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Area forecast discussion
National Weather Service Buffalo New York
342 am EST Thu Nov 21 2019

Synopsis...
after a few days of uneventful weather...our attention will turn to
a storm system that will track well to our west and north later
today through Friday. This will generate a period of very unsettled
weather across our region...including the likelihood for rain
showers along with some gusty winds. High pressure will then build
across the lower Great Lakes to support fair dry weather for at
least the first day of the weekend.

&&

Near term /through Friday/...
generally fair dry weather will remain in place across our forecast
area today...as the axis of a large area of high pressure will exit
our region and make its way across New England. While the majority
of the vast majority of the region will be rain free...any sunshine
that we get during the first half of the day will give way to
increasing cloud cover. The only exception to the dry weather today
could be over Niagara County where some late day rain showers will
be possible...and even those should wait til nearly nightfall.

Temperatures today will be several degrees warmer than the past
few...as warm advection on the backside of the exiting high and
ahead of an approaching cold front will encourage afternoon highs to
range the lower 40s east of Lake Ontario to near 50 in the Genesee
Valley and warmer valleys of the southern tier.

Conditions will then deteriorate tonight...as a deepening sub 1000mb
surface low moving from Lake Huron to western Quebec will push a
relatively strong cold front across our region. While the frontal
boundary will support likely to Cat pops for rain showers...the main
concern will be the intensity of the accompanying winds. Supporting
advisory criteria wind gusts will be the climatologically favorable
track of the deepening storm system to our west and north along with
a 50 to 55kt low level jet. While the strongest winds aloft will be
found ahead of the cold front (as is typically the case with 'cutter
storms')...there will be the risk for some of the strong gusts to
mix to the surface near or immediately behind the sfc front.
Subsidence needed to more efficiently mix the winds to the sfc will
not really be in place until the strongest low level jet passes.
This is also be inferred by forecasts of a weak pv intrusion with
the 1.5 pv sfc only expected to extend as low as 700mb. In any
case...winds will increase overnight with gusts as high as 50 mph
after midnight in a corridor from Lake Erie and the Niagara Frontier
to the Thousand Islands region. Will thus maintain the current Wind
Advisory for these areas.

As the day starts on Friday...the cold front will be in the process
of pushing across the Eastern Lake Ontario region. Strong cold
advection in the wake of the front will allow for at least a mix of
rain and wet snow across the southern tier and Eastern Lake Ontario
regions...where some lake enhancement will delay the ending of the
precipitation. Any snow accumulations in these areas will be under
an inch. Otherwise...the bulk of the shower activity will be done by
midday as much drier air in the mid levels will spread across the
lower Great Lakes. Again...the main concern Friday will be the gusty
winds...which will be strongest during the morning. While the low
level jet in the wake of the front will average 40 kts...diurnally
induced mixing enhanced by stronger subsidence will more easily mix
some of this wind to the sfc. This will result in widespread wind
gusts of 30-35 mph with gusts well into the 40s in the advised
areas. These winds will subside during the course of the afternoon
and evening.

Temperatures will be nearly steady during the midday hours with
slowly falling Mercury readings during the afternoon.

&&

Short term /Friday night through Sunday night/...
Friday night the storm system responsible for friday's windy
conditions will exit across the Gulf of Saint Lawrence...while
broad high pressure and much drier air over the Ohio Valley builds
across our region. This will result in a return to much quieter
weather...with the dry incoming airmass and rather low inversion
heights conspiring to keep any lake response over and southeast of
the lakes confined to a few flurries. Otherwise...nocturnal cooling
of our colder airmass will help to send temperatures down into the
lower 20s across the north country...and to the mid to upper 20s
elsewhere.

On Saturday the surface high will only begrudgingly drift eastward
to New England...while a southern stream trough and associated
modest wave of low pressure develops into the Ohio Valley. While
this latter system should draw close enough to spread some clouds
into our region Saturday afternoon...the departing ridging should
remain dominant enough to result in a mainly dry day...with only an
outside chance of a rain or snow shower reaching the southern tier
toward sunset. Otherwise...expect slightly below average high temps
ranging from the upper 30s/lower 40s east of Lake Ontario to the
lower to mid 40s elsewhere.

