NWS Technical Forecast Discussion

000
FXUS61 KBOX 210805
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
405 AM EDT Mon Sep 21 2020

.SYNOPSIS...

Cooler than normal temperatures continue today with a gradual
warming trend Tuesday followed by warmer than normal temperatures
beginning Wednesday and likely persisting into next weekend.
Hurricane Teddy will pass well east of our coastline Tuesday as it
heads into the Canadian Maritimes. While we will remain dry, it will
bring a period of windy conditions along the coast, especially Cape
Cod and Nantucket where a wind advisory is posted. Minor
coastal flooding and some beach erosion may also occur during
the Tuesday afternoon and early evening high tide. Otherwise,
the prolonged period of dry weather continues right through the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

4 AM Update...

* Sunny but still unseasonably cool today

A 1035+ MB high pressure system centered over northern New England
will continue to result in unseasonably cool temperatures today.
After another frosty morning away from the coast and urban centers,
sunny skies will allow for a nice day despite cooler than normal
temperatures. High temperatures should top out mainly in the lower
to middle 60s. A bit of a northeast breeze along the coast and
especially the Cape/Islands where winds will gust between 25 and 30
mph. Nonetheless, not a bad day in the sunshine at least away from
the gusty winds along the southeast New England coast.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

* Northerly Wind Gusts of 40-50 mph on Tue across the Cape/Nantucket

* Pockets of Minor Coastal Flooding/Beach Erosion late Tue
  afternoon/early evening along the eastern MA Coast, Cape & Islands

* Dangerously High Surf on Tuesday along ocean exposed beaches

Tonight...

High pressure will remain in control of our weather tonight. The
result will be mainly clear skies and another unseasonably chilly
night for most locations. Light winds inland from the coast will
allow for temperatures to bottom out well down into the 30s across
many locations. Some of the normally coolest locations may drop to
or just below the freezing mark. Additional frost headlines will be
needed where the growing season is still technically active.
Meanwhile, across the Cape/Islands the pressure gradient will be
increasing as a result of distant Hurricane Teddy. This will result
in northeast wind gusts of 20 to 30 mph developing after midnight
across the Cape/Islands and perhaps a bit stronger in Nantucket. The
NE winds blowing off the ocean may hold overnight low temps above 50
in many of these locations.

Tuesday...

Hurricane Teddy will be lifting north across the open Atlantic,
approximately 300 to 400 miles east of our coastline. While no rain
is expected from Teddy in our region, a very strong pressure
gradient will develop with high pressure across northern New
England. Bufkit soundings indicating good mixing on northerly winds
across Cape Cod and Nantucket. Therefore, we are hoisting a Wind
Advisory for the Cape and Nantucket for expected northerly wind
gusts of 40 to 50 mph. There is a low risk Nantucket could flirt
with high wind warning criteria, mainly for a period of sustained 35
knot wind gusts. At this point, we felt a Wind Advisory would be
more representative based on model consensus but something later
shifts will have to monitor.

Across the rest of the region, northerly wind gusts of 20 to 30 mph
with the strongest of those winds towards the coastal plain. Plenty
of sunshine will allow for high temperatures to reach the upper 60s
to the lower 70s.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

Highlights...

* Strong NNW winds Tue evening slowly diminish into Wed as Hurricane
  Teddy exits thru the Maritimes

* Warm/Mild days and cool nights

* Dry weather continues into much if not all of next weekend. A
  potential pattern next week may bring some much need rainfall

Precipitation...

Good model agreement among ensembles and deterministic guidance on
Hurricane Teddy tracking well east of New England into the
Maritimes. Strong subsidence on the backside of Teddy will result in
dry weather continuing Tue night into Wed. Low amplitude short waves
moves across the region Thu and Fri, but given both of these
features are moisture starved, other than a spot shower dry weather
will prevail. Then maritime ridging develops this weekend,
continuing our stretch of dry weather at least into Saturday.
Finally by early next week (possibly as early as Sunday) all model
guid including ensembles and deterministic guidance suggest a
pattern change with a full latitude trough evolving over the Great
Lakes/OH Valley. This should result in at least some much needed
rain for southern New England early next week, possibly as early as
Sunday.

Temperatures...

Tuesday night...not as cool as previous nights given well mixed
boundary layer in response to gusty NNW winds on backside of Teddy.
This mild NW flow continues into Wed with 925 mb warming to +16C to
+17C. This combined with downslope flow should support highs Wed
afternoon of 75-80. Thus will derive highs from the warmest
guidance. Not as windy as Tue but NW winds 15-25 mph expected Wed
with highest winds Cape Cod and Islands. Remaining warm Thu with
similar temps aloft although west winds will diminish. 1030 mb high
pressure builds into the maritimes Fri into the weekend and will
knock temps down a bit across eastern MA but temps still near
normal. Inland +16c 925 mb temps will support highs in the mid to
upper 70s, thus warmer than normal Inland. Not much change into
Sunday with high pressure likely lingering but return flow will
increase humidity somewhat with dew pts creeping up into the low
60s.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

06Z Update...

Today...High Confidence in VFR conditions. NE winds gusts of 20
to 30 knots develop after sunrise along the coastal plain with
the strongest of those winds across the Cape/Nantucket.

Tonight...High confidence in VFR conditions. Light N winds
inland from the coast. However, N wind gusts of 25 to 30 knots
develop toward morning on the Cape and up to 35 knots across
Nantucket.

