Flash Flood Warning 46, NWS Gray, ME

Area Forecast Discussion
National Weather Service Gray ME
756 AM EDT Sun Jul 16 2023

.SYNOPSIS...
Heavy rainfall moves in today bringing the threat of flash flooding
along with isolated strong to severe thunderstorms. Heavy rain moves
out tonight while runoff and rises on rivers and streams continues
hydro concerns into Monday morning. The next upper trough in this
active pattern arrives Tuesday bringing the threat of thunderstorms
that could produce torrential rainfall along with the threat for
some strong to severe storms. Drier air slowly works in Wednesday
into Thursday for mostly dry conditions. Yet another trough arrives
late week bringing chances for showers and storms Friday into
the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Update...Mostly minor changes to account for latest radar
trends. As showers and embedded storms have moved thru NH many
gauges are picking up a quick 0.5 inches to 1 inch in less than
an hour. In addition embedded cells across southern NH have
been rotating...in a fairly broad area of 0-500 m helicity over
100 m2/s2. The duel tornado/flash flood threat is expected to
continue thru midday across that area.

Previous discussion...The first bands of showers are sliding
thru parts of the forecast area this morning...with much more to
come today.

Flood watch remains in effect...with little overall change in
the forecast. Upper jet entrance region will move over the
forecast area by midday...supporting the continuation of rain
with embedded torrential downpours. LLJ will increase thru the
day as well...aligning itself parallel to the approaching front.
This will favor more narrow bands of heavy rainfall
amounts. However the extreme moisture of the air mass...PWATs in
excess of 2 inches...means some of this rainfall could
potentially be significant. The HRRR has been consistently
spitting out 5 to 7 inches QPF for several runs now. Despite the
location of the maxima shifting around New England...this is a
clear signal for the upper end of rainfall totals today.

Finally one thing to note is the conditional severe threat
today. Lapse rates are poor given how moist the environment
is...but high dewpoints are supporting an uncapped roughly 1000
J/kg CAPE. If we can maintain the 0-3 km CAPE around 100 J/kg
the low level updraft strength may be robust enough to tap the
shear in the lowest 500 m. Helicity around 100 m2/s2 would be
enough to spin up a brief tornado or two. This threat is
supported by machine learning guidance. It is important to note
that this really is a conditional threat...and other severe
hazards like wind and hail are less likely given the unfavorable
environment for deep intense updrafts.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Most of the worst of it will be over by this evening. Bands of
convection may still linger thru the late evening and the
biggest concern will be whether they track over areas that
received the heaviest rainfall earlier in the day.

Not much focus on Mon given the expected rainfall today...but
overall theme will be clearing but hot and humid. Heat index
values across southeastern NH may push into the upper 90s during
the afternoon.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Deep layer moisture will be on the increase Monday night into
Tuesday ahead of the next upper wave and surface front that will
bring thunderstorms Tuesday afternoon and evening. This moisture
plume will not be as extreme as the plume moving in this morning,
while it will still be anomalously high bringing the threat for
torrential rainfall with thunderstorms. The convective parameter
space also looks favorable for a few strong to severe thunderstorms
Tuesday. Drier air works into the region Wednesday for mostly
dry conditions. The next disturbance approaches for Thursday
although the 00Z guidance keeps it far enough south for another
mostly dry day. Another more robust trough approaches late in
the week for renewed chances of showers and thunderstorms Friday
into Saturday with the large scale pattern favoring troughing
over the Northeast into early next week.

Another active weather day is possible Tuesday as an upper wave goes
negative tilt sending a cold front across the region. While the
threat for heavy rainfall is on the table, there also looks to be
good alignment of shear and instability for some strong to severe
storms. PWATs will again be high with ensemble means around 1.5
inches, while storm motion looks swift enough to keep any storm over
a particular region for a prolonged duration. Regardless, after
the excessive rainfall that is expected today any additional
heavy rainfall will likely lead to hydro issues. The convective
parameter space will be characterized by ML CAPE approaching
1500 J/kg along with deep layer shear approaching 40 kts. Mid
level lapse rates are sub-par, peaking out around 6C/km while
there does seem to be decent CAPE in the hail growth zone.
Latest output from machine learning probabilities out of CSU
does bring both a hail and a wind threat to the region with
slight uptick in probabilities from previous runs. Will continue
to monitor trends in this threat as we enter the window of CAM
guidance over the next 12 to 36 hours for more clarity.

The 00Z guidance continues to suggest the cold front will clear the
coast by Wednesday morning. However, a potent ridge in the western
Atlantic leaves some trepidations with the eastward progression of
the upper wave and how quickly drier air can move into the region. A
mostly dry day looks to be in store with dewpoints dropping into the
low 60s to 50s across much of the north and western NH. Along the
coast, in particular the Mid Coast, dewpoints may remain elevated
continuing humid conditions before drier air eventually works in
Wednesday night with dewpoints across the area Thursday in the low
60s. Thursday also looks to mainly precipitation free with weak
high pressure nearby and one upper wave passing to our south
with another wave back over the Great Lakes. The upper wave over
the Great Lakes approaches Friday and brings chances for
showers and storms late Friday into Saturday.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
Short Term...Widespread IFR and LIFR this morning will continue
ahead of an area of rainfall approaching from the west. Embedded
within the RA will be locally torrential downpours...with low
VIS expected. TSRA will also be possible...most likely across
the southern half of NH into far southwestern ME. IFR or lower
conditions will likely linger thru the night before clearing Mon
back to VFR.

