Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Gray ME
1209 PM EDT Friday Jul 31 2015
high pressure will build in from the west today...providing a day
with very little chance of showers or thunderstorms for a change.
It will still be warm today...but noticeably less humid than
previous days. A weak cold front will cross the area Saturday...bringing
another threat for showers and thunderstorms. Behind this front we
will see another drop in temperatures and humidity. Weak low
pressure aloft...situated over Quebec...will bring a threat for
showers or storms Monday and Tuesday. Cooler air moves for the
middle to end of next week.
Near term /until 8 PM this evening/...
1202 PM...minor estf update to reflect satellite trends and to
ingest current mesonet into short term grids.
925 am...quiet day on tap with westerly flow allowing even the
coast to warm well into the 80s today. Only minor adjustments for
this estf update to reflect the 13z mesonet in 1st period grids.
update: lowered sky cover somewhat and allowed patchy fog to
continue through 12z based on early morning observations.
Otherwise...no other changes needed to the going forecast.
Previous discussion below...
High impact weather potential: patchy dense fog through early
Current pattern: Hudson Bay low has become re-established as of this
writing as potent shortwave and associated surface low have become
vertically-stacked and stationary in this region. South of this
feature a cold front is draped from Quebec southwest along the East
Coast of the United States. Behind this feature...early morning
water vapor imagery reveals substantial midlevel drying is taking
place...with llevel drying occurring just a tad slower in a broad
subsident regime. Looking upstream...additional low-middle level
moisture can be seen associated with a weak shortwave over the
northern Great Lakes pinwheeling east. In the Post-frontal
airmass...a generally quiet day is expected today...with forecast
concerns centered around the aforementioned upstream shortwave and
it/S implications for cloud/shower potential this afternoon.
Fog: a handful of locations have seen visibilities drop below a mile early
this morning. Expect visibilities to improve towards daybreak as llevel
dry advection continues...with dewpoints falling through the 60s and
into the upper 50s.
Clouds/precip: middle level shortwave now crossing from Ontario into
western Quebec will race rapidly east-northeast today with an uptick
in qg forcing for ascent spreading across northern New Hampshire and northwest Maine
early...but then continuing east with our region in anticyclonic
DPVA for the afternoon. Model soundings show a non-zero buildup of
cape during the afternoon with the relatively cool temperatures
aloft... however...substantial capping will be in place in the
h6-7 layer. Thus...have a hard time seeing much if any rain showers
activity developing...with perhaps a sprinkle in the mountains
during the afternoon. A much better bet is some increase in
cloudiness by afternoon. Given the lower dewpoints...it will be
very pleasant day to end July.
Temperatures: 800 mb temperatures are slowly dropping in the Post-frontal
airmass...but will remain in the middle teens...supporting highs well
into the 80s particularly in the downslope regimes...with less
impacts from any sea-breezes given decent westerly gradient.
Short term /8 PM this evening through 6 PM Saturday/...
high impact weather potential: threat for an isolated severe
storm particularly northern areas on Saturday.
Pattern/implications: cyclonic flow aloft will dominate the pattern
throughout much of the column through the short term forecast period
with one primary disturbance in this cyclonic flow pushing into the
area on Saturday. Forecast challenges primarily center on this
feature...and it/S ability to bring showers and thunderstorms to the
Friday night: good flow off the deck and a drier evening airmass
suggests much less fog potential overnight...especially after a day
that should be free from rain. Expect some increasing middle and upper
level cloudiness from upstream shortwave late...with a partly cloudy
night on tap throughout the region. With the drier llevel
airmass...a cooler night looks to be in store...with lows ranging
from the middle 50s north to the lower 60s coastal areas and southern
Saturday: robust shortwave rotates into the region on Saturday with
associated middle level height falls overspreading the region. This qg
forcing will be working with modestly increased moisture as compared
to Friday as precipitable waters move back to around one inch /right around normal
for the first day of August/. Low and middle level lapse rates are
decent...assisted by middle level cool pool to the north of the region.
Thus...fully expect showers to expand across the region during the
morning as daytime heating leads to growing llevel instability.
As for the thunder threat...instability is marginal as surface
dewpoints struggle back to the 60 degree mark. In addition...there
is plenty of dry air aloft...which...when combined with Stout deep
shear /0-6km bulk shear 40-50 kts/ could potentially portend a
hostile environment due to entrainment. That said...with enough
instability...any storms that develop would have access to ample
shear and lower freezing levels than of late...and thus a
conditional severe threat. Best moisture resides further
north...which agrees well with Storm Prediction Center/S introduction of a marginal
risk over Coos County New Hampshire and much of northern Maine. Will
introduce mention of this potential in the morning hazardous
weather outlook. Temperatures will likely be a tad cooler than on
Friday...particularly further north where the greatest cloud cover
and precipitation coverage is expected. Expect temperatures to
range from the 70s in the mountains the Lower/Middle 80s over
southwest Maine and southern New Hampshire.
Long term /Saturday night through Thursday/...
500 mb continues to tend back toward a more +pna
pattern...although skewing more towards an Omega block by middle to
late week with troughing firmly ensconced over James Bay and northern
Quebec. This trend will lead to an unsettled pattern through the
period...with the threat for shra/tsra...highest in the northern
zones. However...fast flow will keep things moving and prevent
any significant rain events.
Sunday will see partly sunny skies and chance for rain showers/thunderstorms and rain in the
mountains Monday afternoon into Tuesday look like the best chance for County Warning Area wide
rain showers/thunderstorms and rain as troughing sags S...and pulls a cold front through northern
New England. Wednesday /Thu will keep US on the cooler side of the
front...as it stalls across the middle Atlantic. Some mountains showers
will be possible as upper level trough and associated cold pool will
be just to the north of the County Warning Area.
Temperature will Sunday will be near normal with highs in the middle
70s to middle 80...but will climb back above normal on Monday with
highs in the 80s...and approaching 90 again in interior southern New Hampshire.
Overnight lows Sat and Sun night will range from the upper 50s to
middle 60s...and Monday night will be more muggy with lows in the 60s
to around 70. Temperatures may fall to slightly below normal by middle to
late week. Stayed closer to the Euro though as the GFS is probably
overdoing the cool air incursion late in the week.
Aviation /16z Friday through Tuesday/...
short term...VFR conditions expected to dominate the coming taf
period. Expect few/scattered cumulus to develop during the day today...with a
transition to more middle level cloudiness tonight ahead of an
approaching middle level disturbance.
Only threat for restrictions through daybreak Saturday will be in
the form on fog tonight at leb/hie. Fog will be less widespread and
dense than it was this morning given drier llevel airmass...moderate
winds aloft...and no rain during the afternoon hours. Thus...will
only carry some MVFR restrictions in the hours leading to daybreak
at these sites.
Winds: westerly winds will increase to 10g18kts for the day today
before diminishing to 5kts or less tonight.
A middle level disturbance will bring scattered showers and
thunderstorms on Saturday...particularly for our northern New Hampshire
terminals. Brief MVFR/IFR restrictions possible in any of these
showers and storms with conditions remaining VFR elsewhere.
Long term...mainly VFR Sat night through Tuesday... with valley fog
possible each night. Scattered rain showers/thunderstorms and rain will impact most of the
terminals late Monday and Monday night.
short term /through Saturday/...winds/waves look to remain below
small craft levels through the short term forecast period under
generally light southwesterly winds.
Long term....persistent SW flow Sunday and Monday could bring
swell to 5 feet or so by Sunday night....which would linger through