Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 162202 AAA

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
602 PM EDT Mon Jul 16 2018

Very warm and increasingly humid conditions are expected to
continue into Tuesday across New Hampshire and western Maine.
The large bank of fog and low clouds over the Gulf of Maine is
forecast to move back inland tonight. Areas of dense fog are
likely along the coast...and may extend well inland by morning.
A cold front will approach through the day...with showers and
thunderstorms expected along and ahead of it. Some early day
rain and embedded thunder will give way to partial sun and allow
for another round of afternoon storms. Some of these may be
severe...with damaging winds the main threat along with
torrential rainfall. Showers and storms push off the coast
Tuesday evening...with cool and drier air moving in behind it.


650 PM Update...
Have added a few more counties to the dense fog advisory:
interior Rockingham, Androscoggin, Kennebec, and interior Waldo.
These counties had fog or dense fog last night and moisture will
be increasing throughout the column through the morning hours
ahead of the approaching cold front. Fog back has already moved
onshore via webcams and satellite imagery. Otherwise added some
drizzle along the coast where webcams are showing some droplets.
Clouds and temperatures were changed to better match the
expected conditions overnight.

Previous discussion...
A humid airmass continues to move into the region overnight
resulting in fairly widespread fog. Satellite shows a stratus deck
just offshore which will move inland after sunset with the coastal
plain rapidly dropping visibilities. Coastal cameras indicate low
visibility already present along the immediate coastline. A Dense
Fog advisory has been issued for the coast as well as portions of
interior southwestern Maine. Further east through Androscoggin and
Kennebec county it is less confident that the marine layer will make
it all the way inland again as weak westerly flow begins overnight
and this area will need to be monitored. Similarly across southern
New Hampshire the marine layer is currently suppressing
convection in portions of eastern Rockingham county but there is
less confidence of the fog extending eastwards towards
Manchester and Nashua. This area will also need to be monitored
for an expansion of the dense fog advisory.


Tuesday will be a classic severe weather set up. The morning starts
off with fog and low stratus along the coast. This fog will
dissipate by mid morning leaving a humid airmass primed for
convection. A cold front approaching from the west will be the
trigger needed to get storms going. The front will arrive into our
far northwestern zones in the early morning crossing the entire
area from west to east reaching the coastline around 22Z. With
CAPE values of 1500 J/kg along with shear of 30+ kts expect an
organized convective line with severe wind gusts as the main
threat. Ahead of the line the humid airmass and low level
southerly and onshore flow will provide some low level turning
of the winds. If any storms are able to initiate ahead of the
main line there will be a threat for tornadoes within these

In addition to the severe threat, flash flooding will be a concern.
Dewpoints in the upper 60s to near 70s with PWAT values approaching
2" will provide plenty of moisture for storms. Coupled with a deep
warm rain layer and you get very heavy rain rates. The big issue
here will be whether the storms are training in one location. If the
line is able to remain progressive through the entire region
significant flooding would likely be avoided however even minor
changes in direction of the line will serve to focus the
precipitation in one area. Despite the potential for flooding have
opted not to issue a Flash Flood watch at this point as the total
area likely to experience flooding will likely be relatively small
and with the line passing through the entire region its hard to
favor one portion of the CWA over another for flood risk.


Generally quiet extended forecast behind Tue cold front. High
pressure will build in in its wake...and more or less hold thru
the end of the week. Ridging build in aloft...and allows surface
high to hold over the region. Model guidance is hinting at NWrn
Atlantic ridging to try and build Wwd towards the end of the
period...which will help to keep the next trof delayed. The
multi-model consensus however does have the next rain chances
moving in over the deep SWly flow sets up on the
Wrn side of the ridge. Though it is at the tail end of the range
of guidance at this time...NAEFS mean PWATs are already forecast
to climb towards +2 standard deviations. That would mean after a
brief break...a return to humid conditions and possible
torrential rainfall in showers/storms for the weekend.


Short Term...
Fog and status visible just offshore on satellite will move
inland rapidly after sunset tonight dropping coastal locations
to LIFR in fog. Further inland IFR fog is possible but less
confident as the Marine airmass may not extend inland as far as
AUG or IZG. The far northern valleys will also see fog extending
from BML to 4B0 and throughout the Androscoggin and Connecticut

Long Term...Expecting VFR conditions to hold thru the end of the
week. As high pressure moves in for the latter half of the
week...afternoon sea breezes are likely as well as nocturnal
valley fog inland.


Short Term...Dense fog will remain over the coastal waters
through tomorrow. Southwesterly flow will increase ahead of a
cold front moving through Tuesday evening and and SCA has been
issued for the outer waters.

Long Term...Winds and seas expected to remain below SCA
thresholds thru the end of the week.


ME...Dense Fog Advisory from 10 PM this evening to 8 AM EDT Tuesday
     for MEZ018>028.
NH...Dense Fog Advisory from 10 PM this evening to 8 AM EDT Tuesday
     for NHZ010-013-014.
MARINE...Small Craft Advisory from 10 AM Tuesday to midnight EDT
     Tuesday night for ANZ150-152-154.



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