NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Mar 1, 10:25 PM EST, 1 hour 35 minutes earlier.
Area forecast discussion, Mar 1, 10:25 PM EST, 1 hour 35 minutes before
Area Forecast Discussion
National Weather Service Taunton MA
1025 PM EST Thu Mar 1 2018
.SYNOPSIS...
A powerful coastal storm will impact the region with major
coastal flooding, damaging winds, possibly heavy wet snow to
the high terrain and fresh water flooding possible across Rhode
Island and eastern Massachusetts. Drier weather returns this
weekend, but with gusty northeast winds lasting through Monday.
Another weather system races east from the Pacific Coast and
brings rain or snow to our area midweek.
&&
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
1030 Pm Update...
A couple of the changes to the forecast to account for current
trends. Pressure falls continue across the Delmarva as secondary
low begins to take shape. High pressure to the north with the
developing low to the south of the region is beginning to
tighten the pressure gradient across the region. This has been
noticed in the past hour as northeast wind gusts have recently
jumped to 20-25 kts across the region. This is only the
beginning to what looks like an epic wind event.
Aside from the wind, precip is beginning to move into
Connecticut and southern RI. While rain is light at first, some
moderate rainfall across NYC and northern NJ will move into the
region over the next few hours. This rainfall will drop vsbys
down to 3-4 SM. Temperatures across the east slope of the Berks
are still in the upper 30s to low 40s as of 1000 PM. As we go
through the evening into the overnight hours, BUFKIT soundings
show that thermal profile will begin to cool as secondary low
drags cold air into the region. Thus could see a few inches of
on and off rain and wet snowfall across the east slope of the
Berks. Will continue to fine tune this portion of the forecast.
Overall the main hazards with this storm remain on track tonight
and into tomorrow. This is shaping up to be a dangerous storm
and encourage all to follow local officials advice and
statements.
&&
.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/...
*** Damaging Hurricane Force Wind Gusts Cape Cod & Islands ***
*** Major/Severe Coastal Flooding Friday & Saturday ***
*** Heavy Wet Snow and Power Outages High Terrain ***
*** Fresh Water Flooding Possible Rhode Island & eastern MA ***
Overview ...
Ingredients coming together for an extreme weather event with
multiple hazards across SNE Fri into Sat. Deepening mid level
trough diving to mid Atlc coast will result in near 300m height
fall center which will induce cyclogenesis near the benchmark
Fri with central pressure down into 970s. Low pres quasi-
stationary Fri as it gets captures under mid level low before
drifting slowly SE Fri night into Sat. Extremely dynamic system
with DT on GFS/NAM down to 700- 800 mb which is a pure
indication of how anomalous this storm is. In fact, 850/925 mb
wind anomalies are 6+ SD above normal which is about as extreme
as it gets. Extreme anomalies in the model atmosphere often
translate to extreme events in the real atmosphere. Major to
severe coastal flooding and damaging winds will be the most
significant aspects of this storm, life threatening in some
cases, but heavy rainfall and heavy wet snowfall are also a
concern and will also be significant.
Coastal flooding threat will be described in a separate section
below.
Damaging Wind Threat...
As mentioned above, low level wind anomalies up to 6+ SD above
normal which is a strong signal for an extreme wind event,
especially for the coast. Rather impressive NE low level jet
75-85 kt sets up across eastern New Eng and the adjacent waters
with 90+ kts east of New Eng. There are still some subtle
differences where the core of strongest winds set up, but the
general theme is the jet will be focused across eastern MA
during much of Fri as it lifts north of the Islands, before
shifting south across Cape/Islands late Fri/Fri night. Winds
will be ramping up in the morning but initially mixing may be
limited could prevent strongest winds from mixing down. Mixing
depth increases afternoon and Fri night when strongest winds
will be realized with potential for peak gusts up to 70-80 mph
over Cape/Islands. Wind field is quite expansive so much of SNE
will experience strong to damaging wind gusts, especially
eastern New Eng. Gusts to 50-65 mph possible in the coastal
plain from BOS-PVD and SE MA with 40-50 mph further in the
interior. High wind warnings expanded to all of RI through NE MA
with wind advisories further west in the interior. At least
scattered power outages with numerous trees down is likely which
will be exacerbated by any accumulation of heavy wet snow.
Heavy Wet Snow Threat...
