Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 191120

Area Forecast Discussion
National Weather Service Boston/Norton MA
720 AM EDT Thu Apr 19 2018

Low pressure over Virginia will pass south of Nantucket this
afternoon and pass south of Nova Scotia tonight. This will bring
a period of chilly rain today. The rain may mix with wet snow,
especially in the higher elevations. Drier air moves in on
Friday, but with isolated showers possible. Building high
pressure will bring gradually warming temperatures and dry
weather into early next week. Storm system expected for mid to
late week.


7AM update...
MSAS already indicating low pres near 1000hPa off the NJ coast
this morning. Precip shield associated with a combination of mid
lvl f-gen and overrunning in advance of the this developing low.
Radar suggesting a SN to PL to RA setup per dual-pol data with
several OBS sites reporting a mix of RA/PL at onset where not
currently SN. Based on early AM wet-bulb temps, areas near or
within a two degrees of freezing are mainly SN with mixed RA/PL
as far S as CT/RI and SE MA. Expecting a continued warm trend as
the warm conveyor develops with the deepening low pres.

One thing to note through the morning is the intrusion of dry
air as the low pres deepens, this may actually transition precip
to more of a DZ/flurry scenario across NW MA before the upper
lvl shortwave rotates later today. POP update will reflect this

Previous forecast discussion follows...

Low pressure along the Mid Atlantic coast will feed upon
favorable dynamics and a sharp baroclinic zone covering the Mid
Atlantic and Southern New England. This favors strong lift, with
models showing 40-50 mb/hr rates during the day. Cross sections
show moisture reaching to 500 mb. While not deep, this is
probably deep enough. Favorable lift and moisture move over our
area starting between 09Z and 12Z and reach maximum values
during the morning, then diminish during the afternoon.

Also of concern is precip type. Temperatures start the day
mainly in the 30s while dew points will be in the upper 20s and
lower 30s. As precip rates increase this morning, this may
induce wet bulb cooling which may bring temps down a few
degrees. Forecast profiles of temperature show borderline
rain/snow profiles, and any additional cooling would only
increase the potential for snow. Profiles most favor snow over
the highlands as well as across Western and Central Mass, and
maybe the Merrimack Valley. An inch or two accumulation is
possible in the highlands, with an inch or less elsewhere in the
favorable area. Mainly rain expected in CT/RI/Southeast Mass
except in the highlands along the CT/RI border.

The low passes Nantucket around 18Z/2PM, at which time the
favorable lift starts moving off to the northeast. Surface winds
will start shifting from the northwest, bringing in drier air.

Low level temps aloft support max sfc temps in the low to mid
40s. The wet bulb cooling may keep some areas in the highlands
in the 30s.



Even as the surface low moves off to the east tonight, the
upper trough will be swinging overhead. The axis is sharp, and
carries a -24C to -26C cold core. This may be sufficient to
maintain clouds and a chance of showers through the evening. The
trough moves off after midnight, after which we may see some
clearing especially in southern sections.


Cyclonic flow around the upper low continues on Friday. The
moisture aloft diminishes through the day, which should allow at
least partly sunny skies. The diminishing moisture will reduce
the development of showers, but we expect at least widely
scattered showers during the afternoon.

The cold advection and cold temperatures aloft should allow
mixing to at least 850 mb. Winds in this layer should reach 25
knots, so we expect gusts of 25-30 mph. Temperatures in the
layer support mid to upper 40s, and if mixing goes above 850 mb
then highs in the low 50s are possible.


*/ Highlights...

 - Near-seasonable, dry conditions over the weekend
 - Warmer conditions, dry, Monday into Tuesday
 - Mid to late week storm system

Overview and model preferences...
A couple of slow meandering cutoffs to monitor through the long
term. The first, is the one that will most effectively define
sensible wx details through the weekend, formed in N New England
and shifting into the Maritimes. The slow movement maintains
below normal H5 height anomalies through early next week,
yielding temps near or below normal in spite of the continued
modification. As this first cutoff shifts E, riding will build
in its wake thanks to warm advection from the W. This ridge will
dominate the forecast through the weekend and into early next
week. The second cutoff meandering through the S stream all
weekend lead to a merger point for Pacific energy mid-week
yielding another risk for wet and more unsettled wx. Given
synoptics are in fair agreement between both operational and
ensemble data, will continue to use a blend for this forecast


Upper lvl cutoff and attendant trof remain close enough to S
New England that upper lvl flow is cyclonically curved through
the period. This may assist in diurnal cloud development.
Combine this with H85/H92 temps a full std deviation below
normal (H85 roughly -4C to -6C while H92 temps run near -2C),
and expect both highs and lows to remain below seasonal normals.

