Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 212057
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
257 PM CST Wed Feb 21 2018

.SHORT TERM...(Tonight through Friday
Issued at 256 PM CST Wed Feb 21 2018

No changes to the Flood Watch planned this afternoon. Two
disturbances will bring swaths of moderate to heavy rainfall to
the region tonight and again Thursday night into Friday. This will
eventually create more runoff than can be handled by already
swollen rivers and streams across the region.

The bigger concern in the short term is icing potential this
afternoon into the evening along the northwestern periphery of the
forecast area. In this area temperatures are hovering near the
freezing mark and some light glazing of elevated objects has
already been reported in a few locations. Very little measurable
rain has fallen across this area so far today, but a swath of
steady light to moderate rain is beginning to spread northeast
over it.

The warm ground should prevent roads from icing up this afternoon,
but elevated surfaces including bridges could become a problem.
The consensus of short range temperature guidance has temperatures
climbing sufficiently above freezing by around 03Z to get rid of
the freezing rain potential, but if the cold air lingers much
longer more substantial issues could develop. We will continue
with the Special Weather Statement approach for now, and if we get
reports of significant travel difficulties we may need to issue a
short Advisory.

As for the Flood Watch, we have had numerous reports of ditches
and creeks at bankful across portions of the area, mainly
southeast Missouri, but no major flood issues as of yet. The
consensus of 12Z model guidance brings a swath of moderate to
heavy rain northeast across the region this evening into the
overnight hours. The 12Z NAM is probably a little overdone as it
deepens a surface low and lifts the old cold frontal boundary
northward close to Ft Campbell overnight.

WPC is playing a more southeasterly track for the best rains
tonight, which sounds good given that the boundary currently
extends from northwest Alabama into central Mississippi. Tried to
introduce a healthy dose of WPC QPF to our superblend of
everything for this event. This will put the heaviest rains over
the Bootheel and west Kentucky. It appears that the rains will
push east of the area by 12Z Thursday, but some lighter showers
may linger along the TN border through the day Thursday. Some
elevated instability will be available so some rumbles of thunder
will also be possible.

It appears that most, if not all of the area will be dry Thursday,
but little relief from the ever increasing water levels across the
area is expected. With the Ohio River rising higher in flood,
tributaries will get backed up leaving nowhere for the runoff to
go.

Our next upper-level impulse will be strong enough to induce a
weak surface low that will track over or just north of our
forecast area late Thursday night. There will be plenty of
elevated instability to support a chance of thunderstorms, as a
swath of heavy rainfall pushes northeast across the region,
especially north and west of the Ohio River overnight Thursday
night. As this system moves east of the area, the cold front
trailing behind the surface low will hang up along or just south
of the Ohio River, so moderate rains could continue through the
day Friday in west Kentucky. Flooding issues are likely to develop
or worsen by Friday morning. It looks like northwest portions of
the area may see another dry period Friday afternoon, but as
stated above there will still be plenty of standing water around
as it will have no where to go.

.LONG TERM...(Friday Night through Wednesday)
Issued at 256 PM CST Wed Feb 21 2018

The extended portion of this forecast package introduces the last
and most dynamic of the series of systems bringing rain to the
WFO PAH forecast area for Friday night into Saturday night.

The 12z Wednesday deterministic 20km GFS and 12km NAM-WRF model
suite seemed to initialize well and have similar solutions on
instability for thunderstorms Saturday into early Saturday evening.

At this time, the greatest instability works along the AR/MO and
KY/TN borders by mid-morning on Saturday with most unstable CAPE
remaining below 600 j/kg,increasing to 600-1000 j/kg mainly over west
Kentucky by noon, then peaking between 600-1200 jg over southeast
IL, southwest IN, and west Kentucky by mid-afternoon on Saturday.

The aforementioned areas of potential instability coincide with the
sharpening and ejection of a close low from the four Corners of the
Southwest U.S. Friday night,  Colorado and Iowa, onward to Iowa
Saturday afternoon, and northern Michigan early Sunday. On Saturday
afternoon, the upper low takes on a slight negative tilt, enhancing
warm air advection and instability through the area and leading to
some potential for strong to severe storms in the WFO PAH forecast
area.

Prior to that time, the GFS and NAM-WRF focus most of the heavy
rain potential into northern sections of southeast Missouri and
through southern Illinois Friday night into Saturday morning. as
deep layer moisture is advected back into the region and interacts
with northward moving warm front.

Periodic and scattered convective activity will dominate the rest of
the WFO PAH forecast area Saturday morning, with the focus of the
heaviest rainfall moving into Southeast Missouri with a potential
line of thunderstorms, advecting northeast through the afternoon
before leaving by day break Sunday.

The overall rainfall from Friday night through Saturday night will
range from one and a half to two inches, with the heaviest QPF
(rainfall amounts) over west Kentucky and southwest Indiana.

Dry conditions should persist from Sunday through next Wednesday,
with another chance for rain late next week. At this point, there is
low confidence on any chances for rain after Wednesday.

Temperatures will remain at or above normal through the extended
forecast period.

&&

.AVIATION...
Issued at 1136 AM CST Wed Feb 21 2018

Fairly high confidence forecast with IFR conditions and northeast
winds the general rule through the period. Light rains will
linger through the afternoon and then another round moderate to
heavy rainfall will move through the region in the 03Z-09Z time-
frame. MVFR visibilities are expected with the heavier rains, and
then fog will keep them going into Thursday morning. LIFR ceilings
will be a possibility at this time as well. Northeast winds may
gust at or below 20kts for awhile over west Kentucky during the
heavier rains late this evening into the early morning hours.


&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through late Saturday night for ILZ075>078-080>094.

MO...Flood Watch through late Saturday night for MOZ076-086-087-100-
     107>112-114.

IN...Flood Watch through late Saturday night for INZ081-082-085>088.

KY...Flood Watch through late Saturday night for KYZ001>022.

&&

$$

SHORT TERM...DRS
LONG TERM...Smith
AVIATION...DRS


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