269 
FXUS61 KBGM 240607
AFDBGM

Area Forecast Discussion
National Weather Service Binghamton NY
107 AM EST Thu Dec 24 2020

.SYNOPSIS...
Rain develops this morning, becoming heavy at times this
afternoon through tonight as a strong storm system brings much 
warmer temperatures and significant snow melt. Flooding will be 
a big concern for much of the region. Winds also get quite 
gusty, especially higher elevations of the Poconos and 
Catskills. Falling temperatures on Christmas may ice up roads.

&&

.NEAR TERM /THROUGH TODAY/...
730 PM update...
A quiet evening before the storm. 

Temperatures are holding between 34 and 39 degrees in most 
places, continuing a slow melt of the existing snow pack. 
Temperatures will hold steady for most of the night, then begin 
to rise between 1 AM and 4 AM.

Rain will arrive in earnest across our western forecast area
between 8 AM and 10 AM Thursday. Expect 0.10" to 0.20" of rain
accumulation by midday.

Previous discussion... 
Changes to the forecast include adding Oneida and Madison to 
the flood watch. Expected rainfall amounts have increased. Parts
of these counties got the heavy snow from last week.

Otherwise more certainty that flooding will happen late Thursday
into Friday. Rainfall amounts have increased again as the models
slow the timing of the rain exit with the strong cold front. 

By Thursday morning, a very amplified upper trough will be
located over the Mississippi Valley, connecting a conveyer belt
of anomalously high moisture levels coming up from the Gulf of 
Mexico, right across the Appalachians to the Northeast. Rain 
will start to develop in the morning, but then becomes heavy at
times Thursday afternoon through Thursday night. Temperatures 
will also surge into the 50s with dewpoints following similarly
as south-southeasterly winds increase across our region. This 
will cause snowmelt of most of the heavy snowpack that exists 
across the area. 

A strong low level jet of 60-70 knots will transport moisture to
boost precipitable water values to around or slightly in excess
of an inch, which is very high for late December. Surface winds
will not be as strong as that due to low level stability, but
the will still get quite stiff. Late Thursday into Thursday
night, gusts could easily hit 30-40 mph in higher terrain
portions of the area; and even 45 mph or so for the Poconos-
Catskills. In addition to potentially causing scattered power
outages, this wind driving warm and moist air into the snowpack 
will further enhance rapid melting. Seeing how this is a late
second to third period borderline wind advisory will hold off on
the issuance. 

Models have skewed just a few hours later with the eventual
cold frontal passage of this system. That may not sound like 
much, but unfortunately this widens the window for significant 
snowmelt from high dewpoints driven by steady south-southeast 
winds, as well as prolonging rainfall a bit more. The reason for
this delay, is a wave of deepening low pressure that forms in 
the southern Appalachians late Thursday, as the amplified upper 
trough to the west cuts off an upper low. This will hang up the 
front for a time, until the surface low passes directly through 
our region late Thursday night. Only after that, will the now 
negatively-tilting cold front go through the region, early 
Christmas morning. 

All told we are looking at a widespread 2 to 3 inches of 
rainfall, with localized maxima around 4 inches in highest 
terrain, especially Poconos-Catskills where topographic upslope 
enhancement will occur. This coinciding with most of the 
snowpack melting, and rapidly, points to a significant flood 
threat beyond just ponding of water. Flash flooding could occur 
in urban/low lying areas, especially where snow is blocking 
drains. Meanwhile, small streams and creeks will quickly 
respond. Rivers will take more time and will continue to rise 
through Friday. Many rivers in the Susquehanna and Delaware
basins will get above flood stage and could hit major levels. 

Temperatures late Thursday night fall from west to east behind
the front changing rain to snow. By 7 AM only Steuben County is
below freezing. An inch or two of snow is possible there.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/...
330 PM Update... 
Frontal rain will lift out Friday morning, to be replaced by 
snow showers but with light amounts. The bigger problem will any
wet roadways freezing over, along with those light amounts of 
snow on top, to create potentially hazardous travel. Timing of 
the cold front looks to be Christmas morning, traversing quickly
from west to east. By sunset on Christmas, temperatures will be
in the lower 20s to near 30, and falling rapidly.

Cold air continues to advect into the region with 850 mb temps down 
to around -13C. With Great Lake temperatures still in the 40s, 
some lake effect snow is likely going to develop, with Steuben 
getting some snow from Erie and then the NY Thruway corridor 
getting some snow off of Ontario. A more defined band of lake 
effect may develop off of Ontario, and slight shifts in the wind
may bring that band into the southern Tug Hill in northern 
Oneida.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
330 PM Update... 
An 850 mb ridge builds in Sunday and brings an end to the lake 
effect snow on Sunday with warming temperatures. Upper level 
ridge builds in late Sunday into early Monday with dry weather 
expected along with warmer temperatures. Highs on Monday will 
climb into the 30s to near 40. Conditions become more active 
late Monday as a cold front approaches from the west and ushers 
in a chance of rain and snow...changing to all snow by Monday 
night into Tuesday as another cold blast of air arrives, with 
possibly some lake effect snow through the day Tuesday.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
An incoming strong storm system is already resulting in 
widespread southerly low level wind shear, and will cause
restrictions and rain to develop south to north between 10-16Z.
Conditions further deteriorate for this afternoon through
evening with moderate to heavy rain, on top of existing snowpack
for most terminals, with resulting fog and low ceilings. There
is some uncertainty as to whether terminals with thicker
snowpack, especially KBGM, could get worse restrictions than 
already forecast - perhaps even to near airport minimums.
Surface winds will generally be southeasterly 10-15 knots with
gusts increasing to 22-30 knots. Low level wind shear meanwhile
will reach 40-55 knots within the lowest 2 kft agl, with
significantly stronger winds just above that.

Outlook... 
Late Thursday night through Friday morning...Low level wind 
shear abates predawn Friday morning, but restrictions persist 
as strong system gradually lifts out of the area. Rain may 
briefly turn to snow before ending.

Friday afternoon through Saturday night...Restrictions possible
in scattered lake effect snow showers, especially KSYR-KRME.

Sunday through Sunday night...Mainly VFR.

Monday...Restrictions possible with a chance of mixed rain-snow.

&&

.BGM WATCHES/WARNINGS/ADVISORIES...
PA...Flood Watch from noon EST today through Friday morning for 
     PAZ038>040-043-044-047-048-072.
NY...Flood Watch from noon EST today through Friday morning for 
     NYZ009-022>025-036-037-044>046-055>057-062.

&&

$$
SYNOPSIS...MDP
NEAR TERM...DJP/TAC
SHORT TERM...AJG/MDP
LONG TERM...AJG/BJT
AVIATION...MDP