Mon and Tue...
Moderating temperatures as ridging gains control. H85 temps
shift above 0C by Mon afternoon, with H92 temps approaching +4
to +6C. This will allow highs to reach near or even above
seasonal normals, limited in part by the strengthening
subsidence inversion as high pres crests across the region.
Also, coastal areas limited by sea breezes thanks to low lvl
flow SSTs still in the 40s. Overnight lows cool thanks to
radiational cooling setup.

Mid-late next week...
A potential merger of Pacific, S stream and even N stream
energy will redevelop longwave trof across the E CONUS, agreed
upon by most ensembles as mean height anomalies once again drop
below normal. This will yield another round of wetter and more
unsettled wx. Timing could be as early as Wed for a new round of
wet wx, but exact timing/details will be better defined as we


Forecaster Confidence Levels...

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

Short Term /through Friday/...

Today and tonight...Moderate confidence.
Areas of MVFR spread E (low CIGS and VSBYS in precip) through
the morning with a risk for IFR (mainly due to VSBY in SN)
across the higher terrain of interior MA/CT. This changes to
rain all TAF sites through the morning, with mainly MVFR
conditions. However, through the afternoon will see some
improvement from NW to SE although the SHRA/SHSN risk will
linger into the evening hours. Note that the timing of the
improvement may be off in the TAF.

Winds shift to the N-NW through the day, peaking during the
evening hours.

Friday...High confidence.

VFR. Northwest winds will gust to 25 knots during the day.
Widely scattered showers during the day, and some of this could
briefly cause MVFR cigs/vsbys.

KBOS Terminal...High confidence in TAF.
Precipitation mainly rain, but some snow/sleet possible for the
first few hours of the TAF. Then mainly rain and MVFR conditions
until some improvement during the evening hours.

KBDL Terminal...High confidence in TAF.

Light precipitation develops around 09Z/5 AM, increasing to max
intensity 13Z to 18Z. Lowest cigs/vsbys expected at that time,
with some improvement during the mid afternoon, then briefly
lower again during the evening. Any snow accumulation is
expected to be confined to grassy surfaces.

Outlook /Friday Night through Monday/...Moderate confidence.

Friday Night through Saturday: VFR. Breezy.

Saturday Night through Monday: VFR.


Forecaster Confidence Levels...

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

Short Term /through Friday/...High confidence.

Today... A coastal weather system moves northeast from the
Delmarva coast and passes near the benchmark mid afternoon, then
moves south of Nova Scotia tonight. Rain will lower vsbys to
around 2-3 miles today. Winds remain below 25 knots and seas
below 5 feet.

Tonight... Increasing northwest winds tonight as the offshore
weather system moves off past Nova Scotia. Winds will reach 30
knots at times. Seas will build on the southern waters late at
night, with five to seven foot heights overnight. Small craft
advisory has been issued for all waters.

Friday... Northwest winds gusting to 25 knots, with seas near 5
feet on the outer waters. Small Craft Advisory in effect.

Outlook /Friday Night through Monday/...Moderate

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft.

Saturday through Monday: Winds less than 25 kt.


MARINE...Small Craft Advisory from 2 AM to 3 PM EDT Friday for
     Small Craft Advisory from 4 AM to 6 PM EDT Friday for ANZ230.
     Small Craft Advisory from 4 AM to 3 PM EDT Friday for ANZ236.
     Small Craft Advisory from 9 PM this evening to 3 PM EDT Friday
     for ANZ235-237.
     Small Craft Advisory from midnight tonight to 3 PM EDT Friday
     for ANZ250.
     Small Craft Advisory from 9 PM this evening to 8 PM EDT Friday
     for ANZ254>256.


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