A strong cold front was dropping south through the region on the morning of July 17th. Numerous severe thunderstorms developed ahead of the front over southern New Hampshire.
Every event with a place to put it. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS GYX wrote it up.
A strong cold front was dropping south through the region on the morning of July 17th. Numerous severe thunderstorms developed ahead of the front over southern New Hampshire. Precipitable water values approaching 2 inches over the region contributed to heavy rainfall with these storms. Two to 4 inches of rain fell in the Manchester area in 3 hours resulting in flash flooding.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Gray, ME wrote its last area forecast discussion before the first event at Jul 17, 10:19 AM EST, 1 hour 31 minutes earlier.
Area Forecast Discussion
National Weather Service Gray ME
1119 AM EDT Tue Jul 17 2018
.SYNOPSIS...
A strong cold front will bring numerous showers and
thunderstorms to the region today with severe storms
possible...along with an increased risk of flash flooding.
Storms will come to an end this evening with the cold front
pushing offshore overnight. High pressure will build in from
the west Wednesday through Thursday and will shift offshore
Friday. High pressure will hold offshore on Saturday. A warm
front will approach from the south on Sunday.
&&
.NEAR TERM /TODAY/...
11AM UPDATE...
Have updated the forecast for new Severe Thunderstorm Watch
covering southern New Hampshire and far southwest Maine. This is
the area with the most destabilization and will likely see a
line of storms move through over the course of the next few
hours. Damaging winds is the primary threat.
9AM UPDATE...
Have decided to let the Dense Fog Advisory drop at 9AM as any
remaining 1/4SM fog is dissipating fast from southwest to
northeast. Made minor adjustments to hourly temperatures and
dewpoints through the day, but no other significant changes were
needed.
715AM UPDATE...
Have tweaked the dense fog advisory based on morning
observations and the first few GOES 16 visible images which
suggest improvement over southeast NH...but more stubborn fog to
the north and east. Initial convection arriving from the west
has provided some heavy rainfall but in general has
decayed...with anything more than an isolated severe threat
having to wait until closer to the noon hour when we realize
significant destabilization. Only very minor adjustments
elsewhere to match morning observations.
Previous Discussion Below...
High Impact Weather Potential:
* Dense Fog through daybreak
* Severe thunderstorms expected this afternoon
* Heightened flash flood risk over southern NH and extreme SW
Maine
* Parts of southern NH may reach near heat advisory criteria
/heat index of 95/ this afternoon.
Pattern: Primary feature of interest early this morning is
impressive mid-July shortwave moving east from Ontario into Quebec.
A 50kt jet at H5 is oriented southwest to northeast across the
western Lakes...nearly collocated with 75kt jet at H3. GOES 16 water
vapor imagery shows substantial moisture plume advancing north and
east ahead of this feature with a shortwave ridge now over the Gulf
of Maine...exiting east. Blended TPW product shows 2" PWAT axis now
over the eastern Great Lakes and the juxtaposition of this moisture
and arriving dynamic forcing from the west /along with attendant
surface front/ has yielded a broad area of showers and thunderstorms
across upstate New York. Further east...a few isolated showers and
thunderstorms have been pulsing up and down at the leading edge of
better theta-e at H8. Clearly...the primary forecast concern
centers around the evolution of convection through the day...its
associated severe weather threats...and the potential for flash
flooding. Of secondary concern is the ongoing dense fog as well as
some potential for southeastern NH and far SW Maine to reach Heat
Advisory thresholds before the convection arrives.
Through daybreak: Warm advection pattern continues...but with
theta-e surge now moving north of the region we/re in no man/s
land in terms of forcing with warm and humid...but
precipitation-free conditions attm. Dense fog continues over
coastal Maine and will not budge through daybreak. Regional
radar composite suggests shower/storm activity will make a run
at the CT Valley by 7-8am in line with recent HRRR runs.
