Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 250457
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1157 PM EST Mon Feb 24 2020

.SYNOPSIS...
After a couple days of quiet and mild weather...a slow moving
low pressure system will bring a prolonged period of unsettled
weather for the rest of the week. Expect an initial mix of light
rain and snow to develop later on tonight and transition over to
rain Tuesday morning...with another mix of rain and snow Tuesday
night then giving way to occasional rain on Wednesday. A strong
cold front will then cross the area Wednesday night and usher in
a much colder airmass that will change the rain back to snow by
Thursday morning...and also bring the potential for heavy lake
effect snow to areas east of Lakes Erie and Ontario for the
rest of the week.

&&

.NEAR TERM /THROUGH TUESDAY/...
Tonight a broad surface low will slowly weaken while making its way
northeastward from southern Missouri to southern Indiana...while
pushing its attendant warm front toward our region. Out ahead of
this system...deepening moisture and developing weak isentropic
ascent has resulted in thickening and lowering clouds to our region
this evening. This will be followed by an increasing potential for
some light precipitation (currently as close as eastern Ohio and
central Lake Erie) across areas south of Lake Ontario overnight...
particularly from about the Genesee Valley westward. Model
temperature profiles continue to suggest that the precipitation will
primarily be in the form of a rain/wet snow mix...with wet snow
becoming a bit more prevalent over time...at least for a
while...with evaporational and nocturnal cooling of the lower levels
of the column. This said...given the light and mixed nature of the
precip and the still warm sfc temps this evening should keep any
snow accumulations should be very minor...and these may well be
largely confined to just the higher terrain. Otherwise nighttime
lows will range from around 30 across the North Country to the lower
to mid 30s elsewhere.

On Tuesday the surface low will continue to weaken as it makes its
way further up the Ohio Valley...with the initial shot of isentropic
ascent across our region lifting a bit further northeastward early
in the day...before tending to weaken and slide off to our east
through the late morning and afternoon hours. Coupled with some
warming of the column...this should result in the initial mix of
rain and wet snow changing back over to all rain during the
morning...before at least temporarily becoming more spotty in nature
during the afternoon. Trended toward more of a drier idea in
pops and wx 17z-21z before next round of rain moves back in from
the south and west. Otherwise plenty of cloud cover and the
cool northeasterly low level flow across our region should help
to mostly confine afternoon highs to around 40F...with some
upper 30s found across the higher terrain.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
Tuesday night an elongated and weak area of low pressure will drift
slowly northeast through the Ohio Valley. A broad low/mid level
baroclinic zone will exist to the north of the low from the Midwest
States into the eastern Great Lakes. A broad area of differential
thermal advection will remain in place Tuesday night through
Wednesday morning in the region of this boundary, with low level
northeast flow and cold advection topped by mid level southwesterly
flow and warm advection. This will continue to support frontogenesis
and isentropic upglide, which will be enhanced by additional ascent
from DPVA ahead of a mid level vorticity maxima later Tuesday night.

Precipitation amounts will be relatively light Tuesday night through
Wednesday morning, with a few tenths of an inch in most locations,
less east of Lake Ontario. Precipitation type will be tricky, with
nearly isothermal near freezing profiles through the lowest layers
of the column. The latest 12Z model guidance has trended a little
colder for this time period, especially the NAM and WRF packages and
subsequently the HREF ensemble mean. Given this trend, adjusted
precip types more towards wet snow for Tuesday night through
Wednesday morning. There may be some minor 1-2 inch accumulations
across Western and Central NY with this, especially across the
higher terrain to the south and east of Buffalo and south of
Rochester.

The weak/elongated low will finally deepen into a stronger, more
consolidated surface low Wednesday afternoon across Ohio in response
to a strong trough digging into the Ohio Valley. Increasing SSW flow
ahead of this system will force the baroclinic zone northward across
the eastern Great Lakes, temporarily ending the steadier light
precipitation in favor of scattered showers for much of Wednesday.
Steadier precipitation will move back into far Western NY late in
the day with the approach of the strong trough. Temperatures will
also surge warmer again Wednesday afternoon in strengthening warm
advection ahead of the system, with highs in the low to mid 40s in
most areas.

A very complex synoptic evolution will then unfold Wednesday night
and Thursday. The sharp Ohio Valley trough will deepen further, and
take on a negative tilt as it phases with additional shortwave
energy diving into the western portion of the trough. The surface
low will move northeast across Lake Erie Wednesday night, moving
north of Lake Ontario by Thursday. The associated cold front will
cross our region slowly from west to east Wednesday night, with most
of the operational guidance suggesting one or more baroclinic waves
running northward along the advancing front. The presence of these
waves greatly complicates the forecast, as the waves may force the
front to take on more anabatic characteristics with steadier,
heavier precipitation developing on the cold side of the frontal
zone.

Expect a period of widespread precipitation Wednesday night with the
passage of this system, generally advancing from west to east across
the eastern Great Lakes. Forcing is very strong, and the system will
capture an impressive plume of deep moisture with Gulf of Mexico
origins. The quality of moisture and dynamics coming together will
produce a general 0.50"-0.75" of liquid equivalent across the region
Wednesday night. Precipitation type will generally be rain ahead of
the front, then transition quickly to wet snow behind the frontal
zone from west to east overnight. There may be a period of
accumulating wet snow in some areas behind the front, although
amounts will be highly dependent on the fine details of how the
aforementioned frontal waves develop.

