LexMAWeather

Area Forecast Discussion
for Lexington MA

        
000
FXUS61 KBOX 200823
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
423 AM EDT Sat Oct 20 2018

.SYNOPSIS...

Today/s mild weather will come to an end tonight as a cold 
front crosses the region. This will bring the return to blustery
and chilly weather overnight and into Sunday. Temperature 
swings roll on through the following week. Warm ups out ahead of
sweeping cold fronts associated with light showery weather and 
breezy conditions, frost / freeze conditions following as high 
pressure builds across the region. A potential Nor'Easter for 
the following weekend. 

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

420 am update...

An approaching shortwave has induced a modest southwest low 
level jet. This has resulted in much milder temperatures 
compared to this time yesterday. Most locations had temperatures
in the 50s early this morning with readings around 60 across 
southeast New England.

Deep moisture is rather limited with this shortwave...but the
forcing was enough to generate a few showers early this morning.
The bulk of these showers will focus along the southeast new
England coast towards 12z on the nose of the LLJ and where a bit
better moisture resides. We also will continue the marginal wind
advisory for the Cape/Islands this morning. So far...we
generally have seen some southwest wind gusts in the 30 to 40
mph range in this region. Winds have actually diminished a bit
in this region over the last hour or two...but may see another 
surge of wind towards 12z as some showers may interact with some
low level dry air. Winds should diminish by afternoon as the low
level jet exits the region.

As mentioned...the bulk of the showers should focus across far
southeast New England after daybreak with mainly dry weather
elsewhere. Even across far southeast New England any showers
should pretty much be over by afternoon. Otherwise...some
partial clearing and a mild start should allow afternoon highs
to reach well into the 60s in many locales. Dry weather will
dominate...but can not rule out a few spot showers during the
late afternoon particularly across the interior.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...

Tonight...

A strong cold front will cross the region this evening with 
perhaps a few showers. However...the better chance for more 
widespread showers will be overnight as a strong upper level 
shortwave/cold pool aloft moves into southern New England. 500T 
drop below -30C and there is a period of decent forcing with a 
bit of elevated instability. We think some of the model 
guidance is too dry tonight and feel the GFS/ECMWF are more 
realistic with QPF. Certainly not expecting a lot of 
precipitation...but do expect some showers across the region. In
fact...temperatures may become cold enough for even some wet 
snow flakes across the high terrain. 

Low temperatures by daybreak should drop into the 30s across
portions of the interior with lower 40s elsewhere.

Sunday...

A blustery and much chillier day across the region. We may have
a few left over showers into part of the morning and perhaps
even a few wet snow flakes in the high terrain.
Otherwise...partial clearing expected with 850T dropping to
between -6C and -8C. This should hold high temperatures mainly
in the upper 40s to the lower 50s. Good mixing should yield
northwest wind gusts of 25 to 35 mph.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

*/ Highlights...

 - Coastal frost / freeze headlines Sunday night - Monday  
 - Rolling temperature trend through midweek  
 - Best wet-weather chances Tuesday - Wednesday with clipper low
 - First-season Nor'Easter potential the following weekend

*/ Overview...

A first-season late week / following weekend Nor'Easter? Very wavy 
longwave pattern into October-end. Persistent NE Atlantic block that 
promotes -NAO / -AO trends, a N Atlantic ridge and upstream Eastern 
N American H5 trof. Meanwhile a progressive Eurasia to NE Pacific H3 
jet max, flipping EPO to positive, +PNA maintained while the split-
flow W CONUS regime deamplifies. The longwave pattern amplifying and 
tucking equatorward into the S-stream per tag-team teleconnections, 
an opportunity emerges in where ejecting NE Pacific energy carves S 
within preferred Eastern N America H5 trof, promoting height falls, 
usurping while stretching / ejecting E Pacific tropical energy. Any 
interaction / phasing unclear, there's increasing signal of S-stream 
obtaining greater cyclonic curvature within the aforemention H5 trof 
promoting potentially our first-season Nor'Easter by the weekend, 
more warm-core / maritime-tropical, potential significant rains as 
CPC notes a moderate risk for heavy precipitation within their 8-14 
day forecast, EC preference in handling the -AO with their forecast. 

Until then, an undulating temperature trend. Clipping Canadian lows 
and highs up against an upstream split-flow. NW vs. SW flow. Flip-
flop back and forth, temperatures averaging overall below-seasonable 
with the best chance of wet-weather Tuesday - Wednesday per Canadian 
clipper-low. Breezy conditions at times but seemingly below advisory-
level criteria. Of remaining forecast areas where the growing season 
is still considered, only Sunday night - Monday there's the greatest 
chance of closing out frost / freeze headlines for those areas, the 
end of their growing season. Otherwise, not much else with
respect to targets of opportunity. 

Consensus weighted forecast with greater weighting to EC / ECens out 
at longer ranges into late October. 

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...

Today...Moderate confidence. A few showers early this
morning...which should focus themselves near the far southeast
New England coast by 12z before moving offshore by afternoon.
MVFR cigs have been pretty limited to far...but may see them
become a bit more widespread for a few hours this morning.
Otherwise...expect VFR conditions to dominate except perhaps the
Cape/Islands where cigs may flirt with MVFR thresholds. SW wind
gusts of 20 to 30 knots on the coastal plain and a bit stronger
at times across the Cape/Islands will diminish this afternoon.

Tonight...Moderate to high confidence. Mainly VFR but some
showers overnight may result in briefly lower conditions.

Sunday...Moderate to high confidence. Any left over showers
should exit the region Sunday morning. Otherwise...VFR with NW
wind gusts of 25 to 30 knots.

KBOS Terminal...Moderate to high confidence in TAF.

KBDL Terminal...Moderate to high confidence in TAF. 

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Windy with areas gusts to 30 kt. Areas
frost.

Monday: VFR. Breezy. Areas frost.

Monday Night: VFR. 

Tuesday: VFR. Breezy. Slight chance SHRA.

Tuesday Night through Wednesday: VFR. Breezy. 

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...

Today...Moderate to high confidence. Gale warnings continue for
all water this morning except for Boston Harbor/Narragansett
Bay. Southwest winds will gust up to 35 knots at times
especially in any showers. Otherwise...winds should diminish by
afternoon as low level jet exits the region.

Tonight...Moderate to high confidence. A cold front crosses the
region this evening. This should allow small craft wind gusts 
to redevelop towards daybreak. 

Sunday...High confidence. Strong cold advection over the
relatively mild ocean will result in northwest wind gusts of 30
to 35 knots. Strong small craft or gale headlines will be
needed...but given current headlines will let the next shift
decide based on the latest data.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. 

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. 

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers. 

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Local rough seas. Slight chance of rain showers. 

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. 

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 20 kt. Seas locally approaching 5 ft. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until noon EDT today for MAZ022>024.
RI...None.
MARINE...Gale Warning until noon EDT today for ANZ231>235-237-250-251.
     Small Craft Advisory until 2 PM EDT this afternoon for ANZ230-
     236.
     Gale Warning until 1 PM EDT this afternoon for ANZ254>256.

&&

$$
SYNOPSIS...Frank/Sipprell
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Sipprell
AVIATION...Frank/Sipprell
MARINE...Frank/Sipprell