NWS Technical Forecast Discussion

000
FXUS61 KBOX 151740
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1240 PM EST Fri Nov 15 2019

.SYNOPSIS...
Milder air moves into the region today ahead of a cold front
dropping south from Northern New England. This front will
sweep across the region tonight and will be followed by much
colder air and gusty north winds into Saturday. High pressure
will bring a brief period of dry weather Sunday. A coastal storm
will then bring rain and wind, possibly some interior ice
Sunday night and Monday. A second storm may impact the region
Wednesday followed by high pressure on Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1230 PM Update:

Forecast appears to be holding up well, with only small changes
needed at this point. Clouds have finally begun to clear out
across Cape Cod and the Islands and will continue to do so over
the next hour or so. Otherwise, pretty pleasant conditions
outside with current temperatures in the mid 40s to lower 50s
under plenty of sun. Highs in the upper 40s to mid 50s still
look on track.

Previous discussion follows...

Axis of scattered showers early this morning from the NJ coast
to vicinity of Nantucket along a weak boundary. This boundary
will keep clouds and a low risk of a shower over Nantucket this
morning before clearing this afternoon. Otherwise, mostly sunny
skies across SNE as the column remains rather dry. But some
increase in cloud cover over northern and western MA later in
the day as low level moisture increases. Modest W/SW flow ahead
of arctic front dropping south into northern New Eng will bring
milder air into the region with high temps reaching mid 40s to
lower 50s, which is near seasonable for this time of year. SW
winds may occasionally gust to 20 mph.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...
Arctic front moves north to south across SNE around 03-06z,
perhaps an hour or 2 later along the immediate south coast and
Islands. Moisture and convergence are limited given westerly
flow ahead of the front so not expecting much more than a band
of sct-bkn clouds with the front. Much colder airmass advects
into the region late tonight on gusty N winds. Gusts to 25-35
mph expected, strongest along the coast. Low temps will settle
into upper teens to mid 20s by daybreak except closer to 30 over
the outer Cape/Islands. Wind chills bottom out in the single
numbers and teens late tonight and Sat morning.

Saturday...
High pres builds across southern Quebec with winds gradually
diminishing in the afternoon. But a period of 30-40 mph wind
gusts possible along the coast in the morning. Very dry airmass
in place with PWATs less than 0.1", about as dry as it gets. We
will have full sunshine but unseasonably cold temps. Highs
mostly in the mid 30s, closer to 30 over higher elevations, but
the gusty N winds will make it feel considerably colder.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
00z guidance still showing next week will feature an active weather
pattern, with the headliner being a coastal low/nor`easter that will
move up the east coast to start the work week. As is always the case
this far out, there are model differences, some of which lead to
fairly significant differences to the weather that we would
experience. For the most part, continued with a model blend, though
leaned a little more toward the 00z ECMWF with regard to
precipitation timing Sunday night -- which had the net effect to
slow things down about 6-8 hours from the straight blended approach.
The potential for some interior freezing rain and light ice
accumulation Sunday night still remains. In addition, latest storm
surge guidance indicates the potential for very minor coastal
flooding with Monday afternoon high tide. For those AFD readers out
there who want more 3am-ish ramblings, continue on for the daily
details...

Saturday Night: Another cold, clear, dry night.  With dewpoints in
the single digits (especially inland) and winds diminishing
(especially inland), lows will bottom out in the 15-20F range
(especially inland). Winds along the east coast and Cape and Islands
will keep going all night as they start to turn more NE. This will
keep it marginally warmer in those areas with 20s to lower 30s.

Sunday: Coastal low will be in full swing off the Carolinas coast.
Low level easterly flow will get established across the region, and
be a little gusty along the east coast, Cape and Islands. Expecting
mainly high clouds to increase during the day. Highs will moderate
some from Saturday, but still below normal with mid/upper 30s
interior to low/mid 40s eastern coastal sections. Still much
uncertainty in timing of precipitation. 00z NAM brings
precipitation northward during the afternoon. This model is most
aggressive, and given the dry airmass that will be in place,
don`t see that happening. Liked the depiction the 00z ECMWF has,
so used that to drive the hourly PoPs. GFS was slower, and
appears to be a little too dry. Net result is to slow the
precipitation a few hours from our previous forecast. Light rain
should spread to Nantucket late in the afternoon and slowly
move north and west. By late Sunday evening it should be to a
Boston-Hartford line.

Sunday Night: Here`s where things get very tricky. Across interior
areas (central/western MA and northern CT) where high temperatures
were only in the 30s, by the time the precipitation has made it into
this region, they will likely be in the 28-32F range (helped by wet-
bulb cooling effects). Forecast temperature profiles aloft show both
a lack of moisture in a snow growth region and warming temperatures
a few thousand feet above ground which scream to me that liquid will
be falling from the sky. Thus it`s all about the temperature at the
surface. The blend I used gives me cold enough temperatures that the
rain will freeze upon contact.  However just a few degrees warmer
changes everything. Pretty confident future model runs will help
zero in on the truth. At this point it doesn`t appear that we are
looking at heavy rainfall during the period of time icing could
occur. I ended up with maximum values of 1/10" of ice. If that is
what happens, it will be slow going for travel Monday morning, but
that`s about all.

