Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 241440
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1040 AM EDT Mon Jul 24 2017

.SYNOPSIS...
Upper low pressure will cross the area today into tonight with
widespread rainfall and cool temperatures before exiting east on
Tuesday. High pressure brings a brief stretch of fair weather
on Wednesday before a weak cold front brings a renewed threat of
showers and storms on Thursday. Fair weather then returns for
Friday into next weekend with temperatures near seasonal late
summer norms.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 1010 AM EDT Monday...no substantial changes for the mid-
morning update. Folded in latest satellite, temperature and
radar trends. Like how the HRRR and BTV4 WRF model are handling
the "now", so blended in their output for the next 6 to 12 hours
as well.

Issue the next few hours will be threat for additional flooding
in southern parts of St Lawrence County. Radar estimates
indicate a few locations in excess of 6" since 1am this morning
within a 40 mile long/10 mile wide east-west oriented heavy
rainfall footprint. This seems quite reasonable given 5.65"
measured at the NY State Mesonet site in Hammond as of 1005am,
and a few other mesonet sites in the area showing almost 4".

Still have another round of moderate to briefly heavy rain
approaching the southern parts of St Lawrence Valley as I type
with rainfall rates about 0.2 to 0.4" per hour. Lightning data
show electrical activity with that convection, though most of
the lightning is just south of the region that has had the most
rain. Hopefully it stays that way.

All of this precip is thanks to a vigorous shortwave embedded
in a negatively tilted trough swinging across the eastern Great
Lakes right now. GOES-16 mid-level water vapor shows that
shortwave very nicely. That puts our region on the eastern
diffluent side of that trough. That shortwave will be reaching
the St Lawrence Valley mid-afternoon. Expect that as happens,
chances for heavy rain will start to diminish as drier air aloft
moves in. Precipitable water values fall to <1.0" with and
behind that trough, compared to the current 1.5" across the
region per latest GPS-Precipitable Water measurements in
Burlington, Plattsburgh and Watertown.

Clouds and precipitation will keep temperatures pretty much
steady where they are. With moist east/southeast flow, did knock
highs down a few degrees, especially eastern Vermont.

Chances of thunder seem minimal for this afternoon. All models
indicate the weak surface based and instability aloft will
remain from the southern St Lawrence County and extend south. As
such, have kept slight chance of thunder only for southern parts
of St Lawrence, Franklin and Essex Counties in NY. Those are
also the areas at most risk for locally heavy rainfall for the
next few hours.

Don`t mean to ignore Vermont. For the most part, it will be on
and off light to occasionally moderate showers all day. With
east/southeast flow, there is a little bit of downsloping along
western slopes of the Green Mountains. Max gusts I`ve seen have
been 25-30 mph. Don`t anticipate anything more than that since
there is only 25 knots or so at 850mb.

Prior discussion from the midnight shift forecaster for tonight
and Tuesday...


By tonight showers will remain quite abundant in coverage,
though wane in overall intensity as the system continues to
pivot slowly east. Patchy br/fog also a good bet just about
anywhere as temperatures hold in the 50s. Winds light.

On Tuesday widespread morning showers gradually move east
and/or fade away as longwave trough exits east and building high
pressure advances east from the Great Lakes and Ohio Valley.
Plenty of clouds should be the rule most of the day, though some
partial sunshine will be possible from the Champlain Valley
west by later in the afternoon. Temperatures remain on the cool
side in the 60s, possibly touching the 70F mark in the SLV where
partial sun may arrive a bit earlier than other areas.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY/...
As of 333 AM EDT Monday...Dry weather will persist Tuesday
night and Wednesday with upper level and surface ridging over
the area. Decreasing cloud cover Tuesday night will set us up
for some fog formation, locally dense, from 06-12z. Surface high
crests over the region on Wednesday, with warm southwesterly
flow developing. Cooler than normal temperatures Tuesday night,
mainly 50s with some 40s in the Dacks. Temps rebound nicely on
Wednesday with near seasonal normals expected, upper 70s to
around 80.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 333 AM EDT Monday...All of the weather in the long term
will be in the first few periods, mainly Wednesday night and
Thursday. A cold front will push across the region with showers
and perhaps thunderstorms Thursday afternoon. This will depend a
lot on the timing of the front which will become more clear as
we get closer to Thursday. Have mentioned likely pops for
Thursday, because this looks like the time period when the fropa
will happen. Showers should be ending during the first part of
Thursday night, though Euro does depict a slower system. Right
now the Friday through Sunday timeframe looks quiet with large
ridge of surface high pressure over the region. Temperatures
will be near normal through the week.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
Through 12Z Tuesday...Deteriorating conditions expected across
the area today into tonight as upper low pressure brings
widespread rainfall to the region. Thunder threat rather low and
confined to northern New York counties. Lack of expected
coverage warrants omission from NY terminal forecasts at this
point. Generally looking at VFR trending MVFR at all terminals
in the 09-18Z time frame as rain arrives from southwest to
northeast. Exception at KSLK where patchy IFR stratus will
linger a few hours this morning before trending MVFR. Brief IFR
visibilities possible in heavier showers, especially in northern
NY. Showers trend a bit lighter after 00Z Tuesday, though
scattered coverage remains across the area as cigs lower into
the 005-015 AGL range (IFR/MVFR) with widespread 3-5sm br and
localized areas below 2sm. Winds east to southeasterly through
the mainly in the 5 to 15 knot range, though occasionally gusty
to 30 kts at KRUT in the 12-00Z time frame, trending light after
00Z.

Outlook...

Tuesday: VFR. Chance SHRA...Patchy morning FG.
Tuesday Night: VFR. Patchy dense FG.
Wednesday: VFR.
Wednesday Night: VFR. Slight Chance SHRA.
Thursday: VFR/MVFR. Likely SHRA...Chance TSRA.
Thursday Night: VFR. Chance SHRA.
Friday: VFR/MVFR. Slight Chance SHRA.

&&

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

&&

$$
SYNOPSIS...JMG
NEAR TERM...JMG/Nash
SHORT TERM...Neiles
LONG TERM...Neiles
AVIATION...JMG



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