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  Thursday July 19, 2018

 

NWS Area Forecast Discussion


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FXUS61 KBTV 171450
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1050 AM EDT Tue Jul 17 2018

.SYNOPSIS...
A cold front moving across the North Country today will produce
showers and thunderstorms. Some storms will contain localized gusty
winds, heavy down pours, and frequent lightning. Much cooler and
drier air arrives tonight on northwest winds and building high
pressure. This trend of cooler temperatures and lower humidity
values will persist for midweek, before some warmer temperatures
arrive by late this week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 1050 AM EDT Tuesday...Cold front now moving into the St.
Lawrence valley with showers developing along it while a area of
prefrontal thunderstorms now moving into southern New England
taking the deeper moisture and best instability out of our
region limiting threat of severe storms. The main forcing for
precipitation will be with the front as it moves through the
area followed by drier air and subsidence. Current stability
analysis shows MLCAPE around 1000 J/kg in the St. Lawrence
valley ahead of the front and expecting this maintain similar
values ahead of the front through mid afternoon. With 0-6km
shear of 25 to 30 knots and limited instability the severe
threat is not zero but marginal at best as indicated in the
latest SPC outlook. Still maintained gusty winds and brief heavy
rains possible with any storms.

Previous discussion...Have adjusted pops/wx and temps/dwpts to
capture current trends. This included to decrease pops across
northern ny into the cpv by mid morning, as radar is showing a
sharp back edge approaching the cpv as of 7 AM. Also, have
reduced the areal coverage of heavy rainfall and gusty winds
this aftn to southern and eastern vt. Still expecting another
broken line of showers/storms to develop across the cpv and
parts of vt ahead of approaching cold frnt later today. However,
some uncertainty on amount of instability due to clouds and
areal coverage of convection because of developing dry slot.
Thinking pockets of instability develop and this combined with
height falls and better jet dynamics will help in development of
a few additional storms, with isolated wind gusts and brief
heavy rainfall possible. Temps with some clearing should warm
back into the upper 70s to mid 80s.

Water vapor shows digging mid/upper level trof across the
central Great Lakes with deep moisture and embedded short waves
in the southwest flow over the NE Conus. Forecast challenge
today will be evolution/timing of convection ahead of cold front
and potential impacts. Thinking the overall threat for severe
is minimal with a few stronger storms possible over
central/southern VT this aftn. Once again, system is coming thru
in parts, which combined with plenty of mid/upper level clouds
will limit instability and impacts across our cwa today. Water
vapor shows best forcing associated with pre-frontal 5h vort
moving across our cwa thru 14z this morning...with elevated CAPE
values btwn 800-1200 j/kg. Also...sfc temp still 80 degrees as
of 3 AM this morning.

Meanwhile... subsidence/dry slot behind initial moisture surge
will develop from west to east across our fa, which will cause
some breaks to occur in the overcast and allow sfc temps to warm
back into the upper 70s to mid 80s. This will create pockets of
sfc based cape values of 1200 to 1500 j/kg mainly along and
east of the cpv btwn 18z-21z, with developing 0 to 6 km shear
values of 30 to 40 knots. The shear is mainly driven off
approaching 70 to 90 knot right rear quadrant of an upper level
jet with digging mid/upper level trof. However, best short wave
forcing is lagging across the northern Great Lakes while highest
pw axis is quickly shifting into southern New England. So
bottom line expecting a few stronger pulse type storms to
develop over our central/southern cwa, which will become better
organized with greater severe weather potential just south of
our cwa.

Have continued to mention likely to cat pops with gusty
winds/heavy rainfall in grids, but not overly concerned about
either element given synoptic and mesoscale setup. Some of the
stronger convective elements will be capable of torrential
downpours, but anticipated storm motions of 25 to 30 mph and
recent dry spell will limit impact. Expect qpf generally in the
0.40 to 0.75 range with isolated higher amounts possible in
areas that received several rounds of convection. Temps are
tricky with such a warm start to the day, but rain cooling
values this morning, but additional breaks develop this aftn,
allowing temps to warm back into the upper 70s to mid 80s.
Thinking dry slot will produce enough breaks in the overcast
prior to fropa to cause a quick bump in midday temps, before caa
develops by sunset.

