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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBTV 161943
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
343 PM EDT Thu Aug 16 2018

.SYNOPSIS...
Clouds will increase quickly tonight with rain showers arriving
toward early Friday morning associated with a surface warm front.
Additional showers and thunderstorms are anticipated throughout
Friday into Friday night with some localized heavy rainfall likely
in the stronger storms. Temperatures will be in the 70s, but
humidity levels will be high on Friday, before cooler and drier
weather returns by late Saturday.

&&

.NEAR TERM /THROUGH FRIDAY NIGHT/...
As of 319 PM EDT Thursday...Water vapor shows deep subsidence/dry air
aloft across northern New England with deeper moisture already
returning to western NY/PA. First embedded vort and mid level
moisture over the Ohio Valley will lift across our cwa on Friday
Morning associated with warm front feature. Meanwhile, next
potent s/w energy over the central Plains will impact our fa on
Friday aftn/evening with additional showers/storms. Bottom line
expect an active Friday with multiple rounds of showers/storms
likely with localized heavy rainfall possible.

Tonight...forecast challenge is fog coverage with clear skies and
light winds thru midnight, before clouds increase and rain showers
arrive toward sunrise. The longest period of clear skies and light
winds will be central/eastern vt, where cross over values should be
reached and fog development is likely. Less confident in dacks given
increasing clouds and 10 to 15 knots developing btwn 500-1000 feet
agl. Temps generally in the upper 40s nek/slk to lower 60s
cpv/southern slv. RAP13 shows 5h vort over Ohio Valley reaching slv
by 11z Friday, along with deeper 1000 to 500mb rh, so expect a sharp
increase in pops on Friday morning from sw to ne. Soundings show
limited instability so anticipating main rain showers.

Friday/Friday Night...First round of showers associated with
embedded 5h vort...good ribbon of 1000 to 500mb moisture...and 850
to 700mb uvv`s signature lifts from sw to ne across our cwa btwn 12z-
16z Friday. This warm front feature is progged to become stationary
along the international border by 18z...as sfc low pres approaches
the eastern Great Lakes.

The question on Friday aftn will be amount of clearing/instability
and potential for strong/severe storms. Models show best chc
for breaks in the overcast with cape values btwn 800-1200 j/kg
developing over central/southern VT...with sharp instability
gradient over central/northern VT. This boundary...combined with
nose of 850mb jet of 40 to 45 knots angling toward the dacks
into central VT will help to enhance deep theta e moisture
advection. This low level convergence/advection, combined with
additional s/w energy and pws >2.0 will increase the threat for
heavy rainfall and potential for localized flash flooding on
Friday aftn/evening. Also, warm cloud depths are ~12,000 feet
and mean storm vector analysis shows potential for
backbuilding/training storms given west/east orientation of
boundary and nose of 850mb jet.

Not anticipating main stem flooding, but high rainfall rates,
especially in trrn or urban areas could cause some issues. New
WPC Excessive Rainfall Outlook has placed our cwa in slight risk
of exceeding flash flood guidance. Given the uncertainty on
amount of instability/clearing and soundings showing mainly
moist adiabatic profiles...will not mention gusty winds in
zones...but just heavy rainfall possible. Also, best 0 to 1 km
and 0 to 3 km shear is displayed near boundary over northern VT,
while best sfc based cape is over southern sections. Progged
850mb temps near 16c and 925mb temps of 20c would support highs
well into the 80s on Friday, but with clouds and southeasterly
flow...thinking mixing will be limited with highs mainly in the
70s. Our streak of 80 degrees or higher could be in jeopardy at
BTV. I currently have 77f for the high. Rain showers continue
overnight with embedded storms...as short wave energy and trof
axis is overhead along with high pw values associated with warm
air conveyor. Temps fall back into the upper 50s to mid 60s by
early Saturday morning.

&&

.SHORT TERM /SATURDAY/...
As of 343 PM EDT Thursday...Low pressure will depart Saturday
morning, but with mid and upper level trough axis still to our
west some scattered showers will be possible through Saturday
afternoon. Mid-level dry air will work its way south and still
believe that clouds will begin to diminish as this drier air
mixes down. Temperatures on Saturday will be in the mid 70s for
valley locations and low to mid 60s across the higher terrain.
Additionally, still watching the potential for some fog develop
on Saturday night with plenty of dry air in the mid levels
allowing us to radiate out.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
As of 343 PM EDT Thursday...Although surface high pressure will
be in place Sunday morning, shortwave energy will still be
moving through aloft...guidance suggests that a piece of energy
will pass near/just south of southern New England Sunday night,
with the potential for a weak coastal low to develop. ECMWF is
the most robust with this system showing sufficient moisture
wrapping around the closed 850 mb low. At this time have trended
somewhere between GFS/NAM and ECMWF with just a mention for a
slight chance of rain for eastern VT. For our area, the best
chances will likely be across southern Windsor county.

For the rest of the extended, high pressure will remain in place
ahead of our next system late Tuesday into Wednesday.
Temperatures look to be slightly above normal through this time
with high temps near to just above 80 degrees. Still not very
good agreement on the exact timing for rain to arrive, but
certainly looks like it will overspread the area by Wednesday.
Details on this system will depend on how the overall pattern
reacts to a series of troughs advancing through the flow from
the Pacific...following this system, another brief shortwave may
pass through on Thursday so have included a mention for just a
chance of rain on Thursday afternoon.

&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
Through 18Z Friday...VFR conditions prevail at all taf sites
thru this evening as sfc high pres continues across the north
country. Forecast challenge is potential for fog/br development
before high clouds and rain showers arrive on Friday Morning.
Thinking best chance for ifr/lifr in fog/br will be at mpv,
where clouds will be last to develop and better chance for site
to reach its cross over temp. Otherwise, thinking a brief period
of ifr is possible at slk btwn 05-08z, before high clouds and
winds increase from the southwest. On Friday, a band of showers
with embedded rumbles of thunder will lift across our taf sites
btwn 12-16z with periods of mvfr cigs/vis possible. Developing
northeast flow and plenty of moisture will produce mvfr to
locally ifr cigs at kmss by midday. Southeast flow develops at 5
to 15 knots with localized gusts up to 20 knots possible at
Rutland on Friday. Additional showers and storms expected on
Friday aftn.

Outlook...

Friday Night: MVFR/IFR conditions possible. Chance SHRA, Slight
chance TSRA.
Saturday: Mainly VFR, with local IFR possible. Slight chance
SHRA.
Saturday Night: VFR. NO SIG WX.
Sunday: VFR. NO SIG WX.
Sunday Night: VFR. NO SIG WX.
Monday: VFR. NO SIG WX.
Monday Night: VFR. NO SIG WX.
Tuesday: VFR. Slight chance SHRA.

&&

.HYDROLOGY...
Several rounds of showers and thunderstorms are expected on
Friday into Friday night with localized heavy rainfall likely.
We are not expecting widespread main river stem flooding, but
isolated very high rainfall rates of 1 to 2 inches per hour are
possible. These high rainfall rates could produce sharp rises on
smaller streams and rivers, along with the potential for
isolated flash flooding, especially in mountainous terrain or
poor urban drainage areas. Basin average rainfall of 0.50 to 1.5
inches is expected, with localized amounts up to 3 inches
possible in areas that receive multiple rounds of storms.

&&

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

&&

$$
SYNOPSIS...Taber
NEAR TERM...Taber
SHORT TERM...LaRocca
LONG TERM...LaRocca
AVIATION...Taber
HYDROLOGY...Taber