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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KGYX 072315
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
715 PM EDT Tue Apr 7 2020

.SYNOPSIS...
A weak area of low pressure will pass south of the region
tonight with a some light rain possible over southern New
Hampshire towards daybreak. High pressure returns on Wednesday
before the weather turns significantly more active on Thursday
as low pressure develops over the Gulf of Maine with rain and
mountain snow expected. High pressure returns on Friday into
Saturday with temperatures a bit below normal for this time of
year.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
710 PM...Weak sfc low passing well S of the CWA may back some
light SHRA NE into SW NH, and maybe along NH/MA border to the
coast late tonight. Forcing is weak on the NE edge, and airmass
is dry, so may not much at all. Air mass is dry enough to see
maybe some snow/sleet mix in at the start, but this is unlikely
outside of the higher terrain. Rad cooling N outside of srn NH
and SW ME should be strong thru midnight or later so temps drop
off quickly, with lows in the low to mid 20s in the mtns, and
25-30 inland areas away from the coast. In srn NH the clouds
limit the lows to the mid to upper 30s.


Previously...
High Impact Weather Potential: None.

Pattern: Pattern across Canada early this afternoon shows an omega
block breaking down as western trough member slips under a cutoff
high over northern Hudson Bay which will bring a growing northern
stream influence over the flow across across much of the
northeastern half of the lower 48 as we move through the remainder
of the week. At the moment northern New England finds itself under
dry northwesterly flow with a frontal boundary separating this
region from significantly more moisture over much of the Mid-
Atlantic and Ohio river valleys.  A wave of low pressure along this
boundary will work south and east through tonight...with clouds and
precipitation chances over our southern areas being the primary
forecast concerns tonight.

Through this evening: Little in the way of sensible weather concerns
through the evening with high clouds beginning to arrive over
southwestern areas. 8pm temperatures will rapidly be falling
into the 30s in our cooler mountain valleys...but remain in the
mid 40s to around 50 for the coast and foothills.

Tonight: Weak shortwave embedded in the northwest flow aloft moves
south and east into the region along with an associated 80kt H5 jet
streak with southern New England in it/s right entrance region.
Surface low coaxes a modest 20-30kt southwesterly LLJ at H8
advecting 1" PWATs north and east to a position south of New England
before stalling.  Airmass well north of the boundary will remain
very dry with PWATs below 0.25".  There is a pretty solid guidance
consensus that precipitation associated with the surface low will
reach into far southern NH after midnight through daybreak.
HRRR/NAM/GFS are all agree with wet bulb zero heights 2500-3000
feet...suggesting that outside of a few possible wet snowflakes over
the hilltops...precipitation should take the form of rain.  Very
sharp cutoff /likely no precip south of a LEB-SFM line/ with no more
than partly cloudy skies should allow lows into the 20s and
30s...with upper 30s likely under the cloud cover and precipitation
over southern NH.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
High Impact Weather Potential: None.

Pattern: Weak shortwave ridge axis builds over the region as high
pressure noses in from the north. This will allow dry air to make
another push with PWATs dropping to near -1 sigma. The arriving
shortwave ridge axis will slow as it moves overhead with the pattern
amplifying ahead of a mobile shortwave trough carving out over the
Great Lakes region Wednesday night.  The result will be one last
quiet weather day with not significant impacts expected.

Wednesday: Low pressure quickly passes south and east of the region
with dry air returning to the entire area under nearly neutral
temperature advection as a surface high noses in from the north.
This dry push should lead to another mostly sunny day across the
region. With onshore flow developing in the afternoon...expect
coastal areas to be cooler than today...likely reaching no
higher than 50. 40s will also be common in the mountains...with
highs in between over southern NH and into the foothills of
western Maine reaching into the 50s again given similar
temperatures aloft. The somewhat cooler temperatures and
increased llevel dewpoints compared to 24 hours previous coupled
with light winds should allow for somewhat diminished fire
weather concerns although afternoon RHs could still fall to near
30 percent in western Maine away from the coast.

Wednesday Night: Flow throughout the column backs in response to
shortwave digging across the Great Lakes.  Primary forecast concerns
center around precipitation potential as dynamic forcing for ascent
increases atop strengthening southeasterly boundary layer flow. With
dry wedge in place ahead of developing warm advection pattern...tend
to believe that the 07.12Z GFS is overdone in it/s extent of upslope
precipitation...with all high res guidance maintaining a more
substantial wedge of dry air in the 5-10kft region. Tend to like the
HREF placement...showing some light rain/snow showers along the
southeastern slopes of the Whites towards daybreak Thursday.
So...expect increasing clouds with some potential for a rain or snow
shower along the southeastern slopes of the terrain with lows in the
20s in the mountains and 30s elsewhere.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
*** Rapidly intensifying coastal low will bring rain, snow, and
 coastal flooding Thursday into Friday***

The focus of the long term forecast will be on a rapidly
intensifying coastal storm impacting the region on Thursday.This
system can be traced back to the merging of two different
waves, one from the southern stream over the four corners and one
from the northern stream over British Columbia. These two jet
streaks combine over the southern Ohio valley/Appalachians with the
resulting 160kt jet streak triggering  rapidly intensifying cyclone
in the left exit region over southern New England.

