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Area forecast discussion 
National Weather Service Gray ME
939 am EDT Tuesday Jul 28 2015

warm and humid air will remain over Maine and New Hampshire for
the next few days. Showers and thunderstorms will be possible
mainly during the afternoon and evening hours today... with some
severe thunderstorms possible. A cold front will cross the region
on Thursday with more thunderstorms possible. A weak disturbance
may bring a few showers to the mountains Friday. Upper level
trough remains in place through early next week bringing periodic
frontal passages and unsettled weather.


Near term /until 8 PM this evening/...

928 am...minor estf update to reflect latest satellite trend and
to ingest current mesonet in first period grids. Shortwave along
the Ontario-Quebec border on GOES water vapor imagery...and
associated convective cluster...was racing southeast and should
be vicinity of our international area near 23z. This feature could
provide an extra kick to evening convection across northern
sections of the forecast area. Otherwise...modified 12z kgyx radiosonde observation
shows convective available potential energy at or above 2000 j/kg which agrees well with forecast
soundings from NAM/GFS for later today...with strong winds
primary concern along with marginally severe hail.

Previous disc...
6am update...
line of showers and storms has made it to the coastline a little
bit earlier than forecast. Have updated precipitation chances to
reflect this. Still a chance of a shower or isolated storm over
southern New Hampshire for the next few hours. Then eyes turn
toward afternoon thunderstorms.

Original discussion...
showers and thunderstorms are moving southeast across the
forecast area early this morning. This is in association with a
sharp shortwave trough rotating around an upper low over eastern
Quebec. Given a moist low level environment and cooler/drier air
aloft... elevated instability is being tapped. These showers and
storms should move off the coast this morning as the shortwave
trough exits.

Behind this trough will be a substantial area of subsidence which
will act to suppress convective activity today. Increasingly warm
and moist low level environment will lead to a good amount of
instability this afternoon on the order of 2000 or more cape. This
is quite strong by northern New England standards. In part this
large cape value is a result of the subsidence behind the trough
which causes drying in the middle levels in an elevated mixed layer.
The result is a capping inversion which helps suppress convective
activity... and allows the area to bake in the heat longer before
afternoon convection is able to stall heating. This high amount of
instability could well go untapped this afternoon in the absence
of a trigger mechanism to focus convection and initiate lifting.

In the search for a trigger we find a surface front / wind shift
line which will have moved south into the forecast area by this
afternoon. It should reach the foothills region of Maine perhaps
near Augusta before it collides with the afternoon sea breeze.
The surface convergence generated by these two boundaries... and
their possible collision... could serve as the triggering
mechanism needed to get thunderstorms going this afternoon.
Another potential source of lift would be heating over the higher
terrain which leads to more focused rising motion over the peaks.
Of these two sources... the colliding boundaries seem to be the
most likely to help overcome the cap and overall subsidence in
this northwest upper level flow regime and lead to thunderstorm

Overall precipitation chances are a bit lower due to the overall
subsidence and are now generally in the chance range over the
coastal plain. However... storms that do form will have the
potential to be strong to severe. Overall wind shear might be just
under what is needed for classic supercells... however it may be
enough to support multicell clusters or supercells within locally
enhanced regions of shear. The total instability available also
suggests hail to be a possibility... and the well mixed low levels
which will be needed to overcome the cap should be supportive of
strong downburst winds. Heavy rain may also be a threat in the
moist environment although storm motion will be fairly brisk
except for potential areas of back building along the colliding

Temperatures will be quite warm today with highs in the 80s and
90s and heat index values rising into the middle 90s over southern
New Hampshire.


Short term /8 PM this evening through 6 PM Wednesday/...
convective activity should begin to wane in the early evening
hours with the loss of heating. However... elevated instability
still exists so any storms which already exist could maintain
themselves into the night. Expect low temperatures in the 60s.
Marine fog could make a return to the Maine coastline. The frontal
boundary that had moved south during the day will likely get
stalled by the collision with the sea breeze front.

Stalled frontal boundary over the coastal plain of Maine and
extending toward the mountains of eastern New Hampshire could
serve as the focus for a few thunderstorms again on Wednesday...
however overall storm chances are low. There should be a better
wind shear environment for storms on Wednesday so if storms do
form there will be the potential for supercells. But the
confidence in storm formation is fairly low as warm air continues
to move in aloft... making it a little more difficult to break the
cap. The warmer air aloft and the continued subsidence in
northwest flow should allow strong heating to occur. Just about
everyone will exceed 80 degrees... even typically cool Rockland.
Portland may flirt with 90 degrees before a sea breeze cuts
heating short. Inland areas and most of New Hampshire should be in
the 90s with heat index values reaching the middle to upper 90s...
but falling short of heat advisory criteria (100 degrees).


Long term /Wednesday night through Monday/...
upper level ridging and warmer 850mb temperatures will be on
their way out Thursday as a cold front approaches from the west.
We should be able to reach the middle 80s to lower 90s ahead of the
boundary before precipitation cools things down. Storms will
benefit from warm muggy air but low level jet will be stronger in
Canada and only on the order of 15 kts here. We could still see
some marginally severe storms with impressive instability over
southern sections especially.

Friday will be just a couple degrees lower than Thursday with
cooler and drier air moving in behind the front. A weak
disturbance in the cyclonic flow aloft keeps partly cloudy skies
and showers over the Carrabassett Valley and to the NE. A weak
cold front crosses the region Saturday with its parent surface low
well removed to the north. This should bring scattered showers and
thunderstorms to mainly the mountains of New Hampshire and Maine.

Another seemingly unanchored area of precipitation arrives for
early next week. These precipitation events tend to be light to
moderate showers and any heavy rainfall will be associated with
stationary or slow moving boundaries (sea breezes... outflows...
etc) that will be hard to define ahead of time. Thus it will be
hard to pinpoint who will get rain and who wont on any given
day... such is the nature of summertime convection.


Aviation /14z Tuesday through Saturday/...
short term...should see VFR conditions return area wide today.
Afternoon and evening thunderstorms are possible mainly over the
coastal plain of Maine and possibly into New Hampshire. Could see
the return of marine fog and clouds tonight to the Maine coast and
valley fog in the New Hampshire valleys.

Long term...prevailing VFR conditions except for Thursday
afternoon to Friday morning MVFR or lower in showers and
thunderstorms as a front moves from west to east. On
Saturday... scattered MVFR in scattered showers and isolated
thunderstorms mainly for khie and kleb.


short term...weak pressure gradient keeps wind flow light and seas
fairly calm.

Long term...winds and seas remain below criteria but may brush
near 5 feet in the far outer waters early Friday. There will be
several wind shifts over the waters this week even though speeds
should be at or below 12 kts.


Gyx watches/warnings/advisories...
New Hampshire...none.



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