Old Res Weather

Area Forecast Discussion
for Lexington, MA

        
000
FXUS61 KBOX 040335
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1035 PM EST Thu Dec 3 2020

.SYNOPSIS...
Increasing cloud cover tonight with quiet weather as high pressure 
nudges in from the south. Light rain spreads in later on Friday and 
Friday night as a warm front lifts in from the incoming system. A 
strong storm system will impact the region Saturday into Saturday 
night bringing the heavy rain, and strong winds along the coast, and 
accumulating snow. The storm exits Sunday, then mainly dry and 
colder weather Monday and Tuesday with a moderating trend
Wednesday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

1020 PM update...

Current forecast is largely on track so only made minor changes
to bring near-term forecast up to date with current obs. 
Satellite shows upstream mid to high- level clouds headed our 
way, which would limit the extent of radiational cooling and 
keep temperatures milder than the previous night. The clouds are
associated with an approaching shortwave from a closed H5 low 
over Oklahoma/Arkansas. This feature could be a key ingredient 
for the potential weekend winter storm across S New England. 
Stay tuned! 

PREVIOUS DISCUSSION...

* Dry and quiet weather expected tonight with increasing cloud 
  cover. 

Mid level ridge axis flattens over the TN/OH Valley due to the 
cutoff over the Mid Mississippi River Valley and cyclonic flow over 
the Great Lakes. Surface high will nudge into southern New England 
from offshore of the Carolinas tonight.

Quiet weather across the region tonight. Will see mid to high clouds 
moving in due to southern cutoff and northern stream mentioned 
above. The high will keep the pressure gradient relaxed, but cloud 
cover should hinder strong radiational cooling. Went with the 25th 
percentile of guidance, which results in lows generally in the 30s 
across much of the region. The coastal plain will see readings in 
the low to mid 40s.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday...

* Mild and cloudy with light rain spreading in from southwest to
  northeast on Friday. Light rain continues Saturday night.

Northern stream shortwave trough will be located near Upper Michigan 
and will dig into the eastern Great Lakes. This is all while the 
southern stream cutoff begins lifting into the TN/OH Valley and 
interacting with the northern stream trough. Low pressure will 
deepen over the TN/OH Valley and a warm front will lift into the Mid 
Atlantic. 

Becoming cloudy across southern New England as the warm front lifts 
into the Mid Atlantic. Aloft flow will become more south-
southwesterly, which will advect more moisture into the region. 
Should see light rain chances increase from southwest to northeast 
during the afternoon as moisture advects in. Good consensus amongst 
guidance for this, but any precipitation should be light. Expect it 
to be above normal temperature wise for southern New England with SW 
to SSW warm air advection at 850 and 925 hPa. Highs will generally 
be in the 50s though there will be some mid to upper 40 degree 
readings across the higher elevations of western portions of the 
CWA. 

Friday Night...

* Light rain continues, but some of the higher elevations across the 
  eastern slopes of the Berks may see precipitation change over to a 
  rain/snow mix. 

Southern stream cutoff continues to phase with the northern stream 
trough as it digs into the eastern Great Lakes. The surface low will 
deepen as it moves into the Mid Atlantic. The warm front associated 
with the low will lift to just south of southern New England.

Still good consensus amongst guidance for light rain across southern 
New England. Though the intensity may pick up toward late Friday 
night/early Saturday as the surface low moves near VA/DE. Should 
stay fairly mild as southwesterly warm air continues to advect in. 
Lows will be in the 30s across the interior and 40s elsewhere.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
500 pm update...

Mother Nature is making us use all of our winter storm
forecasting tools and definitely not making it easy. A very
highly uncertain forecast for Saturday indeed. Let me spend most
of my time focusing on this event, since it's really the 
highlight in the long term. 

Models: still quite a bit of variation and the models are 
struggling with exactly how a northern stream disturbance 
currently diving south along the Ontario/Manitoba border and 
southern stream energy across the ArkLaTex region will merge 
later tomorrow. The model sensitivities to small differences in 
analyzed intensity of these systems is huge. So although 
confidence is high that rapid cyclogenesis will occur somewhere 
along the mid-Atlantic Friday night/early Saturday, those 
important details of exactly when, where and what path elude us.
A 50 mile difference in track, or 6 hour timing difference in 
development means all the difference. What I'm trying to say 
here is make sure you have big error bars around the forecast. 
This is a great situation where probabilistic forecasts are much
better than a single deterministic one. But since I've got to 
make a deterministic forecast, we'll just have to work with it. 

Scenario: so after quite a bit of coordination with neighboring
offices and WPC, generally taking a middle of the road 
scenario. NAM is believed to be a little too far east. More or
less went with a 12z Canadian/ECMWF concept. This would bring 
developing low pressure inside of the 40N/70W benchmark, and 
just from pattern recognition, the heaviest snows would be 
inland. Also looks to be a big slug of moisture with the system,
but most of this will stay out in the warmer side of the storm.
2" of precipitation across parts of RI and SE MA is not out of 
the question. During the day Saturday we should see the lower 
levels cool thanks to dynamic processes and wet bulb effects. So
initially most everyone starts as rain, and by mid- day we 
should see a change to snow across the interior -- mainly ORH 
westward. Boundary layer temperatures are marginal for snow, but
would think that as heavier precipitation starts to fall, the 
cooling should turn it over to snow. With temperatures in the 
lower to mid 30s, it will be a wet snow. As we get into the 
evening, the low will be moving rapidly east of the region and 
colder air starts filtering in. Think that even at the coast the
precipitation ends as some snow. 

