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Area forecast discussion 
National Weather Service Burlington Vermont
812 PM EDT Tuesday Sep 2 2014

Synopsis...
showers and thunderstorms will push across the north country
through the evening hours ahead of a cold front. Expect locally
heavy downpours and gusty winds with the stronger storms. Sunny,
dry and cooler weather will last into Thursday. Warm Summer weather
returns Friday but will be short lived as another cold front sweeps in
early Saturday giving a taste of Fall-like weather for Sunday and
early next week.

&&

Near term /until 8 am Wednesday morning/...
as of 755 PM EDT Tuesday...cold front moving through northern
portion of Champlain Valley at this time...will continue to make
its way east this evening and exit the forecast area between 04z-
06z. Heaviest precipitation with some embedded thunder along the
cold front...with Post frontal rains across northern New York
associated with trailing middle level shortwave. Main threat this
evening will be heavy rainfall and threat of flash
flooding...mainly from Champlain Valley eastward. Only minor
adjustments made to forecast with latest update.

Previous discussion... well, the atmosphere is fairly primed. Very
warm and humid airmass is resulting in modest surface based
instability (cape pushing 2000 j/kg and lifted index of -2 to
-4c). So far the activity has been sparse across the forecast
area, however radar shows the building line of showers and
thunderstorms out across Ontario. These storms should be crossing
into the United States in the next couple of hours. Still like how
the hrrr and our local 6km WRF are handling the evolution of the
convection. Still looks like an Eta model into the Champlain Valley
during the evening (let's go with 6-8pm) and then into eastern
Vermont by 10pm. Upper level shortwave which is the primary
trigger for the convection is now over Michigan and starting to
lift northeast. Out ahead of that short wave the wind fields and
related shear increase, but at the same time we are getting closer
to sunset and loss of heating. So it's a battle between timing of
the best dynamics vs best instability.

At this point still see nothing to change the idea that we'll have
at least a few strong T-storms around, especially this evening and
mainly west of the Green Mountains. Have maintained the idea of
gusty winds and heavy rain as the primary threats.

The activity winds down by middle evening in the St Lawrence Valley
to just after midnight in eastern Vermont. Depending on how much
clearing we get, we could have a bit of patchy fog late tonight.

The cooler air lags the weather a bit, so it will still be a muggy
night with lows in the 60s.

&&

Short term /8 am Wednesday morning through Thursday night/...
as of 355 PM EDT Tuesday...it will be a quiet 48 hours. Short term
models in pretty good agreement with regard to ridging over the region.

Daily specifics...

Wednesday: flow turns northwest during the morning. 850mb
temperatures fall to 11-12c which will support high temperatures
in the middle-upper 70s. With some weak cold-air advection and a good
deal of sun, we'll have some excellent mixing during the
afternoon. We should be able to tap some of the 20kt or so of wind
that will be a few thousand feet up, so it should get a little
breezy in the afternoon.

Wednesday night: high pressure overhead. Clear and calm, so patchy
fog looks like a good bet, especially since the ground will still
be a bit damp from the rain this evening. Temperatures mainly in
the 50s, though upper 40s possible in cold spots like Saranac Lake.

Thursday: shortwave ridging and surface high pressure still nearly
overhead, but starting to push east of the area. 850mb
temperatures up to 12-13c, which supports upper 70s to low 80s for
highs. Plenty of sun as well.

Thursday night: discarded the NAM as it seems too agressive with
the leading edge of warm air advection and produces thickening
clouds and even some light showers. Think it will remain mostly
clear and dry per other models. However with southwest flow
developing, it won't be as cold, lots of 50s and even low 60s for
lows.

&&

Long term /Friday through Tuesday/...
as of 350 PM EDT Tuesday...extended period remains predominately
quiet...largely dominated by high pressure starting the latter
part of the weekend...but wet to begin. For the start of the
period...rising heights and warming temperatures continue Friday
as an upper ridge crests over the northeast in advance of our next
surface front. Mean 925mb temperatures of +20-23c will support highs well
above normal for early September with widespread 80's expected.
It'll be muggy too with dewpoints ranging through the 60s which
will create ample surface instability to support shower and
thunderstorm development along/ahead of the aforementioned front
which will shift into the btv County Warning Area late in the day and through the
evening. Still looks like the best upper dynamics passes to our
north...with the surface boundary parallel with upper flow
supporting the idea of a slow eastward progression which is
maintained by the latest European model (ecmwf). GFS remains a bit faster pushing
the boundary through the County Warning Area by Saturday morning so have continued
on with the previous forecast thinking leaning towards the European model (ecmwf).

By late Saturday trend will be towards clearer and drier weather as
the upper flow turns zonal and surface high pressure builds
eastward out of the central Continental U.S.. this high then slowly shifts east
through the northeast and off into the Canadian Maritimes by late
Tuesday offering a nice stretch of late Summer weather. Temperatures return
to more seasonable levels to maybe slightly below normal with
expected highs Saturday through Tuesday in the upper 60s to low
70s...and lows in the middle 40s to low 50s.

&&

Aviation /00z Wednesday through Sunday/...
through 00z Thursday...variable flight conditions across the
forecast area this evening as showers and thunderstorms
along/ahead of a cold front advance eastward across forecast area
through midnight. Ceilings will primarily remain VFR but visibility will be
reduced to MVFR/IFR in heavier showers and embedded storms. After
06z...precipitation is east of the region leaving generally cloudy skies.
Ceilings overnight will generally be VFR at kmss/kpbg/kbtv...but MVFR
at krut/kslk/kmpv with possible IFR at krut under blocked flow.
Some patchy fog also possible...especially at kslk/kmpv.
After 12z...all ceilings lift to VFR with scattered-broken diurnal
stratocumulus expected through the day.

Outlook 00z Thursday through Sunday...

00z Thursday - 00z Sat...VFR under high pressure. Early morning IFR
fog possible...especially at kmpv/kslk.

00z Sat - 00z sun...possible MVFR and brief IFR in scattered showers
and thunderstorms.

00z sun - 00z Monday...VFR under high pressure. Early morning IFR fog
possible...especially at kmpv/kslk.

&&

Btv watches/warnings/advisories...
Vermont...none.
New York...none.

&&

$$
Synopsis...Nash
near term...rjs/Nash
short term...Nash
long term...lahiff
aviation...rjs/lahiff

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