Confidence continues to grow on both the timing track of the storm
with ensembles showing a tight cluster of low centers just offshore
in the Gulf of Maine by 00Z Friday. Thus have fairly high confidence
in the timing of the precipitation. What remains a tougher call is
the precipitation type. Along the immediate coastline rain will
dominant as the warm air aloft wraps into the center of the low
and surface temperatures remain above freezing. As we move
inland and up in elevation snow becomes increasingly likely with
elevations above 2000ft seeing all snow. The transition zone
remains highly uncertain and foothill locations may go back and
forth between rain and snow multiple times. With precip moving
in in the middle of the day expect an initial push of rain for
all but the highest summits. As the sunsets the cooling
temperatures will combine with the intensifying storm to push
colder air down into the region from the northeast resulting in
a transition to snow with accumulations possible as far south
as Fryeburg to Lewiston to Augusta in Maine. Further west into
New Hampshire, most of the state will hold as rain as the source
of cold air over New Brunswick and eastern Quebec isn`t readily
available to push south of the Whites. With the rapid
intensification and strong upper level support expect to see
some banding locally increasing snowfall as the dynamics within
the band help to further the cooling and help to switch over to
snow. Overall the area of highest threat for intense snow aligns
well with the current Winter Storm Watch and have opted not to
change headlines at this time.

Precipitation is not the only threat with this storm, stronger
wind gusts are also a concern. Forecast soundings show the best
mixing on the backside of the system where northwesterly cold
advection sets up and especially through the foothills and Lakes
Region of NH where downsloping may enhance the flow. Gusts to
40mph are possible and a wind advisory may be needed for a
portion of the area. Along the coast the extent of the strong
winds will depend more on the low track. The greatest wind
impacts will be if the low is able to stay offshore keeping the
gusts into the coast. If the low tracks along the coast, the
calmer conditions at the center will limit gusts. While a very
robust low level jet looks likely ahead of the system, feel
there will be enough of a surface inversion, albeit weak, to
prevent this from mixing down and thus have concentrated the
stronger winds on the backside of the system for Thursday night.

The storm will move out into the Maritimes on Friday setting us
up for northwesterly flow with some isolated showers in the
mountains for Saturday. This flow will continue through the
weekend before another coastal low looks to move up the eastern
seaboard for the start of next week.

&&

.AVIATION /23Z TUESDAY THROUGH SUNDAY/...
Short Term...

Summary: High pressure overhead will give way to a weak area of
low pressure moving south of the region tonight before high
pressure noses back into the region from the north on Wednesday.


Restrictions: VFR conditions will dominate the period from now
through Wednesday night.  There is the potential for a period of
MVFR CIGS in -SHRA from MHT to PSM in the 08-12Z period Wednesday
morning.

Winds:  Northwest winds 10g16kts /with local seabreezes developing/
will diminish to calm/light-variable overnight before starting the
day from the northwest <10kts Wednesday before shifting to the east
and southeast Wednesday afternoon /5-10kts/.  Winds will go calm ore
remain light easterly overnight Wednesday night.

LLWS: No LLWS is expected through Wednesday night.

Long Term... A coastal low will bring precipitation into the
region starting midday on Thursday, expect conditions to rapidly
deteriorate to IFR with rain for all but the highest elevations.
Thursday night rain will transition over to snow moving
southwards into the foothills. LLWS will be a concern for the
midcoast through Thursday and thursday night with a LLJ of up to
60kts expected to remain above the boundary layer. On the
backside of the storm strong northwesterly winds will mix down
to 35kts in the wake of the storm.

Conditions will subside for the weekend with just isolated
showers and MVFR cloud cover remaining in the mountains.

&&

.MARINE...
Short Term...

Quiet on the waters through Wednesday night with onshore winds this
afternoon shifting northeast tonight and strengthening through
Wednesday before shifting southeast Wednesday night.

Long Term... A coastal low will develop near Nantucket and
rapidly intensify as it moves through the Gulf of Maine on
Thursday night. Strong flow around the center of the low will
likely push winds to Gale with a few gusts to storms possible
offshore in the Gulf of Maine. With the low potentially tracking
through the middle of the forecast area, the potential exists
for Gale force conditions both ahead of and behind the system
and so have opted to issue a Gale watch for the 24hr period
encompassing both these scenarios, although there may be a lull
in the middle.

&&

.HYDROLOGY...
A period of heavy rain is likely along the Maine coast Thursday
evening...transitioning to snow over the interior. Given snow in
the mountains and a mix into the foothills...do not expect
significant river flood issues. May have some short-fused issues
along the coast esp with water levels rising towards the ~1AM
Friday high tide which may prevent efficient surface water
drainage along the coast.

&&

.TIDES/COASTAL FLOODING...
Have issued an extended coastal flood advisory for high tides
beginning tonight and continuing through early Friday.
Astronomical tides are near minor flood levels without wind
impacts already...and with surge levels expected to increase
ahead of Thursday/s storm...minor impacts are likely. There is
some potential that the Thursday night high tide could see
moderate /warning level/ impacts depending on exact
timing/location of storm development and how long winds remain
onshore. Still time to examine this tide cycle more closely.

&&

.EQUIPMENT...
The last GYX upper air observation was March 25 at 12Z.
Unfortunately, a disruption in gas supply has temporarily halted
observations from GYX. The order has been placed but a date of
delivery remains TBD.

The Sugarloaf NWR transmitter remains off the air with an
unknown restoration time.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Coastal Flood Advisory until 3 AM EDT Friday for MEZ023>028.
     Winter Storm Watch from Thursday morning through Friday
     afternoon for MEZ007>009-012>014.
NH...Coastal Flood Advisory until 3 AM EDT Friday for NHZ014.
MARINE...Gale Watch from Thursday afternoon through Friday afternoon
     for ANZ150>154.

&&

$$
SYNOPSIS...Arnott
NEAR TERM...Arnott/Cempa
SHORT TERM...Arnott
LONG TERM...Curtis
AVIATION...Arnott/Curtis
MARINE...Arnott/Curtis
HYDROLOGY...
TIDES/COASTAL FLOODING...
EQUIPMENT...