Long Term...Fog will bring the threat of flight restrictions
Monday night. Conditions improve Tuesday morning before a cold
front brings the threat of thunderstorms. Flight restrictions
will be likely in storms Tuesday afternoon and Tuesday night
with the threat of fog into Wednesday morning. Mainly VFR looks
to prevail Wednesday into Thursday.

&&

.MARINE...
Short Term...Winds will gradually increase ahead of the
approaching front today. SCA remains in effect for seas building
to 5 ft and a few gusts around 25 kt. Areas of fog...some
locally dense...will continue into Mon.

Long Term...Marine fog will be likely Monday night through
Tuesday night before a cold front crosses. This front will also
bring the threat of thunderstorms over the waters late Tuesday
into Tuesday night. Southerly flow prevails before the frontal
passage while winds and seas remain below SCA thresholds. Winds
shift more southwesterly Wednesday and briefly northwesterly
Thursday morning before shifting onshore Thursday. Winds and
seas continue to remain below SCA thresholds.

&&

.HYDROLOGY...
Overview: Widespread heavy showers and thunderstorms may cause
potentially significant rainfall/flooding today.

The main push of rainfall is just now beginning to enter the
forecast area and flash flood watch remains in effect for all
zones. Soil moisture remains quite high across much of the
region, resulting in quicker hydrologic responses when rain does
start falling. PWATs increase to near record levels during the
day as an upper level jet streak approaches from the west. A
slow moving area of rainfall with embedded torrential downpours
is expected to cross the region thru the overnight. With warm
cloud depths in excess of 13 kft rainfall may easily be tropical
in nature and efficient enough for extreme rainfall rates. At
this time, most of the area should be preparing for 2 to 3
inches of rain with the expectation that some areas could see
values much higher depending on repeating activity and training.
The rapid runoff response from intense rainfall rates will be
exacerbated by saturated ground conditions. Storage in
streams...reservoirs...and lakes are likewise reduced across
much of the area and could quickly become overwhelmed. The most
vulnerable areas will be recently flooded locations and areas
along the higher terrain with steep slopes. The localized flash
floods of the last several weeks have stressed infrastructure
from roads...lake levels...dams...and embankments. All of these
already impacted areas will be susceptible to more damage if hit
with the higher rainfall amounts...or even some brief but
intense rainfall rates.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Flood Watch through late tonight for MEZ007>009-012>014-
     018>028-033.
NH...Flood Watch through late tonight for NHZ001>015.
MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT
     Monday for ANZ150-152-154.

&&

NEAR TERM...Legro
SHORT TERM...Legro
LONG TERM...Schroeter

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Special Weather Statement
National Weather Service Gray ME
718 AM EDT Sun Jul 16 2023

MEZ007-012-013-018>022-024>027-033-NHZ002>006-161200-
Northern Oxford ME-Interior Cumberland Highlands ME-Kennebec ME-
Central Interior Cumberland ME-Coastal Cumberland ME-Sagadahoc ME-
Southern Oxford ME-Androscoggin ME-Interior York ME-
Interior Waldo ME-Southern Franklin ME-Lincoln ME-Knox ME-
Southern Coos NH-Northern Carroll NH-Northern Grafton NH-
Southern Carroll NH-Southern Grafton NH-
718 AM EDT Sun Jul 16 2023

...A line of heavy rain moving northeast will impact portions of
Oxford, west central Waldo, Androscoggin, southeastern Franklin,
Kennebec, northwestern Knox, north central York, Lincoln,
Cumberland, Sagadahoc, southern Coos, northeastern Grafton and
northern Carroll Counties through 800 AM EDT...

HAZARD...Torrential rain with rainfall rates over 2 inches per hour.
         This will reduce visibility and could cause hydroplaning
         for motorists. More rain is expected throughout the day
         which could result in flash floods as repeating shower
         activity is expected. Please stay tuned to the latest
         warnings from the National Weather Service.

SOURCE...Radar indicated.

IMPACT...Use caution if traveling through into this drenching
         rainfall. Visibility will drop to under a quarter of an
         inch cause rapid ponding on roadways and low lying areas.

Locations impacted include...
Lewiston, Auburn, Brunswick, Augusta, Westbrook, Waterville, Conway,
Berlin, Bath, Gardiner, Bridgton, Lancaster, Fryeburg, Mechanic
Falls, Bethel, Bethlehem, Damariscotta, Lincoln, Naples and Oakland.

This includes the following highways...
 Interstate 295 between mile markers 12 and 51.
 Interstate 93 between mile markers 86 and 114.
 Interstate 95 between mile markers 54 and 128.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4466 7142 4454 6933 4380 6951 4372 6956
      4375 6969 4370 6973 4386 7193
TIME...MOT...LOC 1114Z 240DEG 49KT 4398 7142

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...<30 MPH

SJ