Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 260221
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1021 PM EDT Tue Jul 25 2017

.SYNOPSIS...
High pressure will continue to build into the region tonight and
will bring drier weather and clearing skies. It will shift
offshore on Wednesday...allowing some warmer air back into the
region. A cold front pushes toward the area on Thursday. High
pressure will attempt to build into the region Friday and
Saturday as low pressure forms south of New England.

&&

.NEAR TERM /TONIGHT/...
1015PM UPDATE...
Have updated the forecast to adjust hourly temperature and
dewpoint forecasts toward latest observed values. Already seeing
many automated observing sites with temperatures at their
dewpoints, so each additional degree of cooling from here on out
will contribute to deposition (dew) and condensation (fog).
Still think the most likely fog areas will be the valleys, with
temperatures broadly on track to reach forecast lows. No
significant changes were needed.

7PM UPDATE...
Have updated the forecast based on latest observations and
expected conditions overnight. High pressure nestling down into
our area tonight will bring calm winds with sky cover
increasingly becoming clear. This has already been observed in
the late afternoon visible satellite as earlier cloud cover
over New Hampshire is rapidly dispersing. This will set the
stage for an excellent radiational cooling night, with low
temperatures reaching a base of about 50 degrees, with typical
cold spots in valleys getting significantly colder, and urban
warm spots staying slightly warmer. Expect a good deal of
radiation fog to develop overnight, with much of this
accumulating in the valley locations, becoming dense in the
sharper valleys by daybreak. Given the spotty and localized
nature of the dense fog, will not issue a Dense Fog Advisory,
however, we do expect many valley locations to have very low
visibility by morning.

ORIGINAL DISCUSSION...
Clouds will continue to dissipate as drier air builds into the
region from the northeast. High pressure will continue to allow
for clearing during the overnight period with southwestern New
Hampshire being the last to clear.

With high pressure overhead and light winds, some radiational
cooling will take hold. Overnight low temperatures will drop
into the 40s in many areas. As temperatures fall towards the
dew points, patchy fog will form. Inland valleys will have the
lowest visibilities.

&&

.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
Patchy fog will gradually mix out Wednesday morning across
Maine and New Hampshire. There will be ample sunshine when
compared to today. With modest warming aloft, surface
temperatures will climb through the 70s with many communities
topping out at 80 degrees away from the shoreline. As usual, the
Midcoast region will be the coolest location as a weak gradient
will allow for onshore sea breezes to form.

A cold front will approach from the west Wednesday night. A
southwest gradient will develop, allowing for a breeze to
develop late at night. Increasing cloud cover may prevent as
much fog compared to what is expected tonight.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
The extended forecast will be characterized by large scale
troughing over the eastern CONUS which will move southeast from
northern Canada Thursday into Friday. The system will then stall
over the region due to downstream blocking through at least the
weekend. Several spokes of short wave energy rotating around the
upper low as well as a significant southern stream short wave
will bring clouds and at least one shot of showers and
thunderstorms to northern New England.

Pacific/NA teleconnection has recently been in a negative phase
corresponding to negative temperature anomalies at both 500mb
and and the surface. The good news is temperatures will be just
a few degrees above normal for highs Thursday and Friday with a
warming trend thereafter.

The main concern for the Thursday through early next week time
frame will be the short wave and surface boundary affecting the
region Thursday. The boundary should enter the northern zones
Thursday morning triggering numerous showers and thunderstorms.
This airmass does not look particularly moist so not expecting
any particular hydro issues with this boundary, but localized
heavy rain is possible. The front should clear the coast late
Thursday night.

Instability will be mostly concentrated in southern NH where we
may get more SBCAPE depending on antecedent clouds and showers.
Have included thunderstorms but not any particular enhanced
wording at this time. It is noted a portion of NH is under a
marginal risk of thunderstorms while the remainder of the area
is in general thunder per SPC. Baroclinic zone will shift south
on Friday as secondary cyclogenesis occurs over the Mid Atlantic
region. Some rain showers could brush southern sections early
but dense Canadian high pressure building in from the northwest
will suppress precipitation quickly during the day.

Sunday through early next week look tranquil with a weak system
possible for Tuesday/Wednesday.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Short Term...Patchy fog tonight, mainly across the inland
valleys. A few locations will drop all the way down to LIFR
conditions, namely LEB and possibly HIE and CON. Conditions
improving shortly after sunrise on Wednesday.

Long Term...Areas of MVFR conditions are possible at all
terminals in areas of heavy rain/TSRA Thursday into early
Friday morning. Becoming VFR Friday midday into Saturday.

&&

.MARINE...
Short Term...Winds becoming calm overnight as high pressure
builds in. Wave heights will continue to diminish as well.

Long Term...Seas and winds remain below small craft advisory
levels Wednesday through the extended. Heavy rain/fog/poor
visibilities are likely over the waters Thursday into Saturday.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$

NEAR TERM UPDATE...Kimble
SHORT TERM...Cannon
LONG TERM...Hanes



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