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Area forecast discussion
National Weather Service Burlington Vermont
1030 am EDT Fri Jun 23 2017

Synopsis...
showers and thunderstorms will move through the area today.
Locally heavy rainfall is possible with stronger showers and
storms. A cold front will move through the region tonight,
reducing the humidity and bringing a brief return to dry weather
on Saturday. More showers return early next week. Temperatures
will be near to a bit below their seasonal normals, with highs
in the 70s to low 80s and lows in the 50s and low 60s.

&&

Near term /through Saturday/...
as of 1030 am EDT Friday...Flash Flood Watch issued for the
Adirondacks and portions of central/northern Vermont through 1100 PM.
Mrms estimates fairly accurate this morning showing a swatch of
0.75 to 2 inch rainfall along and 30 either side of a line from
Edwards, New York through Saranac Lake, Burlington and St. Johnsbury.
This has saturated soils in this area and with additional
showers/scattered storms with heavy downpours in existing
1.7-1.9 precipitable water airmass the potential for flash flooding in the
watch area has increased (see Hydro disc below for further
details). Otherwise, did opt to lower maximum temperatures
slightly based off higher degree of clouds and most recent lamp
output. Rest of forecast remains in good shape. Have a good day.

Previous discussion...warm front analyzed running east-west
from southern shore of Lake Ontario and across southern
Vermont. Area of showers and thunderstorms along and to the
north of the front, aided by area of 500mb positive vorticity advection moving into the
region in west-southwest flow out of deepening trough to the
west. Main body of rainfall will lift north this morning through
12z following track of the vort center and on nose of low level
southwest jet of 30-40 knots. Expecting a lull after precip
lifts north, then daytime destabilization will renew chances for
convection through the day. With upper trough to west, and warm
front and shortwave trough moving north there isn't organized
forcing thinking is that showers and thunderstorms will have to
be all instability driven.

Thermodynamic profile continues to show very efficient rainfall
processes, with pwats near 2 inches, warm cloud depth over 12k
feet, and a tall skinny cape. Given southwest unidirectional
flow there is the potential for storms to train over the same
location and produce heavy rainfall rates.

Severe threat less straightforward, with parameters pointing
toward marginal risk from Storm Prediction Center. Best 0-6km shear of 40-50 kts
lifts north this morning with the warm front and low level jet,
then settles down to 35-40 kts for the afternoon storms.
Likewise cape is a modest 1500 j/kg based on Max surface temp in
the low 80s. Kept gusty winds/small hail in forecast, but feel
the biggest threat today is heavy rainfall.

For tonight a cold front moves northwest to southeast bringing
rain chances to an end from west to east. Saturday will be
mostly dry, however a chance for showers remains mainly in the
northern higher terrain with passage of a shortwave trough in
southwest flow as trough to the west deepens. Lows tonight
still in the 60s, then cooler air trickles in for Saturday with
highs in the 70s.

&&

Short term /Saturday night through Sunday/...
as of 430 am EDT Friday...any showers Saturday evening should
end before midnight as any convection will mainly be instability
driven. Dry weather is then expected during the overnight hours
Saturday night through much of the morning hours on Sunday.
Shortwave trough will move across the area Sunday afternoon and
enhance the potential for showers...especially over the northern
half of the area. Lows Saturday night will be in the 50s to
around 60 with highs on Sunday in the 70s.

&&

Long term /Sunday night through Thursday/...
as of 430 am EDT Friday...area remains under a broad upper
level trough of low pressure through about midweek. Shortwave
troughs moving through the large scale trough along with daytime
heating to destabilize the atmosphere will be enough to keep a
chance of showers going just about every day of the first half
of the week. The pattern will change as upper trough moves east
by Thursday and upper ridging builds in. Looking at drier
weather for the Thursday/Friday time period along with a drying
trend.

&&

Aviation /14z Friday through Tuesday/...
through 12z Saturday...rain creating MVFR to IFR conditions for
most taf terminals, except krut far enough south to be on the
fringe. As the rain area lifts slowly north through late morning
conditions to improve to VFR cigs. After the morning rain kept
vcsh for the rest of the day. Uncertainty remains for forcing of
afternoon convection, and until we're able to pinpoint locations
as storms fire vcsh will suffice. Cigs drop to IFR after 00z
with passage of cold front. South winds with gusts in the high
teens today turn southwest and west overnight with passage of
front.

Outlook...

Saturday: VFR. Slight chance rain showers...slight chance thunderstorms and rain.
Saturday night: VFR. No sig weather.
Sunday: VFR. Breezy. Chance rain showers.
Sunday night: VFR. Breezy. Chance rain showers.
Monday: VFR. Chance rain showers.
Monday night: VFR. Chance rain showers.
Tuesday: VFR. Chance rain showers.

&&

Hydrology...
as of 1030 am EDT Friday...storms will have high rainfall rates
today, and training storms moving over same location could
produce areas of flash flooding in the Flash Flood Watch area,
where 0.75 to locally 2 inches of precipitation fell earlier
today. With the overnight rains, 1 hour ffg has decreased into
the 1.0 to 1.5 inch range, with 3-hourly guidance running from
1.5 to around 2 inches. With lack of organized forcing mechanism
for storms this afternoon heavy rainfall will be hit or miss,
but latest output suggests instability on the south side of warm
front to our immediate north may drive additional activity into
the watch area. Per coordination with nerfc additional amounts
of 1 to 2 inches is forecast, which raises concerns for smaller
watersheds. Larger rivers and will see modest rises but should
remain within banks.

&&

Btv watches/warnings/advisories...
Vermont...Flash Flood Watch through this evening for vtz005>010-017-018.
New York...Flash Flood Watch through this evening for nyz028>031-034-035.

&&

$$
Synopsis...Hanson
near term...jmg/Hanson

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