Tuesday...High confidence in VFR conditions. N winds gusts of 20
to 25 knots across the interior, 25 to 30 knots near and
southeast of the Boston to Providence corridor, and 35 to 45
knots across the Cape and Nantucket.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR. Strong winds with areas of gusts up to
45 kt.

Wednesday: VFR. Breezy.

Wednesday Night through Friday: VFR.

&&

.MARINE...

* Dangerously High Seas of 15-20+ feet Tue into Tue night for the
  eastern MA coastal waters

* Storm Watches Nantucket Sound points south and east with Gale
  Warnings for most other open waters

Today and tonight...High confidence. High pressure over northern New
England will continue to result in cool advection over the waters.
This should generate NE wind gusts of 20 to 30 knots and perhaps bit
stronger southeast of Nantucket especially later tonight. 7 to 10
foot seas across the open waters.

Tuesday...High confidence. Hurricane Teddy will pass hundreds of
miles east of our waters. That being said, a strong pressure
gradient with high pressure over northern new England will result in
strong Northerly winds and dangerously high seas for mariners. We
opted to go with a Storm Watch Nantucket sound and our outer-waters
east of Provincetown to Nantucket. Northerly wind gusts of 40 to 50
knots with seas reaching 20+ feet. Later shifts can decide if its
worthy of a Storm Warning or convert to a strongly worded Gale
Warning. Across most of our other waters, opted for Gale Warnings
for northerly wind gusts of 30 to 45 knots. This will be a dangerous
storm for mariners, especially in our waters east of the outer-Cape
and Nantucket.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Low risk for gale force winds with gusts up to
45 kt. Rough seas up to 23 ft.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 14 ft.

Wednesday Night: Winds less than 25 kt. Local rough seas.

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

Thursday Night: Winds less than 25 kt.

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

&&

.FIRE WEATHER...

* Fire Weather Concerns Especially on Tuesday

We continue with the special weather statement for today. The
unusually prolonged stretch of dry weather continues and minimum RH
values will drop to between 25 and 35 percent across much of the
interior and 40 to 45 along the southeast coastline. It will be a
bit gusty near the coastline too.

There are even greater fire weather concerns on Tuesday and it is
possible we may need to consider Fire Weather Headlines. Distant
Hurricane Teddy should generate 20 to 30 mph wind gusts over most
areas but between 40 and 50 mph for Cape Cod and Nantucket. Minimum
RH values will drop to between 20 and 30 percent in most locations
with 35 to 45 percent across the Cape and Islands.

&&

.TIDES/COASTAL FLOODING...

* Coastal Flood Advisory In Effect Tuesday Afternoon/Early Evening

* High Surf/Dangerous Rip Currents especially on Tue

High astronomical MLLW tides of 11.6 feet are on tap for this
afternoon. This may result in splash over and minor pockets of very
minor coastal flooding/beach erosion along the eastern MA coast
given gusty NE winds. Based on this in combination with local
matrices and the Stevens Institute guidance have opted to continue
the Coastal Flood Statement for the eastern MA coastline including
Nantucket.

There is more of a concern on Tuesday as distant Hurricane
Teddy results in some pressure falls and the potential for
northerly wind gusts of 40 to 50 mph across the Cape/Nantucket. This
combined with very highs seas of 15-20+ feet offshore may result in
pockets of minor coastal flooding along the eastern MA coast. This
is especially true on the north facing coastline across Cape Cod Bay
and Nantucket Sound given the wind trajectory will be favorable.
This will likely bring a 1 to 2 foot storm surge. While we are not
expecting a significant/damaging coastal flood event, inundation on
vulnerable shoreline roadways will likely occur within a few hours
of the high tide along with beach erosion. A coastal flood advisory
has been issued for the eastern MA coast, Cape and Islands
for the Tuesday afternoon and early evening high tide.

Lastly, High Surf Advisories remain in place through Tuesday across
all ocean exposed beaches. The worst of the conditions will be on
Tuesday when very high offshore seas may result in extremely high
breakers on beaches. Dangerous rip currents expected too.


&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Frost Advisory until 8 AM EDT this morning for CTZ002>004.
MA...High Surf Advisory until 8 PM EDT Tuesday for MAZ007-019-020-
     022>024.
     Coastal Flood Advisory from 2 PM to 6 PM EDT Tuesday for
     MAZ007-015-016-019-022>024.
     Frost Advisory until 8 AM EDT this morning for MAZ004>006-
     009>013-017-026.
     Wind Advisory from 9 AM to 7 PM EDT Tuesday for MAZ022.
     Wind Advisory from 6 AM to 8 PM EDT Tuesday for MAZ024.
RI...High Surf Advisory until 8 PM EDT Tuesday for RIZ006>008.
     Frost Advisory until 8 AM EDT this morning for RIZ001.
MARINE...Storm Watch from Tuesday morning through Tuesday evening for
     ANZ232.
     Gale Warning from 8 AM to 11 PM EDT Tuesday for ANZ231-233>235-
     250-251-256.
     Small Craft Advisory from 9 AM to 7 PM EDT Tuesday for ANZ230-
     236.
     Small Craft Advisory until 8 PM EDT Tuesday for ANZ237.
     Storm Watch from Tuesday morning through Tuesday evening for
     ANZ254-255.

&&

$$
SYNOPSIS...Frank/Nocera
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Nocera
AVIATION...Frank/Nocera
MARINE...Frank/Nocera
FIRE WEATHER...Frank
TIDES/COASTAL FLOODING...Frank

NWS BOX Office Area Forecast Discussion

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