Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 171521

Area Forecast Discussion
National Weather Service Gray ME
1121 AM EDT Wed Jul 17 2019

A warm humid airmass develops today and tonight, with a cold
front moving slowly south across the area. The front will
produce widespread showers and thunderstorms along with heavy
rainfall and the potential for localized flooding. High pressure
and drier air will briefly build in from the north for
Thursday. The heat and humidity along with unsettled conditions
will return for Friday and the upcoming weekend.


1100 Update...
First batch of rain showers ahead of an approaching cold front
is currently departing eastern zones late this morning. Area of
showers and some thunderstorms across upstate New York will be
pushing in from the west through early afternoon. PWAT`s
approaching 2 inches will likely result in heavy downpours this
afternoon in showers and thunderstorms. Best chance of
convection will be southern New Hampshire and the mountains of
Maine and New Hampshire. Have updated pops/wx grids based on
latest radar trends. Have also tweaked area temps/tds but no
major changes to current forecast planned.

Prev Disc...
6am update.. Rain has just crossed the CT river valley and have
adjusted PoP to reflect this timing. A few stronger convective
showers but mostly light rain within this first batch.

Prev disc... A very humid airmass will move into place across
the region making for a wet day with scattered showers and
thunderstorms. Currently dewpoints are in the mid 60s across the
region and will rise to the low 70s by afternoon. PWAT will
climb to near 2 inches. Water vapor imagery shows and short wave
approaching the region. This impulse is associated with shower
activity currently in New York which will move into Western NH
this morning. These storms will produce small areas of heavy
rain, but should not be widespread enough to cause flooding.

 As we  move into the afternoon a cold front drops down
through the region from Canada. This front will initiate another
round of showers along the Quebec border. The placement of these
showers looks to be mainly north of the morning rainfall, keeping
the flooding threat to a minimum.

Meanwhile a warm front will advance into Massachusetts.  This leaves
our area sandwiched between two forcing mechanisms in a very moist
airmass. While the morning showers are not likely to produce
flooding, localized flash flooding becomes more likely through
afternoon as we see a second round of precipitation. The MA/NH border
will be the focus of the heaviest rainfall, and Flash Flood guidance
in this area is around 2-3 inches, with lower values (closer to 1")
in the Nashua-Manchester corridor. This southern portion of the CWA
is thus the area of greatest concern. Have opted not to issue a
Flash Flood watch for two reasons. First, while rainrates will be
high, the storms themselves will be small and the area impacted will
also be small. Secondly the threat is mainly determined by where the
front sets up across Massachusetts. It is possible this round of
showers may stay completely south of the forecast area.


Tonight the humidity will linger keeping overnight lows in the upper
60s to upper 50s across the region. For Thursday, the frontal
boundary will remain stalled to our south and again showers will
skirt along the MA/NH border. Again, the humid airmass will be
in place making heavy rain possible however currently the rain
looks to stay south of us into Mass. Thursday will be the
coolest day of the week as widespread cloud cover keeps
temperatures confined to the upper 70s.


Oppressive and Dangerous Heat Likely for the Weekend...

The remnants of T.S. Barry will be passing by to our south in
westerly flow Thursday night into Friday morning. This will
bring cloud cover and showers to mainly southern NH and coastal
Maine through the day, before moving into the Atlantic Ocean late
Thursday night.

Dew points jump into the mid 60s to lower 70s by Friday
afternoon with southwest flow bringing subtropical moisture
northward. This combined with 850mb temperatures in the 18-22
degree C range will make for an extremely warm weekend. A sea
breeze Friday will help knock down temperatures in the afternoon
along the coast, but expect highs in the mid to upper 80s over
most of Maine and NH. Southern NH will see some lower 90s. All
this will result in heat indices in the mid 90s for portions of
southern NH and SW ME...likely resulting in a heat advisory.

On Saturday, temperatures will really get cooking ahead of a
southward moving cold front, with widespread mid to upper 90s.
Once again locations in southern NH, the Connecticut River
Valley, and southwest Maine will be the warmest, with the
highest chances of hitting the century mark. Dew points will
also be a couple degrees higher. If this scenario holds true we
may end up issuing a heat watch/warning as heat indices cross
the criteria of 105F for 2 hours. This would be a very dangerous
situation for the elderly without air conditioning and people
working outside.

Although there will be plenty of instability on Saturday, there
will be warmer air aloft which will cap it, at least
initially. Unidirectional shear vectors and an increasing LLJ
may just be enough where they intersect better CAPE for a strong
storm or two capable of producing gusty winds. This would be
primarily along the front, from the Connecticut River Valley
into portions of southwest NH where CAPE will be maximized.
Otherwise very high PWATs and deep warm cloud depths point
towards torrential rain and the possibility of flash flooding as
the main threat.

A secondary cold front will follow quickly Sunday into Monday.
There may be a small area near the Manchester/Nashua corridor
that could reach heat advisory Sunday. There will be a chance
of thunderstorms ahead of the second frontal passage on Sunday,
but there will be less instability and less shear available.
Heavy rain will remain a threat. Temperatures and moisture will
be greatly reduced on Monday with the passage of the second
front, with cloud cover and antecedent convection only leading
to weak showers. Building high pressure will bring a decreased
chance of showers Tuesday and Wednesday, with temperatures


Short Term /through Wednesday Night/...Showers and thunderstorms will affect all terminals
through the day today. Clouds will move in from the west this
morning but ceilings will remain largely VFR, with brief
reductions to MVFR during the showers. Thunderstorms are
expected in two regions this afternoon - along the Canadian
border and along the MA border

Long Term...VFR Friday with some MVFR ceilings on Saturday and
Sunday/Monday morning as a couple of fronts move through the
region, bringing heavy rain and some thunderstorms.


Short Term /Through Wednesday Night/...Showers and thunderstorms
will cross the waters today as a front sags south through the
gulf of Maine. The front will linger to the south of Caches
ledge through Thursday.

Long Term...No problems noted.


The Concord, NH ASOS remains out of service. At this time a
return to service date is unknown. During the outage, TAFs will
continue to be issued for Concord without amendments scheduled.
Climate data for Concord will also be affected, although backup
sources may be used to fill in data after the fact.

The Sugarloaf NWR transmitter is off the air until further
notice. This will be an extended outage as the tower, which was
severely damaged in a winter storm, is rebuilt.




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