Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KBOX 230258
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
958 PM EST Mon Jan 22 2018

.SYNOPSIS...
Scattered showers along with areas of drizzle and fog persist
tonight with some light icing possible across interior northern MA.
A fast moving storm system will bring a period of heavy rain and
perhaps even an isolated thunderstorm or two Tuesday along with
strong winds across portions of eastern MA and RI. Blustery,
dry and colder weather follows Wednesday through Friday, then a
warming trend next weekend. A cold front may bring showers next
Sunday or Monday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

10 PM update...

No wholesale changes with this update as previous forecast
matching up well with current conditions.

Freezing rain continues to be observed across central and
northern Worcester county into northwest Middlesex county where
temps are in the upper 20s and lower 30s. Good drainage flow
with NNE winds persisting across this region. Also getting
reports as far west as Belchertown in eastern Hampshire county.
Thus may extend the Winter Weather Advisory slightly westward to
account for this shallow cold air remaining entrenched into the
early morning hours.

Otherwise previous forecast on track for the overnight with
mainly areas of drizzle with steady/heavier precip holding off
until after daybreak for western MA/CT and not until midday or
so farther east into RI/eastern MA.

Dense fog continues across the hilly terrain of northern RI (SFZ
down to 1/4SM) and ORH down to 1/2SM. Thus dense fog advisory
continues.

Previous discussion...

Highlights tonight...

* Light icing threat across interior northern MA tonight
* Areas of Dense Fog especially across the high terrain

Details tonight...

A very subtle wave that passed east of our region earlier today and
has allowed winds to shift northeast over the last few hours.
Portsmouth NH has seen their temp drop from 38 to 30 in the last few
hours. This is often a warning sign for portions of northeast MA, so
opted to extend winter weather advisory into interior Essex as well
as Middlesex counties.  The greatest risk will be near the NH
border...where temps have already dropped to between 32 and 33 and
may see them fall another few degrees over the next few hours.
Another area of concern remains the high terrain of the Worcester
Hills and east slopes of the Berkshires...where pockets of light
freezing rain/freezing drizzle will continue tonight.  Main idea is
if traveling tonight across interior MA...untreated roads/walkways
may be icy even if they look wet.

The other big concern tonight will be areas of fog...which will be
locally dense.  We have already gone ahead and issued a Dense Fog
Advisory for the high terrain given an ideal setup with light moist
northeast winds.  Guidance indicates that dense fog may overspread
other portions of the region especially across the interior...so
the dense fog advisory may need to be expanded.

As for precipitation tonight...we are looking at a few showers along
with areas of drizzle and fog.  However...high resolution guidance
shows a steadier band of showers moving through the region this
evening.  Will also probably also see more showers develop toward
daybreak and low level jet begins to increase.  Again...ptype mainly
liquid with -fzra/-fzdz risk across parts of interior MA especially
near the NH border and across the high terrain.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...

Tuesday Highlights...

* Heavy rain/isolated thunderstorm risk
* Brief poor drainage street flooding possible
* Period of strong wind gusts along and southeast of BOS-PVD corridor

Tuesday Details...

Perhaps some pockets of left over light freezing rain across
interior MA early Tuesday morning...but that should quickly be
changing over to plain rain.  The main story will be surface low
pressure tracking northwest across the Great Lakes allowing most of
our area to gradually warm sector.  This will result in both Pwats
and southerly LLJ 3 standard deviations above normal.  Will break it
down a bit more below.

Heavy Rain/Isolated Thunderstorm Concerns...

Given the above ingredients combined with strong forcing...heavy
rain will overspread the region Tuesday morning into the early
afternoon from west to east.  The system is quite progressive...so
the bulk of the heavy rain will fall within a 6 hour period.
Rainfall amounts on the order of 0.75 to 1.5 inches are expected.
Model soundings also indicate a few hundred J/KG of MUCape along
with steep mid level lapse rates...so an isolated thunderstorm or
two is certainly possible.  The greatest risk will be near the
southeast New England coast.  The heavy rain will mainly just result
in typical brief nuisance poor drainage street flooding. However...a
Flood Watch has been issued for the Millers River near Athol where
an ice jam remains frozen in place and has the potential to cause
additional flooding.  A Flood Warning also remains in effect for the
CT River at Middle Haddam...where this heavy rain may result in
increased flooding problems as a result of the ice jam.

Fortunately...this system is fairly progressive and the bulk of the
rain should have exited the southeast New England coast by 00z.

Strong Wind Potential...

A potent southerly low level jet around 80 knots at 925 mb will move
across the southeast New England coast Tuesday afternoon and early
evening.  Given surface temperatures expected to warm well into the
50s combined with heavy rain and potential convective elements a
wind advisory has been issued.  We are expecting some gusts to 50
mph Tuesday afternoon and early evening along and southeast of the
Boston to Providence corridor.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...

Big Picture...

Longwave pattern starts the period with a ridge over the Plains and
shallow troughs over the Pacific and Atlantic coasts. The Atlantic
trough lingers through the week.

Shortwave scale shows multiple troughs crossing the northern USA
during this period.  A small but sharp trough moves across New
England Tuesday night, while a broader trough sweeps across on
Thursday and Friday.

Height fields at 500 mb and thermal fields at 850 mb forecast at
below normal January levels Wednesday night through Friday. Fields
then climb above normal Saturday and Sunday. This supports a colder
than normal period Thursday-Friday, trending milder than normal over
the weekend.

Model mass fields and thermal fields are in agreement through
Friday, followed by differences upstream over the Plains over the
weekend. This brings high confidence to the forecast through Friday
and low-moderate confidence Saturday through Monday.

Details...

