Flash Flooding in New Hampshire, July 29 to 30, 2021

Area Forecast Discussion
National Weather Service Gray ME
626 PM EDT Thu Jul 29 2021

.SYNOPSIS...
An area of low pressure will pass through the region tonight
bringing showers and a few thunderstorms along with the
potential for locally heavy rainfall. Cool and drier air will
arrive along with gusty northwest winds on Friday with
temperatures below normal continuing through the weekend. The
next chance for rain comes late Sunday and Sunday night with
drier conditions and gradually moderating temperatures expected
next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
625 PM Update...Have adjusted the forecast a little bit, mainly
with PoPs and QPF for the overnight area to tighten gradients a
bit. Convective 1-hr rainfall amounts of around 2" have been
observed in a few spots this far. Looking like the stripe of
highest rainfall amounts will be in south-central NH eastward
into the lakes region of Maine where an additional 1 to 2 inches
of rain is possible before tapering off from west to east later
this evening. Will continue to monitor for localized pockets of
flash flooding.

Previously...

*A narrow band of localized heavy rain possible this evening
 through tonight along and/or south of the mountains*

Mid level trough with embedded subtle short waves will advance
eastward this evening as surface high pressure shifts further
offshore. The subtle short waves will lead to one low passing to our
north with a secondary low tracking overhead tonight and then
strengthens as it reaches Downeast Maine tomorrow. A surface warm
front will advance northward through southern New England this
evening and will eventually come to a halt somewhere in central New
Hampshire through the coastal plain of Maine. It is along this warm
front where the potential exists for localized heavy rain.

There continues to be run to run and model to model variability
with regards to where the heaviest rain will set up through
tonight. The general consensus is that areas along and south of
the White Mountains and western mountains of Maine have to
potential to see localized heavy rain while extreme southern New
Hampshire looks to be spared from the heaviest of rainfall. The
ingredients involve PWATs around 1.5 inches, which is above
normal, but not exceptionally high around +1 sigma. Warm cloud
depths greater than 10KFT and some elevated instability will
also help contribute to heavy rainfall rates. Although the warm
front is expected to stall tonight, the overall system will be
progressive as well as relatively swift storm motion helping to
reduce flash flood potential. Mesoscale guidance suggests that
the heaviest cells will be capable of rainfall rates around 1
inch per hour, but overall these heavy cells will have low
residence times over a specific area. The main concern will be
training cells as storm motion will be somewhat aligned with
the mean flow.

As confidence is not there for specific areas to receive rainfall
that will exceed flash flood guidance and in collaboration with
neighboring offices have decided to hold off on a Flash Flood
Watch for tonight. Current QPF forecast is swath of 1 to 1.5
inches along and south of the mountains with lesser amounts in
extreme southern New Hampshire and near the International
Border. For areas that do experience the heaviest cells,
localized amounts will approach 2 to 3 inches and as long as
these amounts fall over a period greater than 6 hours flooding
should be minimized with current 6 hour flash flood guidance
generally greater than 2.5 to 3 inches.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Mid level trough axis will cross the area early Friday morning
bringing an end to the rain for areas south of the mountains with
low pressure deepening as it races toward Downeast Maine. A cold
front will be pushing offshore towards mid-day with winds turning to
the northwest setting up cold air advection over the area through
the afternoon. This CAA regime will lead to gusty NW winds with gust
around 25 to 30 mph. As lapse rates steepen over the area there will
be low chances for scattered showers or thundershowers, but
generally expecting dry and blustery conditions for areas south of
the mountains. In the mountains upslope flow will provide better
chances for showers and thundershowers through Friday afternoon.
Downsloping winds will allow coastal areas to be the warm spot with
highs in upper 70s with low 70s along the interior and northern
areas staying in the 60s.

Cyclonic flow aloft and cold air advection will continue Friday
night. Lows will drop into the 40s north to low 50s south with
highest summits approaching the freezing mark. Upslope flow will
continue overnight in the mountains and with cooling temperatures
there will be a chance for some wet snow mixing in atop Mt
Washington by early Saturday morning.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
High Impact Weather Potential: Minimal.