Temperatures never get as warm east of Lake Ontario, and its
possible some, or even most of the precipitation may fall as wet
snow across the higher terrain of the Tug Hill Plateau and western
foothills of the Adirondacks, even on the warm side of the frontal
zone. The potential for accumulating synoptic snow Wednesday night
will need to be watched closely in future model runs, given the
complexity and strongly forced nature of the system.

Cold air will pour into the eastern Great Lakes Thursday as the
surface low deepens further across the Ottawa Valley. General wrap
around snow showers will continue as the deep, closing off mid level
trough crosses the eastern Great Lakes. Lake effect/lake enhanced
snow will also develop east of the lakes. Given the proximity of the
upper level low, there may be some shear initially that inhibits
band organization. Even so, lake enhanced and orographically
enhanced snow will likely produce moderate accumulations across the
higher terrain east of Lakes Erie and Ontario. It will also be quite
windy Thursday, with wind gusts in the 35 to 45 mph range common
across the region. Advisory level gusts in excess of 45 mph are a
possibility. The wind will also produce significant blowing snow
where accumulating snow is falling east of the lakes.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
...Significant Lake Effect Snow Possible Late This Week...

A mid level trough will evolve into a cutoff low Thursday night over
southern Quebec, then remain in place through Saturday before slowly
moving east towards the Canadian Maritimes later Sunday. A strong
surface low will become vertically stacked beneath its mid level
counterpart Thursday night, then gradually fill as the system
occludes through the end of the week. The overall synoptic scale
pattern strongly suggests the potential for a long lasting,
significant lake effect snow event east of Lakes Erie and Ontario
for the latter portion of the week, matching well the analog pattern
of past significant westerly flow lake effect snow events. CIPS
analogs also suggest a strong potential for a significant lake
effect snow event east of the lakes, with the relative greatest
potential east of Lake Ontario. It will be quite windy, especially
early in this event Thursday through Thursday night, with
significant blowing and drifting snow.

This will be a long lasting event, starting Thursday morning and
lasting through Saturday night. The best setup may end up being
later Friday and Friday night, when the coldest air aloft, deepest
moisture, and strongest synoptic scale support all come together.
This may be the period of greatest snowfall rates and accumulation,
flanked by periods of light to moderate snow before and after.
Diurnal impacts will need to be considered given how late in the
season we are. The higher sun angle and stronger daytime diurnal
mixing can disrupt the band scale ageostrophic circulations,
resulting in less organizational structure during the afternoon and
early evening in late season events. A more dense synoptic scale
overcast would help alleviate these concerns, and it appears that
may be possible Friday.

The long lasting nature of this event and quality of the setup
suggest this has the potential to be a major event. It remains too
early for specifics, but snowfall accumulations over the 3 day
period from Thursday through Saturday may be measured in feet. Off
Lake Erie the greatest amounts will focus across the higher terrain
east of the lake, and well south of the Buffalo Metro Area. Off Lake
Ontario, the greatest amounts will focus across the higher terrain
of the Tug Hill region. Of the two lakes, snowfall will likely be
highest east of Lake Ontario where the best setup appears to last
the longest. Boundary layer flow will become northwesterly towards the
end of the event later Saturday or Saturday night, spreading
multiple bands and lighter accumulations to areas southeast of Lake
Ontario.

The lake effect snow will finally taper off and end Sunday as high
pressure moves into the eastern Great Lakes. This will be quickly
followed by the start of a warming trend Monday.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
For the 06Z TAFS, VFR flight conditions are found, but these
will deteriorate to MVFR/IFR through the next 6 hours as low
pressure over the Lower Ohio Valley spins northeastward. This
surface low will send a batch of mainly rain, and higher
elevations snow (KJHW) over our WNY TAF region this morning. A
light northeast wind will prevail this morning and afternoon.

The precipitation combined with a northeast flow will bring at
least MVFR and likely IFR flight conditions this morning across
WNY. With the warm frontal precipitation stalling and then
weakening today...KART will likely remain dry with the
precipitation just to the south of the airfield.

Tonight another wave of warm frontal precipitation will pass
across the TAF region with widespread IFR flight conditions.


Outlook...

Wednesday...MVFR/IFR with morning rain and snow giving way to
periods of rain Wednesday afternoon...then changing back to all
snow Wednesday night.
Thursday and Friday...LIFR/IFR in potentially heavy lake effect
snow east of the lakes...with MVFR/VFR and scattered snow showers
elsewhere.
Saturday...IFR/MVFR with lake effect snow southeast of the lakes...
and scattered to numerous snow showers elsewhere.

&&

.MARINE...
Light winds will become northeasterly and slowly increase overnight
and Tuesday as low pressure slowly makes its way into the Lower Ohio
Valley. Further strengthening of the northeasterly flow is then
expected as the low tracks further up the Ohio Valley Tuesday night.
Have issued a new round of Small Craft Advisories on Lake Ontario as
outlined below. Appears that winds and waves may come up just shy of
criteria on Lake Erie.

These initial Small Craft Advisories are just the open of what will
be a very active mid to late week period on both Lake Erie and Lake
Ontario as a deepening storm system brings an increasing potential
for gales to both lakes beginning late Wednesday night into Friday.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory from 7 PM Tuesday to 7 PM EST Wednesday
         for LOZ042-043.
         Small Craft Advisory from 7 PM Tuesday to 10 AM EST
         Wednesday for LOZ044.

&&

$$

SYNOPSIS...AR/JJR
NEAR TERM...JLA/JJR
SHORT TERM...Hitchcock
LONG TERM...Hitchcock
AVIATION...Thomas
MARINE...JLA/JJR


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