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Area forecast discussion 
National Weather Service Louisville Kentucky
633 PM EST Mon Mar 2 2015

..updated aviation discussion...

Short term (now through Tuesday night)...
issued at 300 PM EST Mon Mar 2 2015

Sharp upper trof digging into Southern California today will be our
main weather player heading into the middle of the week. Surface
high pressure centered over central Illinois and Indiana has
established some fairly cold, albeit not truly Arctic air over the
Ohio Valley as temps this afternoon should just crack the 40 degree
mark. Main challenges in this complex forecast scenario include
precip type at onset late tonight, and flood potential as we head
into Tuesday night.

As the upper impulse gets ejected across the Desert Southwest,
surface cyclogenesis gets underway in the Lee of The Rockies
tonight. Our surface high will retreat toward the mid-Atlantic
coast, opening the door for low-level warm advection. Light precip
could start to develop prior to daybreak while temps are still below
freezing, but it will be a race between the moisture and the
boundary-layer warm advection. Temps will be rising before sunrise,
but there could be a brief window for light freezing rain mainly
north of I-64. Between low confidence and the more impactful weather
that follows, will not issue any headlines for this activity, and
cover it with an severe weather potential statement mention and statements as needed.

Strong SW low-level jetting on Tuesday will tap into warm and moist
air from the Gulf of Mexico, pushing temps well into the 50s and
precipitable water values well over an inch. This is at least 2
Standard deviations above normal for Feb/March at ohx, and pushing 3
South Dakota above normal for iln. Heaviest precip will be Tuesday night,
initially from the Heartland into the Bluegrass region, then shifting
south of the Bluegrass Parkway as the night progresses. Moderate to
heavy rain will continue into Wednesday morning, with quantitative precipitation forecast in the Kentucky
counties ranging from roughly 1.5 to over 2 inches. Therefore after
coord with iln and jkl, will hoist a Flood Watch for central
Kentucky from 00z to roughly midday Wednesday, with the end time to
be aligned with the widespread change to snow.

There will be enough elevated instability Tuesday afternoon and
Tuesday night to support embedded thunder. However, even with
surface temps well into the 50s or even lower 60s, the boundary
layer will remain too stable to support surface-based storms, so
there would be little or no severe threat.

Long term (wednesday through monday)...
issued at 300 PM EST Mon Mar 3 2015

..significant winter storm possible Wednesday - Thursday morning...

The focus is on a possible major late season winter storm across
much of Kentucky and southern Indiana.

Synoptic / mesoscale setup

In short, the synoptic setup for a significant snow storm is
impressive for the lower Ohio Valley. During the day Wednesday, a
strengthening 300 mb jet peaks at 180 kts across the lower Great
Lakes with the right entrance region favorably located over the
lower Ohio Valley while the positively tilted 500 mb trough swings a
pv anomaly through Wednesday evening. There is more than sufficient
upper level support.

The mid levels will be characterized by a strong 850 mb jet pumping
anomalously high amount of moisture into Tennessee and southern
Kentucky. Precipitable waters will remain noticeably high, between 0.8 and 1.0
inch, throughout the day before falling off Wednesday night.
Moisture is readily available.

A cross section from southern Indiana to northern Tennessee reveals
a long period of strong, sloping frontogenesis with ample lift
throughout. This peaks Wednesday afternoon, continuing well into
Wednesday night. Additionally, there are periods of negative
equivalent potential vorticity (neg epv) which would support more
intense snow rates, even thunder. Periods of 1-2" per hour not out
of the question. This is most likely along and just north/south of
the Kentucky parkways, late Wednesday afternoon through Wednesday evening.

Concurrently, significant low level cooling arrives on the heels of
the strong Arctic high pressure Wednesday morning across southern
Indiana and northern Kentucky. The models are extremely impressive
with this cold surge, sending readings into the low 20s or teens by
Wednesday evening! The cold air will take its time to reach southern
and southeast Kentucky, possibly not until mid or late evening. This
cold air surge is one critical element of the event.

Model trends / preferences

A time-trend analysis of the past 24 to 36 hours shows that models
have consistently shown this banded, overrunning precipitation as a
secondary wave develops across the south. On top of a rapidly
cooling boundary layer, all of the deterministic models are showing
warning level snowfall across the area. Previously, the faster, more
progressive GFS has trended slower and back to the northwest with
its f-gen band. This adds confidence to the European model (ecmwf)/NAM/Gem solutions.
A blend of wpc/GFS/NAM quantitative precipitation forecast was used.

Wild cards

The main wild card will be how fast the low level cold surges
southward. The GFS by far is very fast with the cold air arrival,
while the NAM/Gem and blended guidance are several hours slower. This
will have huge impacts on the snow/ice amounts. There is potential
for a significant amount of sleet across southern Kentucky where the
warm nose aloft lags the most while the boundary layer cools. This
may significantly reduce snow amounts through 06z Thursday.

Sensible weather / snow amounts / impacts

Wednesday morning precipitation will likely start off as rain then
the changeover to brief sleet/freezing rain then quickly to snow
along and north of the Kentucky parkways. The changeover will then take
place across the Cumberland and southern/southeast areas later in
the afternoon or evening.

