NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Apr 1, 6:08 AM EST, 2 hours 22 minutes earlier.
Area forecast discussion, Apr 1, 6:08 AM EST, 2 hours 22 minutes before
Area Forecast Discussion
National Weather Service Taunton MA
708 AM EDT Sat Apr 1 2017
.SYNOPSIS...
Colder air will move south later today, changing rain and wintry
mix back to snow in many areas before it tapers off. Low
pressure moves east of Nantucket this afternoon. High pressure
brings dry weather Sunday and Monday. A new storm approaches
from Texas on Tuesday with rain and a chance of sleet inland.
Another storm may affect Southern New England late Thursday and
Friday.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
7 am update...
Low pressure south of Block Island will continue to move east
this morning. Extrapolating its movement would bring it east of
Nantucket around 18Z/2 PM. As it does so, winds will turn from
the north and draw colder air south across our region. With
borderline temperatures, such cooling should be enough to change
the rain to snow/sleet, especially across the North
Shore/Merrimack Valley, possibly the Boston Metrowest.
Note that 6 am temps were frequently 2F milder than forecast.
This may change as the colder air moves in. Also note winds
increasing on Cape Cod and Islands with gusts up to 50 mph. This
will continue to be a concern through the morning.
We have made adjustments to temps and dew points. The forecast
of additional snow looks good, although accumulations near the
coast may be more on grassy surfaces than on paved areas.
Previous discussion...
* Sleet/Rain Changes Back to Snow Over Next Few Hours *
Overview...
Low pres has already dropped below 1000mb as of 3AM per latest
MSAS data and is now about 100sm S of the Forks of Long Island.
This has had two primary impacts. Dry air is already spilling
into the W reaches of New England and E NY as trowal/CCB
continues development. Meanwhile, per 0.5 degree CC product,
warm air intrusion is starting to once again pivot slowly S, and
will accelerate as the low deepens and passes closer over the
next few hours. Therefore, outside ft DZ/FZDZ in the dry air
from the Worcester Hills W, a gradual shift from rain/sleet mix
elsewhere will give way back to SN as trowal/deformation banding
picks up.
Precipitation...
Mix snow/sleet will gradually give way back to SN. This has
already started in Essex CO, and will gradually shift SSW with
time through mid morning. Noting signature for decent banding
given nearly 15microbars/sec lift in the dendrite zone
especially across N/NE MA, typical of a comma-head on deepening
low pres. This is likely to yield some periods of 1-2 inch per
hour snowfall rates, as far S as the Boston metro. For the
immediate coast though (the city of Boston included)
accumulation on roads may be limited due to warm intrusion of
marine flow unless these higher rates are fully realized. This
will need to be watched.
Overall, mod/hvy SN can be expected at times especially N of
the Pike and across NE MA. This may overcome the temps near or
even above 0C at the SFC thanks to dynamic cooling. Therefore,
especially form the Worcester Hills E, the bulk of the snowfall
is still yet to come, and, although still generally wet, SLRs
will be a bit higher with this SN. Elsewhere, it will be a race
between diurnal warming and crashing heights this morning which
will ultimately determine both precip types and amounts yet to
fall. This will be especially true for areas S of the Mass Pike
where additional light snow cannot be ruled out, along with a
low risk for some sleet before ending given the gradual cooling
of the lowest levels prior to the cooling of the warm layer
between H85 and H7.
Given that more SN is expected where warnings currently sit,
will maintain headlines through the morning, allowing day shift
to get another look as the low pres passes and then transitions
out to sea. As much as another 6-8 inches are still possible
particularly from N Worcester CO E into Essex CO.
Winds...
