Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Buffalo New York
951 am EST Friday Dec 20 2013
a cold front will stall across Lake Ontario today...bringing periods
of rain with freezing rain developing over the north country. The
front will remain stalled this weekend and bring freezing rain to
the north country and possibly the South Shore of Lake Ontario.
There will also be a potential for heavy rainfall and flooding.
Near term /through tonight/...
a frontal boundary will remain stalled across Lake Ontario with a
weak wave of low pressure forecast to track along this boundary
today. Meanwhile...an area of warm air advection in the middle levels
will spread ahead of this low along the stalled front and bring periods
of rain to most of western New York. However far northern portions of
Jefferson and Lewis counties can expect 1 to 3 inches of snow
today. Temperatures will rise into the 40s in southern locations south
of the front...but temperatures will remain in the 30s near the
frontal boundary...gradually falling this afternoon as the front
sags slightly southward.
The frontal boundary will sharpen tonight...with its position
critical to the forecast. Although model guidance is in fairly close
agreement...even subtle differences are important since surface
temperatures on the cold side of the front will drop below
freezing tonight. 00z sseo shows several ensemble members with
freezing rain over portions of the Lake Ontario shoreline...with
more significant freezing rain over the St Lawrence Valley.
Current watch and Ice Storm Warning covers this nicely.
Currently...highest confidence for significant icing will be over
portions of the north country on the cold side of the sharpening
boundary. This will result in surface temperatures dropping below
freezing with air aloft warming well above freezing tonight. Some
observations in neighboring Canada are already showing freezing
rain...and expect this to work its way southward into the north
country as a cold northeast flow starts to drain into the region.
Short term /Saturday through Sunday night/...
a complex forecast this time period as a frontal boundary...rich in
moisture will stall over our region bringing heavy precipitation...and
also concerns for freezing rain and ice accumulation where shallow
cold air can undercut the warmer air aloft.
On Saturday the frontal boundary will likely sag southward as a weak
wave of low pressure rides along it over New York state Saturday morning. This
weak wave will bring light precipitation through the day...with
precipitation rates increasing later in the day. A large spread in
850 hpa temperatures across the region...ranging from +4 to +6c near
the state line to -3c across the north country. However a shallow
layer of colder air...advected southward by a northeast wind will
undercut the warmer air. Just how cold this shallow layer of cold
air gets will play big implications on the p-type later Saturday and
into Saturday night.
For Saturday we will have highest chances for precipitation along
the frontal boundary...with categorical across southern
regions...while lowering to low likely over Lake Ontario and the
Saint Lawrence valley.
Saturday night a cutoff low...currently seen on water vapor imagery
over Southern California...will phase with a subtle shortwave in the
northern branch of the jet stream and deepen a surface low that will
advance from near Arkansas Saturday evening to the Ohio Valley by
Sunday morning. Ahead of this surface low a deep southerly flow
around a western Atlantic high will bring Gulf moisture
northward...with precipitable waters rising towards +1.50 inches which is about
+4sd above normal. Isentropic upglide along with the region under
the favorable right entrance region of a +3sd 200 knot upper level
jet over southern Canada at 250 hpa will produce synoptic lift
generating a period of rain. With the high amount of moisture drawn
into this system this rain could become significant beginning
Saturday night...especially when coupled with recent snows
increasing the water in the snowpack.
Recent runs of the meteorological model ensemble river forecasts
continue to highlight the watersheds of western New York as possible
locations of concern of river flooding. Will continue to elaborate
on this concern in the hydrology section below...as well continue
with a Flood Watch for the area.
To the north of this boundary Saturday evening and night freezing
rain will be a concern towards the north country and the Saint
Lawrence valley...as well as along and just southwest of the Lake
For the north country...we have upgraded the headline to a Ice Storm
Warning. A northeast flow will dam the cold air up through the Saint
Lawrence valley will likely keep a shallow layer of sub freezing air
through the night and into Sunday. With air aloft warming as the
baroclinic boundary begins to push northward snow across the region
will likely change to wintry mix/plain rain. With surface
temperatures remaining below freezing an significant ice
accumulation is likely Saturday night...especially through the Saint
Lawrence valley region and even to points such as Watertown...and
also the Black River valley. As deeper moisture advects northward
Saturday night precipitation rates will increase and upwards to a
half inch of ice accumulation on objects is likely. We will carry
the Ice Storm Warning through the day Sunday...with likely just the
Saint Lawrence valley receiving additional ice accumulation as
locations such as Watertown and much of Lewis County may warm into
the middle 30s...enough to limit the ice potential.
For the northern Niagara Frontier and Monroe County...we have issued
a Winter Storm Watch for ice potentially accumulating up to one half
inch. Northeast flow in the lowest 5k feet of the atmosphere will
likely build a subfreezing layer just south of Lake Ontario...and
southward towards the New York state thruway. Believe models will have a hard
time picking up on such a shallow layer...though the GFS with
support from the 00z gefs/European model (ecmwf) and regional Canadian model all hint
at an ice potential with the cold front sagging southward allowing
the cold air to build Saturday night. Confidence is still medium at
best for this event here for if the warmer 00z NAM were to
verify...there would very little ice accumulation. Also the
northeast flow over the middle 30s Lake Ontario may limit the ice
accumulation along the immediate Lake Ontario shoreline.