Saturday night and Sunday the various guidance packages continue to
exhibit notable differences with respect to just how much the
advancing southern stream trough will phase with a weaker northern
stream counterpart that will be simultaneously sliding across the
Great Lakes region. The GFS remains the most aggressive at phasing
these two features...and consequently tracks the initial surface low
furthest to the north...with this feature tracking directly across
the lower Great Lakes later Saturday night before giving way to a
developing coastal system along the New England coast on Sunday.
Were such a track to be realized...most if not all of our region
would likely see a period of light to moderate precipitation between
Saturday night and the first half of Sunday. On the other end of the
guidance envelope is the Canadian Gem...which shows no such phasing
and subsequently tracks the main surface low eastward well to our
south Saturday night and then northeastward along the Atlantic
coastline on Sunday...with only some light precip brushing our
southeastern periphery. Meanwhile...the European model (ecmwf) allows for some
partial phasing between the two streams and consequently lies in
between these two extremes.

All the above said...it should also be noted that the overall
guidance trend during the last 24 hours has been toward at least a
somewhat more phased system tracking a bit further to the north.
Taking both this and the aforementioned model differences into
account...have elected to bump precip chances up a little higher
overall from continuity and also extend these a little deeper into
Sunday...while still keeping these confined to the chance range for
now given the lingering forecast uncertainty. As for ptype...for the
time being have kept this a general chance of light rain and snow
considering both the expected time frame of the event and the still-
uncertain low track...though some light freezing precipitation
definitely cannot be ruled out...with the potential for this
ultimately dependent on the eventual low track and its resultant
influence on thermal profiles across our region.

Later Sunday and Sunday night this system will exit off to our
east...with a marginally cold enough airmass in its wake potentially
supporting a weak lake response east of the lakes...though this will
again likely be greatly inhibited by building ridging/drier air and
corresponding low inversion heights. Otherwise Sunday night should
be dry and quiet...with lows mainly in the upper 20s to lower 30s.

&&

Long term /Monday through Wednesday/...
on Monday modest ridging at all levels will build eastward across
the lower Great Lakes and into New England. This should result in
a dry day...with a general warm air advection pattern allowing
daytime highs to climb back into the lower to mid 40s across the
north country and southern tier...and to the mid to upper 40s
elsewhere.

Monday night and Tuesday longwave troughing will begin to reload
across the northern plains and upper Midwest...with a corresponding
surface low tracking from the upper Great Lakes across western and
central Ontario province. During this time frame the model consensus
keeps the bulk of any associated precipitation to our north and
west...with a general warm advection pattern persisting across our
region. This should translate into continued mainly dry weather for
our area...with highs on Tuesday surging into the upper 40s and
lower 50s in most areas.

Tuesday night and Wednesday the large-scale trough will extend
eastward across the Great Lakes...though as noted previously model
discrepancies quickly arise in the handling of the main surface
features. While the GFS continues to insist on tracking one strong
low across the upper Great Lakes...the ecwmf is much weaker and
further east with this system...and tracks it directly across New
York state. While both of these scenarios would bring increasing
chances of rain and eventually a downward trend in temperatures on
the backside of this system on Wednesday...the former would also
pose the potential for another round of strong winds across our
region.

&&

Aviation /09z Thursday through Monday/...
while most of western and central New York will experience VFR
weather for the remainder of the pre dawn hours...moisture trapped
beneath a staunch subsidence inversion will encourage areas of
strato-cu that will be based between 2500 and 3500 feet. This will
result in IFR to MVFR cigs at times...mainly across the southern
tier.

The lower cigs will lift during the first several hours of daylight
this morning...as a strengthening southerly flow will help to mix
out the low level moisture. This flow will also advect increased mid
level moisture across the state. This will translate into mainly VFR
conditions with surface winds picking up during the afternoon.

