Lakeshore Flood Warning 6, NWS Buffalo, NY

Area Forecast Discussion
National Weather Service Buffalo NY
201 AM EDT Mon Jun 10 2019

.SYNOPSIS...
A complex frontal system will slowly move across the region
through tonight. This will bring showers and possibly
thunderstorms to the region, with a locally heavy rain possible.
High pressure will move across our region behind this front and
bring dry but cooler weather for Tuesday and Wednesday.

&&

.NEAR TERM /THROUGH TODAY/...
High pressure off the New England coast will circulate increasing
amounts of moisture across our region overnight...particularly over
the western counties (west of Finger Lakes). Early this morning
radar shows showers developing along the leading edge of a 40
knot low level jet, which is transporting moisture into the
region. These showers will gradually spread eastward as the LLJ
moves eastward overnight. Otherwise, the increasing cloud cover
and a south to southeast flow helping to hold min temperatures
in the upper 50s to lower 60s for most areas.

Monday will start off with the aforementioned 35 to 40 knot LLJ
centered over the Finger Lakes region. Scattered showers over the
western counties will gradually transition to the Eastern Lake
Ontario region during the midday and afternoon as the strengthening
forcing moves east. While the bulk of the 'heavy lifting' will be
done by the LLJ...there is a suggestion that a subtle 60kt H25 jet
will aid in the lift. Have thus raised pops to likely for several
hours for the Eastern Lake Ontario region.

As the loosely organized shower activity pushes east during the
midday and afternoon...there will be a 3-5 hour lull in pcpn from
the Finger Lakes westward. There should be a thinning of the cloud
cover during this time period...and this will allow things to become
a little more unstable over the western counties. It will also allow
temperatures to climb into the mid and upper 70s...above where most
guidance packages are advertising.

During the late afternoon...a wavy cold front will FINALLY start to
make its way across Lake Erie and southern Ontario. Strong forcing
along and immediately ahead of this boundary will be supplied
primarily from a 120kt H25 cyclonic jet over Ontario and to a lesser
extent a divergent upper level flow and low level convergence. This
will all come together within 30-50 miles of the sfc front...which
should be in the vcnty of the IAG River between 22-00z. While the
synoptic forcing will nearly guarantee rainfall with the frontal
passage...it will be the added components from a marginally unstable
but saturated environment that could support problematic rains.

SBCAPE values late Monday and Monday evening will be in the vicinity
of 500 j/kg...which is more than enough to encourage some true
convection to develop ahead of the stronger synoptic forcing. PWAT
values will be approaching 2 inches while the freezing level will be
around 13kft. Personally...I find it more representative to look at
the hgt of the -10c level when discussing warm cloud depth and the
potential for enhanced warm rain processes. In this case...the -10c
level will be close to 20k ft. This will certainly favor warm rain
processes within the convection...and with very short MBE vectors...
there will be the threat for training cells with torrential
downpours. The cells themselves should be moving at 20 to 25 knots
but the propagation of the convection ahead of the slow moving front
should be slower. Have thus added heavy rain to the package for the
western counties. QPF for Monday will range from a tenth or so over
most areas...to a half to one inch for the FAR western basins (near
Lake Erie). Significantly higher amounts (ex. an inch or two) cannot
be ruled out for these far western areas by evening. Its not out of
the question that a flash flood watch could be considered in a
future package. The heavy rains will then push east Monday night
(see next section for details).

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/...
The cold front will sweep across the lower lakes Monday night with
showers gradually ending from west to east overnight. A brisk west
to northwest flow will follow behind the front with cold air
advection ushering in a cooler and drier air mass as we head into
Tuesday.

Surface high pressure over the Mississippi Valley with build into
the Ohio Valley on Tuesday. With a much cooler air mass in place and
H850T forecast to be in the single digits of +4 to +6C, this will
support highs only in the 60s. What a difference behind the front, a
15F to 20F drop in high temperatures and less humid. As the high
builds into the Ohio Valley, the tight pressure gradient residing
across the lower lakes will gradually relax with the brisk wind flow
gradually subsiding by late in the day.

Tuesday night, surface high pressure over the Ohio Valley will track
east across Pennsylvania. With the loss of daytime heating, mixing
heights will fall and winds will quickly diminish becoming light and
variable shortly after sunset. With clear skies expected and light
winds good radiational cooling will take place with lows dropping
into the 40s away from the influence of the lakes.

High pressure will exit off the coast and then out to sea on
Wednesday. However, it will maintain dry conditions areawide, but
there will be some increase in high clouds late in the day with the
next system approaching the region. Speaking of which, a deepening
trough and its surface low pressure over the Upper Midwest is
forecast to track towards the lower lakes Wednesday night. As the
trough and cold front nears our area, look for increasing chances
for showers as we head into Thursday.