This is the most difficult aspect of the storm, due to thermal
differences in the models as well as marginal BL temps. NAM is
the coldest of the guidance while GFS is warmest with ECMWF in
between but closer to the NAM. Most of the HREF suite of hi-res
guidance is also cold and close to the NAM. Given we are
dealing with a situation with cooling temps aloft and marginal
BL temps within area of strong omega, potential for dynamic
cooling of the column suggests colder scenario may play out
where heavy precip and strong omega occurs. Very difficult
snowfall forecast as marginal change in BL temps as well as
changes in intensity of precip will result in precip flipping
between rain and snow and vice versa. Elevation will certainly
play a major role in snow accum but can't rule out some accum in
the coastal plain. Ptype may be mostly snow mixed with rain at
times over higher elevations of Berkshires during Fri, but
thermals are warmer to the east suggesting more rain than snow
Worcester hills. However, the guidance is suggesting cooling in
the lower levels from mid afternoon into Fri night with
continued strong omega resulting in rain likely flipping to snow
rest of higher terrain and eventually even across the coastal
plain including BOS-PVD and SE MA after 00z when there is still
a decent slug of precip to move through. Should see a good thump
of snow in the Berkshires and possibly northern Worcester Hills
with potential for 6-12 inches, heaviest in the Berkshires
where there will be some upslope. In a worst case scenario there
is a low risk for amounts up to 20 inches in the Berkshires but
everything would have to come together with precip remaining
mostly all snow and not confident in that. Elsewhere, we think
there is potential for 3-6 inches higher terrain in northern CT
and NW RI with 1-3 inches coastal plain from BOS-PVD and CT
Valley. Most of the accum will likely be on non paved colder
surfaces as temps will be above freezing, except higher
elevations where it'll be a but colder.
Confidence in snow amounts is moderate in the Berkshires and
northern Worcester hills but low everywhere else due to ptype
uncertainty. It is possible the coastal plain sees little
snowfall. It only takes about 4 inches of heavy wet snow to
bring down tree limbs resulting in isolated power outages so
potential for significant impact in the higher elevations if
higher snowfall is realized. Watches were converted to warnings
for the Berkshires and higher elevations northern Worcester
county, but continued the watch for southern Worcester county
where confidence is lower.
Heavy Rain Threat...
Still looking at the risk for 2-4 inches of rainfall with
greatest threat over RI and southeast MA where isolated amounts
could reach 5 inches. Tremendous low level convergence is noted
at the nose of a persistent low level jet which will result in
heavy rainfall during most of Friday. Flood Watch continues with
greatest risk for flooding across RI and southeast MA.
Saturday...
Some leftover rain/snow showers possible in eastern New Eng,
otherwise clouds should give way to partial clearing by late in
the day. Still very windy, especially in the morning with strong
gradient in place and steeper low level lapse rates. Soundings
show potential for 50-65 mph gusts Cape/Islands, 40-50 mph
BOS-PVD corridor and SE MA and 25-40 mph to the west of I-95.
Winds will be diminishing in the afternoon.
&&
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Big Picture...
Longwave pattern shows ridge east/trough west this weekend,
then flattens during the week. Shortwave pattern shows one
storm during the long-term period. This system enters the
Pacific Coast Saturday and then moves east across the nation
and reaching the East Coast midweek.
The model mass and thermal fields are similar Sunday and show a
similar setup of ridges and troughs during the week. However,
even on Monday there are significant differences in the
placement of those ridges and troughs. Thermal fields also show
differences by Monday. Thus confidence in the forecast is
moderate at best and potentially low for Wednesday.
Details...
Sunday through Tuesday...
Offshore storm continues to move out to sea while high pressure
builds south into Quebec. The pressure gradient between the two
systems will maintain a northeast flow across Southern New
England Sunday and Monday. This flow is strongest Sunday, and
diminishes Monday. Moisture profiles show dry air above 750 mb
and very moist air below 850 mb. This suggests mainly cloudy
skies, especially in Eastern Mass where the northeast flow is
off the water. There is a low chance of showers, primarily in
the onshore northeast flow over Cape Cod and Plymouth County.
Low confidence, and individual models show at most 0.01 inch at
any one time, so we will show slight chance pops in that area.
Otherwise dry weather.
Low level mixing profile reaches at least 925 mb and likely 900
mb, with temperatures at that level supporting max sfc temps in
the low 40s each day. Dew points in the 20s to around 30 should
support min sfc temps roughly 25-35.
Sunday winds in the mixed layer will be near 25 knots in
Western areas and near 40 knots Cape Cod and Islands, so we
have adjusted gusts to these levels. Monday winds in the
similar mixed layer are a little less.
Weak high pressure ridge builds over the region Tuesday and
maintains the dry weather and similar temperatures.
Wednesday-Thursday...