The main issue tonight will be related to winds. A deepening storm
system passing over the upper Great Lakes will push a cold front
across our forecast area. A 50-55 kt low level jet will precede the
front...and while the bulk of these winds will remain aloft...this
will support some low level wind shear across the region. In the
wake of the front...winds of up to 40 kts will impact the usual
corridor from Lake Erie and the Niagara Frontier to the Thousand
Islands region. Meanwhile...MVFR cigs will be accompanied by the
likelihood of rain showers.

Outlook...

Friday...MVFR/VFR with a chance of showers. There will be enough
wind to impact ground operations...especially through the midday
hours.
Saturday and Sunday...mainly VFR.
Monday...VFR.

&&

Marine...
gale warnings are in place across the lower Great Lakes.

The axis of a large area of high pressure will exit across New
England during the midday and afternoon. This will allow winds to
freshen as the day progresses...although the predominant southerly
flow will keep the highest wave action and associated choppy
conditions well offshore or in Canadian waters.

Tonight...deepening low pressure moving from Lake Huron to western
Ontario will push a relatively strong cold front across the lower
Great Lakes. Winds will strengthen in the wake of this front...with
west to southwest winds reaching gale force intensity after 03z on
Lake Erie and on the western third of Lake Ontario...and after 09z
on the eastern two thirds of Lake Ontario. A Gale Warning has now
also been issued for the upper iag river and Buffalo Harbor...with
a high end Small Craft Advisory being issued for the St Lawrence River.

As the still deepening storm system makes its way across northern
Quebec on Friday...gale force winds will remain in place across the
lower Great Lakes through at least the morning hours. Winds will
gradually subside from west to east during the midday and afternoon.

Expansive high pressure centered over the upper mid west Friday
evening will build across the lower Great Lakes Friday night. This
will continue to allow winds to weaken Friday night into Saturday.

&&

Tides/coastal flooding...
another Lakeshore flood event is expected tonight through Friday on
both lakes Erie and Ontario. Deepening low pressure will pass by to
the north of the lakes, with several hours of strong southwest to
westerly winds expected behind a strong cold front.

For Lake Erie, the strongest winds will occur late tonight before
quickly diminishing during the day Friday. Sustained winds will
likely reach between 30 and 35 knots for a few hours on Lake Erie.
In periods of normal lake levels these winds would not be strong
enough to produce a warning criteria seiche on Lake Erie, however
all of the Great Lakes are at abnormally high levels. Given this,
levels on Lake Erie are expected to just exceed the 8 foot level
late tonight.

For Lake Ontario, the strongest winds will occur Friday morning
before slowly diminishing later in the afternoon and evening.
The combination of High Lake levels, strong onshore winds, and high
wave action will result in Lakeshore flooding and shoreline erosion
on the east half of the lake on Friday.

It should be noted that while flooding may occur, we do not
anticipate impacts as significant as the Halloween night Lakeshore
flood event.

&&

Buf watches/warnings/advisories...
New York...Lakeshore Flood Warning from 4 am to 7 PM EST Friday for
nyz006-007.
Wind Advisory from 4 am to 4 PM EST Friday for nyz006-007.
Lakeshore Flood Warning from 8 am to 7 PM EST Friday for
nyz004-005.
Wind Advisory from 11 PM this evening to 10 am EST Friday for
nyz001-002-010-011-019-085.
Lakeshore Flood Warning from 11 PM this evening to 10 am EST
Friday for nyz010-019-085.
Marine...Gale Warning from 11 PM this evening to 10 am EST Friday for
lez020-040-041.
Gale Warning from 4 am to 7 PM EST Friday for
loz043>045-063>065.
Gale Warning from 1 am to 1 PM EST Friday for loz042-062.
Small Craft Advisory from 1 am to 4 PM EST Friday for
slz022-024.

&&

$$

Synopsis...rsh
near term...rsh
short term...jjr

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