Area Forecast Discussion
Issued by NWS Buffalo, NY

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680
FXUS61 KBUF 300055
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
855 PM EDT Wed Mar 29 2017

.SYNOPSIS...
High pressure north of the region will move east tonight while a
low pressure system over the Mississippi Valley draws moisture
northward toward the New York on Thursday. Chances for rain
will increase over far Western New York Thursday afternoon, with
precipitation spreading across all of Western and Central New
York Thursday night. This complex system may bring some thunder
to far Western New York while a brief period of snow will be
possible east of Lake Ontario. The system will change to all
rain by Friday and move east of the region for the weekend.

&&

.NEAR TERM /THROUGH THURSDAY/...
Tonight...high pressure settles across the region with
increasing high clouds. These will be thin for much of the night,
resulting in fair radiational cooling conditions. Low
temperatures will drop below freezing for most locations, with
the coldest readings in the mid 20s in the North Country.

Friday...model consensus has trended faster with the onset of
precipitation on Thursday. The 18Z NAM in particular is faster,
with the HRRR and current radar trends also supporting the
faster trend. Expect rain to enter the Western Southern Tier
around noon, spreading into Buffalo by mid-afternoon and to
Rochester late afternoon. Latest model consensus supports a
start time about 3 hours faster than previous guidance.

This faster timing has other impacts to the forecast, with
cooler high temperatures expected. It`s also interesting to note
that BUFKIT thermal profiles support a brief period of rain and
snow mix across the lower Genesee Valley and Finger Lakes
regions late Thursday afternoon. Given the March sun, this is
not likely to produce any accumulation, but it is possible a few
snow flakes will be mixed in.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
Let there be no doubt...it is going to be wet and unsettled for much
of this period as another soaking rain will be in the offing. The
good news is that the rain will pull away from the region just in
time to give us a fairly nice weekend.

A storm system...currently spawning severe weather across the Lower
Mississippi Valley...will approach our region from the Ohio Valley
Thursday night. A 40kt low level jet ahead of this stacked low will
pump abundant GOMEX moisture up and over a tight H925-70 baroclinic
zone that will be in place over the Lower Great Lakes. A weakly
coupled H25 jet will accompany the isentropic lift...so plenty of
forcing will be in place to interact with a moisture rich (PWAT arnd
1") environment. This will encourage some locally moderate to heavy
rain...and as mentioned in a previous discussion...could support the
potential for elevated convection. Pattern recognition...including
the strength of the warm frontal boundary...has looked impressive
for several days. FINALLY...forecast soundings are suggesting steep
enough lapse rates aloft for elevated CAPE values in excess of 100
j/kg. Will add the chance for thunderstorms to the western zones...
with some enhanced wording for some heavy rain. These should be
focused along an axis of mid-level warm air advection which will
spread from SW-NE Thursday evening. Rain may briefly taper off
behind this across the Western Southern Tier Thursday evening
before the next round of steady rain moves in later in the
night.

For sites east of Lake Ontario...the pcpn could start off as a
wintry mix...so will maintain that wording. Snow should struggle
to accumulate in most areas, but precipitation rates should be
ample for some accumulation across higher terrain. Several
inches are possible across higher terrain before warmer mid-
level air changes precipitation over to rain late in the night.
This is a difficult snow forecast since temperatures will be
very marginal.

The stacked low will drift across the Upper Ohio Valley on Friday...
while its associated warm frontal boundary will push north across
Lake Ontario and the North Country. This will encourage the
steadiest and `heaviest` rain to move across the eastern Lake
Ontario region in the vcnty of the strongest isentropic lift...while
mainly hgt falls and a divergent upper level flow will drive lesser
rains (including some drizzle) over the western counties...and in
particular across the Southern Tier. Will use cat pops across all of
the forecast area. Any mixed pcpn at the start of the day over the
North Country will change to just rain by late morning. Afternoon
temperatures will range form the upper 40s near the Pennsylvania
border to the upper 30s across the eastern Lake Ontario region.