The juxtaposition between the heaviest quantitative precipitation forecast and sufficient cold air,
both at the surface and aloft, is the biggest challenge, especially
in a strong frontogenetical banding event like this. However, the
synoptic setup is very impressive for this area. Despite little
historical guidance for these types of events of this magnitude so
late in the season, the atmospheric dynamics and setup are very
unusual which makes for an realistic but unusually challenging
forecast.

With that said, confidence is certainly high enough to issue a
Winter Storm Watch for the entire area Wednesday through Thursday
morning. Amounts of 4 to 8 inches are conservatively reasonable
across the entire area. However, locally higher amounts 10 to 12
inches not out of the question across central Kentucky.

If this scenario pans out, there would be significant travel impacts
Wednesday and Thursday for the area. Compared to the recent Feb 16
significant snow storm, this snow will be of a heavier, wetter
variety. This could put excess weight on power lines and trees.
Also, significant sleet and/or freezing rain could create additional
problems closer to the Tennessee border.

Thursday

Following the mid-week system, there's good consensus among the
02.12z guidance that Canadian high pressure builds into the Central
Plains and Ohio Valley. With northwest flow aloft and cool high
pressure, it will be another unseasonably cold day with highs only
in the 20s to near 30 degrees.

Thursday night through Monday.

A split flow will preside over the western Continental U.S. Friday with 500mb
ridging expected over northern California northwest of a practically
cutoff trough south of Arizona. The northern branch of the jet will
dive southeast across the northern rockies into confluent flow
across the plains.

Troughing will continue over the Great Lakes and the Ohio Valley,
but its amplitude will diminish, allowing our recent cold
temperatures to slowly moderate towards more seasonal values for
early March (when high temperatures average in the lower 50s).

High pressure will lie right over the commonwealth early Friday,
with cold temperatures and light winds likely during the morning
hours. If we have fresh snow cover and perfect radiational cooling,
temperatures early Friday may fall to near zero across our colder
locations. Despite mostly sunny skies, highs on Friday will only
rebound only into the lower to mid 30s. Surface ridging over the
Ohio Valley will continue Friday night into Saturday. Under partly
cloudy skies, Saturday will warm well into the 40s.

Within this evolving northwestern flow pattern, Gulf moisture will
stay confined to near the coast. Therefore no precipitation is
expected with the passage of a weak surface trough late Saturday. This
cold front will be quite weak, only resulting in a wind shift to the
northwest.

The European model (ecmwf) shows moisture returning north through Tennessee late
Sunday associated with a southern stream disturbance. Although the
GFS and the Canadian models show dry weather late Sunday into early
Monday, have left in just a slight chance for light precipitation
late Sunday.

Highs Monday will rise into the lower to mid 40s.

&&

Hydrology...

Issued at 310 PM EST Mon Mar 2 2015

Rivers have returned to normal winter flow, but soils remain
saturated from recent snowmelt. The rest of the snowpack will be
added to the mix with tuesday's warm temps, and rises from heavy
rainfall will be quick due to saturated soils.

Still expecting 1.5 to over 2 inches of rain across central Kentucky
before the changeover to snow on Wednesday. Heaviest amounts will
be across south central Kentucky, and that coincides with the most
saturated soils from snowmelt. This will likely produce
widespread minor flooding in the salt, green, Licking, and
Kentucky basins. Residents in flood prone areas should closely
monitor water levels over the coming days and be prepared for
possible flooding by mid to late week.

&&

Aviation (00z taf update)...
issued at 630 PM EST Mon Mar 2 2015

VFR conditions are expected this evening through the early morning
hours before an approaching storm system spreads precipitation into
the region tomorrow morning. Low level moisture will begin to
increase around 9-10z with MVFR conditions above fuel alternate
resulting. Rains and low end MVFR conditions look to arrive around
11-12z at the taf sites. At Lex/sdf, sfc temps may be close to
freezing or just below freezing at the time of rain onset so
freezing rain/fzdz may be possible for a brief window before temps quickly
warm just after sunrise. Still not confident enough to include a
freezing p-type group in the taf sites at this point due to the
short window of opportunity but will monitor closely overnight.
Rain will continue for much of the day tues especially near sdf
where a band of moderate to heavy rainfall may set along the Ohio
River late in the day. Think that sdf/Lex terminals will probably
go to IFR conditions during the afternoon hours. Bwg may go IFR
also but not as confident here where precip coverage/low level
moisture depth may not be quite as ideal. Easterly winds this
evening/overnight will become predominantly south-southwest between 8-12 kts
tomorrow.

&&

Lmk watches/warnings/advisories...
Kentucky...Winter Storm Watch from Wednesday morning through Thursday
afternoon for kyz023>043-045>049-053>057-061>065-070>072.

Flood Watch from Tuesday evening through Wednesday afternoon for
kyz023>043-045>049-053>057-061>067-070>078-081-082.

Winter Storm Watch from Wednesday afternoon through Thursday
afternoon for kyz066-067-073>078-081-082.

In...Winter Storm Watch from Wednesday morning through Thursday
afternoon for inz076>079-083-084-089>092.

&&

$$

Short term........Ras
long term.........Zbt
hydrology.........Ras
aviation..........ams

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