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Area forecast discussion
National Weather Service Buffalo New York
432 am EDT Thu Mar 23 2017

Synopsis...
high pressure across the lower Great Lakes will result in fair
weather today with temperatures rebounding a bit. A warm front will
bring a period of rain or mixed precipitation late tonight and
Friday, followed by warmer temperatures. This front will stall
across the region this weekend, with several disturbances moving
along the front and bringing periods of rain over the weekend and
into the first half of next week.

&&

Near term /through tonight/...
today will get off to a chilly start with Arctic high pressure
across the region. This will provide good radiational cooling
conditions, especially where there is still snow pack in place.
Temperatures at daybreak will be in the teens and single digits
across most of western New York, with some below zero readings
across the north country.

This high will shift to the mid Atlantic states today, but still
will provide fair weather and sunny skies for most of the day. High
clouds will approach from the west in advance of our next system
late in the day. Temperatures will be much warmer today, with highs
around 40 across far western New York with lower to mid 30s
elsewhere.

Things will get more active late tonight when a warm front extending
from a system across Colorado approaches the region. This boundary
will be best defined in the mid-levels with a sharpening thermal
gradient and southwesterly 850mb winds increasing to around 45 kts
by 12z Friday. This will result in a round of precipitation late
tonight.

Model guidance has trended slightly slower with this, with a
consensus bringing this precipitation into western New York well
after midnight. Despite minor shifts in timing, confidence in
measurable precipitation is high given the synoptic scale lift and
good model agreement. Although this should primarily fall as rain,
it still may start off as a brief period of mixed precipitation.
Forecast thermal profiles suggest snow, sleet, and freezing rain are
all possibilities across the interior with warmer surface
temperatures across the lake plains west of Rochester likely to
result in just plain rain.

Of these precipitation types, the most critical is the chance for
freezing rain since even trace amounts of ice can be problematic.
Although warming mid-level temperatures will eventually rule out
snow or sleet, surface temperatures will be the key with these
expected to be near freezing across the interior at the onset.
However, if high clouds move in a bit early this evening, surface
temperatures may be warmer which would lower the risk for any
freezing rain. Following coordination with neighboring offices, we
will continue to mention this in the severe weather potential statement outlook.

&&

Short term /Friday through Saturday night/...
this period will feature a prolonged period of precipitation as a
frontal boundary approaches and then stalls across the region.
Rainfall amounts of one to two inches over a 48-hr period starting
12z Friday will produce a soggy ground and low-lying ponding of
water. With the rain spread out enough through the period it will
limit any flash flooding concerns.

Looking into the details this early Thursday morning, a surface
anticyclone is found over the northwest territories, and this feature will
supply cold air, with the leading edge of the cold airmass now
dipping southward over the southern Canadian prairies. Meanwhile
water vapor imagery displays a deep trough of low pressure over
Southern California, heading towards the desert SW. The sprawling
area of surface high pressure that is over US now, will slide to the
east today. A southerly flow around the high pressure will be
increasing moisture, with a 50 knot low level jet bringing deeper moisture
northward. Through this time period the Canadian cold front will
slide towards the eastern Great Lakes region, stalling as the SW
upper level low begins to pull milder, and more moist air northward
across the Ohio Valley region.

Initially isentropic upglide motion will bring precipitation that
will continue to spread across our region Friday morning. There will
be a concern for freezing rain briefly at the onset of falling
precipitation as lingering cold air leaves a surface that will be
near freezing. There will not be a surface high pressure over Quebec
freshening this cold air, so the southerly winds should be able to
scour out any subfreezing temperatures quickly, but still a threat
will be there for freezing rain, especially across the southern
tier, and northeastward across the Genesee Valley/Finger Lakes
region and into the Eastern Lake Ontario region. This area of mainly
rain will push northward, possibly exiting the so. Tier by early
morning, while then reaching the north country.

As the cold front from Canada drops towards the region Friday night,
a baroclinic frontal zone will tighten over the region. The threat
for rain will continue along the boundary. Rainfall could be
moderate as +2sd precipitable waters develop across the region and along the
frontal boundary. This frontal boundary will sag southward some
later Friday night and Saturday as the Arctic high pressure pushes
colder, denser and drier air southward. As the boundary shifts
southward, so will the axis of precipitation, with rainfall at this
time largely falling along and behind the cold front. A northerly
flow may bring areas of drizzle and fog south of Lake Ontario Friday
night and into Saturday. The lower boundary layer across the Saint
Lawrence valley and Eastern Lake Ontario region may become cold
enough to support some wintry mix precipitation Friday night.

