Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 211409
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1009 AM EDT Tue Aug 21 2018

.SYNOPSIS...
An area of low pressure over southeast Michigan will deepen as
it moves across southwest Ontario into Quebec through tonight.
This storm will bring widespread showers and embedded
thunderstorms along its warm and cold fronts with heavy
rainfall and a few strong to severe storms possible. A few
showers may linger into Wednesday before another area of high
pressure brings a return of fair weather Thursday and Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Regional radars showing scattered to numerous showers and a few
embedded thunderstorms are ongoing north and east of the low
center as well as along/ahead of the warm front. The warm front
will lift across the region this afternoon with additional
showers and thunderstorms. There should be a short window of
rain-free or just isolated convection within the warm sector
this afternoon ahead of the system cold front. There are two
main threats with storms today, heavy rain and severe storms.
High temps are forecast to reach into the 70s with uncomfortable
humidity as dewpoints reach to around 70F behind the warm
front.

Heavy Rain... The warm front will usher in a tropical airmass with
Pwats per SPC Mesoanalysis ranging between 1.7-2 inches. These
values are near to about a third of an inch above the daily maximum
for KBUF April 21st. Tall skinny CAPE profiles with thick warm cloud
depths and short MBE vectors during the afternoon hours will yield a
heavy rainfall threat with any thunderstorms. WPC has included WNY
in an excessive rainfall threat for slow moving storms. The heaviest
storms and highest QPF (2+") per WPC should occur just west of NY in
Southwest Ontario. Basin average rainfall totals from our official
forecast range from one and a quarter inches for far western NY down
to around three-quarters of an inch from the Genesee Valley to the
North Country. Locally higher amounts are very possible in slow
moving heavy storms which could bring flash flooding if training
develops.

Severe Storms...
While much of the warm sector is forecast to be cloudy this
afternoon, the combination of around 30kts of bulk shear with
even limited surface CAPE could promote a risk for severe
thunderstorms ahead of and along the cold front late this afternoon.
As the surface low tracks across southwest Ontario, veered low level
wind profiles might support a risk for a few rotating thunderstorms.
The tropical airmass with dewpoints around 70F will allow for low
LCLs of 1000m or lower. While any strong/severe storm could bring
strong/damaging winds, the low LCLs and 0-1km SRH of 200-250 m2s2
may also be favorable for an isolated tornado. SPC has placed
western NY and the Genesee Valley in a Day1 Slight risk for severe
weather with a Marginal risk further east into CNY due to later
diurnal timing. A 2% tornado risk was included in the western
Southern Tier closer to the surface low track.

Tonight, as the 500mb trough amplifies, the surface low is forecast
to rapidly deepen below 1000mb and perhaps down to below 995mb while
rapidly shifting north of New York into central Quebec. As this
occurs, the low will drag its trailing cold front west to east
across New York. Expect widespread showers and thunderstorms along
the front along with a continuing heavy rain and strong wind threat
at least closer to the front. Behind the front, cooler air will
arrive with wrap around moisture and upslope flow allowing for
lingering chances for showers. Drying will begin in the wake of the
front but dewpoints will only slip back into the mid to low 60s by
daybreak Wednesday. This will keep overnight temps on the mild side
for one more night bottoming out in the mid 60s. Steady/Gusty winds
with a surface pressure gradient will likely prevent fog.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
A pattern change will occur at the start of this period as a cold
front and much deeper moisture shifts eastward, and an upper level
trough brings a pool of cold air aloft over the Eastern Great Lakes.

Under this trough, with 850 hPa temperatures dropping into the upper
single digits C there will be a lake response, with enhanced clouds
southeast of the lakes, and lake effect rain. Will place likely Pops
SE of the Lakes where combination of lake effect and orographic lift
rain showers are likely...and maintain chances PoPs most everywhere
else where a combination of increasing lapse rates and cyclonic flow
aloft may bring scattered convective showers through the daylight
hours. The bulk of showers will end early Wednesday evening...with
lake effect rain showers tapering down last SE of the lakes.

