Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 251531
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1131 AM EDT Tue Apr 25 2017

.SYNOPSIS...
An onshore flow will transport cool, moist air off the Gulf of
Maine today into Thursday. Today drizzle and rain will begin to
overspread the region with rain becoming heavier tonight into
Wednesday as low pressure moves north along the East Coast. The
cool, moist maritime air may linger into Thursday east of the
mountains, but warm air will begin to move into western New
England and eventually even make it to the Maine coast this
weekend.

&&

.NEAR TERM /TODAY/...
1130AM UPDATE...
Have made another attempt at refining hourly POPs to reflect
most likely timing of precipitation. It will never be perfect,
but hopefully latest update reflects this timing better. First
round of rain is falling out of a mid level cloud deck which has
advanced northward to about the center of the forecast area at
this time. This hasn`t amounted to more than a few hundredths of
an inch of precipitation in most spots. Further south there will
be a break lasting several hours before the next round arrives
in the late afternoon and early evening.

730AM UPDATE...
Have updated the grids for current conditions. Latest radar
imagery and surface observations indicated very light rain, with
drizzle and patchy fog has reached southern New Hampshire and
portions of coastal Maine, mainly over southern coastal York
County. This precipitation is entering southern portions of the
forecast area quickly this morning. Have upped pops for the next
few hours.

Interior and central/northern areas have a slightly different
airmass in place with large dew point depressions as noted by
surface observations and mesoscale models. Therefore, it will
likely take several more hours steady light precipitation to
reach areas from about Augusta and points north.

Made some adjustments to cloud cover and fog locations with this
forecast update. There was some brief patchy spots with 1/4
visibility, but they have since then come up in most places.

ORIGINAL DISCUSSION...
Areas of maritime drizzle and fog will remain off the coast
early this morning, before coming onshore today. Meanwhile, rain
will continue to spread northward from southern New England
today as a surface area of low pressure reaches the Mid Atlantic
coastline. With easterly winds developing, temperatures will be
much cooler along the coastline, with only 40s for highs in
most areas.

&&

.SHORT TERM /TONIGHT AND WEDNESDAY/...
Rain will continue tonight with widespread drizzle and fog as
a deep maritime influence will overspread much of the region.
Low pressure will slowly fill as it moves up the New Jersey
coastline. Nevertheless, trajectories over the open Atlantic and
slow movement of the system will increase the rainfall totals
over the region despite not an overly impressive dynamic
pattern.

By Wednesday, the upper level low pressure system begins to open
with time as a surface low dissipate as it approaches Nantucket
Island. The heaviest rain will fall in the morning, before the
precipitation begins to lighten with time.

Temperatures will struggle to climb through the 50s in most
sections with a southeast flow continuing over the region,
albeit light. Some warmer temperatures may occur in the
downslope regions of the Connecticut River Valley.

QPF: Rainfall totals over 1 to 2 inches are still expected over
southern areas. There may be a few local spots that exceed 2
inches of rain in any persistent periods of heavy rain, mainly
along and or near the coastline.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Low pressure drifts northeast into the Gulf of Maine Wednesday
night and Thursday. This will continue a persistent onshore flow
keeping low levels saturated. The stratiform rain, drizzle and
areas of fog will continue Wednesday night and by Thursday only
the drizzle and areas of morning fog should remain. Models
indicate by Thursday night the low moves far enough offshore to
allow some clearing in the mid/high levels but lack of any
ventilation of drier air in the lower levels should keep low
levels moist with stratus remaining, especially southern and
eastern areas. Temps warm aloft Friday but unfortunately the
combo of the clouds and a developing south flow will keep temps
from reaching their potential, although southern areas of NH may
reach into the 70s. A weak short wave moves through late Friday
so scattered showers and even the chance of afternoon
thunderstorms may occur.

Over the weekend a large broad Bermuda high sets up off the east
coast with its northern periphery of a warm southwest flow
aloft reaching into northern New England. Models are having a
tough time trying to resolve any short waves traveling around
the outer periphery of the large high into New England. This
will make or break the weekend with either a mild nice weekend
versus a wet damp weekend. Confidence at this time is low on the
outcome.

&&

.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
Short Term...VFR conditions this evening will begin to
transition to MVFR and then IFR conditions from south to north
during the day Tuesday as low level moisture spills in off the
Gulf of Maine. There could be an area of lower clouds arriving
along the immediate coast as early as tonight which would bring
a threat for LIFR conditions. But the broader forecast is for
worsening conditions on Tuesday with just about all areas IFR or
worse by Tuesday evening in drizzle and light rain which will
likely last through Wednesday. To the northwest of the
mountains up in Whitefield, conditions may stay VFR to MVFR
until the better forcing for rain arrives Tuesday night.

Long Term...Lingering IFR/LIFR conditions likely through
Thursday. Improvement to VFR/MVFR likely for Thursday night
into Friday...with occasional restrictions in SHRA and possible
TSRA possible Friday afternoon. Frontal boundary remains across
the region through the weekend with VFR likely dominating...but
scattered MVFR/IFR restrictions in SHRA possible.

&&

.MARINE...
Short Term...Expect an increasing easterly flow tonight into
Tuesday, with winds as high as 25 KT beginning Tuesday
afternoon. Winds shift to the southeast on Wednesday and
decrease in intensity Wednesday night. The onshore fetch will
allow wave heights to build up to 5 to 9 FT through Wednesday
night.

Long Term...While winds will subside...lingering deep water
swells will likely necessitate SCAs over the outer waters into
the weekend.

&&

.FIRE WEATHER...
Moist onshore flow will bring high humidity and a wetting rain
to all areas today through at least Wednesday.

&&

.HYDROLOGY...
Continued snow melt in the mountains will keep river levels
high. Rain expected this afternoon and Wednesday will add runoff
to area rivers already at high levels. While widespread
flooding is not expected, it is not out of the question that a
few rivers could reach minor flood stage.

&&

.TIDES/COASTAL FLOODING...
High astronomical tides begin tonight and last through the
weekend. The combination of high tides and an onshore flow with
the coastal low will bring the chance of beach erosion, splash
over, and minor coastal flooding during the time of high tide
tonight and again Wednesday night. There is relatively good
agreement between the WRMGYX forecasts and in-house matrices for
this event. Minor expects will be possible, from Portland
Harbor and points south.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory from 4 PM this afternoon to 6 PM EDT
     Wednesday for ANZ151-153.
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ150-152-
     154.

&&

$$

NEAR TERM UPDATE...Kimble


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