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Area forecast discussion
National Weather Service Buffalo New York
253 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/...
high pressure to the north of the state is providing the area
with northerly flow. A thin layer of moisture has morphed from
stratus along and north of Lake Ontario to clearing while inland
areas have experienced a flat cumulus field. Expect this to
dissipate later this afternoon with clearing over much of the
region by sunset.

Tonight...high pressure settles across the region with mostly
clear skies and light winds resulting in decent 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...increasing clouds from west to east will be the main
feature for much of the day as moisture streams northeastward
in a warm advective pattern. However, models continue to
struggle with the depth of the moisture with the operational NAM
and GFS differing toward the onset of precipitation. Ensemble
data of course is somewhere in between. There will be a
tightening baroclinic boundary moving in and this will help
moisten up the atmosphere, but at this time cannot fine tune the
rain start time in terms of rain starting in the early
afternoon, late afternoon, or early evening for at least western
New York. Will therefore continue with a increasing probability of
rain with time, starting out as sprinkles due to the initial dry
air near the surface.

&&

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. For sites east of
Lake Ontario...the pcpn could start off as a wintry mix...so will
maintain that wording. Otherwise...near 100 pops will remain intact
across the western counties while high likely pops will be in place
for the north country.

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 Tuesday/...
ridging moving across the lower Great Lakes should supply US with a
nice ending to the weekend...as at least partial sunshine and 850 mb
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 probability of precipitation and quantitative precipitation forecast. 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 European model (ecmwf).

&&

Aviation /19z Wednesday through Monday/...
VFR conditions are expected through Thursday. High pressure
will be north of Lake Ontario with light northerly flow today.
There is a stratus deck over Lake Ontario and Eastern Lake
Ontario, but ceilings are near 5000'. This will not make much
progress to the south today, and it should start to mix out in
the dry airmass later this afternoon. Winds will shift to the
east overnight and Thursday. Eventually higher clouds will move
into the region, with slowly lowering cigs during Thursday
afternoon.

Outlook...
Thursday night into Saturday...MVFR/IFR with rain showers.
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.

&&

Buf watches/warnings/advisories...
New York...none.
Marine...none.

&&

$$

Synopsis...zaff

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