291
FXUS63 KPAH 131104 AAA
AFDPAH

Area Forecast Discussion...UPDATED
National Weather Service Paducah KY
604 AM CDT Tue Aug 13 2024

.KEY MESSAGES...

- Humidity increases west to east through the day.

- An axis of heavy rain may set up over southeast Missouri
  tonight, leading to flash flood potential - but the exact
  placement remains uncertain.

- Scattered thunderstorms will probably become more numerous by
  Thursday night as a front approaches. There is some severe and
  flash flood risk with this activity although things are not
  set up exactly right for it.

&&

.DISCUSSION...
Issued at 207 AM CDT Tue Aug 13 2024

Flow aloft this morning is out of the northwest with a distinct
shortwave over SWIN. This seems to be responsible for some very
light showers over this region but they only have an extremely
thin layer of moisture just below the residual subsidence
inversion to work with and most rain is probably not quite
reaching the ground. That activity should be well clear by
sunrise with residual high clouds rounding the trough. As often
occurs with this pattern dewpoints range from 60-62 over Indiana
to the lower 70s over central Arkansas.

As the primary ridge axis aloft moves eastward through the day
this area of richer low and deep column moisture moves with it,
moving over southeast Missouri by late afternoon. This increase
in moisture may spark showers and elevated thunderstorms by
late afternoon but it will have to fight against some initially
marginal thermodynamics. As the ridge axis moves further east by
sunset the region falls more under the right exit region of the
polar jet maximum oriented to our north. Most guidance then
develop two distinct shortwave troughs moving through late
Tuesday afternoon and then early Wednesday morning. This should
help improve our thermodynamic situation to the point
convection becomes more likely in the areas of deeper moisture
over southeast Missouri. Northwest flow over a moisture gradient
with PWATs >2.00+ is a common heavy rain/flash flooding pattern
for us. HREF 24 hr PMMs show 4 to 7 inches over Carter and
Ripley Counties. Discussed a flood watch with neighbors and WPC
but the marginal thermodynamics and uncertainty on exactly
where an axis of heavy rain would set up precludes us this
morning. Will however increase the tone of flood messaging with
this forecast package.

The large scale ridge axis moves roughly overhead Wednesday.
Some guidance show a weak shortwave moving through this flow but
with the subsidence of the large scale ridge - think the
thermodynamics may be more unfavorable for convection. Although
the depth/richness of the column moisture precludes us from
keeping the forecast dry.

All of this is being nudged eastward by a zonal polar jet max
capped off by a fairly strong shortwave trough. Guidance is
coming into better agreement on the timing and details with this
trough moving through Thursday night. Instability is marginal
on weak mid level lapse rates, but its conceivable that a window
opens up late Thursday afternoon and evening for at least some
severe risk given the favorable deep layer shear profile and
marginal low-level shear profile coupled with available
moisture. High column moisture will make flash flooding a
potential concern as well. The surface front however is slow to
move through weak convergence in the warm sector during peak
heating. GFS/NAM also show DCVA behind a leading shortwave
trough which may lead to poorer thermodynamic instability
further limiting severe/convective potential.

00z ECMWF is a little slower with the frontal passage where the
GFS brings it through cleanly by Saturday owing to more
sharpness in the overall trough. I tend to prefer the faster
solution given ensemble members from both camps. A new trend in
the 00z suite is to be stronger with the advancing upper ridging
over the four corners region which puts us back in a boundary
northwest to southeast with thunderstorms under northwest flow
by Sunday but ridging builds much stronger through Monday and
breaks this pattern up. NBM doesn`t bite off on any PoPs with
this pattern and its close enough to not argue too much at this
point, will see if that trend continues through the next 12-24
hours.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 604 AM CDT Tue Aug 13 2024

VFR conditions are expected today under passing high clouds and
light winds. Tonight convection may become a factor over
southeast Missouri or slightly further east. Fog may settle in
to parts of southeast Missouri as well as increased low level
moisture works its way eastward through the overnight hours.
More widespread convection is possible further east by around
daybreak. &&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.

&&

$$

DISCUSSION...JGG
AVIATION...JGG