Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 230219

Area Forecast Discussion
National Weather Service Buffalo NY
1019 PM EDT Wed Mar 22 2017

High pressure will build over the lower Great Lakes tonight and then
east of the region on Thursday. One more night of very cold
temperatures will give way to a moderating trend Thursday into the
weekend as warmer air moves into the region. A frontal boundary will
stall across the region this weekend, with several disturbances
moving along the front and bringing periods of rain over the weekend
and into the first half of next week.


Sprawling arctic high pressure will continue to build in across the
Great Lakes through tonight. A few scattered narrow bands of lake
effect clouds will persist southeast of Lake Ontario supported by
the cold airmass, but the very shallow inversion and dry synoptic
scale environment will only permit partly cloudy skies at most.
Otherwise for the rest of the area expect clear skies overnight.

High pressure will move over the forecast area overnight setting the
stage for optimal radiational cooling with light to calm winds and
clear skies. Locations with lingering snowpack, such as the North
Country and interior portions of the western Southern Tier will see
temperatures plummet overnight, with lows bottoming out in the
single digits to near zero. Meanwhile, lake plains locations, those
with limited snow pack, and urban areas will see temperatures mainly
in the low to mid teens.

The large high pressure system will slide to our east on Thursday,
with some limited return flow developing by mid-day and the
afternoon. This will allow for some moderation in temperatures as
the core of the cold airmass exits the region. High temperatures
will run a fair bit warmer than Wednesday, as temperatures reach the
low 30s in the North Country to within a few degrees of 40 in
Western NY. Full sunshine in the morning will give way to a gradual
increase in mid/high clouds later in the afternoon as mid level warm
advection increases from west to east.


After a midweek dry spell, the region will enter a prolonged
unsettled and relatively mild period that will last at least through
the weekend as a slow moving upper level low meanders across the
nation`s mid-section. Southerly return flow around high pressure
stationed off the Eastern Seaboard, enhanced by a strengthening low-
level jet will pump moisture and mild air northwards across the
Great Lakes. Meanwhile, a northerly flow of cooler and drier
Canadian air be found just to our north, sharpening a frontal
boundary across the lower Great Lakes. This front will then serve as
the focus for a band of precipitation across the forecast area that
will develop starting late Thursday night as the warm front lifts
across the region. With warm air initially overrunning the initially
sub-freezing surface air, we may see a brief period of mixed
precipitation, particularly across the Southern Tier and North
Country, late Thursday night into Friday morning before the arrival
of much warmer air on southwesterly flow changes precipitation over
to rain. Temperatures Thursday will then warm into the 50s across
Western NY, with low 40s in the North Country.

Currently, the model consensus places the frontal precipitation band
along and north of the Thruway, with around a half to three quarters
of an inch of precipitation possible, heaviest across the north
Country. Temperatures across the forecast area should hold above
freezing Friday night, allowing for ongoing frontal precipitation to
persist as rain, with the possible exception of the St. Lawrence
valley, where a possible backing northerly flow may bring in some
sub-freezing air and result in mixed precipitation early Saturday

As we move into Saturday, diurnal effect should keep precipitation
mostly rain, however, as the broad surface low wandering across the
Mississippi valley draws closer to the region, increasing
northeasterly flow into this low may draw colder surface air over
Canada back across the forecast area, potentially undercutting the
warm air aloft.  This may result in more substantial sleet and/or
freezing rain across the North Country, and possible as far south as
the Lake Ontario plain. Model guidance differs somewhat in how far
north the warm front travels into Canada Friday, and hence how far
south it will retreat Saturday, so all mixed precip grids are being
left as a chance for the time being.


An active wet pattern will continue Sunday into at least the middle
of next week as western and north-central NY will remain within the
track of at least two mid-level troughs. Each of these troughs will
provide synoptic lift over and moist southerly flow into our region
that will interact with a low frontal boundary. The first will shift
over our region in the Sunday-Monday time frame while the second
looks to shift over our region Tuesday. It appears now that this
boundary from Sunday into Monday will lie further to the north
towards or across the Saint Lawrence River Valley then should be
shoved even further northeast by the second trough/surface low. For
western NY into the Finger Lakes, the position of this front will
allow for plenty of low level warm air to support high confidence
for a soaking plain rain. Looking across the North Country into the
Saint Lawrence Valley, the position of the front will be more
sensitive to p-type and a risk for some freezing rain Sunday night
and Monday night with cold northeasterly flow down the SLV. Have
kept any wording for freezing rain at chance POPs due to the
uncertainty in frontal position this far out.

In terms of surface temperatures, areas to the south of the front
will mainly feature warmer temps upper 40s into the 50s while areas
north of the boundary will see cooler temperatures 30s and lower 40s
with some 20s possible in the North Country during the overnights.

In terms of QPF, a rough average accumulated total among the models
Sunday through Tuesday ranges from around a half inch on the ECMWF
to around 2 inches on the GFS. While the long duration of this event
should not bring any flash flooding concerns, river and stream rises
and areal flooding of low-lying areas may be possible. If sub-
freezing air remains locked at the surface on the north side of the
front there could also be a threat of some significant ice
accumulations but the lowest confidence is found in that scenario.
Overall, confidence on the specific placement of any weather hazards
during this period is fairly low at this range which has precluded a
mention in the Hazardous Weather Outlook.

On Wednesday, the second surface low will shift off into New England
with a cooler and perhaps very dry northwest flow in its wake. The
12z ECMWF shows very dry air with just a cool northwest flow on
Wednesday while the GFS shows some lingering wrap around moisture
may lead to some upslope/lake enhanced showers. Have gone with
slight/low chance POPS for Wednesday under a more clear signal shows
up in the guidance.


High pressure will build across the region tonight with SKC in most
areas. A few scattered lake effect clouds will persist in a small
area southeast of Lake Ontario with bases around 4K feet. The
surface high will drift to the Mid Atlantic later Thursday. VFR will
continue with a modest increase in mid/high clouds from west to east
later in the afternoon.


Friday...Deterioration to MVFR/IFR with rain. The rain may begin as
a brief period of freezing rain or snow in some areas.
Saturday thru Monday...MVFR/IFR with rain likely at times.


Moderate northwest winds will maintain small craft advisories on
Lake Ontario through late this evening. High pressure will build
towards the lakes overnight, allowing for winds and waves to
subside, with small craft conditions lingering longest along the
south shore of Lake Ontario.

This high pressure will then build across the region Thursday while
bringing a return to light winds and negligible waves.


MARINE...Small Craft Advisory until 2 AM EDT Thursday for LOZ042>045.



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