Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 272023

323 PM EST Fri Feb 27 2015

.SHORT TERM (Now through Saturday Night)...
Issued at 250 PM EST Fri Feb 27 2015

...Unsettled Period of Weather Coming...

After a few early morning flurries/light snows, the rest of the day
has turned out pretty tranquil, though well below normal for
temperatures. Highs today are some 20-30 degrees colder than normal!
Clouds will be on the rise tonight as a winter weather system across
the Southern Plains sends its clouds our way. These clouds should
help to keep our lows from quite as low as they did last night, and
a little bit of a northeast wind should help with that as well. Thus
have forecast lows in the single digits only for our far
northeastern counties, whereas most locations should be in the teens.

Latest high-res models hint at a few radar returns possible as these
clouds move through. The moisture will be fairly high level
according to soundings, but would not be surprised to see some
flakes make it to the surface after midnight and into the morning
hours. Have added a period of scattered flurries areawide.
Temperatures during the day should warm to or above freezing for
many locations, as wind shifts to easterly and thicknesses come up.

Further warm air advection Saturday night will combine with better
moisture to bring a more solid chance for precipitation. Forecast
soundings indicate the transition zone from rain to a wintry mix and
snow will be right over our area concurrently. Used a blend of
NAM/GFS max temperatures aloft as well as forecast surface
temperatures to approximate where those zones will be. As such, came
up with a narrow stripe of around an inch of snow, for the north
part of our northern counties in Indiana, with a few tenths of an
inch possible along the Ohio River. South of the snow zone in our
north, we could see some freezing rain. A lot will depend on
temperatures, but for now have a zone from the Bluegrass and Western
KY Parkways with some light ice accumulations, under a tenth of an
inch. Not sure how much of an impact this will be given the time of
day and it being a weekend, the antecedent wet grounds, which should
aid in slowing the freezing down, and with the warm air coming in we
should transition quickly to rain by late Sunday morning, see
discussion below.

.LONG TERM (Sunday through Friday)...
Issued at 245 PM EST Fri Feb 27 2015

...Period of Active Weather Setting Up For Ohio Valley...

The long term forecast period is focused on two weather systems that
will bring a variety of weather to the Ohio Valley through next week.

System 1: Sunday - Monday night

As a weak area of low pressure moves from the central Plains to
southern Great Lakes, a band of precipitation is likely to break out
early Sunday morning across the northern forecast area. Soundings
show that a tight gradient between all liquid, a wintry mix and all
snow along the IND/LMK forecast area. Included a rain/snow mix with
just a slight chance of freezing rain across the northern forecast
areas where additional light accumulations are possible. Between
12-15z Sunday, southerly flow is expected to warm the boundary layer
temperatures sufficiently to support a changeover to all liquid
across the entire area.

The atmosphere becomes increasingly saturated during the day Sunday
with weak but persistent isentropic lift on the 290-305k surfaces.
Expecting periods of rain showers, highest coverage to be north of
the KY parkways during the day. Plan on highs to top out around 40
across southern Indiana to near 50 across south-central Kentucky. By
late afternoon into Sunday night, as the low pressure lifts
northeast, a front is dragged across the forecast area. This will
bring a band of showers to the entire area over the course of the
night. There is support for a changeover to a wintry mix of
rain/snow/freezing rain late Sunday night into very early Monday
morning across southern Indiana and far northern Kentucky as the
column loses ice saturation aloft coupled with surface temperatures
falling below freezing. It looks to be a short window of opportunity
before moisture really dries up. For the rest of the area, lingering
precipitation will begin to exit Monday morning.

A modified Arctic air mass spills into the region Monday into Monday
night, giving the region a break in between systems. This will be
short-lived as the next system is quick on its heels. Look for highs
mainly in the 40s, still 5 to nearly 10 degrees below normal.

