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  Saturday May 26, 2018

 

NWS Area Forecast Discussion


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FXUS61 KBTV 261718
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
118 PM EDT Sat May 26 2018

.SYNOPSIS...
Unsettled weather is expected for the Memorial Day weekend as a cold
front moves slowly southward out of Canada today. This will allow
showers to move across the area...especially over northern New York
and the northern half of Vermont. Thunderstorms will be possible
during the afternoon hours, particularly for central and southern
Vermont and parts of the Adirondacks.  Cooler and drier weather
moves into Vermont on Sunday...but there will still be a threat of
showers and thunderstorms across portions of northern New York as an
upper level trough of low pressure moves toward the region. This
feature will move across the area on Memorial Day and will continue
to bring a chance of showers to the entire North Country.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 100 PM EDT Saturday...Have updated forecast to integrate
some high resolution data into the grids to capture pop
associated with boundary dropping south. Latest analysis shows
the boundary has moved thru BTV with wind shift to the north and
temp dropped by 4 degrees. This boundary will continue to drop
south across central/southern Vt and northern dacks this aftn.
LAPS shows sfc based CAPES have increased to 500 to 1000 ahead
of boundary with temps warming into the mid/upper 70s and dwpts
near 60f. Vis satl pic shows some vertical development to clouds
ahead of this boundary indicating some instability...and would
expect better chances for thunderstorms btwn 18z-21z this aftn.
SPC has upgraded our south-central cwa into marginal risk, but
given the weak forcing and limited available instability only
expecting localized heavy downpours and maybe an isolated wind
gust to 40 mph in the stronger convective elements. A sharp
north to south gradient in temps will continue with lower 60s
north to l/m 80s south, along with chances for precip as the
boundary impacts central/southern section this aftn.

Previous discussion below:
Have made some adjustments to the PoPs and expected
thunderstorm coverage for today. Main gist is that some of the
06z model guidance has pulled back on coverage of showers for
today, as well as instability and convective potential for this
afternoon. Main factor at play is the ongoing shower activity
early this morning, with radar indicating scattered showers
along and north of a Gouverneur to Lincoln to Corinth line. Even
seeing one lone thunderstorm that is just now crossing the
border up by Berkshire/Highgate. Note that much of the hi-res
guidance now indicates that once this activity moves by, the
atmosphere will not be able to recover enough to allow more
widespread showers/possible thunder with the moisture
axis/frontal boundary later this afternoon. Meanwhile, a few
members such as NAM nest and ARW indicate more robust
convection, if a bit less than shown with the 00z runs. SBCAPE
progs are a little less generous as well, though there are still
some areas that could approach 800-900 J/kg. Have added a
mention of a slight chance of thunder over the northern tier of
VT counties this morning for the aforementioned border crosser,
but have pulled back a bit on thunder coverage owing to the
aforementioned trends. Later shifts will need to keep a close
eye on trends as subsequent model runs and radar/observation
trends become clear.

Previous discussion...We are still anticipating a less-
than-stellar holiday weekend with unsettled weather to prevail,
especially on Saturday. A cold front currently lies poised just
north of the international border, and it will slowly move
southward into and across the North Country through the day.
Moisture will be plentiful ahead of the front; PWATs already
exceed 1.25 inches across the region, and this looks to increase
to 1.5-1.75 inches just ahead of the front as moisture streams
northward around high pressure located well to our southeast.
Much of the hi-res guidance, including the local 4km WRF, RAP,
HRRR, NAM 3, and NAM Nest all indicate showers firing up along
and just ahead of the front as it crosses the region. Scattered
showers are ongoing across the northern portions of the area at
this hour associated with a moisture axis, and these will
continue through mid morning or so. Then more robust activity
associated with the frontal passage will spread from north to
south, roughly noontime through sunset or so. Guidance also
indicates there will be instability available for convection,
with both the NAM and GFS spitting out SBCAPE of 500- 1000+ J/kg
this afternoon. The highest chances for any thunder will lie
south of a St Johnsbury to Burlington to Saranac Lake line as
these areas will have the best opportunity to destabilize ahead
of the frontal passage. With abundant moisture, any thunderstorm
would be capable of producing briefly heavy rainfall. Today`s
high temperatures will depend on how early the front goes
through; far northern NY and VT will top out around 70 while
south-central VT will be in the upper 70s to lower 80s.

