Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 300200

Area Forecast Discussion
National Weather Service Burlington VT
1000 PM EDT Fri Jul 29 2016

Canadian high pressure will bring dry weather tonight through
Saturday. High temperatures on Saturday will range from the upper
70s to lower 80s, with pleasantly low humidity levels. The next
upper level disturbance approaching from the Great Lakes will
bring the next chance for showers for Sunday into Monday, possibly
lingering into Tuesday along with cooler temperatures. High
pressure Wednesday through Friday will bring warming temperatures
and mainly dry weather.


As of 1000 PM EDT Friday...A pretty nice mid-summer evening with
synoptically quiescent conditions in place, as ridge of high
pressure continues to build into the North Country. Based on METAR
data trends showing sizable T/Td spreads of 10 to 15 degrees and
some weakening northerly flow, I`ve largely just shifted the onset
time of radiational river valley fog back by a few hours with this
update. Think the earliest start time would be after midnight
given the still rather dry current conditions. Expect temps to
continue to fall pretty sharply through the next couple of hours
as winds go light to calm. Lows in the 50s to around 60 appear on
track, with a few upper 40s possible in the Adirondacks.

Previous near-term discussion issued at 415 PM Friday follows...

Quiet weather conditions tonight with light N-NW winds and mostly
clear skies. Should see min temps a bit cooler than recent days
with advection of lower dewpoints/clear skies/good radiative
cooling. Lows mainly in the 50s to around 60 near relatively warm
Lake Champlain. Looks like a good night for radiation fog to
develop around midnight in the favored river valleys of central
and eastern VT, and also across the valley locations within the
northern Adirondack region.

Pleasantly dry conditions for Saturday with weak surface high
pressure across southern Quebec providing light north winds and
sfc dewpoints in the low-mid 50s. Model RH, cloud fields, and
simulated satellite all show some areas of high clouds from time
to time through Saturday so skies will partly to mostly sunny. 850mb
temperatures of +11 to +14C depending on the model of choice but
all should translate to afternoon highs in the upper 70s to lower
80s. Winds light and variable.


As of 415 PM EDT Friday...12z guidance not all that consistent,
but consistent enough to indicate that there will be period of
showers (perhaps a steady moderate rain) sometime between late
Sunday into Monday, all in association with a trough swinging
through. That trough and associated energy is currently back
across Nebraska. Timing differences between the models, with ECMWF
being the fastest (showers by Sunday morning), the GFS with it`s
showers coming in during the afternoon, and NAM the slowest with
nothing really until early evening. Stuck a little closer to the
GFS as it seemed like a good compromise. Thus painting in
increasing chances of showers from southwest to northeast during
the day.

Some interesting signals in the models for late Sunday/Sunday night.
All models show some precipitation "bullseyes" where they indicate a
small area of locally heavy rain. Doesn`t appear to be convective
feedback, as models keep instability very minimal during the period.
However over the course of 18 hours or so, QPF adds up in the
models. GFS has a swath of 2-3" from south-central VT back across
central NY. ECMWF has 2-3" almost everywhere across VT, with lesser
totals west. NAM has 1-2" from central VT extending southwestward.
Great variability in the model depiction. Think this is due to how
each model handles the shortwave energy, which appears to come
through in pieces, combined with convergence aloft around the 700mb
level. Soundings do indicate a fairly juiced atmosphere, with
Precipitable Water values around 1.5", fairly deep warm cloud depth
(11-12k ft) along with rather weak winds, with 850mb trending
southeasterly (hint of atlantic moisture tap). Bottom line, could
see some downpours, perhaps Sunday night.

For precip totals in the forecast, I went with an overall model
blend including output from 00z model suite. End up with rainfall
totals of 1-1.25" across South-Central VT. Lower amounts the further
north you go -- just 0.25-0.33" or so up along the Canadian border.
At this point don`t expect any flood issues. Ground in that part of
the state is very dry (borderline moderate drought), and as long as
the rain falls over several hours, the ground will absorb it.
However, something that will need to be watched in later forecasts.

As mentioned above, instability is very minimal, so primarily rain
showers with only isolated rumble of thunder expected. Precipitation
will be slow to end on Monday, with lots of clouds around.
Temperatures Sunday 70s to perhaps lower 80s (depends on how quick
the showers get in). Will be on the cool side Monday with all the
clouds, just 70s.


As of 415 PM EDT Friday...okay agreement between 12z suite of GFS
and ECMWF. Both show a weak shortwave moving across early on
Tuesday that should result in a few showers first thing in the
day. Ridging aloft develops mid/late week, though ECMWF is much
more pronounced with it. GFS is, like the 00z run, showing a
flatter ridge. Even so, it should be dry Wednesday and Thursday.
Perhaps an isolated shower in some of the higher terrain, but have
PoPs less than 20% both days. A front will be approaching on
Friday, though at this point the 12z runs keep us dry. With 00z
runs having a slightly faster timing, have kept some 20-30 PoPs in
for Friday, in the event later models speed up again.

Probably the most noticeable aspect of the weather next week will be
the increasing heat. GFS has 925mb temperatures of 20-22C on
Wednesday and increases it to 24-26C by Friday. ECMWF is a little
cooler. Thus looking at highs well into the 80s to even lower 90s by
Thursday and Friday. 25C at 925mb typically translates to 35C (95F)
at lower elevations.  Not ready to go that warm yet, but followed
the previous forecasters idea of upping high temperatures a few
degrees from the base model blend.


.AVIATION /02Z Saturday THROUGH Wednesday/...
Through 00Z Sunday...Mainly VFR except local LIFR/VLIFR in FG at
KMPV/KSLK 06-12z Sat. Afternoon cumulus clouds dissipating with
skies becoming mostly clear overnight. Locally dense radiation
fog at KMPV/KSLK by about 06z and should be gone by 12-14Z Sat.
Some cumulus at 3500-4500 ft aft 14z Sat with some high clouds
moving in from the southwest in advance of a warm front aloft.

Light to calm wind overnight becoming light northwest during the
day Saturday.

Outlook 00z Sunday through Tuesday...

00z Sun-00z Mon: Mainly VFR, possible MVFR in evening showers
with approaching weak low pressure.
00z Mon-12z Tue: Areas of MVFR in showers. Local IFR possible
with weak low pressure.
12z Tue through Wednesday: Becoming VFR with building high




NEAR TERM...Loconto/Sisson
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