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Area forecast discussion
National Weather Service Taunton Massachusetts
721 am EDT sun Sep 24 2017

Synopsis...
high pressure covering much of the eastern USA will bring warm and
dry weather to southern New England the first part of this week.
Cold front approaches southern New England during Wednesday
afternoon and triggers scattered showers. Cold front slowly moves
through southern New England on Thursday and offshore Thursday
night. Maria will bring swells to the South Coast, but the center is
expected to pass well southeast of New England late in the week.

&&

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

715 am update...

Main near term issue remains the dense fog over the cape and
islands. It has been slowly eroding away on the northwest edge and as
of 7 am the upper cape was beginning to break out per
observations. The GOES 16 satellite imagery has been painting a
similar picture this morning. The fog should gradually lift
through mid morning with the Outer Cape and Nantucket being the
last to see sun. There is a risk of the fog settling back in
across the cape and islands around or a little after sunset.

Otherwise, we are on tap for hot temperatures as noted in the
prior discussion although tempered some with sea breezes along
the coastal plain. Have kept high temperatures from the previous
forecast and just tweaked hourly temperatures a little to
reflect current observations, trends, and guidance. As per the
prior forecast, the gradient will be very slack with light
surface winds. This will set the stage for sea breezes along
essentially the entire coastline.

Prior discussion...
fog lingers early today, both as a broad area over southeast mass
including the cape and islands and as smaller patches inland. The
fog will dissipate during the morning. All areas will either start
clear or become clear this morning. Expect sunny skies through the
afternoon.

A weak shortwave is projected to move across northern New England,
and could spread a few clouds our way. Subsidence under the ridge is
likely to diminish any clouds and favor the mostly sunny forecast.

Full sun will mean deep mixing, with the layer expected to reach to
at least the 850 mb pressure level. The models have been known to
underestimate this depth, so mixing above that level is a reasonable
scenario, and would mean surface temperatures warmer than our
estimates. Both the GFS and European model (ecmwf) project 850 mb temps of 17-19c,
the NAM and ggem 18-19c. This suggests inland areas will reach Max
sfc temps in the upper 80s and lower 90s. Light wind under the high
pressure will allow sea breezes to develop later this morning. This
will buffer temps, so expect lower Max temps in eastern mass and Rhode Island
especially along the coast.

Interior climate sites are expected to reach Max temps at or near
record levels. The sea breeze at Boston should keep Logan Airport
below its record.

We do expect coolest temps to be found on Cape Cod. Should the fog
linger longer than anticipated, temperatures may be a few degrees
cooler still.

&&

Short term /6 PM this evening through 6 PM Monday/...
high pressure remains over the region tonight and Monday. That means
mostly clear skies and light winds. Dew points will mostly be in the
60s much of this time, so min temps tonight will be in the upper 50s
and 60s. Expect a similar mixed layer Monday, with temps at 850 mb
of 17-18c. So Max temps should be similar to today, upper 80s and
low 90s inland and cooler near the shore. Light flow will again
allow sea breezes to form, buffering temperatures along the
coastline.

Patchy fog will again be possible overnight and early morning,
favoring the usual inland fog spots as well as the South Coast.

&&

Long term /Monday night through Saturday/...
highlights...

* very warm temperatures persist into Wednesday
* scattered showers Wednesday afternoon through Thursday night
* Maria will bring swells to the South Coast but will sharply
recurve out to sea southeast of New England late in the week

Tuesday...850 mb temperatures 16c to 18c will support very warm
temperatures into Tuesday with surface temperatures reaching well
into the 80s throughout most of the region. The light gradient flow,
however, will set the stage for sea breezes along the coastal plain.

Wednesday through Thursday night...a surface cold front and upper
short wave trough will approach southern New England Wednesday
afternoon. We anticipate scattered showers to develop ahead of the
front Wednesday afternoon in moist, marginally unstable air mass.
Model guidance suggests showalters near zero and k indices abov 30.
Models have been giving a signal that tropical moisture from Maria
may become entrained in the air mass just ahead of the front and
increase the risk of a few heavy downpours, although there is
uncertainty in the timing/duration of Maria's moisture entrainment.
The air mass appears to be unstable enough to support isolated
thunder at least for Wednesday afternoon/evening. Due to the upper
shortwave trough having to erode a very strong upper level ridge,
the surface cold front will probably move slowly across southern New
England and may not pass offshore until Thursday night. Thus, we
anticipate scattered showers to persist Wednesday night into
Thursday night. With tropical moisture from Maria and precipitable
waters 1.7 to 2 inches, some the showers could contain very heavy
downpours anytime late Wednesday to Thursday night.

The air mass remains mild Wednesday into Thursday, but clouds will
likely temper the high temperatures some. May still experience 70s
to near 80 on Wednesday and probably mid to upper 70s on Thursday.

