Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 241145
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
645 AM EST Sat Feb 24 2018

...Updated Aviation Discussion...

.Short Term...(Today through Sunday)
Issued at 321 AM EST Sat Feb 24 2018

...FLOOD WATCH FOR ALL OF SOUTHERN INDIANA AND CENTRAL KENTUCKY
THROUGH SATURDAY NIGHT...
...SIGNIFICANT FLASH FLOODING LIKELY SATURDAY NIGHT INTO SUNDAY
MORNING...
...STRONG TO SEVERE THUNDERSTORMS LIKELY SATURDAY EVENING THROUGH
SATURDAY NIGHT...

A fairly active and significant event will unfold over the next 24
hours. Details regarding flooding and severe weather have been
broken down into separate categories.

***Flooding Potential***

Soils remain very saturated heading into today. The KY Mesonet and
local obs show rainfall amounts over the past 3-4 days exceeding 5
inches from the western end of Kentucky into southern Indiana. Many
rivers are already in minor to moderate flooding, and some are
forecast to approach or exceed major flooding levels. The ground at
this point just can`t contain much in the way of moisture, so the
flood and flash flood threat will remain high. WPC has included
parts of southern Indiana and central Kentucky in a rare Day 1 High
Risk for excessive rainfall given the antecedent conditions and
forecast rainfall. The current flood watch that is in place was
expanded eastward to cover the entire CWA. While rainfall in the
areas that were previously not under a flood watch were generally an
inch or less, flash flood guidance remains around 1" to 1.5", and
given the convective nature of the event, some flooding issues could
arise so feel the expansion is warranted.

We`ll start off this morning with scattered rain showers as a warm
front lifts northward. Models have the warm front stalling near the
I-64 corridor by late morning, and we should see some drying south
of the warm front. As the atmosphere destabilizes within the warm
sector, showers and storms will develop during the afternoon hours.
Rainfall rates could be heavy within these storms, as a plume of
+1.3" PWATs work in from the southwest. Showers and storms will
increase in coverage during the evening and overnight hours as a
cold front approaches from the west. Again, moderate to heavy rain
rates are expected with the overnight showers and storms. It is
during this timeframe that we expect our most significant flood and
flash flood problems to arise.

By the time the cold front has cleared the CWA Sunday morning, total
rainfall of 1 to 2 inches will be possible across southern Indiana
and central Kentucky, with the highest amounts residing across
northern Kentucky and southern Indiana. Given the convective nature
of precipitation and very moist air in place, heavy rainfall rates
are likely across parts of the area, so isolated +3" amounts are not
out of the realm of possibility. Flooding (areal and river) and
flash flooding problems are a near certainty, with significant
issues likely across parts of southern Indiana and western Kentucky.
Extreme care and attention should be given to any areal flood or
flash flood warnings that are issued. Remember, flash flooding is
the leading cause of weather related deaths in the United States!

***Severe Potential***

There may be two opportunities for severe weather in the short term.
Low/Mid level lapse rates begin to steepen in the warm sector this
afternoon as the warm front lifts north and stalls near the I-64
corridor. Several models show decent values of CAPE for late
February standards by 21z today(300-700J/KG), with the NAM being an
outlier (+1,000J/KG). Deep layer shear will be modest, with 0-6km
bulk shear ranging from 40-50kts within the warm sector. Nearly all
models fire off convection during the afternoon (after 18z), and
given the environment, there could be some organized convection,
including that of supercelluar nature. The one thing that may keep
severe convection in check will be a lack of a cap -- IF, and this
is a big IF, lots of showers and storms fire off and start competing
for limited instability, it might be enough to keep convection sub-
severe. However, some of the latest runs of the HRRR are showing
isolated to scattered convection developing across south central
Kentucky after 20z, and some of these cells are showing positive
values of updraft helicity, indicative of supercells. All facets of
severe weather would be possible with these supercells, including
tornadoes. A window for a small (but increased) tornado threat will
occur near sunset, as storms still remain surface based within the
warm sector and the LLJ begins to ramp up and lead to increased
clockwise low-level hodographs (i.e., increased storm relative
helicity). Storms approaching the warm front will also have to be
watched closely, as a corridor of enhanced storm relative helicity
could aid in development of low level mesocyclones.

The second, greater opportunity for severe weather will come
Saturday evening into Sunday morning. A line of strong to severe
storms will form in MO/AR/TX this afternoon and push into the lower
Ohio Valley. Confidence in the timing of this line isn`t the
greatest, as some high-res models start bringing this line in
shortly after 00z while others hold off until after 07z. The
majority of models hold off on bringing this line to western
portions of the CWA until after 04z. Deep layer and low level shear
increase substantially during the overnight hours ahead of the line,
with 0-6km shear approaching 60kts while 0-1km shear approaches 40-
50kts. While instability will remain low (<500J/KG), it will
probably be sufficient to keep storms strong to severe west of I-65.
As they pass east of I-65, instability drops to near zero values,
and most models begin to weaken the line. With such a strong low
level wind field, straight line damaging wind gusts will be the most
likely severe threat. With very saturated soils, would not be
surprised to see reports of trees toppling over even in sub-severe
winds.

