NWS Forecaster Discussion
BOX
Area Forecast Discussion National Weather Service Boston/Norton MA.
000
FXUS61 KBOX 161122
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
722 AM EDT Sun Jun 16 2019
.SYNOPSIS...
A cold front sweeps south from Canada, bringing showers and
local downpours. An isolated thunderstorm may cross the South
Coast. Weak high pressure brings clearing conditions tonight
and Monday. Then bouts of rain, embedded heavier showers,
possible thunderstorms beginning Monday night through Friday
morning as a wave train of storm systems slides west to east
along a stalled surface frontal boundary across or in vicinity
of southern New England. Next weekend could end up warmer and
dry.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
730 am update...
Satellite imagery and surface observations this morning indicate
the cold front currently draped from southwest to northeast over
NY. Our first batch of rain showers has become more scattered in
nature as it continues to move to move east, mainly south of the
MA Pike. Our next round of rain is visible upstream over eastern
NY/PA this morning associated with the low that will cross our
region ahead of the cold front later today. Adjusted POPs a bit
to bring in line with latest hi-res guidance bringing the
majority of this activity through mid-late afternoon.
Previous discussion...
A cold front moving through the St Lawrence Valley and Great
Lakes settles south into northern New England. Meanwhile a zonal
flow aloft circulates higher humidity air across NY and PA,
eventually into Southern New England this afternoon/evening. An
upper jet will move through the zonal flow, with the right
entrance region in position to lift and the formation of a wave
along the front. The front becomes parallel to this zonal flow,
and stalls over northern Mass late in the day.
The increased moisture and the lift will favor the development
of showers to our west that move into Southern New England. The
best chance will be along/north of the Mass Pike during the
afternoon. A plume of 1.8 inch PW values sweeps east at this
time...the NAM suggests 2 inches...and may bring enough moisture
to support local downpours.
Convective parameters are for the most part mediocre. But some
parameters do become supportive of thunderstorms late in the
afternoon... LI values dip to around zero along Southern CT and
the South Coasts of RI and MA, K indices are in the 30s, CAPE
values reach 200-300 J/KG along/south of the South Coast. We
will indicate widely scattered or isolated thunder in this area.
The expected sky cover will limit daytime heating/mixing. If we
get more sun than expected, a deeper mixed layer would support
warmer daytime temperatures. We generated max sfc temps off the
925 mb values, the result covering the upper 60s to mid 70s.
With more sun and mixing to 850 mb, surface temperatures would
climb to the lower 80s and greater instability would be
generated.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Tonight...Upper jet and right entrance region move off to the
east tonight. The surface low pressure wave may be enough to
push the stalled front to the South Coast, but only low
confidence in the positioning of the front during the night. Dew
points will be around 60...but it is possible the front remains
farther north over Southern New England with local dew points
in the low to mid 60s. Expect min sfc temps in the upper 50s to
mid 60s.
Showers linger through the evening but then move off to the
east by midnight. If the front remains over Southern New England
then there would be a chance of showers overnight.
Monday...
High pressure moves across Northern New England should bring a
brief break to the showers. The ridge and its subsidence will
allow for daytime heating, with mixing to 850 mb. This will tap
850 mb temps of 10-12C, supporting max sfc temps of roughly
75-80F.
However the right entrance region of the next upper jet moves
into position over New York and PA late in the day, favoring a
slight chance of showers in CT/RI at that time.
&&
.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights...
* Bouts of rain, embedded heavier showers, possible thunderstorms
beginning Monday night continuing through Friday morning
* Seasonable temperatures throughout
* By late June could see warmer than average conditions
Details...
Monday Night and Tuesday...
Our next chance at wet weather comes sometime late Monday
night/Tuesday morning as a surface low pressure system strengthens
over the Delmarva peninsula and crosses southern New England. Mid
and upper level moisture out ahead will bring increasing cloudcover
Monday evening, and rain chances increase with time. All the
ingredients should come together for rain Monday night and Tuesday
given ample moisture through the column (PWATs 1.5-2"), isentropic
lift along a warm frontal boundary, and placement under the RRQ of a
100kt 300mb jet. However, there still exits some uncertainty as to
how widespread it will be for a few reasons. Mainly, the exact track
of the low will play an important role. NAM guidance wants to keep
the low and best forcing for precip further south, mainly impacting
areas south of the MA Pike, while global models bring rain further
north. Given the difficulty models are having with resolving exact
details in a quasi-zonal pattern, will stick with a consensus of
chance POPs through Tuesday for now. Increased clouds and rain
should keep temperatures cooler on Tuesday, in the upper 60s and low
70s.
