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Area forecast discussion
National Weather Service Taunton Massachusetts
651 am EST Sat Nov 18 2017

Synopsis...
high pressure over New England this morning provides dry
weather much of the day. A warm front approaches the region late
in the day with rain overspreading the area toward sunset and
into the evening. Strengthening low pressure moves across
western New York state tonight with warm air, strong winds and showers
impacting southern New England. The attending cold front sweeps
across the area midday Sunday with strong winds ahead and
behind the front. Dry and cold conditions move across the
region Sunday night and Monday. Mild and dry weather Tue ahead
of a cold front which brings a risk for showers Tue night and
Wed, followed by dry and colder weather Thanksgiving day.

&&

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

630 am update...

Mainly clear skies noted across central and eastern areas via
11z observations and latest GOES-13 infrared satellite imagery.
Leading edge of high clouds have started to push into the east
slopes of the Berkshires and western CT River Valley, which
will continue to quickly push east through the day.

Leading edge of precip has reach into western New York state/west PA,
with a few reports of -ra over far West New York state and northwest and west
central PA where the higher radar reflectivities are noted on
regional composite radar. With the very dry air in place and an
approaching mid level disturbance weakens in the fast flow
aloft, it should remain dry through late morning.

Rather cold start to the day with the mainly clear skies,
dewpoints down to the teens and light/variable or calm winds.
Temps at 11z were generally in the upper teens and 20s, except
around 30 along some of the immediate S coast as well as near
kbos. Expect S-SW winds to pick up during the morning, with
gusts up to around 20 kt along the S coast by midday. This will
bring temps up rather quickly, as readings recover to around 50
degrees along the S coast, but only the mid 30s to lower 40s
across north central and west mass.

Previous discussion...

Cold Ridge of high pressure across the region this morning.
Sunshine to begin the day, but clouds will quickly increase as
mid level ridge axis moves offshore and flow becomes cyclonic
with warm air advection pattern developing. Despite increasing clouds today low
level warm air advection will provide a nice temp recovery from this morning's
cold with highs this afternoon 50-55 along the coastal plain,
extending into the Boston-Providence-Hartford corridor. Farther
inland shallow cool air will be more stubborn to modify and
highs there rising into the mid and upper 40s.

Area of rain showers across west PA/New York will weaken and mainly pass
northwest of our region as this lead short wave trough deamplifies
as it runs into the ridge over New England. Thus expecting
mainly dry weather today with any rain holding off until 21z-00z
and mainly confined to CT and western-central Massachusetts. Therefore
expecting dry weather to prevail today across Rhode Island and eastern Massachusetts.

&&

Short term /6 PM this evening through 6 PM Sunday/...

Early am update...

*** strong south-southwest winds south coastal Massachusetts and Rhode Island ***
*** possible fine line of low convection ***

Tonight and Sunday morning...

Wind threat...

Strong low level jet overspreads the South Coast tonight. Good
agreement from models on magnitude of jet 55-60 kt at 925 mb. Model
soundings initially Show Low level lapse rates favorable this
evening for 35-40 kt Transfer in gusts. But as the night progresses
strong warm air advection steepens low level inversion with gusts becoming less of
a factor and sustained winds becoming the main issue with speeds 25-
30 kt. This will be sufficient to support Wind Advisory for the
entire South Coast including Cape Cod and the islands.

Rain and convective threat...

Periods of rain with warm front lifting north across the region.
Impressive warm sector for mid to late Nov overspreads the area late
tonight with dew pts surging to 55-60! Thus temps will rise
overnight toward 60 in the coastal plain. This will set the stage
for a potential fine line of low top convection toward daybreak
Sunday from west to east. Short wave energy merges into a negative
tilt trough approaching the region toward 12z Sunday. Also a strong
upper level jet streak of 130 kt approaches from the southwest 12z
Sunday with lfq over southern New England, enhancing qg forcing. In
addition very strong frontal convergence as low pres deepens to 982
mb over New York state. Thus very strong forcing for ascent in a high
shear environment. With dew pts rising to near 60 a few hundred
joules of cape advects across the area, anomalous for mid to late
Nov. However limiting factor for convection is marginal low and mid
level lapse rates. So question becomes will there be sufficient
instability to yield a strong response at the surface in the form of
fine low of convection. Unfortunately model timeframe here is 36-42
hrs, on edge of our mesoscale guid with hrefv2 only out to 36 hrs.
However the ncar ensembles, arw and nmmb all hint at possible fine
line in simulated reflectivity fields. For now will insert slight
chance thunder with gusty winds and heavy downpours as pwats climb
to 1.5 inches (+2 Standard deviations).