--Pattern and Implications--

A review of the pattern across NOAM and surrounding waters shows the
dominant teleconnection indices being a -EPO and +PNA. These two
indices essentially point to the fact that Pacific flow is weak
across the central/eastern pacific and does not have a path to
penetrate into the lower 48.  The PNA pattern is the result of a
strong ridge near the spine of the Rockies which reaches nearly to
the Arctic coast of Canada.  This leads to downstream troughing with
a longwave trough axis right overhead....with a Canadian source for
the air coming into our region.  No significant change to this
pattern looks to occur through the weekend and into Monday.  Beyond
this...there is moderate deterministic/ensemble guidance agreement
that the longwave trough axis shifts west of our longitude as
Atlantic high pressure ridge builds a bit westward.  How this trough
evolves during the second half of next week is not well agreed upon
as some ensemble members close it off while others weaken/lift it.
The overall trends is for it/s weakening however.

This overall pattern evolution favors a gradual moderation in
temperatures after a weekend with values a good bit below seasonal
norms.  In terms of precipitation potential...there is good
agreement of a shortwave rotating overhead through the larger
longwave trough on Sunday night-Monday.  Beyond this...there are no
strong signals for precipitation through the remainder of the long
term as we await the eventual evolution of the trough axis to our
west.

--Daily Details--

Saturday:  Somewhat fall like pattern continues on Saturday as heights
throughout the column remain about 2 sigma below climatology with
T8s in the -2 to -3 sigma range.  That's roughly a 1 in 10 year
event...so nothing record breaking...but certainly noticeable.
Northwest winds will continue to be somewhat brisk but weaker than
on Friday as cold advection will be over.  PWATs below 0.5" spell a
dry and sunny day which, with good mixing should allow high
temperatures to ready the upper 60s in the mountains and lower to
perhaps middle 70s to the south given T8s around +7-8C.  Despite the
sun...be aware that if you are going to hike over the higher
terrain...the highest peaks will see wind chills in the 20s with
high temperatures in the 40s and strong northwesterly winds.

Sunday - Monday: Positively-tilted shortwave trough axis will be
oriented over the Great Lakes Sunday morning...sharpening to more of
a neutral tilt as it arrives overhead Sunday night and then moves
rabidly east of the region on Monday.  Ahead of this feature...PWATs
push back above 1" to nearly 1.5".  While the best jet forcing will
be south and east of the region /where a secondary area of surface
low pressure is likely to develop/...the arriving area of mid level
height falls and warm advection over a northeastward advancing warm
front should allow for an increase in shower chances most likely in
the Sunday night period.  Given that the mid/upper pattern involves
the potential for northern/southern stream phasing...there is some
inherent uncertainty...but will continue likely PoP mention in
agreement with the model consensus...with some potential for thunder
over southern sections given the warm frontal placement if forcing
can arrive Sunday afternoon...but beyond that overnight timing would
not favor a significant thunder risk.  Should see improving
conditions by later Monday as the shortwave quickly moves east.
Temperatures remain cool through the period...upper 60s in the
mountains and lower/mid 70s to the south.

Tuesday - Thursday: Low pressure departs the region by Monday night
with a weak ridge of high pressure building back of the area on
Tuesday.  Beyond this expect temperatures and boundary layer
moisture to increase somewhat as we lose the Canadian influence to
the lower tropospheric flow.  There is certainly some room open for
a shortwave to move into the region this period although there is no
strong consensus on this.  Thus...low...but slowly increasing PoPs
are in order...close to 12 hour climo values which are near 20%
While highs will increase...temperatures aloft and the nearby trough
do not indicate that we/ll move much higher than normal by the end
of the period.  Thus...after a day in the 70s throughout the
forecast area on Tuesday...70s north with upper 70s/lower 80s south
looks like a good place to start the forecast and watch how the
guidance envelope evolves.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Short Term...Clouds will thicken and low through this evening
with SHRA and some TS. Lowering cigs and reduced visibility
will bring MVFR to IFR conditions overnight with IFR improving
to MVFR early Friday morning and VFR by mid morning.NW west will
gust to 20 kts Friday afternoon with VFR likely Friday night as
winds inhibit fog formation.

Long Term...

VFR Saturday likely continuing through at least early afternoon
Sunday.  Showers likely with some restrictions most likely in the
Sunday night/early Monday period before improving back to VFR.
Northwest wind gusts 15 to 20kts expected on Saturday with light
winds Saturday night through Sunday night before westerly winds
increase to 10-15kts on Monday.

&&

.MARINE...
Short Term...Southerly winds will gust to 30 kts overnight with
seas building to 5 ft. Southerly winds drop off Friday morning
and then a cold front crosses the waters mid-day Friday with SCA
conditions likely Friday afternoon and night.