Easterlies will be picking up early this morning as low pres
continues to deepen toward the SE coast. Strongest gusts, mainly
50-60 mph are expected across Nantucket as well as the SE
extremities of Cape Cod. This first round of winds drops off
late morning but gives way to another round of stronger N-NW
late this afternoon and evening in the dry intrusion/cold and
strong pres rises on the back side of the low pres. Another
round of 50-60 mph gusts possible Cape/Nantucket so will be
upgrading to a wind advisory there. 40-50 mph gusts also
possible across SE MA/RI with this second round, so wind
advisories continue inland.
Impacts...
The combination of accumulating sleet/fzra overnight will
continue to lead to slippery roadways this morning. This will
only be exacerbated by the gradual return to all frozen
precipitation as comma-head/trowal banding sets up. Therefore,
maintaining the Warnings/Advisories are warranted. The heavy
sleet/freezing rain could also exacerbate risk for power outages
inland in spite of lesser additional snowfall. Meanwhile, across
NE MA Mod-heavy wet snowfall returning will lead to an even
higher risk for downed trees/wires, and this will likely be the
area to watch especially through this afternoon as 1-2 inch per
hour snowfall rates cannot be ruled out, along with gusty ENE
winds.
Otherwise, strong winds Cape/Islands, although amongst mostly
rain could still yield some tree damage and power outages,
especially given the wet soils where total QPF by storm's end
may exceed 2.00 inches.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
Overnight...
Precipitation of all forms should end across E MA/RI between 21Z
and 00Z (5PM and 8PM local), but may linger across the
Cape/Islands a shade longer, but mainly as rain due to strong
marine influence. Strong winds will recede through the evening
and shift W. Although, some clouds may linger early in the
night as moisture remains trapped beneath a building subsidence
inversion. Therefore, this will limit cooling unless clouds
break earlier than currently forecast. Expecting mins mainly in
the upper 20s and low 30s. Will need to watch for patchy ice
where winds are unable to dry out wet area roadways. This is
most likely across E MA/RI where precipitation lingers later in
the day today.
&&
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Big Picture...
Weak longwave trough over the Northeast USA flattens closer to zonal
by late next week. Two southern shortwaves move through this flow
past New England next week, one on Tuesday and the second on Friday.
Models show a general agreement on these trends through Tuesday,
then differences in timing and placement on the shortwaves and
surface systems Tuesday evening through Friday. We used a blend to
average out these differences.
Details...
Sunday-Monday...
High pressure builds over the region. A northern stream shortwave
actually dips in on Sunday with cold air aloft. That combined with
solar heating of the surface and lingering moisture below 850 mb
will mean diurnal mixing and cloud formation. Skies then clear out
Sunday evening. Winds in the mixed layer Sunday reach 20-25 knots,
which will be available as northwest wind gusts at the surface.
Temps in the mixed layer support mid 40s to lower 50s.
Clearing skies and light winds for Sunday night will allow
radiational cooling. With dew points in the mid and upper 20s, min
temps of 25-35 are reasonable.
High pressure still overhead much of Monday, but shifts offshore
toward evening. Overall a dry day with temps aloft supporting max
sfc temps of 45-55. Developing southwest flow aloft will start to
bring some high clouds late in the day.
Monday night-Tuesday...
Shortwave and surface system from Texas Sunday night will move
through the Midwest Monday night and into the Northeast on Tuesday.
Low level south winds increase over the Ohio Valley Monday afternoon
and then shift to the East Coast Monday night after midnight. This
will bring increasing moisture and generate isentropic lift as it
moves over the colder air in place over us. We bring in likely pops
after midnight.
Surface winds from the south should keep most places above freezing.
With the surface high shifting out to sea, there is no significant
signal of cold air damming inland. However low level ageostropic
winds inland are forecast at 40-45 knots from the north, leaving a
concern that some cold surface air may drain into our northern
areas. We are indicating a chance of sleet or freezing rain in
parts of Western and Central Mass, mainly north of the Mass Pike.
