Scientific forecaster C&P

Direct from the NWS:

Area forecast discussion

National Weather Service Gray ME 236 PM EST Wed Dec 21 2016

Synopsis...

an Alberta clipper will swing through the region on Thursday

bringing light snow accumulations throughout the region. Behind

this system...high pressure will bring dry and seasonable

conditions to end the week. Another fast moving weather system

will bring a potential round of snow or mixed precipitation on

Saturday with seasonably mild temperatures and dry weather

expected for Christmas and into Monday of next week.

&&

Near term /through tonight/...

High impact weather potential: light snow develops towards

daybreak.

Current pattern: early afternoon water vapor imagery shows broad

troughing over the eastern half of noam....with one very moisture

starved shortwave over New England as of this writing...with a more

impressive...yet still moisture-starved shortwave upstream moving

into the Great Lakes region. Between these features is a region of

shortwave ridging...with an attended high pressure ridge axis at the

surface. This feature will control our weather through much of the

overnight before the flow backs ahead of the upstream shortwave and

top-down saturation commences...with some snow likely to develop

over western locations by the very end of the near term forecast

period.

Through this evening: initial shortwave is now passing east of our

longitude...and had been responsible for a build up of clouds

upstream of the mountains of New Hampshire and western Maine...with

a few snow showers...but little else. I don't expect much in the

way of changes through this evening...with clear skies south and

east of the mountains allowing temperatures to start dropping

rapidly from their mild afternoon highs in the 30s to around 40 back

into the 20s to around 30 by 7pm. Afternoon westerly winds will

diminish markedly by sunset.

Tonight: height rise center associated with shortwave ridge axis

moves though the region for the first half of the night before warm

advection and associated top-down saturation develops.

Thus...expect temperatures to drop quickly in the evening /with some

bust potential on just how low they go...but some teens possible/

before leveling off and likely rising towards daybreak. Antecedent

airmass is dry...but not terribly so...so have no reason to stray

much from model consensus arrival time of precipitation of 4-6am over

New Hampshire...moving after the daybreak for all but extreme western Maine.

Temps certainly cold enough for snow all locations through

daybreak. Less than one inch of snow by daybreak over New Hampshire is

expected.

&&

Short term /Thursday through Thursday night/...

High impact weather potential: period of light snow Thursday.

Pattern: Great Lakes shortwave sharpens somewhat as it moves

overhead during the day Thursday...with a very weak surface

reflection deepening once qg forcing for ascent reaches the waters

of the Gulf of Maine. While this coastal low will strengthen

rapidly...it will be too little too late for much of a significant

impact over our forecast area. However...a period of warm advection

Thursday morning in the presence of -divq will yield a period of

light precipitation for much of the day. Shortwave ridging and

associated surface high builds in Thursday night...with quiet

weather to end the short term forecast period.

Thursday: upstream clipper will be located near Georgian Bay

Thursday morning before moving into northern New England by

afternoon with a new low getting it/S act together towards evening

in the Gulf of Maine. Upper forcing for ascent is modest...with

-DPVA/height falls associated with positively tilted

shortwave...but poorly placed jet and limited moisture paints a

picture of generally light precipitation due primarily to

fgen/isentropic upglide.

Quantitative precipitation forecast/snow: given very meager pwats to work with /generally less than

one third of an inch/...and fast moving nature of clipper...this will

not be a prolific precipitation producer. Model consensus favors

about 0.10-0.15" liquid...which seems very reasonable...with about

6 hours of precipitation expected for any location.

Vertical profiles do suggest decent snow:liquid ratio potential...

with vertical motion centered within and just above the dendritic

growth zone. Cobb algorithm suggests ratios near 14-15:1 away from

the cost.

Ptype: as indicated by previous shift...likely that nearly all

precipitation that falls will be snow. Wet bulbs near 34-35f along

the coast by late morning suggest that some rain may mix in...as

precipitation comes to an end...limiting coastal accums a tad.

A look at sref and ncar ensemble plumes yields a few members up to

3" over inland areas and see this as the upper limit for snow.

Timing will be problematic for the commute...with all of New Hampshire and some

of western Maine likely seeing snow begin by 7am. Will continue

mention in the hazardous weather outlook and could foresee a Special Weather Statement down

the Road...but no headlines warranted given these amounts.

Thursday night: high pressure builds into the region with building

heights aloft arriving after midnight. Cold advection behind

Thursday/S low will be rather meager...with a seasonably mild

overnight expected. Good gradient through much if not all of the

night with 1000 mb geostrophic winds remaining 15-25kts...which

should keep all but the more sheltered valleys coupled.

Thus...expect a few spots to drop into the upper teens...with 20s

for most locations. If the wind dies off a bit sooner than

expected...more widespread teens are likely.

&&

Long term /Friday through Wednesday/...

models are in decent agreement on the longwave pattern through

the end of the calendar year. We begin the period with a fast low

amplitude flow. However...digging shortwave energy entering the

Pacific northwest will quickly carve out a deep trough over the

western third of the Continental U.S. For the start of the upcoming weekend.

Downstream we'll initially see rising heights and a warming trend

and rather quiet weather for the Holiday weekend. By early next

week...we've transitioned to a high amplitude progressive pattern

that portends a stormy end to 2016. In the dailies...high pressure

builds by to our south Friday and Friday night with quiet weather

and seasonable temperatures. On Saturday...a passing shortwave

will drive an associated cold front across the area with a few

snow and rain showers with p-type driven by boundary layer

temperatures. Rising heights take over with a ridge of high

pressure traversing the area for Sunday into early Monday. A

potent shortwave and associated surface low will track from the

northern plains and western Great Lakes on Monday into eastern

Canada Tuesday. The warm front from this system will bring some

light overrunning precipitation to the area late Monday and Monday

night followed by a cold front early Tuesday. In the wake of this

system...gusty winds along with some upslope snow showers across

the higher terrain for Tuesday and Tuesday night. Rising heights

and a ridge of high pressure arrive by midweek. A deep trough and

associated storm system arrive for the tail end of the work week

with a mixed precipitation event for the region followed by

sharply colder temperatures and a few mountain snow showers to

ring in the New Year.

&&