Wednesday through Friday...
Reiterating the point previously made, the flat, zonal nature of the
steering flow during this period of the forecast keeps confidence
low on exact timing and details of rain chances. Suffice it to say,
it won`t be a dry week for southern New England. A progressive and
unsettled pattern which set up earlier in the week continues through
at least Thursday. With a stalled W-E surface frontal boundary
lingering in the vicinity, a quasi-stationary upper jet in favorable
position for synoptic lift, and precipitable water values
consistently above 1.5", a series of shortwaves will bring periodic
rain chances with potential for embedded downpours and
thunderstorms. Not until Thursday night/Friday do we see signs of a
pattern change. Models continue to be consistent in bringing a
stronger shortwave out of the Ohio Valley into the northeast. This
would bring widespread potentially heavy rain to the region, though
whether it exits by Friday or sometime later is yet to be seen. No
appreciable hot or cold air intrusions foreseen in this time period,
so temperatures should remain pleasant and seasonable, in the
70s...cooler along the coast.
Saturday...
Behind the Thursday night/Friday front drier air filters in and
things look to dry out under weak mid level ridging. Temperatures
still around average, though the CPC and some ensemble guidance is
calling for the return of above average temperatures for late June,
so warmer weather may be just over the horizon.
&&
.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Monday/...High confidence.
Today...
VFR conditions will lower to MVFR during the morning or early
afternoon, with ceilings to IFR in the afternoon especially
along and north of the Mass Pike. Visibility will be 3 to 5
miles in showers, but 1 to 3 miles where fog forms. Isolated
thunderstorms are possible this afternoon along the South Coast.
Tonight...
IFR/MVFR conditions the first part of the night in showers and
fog. Conditions then improve from northwest to southeast as
high pressure builds overnight. Lingering IFR on Cape Cod and
Islands late at night, while all other areas improve to VFR.
Monday...
High pressure over northern New England will bring a brief spell
of dry weather with VFR conditons all areas.
KBOS Terminal...High confidence in TAF.
KBDL Terminal...High confidence in TAF.
Outlook /Monday Night through Thursday/...Moderate confidence.
Monday Night: Mainly VFR, with local MVFR possible. Chance
SHRA.
Tuesday: Mainly VFR, with areas IFR possible. Chance SHRA.
Tuesday Night: Mainly VFR, with areas IFR possible. Slight
chance SHRA.
Wednesday through Wednesday Night: Mainly VFR, with local IFR
possible. Chance SHRA.
Thursday: Mainly VFR, with areas MVFR possible. SHRA likely.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Monday/...High confidence.
Today...
Winds diminished all areas during last night. Rough seas of 5 to
6 feet and gusts to 20 kt linger over the southern outer
waters. These rough seas may spread back into RI and Block
Island Sounds. We will maintain a Small Craft Advisory in these
areas. Winds less than 20 kt and seas less than 5 feet are
expected in the remaining marine forecast zones.
Showers and isolated thunderstorms will develop today,
especially in the afternoon/evening. Increasing humidity in the
air will also favor fog formation, with visibility in rain and
fog 3 to 5 nm.
Tonight...
Showers and fog will continue to affect the waters early
tonight, with vsbys around 3 nm. Expect conditionsl to improve
to unrestricted visibility toward morning.
Monday...
Weak high pressure builds south over the waters. This should
maintain dry conditions through the day. North winds become
variable as the high pressure moves overhead. Winds will remain
less than 20 kt. Most seas will be less than 5 feet, but may be
near 5 feet along the extreme southern edge of the southern
outer waters.
Outlook /Monday Night through Thursday/...Moderate confidence.
Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers.
Tuesday through Wednesday: Winds less than 25 kt. Chance of
rain showers.
Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Chance of rain showers.
Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft. Rain showers likely.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until noon EDT Monday for ANZ254>256.
Small Craft Advisory until 6 AM EDT Monday for ANZ235-237.
&&
$$
SYNOPSIS...WTB/BW
NEAR TERM...WTB/BW
SHORT TERM...WTB
LONG TERM...BW
AVIATION...WTB/BW
MARINE...WTB/BW