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  Thursday September 20, 2018

 

NWS Area Forecast Discussion


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000
FXUS61 KBTV 191924
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
324 PM EDT Wed Sep 19 2018

.SYNOPSIS...
Morning clouds will give way to partial sunshine this afternoon
as high pressure and it`s cooler airmass settle south across
the region. Temperatures gradually warm by Thursday into Friday
before a strong cold front and associated showers push through
the area Friday evening. Behind this front cooler and drier
weather are expected for the upcoming weekend.

&&

.NEAR TERM /THROUGH THURSDAY NIGHT/...
As of 1023 AM EDT Wednesday...Forecast for the rest of today
remains in excellent shape with the main idea of clouds
gradually scattering out and temps rising into the 60s still
looking good.

Previous Discussion... Variable cloud cover continues to advect
slowly south into the forecast area this morning behind a
departing early fall cold front. A secondary, back door front is
also pushing southwestward across eastern New England and will
reach the CT River Valley by sunrise with additional clouds and
perhaps spotty drizzle under a more maritime airmass. The
forecast challenge today will be how soon these clouds mix out
and the degree to which boundary layer warming governs
temperature maxima. I feel the NAM output continues to
exaggerate the strength of the frontal inversion and it`s much
cloudier/cooler thermal profiles are too pessimistic, especially
west of the Greens. Time will tell but for now have leaned more
toward a model-blended solution showing skies trending partly
sunny by afternoon in most areas with temperatures ranging
through the mid and upper 60s under modest boundary layer
mixing. It is September, not December.

By tonight weak surface high pressure bridges into the area with
large scale height rises aloft. Winds will be light north to
northeasterly, with occasional clouds being most prevalent across
eastern VT counties under lingering maritime airmass affects. Again,
some spotty drizzle may be possible in these areas. Low temperatures
a blend of mainly bias-corrected guidance ranging through the 40s to
around 50, though locally milder near Lake Champlain.

By Thursday surface to mid level ridging continues across the area
with light north to northeasterly winds tending to veer more
east/southeasterly toward evening in advance of our next system.
Partly sunny skies should be the rule with some broad increase in
mid to upper level cloud cover across northern NY toward day`s end.
Temperatures to moderate slightly with max values mainly from the
mid 60s to lower 70s.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
As of 323 PM EDT Wednesday...A low pressure system will track from
the northwestern Great Lakes Region to central Quebec Friday,
pushing a series of fronts through New England that will control the
short term forecast. Plenty of baroclinicity associated with
the early fall system, so should see some impressive temperature
advection over the area Friday. The cyclone`s warm front will
cross the forecast area Thursday night through Friday morning,
spreading some rain through the North Country as it does so.
Very little dynamic support in the upper-levels over our area
(much more impressive further north...), but a broad area of
synoptic-scale ascent from isentropic lift will ensure that most
areas see at least some measurable rain. Given the strength of
the impressive 50+ kt LLJ within the warm-sector of the cyclone,
QPF will vary greatly with terrain. The Saint Lawrence Valley
should see the most precipitation under this setup, while much
of the Champlain Valley will get rain- shadowed, resulting in
significantly lower QPF amounts. Overall for many areas, should
be a higher PoP/lower QPF scenario. Some marginal (<500 J/kg)
elevated CAPE will be present over northern New York, so an
isolated rumble of thunder Friday morning can`t be ruled out.

Winds Friday will be quite gusty from the south/southwest. Strongest
winds will be at highest elevations and in channeled valleys.
Areas of the Saint Lawrence Valley and the Champlain Valley will
see gusts in excess of 25 mph during the afternoon hours as
higher momentum air within the LLJ is mixed to the surface.

High temperatures Friday will be highly variable across the CWA.
Strong warm air advection will allow temperatures to climb into
the low 80s in the Saint Lawrence Valley. The Champlain Valley
should also warm up quite a bit, with highs near or at 80
degrees not out of the realm of possibilities with strong
downsloping southwesterly winds in place. Areas east of the
Green Mountains, however, will be noticeably cooler, topping out
in the upper 60s to low 70s.

Friday night, the system will sweep a strong cold front through
the area, reinvigorating shower activity. Better dynamics aloft
associated with this frontal passage, however best upper
support does lag quite a bit behind the frontal passage. Given
the overnight timing of the frontal passage and the subsequent
lack of much instability, along with the offset of surface
forcing and upper-level support, not overly impressed at this
point with thunderstorm potential. If any thunderstorms are able
to develop along the front however, the strong shear/flow aloft
could support some gusty winds reaching the surface. Once
again, best chances of an isolated thunderstorm or two would be
in the Saint Lawrence Valley, where the earlier timing of the
frontal passage would line up with better instability.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 323 PM EDT Wednesday...Large scale pattern shows confluent flow
aloft with general trof across the ne conus as large 1028mb
high pres extents from northern Great Lakes into northern New
England. Overall this pattern supports below normal precip
chances and temps thru early next week. The ECMWF shows a
secondary surge with passage of weak boundary on Sunday, along
with progged 850mb temps dropping to 0c near the international
border. Moisture is very limited, so only expecting clouds
associated with increasing 850 to 500mb rh fields, while best
upper level forcing associated with 5h vort stays north of our
cwa. These northern stream fronts with limited moisture
typically have limited moisture with a wind shift and cooling
low level thermal profiles. Overnight low temps during this time
period will be challenging based on winds with advection and
potential clouds. Have kept very close to guidance, but values
could be 4 to 8 degrees lower if more clearing and lighter winds
develop. Have kept very close to superblend values for days 4
thru 7. This supports highs mainly in the 60s and lowers in the
mid 30s to mid/upper 40s depending upon location, warmest values
near the Lake Champlain. A warming trend with increasingly
moisture arrives on Tues into Weds of next week. Still plenty of
uncertainty on timing of moisture and magnitude of waa.

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
Through 18Z Thursday...Very tricky aviation forecast for the
next 24 hours as despite high pressure building overhead, an
abundance of low level moisture and possible developing
inversion overnight will make for variable cloud cover with the
potential for lots of MVFR/IFR. Highest confidence in the
forecast is this afternoon and early evening where mainly VFR
is expected with current areas of MVFR ceilings lifting to VFR
over the next couple of hours. VFR persists through sunset
before the low level inversion begins to develop trapping low
level moisture at the surface. Beyond 02Z confidence is low as
to the expected ceiling height where currently the forecast is
more pessimistic with ceilings lowering to a mix of VFR to IFR
with areas of FG/BR possible. If low flight categories do
develop, they`ll likely last into mid-morning Thursday before
lifting to VFR by 16-18Z. In regard to winds, should be
generally north to northeast 5-10kts this afternoon, and become
light and variable overnight.


Outlook...

Thursday Night: VFR. Chance SHRA.
Friday: Mainly VFR, with areas MVFR possible. Chance SHRA, Slight
chance TSRA.
Friday Night: Mainly VFR, with areas MVFR possible. Likely SHRA.
Saturday: Mainly VFR, with local MVFR possible. NO SIG WX.
Saturday Night: VFR. NO SIG WX.
Sunday: VFR. NO SIG WX.
Sunday Night: VFR. Slight chance SHRA.
Monday: VFR. Slight chance SHRA.

&&

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

&&

$$
SYNOPSIS...JMG
NEAR TERM...JMG/Lahiff
SHORT TERM...Taber
LONG TERM...Taber
AVIATION...Lahiff



 
 
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