System 2: Tuesday - Wednesday

By Tuesday, high pressure will have pushed to the east coast,
setting up strong southerly flow across the deep south to the lower
Ohio Valley. As upper level energy ejects out of the southwest US, a
surface low develops in the lee side of the Rockies, deepening as it
takes a rough Denver-Kansas City-Chicago-Buffalo track. This puts the
forecast area well into the warm sector and dewpoints are progged to
climb into the low 60s Tuesday afternoon/evening. A near record
amount of moisture could be drawn up ahead of this system and PWATs
off the GFS have consistently been 1.5 to 1.6 inches ahead of the
cold front. This is between 3.0 and 3.5 SD above normal and would be
a record for BNA sounding climatology. We`ll also have surge of
warmer air, bringing milder and spring-like weather to the region.
Highs could reach well into the 60s Tuesday and/or Wednesday,
depending on the frontal passage timing.

There remains timing/placement differences between the forecast
models on the frontal passage and whether any boundaries may stall
out across the area. This of course impacts heavy rain and possible
t-storm, including severe weather, potential. Considering this is
day 5/6, while confidence is above average in seeing a strong storm
system next week, the details and exact placement still need to be
sorted out. The GFS continues to be a faster outlier while the
GEM/ECMWF are roughly 12-24 hours slower with its synoptic features.

Nonetheless, there`s good consensus that the upper levels will be
characterized by a 140-150kt 300 mb jet with a positive tilted upper
trough across the central CONUS. While at 850 mb, a 60-70 kt jet
sets up across the area. MUCAPE values are limited, generally less
than 100-200 J/kg, but the tongue of higher instability wouldn`t be
too far away so timing/location differences could still pull more
instability north. Generally, the high shear low cape environment
looks supportive of elevated thunderstorms with the potential for
strong to severe weather somewhere across deep south to lower Ohio
Valley. CIPS analog for Tuesday evening have the probability of
damaging wind or hail reports between 20-30 percent near the TN

Hydrology will be another main concern as the area could receive
upwards of 2 to 3 inches of rain over the course of the period. See
the hydrology section below for details on how this could impact the
area including river basins.

Thursday - Friday

A modified Arctic air mass noses down into the Ohio Valley in the
wake of the mid-week system, bringing a sharp temperature drop for
Thursday with highs back into the 30s. 27.12z continued to struggle
with how progressive the flow becomes as some of the deterministic
guidance stalls the mid level energy just to our southeast. Given
this moisture would be overrunning over a new, modified Arctic
boundary layer, this could spell concern for wintry weather. Until
these details can be sorted out, a model consensus with slight
chances of primarily rain or rain/snow mix look good at this point.



Issued at 315 PM EST FRI FEB 27 2015

The snowpack in the area contains between a half and two inches of
liquid. The snow will continue to melt into the weekend as
temperatures are forecast to rise above freezing and into the 40s on
Sunday. At the same time, mixed precipitation will move into the
region Saturday night and change over to all rain on Sunday. Up to a
half inch of liquid is expected. The combination of snowmelt and
rain will saturate the ground and cause rises on small streams.

On Monday night, another storm system is expected to move in with
more rain and snow changing over to all rain on Tuesday. There is
even a chance of thunderstorms on Tuesday. The rain will continue
into Wednesday night before it changes back into snow before ending
on Thursday.  The total amount of liquid precipitation is between
one and three additional inches.

This second storm could trigger widespread flooding, especially in
areas with heavier snowpacks. The flooding on the major rivers would
begin on Wednesday. At this time it is too early to predict where
the heaviest rains will fall near the Ohio River. The National
Weather Service will continue to watch these developing storms
closely this weekend. Residents in flood prone areas should expect
to begin to watch water levels next week.

.AVIATION (18Z TAF Update)...
Issued at 1220 PM EST Fri Feb 27 2015

Have pockets of cumulus near the threshold for VFR/MVFR at this time
across the region. Looks like the best chance to go down to MVFR is
at LEX for a few hours, based on latest visible satellite imagery.
These clouds should dissipate as the sun sets, but additional mid
and high clouds will come in here tonight as a disturbance now over
the southern Plains crosses our region. Not expecting precip with
this disturbance but as ceilings lower we could drop down to the
lower VFR range around daybreak Saturday. High pressure northwest of
the region now will shift to northeast of the region by Saturday,
changing our winds from northerly now to easterly by the end of the
forecast period.




Short Term........RJS
Long Term.........ZT
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