For now...believe the southtowns of Buffalo and southern portions of
Genesee County will remain warm enough to likely have just plain rain
There will also be a concern for freezing rain Saturday night across
the southeast Lake Ontario shoreline and northern Finger Lakes.
However we believe the ice accumulation will be less here...as
surface temperatures will likely be a few degrees warmer as the
shallow northeast flow undercutting the warming aloft will be
Sunday a filling area of low pressure will advance from just west of
the County Warning Area towards northern New York by Sunday evening. With the
greatest isentropic lift...as well as the rich moisture now to the
east the period of precipitation will be coming to an end early
Sunday...with just a chance of rain showers lingering through the
day. Quantitative precipitation forecast from Saturday into early Sunday will likely range from one
to two inches across the area...and continue the flood threat.
Behind the area of low pressure cold air will begin to deepen Sunday
night and change the mixed precipitation back over to light snow
showers. Temperatures aloft at 850 hpa only cool down towards around
-8c by 12z Monday morning...still too warm to start to generate lake
snows...and though we will carry a low chance probability of precipitation for Sunday
night...expect the bulk of the night to remain cloudy...and brisk as
the cold air deepens.
Mild temperatures will remain through Sunday...then behind the cold
front overnight lows Sunday night will fall back into the 20s...to
Long term /Monday through Thursday/...
the first couple days of next week leading upon to the Christmas
Holiday look to feature a much quieter weather period...and a return
to more wintery weather as colder air deepens over the eastern Great
Lakes. Falling temperatures Monday will bring back the below normal
pattern with snow showers and lake effect snows as a cold upper
level trough approaches the region. Scattered lake effect showers
will linger into Tuesday night before giving way to high pressure
expected to transit the lower Great Lakes later Tuesday night and
into Wednesday. Southerly flow and temperatures moderating towards
normal return on Christmas day as the high moves to our
east...though conditions should remain dry. Another weak storm
system may bring some snow to the region Thursday from the
Aviation /15z Friday through Tuesday/...
a frontal boundary will cut across our region...which will result
in significantly different conditions by location. In
general...expect IFR/LIFR ceilings to the north of the front /kart/ with
mixed precipitation trending to freezing rain tonight. The most
difficult forecast is near the boundary...where mainly MVFR
conditions should deteriorate to IFR today...however conditions
may vary in and out of rain showers and as the front meanders.
Increasing confidence in IFR conditions at these locations tonight
as the front sags southward toward kiag-kbuf-kroc. Kjhw will
remain south of the front...with conditions forecast to drop to
IFR as the inversion lowers. Despite modest SW surface
winds...winds aloft will be strong resulting in low level wind shear.
Saturday and Sunday...MVFR with periods of rain. Freezing rain at kart.
Monday...mainly VFR with chance of snow showers. Local IFR.
Tuesday...mainly VFR with a chance of snow showers southeast of lakes.
a frontal boundary will be stalled roughly in between Lake Erie and
Ontario through tonight. Winds will be primarily NE on Lake
Ontario...and SW on Lake Erie. The front will then meander southward
a bit on Saturday night and Sunday and then will push to the southeast of
the waters for good late Sunday...with a SW flow developing on all
of the waters. Conditions will approach...but fall just shy
of...small craft criteria several times this weekend.
model consensus suggests the region will likely get between 1.5
and 2 inches of precipitation this weekend. Even if these rainfall amounts
verify...there remains significant uncertainty with respect to snowmelt and
runoff. A nearly stationary frontal boundary will be across the
region for much of the weekend...dividing cool and sub-freezing
temperatures from much warmer and moist air just to the south of
the boundary. North of the boundary...the deep snowpack will tend
to absorb at least some of the rainfall and keep runoff
manageable. However...temperatures and dew points will rise well
into the 40s south of the front...which will bring an even greater
risk of flooding.
Utilizing mmefs ensemble guidance for naefs/gefs/sref
guidance...the greatest risk for river and creek flooding is for
the Buffalo creeks and Allegheny basin. This is because it is these
basins which both have the greatest snowpack and are most likely
to stay on the warm side of the boundary. The risk is slightly
less in the Genesee basin and Black River basin...however it
would only take a small shift in the position of the front for
the greatest risk to shift to these basins.
The worst case scenario would be if the heavier rainfall amounts
occur on the warm side of the front...which is shown by some
individual ensemble members. If this occurs...flooding may be
significant with the potential for some forecast points to reach
moderate flood stage.
In addition to potential river flooding...this amount of rainfall
will bring a risk for minor localized flooding in low-lying and
poorly drained areas...since recent snowfall may block storm
drains in some urban areas. Many creeks already have some ice on
them...which could result in minor ice jams following the initial
If flooding does occur...it would begin with the arrival of the
heaviest rain on Saturday night. The risk would linger through
Sunday as various basins respond to the rainfall and snow melt.
Issues could linger into Monday for slower responding rivers.
the ktyx radar in Montague is out of service due to a hardware
failure. Parts are on order. The radar will be out at least
through this morning.
New York...Flood Watch from Saturday afternoon through Sunday evening for
Ice Storm Warning from 7 PM this evening to 11 PM EST Sunday
Winter Storm Watch from Saturday afternoon through Sunday
morning for nyz001>003-010-011.