NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Jan 12, 1:08 PM EST, 2 hours 28 minutes earlier.
Area forecast discussion, Jan 12, 1:08 PM EST, 2 hours 28 minutes before
Area Forecast Discussion
National Weather Service Taunton MA
108 PM EST Fri Jan 12 2018
.SYNOPSIS...
Multiple low pressure areas along an approaching cold front will
bring heavy rainfall, potential flooding and gusty winds later today
and tonight. A sharp turn to much colder weather occurs rapidly
Saturday which will allow rain to end as a bit of light freezing
rain and sleet ...but more importantly a Flash Freeze is expected
across all of southern New England. Mainly dry but cold weather
is expected Sunday and Monday, but will have to watch the
potential for some snow mid week.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
***Heavy rainfall developing this afternoon across western MA
and northern CT***
***Near record high temps this afternoon***
1 PM update...
Band of heavy rain across far western MA and slowly moving
east. Focus for heavy rainfall will remain across western MA and
northern CT where 1-2 inches is possible. Showers have moved
east across RI and eastern MA where low level jet is supplying
low level convergence, but heavy rain in the coastal plain will
hold off until evening.
Record high temp has been broken at BDL and tied at ORH. Within
one degree at BOS/PVD.
Getting some gusts to 40 mph across RI and SE MA where temps
around 60 enhancing low level mixing. This will continue this
afternoon with gusts 40-50 mph possible.
Previous discussion...
Rain is overspreading western New Eng late this morning and
will continue to slowly move east through the afternoon.
Strongest forcing for ascent focused across the west this
afternoon along the axis of the 850mb jet where heavy rainfall
is likely. In fact, the hi-res guidance is indicating 1-2 inches
of rain across western MA and northern CT with locally higher
amounts. Some marginal elevated instability supports isold
thunder in the west as well. Areas of urban and poor drainage
flooding is expected which will impact the evening commute in
western New Eng.
Across eastern New Eng, mainly dry conditions will prevail into
the afternoon with just a few showers possible. Leading edge of
steadier rainfall will approach the I-95 corridor by late
afternoon but portions of far SE MA and Cape/Islands may remain
mostly dry through the daylight hours other than a few showers.
Temps already well into the 50s and will see some high into the
lower 60s across portions of RI and eastern MA. Near or record
high temps likely across portions of SNE. Winds will be
increasing with the warmer temps helping to enhance low level
mixing in the coastal plain where gusts 30-40 mph possible
mid/late afternoon. Stronger gusts possible tonight as core of
LLJ moves into the region.
Still some patchy dense fog in the CT Valley and higher
elevations in central MA, but vsbys are improving and expect
this to continue as rain overspreads the region.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
***Unseasonably mild this afternoon and into tonight with heavy rain
overspreading the region along with the potential for significant
urban street flooding***
***Flash Freeze Saturday along with a brief period of freezing
rain/sleet***
This afternoon and tonight…
Anomalous high Pwats 4-5 standard deviations above normal coupled
with a southwest low level jet 3+ above normal will set the stage
for heavy rain and potential flooding. While showers will increase
in coverage during the afternoon...the heaviest of the rain will
probably hold off until late afternoon across the interior and the
evening across eastern MA/RI. The rain will continue heavy at times
through tonight. This is when the strong forcing acts on Pwats
around or even a bit higher than 1.5 inches. The strong southwest
low level jet will also allow milder air to mix down from above
bringing afternoon high temperatures into the upper 50s and lower
60s. These temperatures will not fall much tonight leading to rapid
snowmelt.
Putting it all together...heavy rainfall coupled with rapid snowmelt
will result in a significant urban street flooding threat mainly
late this afternoon and into tonight. Dewpoints rising well into
the 50s will result in MUCape Values of a few hundred J/KG, so
certainly the risk for a rumble or two of thunder. Overall...expect
storm total rainfall of 1 to 3 inches with localized 4 inch
amounts possible. Some guidance suggests the best chance for
these higher amounts will occur across southeast New England on
the nose of the low level jet. Some other data suggests it may
occur further back into the interior where some enhanced surface
convergence may occur. Ultimately...this will come down to many
mesoscale factors but the main idea is the potential for
significant urban street flooding exists especially with snow
clogged storm drains. Some minor river and stream flooding is
also possible and we can not rule out isolated ice jam flooding
given the bitter cold of the last two weeks.
We also did hoist a wind advisory tonight for the far southeast New
England coast. Despite the strong inversion in place...most of the
mesoscale model guidance was showing a southwest 925 mb low level
jet of over 80 knots. Any convection or heavier rain elements may
bring some of that wind down beginning this evening and through
daybreak Saturday. Therefore...felt the potential for some 40 to 50
mph wind gusts warranted the wind advisory.
