of snow tonight with
some accumulations expected at least in the high terrain and it
will become windy. Dry weather is on tap for Saturday, but it
remains windy. Milder temperatures arrive Sunday, but it will
turn colder again Monday into the middle of next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...

* Accumulating snow for the high terrain tonight but a touch of
  snow in all locations overnight

* Turning windy across the entire region tonight

10 PM Update...

As expected precip shield is blossoming over southern New
England as comma-head pivots cyclonically across the region.
SPC mesoanalysis displays this nicely with WAA at 500 mb from
north to south across New England with -23C air over northern
NH/VT and -34C over NJ,as potent mid/upper level short wave
moves across NJ/NYC/LI.

Despite radar looking impressive ptype is rain at BDL with a
temp of 40. Even where it is snowing temps are 35 at AQW and
AFN. So rain will eventually change over to snow overnight as
CAA increases. However initially snow will only accumulate on
colder surfaces and then eventually onto paved surfaces. Thus
some accumulations will initially be lost to melting. Therefore
our forecast snow accumulations may be too high by an inch or
so. Nevertheless, no wholesale changes with this forecast
update. Earlier discussion below.

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

Previous Discussion...

Despite the fact that the surface low will be lifting into
downeast Maine tonight, another robust piece of shortwave
energy drops southeast. The result will be an increase in areal
coverage of the precipitation this evening, especially during
the overnight hours. The column will also be cooling, so any
mixed precipitation will quickly change to snow showers across
the high terrain. An initially mild boundary layer will result
in this process taking until later this evening and possibly
after midnight in some lower elevations. However, we do expect
all locations to change to see at least a brief period of snow
showers.

A rather interesting snowfall forecast too with some
rather unusual mesoscale processes in play. Strong low level
northwest flow will result in upslope enhancement in the
Berkshires as well as Worcester/CT Hills. The northwest wind
trajectory will favor the western portions of the Hills, where
the upslope is enhanced.

The exact wind trajectory actually favors Tolland County CT
where upslope flow will be enhanced. They are also closest the
the dynamics of the shortwave passing near the south coast.
Therefore, we opted to go with a Winter Weather Advisory there
for 2 to 4 inches of snow. We also continued those headlines for
western Franklin/western Hampshire county in MA, where an
additional 2 to 5 inches of snow is possible tonight with higher
amounts near and above 1500 feet. Across the Worcester Hills,
the upslope component is not quite as strong and the better
dynamics are to their south. Nonetheless...1 to 3 inches of snow
appear reasonable on the western slopes, but there is a low
risk will need to upgrade to an advisory depending on how things
transpire. Across the rest of the region...generally a coating
to 1 inch of snow is possible mainly on grassy surfaces.
However,there is a low risk for isolated 2 inch amounts across
portions of RI/SE MA where the strongest dynamics appear to
reside.

Lastly, the sub 980 mb low will increase the pressure gradient
and result in westerly wind gusts of 35 to 45 mph. It is on the
cusp of Wind Advisory criteria, but for now opted to only
include the Cape/Nantucket given it is rather marginal.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...

Saturday...

The snow showers will pretty much be over by daybreak Saturday.
While some clouds may linger in the morning, skies should become
partly to mostly sunny by late morning/early afternoon. Strong
mixing supports westerly winds of 35 to 45 mph with perhaps a
few gusts up to 50 mph. Again...we were on the cusp of wind
advisory criteria but held off at this point except for the
Cape/Nantucket. Later shifts may need to consider expanding
dependent on future model guidance. High temperatures should be
well up in 40s to perhaps near 50 across portions of RI/SE MA.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...

Highlights...

* Brisk conditions Saturday evening, then dry and milder Sunday
* Cold front brings rain late Sunday night and Monday, with
  snow across the higher inland terrain
* Colder weather follows through mid week, then temperatures
  look to rebound late next week

Details...

Saturday night and Sunday...

Large high pressure builds SE across mid Atlc on general W-NW
flow aloft. May see some gusts up to 25 kt or so Sat evening
with leftover pressure gradient will diminish overnight. Should
end as low level lapse rates quickly drop below H85. However,
winds will remain gusty across the coastal waters with good low
level mixing there. Skies clear out, which will allow for
radiational cooling away from the coast. Temps will bottom out
in the mainly in the 20s, but only down to the lower 30s along
most of the immediate coast.

Will remain dry on Sunday as the high passing S of the region.
So, will see sunny skies early, but high clouds will start to
push in across western areas during the afternoon. Highs will
range from the mid-upper 40s across the higher terrain and
across most of the Cape and islands, up to the lower-mid 50s
elsewhere.

Sunday night and Monday...

As the high positions off the mid Atlc and Carolina coasts,
Arctic front pushes steadily SE Sun night, while weak low pres
shifts E across the southern Great Lakes and Ohio valley. Should
see the leading edge of the precip shift into N Mass after
midnight Sun night. Temps will be cold enough for precip to
start off as light snow, though QPF values will be less than 0.1
inches, so should not see a lot of snow accumulation.