In terms of high and low temperatures, it will be several degrees
warmer Wednesday as a southerly return flow picks up with 70s
expected areawide. It won't be as cool Wednesday night due to
increasing southerly flow and cloud cover, with low to mid 50s
expected by daybreak Thursday.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
A broad upper level trough over the northern Great Lakes and the
eastern third of Canada will influence most of the weather during
this period. Timing for this period varies between model guidance
for temperatures, rain and frontal boundaries.

On Thursday a shortwave tough within the larger scale trough will
trek southeast across the Northern Plains into the Great Lakes
bringing increased chances for rain. With this shortwave trough a
surface low and cold front will cross the region through the early
afternoon on Thursday. As the trough axis moves northeast out of the
area early Friday, rain will shift northeast toward New England.

Brief and weak ridging from Friday into early Saturday will result
in lower chance POPs across the area, with the best chance for rain
showers along the northern fringe of the region.

Another shortwave trough embedded within the larger scale trough
will shift southeast out of the Northern Plains and into the Great
Lakes Saturday into Sunday. This will cause rain to overspread the
region ahead of and along a cold front that will pass through the
area during this time. It is important to note that with this
passing cold front, timing is around 12-18 hours different between
the GFS and the ECMWF. This timing difference is due to the location
of the upper level shortwave in each model, where around 750 miles
separates the placement of this feature at 12Z on Saturday between
the two models.

Temperatures during this period will be below normal for each day,
with Friday being the coolest day, with temperatures around 10
degrees below normal for the daytime high. High temperatures will
generally be in the upper 60s to near 70 for each day except Friday
when temperatures are only expected to reach the low to mid 60s.
High temperatures on Thursday and Sunday will occur during the late
morning into the early afternoon, before a cold front crosses the
region each of these two days.

Due to the timing uncertainty with the system for the weekend,
temperatures and POPs will need to be adjusted as model agreement
becomes better aligned.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
A 40 kt LLJ will slowly push eastward across the region. This
will result in roughly a 6 hour period with LLWS at TAF sites,
and will also provide the focus for some showers. Behind this, a
showers can't be ruled out but the majority of the time will be
rain-free with VFR flight conditions. The exception is across
the Southern Tier where low level moisture will increase with
MVFR cigs possible today.

A cold front and surface trough will move across the area from
west to east late this afternoon and this evening. This will
bring roughly a 2 hour period with heavy rain to most spots,
with embedded thunderstorms possible. Low moisture will then
increase along the frontal boundary with cigs likely to lower
into the IFR flight category near the boundary before improving
slightly late tonight.

Outlook...
Tuesday and Wednesday...VFR.
Thursday...VFR/MVFR with showers likely and possibly some
thunderstorms.
Friday...VFR.

&&

.MARINE...
As high pressure along the New England coast pushes further
offshore...light to moderate southeasterlies will persist on Lakes
Erie and Ontario through most of Monday. A complex frontal system
will approach from the west late Monday. This will promote winds to
veer to the south while freshening somewhat during the early
evening.

In the wake of the slow moving cold front...winds will shift to the
west and significantly freshen to near 30 knots. This will result in
small craft advisories for all of the NY nearshore waters Monday
night into Tuesday...with a brief period of gale force wind gusts
not out of the question for the open waters of Lake Ontario between
06 and 12z Tuesday.

Winds and waves will then gradually subside during the second half
of Tuesday into Tuesday night.

&&

.TIDES/COASTAL FLOODING...
A strong cold front will cross Lake Ontario Monday evening with
southerly winds quickly veering to the west-northwest and
increasing to 15 to 30 knots in its wake. The significant
freshening of the winds will couple with strong cold advection
to support a substantial increase in wave heights Monday night
into Tuesday. Winds will then subside from west to east late
Tuesday and Tuesday night as a large area of high pressure will
move into the region.

The combination of the increased wave action and high lake levels
will lead to an increased potential for flooding along the south
shore of Lake Ontario from Monday night through at least Tuesday...
for which a Lakeshore Flood Watch remains in effect as outlined
below.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Watch from late tonight through Tuesday
     evening for NYZ005>007.
     Lakeshore Flood Watch from late tonight through Tuesday
     afternoon for NYZ003-004.
     Lakeshore Flood Watch from this evening through Tuesday
     afternoon for NYZ001-002.
MARINE...None.

&&

SYNOPSIS...Apffel
NEAR TERM...Apffel/JJR/RSH
SHORT TERM...AR
LONG TERM...SW
AVIATION...Apffel
MARINE...RSH
TIDES/COASTAL FLOODING...RSH

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