Tonight, any lingering showers/storms will exit our cwa by 00z as
axis of deeper 850 to 500mb moisture shifts south. Moderate low
level caa develops on northwest winds of 5 to 15 knots...as
mid/upper level trof deepens. The forecast challenge will be if any
fog/br can develop, especially with anticipated recent rainfall.
Thinking the combination of gradient winds/mixing and potent s/w
energy will produce enough lift for some clouds to redevelop
overnight and limit fog development. Temps will range from the upper
40s to lower 60s overnight.

Mid/upper level trof axis will be overhead on Weds along with
building 1020mb high pres. Expect cooler temps with some fair wx
cumulus clouds to develop, especially over the trrn during peak
heating. Temps mainly in the 70s with light northwest winds and much
lower rh values.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY/...
As of 338 AM EDT Tuesday...Fair weather continues
through the middle of the week as high pressure slowly crests over
the North Country. Expecting mostly clear skies and light winds
Wednesday night, and this combined with any lingering low-level
moisture from Tuesday`s rains may allow for patchy fog to develop
late Wednesday night into early Thursday, mainly in sheltered river
valleys. Lows will be in the lower to mid 50s in the larger valleys,
while the mountain areas will drop down into the 40s, with a few
upper 30s possible in the Adirondacks. Thursday will see lots of
sunshine once any fog burns off with highs in the mid 70s to lower
80s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 338 AM EDT Tuesday...High pressure will continue to
dominate the region through Saturday, keeping the weather dry
into the weekend. Temperatures will remain warmer than normal
with highs in the mid to upper 80s and lows in the mid 50s to
mid 60s. The next chance of rain begins to threaten on Sunday as
an upper low pinwheels across the Great Lakes. This will place
New England under deep south flow, which will usher moisture
back into the region. This low will meander over the Great
Lakes/Ohio River Valley as it gradually becomes an open wave
through early next week, keeping the threat of showers and
thunderstorms around into Tuesday. Temperatures will remain near
or slightly above normal for late July.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
Through 12Z Wednesday...Challenging next 6 to 12 hours in
aviation as timing of changeable conditions associated with
convection is the main concern. Radar shows scattered activity
across the cpv and parts of vt with some brief periods of mvfr
vis/cigs expected in the heavier reflectivity elements thru 14z.
Additional thunderstorm activity expected this aftn, mainly
along and east of the CPV with brief mvfr/ifr vis/cigs possible.
Some areas of enhanced turbulence and low level wind shear is
possible in and near thunderstorm activity today, along with
gusty outflow of 25 to 30 knots. Winds shift to the northwest by
late aftn with vfr prevailing into the evening hours. Thinking
mainly dry air advection on 10 to 20 knot winds around 500 feet
agl will limit areal coverage of fog/br overnight. Maybe a 1 to
2 hour window near sunrise at slk of some ifr conditions on Weds
morning.

Outlook...

Wednesday: VFR. NO SIG WX.
Wednesday Night: VFR. Patchy BR.
Thursday: VFR. NO SIG WX.
Thursday Night: VFR. NO SIG WX.
Friday: VFR. NO SIG WX.
Friday Night: VFR. NO SIG WX.
Saturday: VFR. NO SIG WX.

&&

.EQUIPMENT...
SLK ASOS has a comms issue with no obs being transmitted since
2:51 PM on Monday. ET`s will examine the problem today.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...None.
NY...None.

&&

$$
SYNOPSIS...Taber
NEAR TERM...Taber/Sisson
SHORT TERM...Hastings
LONG TERM...Hastings
AVIATION...Taber
EQUIPMENT...WFO BTV



 
 
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