There will also be a threat for quick spin-up tornadoes within the
line, especially in bowing segments where the rear inflow jet
descends or surging outflow contribute to the development of
mesovortices. The orientation of the 0-3km shear vector will
generally favor tornadic mesovortices on the northern end of these
bowing segments or broken lines.

***Sunday***

By sunrise Sunday morning, the cold front and showers/storms should
be east of the CWA. Cooler, drier air will work in behind the front,
though with Pacific origins, temperatures will remain in the 50s for
most of the day. High pressure settles over the region by late
Sunday afternoon.

.Long Term...(Sunday night through Friday)
Issued at 250 AM EST Sat Feb 24 2018

...Active more unsettled weather Tue night-Thu night...

Well...the medium range of the forecast begins drying out the
waterlogged ground and then quickly transitions to an active mid
week with more rain and storms.

Sun night - Tue

Major change with surface high pressure moving into the area and the
long last the sun will make a grand appearance. Enjoy Monday and
Tuesday with sunny skies and light winds. Weak zonal flow will and
no strong cold advection will allow temps to stay in the 60s for
high temps.  With the sfc high nearby on Monday night into Tuesday
sunrise, could be a classic setup for areas of fog to develop, and
with saturated ground, could become locally dense. I`ve included fog
in the weather grids.

Tue night - Thu night...

Period is very active with southwest flow bringing strong spoke of
energy up from Joshua Tree NP CA into Grand Canyon NP AZ by Wed am.
In response heights will be on the rise and sfc low will deepen the
area, this surface low will deepen as it moves from AMA towards ORD
Thu evening. Most concerning is GFS presentation of a of upper
trough with strong kinematics taking on a negative tilt by Wednesday
night across I 35 corridor of Srn Plains. This will help lift old
boundary back north as warm front increasing Ohio Valley instability
parameters with plenty of shear in place.

There is the potential for strong storms with this system, however
if the GFS is correct (ECM is much much slower), my main concern is
waves of rain on top of super saturated ground and ongoing river
flooding. This additional additional rainfall will aggravate river
flooding concerns once again.

Fri into next weekend...

Ridging takes over the it appears to be much more placid and drier
weather. Slightly colder air looks to come in behind the front
bringing temps closer to normal by Friday.

&&

.Aviation...(12Z TAF Issuance)
Issued at 640 AM EST Sat Feb 24 2018

Early this morning, light rain showers will continue along with IFR
conditions for SDF/LEX/HNB.  BWG may wobble back and forth between
IFR/MVFR this morning as better rains move over the TAF site. Steady
rains will continue through mid morning before a break or lull in
convection is expected for late morning/early afternoon. A warm
front will sharpen across the region this afternoon bringing a
better chance for storms this afternoon and evening.  Sct-numerous
showers and storms are expected this evening into tonight as a cold
front approaches the region. Cigs should improve as we head toward
this evening with winds becoming gusty from the SSW.

&&

.Hydrology...
Updated at 325 AM EST Sat Feb 24 2018

Copious amounts of rainfall have lead to excessive runoff and river
flooding in many locations over southern Indiana and central
Kentucky. The latest synoptic models indicate an additional one to
two inches of rainfall will be possible across southern Indiana and
central Kentucky. This additional rainfall will result in continued
rises on area rivers, creeks and streams. Continued river flooding
is expected and will likely worsen in coming days. A few river
points have already achieved moderate flood levels, and more are
likely to follow. We could even see the major flood category reached
at Boston. For a complete list with forecast hydrographs, please
visit our website at:
https://water.weather.gov/ahps2/index.php?wfo=LMK

In addition to river flooding, typical low-land and flood prone
areas are likely to experience flooding at times through the coming
days. Widespread showers and storms forecast to move through the
area Saturday through Saturday night will pose an elevated flash
flood risk to much of the area. A Flood Watch is in effect through
Sunday morning.  Stay alert to current flood warnings at
www.weather.gov/lmk.

Additional rains could arrive by midweek and worsen the ongoing
flooding. Stay tuned to the latest forecasts as confidence increases
in the rainfall amounts.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Flood Watch through Sunday morning for INZ076>079-083-084-
     089>092.

KY...Flood Watch through Sunday morning for KYZ023>043-045>049-
     053>057-061>067-070>078-081-082.

&&

$$

Short Term...DM
Long Term...JDG
Aviation...AMS
Hydrology...DM



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