Snowfall: Since there is uncertainty as to how much QPF will be
rain before turning to snow, that makes it tough to come up with
an accumulation forecast. Snow ratios appear to be on the low
side (as mentioned above...a heavy wet snow). I went with about
7-8:1 ratios during the day Saturday. As we get into the
evening, rising to 9-11:1. For those playing at home and viewing
snowfall predictions from online weather models, please be aware
that most use a strict 10:1 ratio which will get you into
trouble more often than not. Although I'm going with the lower
snow ratios, there is an unknown if we end up getting a
mesoscale band or well developed comma head forming. If that
were to happen, that means we've got strong vertical motion in
the snow growth zone resulting in large dendrites and a much
higher SLR. After putting everything in a blender, I came out
with amounts over 6" across higher elevations of MA with 2-4"
widespread at lower elevations. Perhaps an 1" as you get just
inland of the coast. Chances of my forecast being perfect?
Pretty low. Snowfall amount probabilities suggest the potential
for 10"+ across the higher terrain. Later forecasts will
hopefully fine tune things.

Rainfall: Juicy system, so I've got a swath of 1-2" across RI
and SE MA. Not thinking any hydro issues at this point, but if
we see amounts pushing 2.5" then some of the smaller rivers will
be reaching action stage. Again, something for later shifts to
look at. If this storm ends up tracking further east, then those
heavier rainfall totals fall over the ocean.

Winds: Quick look at model soundings in BUFKIT suggest we could
see gusts upward of 50mph across the Cape and Islands. 35-45mph
across eastern MA. As the system pulls away Saturday night,
there could be a fairly strong pressure rise couplet along with
deeper mixing on the backside of the storm to give a period of
some strong winds as the direction switches to the northwest.
Maybe be dealing with a marginal high wind situation requiring
an advisory or warning. Again, later shifts will look at that
detail more as well. 

Coastal Flooding: Although we'll have some strong onshore
easterly winds, and a rapidly deepening low pressure, at this
point it looks like the astronomical tides are low enough that
any surge from the storm won't cause any problems. 

Phew. I think that kinda hits the highlights. thanks for making
it through the rambling. 

For the rest of the extended, I stuck with the NBM. Doesn't look
like any other big storms. Upper flow trends toward a zonal flow
by mid-week, so although cooler than normal to start next week,
we'll get to more or less seasonable temperatures for mid week.

&&

.AVIATION /04Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Overall high confidence. 

Tonight and through about 18z Friday...VFR area wide. Thin
cirrus early overnight will slowly transition to thicker mid
level cloud deck by Friday morning. Southwest winds generally
light, though some gusts near 20kt possible across the Cape and
Islands. 

After 18z Friday...clouds will continue to lower and thicken.
MVFR ceilings develop from west to east. Should start to see
some light rain develop across the western half of the region
just prior to sunset. Shouldn't pose any restrictions to
visibility however. 

After 00z Saturday...expecting MVFR ceilings to develop area
wide. May start to see some MVFR visibilities as well,
especially at BDL terminal as rain gets a little steadier. As we
get later at night, and beyond the TAF forecast period, would
expect IFR conditions to develop. 

KBOS TAF...High confidence in the TAF.

KBDL TAF...High confidence in the TAF.

Outlook /Saturday through Tuesday/...

Saturday: Mainly IFR, with local MVFR possible. Strong winds
with gusts up to 45 kt. RA, chance SN.

Saturday Night: Mainly IFR, with areas MVFR possible. Strong
winds with gusts up to 45 kt. Chance SN.

Sunday: Mainly VFR, with local IFR possible. Windy with local
gusts up to 30 kt. 

Sunday Night through Monday: VFR. Breezy. 

Monday Night through Tuesday: VFR.

&&

.MARINE...
* Gale Watch for all waters Saturday into early Sunday.

Have taken down the Small Craft Advisories and hoisted a Gale Watch 
for all waters for the system expected to impact southern New 
England Saturday into Sunday. Confidence high enough in the majority 
of waters seeing at least Gale Force gusts if not higher, but still 
need to pin point the details given the differences in the track and 
intensity of the system. Mariners will want to stay tuned to the 
forecast. 

Winds will be WSW to SW tonight and on Friday with speeds of 15-20 
kts and gusts of 20-25 kts. Wave heights generally 4-6 feet with the 
highest waves across the outer waters.

Winds diminish to 5-15 kts Friday night ahead of the incoming system 
and the direction gradually shifts to the NE with gusts of 10-15 
kts. Seas falling to 3-4 feet across the eastern waters and remain 
around 4-6 feet across the southern waters.

Outlook /Saturday through Tuesday/...

Saturday: Low risk for gale force winds with gusts up to 45 kt.
Areas of rough seas. Rain. Visibility 1 to 3 nm.

Saturday Night: Moderate risk for gale force winds with gusts
up to 45 kt. Rough seas up to 15 ft. Chance of rain. 

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 12 ft. 

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. 

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

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Storm Watch from Saturday morning through Sunday 
     morning for MAZ002>006-008>010-026.
RI...None.
MARINE...Gale Watch from Saturday morning through Sunday morning for 
     ANZ230>237-250-251-254>256.

&&

$$
SYNOPSIS...BL/Nash
NEAR TERM...BL/Chai/Nash
SHORT TERM...BL
LONG TERM...Nash
AVIATION...BL/Nash
MARINE...BL/Nash