Today: There is a slowly growing consensus in the high
resolution...convection allowing model suite in the convective
evolution for today...with the potential for two rounds of
thunderstorm activity. A subtle pre-frontal trough is evident
over upstate New York...with the surface cold front over the
eastern Great Lakes. Expect convection associated with the lead
trough to arrive late this morning into early afternoon. Hard to
say just how robust this will be...but with MUCAPES already over
1000 J/kg over southern NH...see no reason why isolated
strong/severe storms would not occur. As this feature heads
east...it becomes increasingly WSW-ENE oriented...and becomes
increasingly parallel to the mid level flow...with the cold
front catching up from the west. The details of destabilization
are a bit in question given the initial convection...but early
afternoon HRRR/RAP profiles show MLCAPEs nearing 2000 J/kg with
effective shear reaching 35kts. Shear profiles generally favor a
linear mode /and thus a damaging wind threat/ as they are
largely parallel to the approaching trough and front suggesting
convection will grow upscale. However...expect convection with
the initial pre-frontal trough to initially be more cellular...
which brings a nonzero tornado threat into play...particularly
near the retreating fog deck...given the potentially for locally
backed surface flow and 0-3km shear of 30kts along with ample
llevel CAPE. This threat would thus be greatest over southern
NH and far SW Maine...which lines up well with HREF UH
probabilities and SREF STP probabilities. As the event
continues...the wind damage threat will largely dominate with
the potential for bowing segments. This also agrees well with
morning slight risk area from SPC.
Temperatures: West of the fog bank...temperatures currently
reside in the lower to middle 70s and will have no problem
rapidly increasing this morning with some high res guidance
sources indicating that 90F is in reach. Details with convection
reduces confidence a bit and have tempered these higher values a
bit. However...if they are realized...we'll meet heat advisory
thresholds...and will have to watch this through the morning.
Flash Flooding: While the overall trough/frontal motion is
rather fast from west to east at least initially...have some
concerns over southern zones as we reach mid/late afternoon.
This is where convective line will gain more of an E/W component
with training becoming increasingly likely as PWATs move above
2"...warm cloud depths move to near 13kft. This environment
favors torrential downpours...so it won/t take much training for
an enhanced flash flood threat and in collaboration with
ALY/BOX have issued a flash flood watch for southern areas for
the afternoon.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
High Impact Weather Potential: Severe thunderstorm/flash flood
potential quickly wanes this evening with some areas of fog
overnight although dense fog is not expected.
Tonight: Surface cold front looks to reside near the CT Valley at
00Z...pushing quickly east during the evening and offshore after
midnight. Given primary thrust of convection just ahead of the
front...expect severe threat to largely be over by 8pm with some
residual shower and potential storm activity through the evening
before more substantial drying is realized overnight. Without a
real strong push of dry advection through the night...patchy/areas
of fog are again likely. Lows...albeit a bit cooler than tonight
will remain mild...50s north and lower/mid 60s south given that
we/ll be waiting until towards daybreak Wednesday for more
substantial cold advection.
Wednesday: Rising heights...anticyclonic differential vorticity
advection and low level impressive dry advection should yield a much
quieter day across the forecast area as PWATs fall to below -1 sigma
/towards 0.5"/. Expect surface dewpoints to fall through the 50s
during the afternoon /similar to what we're seeing in the western
Great Lakes at this time/. Thus expect mostly sunny skies and with
T8s in the 10-13C range expect highs in the 70s north with lower 80s
over downslope areas of southeastern NH and just inland from the
coast of Maine.
&&
.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Models remain in fairly good agreement tonight on quiet weather for
the latter half of the week and into the weekend as high pressure
remains in control over the northeast.
High pressure will continue to build in from the west Wednesday
night and Thursday...cresting over the region by late in the
day. Looking for mostly sunny skies through the day with highs
ranging from the mid 70s north to the mid 80s south. An
afternoon sea breeze will cool coastal areas of Maine and New
Hampshire.
Clear skies and light winds will allow for good radiational
cooling Thursday night. Lows will generally range through the 50s
from north to south.
High pressure will shift offshore on Friday with return flow
bringing slightly warmer air back into the region although
relative humidities should remain in the comfortable range. Highs
will range through the 80s from north to south. Quiet weather
will continue Friday night as high pressure holds offshore.
Low temperatures will once again range through the 50s.
Weekend outlook still evolving as high pressure forecast to slide
east and an onshore gradient tightens over the area. Looking like
one more nice day for Saturday as high pressure remains nosed in
from the east for most of the day. Highs will range from the mid
to upper 70s along the coast to the mid 80s inland.