Long Term...

Winds/waves will gradually diminish below SCA levels on Saturday and
likely remain below SCA criteria for Saturday night through
Monday.

&&

.HYDROLOGY...

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory until 7 AM EDT Friday for ANZ151-153.
     Small Craft Advisory until 8 AM EDT Friday for ANZ150-152-154.

&&

NEAR TERM...Ekster/Schroeter
SHORT TERM...Schroeter
LONG TERM...Arnott

Hazardous Weather Outlook
National Weather Service Gray ME
1251 PM EDT Thu Jul 29 2021

MEZ007>009-012>014-018>028-033-NHZ001>015-301700-
Northern Oxford-Northern Franklin-Central Somerset-Southern Oxford-
Southern Franklin-Southern Somerset-Interior York-
Central Interior Cumberland-Androscoggin-Kennebec-Interior Waldo-
Coastal York-Coastal Cumberland-Sagadahoc-Lincoln-Knox-Coastal Waldo-
Interior Cumberland Highlands-Northern Coos-Southern Coos-
Northern Grafton-Northern Carroll-Southern Grafton-Southern Carroll-
Sullivan-Merrimack-Belknap-Strafford-Cheshire-Eastern Hillsborough-
Interior Rockingham-Coastal Rockingham-
Western And Central Hillsborough-
1251 PM EDT Thu Jul 29 2021

This Hazardous Weather Outlook is for Maine, south central Maine,
southwest Maine, west central Maine, western Maine, New Hampshire,
central New Hampshire, northern New Hampshire and southern New
Hampshire.

.DAY ONE...This afternoon and tonight.

Showers and embedded thunderstorms will increase in coverage across
the area late this afternoon and this evening with locally heavy rain
possible through this evening and into the pre dawn hours Friday.
There is some potential for areas of heavy rain to repeatedly move
over the same areas...increasing the risk for localized flash
flooding. Flooding on area rivers is not expected.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Hazardous weather is not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter reports of rainfall amounts are always appreciated.

ANZ150>154-301700-
Coastal Waters from Stonington, ME to Port Clyde, ME out 25 NM-
Penobscot Bay-
Coastal Waters from Port Clyde, ME to Cape Elizabeth, ME out 25 NM-
Casco Bay-
Coastal Waters from Cape Elizabeth, ME to Merrimack River, MA out
25 NM-
1251 PM EDT Thu Jul 29 2021

This Hazardous Weather Outlook is for Atlantic coastal waters.

.DAY ONE...This afternoon and tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAY ME
401 PM EDT THU JUL 29 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0230 PM     WATER SPOUT      6 ESE KENNEBUNKPORT     43.31N 70.37W
07/29/2021                   ANZ154             ME   PUBLIC

            RECEIVED AN IMAGE OF A WATER SPOUT OFF THE
            COAST OF CAPE PORPOISE. ROTATION ON RADAR
            CONFIRMED THE TIME AND LOCATION.

&&

Special Weather Statement
National Weather Service Gray ME
914 PM EDT Thu Jul 29 2021

NHZ007-008-011-015-300200-
Merrimack-Western And Central Hillsborough-Cheshire-Sullivan-
914 PM EDT Thu Jul 29 2021

...A LINE OF STRONG THUNDERSTORMS WILL AFFECT SOUTHEASTERN
SULLIVAN...SOUTHEASTERN CHESHIRE...WESTERN HILLSBOROUGH AND
SOUTHWESTERN MERRIMACK COUNTIES...

At 913 PM EDT, radar indicated strong thunderstorms were located
along a line extending from near Hillsborough to Marlborough to near
Vernon. Movement was southeast at 35 mph.

Winds in excess of 40 mph are possible with these storms.

Locations impacted include...
Keene, Jaffrey, Henniker, Milford, Amherst, Swanzey, Peterborough,
New Boston, Winchester, Antrim, Mont Vernon, Greenfield,
Lyndeborough, Bradford, Dublin, Francestown, Bennington, Richmond,
Harrisville and Nelson.

Torrential rainfall is also occurring with these storms, and may
cause localized flooding. Do not drive your vehicle through flooded
roadways.

LAT...LON 4273 7246 4277 7249 4277 7252 4278 7251
      4279 7252 4293 7223 4326 7200 4292 7161
      4271 7173
TIME...MOT...LOC 0113Z 311DEG 29KT 4316 7199 4289 7217 4274 7248