NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Dec 21, 6:55 AM EST, 35 minutes earlier.
Area forecast discussion, Dec 21, 6:55 AM EST, 35 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
655 AM EST Fri Dec 21 2018
.SYNOPSIS...
A strong low pressure passing west of the region will bring heavy
rainfall and strong coastal winds into tonight. Low pressure
departs on Saturday followed by mainly dry and seasonably cold
conditions Sunday through Thursday. Another low pressure system
may bring rain late next week along with milder temperatures.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
655 AM Update...
Rain has moved in across Southern New England. Some lighter
showers on the Cape and Islands, but expect the rain to expand
over that area this morning. Morning observations show 0.50 to
1 inch in 6 hours upstream in New Jersey. Expect this to move
into our area this morning, with flood advisories likely.
Adjustments have been made to temperatures, but no significant
changes to the forecast.
Previous discussion...
Overview...
With the 00Z guidance updates, the synoptic scale features
remain in play as advertised. Deep trof will gradually take on a
negative tilt as strong vortices rotate S-N. The first round of
-RA having already moved thorugh overnight, associated with
advanced isentropic lift will now give way to baroclinically
forced precip as low-mid lvl warm front slide through between
the time of this writing and 12Z this morning. Following this
the first in a series of reinforcing LLJ will approach the S
coast, with strong convergence allowing for stronger forcing
across CT and W MA through the morning hours. PWAT plumes are
still extreme, nearing CHH/OKX record values for the date and
model prog maximums, so there will be no loss of moisture. The
deep warmth suggest efficient warm cloud processes, which along
with the high humidity and forcing from the LLJ will yield very
high rainfall rates throughout the day, potentially reaching or
exceeding 1.00 in/hr at times. Persistent 50-60 kt H92 LLJ will
be reinforced this evening across the SE, with a boost to 70+
kt. Below is a reiteration of the main impacts:
Rainfall...
The first round associated with the warm front will yield
periods of mod-heavy rain through about mid morning. Then as the
LLJ initially sets up across the coast, this will shift to a
more linear band along the main theta-e and PWAT axis setup
across CT and W/central MA. The combination of the low-mid
convergence as well as enhanced upslope will lead to heavy
rainfall rates. However, one concern is, as the overall mid lvl
trof axis tilts, it will hold this plume/precip shield in place
into the afternoon hours. Recent meso-scale models support this
thinking suggesting that the final highest QPF amounts may fall
across CT and W MA, and will update forecast to reflect this
thinking. Some HREF ensemble members show as much as 3-5 inches
in these areas, so will need to monitor for stream/river
flooding as well as localized urban flooding in the lower
portions of the CT valley. Further E, as the reinforced LLJ
begins to shift E later today, the axis of rain will begin to
shift E and remain at least somewhat progressive. However, all
the players remain for periods of heavy rainfall as it becomes
transitory, so will need to monitor for periods of heavy rain
with poor drainage or urban flooding and even some river
flooding as well, although QPF values will be more between
1.00-3.00in given the somewhat progressive nature. Will need to
monitor for convective elements as MU CAPE values could exceed
100j/kg, which could enhance these rainfall rates. Overall,
given the river/urban flood risk continues, will maintain the
current Flood Watch.
Strong winds...
With the first H92 LLJ impulse moving from the S coast and over
SE New England at least some of the 55-65 kt momentum will be
mixed to the sfc, especially where some of the lower lvl
inversion is eroded or overcome via precip drag. This will yield
at least wind advisory criteria gusts/damage (40+ mph) through
the daylight hours where current advisories reside. However,
latest trends push the axis a bit further W, so will be expand
the wind advisory include NE CT/NW RI and much of central MA
where the axis of higher terrain rests.
The second impulse crosses this same region between 21Z and 03Z
this evening, with a core exceeding 70kt. With convection a
possibility, and some CAMs suggesting a fine line development,
the strongest gusts across SE MA or even RI will more than
likely occur during this secondary impulse. In any case, gusts
50-60 kt are possible, with a low risk for higher should this
convective line develop and cross land from the open waters.
Given the prolonged stress from winds during the day, combined
with soaked soils, this may exacerbate the risk for tree damage.
Will be expanding the high wind warning to include Cape Ann due
to its exposure and proximity to the core of the jet.
Convective impacts...
Unexpectedly, given the model maximum LLJ, 0-3 km helicity
values are very high, on the order of 400+. However, soundings
continue to suggest little in the way of sfc based CAPE given
the moderate inversion. In fact, peak helicity and sfc
instability do not overplap, the former across New England and
the later upstream across PA/NJ. Still, with a pulse of higher
mid lvl lapse rates and MU CAPE values exceeding 100j/kg late
this afternoon and evening, if any stronger convection occurs
there is the low risk for a spin-up. This is most likely over
the waters, suggesting a waterspout risk but the risk is non-
zero over SE MA as well. In fact, have noticed some gate-to-gate
shear couplets offshore of the Carolina's and VA overnight.
However, it should be noted that the development of an
efficient fine line may be the dominant threat here given recent
CAMs suggesting of this possibility. This could more
efficiently bring down the anomalous LLJ.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
Short term discussion is covered by the near term discussion.
&&
.LONG TERM /SATURDAY THROUGH THURSDAY/...
Big Picture...
Upper trough associated with today's storm will move northeast
of Southern New England. The upper flow then becomes zonal, with
at least two shortwaves moving through the flow. Model
consensus would bring them through New England Monday and
Wednesday. But shortwaves forecast in a zonal flow carry some
uncertainty as to how fast they actually move. A third
shortwave, coming off the Pacific and into the Southwest USA,
may eject through the Plains Thursday.
