131
FXUS63 KLMK 170732
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
332 AM EDT Fri May 17 2024

.KEY MESSAGES...

*   Widespread showers and scattered storms today and Saturday, with
    locally heavy downpours possible.

*   Drier Sunday with hot temperatures Monday and Tuesday.

*   Showers and storms return Tuesday with increasing potential of
    strong to severe storms on Wednesday.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 331 AM EDT Fri May 17 2024

A 500mb shortwave trough reaching from the upper Mississippi Valley
to the southern Plains early this morning will push eastward today.
Isentropic upglide and plentiful moisture ahead of this feature
will lead to widespread showers and embedded thunderstorms.
Instability, shear, and lapse rates suggest any thunder will be
rather isolated and sub-severe, which indeed was the case with
upstream convection overnight.

The best lift will be this morning ahead of the incoming upper wave
and simultaneously under the right entrance region of a small upper
jetlet over the eastern Great Lakes and the left exit region of the
main upper jet streak from Texas to Tennessee. Coverage should
lessen slightly this afternoon as upper support weakens a bit,
though scattered showers and storms can still be expected across the
region.

Enough moisture for some heavy downpours today, but parameters for
widespread heavy rain are not especially impressive with
precipitable water around 1.5", K Index in the low 30s, and 850mb
dew points around 10C. QPF is well below FFG numbers. Much of the
region was dry yesterday (other than a few showers mostly along and
north of I-64), and Kentucky Mesonet soil moisture data show that
the ground has dried out a bit at the 2 and 4 inch depths over the
past 24 hours. SPC and DESI HREFs indicate that the most likely
areas for flooding today will be from western Kentucky to northeast
Arkansas. Looks like any heavy downpours here in southern Indiana
and central Kentucky will be localized. The best chance of water
issues would be with training storms as they move from southwest to
northeast around 20-25 mph. Some minor local flooding was reported
in southeast Missouri overnight in an area where a band of heavy
rain persisted.

Despite clouds and rain today, still went a bit on the high side of
guidance for high temperatures. We`ll have a warm start to the day
with temperatures this morning in the 60s, and if the rain does
become somewhat less widespread this afternoon highs in the low and
mid 70s should be attainable.

Widely scattered showers will be possible tonight as a disorganized
surface low heads from Mississippi into Tennessee. Winds will be
very light. With the humid, stagnant air mass in place, models are
showing a strong signal for some fog development late tonight. Lows
will be in the lower 60s.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 331 AM EDT Fri May 17 2024

Synopsis...Extended forecast period begins featuring a split flow
pattern over the CONUS as a weakening shortwave trough moves across
the Southeast US and an upper low meanders over Alberta and
Saskatchewan provinces in southern Canada. Although the southern
streamer and attendant surface low will be weak, the slow-moving
character of the system will allow for showers and potentially
storms to linger through Saturday. Perhaps the biggest change during
the weekend and early next week will be the pattern amplification
over the West as a southern-stream upper low moving towards Baja
California phases up with northern-stream shortwave energy rounding
the southern Canada low. As a result, positive upper height
anomalies will extend from a subtropical high over northern Mexico
to the Central US and into the Northeast, providing mostly dry but
humid and hot conditions. Then, active weather arrives by the middle
of next week as West Coast trough lifts towards the Great Lakes and
a cold front races to the area. Organized strong to potentially
severe convection is possible on Wednesday or Wednesday night with
the frontal passage.

Model Confidence/Evaluation...There is medium to high confidence in
the large scale pattern evolution through early next week. Global
deterministic guidance (GFS/ECWMF/CMC) has come into better
agreement with the track and structure of the southern stream trough
this weekend while the West Coast phasing episode early next week is
handled fairly well in the latest runs with still some differences
in the CMC versus ECMWF/GFS. That being said, confidence decreases
substantially across the board Tuesday and onwards, probably due to
initial differences and additional energy interaction with an
upstream wave approaching from the Pacific Northwest. The
aforementioned uncertainties will limit predictability of frontal
timing and best chances of organized strong to severe storms. Based
on GFDL Shield model and CSU algorithm, severe probabilities are
higher on Wednesday.

Saturday - Sunday...Slow moving system this weekend will allow
moderate PW values to continue on Saturday. A weakly-forced
environment, due to cyclonic convergence, along with surface-based
instability around 1000 J/kg Saturday afternoon could promote
isolated to widely scattered convection in the afternoon and early
evening. Other sources of convective initiation could be
differential heating and outflow boundaries. On the other hand,
young updrafts would have to overcome a capping inversion around 850-
mb and mid-level dry air to become established. If storms do
materialize on Saturday, there is a chance for localized flooding
issues due to heavy, persistent rainfall given weak steering
currents plus gusty winds due to collapsing downdrafts amid no deep
layer shear. Sunday should be drier and warmer, at least for the
western half of the forecast area, as the southern-stream wave
pushes to the east and the mid-level ridge and attendant surface
high pressure move in to the region and filter stable airmass.

Monday - Tuesday...Mostly dry and increasingly warm weather is
anticipated by early next week. The presence of strengthening ridge
overhead and southerly warm winds will enhance warm air advection.
Therefore, raised highs both Monday and Tuesday following BC
guidance. Tuesday should be mostly dry but cannot rule out low
chance of showers to the far west if quicker guidance ends up
verifying. Last but not least, HEFS river guidance is indicating a
low chance (20-30%) of minor river flooding for Rochester and
Woodbury centered on Monday provided that 1.5 to 2 inches of rain
accumulate in their respective basins this weekend.

Wednesday - Thursday...A cold front will move through the area by
the middle of next week carrying a non-zero threat of strong to
severe weather and localized flooding. As mentioned above, there is
low confidence in timing details but best chances for severe
probabilities are concentrated on Wednesday.

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 130 AM EDT Fri May 17 2024

An upper trough over the Mississippi Valley early this morning will
interact with deep moisture to produce widespread showers and
embedded thunderstorms as it moves into the Ohio Valley today.
Coverage may lessen some this afternoon as the upper wave weakens
and moves on to the east of the region. Low level clouds and
moisture will linger into tonight in a moist, stagnant atmosphere.
Can`t rule out some patchy light fog towards Saturday morning in
parts of the area.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...None.
IN...None.

&&

$$

SHORT TERM...13
LONG TERM...ALL
AVIATION...13