The front will slowly push across the region during Monday,
with some enhanced precip as the midwest low slides along it.
Temps will be close for RN/SN mix or light snow in general,
mainly across the higher terrain into N central/NW Mass Sunday
morning before changing over to all rain. As the front moves S,
winds shift to NW but the colder air lags behind. So, expect
temps to top off in the upper 30s to lower 40s across the higher
terrain, ranging to the upper 40s across the coastal plain.
Colder air filters in during late Mon and Mon night, with lows
in the upper teens and 20s, except around 30 along the immediate
S coast.

Tuesday and Wednesday...

Dry but cold conditions in place during this timeframe as
another large high slowly crosses the eastern half of the U.S.
Coldest air expected Tuesday as H85 temps drop to -14C to -18C
(S to N) around 12Z Tuesday. Expect high temps to run 10 to 15
degrees below seasonal normals, mainly in the 30s except around
40 across the lower CT valley and S coastal areas. Winds will
diminish Tue night so radiational cooling will allow temps to
plummet down to the mid teens to lower 20s inland, ranging to
the mid-upper 20s along the coast.

As the center of the 1036 hPa high crosses New England
Wednesday, will see mostly clear skies but it will remain cold.
Temps will be about 5 degrees above Tuesday`s readings, but
still only in the upper 30s to mid 40s.

Thursday and Friday...

The high will remain along the eastern seaboard Thursday as
broad troughing begins to develop across the N central and
western U.S. Some question where the ridge axis sets up, which
will determine the temp profile. For now, looks like temps will
rebound to near or a bit above seasonal levels late next week,
but quite a bit of uncertainty as to the position of the high.
At this point, the dry regime looks to continue.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...

03z update ...

Rain showers change over to snow showers overnight, probably
around 06z or so. Snow accumulations mainly confined to the
higher elevations and non paved surfaces. Thus runways mainly
wet especially at the lower elevations. Improving conditions
from west to east 09z- 12z. Earlier discussion below.

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

Tonight...Moderate to high confidence.
Expect scattered showers to increase in areal coverage through
the evening. PTYPE mainly SHSN across the E slopes of the
Berkshires into S VT, but probably will take until late evening
or even after midnight to reach some of the lower elevations. A
few inches of snow are likely in the higher terrain. In the
lower elevations, expect a coating to 1 inch with a low risk for
up to 2 inches across portions of RI/SE MA. Most runways in the
lower elevations will remain wet, but a brief slushy coating of
snow would be possible in any heavier snow showers. MVFR
conditions will dominate tonight, but IFR thresholds will be met
in bands of snow showers with brief LIFR conditions not out of
the question. Westerly wind gusts of 30 to 40 knots expected.

Saturday...High confidence.
Some lingering MVFR ceilings in the morning will improve to VFR
by afternoon. Northwest wind gusts of 30 to 40 knots expected.

KBOS terminal...Moderate to high confidence in TAF.

KBDL terminal...Moderate to high confidence in TAF.

Outlook /Saturday Night through Wednesday/...

Saturday Night: Mainly VFR, with areas IFR possible. Windy with
gusts up to 35 kt.

Sunday: IFR. Breezy.

Sunday Night: Mainly VFR, with local IFR possible. Breezy.

Monday: Mainly MVFR, with areas VFR possible. Chance RA.

Monday Night through Tuesday: VFR. Breezy.

Tuesday Night through Wednesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...

*** WNW Gales Tonight and Saturday ***

Tonight and Saturday...High confidence.
Strong low pressure lifting into downeast Maine will result in
an increasing pressure gradient. The result will be high
confidence in west to northwest gale force wind gusts of 35 to
45 knots developing tonight and continuing into Saturday. Gale
Warnings remain posted for all waters through Saturday
afternoon.

Outlook /Saturday Night through Wednesday/...

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Visibility 1 nm or
less.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Visibility 1 nm or
less.

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Local visibility 1 nm or less.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain.

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers, slight chance of snow
showers. Local visibility 1 to 3 nm.

Tuesday through Tuesday Night: Winds less than 25 kt. Areas of
seas approaching 5 ft.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 7 AM EDT Saturday for CTZ003.
MA...Wind Advisory until 4 PM EDT Saturday for MAZ022-024.
     Winter Weather Advisory until 8 AM EDT Saturday for MAZ002-008.
RI...None.
MARINE...Gale Warning until 8 PM EDT Saturday for ANZ231>235-237-250-
     251-254>256.
     Gale Warning until 6 PM EDT Saturday for ANZ230-236.

&&

$$
SYNOPSIS...Frank/EVT
NEAR TERM...Frank/Nocera/EVT
SHORT TERM...Frank
LONG TERM...EVT
AVIATION...Frank/Nocera/EVT
MARINE...Frank/EVT

NWS BOX Office Area Forecast Discussion