C&P from NWS Scientific forecaster

From the NWS Scientific Forecaster this afternoon:

Area forecast discussion. updated

National Weather Service Gray ME 12:33 PM EST Thu Dec 22 2016

Synopsis...

an Alberta clipper will swing through the region today bringing

light snow accumulations. Behind this system, high pressure will

bring dry and seasonably cool temperatures for Friday. Another

fast moving weather system will bring a potential round of snow or

rain on Saturday. Quick clearing will be in store Saturday night,

with high pressure building in for Christmas, providing mainly

sunny skies and temperatures near normal. Another low will move

to our west early next week and bring another round or snow,

especially Monday night.

&&

Near term /until 6 PM this evening/...

1031 am update: primary changes this hour are to speed up pop

evolution through the early afternoon as back edge of

precipitation races quickly through southern Vermont/western Massachusetts. Only

another 1-3 hours to go /west to east/ south of the mountains

given extrapolation. Boosted near term temperatures as well...but

nearly all meaningful precipitation will fall as snow.

8:43 am update: going forecast in good shape...with only

substantial adjustment being to give a period of lower pops over

southern New Hampshire before upstream precipitation band now over eastern

New York marches through the area bringing another round of light

snow. Otherwise...have seen snowfall amounts around 1 inch with

the initial warm advection Wing...which should line US up well

with the inherited 1-3" forecast.

6:45am update...

leading edge of snow has moved through southern New Hampshire and

into southwest Maine. Observations have shown visibility reduced

to as little as 1/2 mile as this moves through, though the

heaviest band does not last too long. To the south of this band,

lighter snow will continue to fill in with the back edge of the

snow arriving in New Hampshire shortly after noon. So far this has

been a fairly well-behaved system and earlier adjustments to

hourly precipitation chances seem to be panning out well. Made

minor adjustments to hourly temperatures based on latest

observations this morning. Otherwise there were no significant

changes made.

Original discussion...

an Alberta clipper style system will track through the area today

spreading light snow across most of the area. Regional radar

already shows this snow as it has already moved through the

eastern Great Lakes and into New York. Leading edge of this snow

has developed a linear structure to it associated with the

leading edge of the warm advection aloft and will begin moving

into New Hampshire within the hour. Expect the radar to begin to

fill in behind this as broad synoptic scale lift continues to

generate light snow until the axis of the shortwave trough aloft

passes through the region this afternoon.

Since this is a fast-moving system without a deep moisture

connection, total snowfall amounts are expected to be light. Only

about 1 to 3 inches is expected, with the highest amounts likely

occurring in a band stretching across the central portion of the

forecast area, from central New Hampshire into southwest Maine.

Temperatures aloft are all well below freezing, so temperatures at

ground level will dictate precipitation type today. As snow begins

to arrive, expect temperatures to fall a few degrees and stay near

or below freezing until the precipitation ends this afternoon. As

the system moves east and the trough axis moves by, winds will

shift to the northwest and this is when temperatures will reach

their highs of the day late this afternoon for most areas. By

then, precipitation will be ending, so by in large this should be

an all snow event. The only exception could be along the immediate

coastline particularly near the Rockland area where a southwest

low level wind will be better able to drag in warmer air from the

ocean.

The hourly temperature forecast today shows a non-typical trend

with temperatures holding steady during the morning during the

snowfall before increasing this afternoon as winds become

northwesterly. Hourly precipitation chances reflected the broad

conceptual idea of a warm-frontal snowfall band lifting into the

area from the southwest followed by the passage of the low

pressure center/cold front this afternoon. Precipitation chances

are near 100 percent though amounts are expected to be light.

&&

Short term /6 PM this evening through 6 PM Friday/...

surface low pressure strengthens as it tracks into the Gulf of

Maine and then out to sea tonight. A northwest flow behind the low

will keep clouds and a chance of snow in the mountains through the

evening and decreasing Friday morning. Low temperatures will be in

the 20s due to a relatively moist air mass remaining in place and

winds that do not really go calm due to the strengthening and

departing low.

High pressure to our south will move east toward the Atlantic

coast on Friday with a westerly flow over New England. This will

bring in warmer temperatures from the center of the country, with

highs in the upper 30s to mid 40s and a mostly sunny sky. Much of

the freshly fallen snow may melt/compact with the sun and warm

temperatures expected on Friday, though the sun angle is at its

yearly minimum right now.

&&

Long term /Friday night through Wednesday/...

chance of high impact weather: low.

At 500 mb a weakly amplified zonal flow across North America will

provide changeable conditions through the extended forecast with

the treat of precipitation every 36 hours or so, although all the

systems look fast moving, and should not produce any significant

precipitation. The zonal flow will keep the cold air inside the

Arctic Circle through the period, and temperatures will generally

near to above normal through the period.

Friday will see surface high exiting to the east and weak 500 mb

waves, one in southern stream and one in the north approaching

from the west. Friday night should stay dry with increasing

clouds after midnight. Lows, which will likely occur around

midnight, will range from around 20 in the north to the mid-upper

20s in the south.

Saturday will see the arrival of precipitation from those two

waves early in the day. Models are in fair agreement that those

two waves do not phase until they are just to our east, and best

precipitation will pass to the south and east as well. While a

decent 850 mb jet does develop, mid level temperature gradient is

not so great, at least not this far N, and this will also help

shunt greatest quantitative precipitation forecast to the S and east. Precip will likely begin as

snow everywhere during the first part of the morning, but should

go over to rain on the coast and in southern New Hampshire fairly quickly

with little, if any accumulations expected. As you move further

inland will see some light accumulations on the inland coastal

plain, but should be less than an inch before changeover to rain

occurs. An inch or two is possible in the mountains, but even some

of the mountains may see some mixing with rain by Saturday

afternoon. Highs will be above normal, from the low 30s in the north

to around 40 in the south. Precipitation will wind down during

the latter part of Saturday afternoon, and all precip should be

done around sunset or shortly thereafter /other than some upslope

shsn in the mountains Saturday evening. Skies clear out

overnight, and it looks mild with lows dropping into the 25-30

range.

Christmas will feature a lot of sunshine and highs in the upper

20s to near 40. Sunday night stays dry and with some colder air

in place will see lows closer to normal, generally 5-10 above in

the N, and in the mid to upper teens in the south.

The next system will begin to impact the area on Monday with snow

or rain showers moving in during the afternoon. The best chance

for precipitation will be Monday night, and will likely see a

period snow in many areas, except perhaps at the ME coast and in

seacoast NH, which could stay mostly rain. The system moves out on

Tuesday, and we head into another dry break during the middle of

the week, but turning colder on Wednesday. The next chance of

precipitation will be Thursday.

&&

(And, just to be different with this post)

Aviation /18z Thursday through Tuesday/...

short term...

Summary: low pressure will exit east of the region through tonight

with improving flying conditions expected as high pressure builds

into the region from the west. This high will bring good flying

conditions as it moves south of the region through Friday night.

Restrictions: MVFR/IFR restrictions ongoing in light snow /has

changed to rain at rkd/. Expect improvement to VFR at all sites

except hie this afternoon....with residual MVFR clouds and some

flurries/shsn at hie that will come to an end overnight. VFR all

terminals Friday-Friday night.

Winds: light southerly/southwesterly winds will become northwest

tonight and increase to around 10kts before shifting more westerly

for the day on Friday 5-10kts.

Llws: not expected through Friday night.