141
FXUS61 KGYX 042006
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
406 PM EDT Tue Jun 4 2024

.SYNOPSIS...
Some showers are possible through this evening as high pressure
sinks south of the region. Chances for showers and
thunderstorms increase Wednesday as a trough approaches from the
west. These showers will be slow moving and pose an isolated
flash flood risk. This trough likely brings periods of
widespread rainfall late Thursday which also may pose an
isolated flood risk. Unsettled conditions persist through the
end of the week with daily chances of showers and an isolated
afternoon thunderstorm.

&&

.NEAR TERM /TONIGHT/...
Stagnant boundary layer this evening and overnight will lead to
slow moving showers across western and northern NH. These have
so far formed off terrain lift, forming a neat line along the VT
Green Mtns and now the Whites in NH. Can`t rule out a rumble of
thunder, but the lack of shear and more potent CAPE should keep
these mostly lightning free.

Elevated instability remains overnight, and any energy crossing
the region tonight could pop up some isolated showers after
midnight and daybreak. This seems most likely for portions of
southern and central NH. Between cloud breaks overnight, could
see fog formation in the valleys once again, especially where
rain fell this afternoon.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
Marginal threat for localized flash flooding Wednesday, but it
will be highly dependent on training showers/storms.

Parameters continue to show decent precip efficiency and weak
storm motion Wed afternoon. Surface instability builds late
morning from the mountains across the interior. HREF mean CAPE
develops 700-1000 j/kg here, with the profile long and skinny.
Some patches support up to 1500 j/kg. RH through the column has
improved through the week, but low levels do remain borderline
at around 70 percent. PWATs have slowly been increasing early
this week, and should push above 1.1 inch overnight, the current
75th percentile. Warm cloud layer continues to be an item
holding back precip efficiency, remaining at or below 10kft.
This can prevent warm cloud processes from really boosting
soaking rain.

Looking at storm motion, cloud layer winds are slow, but overall
shear should carry storms towards the coast. Looking at hires
model reflectivity, storm mode appears single cell to multicell,
likely pulsing up then falling apart due to the lack of venting
ability. But, backbuilding or training does look possible as
showers/storms refire behind those that tail off to the SE.

For areas that overcome broader lift, lack of deeper clouds, and
dry antecedent conditions, there will be that chance for
periods of heavy rainfall that could lead to minor flash
flooding. HREF probs of greater than a half inch and inch of QPF
in 3 hours are above 80 percent for much of the Whites and
foothills of ME/NH. Thus the marginal ERO from WPC fits the bill
for tomorrow.

With CAPE outweighing shear tomorrow and small inverted V
soundings, could also see some storms create gusty winds as
they collapse initially. But this is also expected to be limited.

Showers will develop late morning and continue to spread in
coverage into the afternoon and evening hours, tapering after
dark as surface instability weakens. Some showers may hang on
into the mid to late evening hours as elevated CAPE remains.


&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Overview: We return to wetter weather as the 500 mb pattern
shows a ridge over the Atlantic blocking an upper low over the
Great Lakes through the extended period. This will result in
daily chances for rain showers as multiple waves rotate around
this low and over our region. Right off the bat we could see
some particularly heavy rain that may cause some localized
flooding.

Impacts:
*  Slow moving showers containing heavy rain may cause some
   localized flooding on Thursday.

Details: Vertically stacked low pressure approaches the region
Thursday draping an occluded front just to the west of area.
Convergence along this front looks to generate a healthy band of
showers with decent upper level forcing entering into the
region as we end up downstream of a 75 kt jet. A very moist
airmass will also be entering the region with PWAT values of
1.50-1.75" modeled across much of the area. These values are
above the 90th percentile for this date and approaching the
daily max. All these ingredients combined spell a decent setup
for heavy rain and have rightfully warranted a Marginal Risk
from the Weather Prediction Center, however there are some other
uncertainties in this set up that we will have to consider
before we can pin down where the best chance for this to occur
is. As detailed in the short term section, Wednesday poses a
threat for localized flooding as well. Areas that get a soaking
then, will be more prone to flooding during these showers. This
is an important factor because antecedent conditions in our area
are generally dry with rivers running below normal. Ensemble
guidance is showing low level flow of only around 15-20 kts and
with very little instability, mean CAPE values only around 200-
300 J/kg and really only in southwestern New Hampshire, this
points more toward a flooding scenario caused by a slow moving
area of showers that, due to its efficient rain processes and a
plenty moist airmass, will likely contain some heavy rain. With
all that being said GFS and EURO ensembles are spitting out
24-hr rain totals between 0.75 and 1" with less than 25% chances
of going over an inch, which isn`t super impressive. As of
right now this looks like urban and poor drainage areas would
see the best chance of flooding, due to the aforementioned dry
antecedent conditions and rivers running below normal. However,
this is likely to change in some places following Wednesday`s
events.

Vertically stacked low pressure will continue to send fronts
through the region along with shortwaves rotating around the
upper low. This will necessitate shower chances through the
remainder of the week and into early next week, however not
everyday will be a wash and some will be drier than others.
Temperatures look to remain pretty steady with highs in the 70s
and lows in the 50s each day. Can`t speak much on exact details
about the rain day to day at this time range, but it is worth
mentioning that return flow from the high pressure offshore will
keep an onshore component to the wind for a majority of this
period so PWAT values are looking to stay up around an inch. It
is also worth mentioning that the low level pattern continues to
show flow rarely exceeding 20 kts and as we get into the
weekend and early next week shower activity will become more
convective in nature as we see clearer skies and warmer daytime
temperatures. Isolated flooding will have to remain in the backs
of our minds as we continue through this upcoming stretch of
wetter weather.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Short Term...VFR. Some uncertainty on fog/low stratus potential
along ME/NH coast tonight through Wednesday. Confidence in this
isn`t enough to include in TAFs at this time. Valley fog may
develop again across the interior after showers this afternoon
and early Wed AM. Showers and TS possible Wed late morning into
afternoon. These will be focused for interior terminals, but can
expect coastal sites to also see showers moving their way
mid/late afternoon.

Long Term...A mixed bag of restrictions is expected on Thursday
as an area of rain crosses the region. This will likely have
embedded areas of heavier showers and may even have some
embedded thunderstorms as well, but this is less likely and
would be more confined to southern New Hampshire terminals. VFR
looks to prevail through the rest of the week, but isolated to
scattered showers and thunderstorms will be possible each
afternoon. Winds remain 5-10 kts with gusts 10-15 kts through
Sunday.

&&

.MARINE...
Short Term...Below SCA. Warm, moist flow over the cooler SST
will create patches of vis reduction due to fog through at least
the next 36 hours.

Long Term...Conditions hazardous to small craft are not expected
through Sunday. Fog over the waters is likely Thursday along
with rain showers as a system crosses the region. Showers and
thunderstorms are possible each afternoon this week. Winds are
primarily onshore at 10-15 kts through Sunday as high pressure
remains over the atlantic and low pressure remains inland.

&&

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

&&

$$

NEAR/SHORT TERM...Cornwell
LONG TERM...Baron