Friday...model consensus has the cold front passing offshore to the
southeast of New England Friday morning, and Maria passing well
southeast of our region. This will result in drier and cooler air
moving into southern New England, although showers may linger into
the morning for a while in southeast sections. Maria will become
embedded in the westerlies on the leading edge of the mid latitude
upper trough and likely pass out to sea well southeast of New
England. 850 mb temperatures drop about 10c, and friday's high
temperatures will be noticably cooler, albeit just closer to normal,
even with plenty of sunshine and downsloping winds. Dewpoints in the
upper 60s and 70s for most of the week will drop into the 40s most
locations during Friday.

Saturday...for now Saturday looks to be dry with near normal
temperatures and very low humidity.

&&

Aviation /11z Sunday through Thursday/...
forecaster confidence levels...

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

Short term /through Monday/...

Today...high confidence.

IFR-LIFR conditions will slowly improve across the cape and
islands. VFR elsewhere. Light winds with coastal sea breezes.

Tonight...moderate confidence.

Believe we will see a return to stratus and fog across the cape
and islands this evening and possible expansion during the
overnight over other sections of southeast Massachusetts and southern Rhode Island.
Anticipate generally VFR conditions elsewhere except for patchy
valley fog.



Monday... high confidence.

Fog over the southeast and some interior valleys Burns off during the
morning. Most of the day will be VFR with light winds. Sea
breezes again expected along the coastlines, including bos.

Kbos taf...high confidence today for VFR. Moderate confidence
tonight as possible that fog may redevelop for a while.

Kbdl taf...high confidence in taf.

Outlook /Monday night through Thursday/...

Confidence...high.

Monday night through Tuesday...mostly VFR except for patchy
late night/early morning fog.
Wednesday...mostly VFR but scattered MVFR ceilings/visibilities in
showers, main during the afternoon.
Wednesday night through Thursday...MVFR ceilings and areas of IFR
ceilings and visibilities in scattered showers and fog.

&&

Marine...
forecaster confidence levels...

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

Today...high confidence.

High pressure over the eastern USA will bring light wind to the
waters. The remnants of Jose well south of our area may favor a
light east wind over our waters. Daytime heating of land areas will
favor a sea breeze along the immediate coast.

Locally dense fog may start the day over the waters around Cape Cod
and islands. This will mean a period of poor visibility to start the
day. This fog should thin and dissipate over most waters by midday
or early afternoon.

Most seas are at 2-3 feet with a period of 10-15 seconds. This is
leftover swell from Jose. Higher seas are found on the southern
outer waters, and an additional south swell of 3-4 feet leading
north from Maria will start showing itself in our southern waters
later in the day. A high surf advisory remains in place in
anticipation of this swell.

Tonight and Monday...

Light wind continues under high pressure. Seas will continue to
build through the period as increasing swell moves north from Maria.
South swell should reach 6-8 feet by Monday afternoon, affecting all
of our exposed southern shoreline and possibly the southern beaches
of the Outer Cape late Monday. Expect high surf and dangerous rip
currents as a result of this swell.
Outlook /Monday night through Thursday/...confidence...high

Relatively light winds are expected through this period. However,
swells from Maria will propagate into the southern coastal waters,
beginning late Sunday night or Monday. Small craft advisories will
likely need to be extended due to rough seas for the waters south of
the Massachusetts and Rhode Island coast for all of this period.

The swells from Maria will likely produce another round of high surf
and dangerous rip currents.

Outlook...Monday night through Thursday

Confidence...high

Relatively light winds are expected through Thursday. However,
swells from Maria will impact the south coastal marine zones, and
small craft advisories will likely be needed due to rough seas.

The swells from Maria will likely produce continuing high surf and
dangerous rip currents, probably lasting most of the week.

&&

Tides/coastal flooding...
high surf advisories remain in effect through Monday along the South
Coast and the Outer Cape East Coast. Swells from Maria will likely
increase the surf and associated rip current risk across our ocean-
exposed South Coast starting later this afternoon and continuing
through the week. This will be true even though Maria may eventually
recurves out to sea well southeast of New England.

&&

Climate...
best chance for record high temps will be today and Monday, when
records are 85-90. Records Jump Up Tuesday to 89-95, while less
mixing will lead to lower Max temps. So the chance for new records
diminishes Tuesday. The current record Max temps for the three days
are

Bos 90/1959 89/1926 95/1881
orh 85/2010 85/1970 91/1930
bdl 89/1959 90/2007 93/2007
pvd 87/1959 89/1920 89/2007

Also... dew points in the 60s, while high for this time of year,
are not unprecedented. The daily extreme high values for dew points
are still in the low 70s, except for a couple of 68 and 69 values at
Worcester.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...dense fog advisory until 9 am EDT this morning for maz022>024.
High surf advisory until 6 PM EDT Monday for maz020-022>024.
Rhode Island...dense fog advisory until 9 am EDT this morning for riz008.
High surf advisory until 6 PM EDT Monday for riz006>008.
Marine...Small Craft Advisory for hazardous seas from 2 PM this
afternoon to 6 PM EDT Monday for anz235-237.
Small Craft Advisory for hazardous seas until 6 PM EDT Monday
for anz254>256.

&&

$$
Synopsis...wtb/Thompson
near term...wtb/Thompson
short term...wtb

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