As noted above, there is a consensus that a Pacific shortwave
will cross the country, redeveloping off the East Coast either
Tuesday night or Wednesday. The coastal storm then sweeps east
and northeast, passing offshore of Southern New England. Models
agree on the event, but disagree on the timing of the storm
passage as well as the distance offshore that it passes. Model
QPF fields show the highest values along our south coast or
offshore. Temperature profiles show mostly snow, but sfc temps
show potential for rain or a rain/snow mix.
We will indicate chance pops starting Wednesday and diminishing
west to east Thursday. Highest values will be along the South
Coast. The forecast will show potential for both rain and snow
at this time.
&&
.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Saturday/...
Tonight...VFR dropping to IFR through the night. Rain will
overspread the region but some sleet/snow is possible across the
high terrain of the Berks and ORH hills.
NE winds will increase up to 15-30 kt around or after 06Z.
Gusts up to 35 kt possible across coastal terminals toward
daybreak. LLWS likely up to 50-70 kt at 2000 ft after 09Z.
Reduced visibility in rain, mixed with snow at times inland, and
patchy fog.
Friday...Mainly IFR. Strong winds with gusts to 55-60 kt
Worcester Hills and east, including S RI. RA, mixing with SN
across inland areas after 18Z.
Friday Night...Mixed IFR/MVFR. Strong winds with gusts to 55 kt
Rhode Island and eastern Massachusetts. RA along the coast with
mixed RA/SN inland, then changing to SN during the night.
Saturday...MVFR cigs improve to VFR by the afternoon. Lingering
5 mile vsbys in rain and patches of 2 mile vsbys in snow early.
Conditions linger at 2000 foot ceilings and 5 mile vsbys in rain
on Cape Cod and Islands. Over the remainder of Southern New
England conditions will improve to VFR.
KBOS Terminal...High confidence in TAFs.
KBDL Terminal...High confidence in TAFs.
Outlook /Saturday Night through Tuesday/...
Saturday Night: Mainly VFR, with local IFR possible. Strong
winds with gusts to 40 kt.
Sunday: Mainly VFR. Strong winds with gusts to 45 kt, strongest
over Cape Cod and Islands.
Sunday Night: VFR. Strong winds with local gusts to 40 kt,
mainly on Cape Cod and Islands.
Monday through Monday Night: Mainly VFR. Windy with gusts to 35
kt.
Tuesday: Mainly VFR. Breezy.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Saturday/...High confidence.
Tonight...
Low pressure forming off the mid Atlc coast moves south of the
waters overnight, headed toward the 40N/70W benchmark. NE winds
over the waters will increase to gale force across the waters
after 1 AM/06Z. Visibility lowers in rain and fog mostly after
midnight. Seas will build up to 6-10 ft overnight.
Friday through Saturday...
Extremely dangerous marine conditions expected Friday through
Saturday with much of the period characterized by storm force
winds and seas across ocean exposed waters over 20 feet. There
is a period Friday afternoon and Friday night when we anticipate
hurricane force wind gusts and seas 25 to 35 feet across the
ocean exposed eastern coastal waters. Seas may actually approach
40 feet Friday evening just east of Cape Cod.
Outlook /Saturday Night through Tuesday/...
Confidence...High.
Northeast winds and seas will gradually subside Sunday through
Tuesday. Gales will continue through the weekend. We will
probably still have seas high enough to warrant Small Craft
Advisories even into Tuesday.
&&
.HYDROLOGY...
While coastal flooding is one of the main threats with this
system, we will also have to keep a close eye on the river
flooding aspect. Many streams are flowing above to well above
normal for this time of year, due to mild weather which melted
any snow pack and a steady feed of rain events during the month
of February.
A Flood Watch continues for north central to northeast CT, all
of RI, and eastern MA from Friday morning into Saturday morning.
This upcoming storm is expected to bring 1.5 to 4 inches of QPF,
with the highest amounts across RI and eastern MA. The rain is
expected to mix with or change over to snow, so some QPF will
be frozen. The rain that does fall will likely result in areas
of minor stream and river flooding with in the Flood Watch area,
with pockets of moderate flooding possible. A River Flood
Warning has already been issued for the Pawtuxet River at
Cranston with moderate flooding forecast. Significant urban and
poor drainage flooding is also possible within the Flood Watch
area. Minor urban and poor drainage flooding is still possible
across interior MA, although a potential changeover to snow
should limit the extent of flooding.
In addition...due to the extent of storm surge expected with the
midday Friday, Friday night and Saturday high tides, saltwater
will likely elevate river levels along tidally affected rivers
and streams, and could result in flooding away from the
immediate shoreline. This could occur along some east coastal MA
as well as south coastal MA/RI rivers.
