384
FXUS61 KGYX 282013
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
413 PM EDT Tue Jul 28 2026

.WHAT HAS CHANGED...
Forecast rainfall has increased and consolidated across
southern New Hampshire through Thursday, with a building flash
flood threat.

&&

.KEY MESSAGES...
1. Showers and thunderstorms will produce heavy downpours
through early evening across southern and central New Hampshire.
Lingering or training storms may produce localized instances of
flash flooding.

2. A cut-off low develops over the region Tuesday night and
Wednesday and remains in the vicinity through Friday.  This will
bring multiple rounds of rainfall with locally heavy rainfall
possible resulting in isolated to scattered flash flooding.

3. Some periods of drier weather this weekend with warmer
temperatures, but daily chances for precipitation continue,
especially Sunday.

&&

.DISCUSSION...
KEY MESSAGE 1 DESCRIPTION...

Showers and thunderstorms are underway across much of New
Hampshire this afternoon as a moist airmass moves into daytime
instability.

The environment has a number of components supporting heavy
rainfall rates through early evening: high PWATs, low steering
flow, and ample instability. PWAT values around 1.5 to 2" are
being supporting by southerly vapor transport ahead of closed
low exiting the Great Lakes. The broad nature of the low lacks
a strong LLJ at this time, thus low shear and storm motion is
expected. This overlap with daytime instability and generous
dewpoint temps means showers and storms will continue to develop
and wash out into early evening.

The primary threat with these storms will be lightning and heavy
downpours, where repetitive training or slow storm motion could
result in isolated flash flooding. Urban areas will also be
susceptible due to poor drainage should these heavy storms roll
through. Rain rates at area weather stations have generally
been 2 to 4 inches per hour. This is expected given the
environment. The good news is the lack of shear has allowed most
storms to rain into their updrafts, reducing their longevity.
Thus while these rates are concerning, they haven`t been
sustained long enough to overcome otherwise high flash flood
guidance in the region. Will continue to monitor the area
through early evening, as repeat rounds of heavy rainfall over
recently saturated ground will tend to increase this threat.

The large, dynamic system in the region will slowly pivot
eastward overnight. Guidance today has ramped up QPF in New
England overnight, with a pivoting band of forced rainfall
occurs just west of New Hampshire overnight. This will slowly
near western New Hampshire through the early morning hours
Wednesday. Across the rest of the forecast area, onshore flow
increases and will continue to advect moisture inland. It is
expected that stratus will fill in, with fog developing along
the coast and interior overnight.

KEY MESSAGE 2 DESCRIPTION...

Cutoff low pressure will move across the Delmarva on Wednesday
morning to southern New England on Thursday before slowly exiting to
the east over the Gulf of Maine by Friday. This will continue to
bring periods of moderate to locally heavy rainfall to the region
with the most impactful period expected to be on Wednesday.
Temperatures will generally be below avg through the end of the week
given the onshore flow combined with clouds and rainfall.

The H85 SE LLJ will increase during the day on Wednesday, which may
allow for some backbuilding of convective showers within an expected
favorable environment for heavy rainfall. The latest 12Z HREF
ensemble mean 24-hr QPF through 00Z Thursday ranges from between 1-
2" across much of south-central NH with less than an inch elsewhere.
Portions of northwestern ME may mot see much rainfall during the
day. As a result, an isolated to scattered flash flood threat
exists, especially where cells do manage to back build and fall
across both the urban corridors and the mountainous terrain of
NH. River flooding is unlikely given the large available storage
space but rises on rivers and streams can be expected. High
temperatures will be mainly into the 70s but high dew points
will make it quite muggy.

Periods of showers and embedded downpours will continue on
Thursday, with the highest rainfall amounts expected across
south-central NH and extending into southwestern ME. The showers
will be very slow moving with backbuilding possible, and
therefore a flash flood threat will remain... especially given
that soils will likely be already saturated in some locations.
Flow will become more northeasterly on Thursday night, with a
diminishing flash flood threat.

Total rainfall is currently expected to range from less than an inch
across northwestern ME and into northern Coos county NH with
generally between 1-3" of rain further to the south. The 12Z HREF
LPMM does show the potential for localized amounts of up to around
5", with the areas of greatest concern for these amounts across the
Whites as well as across southern NH.

The cutoff low will finally exit to our east on Fri, allowing
for brightening skies and high temperatures of around 80
degrees.

KEY MESSAGE 3 DESCRIPTION...

Saturday should feature drier conditions as the cutoff low
dissipates over the Gulf of ME. Highs will be into the lower 80s
with partly cloudy skies. Sunday into early next week looks to be
potentially unsettled again at times as a series of disturbances
cross down from the north, allowing for scattered showers and
storms. This will help to keep temperatures near seasonable avg
with no significant heat in sight as we head into the month of
August.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Through 18z Wednesday...SHRA and TS will continue across NH
terminals this afternoon and early evening. Decreasing coverage
is expected after 00z. These will bring brief IFR conditions,
mainly due to very heavy downpours lowering vis. Primary
overnight impacts will be broadening IFR stratus, with fog late
until early morning. Lowest ceilings will be towards the
interior and foothills where LIFR conditions are expected. Fog,
potentially dense, develops across much of coastal and interior
Maine.

Ceilings tend to remain IFR Wed morning, with RA increasing
across NH west to east.

Outlook:

Wednesday-Thursday: Periods of RA/TSRA combined with low
ceilings will bring IFR or worse restrictions. Winds will be
primarily out of the SSE at 5-15 kts, gusts to 25 kts. Locally
stronger winds are likely within any thunderstorm.

Friday-Sunday: Expect conditions to slowly improve on Friday as
low pressure moves away from the region though scattered showers and
a thunderstorm are possible.  VFR conditions should dominate
Saturday outside of an isolated afternoon shower/storm. Winds at
or below 10kts.

&&

.MARINE...
Broad low pressure moves east into New England tonight. Onshore
flow will result in abundant marine stratus and vis reducing
fog tonight into Wednesday.

SCA conditions likely Wednesday evening through early Fri
with S-SE wind gusts outside of the bays up to 25 kts with seas
of 3-7 ft.

&&

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

&&

$$

DISCUSSION...Cornwell/Tubbs