Saturday…
Rapidly changing weather conditions as temperatures crash from the
50s to near 60 and into the 20s from northwest to southeast in a
matter of hours. We do not often see temperatures drop that quickly
in this part of the country…but once the cold front crosses shallow
cold air will surge into the region. While the bulk of the
precipitation will likely be over by the time temperatures drop
below freezing...there is likely to be a brief period of light
freezing rain/freezing drizzle and sleet. We are actually more
concerned though about a Flash Freeze given all the rain that were
expecting through daybreak Saturday. Usually most of our flash
freezes occur when winds turn north to northeast as opposed to
northwest as we are expecting on Saturday. However, given the rapid
temperature drop do not think will be able to dry up fast enough to
prevent at least some issues. Therefore...we have hoisted a Winter
Weather Advisory generally along and northwest of a Boston to
Providence corridor. It is mainly for the Flash Freeze concern, but
also for the potential for a bit of light freezing rain/freezing
drizzle.
&&
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
*/ Highlights...
- Cold Sunday through Monday, ocean-effect snows over
Cape/Islands
- Potential winter storm Tuesday into Wednesday
Overview and model preferences...
Shift toward positive NAO/AO and PNA as robust blocking ridges
form across the W CONUS and the N central Atlantic. PV analyses
suggest the resulting phased positive height anomalies between
these two blocks then force a lobe of the Baffin Island vortex S
as a cold upper lvl cutoff, which then settles across the Great
Lakes through early next week. Baroclinic zone E of this cutoff
will provide the lower lvl mechanism by which sfc low pres
develops, beneath the eastward shifting poleward exit region of
the jet rotating S of the cutoff and longwave trof. This low
pres, and the potential for typical coastal energy transfer is
the focus for Tue and Wed, suggesting a risk for a winter storm.
The breakdown of the positive PNA as shortwave shifts onshore
from the Pacific will force the cutoff and longwave trof to
shift to the E, but both deterministic and ensemble progs vary
greatly with the resulting depth of the wave as it shifts toward
New England, which although it allows drier CP air to filter
in, makes for an uncertain thermal forecast. Given the gradual
breakdown in mass/thermal field agreements beginning with the
mid week system, will continue to lean most heavily on ensemble
means to account for the full envelope of potential solutions.
Details...
Sat night into Mon...
Colder airmass associated with the arctic vortex lobe mentioned
above settles over New England through the period allowing H92
temps to dip as low as about -12C by Mon morning. This suggests
a period with below normal temperatures once again, as highs
will remain in the 20s with lows in the single digits and low
teens, coldest Mon night thanks to radiational cooling setup.
While the high pres and strong subsidence inversion implies
mainly dry conditions for most, echo the previous forecasters
concerns for occasional light SHSN across especially the outer
Cape. N-NW flow should allow enough fetch given at its coldest
the SST to H85 delta T-values settle around 15C. However, the
strong subsidence inversion may restrict the mixed layer mainly
below H85, with the steepest lapse rates in drier air and little
to no moisture in the DGZ, so snowfall rates should remain
light. Will continue to highlight this risk into the daytime on
Mon, before the trajectory shifts.
Tue into Wed...
As upper lvl jet streak rounds the base of the developed
longwave trof/cutoff across the Great Lakes, the sfc clipper
will begin to shift into New England. Enough moisture in the
DGZ, combined with the modest clipper lift should yield some
snowfall wit the arrival of this feature as early as later Tue.
However, there is some suggestion of a gradual tilt to the
longwave, enhancing the lower lvl baroclinic zone off the coast,
which may allow for energy transfer in the form of secondary
redevelopment. ECMWF leads the charge here, as it deepens and
tilts the trof more significantly. While this is a typical bias,
it has been most persistent with this thinking. Due to the
further amplification, it also delays the process, potentially
yielding impacts into early Thu. Ensemble means from the ECENS
and GEFS do support this to some extent, but with obvious
deference to some of the more progressive members. Will continue
with this timing and thermal profiles in support of mainly SN
(H92 temps remain below -2C even on the deterministic ECMWF).
While confidence continues to rise in a period of at least
light-moderate snowfall, the amplification and final tilt will
ultimately determine how strong and how much snow ultimately
falls, something that will become better resolved as the arctic
wave becomes better sampled later this weekend.
Thu into Fri...
Should the ECMWF or ensemble members that support the more
amplified pattern be correct, impacts from the potential
snowstorm could linger into Thu. However, at this time, given
the overall lack of confidence due to prog variance will trend
toward somewhat colder than normal conditions given the arctic
wave is expected to open, moderate and shift over New England.