&&
.TIDES/COASTAL FLOODING...
Coastal Flood Warning is now in effect for east coastal
Massachusetts.
Confidence is fairly high for three high tide cycles of moderate to
major coastal flooding and a 4th high tide cycle of minor to
moderate coastal flooding Saturday night along the Massachusetts
east coast. Beach erosion for this event is expected to be extreme.
The main uncertainty is just how fast surge and seas ramp up during
Friday morning and just how high they remain for the Friday night
and Saturday midday high tide cycles.
Our best estimate at this time is that the surge will increase to
near 3 feet about the time of the Friday late morning high tide,
near 5 feet during the Friday late afternoon/early evening low tide,
near 4 feet for the Friday night high tide, and 3 to 3.5 feet for
the Saturday midday high tide. We anticipate waves to build to near
20 feet about the time of the late Friday morning high tide, peak 30
to 35 feet (except 35 to 40 feet east of Cape Cod) for Friday
evening, 25 to 30 feet during the Friday night high tide, and 20 to
25 feet (but with 13 to 14 second periods) for the Saturday midday
high tide.
Many have asked how this event will compare to January 4. Our
thinking is that water levels will most likely approach the January
4 levels for possibly all three high tide cycles from Friday through
Saturday. A worst case scenario would have one of the tides go a
little above the January 4 record level. For Boston, anticipate all
three high tide cycles will have water levels in the vicinity of
14.5 to 15.0 feet MLLW, but a worst case scenario could bring it as
high as 15.5 feet MLLW.
For ocean exposed coastal communities along eastern Massachusetts,
waves on top of elevated water levels will likely result in damage
to shoreline structures, conceivably during all three high tide
cycles. Many neighborhoods will likely become isolated, some for
extended period of time. The water will be extremely slow to
evacuate after the late Friday morning high tide, and pockets of
coastal flooding may persist all the way to the following Friday
night high tide.
Another big issue with this event will be severe to extreme beach
erosion over multiple high tide cycles. This could seriously
compromise protective dunes and sea walls in some spots and increase
vulnerability to some areas during the course of the event.
Along the Rhode Island exposed south coast, elevated water levels
and swells may produce areas of minor coastal flooding and
potentially moderate to rather severe beach erosion. A Coastal Flood
Advisory is in effect for that area.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch from 7 AM EST Friday through Saturday morning for
CTZ002>004.
Wind Advisory from 7 AM Friday to 7 AM EST Saturday for
CTZ002>004.
MA...High Wind Warning from 10 AM Friday to 10 AM EST Saturday for
MAZ013-015>024.
Flood Watch from 7 AM EST Friday through Saturday morning for
MAZ005>007-013>024.
Coastal Flood Advisory from 7 AM Friday to 11 PM EST Saturday
for MAZ020.
Coastal Flood Warning from 9 AM Friday to 3 AM EST Sunday for
MAZ007-015-016-019-022>024.
High Wind Warning from 10 AM Friday to 7 AM EST Saturday for
MAZ005>007-012-014.
Wind Advisory from 7 AM Friday to 7 AM EST Saturday for MAZ003-
004-010-011-026.
Winter Storm Watch from Friday afternoon through Saturday
morning for MAZ012.
Winter Storm Warning from 4 AM Friday to 4 AM EST Saturday for
MAZ002-008-009.
Winter Storm Warning from noon Friday to 4 AM EST Saturday for
MAZ004-026.
RI...High Wind Warning from 10 AM Friday to 10 AM EST Saturday for
RIZ002-004>008.
Flood Watch from 7 AM EST Friday through Saturday morning for
RIZ001>008.
Coastal Flood Advisory from 7 AM Friday to 11 PM EST Saturday
for RIZ002-004>008.
High Wind Warning from 10 AM Friday to 7 AM EST Saturday for
RIZ001-003.
MARINE...Hurricane Force Wind Warning from 1 PM Friday to 7 AM EST
Saturday for ANZ232-254>256.
Storm Warning from 1 PM Friday to 7 AM EST Saturday for
ANZ233>237.
Storm Warning from 7 AM Friday to 1 AM EST Saturday for ANZ230.
Hurricane Force Wind Warning from 1 PM Friday to 1 AM EST
Saturday for ANZ231-250-251.
&&
SYNOPSIS...WTB/EVT
NEAR TERM...Dunten
SHORT TERM...KJC
LONG TERM...WTB
AVIATION...WTB/EVT
MARINE...WTB/EVT/Thompson
HYDROLOGY...NMB
TIDES/COASTAL FLOODING...Thompson