The cold front will return northward as a warm front by Saturday
night as the SW upper level low is now reaching the mid-Mississippi
Valley and lower Ohio Valley. This upper level low will air in
establishing a deep southerly, moist flow that will continue chances
for precipitation through Saturday night.

There will be chances for snow/sleet Saturday night across the
Eastern Lake Ontario region, and possibly southeast of Lake Ontario as the
Arctic high pressure pushes a shallow cold airmass across the
region.

There will be a good spread in temperatures across the region this
period. Temperatures will climb into the low to mid 50s south of the
frontal boundary, while to the north highs will only manage to reach
the 40s.

&&

Long term /Sunday through Wednesday/...
an active wet pattern will continue Sunday into at least the middle
of next week as western and north-central New York will remain within the
track of at least two mid-level troughs. Each of these troughs will
provide synoptic lift over and moist southerly flow into our region
that will interact with a low frontal boundary. The first will shift
over our region in the Sunday-Monday time frame while the second
looks to shift over our region Tuesday. It appears now that this
boundary from Sunday into Monday will lie further to the north
towards or across the Saint Lawrence River valley then should be
shoved even further northeast by the second trough/surface low. For
western New York into The Finger lakes, the position of this front will
allow for plenty of low level warm air to support high confidence
for a soaking plain rain. Looking across the north country into the
Saint Lawrence valley, the position of the front will be more
sensitive to p-type and a risk for some freezing rain Sunday night
and Monday night with cold northeasterly flow down the slv. Have
kept any wording for freezing rain at chance pops due to the
uncertainty in frontal position this far out.

In terms of surface temperatures, areas to the south of the front
will mainly feature warmer temps upper 40s into the 50s while areas
north of the boundary will see cooler temperatures 30s and lower 40s
with some 20s possible in the north country during the overnights.

In terms of qpf, a rough average accumulated total among the models
Sunday through Tuesday ranges from around a half inch on the European model (ecmwf)
to around 2 inches on the GFS. While the long duration of this event
should not bring any flash flooding concerns, river and stream rises
and areal flooding of low-lying areas may be possible. If sub-
freezing air remains locked at the surface on the north side of the
front there could also be a threat of some significant ice
accumulations but the lowest confidence is found in that scenario.
Overall, confidence on the specific placement of any weather hazards
during this period is fairly low at this range which has precluded a
mention in the hazardous weather outlook.

On Wednesday, the second surface low will shift off into New England
with a cooler and perhaps very dry northwest flow in its wake. The
12z European model (ecmwf) shows very dry air with just a cool northwest flow on
Wednesday while the GFS shows some lingering wrap around moisture
may lead to some upslope/lake enhanced showers. Have gone with
slight/low chance probability of precipitation for Wednesday under a more clear signal shows
up in the guidance.

&&

Aviation /09z Thursday through Monday/...
high pressure across the region will result in clear skies
through early afternoon, with VFR conditions. The surface high
will drift to the mid Atlantic later today with a modest
increase in mid/high clouds from west to east this afternoon.

Clouds will thicken and lower tonight, but it should generally
remain dry and in the VFR category until just before 12z Friday
when precipitation moves in. This should fall mainly as rain,
but may start as snow, sleet, or freezing rain at Art/jhw. There
is also a potential for low level wind shear late tonight.

Outlook...

Friday...deterioration to MVFR/IFR with rain. The rain may begin as
a brief period of freezing rain or snow in some areas.
Saturday thru Monday...MVFR/IFR with rain likely at times.

&&

Marine...
high pressure across the lower Great Lakes will slide to the mid
Atlantic states today and maintain light winds and negligible waves
through this evening.

S to SW winds will increase Friday, and will approach small craft
criteria across eastern portions of Lake Erie and Ontario. After
this, a boundary will stall across the region, with a northeasterly
flow likely on Lake Ontario and variable winds on Lake Erie. This
may require small craft headlines on Lake Ontario at times this
weekend.

&&

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

&&

$$

Synopsis...apffel
near term...apffel

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