Lower end POPs will cover any precip risks at this time.
&&
.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Saturday/...
Through tonight...Moderate confidence.
Conditions lowering with widespread IFR/LIFR expected through
tonight as rain overspreads the region from west to east. Rain
changing to a period of FZRA western MA 09-12z. South wind
gusts to 40-45 kt through tonight across RI and eastern MA with
winds likely peaking between midnight and daybreak Sat.
Widespread LLWS.
Saturday...Moderate confidence.
Rain changes to a brief period of FZRA interior before ending in
the morning. Low risk of brief FZRA making it into the coastal
plain before the precip ends. Greater concern is flash freeze as
temps rapidly drop from 50s to below freezing in 2-3 hours.
Widespread IFR/LIFR improving to VFR in the afternoon from west
to east.
KBOS Terminal...Moderate confidence in TAF.
KBDL Terminal...Moderate confidence in TAF.
Outlook /Saturday Night through Tuesday/...
Saturday Night: Mainly VFR, with local MVFR possible over the
Cape/Islands. Breezy. Slight chance FZRA.
Sunday through Martin Luther King Jr Day: Mainly VFR, with
local MVFR possible over the Cape and Islands.
Monday Night: MVFR/IFR conditions possible. Slight chance SN.
These depend on the timing of a potential snowstorm.
Tuesday: Mainly IFR, with local MVFR possible. Chance SN. These
depend on the timing of a potential snowstorm.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Saturday/...
Today and Tonight…Moderate confidence. Winds increase later this
morning and afternoon to strong small craft thresholds. The low
level jet really cranks tonight and despite the inversion some 35 to
40 knots wind gusts may mix down especially in any convective
elements. Therefore...opted to hoist marginal Gales and expect seas
to build to between 8 and 13 feet. Vsbys may drop to under a mile
at times in areas of heavy rain and fog.
Saturday...Moderate to high confidence. Marginal southwest gale
force southwest wind gusts will shift to the northwest behind a
strong cold front. Winds should gust from the northwest between 20
and 30 knots.
Outlook /Saturday Night through Tuesday/...
Saturday Night: Winds less than 25 kt. Rough seas up to 12 ft.
Slight chance of freezing rain, slight chance of snow showers,
slight chance of sleet. Areas of visibility 1 to 3 nm.
Sunday: Winds less than 25 kt. Rough seas up to 10 ft. Local
visibility 1 to 3 nm.
Sunday Night: Winds less than 25 kt. Areas of rough seas.
Chance of snow showers. Areas of visibility 1 to 3 nm.
Martin Luther King Jr Day: Winds less than 25 kt. Seas up to
5 ft. Slight chance of snow showers. Areas of visibility 1 to
3 nm.
Monday Night: Winds less than 25 kt. Seas up to 5 ft. Local
visibility 1 to 3 nm.
Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain, slight chance of snow. Visibility 1 to 3 nm.
&&
.CLIMATE...
Record high temps for today January 12
BOS 61 in 2017,1975
PVD 60 in 2017
BDL 57 in 1913
ORH 57 in 2017
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through Saturday afternoon for CTZ002>004.
Winter Weather Advisory from 9 AM to 4 PM EST Saturday for
CTZ002>004.
MA...Flood Watch through Saturday afternoon for MAZ002>024-026.
Winter Weather Advisory from 11 AM to 7 PM EST Saturday for
MAZ005>007-013>019.
Wind Advisory from 5 PM this afternoon to 9 AM EST Saturday
for MAZ018>024.
Winter Weather Advisory from 5 AM to 4 PM EST Saturday for
MAZ002-003-008>011.
Winter Weather Advisory from 9 AM to 4 PM EST Saturday for
MAZ004-012-026.
RI...Flood Watch through Saturday afternoon for RIZ001>008.
Winter Weather Advisory from 11 AM to 7 PM EST Saturday for
RIZ002>005.
Winter Weather Advisory from 9 AM to 4 PM EST Saturday for
RIZ001.
MARINE...Gale Warning from 5 PM this afternoon to 9 AM EST Saturday for
ANZ231>234-250-251-254>256.
Small Craft Advisory until 7 AM EST Saturday for ANZ230-236.
Small Craft Advisory until 7 PM EST Saturday for ANZ237.
Gale Warning from 5 PM this afternoon to 7 AM EST Saturday for
ANZ235.
&&
SYNOPSIS...Frank/Doody
NEAR TERM...KJC
SHORT TERM...Frank
LONG TERM...Doody
AVIATION...KJC/Doody
MARINE...Frank/Doody
CLIMATE...