A convective shortwave will slip down the backside of the upper
level trough aloft Wednesday night, bringing a layer of moisture
across the region near 5K feet Wednesday night. While this feature
may just increase clouds...with a partly to mostly cloudy sky
Wednesday night...it may yield an isolated shower over the warmer
lake bodies. Under the pool of cold air aloft Wednesday and Thursday
morning the convective cloud depth should be sufficient to support
an environment that could spawn a waterspout or two.

Later Thursday and through the day Friday surface high pressure will
build towards and over the region. Subsidence and much drier air
should support mainly clear skies, with day to day warming. Typical
valley fog is likely across the So. Tier, with some patchy fog also
possible SE of Lake Ontario.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
A shortwave will advance across the Great Lakes to start this
period, dampening as it moves towards a ridge of high pressure that
will be in place across the East. This ridge will deflect the
shortwave northeastward towards Canada, but its influence will be
close enough to our region for scattered showers Saturday...with a
threat for showers and thunderstorms continuing Sunday as a weak
cold front moves into the region, and possibly stalls near the
region. Another wave of moisture will bring additional chances for
rain showers and thunderstorms Monday to the region.

Temperatures will be seasonable to start the period, in the mid 70s
to lower 80s. Sunday and Monday warming aloft will allow for warmer
surface temperatures that will soar well into the 80s...provided
that ample sunshine occurs. If thicker clouds linger then
temperatures will not be as warm as forecasted.

&&

.AVIATION /14Z TUESDAY THROUGH SATURDAY/...
Lower end VFR and MVFR cigs will be common with tempo reductions to
IFR cigs/vis in passing heavy showers and storms associated with
a northeast moving warm front. During the course of the midday
and afternoon, the convection will move across the eastern Lake
Ontario region where conditions will similarly deteriorate to
MVFR levels.

In the wake of the warm front, there looks to be a short few hour
period where showers and storms will become just isolated until a
strong cold front arrives moving toward the evening hours. Showers
and thunderstorms just ahead of and along this cold front could
become strong to severe with gusty winds/heavy rain and IFR or lower
restrictions. The front will pass east across WNY and CNY tonight
with winds shifting to westerly in its wake. Expect precipitation to
taper down behind the front with just chances of showers in place
for WNY by 06z Wednesday while KART should still likely see showers
and storms through 09/12z.

Outlook...

Wednesday...Mainly VFR. A chance of showers.
Thursday and Friday...VFR.
Saturday...Mainly VFR.

&&

.MARINE...
An unseasonably deep surface low will track north across the central
Great Lakes with southerly winds picking up on the eastern Great
Lakes today. Widespread showers and embedded thunderstorms will be
accompanied by near advisory level winds and waves today as the
southerly direction will direct the highest waves into Canadian
waters.

A sharp cold front will cross the lower Great Lakes tonight which
will bring gusty westerly winds and higher waves. Small craft
Advisories for the Nearshore waters of Lake Erie and Ontario
remain in effect tonight into Wednesday.

Waterspouts are a possibility late tonight through Wednesday morning
and then again Wednesday night in any residual showers. Cold air
aloft will help support the instability to produce the waterspout
potential.

High pressure will return for the end of the week with a return of
fair conditons.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory from 8 PM this evening to 8 AM EDT
         Wednesday for LEZ040-041.
         Small Craft Advisory from 2 AM to 8 PM EDT Wednesday
         for LOZ043>045.
         Small Craft Advisory from 8 PM this evening to 2 PM EDT
         Wednesday for LOZ042.

&&

$$

SYNOPSIS...Smith/TMA
NEAR TERM...Smith/TMA
SHORT TERM...Thomas
LONG TERM...Thomas
AVIATION...Smith/TMA
MARINE...Smith/TMA


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