For tonight into Sunday...the threat of showers will wind down
across the region as the front pushes south and daytime heating is
lost. Cooler air will make it into eastern portions of VT, allowing
temperatures to drop into mid 40s to around 50, while areas from the
Greens west will remain the mid 50s. Sunday will be cooler as
maritime air wraps in from the east. Although there will be
plentiful cloud cover, Sunday will generally be a dry day with highs
in the 60s along and east of the Greens, to around 70 in the
Champlain Valley and Adirondacks. The exception will be out toward
the St Lawrence Valley where an approaching upper wave along with
increasing moisture on southerly flow will allow showers to develop
during the afternoon. Note that only some of the guidance indicates
there will be enough instability for thunderstorms, so have stayed
with just a slight chance for now. Also noted that MOS is split on
high temperatures for this area with the MET giving Massena a high
of 65 on Sunday, while the MAV is showing a significantly warmer 76.
Have stayed with the warmer solution, and even bumped up a degree or
two, but did lower just a couple of degrees from the previous
forecast owing to the cooler solutions.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY/...
As of 343 AM EDT Saturday...Inverted surface trough as well as
weak shortwave moving in flow with PWATS still in the 1.25 inch
and some instability with Most unstable cape around 500 j/kg,
can`t rule out scattered t-storm on Monday focused across NY as
backdoor front still has stabilizing influence across VT.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 343 AM EDT Saturday...Northern stream shortwave and weak
front move across Monday night with a few showers possible but
what comes after for mid week is worth it.

High pressure aloft and at the surface will mean pretty much
dry and warmer than normal Tuesday through first half of
Thursday. Thereafter, a northern stream shortwave and surface
front threatens and possibly interacts with remnants from
Alberto with PWATs increasing to 1.5-2 inches for increase
threat of showers with possible local downpours Thu ngt-Fri and
perhaps lingering into beginning of next weekend. Still plenty
of time to watch and really with a tropical
system...timing/location not reliable until it makes landfall.

&&

.AVIATION /17Z SATURDAY THROUGH THURSDAY/...
Through 18Z Sunday...Forecast aviation challenge will be cigs
and timing of precip associated with boundary dropping across
our taf sites this aftn. Thinking a period of mvfr cigs are
possible between 18z-21z at PBG/BTV/MPV with brief ifr cigs
possible based on upstream conditions at FSO. Otherwise, a 30
minute to 1 hour window of rain showers with a few rumbles of
thunder can be anticipated at btv/slk/rutland/mpv btwn 19z-21z
this aftn with boundary, along with a wind shift to the north at
5 to 10 knots. North winds will shift to the southeast tonight
with some localized gusts possible at Rutland up to 20 knots
anticipated overnight. These winds will help to advect a
maritime airmass into central/eastern tafs overnight into Sunday
with some mvfr cigs likely, especially at mpv. These lower
clouds may impact rutland/btv/pbg and slk at times with some
mvfr conditions. Otherwise, any leftover showers will dissipate
by 22z this evening with mainly dry conditions prevailing
through 18z Sunday.

Outlook...

Sunday Night: Mainly VFR, with local MVFR possible. Slight chance
SHRA.
Memorial Day: Mainly VFR, with local MVFR possible. Chance SHRA,
Slight chance TSRA.
Monday Night: Mainly VFR, with local IFR possible. Chance SHRA.
Tuesday: VFR. NO SIG WX.
Tuesday Night: VFR. NO SIG WX.
Wednesday: VFR. NO SIG WX.
Wednesday Night: VFR. NO SIG WX.
Thursday: VFR. Slight chance SHRA.

&&

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

&&

$$
SYNOPSIS...Hastings
NEAR TERM...Hastings/Taber
SHORT TERM...SLW
LONG TERM...SLW
AVIATION...Taber



 
 
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