Winds turn from the east on Tuesday with south winds above that and
strong isentropic lift indicated through much of the day. We will
show categorical pops for rain. Precipitable water values are
indicated to reach at least 1 inch, possibly 1.25 inches. These
values are well above average. For that matter they are above the
90 percent value for the date. This indicates potential for a
soaking rain on Tuesday. Winds in the low level jet reach 60-65
knots at 2000-3000 feet AGL. This suggests wind gusts in any heavier
downpours may reach 35-40 knots.
One concern is the handling of the surface low on Tuesday by the
GFS. The 00Z run shows the primary surface low moving west-east
across NY and VT to near Portsmouth New Hampshire. This might be due
to a shallower depiction of the upper shortwave. The 00Z ECMWF
redevelops the low along the Long Island coast and slides it around
SE Mass early Tuesday night. Tend to favor the ECMWF handling, but
as it is 96 hours out there is time for this to change. Expect a
cloudy wet day with strong gusty ESE winds along the coast.
The low moves off through the Gulf of Maine Tuesday night, at which
time the rain should taper off from west to east.
Wednesday through Friday...
High pressure surface and aloft build overhead Wednesday and
Wednesday night. Expect dry weather during this time. Temps aloft
support max temps in the upper 40s and 50s.
Another weather system comes out of the Southern Plains midweek. The
long range models are showing a higher amplitude flow mid-late week,
and thus a slower progression of this later weather system. Current
consensus brings the system to New England from late Thursday
through Friday. If there is any cold air damming at that time, it is
not showing well in the data. But this is also seven days in the
future with time to evolve. We expect another rainy forecast for
late Thursday-Friday.
&&
.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Tonight/...
Through 18Z...Moderate confidence.
Mostly IFR conditions across the region with some MVFR in
Western Mass. Mix of RA/PL this morning will gradually give way
to SN from N-S through mid day. Some accumulations of SN/PL are
likely especially N of the Mass Pike from Worcester County MA
to the E. Expect lesser amounts along the immediate coast,
especially Boston to Gloucester. Some lighter accums further S.
E to NE winds increase with time, reaching 20-30 kt inland then
40-50 kt mainly SE MA.
18Z this afternoon through tonight...Moderate confidence.
High confidence in TAF trends lower confidence in timing. Last
remnant SN/RA comes to an end from W-E through 00Z, lingering
longest over Cape/Nantucket. MVFR conditions linger thanks to
low CIGS past 00Z for many although some improvement especially
W MA/CT are possible. N-NW wind gusts increase during the late
afternoon/evening especially Cape/Islands where another round of
45-55 kt are possible.
KBOS Terminal...Moderate confidence in TAF. Change back to SN is
expected but timing may be off in TAF.
KBDL Terminal...Moderate confidence in TAF.
Outlook /Sunday through Wednesday/...
Sunday-Monday...
VFR. Northwest winds Sunday may gust to 25 knots. Winds become light
and variable Monday.
Monday night and Tuesday...
Lowering cigs Monday night, especially after midnight. MVFR-IFR cigs
and vsbys expected after midnight and through the day Tuesday.
Pockets of snow/sleet/freezing rain possible initially in Northern
Mass and along the East Slopes of the Berkshires, but should change
to rain during Tuesday. East-Southeast winds at the surface. Strong
east winds expected at 2000 feet AGL with speeds of 60-65 knots.
This would lead to low level wind shear in the coastal plain.
Tuesday night-Wednesday...Rain tapers off Tuesday night. Conditions
improving to VFR from west to east by Wednesday. North-northeast
winds along the coast with gusts near 25 knots.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Tonight/...
Today...High confidence. E winds continue to increase this
morning with the expectation for gusts around 50-55 kt across SE
open waters. Storm Warnings continue there with Gales elsewhere
with gusts 35-45 kt. A brief lull is possible before winds
shift to the N this afternoon and increase once again to near
storm force.
Gale force gusts drop rapidly overnight tonight, but small craft
advisory for lingering N-NW winds and very high seas will still
be needed even as Gales end.