Sunday afternoon...

*** strong Post frontal winds likely ***

979 mb low traversing northern New England/St Lawrence River valley
will yield strong pres gradient, Post frontal cold air advection and impressive
pres rise-fall couplet for a period of strong west-northwest Post frontal winds
Sunday afternoon with speeds of 40 to 50 mph possible. Later shifts
will have to evaluate model trends for possible issuance of wind
advisories for Sunday afternoon. Drying trend behind the fropa along
with temps falling from morning highs near 60.

&&

Long term /Sunday night through Friday/...

Highlights...

* strong west-northwest winds linger Sunday night into Monday
* some question whether moisture tries to work up the eastern
Seaboard around the Tue night/Wed timeframe as cold front passes
* temperatures fluctuate day to day, from mild to cold back to mild
again through most of the work week

Overview...

Overall mid level steering pattern across the lower 48 remains
progressive through most of the long term periods. However,
noting some members of the 00z model suite try to dig a long
wave 500 mb trough along the eastern Seaboard around the late
Tuesday into Wednesday timeframe, which may try to capture some
southern stream moisture and push it up the eastern Seaboard as
a cold front passes across the region. The trough appears to
retreat northward late Wed night or Thu, then becomes nearly
zonal in the northern stream by the end of next week while
another northern stream trough starts to dig into the Pacific
northwest.

Models and ensembles continue to signal fluctuating temperatures
as progressive short wave troughs and ridges move across the
region. So, will see mild temps ahead of approaching frontal
systems, then a cool down after the systems pass. By the end of
the week, temps may feel more like early winter than late
autumn.

Used a blend of available guidance for the majority of the
forecast, though opted closer to a 00z ggem/European model (ecmwf) blend for Tue
night/Wed with possible influx of moisture into the region ahead
and with the approaching cold front.

Details...

Sunday night through Monday night...

500 mb short wave digs across into east PA/north New Jersey and steadily shifts east
which will bring another shot of cooler air as well as gusty
west-northwest winds. As the strong surface low, on order of sub-980 hpa
across north Maine into the Gulf of St Lawrence will exit with a
tight pressure gradient in place. Also, both the GFS and NAM
BUFKIT soundings showing deep momentum Transfer, possibly up to
800 hpa across inland areas from Monday morning through early
afternoon. There is also excellent low level lapse rates up to
h85, up to 8-9c/km. So, could see gusts up to 35 to 45 mph,
possibly higher across the higher inland terrain and along the
coast at times. Have kept mention of the potential of strong
gusts in the severe weather potential statement for this timeframe.

Expect dry conditions during this time, but some question
whether any lake effect snow bands may reach into western areas
during Monday on the strong west-northwest winds. At this point, kept a
dry forecast going.

Strong cold air advection continues behind the departing low,
with 850 mb temps down to -10c to -12c overnight into Monday
morning and not changing much during the day Monday. So, expect
highs possibly holding in the lower-mod 30s across the higher
terrain of north central and northwest mass, ranging to 40-45 across the
coastal plain. With the gusty winds, it will feel more like the
upper teens to mid 20s inland to 30-35 along the shore.

Tuesday...

High pressure builds across the mid Atlantic and southeast states,
with northern ridging passing across the region. Winds will back
to SW by Tue afternoon as the high pushes off the coast. Could
see gusts up to 25-30 kt along the immediate S coast, Cape Cod
and the islands late in the day. May see some mid and high
clouds work into western areas Tue afternoon. Will likely see
the mildest temps of the week during this time as highs are
forecast to be in the 45-50 degree range inland and the mid 50s
across north CT to the coastal plain.

Tuesday night and Wednesday...

Cold front approaches, so will see clouds increase. Associated
low pressure will shift across southern Ontario and Quebec in
the fast flow aloft. There are some questions as to the
evolution of the eastern U.S. Pattern during this timeframe, as
there is wide model solution spread handling the 500 mb pattern.
Some models trying to dig an 500 mb trough to the mid Atlantic or southeast
U.S. While one model member actually cuts off 500 mb low across S
GA/FL.

At this point, the front should push offshore, but the big
question will be whether the timing of the front and a weakness
in the 500 mb pattern may be enough to allow some southern stream
moisture to work NE along the eastern Seaboard across the
region. At this point, have introduced chance pops Tue night into
Wed, then shift east late Wed into Wed night. Have lower confidence
due to wide solution spread.