Upper contour fields diminishes below normal over the weekend,
recovers to normal Sunday, then cools again Monday. A broad upper
ridge trends rising heights through midweek.
Mass and thermal fields on the various long-range models show
agreement on the big picture much of the period. For example,
they agree on the building upper ridge next week. But there remain
differences in details in positioning of the various shortwaves
next week.
Concerns...
Saturday...
Cold front moves through by Saturday midday. This passage may
generate one more batch of showers, followed by a wind shift
from the west and bring in colder drier air. Mixing in the cold
advection behind the front should reach 900 mb or a little
above. Winds in the mixed layer will be 30-35 kt. Temperatures
in the mixed layer will support mid 50s in the east, mid 40s in
the western hills.
Sunday through Wednesday...
Dry weather with seasonably cool temperatures. Shortwave on Monday
suggests a chance of rain/snow showers, although moisture and
lift are rather limited and suggest slight chance pops at best.
RH cross sections do support a period of sky cover Monday.
Partly to mostly clear Tuesday, then another batch of clouds for
Wednesday.
Thursday...
GFS shows ejecting shortwave reaching the Midwest and drawing
southerly flow and overrunning Thursday night. ECMWF shows high
pressure dominating through the day, with fair skies and light
winds. All suggest dry weather during the day. The GFS is
fastest with onset, while the GGEM and ECMWF suggest onset
toward Friday morning. Low confidence in timing of the rain, but
we trended toward the slower timing.
&&
.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 Tonight/...High confidence.
Today... IFR even occasional LIFR conditions in periods of
heavy rain. Strong E-SE winds gusting 30-40 kt inland, 40-50 kt
across E MA and RI, and 50-60 kt across SE MA (strongest this
evening). LLWS, 40-50 kt through the day, then 50-65 kt
especially across the E this evening. Low risk for TS,
especially across the SE this evening.
Overnight tonight...
Gradual improvement, winds slacken and rains taper after 03Z
this evening although lingering lighter rain is likely until the
early AM hours. IFR/MVFR conditions continue after a brief lift
to VFR following the heaviest rain. S winds around 10 kt with
gusts around 25 kt through 12Z.
KBOS Terminal...High confidence in TAF trends, timing may be off
a bit.
KBDL Terminal...High confidence in TAF trends, timing may be off
a bit.
Outlook /Saturday through Tuesday/...
Saturday: Mainly VFR, with local IFR possible early. Windy with
gusts to 35 kt. Slight chance SHRA.
Saturday Night: VFR. Windy with gusts to 30 kt.
Sunday: VFR. Breezy.
Sunday Night: VFR. Slight chance SHSN.
Monday: VFR. Breezy. Slight chance SHSN.
Monday Night through Christmas Day: VFR. Breezy.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Tonight/...High confidence.
Winds will begin to increase through the morning hours reaching
reaching gale force later this morning and storm force this
afternoon. These winds will then linger into the overnight
hours. Heavy rain/fog will accompany these winds with resulting
visibility below 3 nm.
All rain/winds dissipate this evening, although high seas will
take some time to diminish. Gale conditions linger into Saturday
for some areas, while others can be converted into Small Craft
Advisories.
Outlook /Saturday through Tuesday/...
Saturday: Moderate risk for gale force winds with gusts up to
35 kt. Rough seas up to 17 ft. Slight chance of rain showers.
Saturday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 16 ft.
Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas.
Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.
Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Monday Night: Winds less than 25 kt. Local rough seas.
Christmas Day: Winds less than 25 kt. Seas locally approaching
5 ft.
&&
.TIDES/COASTAL FLOODING...
* Coastal Flood Advisory for South Coast This Evening *
Early AM surge values from Sandy Hook, NJ to Montauk Pt
including CT along LI sound is generally running about 0.6-0.9,
highest close to the developing LLJ to the S. This is about
130-140 percent of current ESTOFs values. However, these values
are also low enough such that the early AM S coastal tide should
pass with little impact in spite it being high astronomically.
However, the combination of persistent S-SE fetch and a couple
strong LLJ impulses surge values could reach 1.5-2.0 ft along
the S coast, along with 12-15 seas offshore. This will continue
to risk for some minor coastal flooding mainly across S coastal
locations with this evening's high tide. Current Advisory covers
those areas at risk.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through late tonight for CTZ002>004.
Wind Advisory until 10 PM EST this evening for CTZ004.
MA...High Wind Warning until 10 PM EST this evening for MAZ007-
019>024.
Flood Watch through late tonight for MAZ002>024-026.
Wind Advisory until 10 PM EST this evening for MAZ005-006-
012>018.
Coastal Flood Advisory from 4 PM this afternoon to 9 PM EST
this evening for MAZ020>022.
RI...High Wind Warning until 10 PM EST this evening for RIZ006>008.
Flood Watch through late tonight for RIZ001>008.
Wind Advisory until 10 PM EST this evening for RIZ001>005.
Coastal Flood Advisory from 4 PM this afternoon to 9 PM EST
this evening for RIZ004>007.
MARINE...Storm Warning until 9 PM EST this evening for ANZ231-232-256.
Gale Warning until 1 AM EST Saturday for ANZ233>235-237-251.
Gale Warning until 10 PM EST this evening for ANZ230-236.
Storm Warning until 1 AM EST Saturday for ANZ250-254-255.
&&
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Doody
SHORT TERM...Doody
LONG TERM...WTB
AVIATION...WTB/Doody
MARINE...WTB/Doody
TIDES/COASTAL FLOODING...