As the upper level support for the complex storm system drifts east
across Pennsylvania Friday night...the initial sfc reflection will
weaken then `jump` (redevelop) off the New Jersey coast. While the
bulk of the moisture will remain in place over the region during the
storms transition to the coast...only limited low level forcing will
persist. This will allow the widespread rain to taper off as a bit
of light rain and/or drizzle...again mainly over the Southern Tier.
Given the light sfc gradient and near saturated conditions...there
will likely be some fog as well...with dense fog possible over the
higher terrain.

While mid level ridging will make its way across the Lower Great
Lakes on Saturday...guidance is suggesting that a weakness in the
sfc pressure field will remain in place over our forecast area. This
will promote enough of a cyclonic flow in the low levels to combine
with leftover low level moisture (trapped beneath a subsidence
inversion) to keep much of the day shrouded under clouds. Temps
Saturday afternoon will be in the mid to upper 40s.

A shortwave in the northern branch will sweep across the St Lawrence
Valley Saturday night...glancing by our forecast area in the
process. While there will still be a fair amount of low level
moisture in place...high pressure nosing south from Hudson Bay
should supply us with fair dry weather. Temps Saturday night will
generally settle into the low to mid 30s.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Ridging moving across the Lower Great lakes should supply us with a
nice ending to the weekend...as at least partial sunshine and H85
temps near zero C will enable afternoon temperatures to get into the
50s (up 40s Ern Lake Ont Region).

Another southern stream closed low over the southern Plains and
Lower Mississippi Valley will amplify a downstream ridge over the
Ohio Valley Sunday night and Monday. This will keep fair dry weather
in place with temperatures remaining a few degrees above early April
norms.

The various medium range guidance packages diverge with their
solutions at this point...as one forecast `camp` opens up the
southern stream closed low and drifts it across our forecast area
late Monday night and Tuesday. The other `camp` keeps the bulk of
the system to our south...with notably lower POPS and QPF. Will
maintain the likely pops over our region on Tuesday to avoid flip
flopping.

While there is low confidence in the guidance for Wednesday...there
is general consensus that our forecast area will be in a lull as far
as pcpn is concerned. Will refrain from the details due to the large
variance among the various ensemble members of the GEFS and ECMWF.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
VFR conditions are expected through Thursday morning with high
pressure across Ontario ridging southward into the region.
High clouds will move into the region overnight, with these
lower Thursday morning but still staying firmly in the VFR
category.

Precipitation will spread across the region from west to east
on Thursday afternoon. After a couple hours, this should lower
cigs to MVFR.

Outlook...
Thursday night into Saturday...MVFR/IFR with rain. A chance of
thunderstorms Friday night.
Sunday and Monday...VFR.

&&

.MARINE...
Winds will turn easterly by Thursday and southeasterly Friday
as our next storm system passes south of the Great Lakes. Winds
will approach 15-20 knots but higher waves should remain in
Canadian waters. Winds should be out of the northwest following
the passage of the low for the weekend.

&&

.HYDROLOGY...
This system will produce several rounds of rain, with storm
totals generally expected to average in the 1-2 inch range.
Model consensus suggests the highest amounts will be along the
south shores of Lake Ontario. MMEFS ensembles show there is a
good chance that many of the Buffalo and Rochester creeks will
reach action stage, but low probabilities that they will flood.
This said, if QPF exceeds the forecast by even a half inch in
any particular basin, this would likely be enough to reach minor
flood stage since flows are already high going into the event.

Based on this, will add a mention of this possibility to the
HWO.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...None.

&&

$$

SYNOPSIS...ZAFF
NEAR TERM...APFFEL/ZAFF
SHORT TERM...APFFEL/RSH
LONG TERM...RSH
AVIATION...APFFEL
MARINE...ZAFF



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