Expect highs on Wed to run close to or just above seasonal
normals, though temps will start to fall as northwest winds take over
later Wed.

Thanksgiving and Friday...

At this point, looks like dry conditions will be in place at
the end of next week as high pressure ridge builds in from the
west as the northern stream steering flow becomes nearly zonal.
Some clouds may push in across the cape and islands on Friday as
low pressure passes well southeast of the region.

Looks to be a chilly Thanksgiving day with highs only in the
mid 30s to around 40 inland and the lower-mid 40s at the shore.
It may be a bit milder on Friday, with highs mainly in the 40s,
but some upper 30s may linger across the higher inland terrain.

&&

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 Sunday/...

1145z update...

Today...high confidence.

Expect VFR with dry conditions. The exception will be across
western Massachusetts/north central CT where MVFR conditions in light rain
will increase between 21z-00z. Light winds this morning
increase sharply this afternoon from the S-SW with gusts
approaching 30 kts by sunset (22z). Elsewhere S-SW winds
increasing to 10-15 kt.

Tonight...high confidence.

VFR-MVFR to start in areas of light rain but trending toward
IFR-MVFR after midnight. Strong S-SW 20-25g45kt winds along the
South Coast including Cape Cod and islands. Elsewhere winds not
as strong and this results in low level wind shear across much of Rhode Island and
eastern Massachusetts including bos.

Sunday...moderate confidence.
Some uncertainty on probability of -tsra and areal coverage
along and ahead of strong cold front.

MVFR-IFR with widespread showers, some with locally heavy
downpours. Could be a fine line of low top showers with embedded
thunder roughly 15z-18z. Gusty winds may accompany this
convection. Strong south-southwest winds 20-25g45kt continue along the South
Coast including Cape Cod and islands. Away from the South Coast
expect low level wind shear to continue thru the morning across Rhode Island and eastern
Massachusetts. Then a strong cold front sweeps across the area late morning
into the early afternoon, with improving conditions behind the
front and a wind shift to the west.

Kbos terminal...high confidence in taf thru 00z, then some
timing issues on arrival of lower ceilings and rain. Period of
rough weather tonight into midday Sunday with low level wind shear along with
+shra. Isolated thunder possible 15z-18z Sunday, then strong
cold front sweeps across the area with improving conditions
after 18z Sunday.

Kbdl terminal...high confidence in taf thru 21z, then some
timing issues on arrival of lower ceilings and rain. Unsettled
weather tonight into Sunday morning with +shra and possible
isolated thunder.

Outlook /Sunday night through Wednesday/...

Moderate confidence.

Sunday night through monday: VFR. Windy with gusts to 35 kt.

Monday night: VFR. Windy with gusts to 30 kt.

Tuesday: VFR. Breezy.

Tuesday night: mainly VFR, with local MVFR possible. Breezy.
Chance shra, slight chance freezing rain well inland.

Wednesday: mainly MVFR, with areas VFR possible. Breezy. Chance
rain showers.

&&

Marine...
forecaster confidence levels...

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

Short term /through Sunday/...high confidence.

Today...

Light winds this morning with high pressure overhead. S-SW
winds increase this afternoon and approach gale force by late in
the day.

Tonight...

S-SW gales along with poor vsby in showers and fog.

Sunday...

Strong cold front sweeps across the region late morning and early
afternoon, with south-southwest prefrontal gales and west-northwest gales behind the front.
Showers, locally heavy in the morning along with fog but improving
with the wind shift.

Outlook /Sunday night through Wednesday/...

Moderate to high confidence.

Sunday night and monday: moderate risk for gale force winds
with gusts up to 35-40 kt. Rough seas up to 12-14 ft.

Monday night: low risk for gale force winds with gusts up to
35 kt. Areas of rough seas.

Tuesday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

Tuesday night: moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Areas of rough seas. Chance of
rain showers.

Wednesday: winds less than 25 kt. Local rough seas. Chance of
rain showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for
maz020>024.
Rhode Island...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for
riz005>008.
Marine...Gale Warning from 5 PM this afternoon to 6 am EST Monday for
anz232>234.
Gale Warning from noon to 11 PM EST Sunday for anz230.
Gale Warning from 1 am Sunday to 6 am EST Monday for anz231-
251.
Gale Warning from 7 PM this evening to 7 PM EST Sunday for
anz236.
Gale Warning from 3 PM this afternoon to 6 am EST Monday for
anz235-237-255-256.
Gale Warning from 7 PM this evening to 6 am EST Monday for
anz250-254.

&&

$$
Synopsis...nocera/evt

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