Monday: The nor`easter will start to accelerate as it passes
by just to the south of the 40N/70W benchmark as it feels the
influence of an approaching upper level trough. Certainly a raw day,
with steady rain and gusty easterly winds. Not a very intense low,
but enough that we should see max gusts over 35mph across SE Mass
and the Cape and Islands.  However, the fetch area of easterly winds
is pretty sizeable, so that should generate a storm surge on the
order of 1.5ft.  When adding that to the Monday afternoon high tide,
we will be very close to the levels where we see minor coastal
flooding. At the very least, some splashover is expected. Again,
stay tuned for later forecasts as we fine tune that aspect.  Storm
pulls away Monday night, but as it does so, we will still have
moisture around and some cooler air coming in on northwest winds. So
the rain may end as a brief period of very light wintry mix across
the higher elevations of central/western MA.

Tuesday: Should be generally dry, though clouds will remain as an
upper level shortwave zips overhead. Temperatures a little below
normal and mostly in the 40s.

Wednesday: GFS and ECMWF both indicate an upper level low
will drift across the region. So can`t rule out another day with
lots of clouds and a few showers. Cold enough aloft (about -4C at
850mb) that this would result in snow showers across the high
terrain. Temperatures again mainly in the 40s.

Thursday: looks like we get to see some sunshine as a shortwave
ridge moves over.  However another fairly robust upper trough and
surface low will be off to our west (or say the 00z models). Could
be the low is reaching us on Thursday instead. In that case, no
sunshine. Later forecasts will work on those details.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

18z TAF Update:

Short Term /through Saturday/...High confidence.

Rest of Today...

Still some leftover MVFR ceilings at ACK, but most areas
otherwise are VFR SKC and these trends will continue through the
early evening. Southwest winds 8-12 kts with gusts 20-25 kts.

Tonight...

VFR and southwest breezes to continue until a strong cold front
moves through Southern New England through the 03-07z
timeframe. Front will be dry, but it will be accompanied by
FEW-SCT VFR low-mid clouds (040-070). Winds to shift from WSW/W
to NW and gusty (20-28 kts inland, nearing 30 kts towards the
coasts) upon and after frontal passage.

Saturday...
VFR, through continued breezy with N wind gusts 25-35 kt through
morning, then subsiding to 20-28 kt by afternoon. Gusts will
trend on the higher end of those ranges toward the coasts.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. Breezy.

Sunday: Mainly VFR, with areas MVFR possible. Windy with local
gusts up to 30 kt.

Sunday Night: MVFR/IFR conditions possible. Windy with gusts up
to 35 kt. Chance RA, chance FZRA.

Monday: MVFR/IFR conditions possible. Windy with local gusts up
to 30 kt. Chance RA, chance FZRA.

Monday Night: MVFR/IFR conditions possible. Breezy. Chance RA,
chance FZRA.

Tuesday: Mainly VFR, with local MVFR possible. Slight chance
RA, slight chance FZRA.

&&

.MARINE...
10 AM Update:

Short Term /through Saturday/...High confidence.

Isolated showers to continue across the southern offshore
waters into Nantucket Sound until about noon but unlikely to
result in restrictions to visibility. Otherwise, SW wind gusts
20-25 kt today will shift to NW this evening to N overnight into
Sat behind an arctic front. Gale force gusts to 35 kt
developing after midnight and persisting into Sat morning before
gradually diminishing. Gale warnings are in effect except SCA
for Boston Harbor and Narragansett Bay. Seas over the eastern
waters will peak around 8-9 ft late tonight and early Sat then
slowly subside. Good vsbys.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Rough seas up to 10 ft.

Sunday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 13 ft. Slight chance of rain.

Sunday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 16 ft. Rain likely.

Monday: Strong winds with gusts up to 35 kt. Rough seas up to
16 ft. Rain likely.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 15 ft. Chance of rain.

Tuesday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 3 AM to 1 PM EST Saturday for ANZ232>235-237.
     Small Craft Advisory from 10 PM this evening to 1 PM EST
     Saturday for ANZ230.
     Gale Warning from 1 AM to 1 PM EST Saturday for ANZ231-250-251-
     254>256.
     Small Craft Advisory from 1 AM to 5 PM EST Saturday for ANZ236.

&&

$$
SYNOPSIS...KJC/Nash
NEAR TERM...KJC/Loconto/Nash
SHORT TERM...KJC
LONG TERM...Nash
AVIATION...Loconto/Nash
MARINE...KJC/Loconto/Nash

NWS BOX Office Area Forecast Discussion

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