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Area Forecast Discussion (AFD)

Issued by Boston, MA (BOX)

FXUS61 KBOX 220830

Area Forecast Discussion
National Weather Service Taunton MA
330 AM EST Thu Feb 22 2018

A wave of low pressure tracking along a stalled frontal 
boundary to the south will bring a period of snow and sleet 
along and north of the Pike with rain and sleet to the south. 
Active weather pattern Friday through the weekend as a series of 
frontal boundaries usher wet weather across the region with the 
threat of freezing rain across the N/W interior, high terrain. A 
near-seasonable, quiet pattern for early next week.


Sct showers moving east across the south coast region within 
the deep moisture axis near and south of the coast. This shower 
threat will continue through daybreak before low level dry air 
begin to push south toward the coast. 

Otherwise, colder air has been a bit slower to arrive across 
SNE with temps still in the 40s north and 50s south, but 30s to
the N/W and good low level dry advection with dewpoints falling
through the 20s. Temps will fall into the 30s north and lower 
40s south around daybreak.

Strong high pres to the north with flat wave tracking along
stalled frontal boundary to the south today as temps gradually 
fall through the 30s. Low level colder air undercutting 
somewhat milder air aloft will bring a decent shot of mid level
omega within a deep moisture plume which will spread precip 
across SNE. Soundings show a rather pronounced low dry layer 
which will have to be overcome and this will likely delay onset 
of precip until around midday and especially during the 

Biggest challenge remains the ptype forecast as cooling aloft is
a bit slower than low level cooling. Pronounced low level cold
air with 925/950 mb cooling to -3 to -5C this afternoon with a 
warm layer above 800 mb which will set up a zone of sleet across
the region. Cold air deep enough far north with soundings 
suggesting mostly snow north of the Pike, mixed with sleet at 
times, with rain mixing with or changing to sleet to the south
Precip could even change to a period of snow/sleet across N CT,
N RI and interior SE MA. Precip may begin as a brief period of
rain northern MA but will quickly change to snow given 
evaporative cooling processes. Not really expecting any 
freezing rain given the depth of the low level cold layer, but
there is a low risk near the MA/CT and northern RI border.

Decent shot of omega through DGZ noted across northern MA this 
afternoon so a brief period of heavier snow is possible. 
Expecting an inch or 2 accum north of the Pike, mainly on colder
non-paved surfaces with locally up to 3 inches possible over 
the Berkshires. Less than an inch elsewhere away from areas 
near the south coast.


Precip will exit the region by early evening with dry conditions
most of the night as surface ridging builds south into SNE.
However, sufficient moisture for cloud cover to persist through
the night, especially south of the Pike. Partial clearing is 
possible across NE MA late. Lows mostly in the 20s to lower 30s.


*/ Highlights...

 - N/W interior freezing rain late Friday into Friday evening
 - Once again Saturday night into Sunday
 - Dry, near seasonable into early next week 

*/ Overview...

Upstream Pacific flow flattening, SE CONUS sub-tropical H5 ridge 
weakening, SW-NE thermal wind axis and storm track along the better 
baroclinic zone shifts S/E along which mid-level impulses eject out 
of a preferred H5 trof pattern across the W CONUS. An active weather 
pattern with a series of disturbances delivering mixed precipitation 
events thru the weekend. Building up over the N Atlantic
signaled by a strongly -NAO, in going with ensemble means, we 
fall within a near-seasonable, dry pattern W of favorable storm 
development over SE Canada. Model forecast consensus through 120
hours (Monday) with preference towards ensemble means 
thereafter. Hit on targets of opportunity below. 

*/ Discussion...

Late Friday into Friday evening...

Over-running precipitation event. Focus on thermal fields below H9, 
especially surface with 2m wet-bulb temperatures, beneath the warm 
nose up around H8. Surface high situated E, less N-funneling winds 
per ageostrophic motions. There's the potential for a brief period 
of freezing rain. Dry air issues, above average heights, mid-upper 
level ridging, absence of deep layer lift, that with precipitable 
waters building towards 0.75 to 1.00 inch, light to moderate precip 
event is forecast with amounts upwards of around 0.25 inches, no 
higher than 0.50, with highest amounts along the W slopes of high 
terrain. A brief period though during the height of precip outcomes, 
ice accretion of 0.1 to 0.2 inches is possible over higher terrain. 
Highest confidence towards N/W MA especially Berkshires, Worcester 
Hills. High-res guidance colder than global. With the high situated 
E, return S flow, prefer to edge away from coldest guidance and not 
be pessimistic. Thus forecast ice accretion amounts up as high as 
around 0.10 inches over high terrain, especially Berkshires. Lean a 
non-GFS consensus with impacts to Friday PM commute. 

Saturday night into Sunday...

Another over-running precipitation event associated with a classic 
warm occlusion into S Canada. Stronger synoptic forcing / lift that 
invokes a secondary surface low along our coast with a synoptically 
favorable cold surface high N/E towards a freezing rain event. More 
likely outcomes of colder temperatures funneling S, maintaining over 
the interior, notably occurring during an overnight period. Prefer 
EC/NAM/Canadian, higher confidence of freezing rain over Berkshires, 
Worcester Hills. Accretion amounts of 0.1 to 0.2 given less dry air 
intrusions, deeper lift / forcing. Precipitable waters once again in 
the 0.75 to 1.00 inch range. 

Early next week...

Keeping with ensemble means, traffic build up across the N Atlantic, 
the strongly -NAO, the initial region of favorable storm development 
over SE Canada, will need to closely monitor for any retrogression 
as energy continues to eject out of a preferred H5 trof pattern over 
the W CONUS. Looking dry and near-seasonable till roughly Thursday 
into Friday, being W on the backside of favorable storm development. 
NW winds prevailing. Overall, a low confidence forecast.  


Forecaster Confidence Levels...

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

Short Term /through Tonight/...Moderate confidence.

VFR in the morning, then lowering to MVFR/IFR in the afternoon
with snow/sleet along and north of the Pike and rain/sleet to 
the south.

Precip moves offshore by early evening. MVFR/IFR improving to
VFR north of the Pike, but MVFR cigs persisting to the south. 

KBOS Terminal...Moderate confidence TAF. Conditions lowering to
MVFR then IFR in the afternoon with a period of rain, then snow
and sleet. Any accum less than an inch. 

KBDL Terminal...Moderate confidence TAF. Conditions lowering to
MVFR then IFR in the afternoon with a period of rain, sleet and
snow. Any accum less than an inch. 

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

Friday: Mainly MVFR, with areas VFR possible. RA, FZRA.

Friday Night: MVFR/IFR conditions possible. RA likely, FZRA

Saturday: VFR. 

Saturday Night: Mainly MVFR, with local IFR possible. Breezy.
RA likely, FZRA likely.

Sunday: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA.

Sunday 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 Tonight/...Moderate confidence.

Northerly winds turning NE with gusts to 20-25 kt at times. SCA
issued for eastern waters due to seas. Vsbys reduced in rain and
snow this afternoon.

Mainly NE winds with gusts to 20 kt at times. Seas mostly below

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Rain, slight chance of freezing

Friday Night: Winds less than 25 kt. Rain likely. 

Saturday: Winds less than 25 kt. 

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain. 

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas. Rain. 

Sunday Night: Winds less than 25 kt. Areas of rough seas. 

Monday: Winds less than 25 kt. Seas up to 5 ft. 


MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ250.
     Small Craft Advisory until 7 PM EST this evening for ANZ254.


LONG TERM...Sipprell