Sunday still looking questionable as a warm front sets up south
of New England and slowly begins to move northward.
Increasingly moist onshore flow will likely bring ocean stratus
into the region at some point in the day but models are
differing on timing and extent. Slow moving trough approaching
from the west will likely bring some light precipitation into
the region Sunday night and into Monday as a prolonged period of
unsettled weather shifts in from the west.
&&
.AVIATION /13Z TUESDAY THROUGH SATURDAY/...
Short Term...
Summary: A strong cold front will approach the region today...
move through the area tonight...with high pressure arriving in
its wake on Wednesday.
Restrictions: Highly variable conditions continue at the moment at
the terminals...ranging from VLIFR-VFR but some slight improvement
at some of the sites in the past 1-2 hours. Expect this improving
trend to continue through mid morning with a period of VFR possible
at all sites except RKD before showers/storms /some severe/ arrive
from the west with periods of MVFR and short-lived IFR
restrictions. After the showers and storms depart this evening...
some residual moisture will allow for MVFR/IFR HZ/FG development
before improvement to VFR comes for all sites Wednesday morning
and continues for the day.
Winds: Southerly winds 5-10kts for the day today diminish to less
than 5kts this evening before shifting northwesterly and increasing
to 10-15kts for the day on Wednesday.
LLWS: No LLWS expected through Wednesday.
Lightning: Primary period of thunderstorm activity with widespread
lightning looks to be in the 11am - 8pm range. Isolated activity is
possible before this.
Long Term...
VFR Wednesday night through Saturday.
&&
.MARINE...
Short Term...SCAs over the outer waters late this morning
through this evening given strengthening southwesterly flow and
building waves ahead of approaching cold front. Winds/waves
diminish for the day on Wednesday.
Long Term...SCA's may be needed over the weekend.
&&
.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Flash Flood Watch until 8 PM EDT this evening for MEZ018-023.
NH...Flash Flood Watch until 8 PM EDT this evening for NHZ008-
010>015.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ150-152-
154.
&&
NEAR TERM UPDATE...KimbleHazardous Weather Outlook National Weather Service Gray ME 1120 AM EDT Tue Jul 17 2018 MEZ018-023-NHZ008-010>015-181530- Interior York-Coastal York-Merrimack-Strafford-Cheshire- Eastern Hillsborough-Interior Rockingham-Coastal Rockingham- Western And Central Hillsborough- 1120 AM EDT Tue Jul 17 2018 ...SEVERE THUNDERSTORM WATCH 278 IN EFFECT UNTIL 7 PM EDT THIS EVENING... ...FLASH FLOOD WATCH IN EFFECT UNTIL 8 PM EDT THIS EVENING... This Hazardous Weather Outlook is for southwest Maine, New Hampshire, central New Hampshire and southern New Hampshire. .DAY ONE...This Afternoon and Tonight. Please listen to NOAA Weather Radio or go to weather.gov on the Internet for more information about the following hazards. Severe Thunderstorm Watch. Flash Flood Watch. Severe thunderstorms are possible today...especially during the afternoon and early evening hours. In addition to very heavy rainfall..the primary severe weather threat is damaging winds. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. MEZ007>009-012>014-019>022-024>028-NHZ001>007-181530- Northern Oxford-Northern Franklin-Central Somerset-Southern Oxford- Southern Franklin-Southern Somerset-Interior Cumberland-Androscoggin- Kennebec-Interior Waldo-Coastal Cumberland-Sagadahoc-Lincoln-Knox- Coastal Waldo-Northern Coos-Southern Coos-Northern Grafton- Northern Carroll-Southern Grafton-Southern Carroll-Sullivan- 1120 AM EDT Tue Jul 17 2018 This Hazardous Weather Outlook is for Maine, south central Maine, southwest Maine, west central Maine, western Maine, central New Hampshire and northern New Hampshire. .DAY ONE...This Afternoon and Tonight. Severe thunderstorms are possible today...especially during the afternoon and early evening hours. The primary severe weather threat is damaging winds. Locally heavy rain is also likely. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. NHZ009-181530- Belknap- 1120 AM EDT Tue Jul 17 2018 ...SEVERE THUNDERSTORM WATCH 278 IN EFFECT UNTIL 7 PM EDT THIS EVENING... This Hazardous Weather Outlook is for central New Hampshire. .DAY ONE...This Afternoon and Tonight. Please listen to NOAA Weather Radio or go to weather.gov on the Internet for more information about the following hazards. Severe Thunderstorm Watch. Severe thunderstorms are possible today...especially during the afternoon and early evening hours. The primary severe weather threat is damaging winds. Locally heavy rain is also likely. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. ANZ150>153-181530- 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- 1120 AM EDT Tue Jul 17 2018 This Hazardous Weather Outlook is for Atlantic coastal waters. .DAY ONE...This Afternoon and Tonight. Areas of dense fog will continue over the waters through tonight. Strong to severe thunderstorms are possible over the waters this afternoon and evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. ANZ154-181530- Coastal Waters from Cape Elizabeth, ME to Merrimack River, MA out 25 NM- 1120 AM EDT Tue Jul 17 2018 ...SEVERE THUNDERSTORM WATCH 278 IN EFFECT UNTIL 7 PM EDT THIS EVENING... This Hazardous Weather Outlook is for Atlantic coastal waters. .DAY ONE...This Afternoon and Tonight. Please listen to NOAA Weather Radio or go to weather.gov on the Internet for more information about the following hazards. Severe Thunderstorm Watch. Areas of dense fog will continue over the waters through tonight. Strong to severe thunderstorms are possible over the waters this afternoon and evening. .DAYS TWO THROUGH SEVEN...Wednesday through Monday. 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
1207 PM EDT TUE JUL 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1125 AM TSTM WND DMG WASHINGTON 43.18N 72.10W
07/17/2018 SULLIVAN NH LAW ENFORCEMENT
WIRES DOWN IN WASHINGTON
&&
EVENT NUMBER GYX1803328
PATTERSONPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAY ME
1211 PM EDT TUE JUL 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1030 AM TSTM WND DMG SALISBURY 43.38N 71.72W
07/17/2018 MERRIMACK NH DEPT OF HIGHWAYS
US 4 CLOSED TREES AND WIRES.
&&
EVENT NUMBER GYX1803329
CURTISPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAY ME
1216 PM EDT TUE JUL 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1010 AM TSTM WND DMG MARLOW 43.12N 72.20W
07/17/2018 CHESHIRE NH DEPT OF HIGHWAYS
TREES AND WIRES DOWN AT JCT RT 10 AND RT 123
&&
EVENT NUMBER GYX1803330
CURTISSpecial Weather Statement
National Weather Service Gray ME
1225 PM EDT TUE JUL 17 2018
MEZ018-023-171700-
Interior York-Coastal York-
1225 PM EDT TUE JUL 17 2018
...A STRONG THUNDERSTORM WILL AFFECT SOUTHWESTERN YORK COUNTY...
At 1224 PM EDT, a strong thunderstorm was located near South Berwick,
or 9 miles south of Sanford, moving northeast at 25 mph.
Winds in excess of 40 mph and pea size hail are possible with this
storm.
Locations impacted include...
Alfred, Sanford, Somersworth, Kennebunk, Berwick, Lebanon, Wells,
South Berwick, York, Eliot, Lyman, Arundel, Ogunquit, North Berwick
and Rollinsford.
This includes Interstate 95 between mile markers 10 and 29.
This also includes...
Ogunquit Beach and Wells Beach.
Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.
LAT...LON 4322 7081 4324 7081 4324 7084 4326 7085
4326 7086 4329 7091 4330 7090 4333 7093
4336 7094 4346 7053 4320 7044 4318 7083
TIME...MOT...LOC 1624Z 247DEG 21KT 4330 7075
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</html>The Boston radar (KBOX), 123 km away, was scanning. The volume nearest the first event began at 16:50 UTC.
1 flash flood. Local standard time, as filed.
Thunderstorms produced 2 to 4 inches of rain in 3 hours causing flash flooding in Manchester. North and Union streets were flooded and cars were submerged on West River Road.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in New Hampshire in 2018Every state in 2018This day in weather historyWarnings in New Hampshire, July 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-30. Each event’s entry at NCEI: 775298.