Outlook /Sunday through Wednesday/...
Sunday through Monday...
Northwest gusts to 25 knots Sunday. These diminish late in the day
as high pressure moves over the waters. Seas remain above 5 feet on
the eastern exposed waters during the day but diminish late Sunday
and Sunday night. Small Craft advisories will be needed Sunday.
On Monday lingering 5-foot seas on the outer waters will diminish by
afternoon. Winds will be less than 20 knots through the day.
Monday night through Wednesday...
Winds increase from the south and southeast Monday night, reaching
25-30 knots early Tuesday south of RI. Southeast winds continue to
build on Tuesday, reaching 35 knots on most of the waters. Winds
turn from the north Tuesday night and northwest Wednesday with
speeds diminishing to 25-30 knots. Seas build to 5-10 feet
Tuesday, then subside Wednesday. A Gale Warning may be needed
for Tuesday.
&&
.TIDES/COASTAL FLOODING...
The Coastal Flood Advisory remains in effect for east and
northeast facing shorelines south of Boston for the afternoon
high tide. Anticipate a surge of 1.5 to 2 feet along the
Plymouth County coast, Nantucket and the east/northeast sides of
Martha's Vineyard. This has been slightly increased given a
higher than predicted surge with the early morning high tide
this morning. However, even with this increase, current location
of Coastal Flood Advisory is good for this afternoon, so will
not be making changes with this morning's update.
Waves of about 10 to 15 feet are expected into Massachusetts
Bay and 14 to 18 feet east of Cape Cod and east of Nantucket
during the this afternoon. The combination of surge and waves
will likely cause areas of minor coastal flooding from Hull to
Plymouth to Dennis, Nantucket Harbor area, and possibly at some
spots along the Vineyard shore. In addition, we expect some
erosion as well, especially along the ocean side of Cape Cod and
the east side of Nantucket. At the very least, there could be
some isolated splashover north of Boston at the time of high
tide but do not expect any significant impact.
We still may need to issue a Coastal Flood Advisory for
portions of the Massachusetts east coast for the early Sunday
morning high tide but have not done so yet, since there is
enough time for changes in the intensity and timing of the
storm through the day today. As of now, winds do not really
turn offshore until after the time of the high tide Sunday
morning and hence may see a residual surge of 1.0 to 1.5 feet.
The surge in addition to a long period swell may be enough to
cause an additional round minor coastal flooding and erosion for
east, northeast, and north facing shorelines south of Boston.
Experimental wave run- up guidance is also showing a risk for
erosion and minor overwash along the Plymouth County coast
during the Sunday early morning high tide.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 11 AM EDT this morning for
CTZ002>004.
MA...Wind Advisory until 8 PM EDT this evening for MAZ013-016>021-
023.
Coastal Flood Advisory from 2 PM this afternoon to 6 PM EDT
this evening for MAZ019-022>024.
Winter Storm Warning until 4 PM EDT this afternoon for
MAZ002>008-010-012-014-015-026.
Winter Weather Advisory until 4 PM EDT this afternoon for
MAZ013-016.
Winter Weather Advisory until 11 AM EDT this morning for
MAZ009-011.
High Wind Warning until 8 PM EDT this evening for MAZ022-024.
RI...Wind Advisory until 8 PM EDT this evening for RIZ002>008.
Winter Weather Advisory until 4 PM EDT this afternoon for
RIZ001.
MARINE...Storm Warning until 8 PM EDT this evening for ANZ232-254>256.
Gale Warning until 11 PM EDT this evening for ANZ231-233>235-
237.
Gale Warning until 8 PM EDT this evening for ANZ230-236.
Gale Warning until midnight EDT tonight for ANZ250-251.
&&
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Doody
SHORT TERM...Doody
LONG TERM...WTB
AVIATION...WTB/Doody
MARINE...WTB/Doody
TIDES/COASTAL FLOODING...