Tuesday night-Wednesday...

Cold front moves well offshore while the parent surface low moves
off through Quebec. Dry slot moves up across New England Tuesday
night. With the upper trough and associated cold pool moving
overhead, we would expect some strato-cu during the day
Wednesday...more than any of the forecast guidance is showing. But
with PW values at 0.25 inch or less it may simply be too dry.

Mixed layer depth should reach between 900 and 925 mb. The
warmer/deeper mixing would support Wednesday highs in the low to mid
40s, while the colder/shallower mixing would support highs in the
mid 30s to around 40. The mixed layer also will contain 20-25 knot
winds, supporting similar daytime wind gusts.

Thursday-Friday...

High pressure builds over the region.  Cold advection and a tighter
pressure gradient will mean a blustery day Thursday with gusts 25-30
knots. Thursday temps around -14C at 900 mb suggest max temps mainly
in the 20s. Temps climb a couple of degrees Friday, so expect Friday
max temps in the upper 20s to low 30s. With airmass dew points in the
single numbers but lingering northwest wind, expect nighttime lows
in the upper single numbers and teens.

Saturday through Monday...

High pressure shifts offshore and surface winds turn from the
southwest. This will bring milder air back to the region. As noted
above, the incoming airmass is forecast to be above normal. Max
temps should at least reach the 40s with some potential to reach the
50s. Another upper trough is currently timed for early next week,
led by an associated surface cold front . There are differences
in model timing of the trough. The GFS shows fropa on Tuesday,
the ECMWF shows Monday, and the GGEM shows Sunday. So there is
uncertainty in timing of the cold front. For now we will show
chance pops Sunday-Monday. Temperatures look to be warm enough
for any precipitation to be rain showers.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

10 PM/03Z update...

No wholesale changes to previous TAFs. As for tomorrow LLWS will
be an issue along the south coast of MA/RI as strong low level
southerly jet up to 85 kts traverses the coastline 18z-23z.
Earlier discussion below.

====================================================================

Tonight...Moderate confidence. IFR to LIFR conditions expected in
showers and fog. Some of the fog will be dense. Pockets of
light freezing drizzle or light freezing rain possible across
portions of interior MA...especially high terrain. Southerly
LLWS concern increases overnight.

Tuesday...Moderate confidence. IFR to LIFR conditions may
improve to mainly IFR conditions by late morning/early afternoon
as the worst of the fog mixes out. However...roughly a 6 hour
band of heavy rain with perhaps an embedded t-storm or two will
work across the region on Tuesday. Activity should be mainly
offshore by 00z. Southwest wind gusts up to 40 knots expected
along and southeast of the Boston to Providence corridor during
the afternoon. LLWS will be a concern for the rest of the
region.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Mainly VFR, with local MVFR possible early.
Windy with gusts to 35 kt. Slight chance SHRA.

Wednesday: VFR. Windy with gusts to 30 kt.

Wednesday Night through Friday: VFR. Breezy.

Friday Night: VFR.

Saturday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Tonight...Moderate to high confidence.  ENE winds will shift to SE
tonight with some gusts to 25 knots across our eastern waters along
with 3 to 6 foot seas.  The rest of our waters should generally have
winds/seas remaining below small craft advisory thresholds...but
areas of fog may be locally dense.

Tuesday...Moderate to high confidence.  Impressive 70 to 80 knot low
level jet lifts across the region from southwest to northeast.
Despite inversion in place...heavy rain and enough mixing should
provide a few hours of southwest 35 to 40 knot wind gusts Tuesday
afternoon into the evening.  Gale warnings are posted for all waters
as seas build between 7 and 11 feet.

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 12 ft. Slight chance of rain showers
early.

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 11 ft.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.

Thursday through Thursday Night: Moderate risk for Small Craft
Advisory winds with gusts up to 30 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Local rough seas.

Friday Night: Winds less than 25 kt.

Saturday: Winds less than 25 kt. Local rough seas.

&&

.HYDROLOGY...
The heavy rain will mainly just result in typical brief nuisance poor
drainage street flooding. However...a Flood Watch has been issued for
the Millers River near Athol where an ice jam remains frozen in place
and has the potential to cause additional flooding.  A Flood Warning
also remains in effect for the CT River at Middle Haddam...where this
heavy rain may result in increased flooding problems as a result of
the ice jam.

For details on specific area rivers, including observed and
forecast river stages, please visit the Advanced Hydrologic
Prediction Service (AHPS) graphs on our website.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Dense Fog Advisory until 9 AM EST Tuesday for CTZ004.
MA...Wind Advisory from 11 AM to 7 PM EST Tuesday for MAZ013-
     015>024.
     Dense Fog Advisory until 9 AM EST Tuesday for MAZ002-004-008-
     009-012-026.
     Winter Weather Advisory until 9 AM EST Tuesday for MAZ002>006-
     008-026.
     Flood Watch from 6 AM EST Tuesday through Tuesday afternoon
     for MAZ003-004.
RI...Wind Advisory from 11 AM to 7 PM EST Tuesday for RIZ002>008.
     Dense Fog Advisory until 9 AM EST Tuesday for RIZ001.
MARINE...Gale Warning from 9 AM to 8 PM EST Tuesday for ANZ232>235-237-
     250-254>256.
     Gale Warning from 10 AM to 6 PM EST Tuesday for ANZ230-231-236-
     251.

&&

$$
SYNOPSIS...WTB/Frank
NEAR TERM...Frank/Nocera
SHORT TERM...Frank
LONG TERM...WTB
AVIATION...WTB/Frank/Nocera
MARINE...WTB/Frank
HYDROLOGY...Staff



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.