Area Forecast Discussion (AFD)
Issued by Louisville, KY (LMK)
237
FXUS63 KLMK 220704
AFDLMK
Area Forecast Discussion
National Weather Service Louisville KY
304 AM EDT Sat Sep 22 2018
.Short Term...(Today and tonight)
Issued at 304 AM EDT Sat Sep 22 2018
A wavy cold front was draped from the northern Bluegrass southwest
through south-central Kentucky this morning. There's a very evident
wind shift and dewpoint gradient across the boundary. A 10-15 kt
northerly breeze north of the front has lowered the SDF dewpoint
from 71 to 56 degrees. Lower 70s dewpoints were still common along
and south of the Bluegrass Pkwy as of this writing.
The main forecast challenge today is precip coverage and timing.
Waves of rainfall are likely today and tonight, particularly in
central KY. A notable shortwave trough has been centered over the
southern Plains, with mid-level energy and deep moisture spreading
ENE along the boundary. Convection has blossomed along the elevated
front from northern Arkansas into Middle Tennessee. Expecting
numerous showers along with a few rumbles of thunder to continue to
lift into southwest and south-central KY between now and 12z, so
will maintain the highest pops across the south through mid-morning.
The thermodynamic environment has diminished considerably over the
past couple of hours, with only 250 J/kg MLCAPE and increased CIN in
that area. So stronger storms are unlikely, and lightning activity
will continue to diminish with the diurnal minimum. Southern KY
could see a brief lull in precip from mid/late morning through early
evening.
Meanwhile, isentropic ascent/overrunning should result in stratiform
rain spreading as far north as southern Indiana (as seen now in
Missouri). Leaned on a consensus of the ARW/NMM/3 km NAM/HRRR for
timing details, so will spread higher rain chances along the Ohio
River from midday through the afternoon hours. This evening and
tonight, the band of deep moisture oriented along the elevated
frontal boundary sinks slowly south and east a bit. In addition to
the favorable low-level convergence, some divergence aloft is
maintained in the right front quadrant of the upper jet. Expecting
numerous showers tonight south of the river.
Temperatures will feature a north-south gradient, with afternoon
highs maxing out in the mid/upper 60s in southern Indiana to the
low/mid 70s in southern KY. Likewise, lows tonight will range from
the mid 50s to mid 60s.
&&
.Long Term...(Sunday through Friday)
Issued at 255 AM EDT Sat Sep 22 2018
...UNSETTLED AND WET SEVERAL DAYS...
Stationary front will be draped across the Ohio and/or Tennessee
Valleys on Sunday, supporting scattered to numerous showers
especially across south-central Kentucky. Depending on the evolution
of precip today and placement of the moisture axis, there could be
some intervals of heavy enough rain in south-central Kentucky to
cause localized flooding. Best chance of actually staying dry
through the day will be over southern Indiana, but expect plenty of
clouds and temps on par with mid-October normals.
Upper flow backs SW late in the day and lifts the front northward
across the area Sunday night. We'll carry high-end chance POPs
heading toward morning, but at this point the heaviest QPF will
shift north and west enough to give southern Kentucky a break.
Broad and deep south/southwest flow on Monday will support at least
chance POPs throughout the area. Precip chances will continue into
Monday night, but still with low confidence in exact timing.
Tuesday through Wednesday should bring the heaviest precip of the
upcoming week, with heavy rain almost a given from what could be
multiple rounds of storms to get the cold front through the area.
Uncertainty in timing was all that kept us from going categorical in
any time period, but we do have an extended period of 60-70% POPs.
Main hazard will be small stream flooding, but if the timing lines
up with peak instability, the upper dynamics could support a few
strong or severe storms.
Low-confidence forecast for the latter part of the week, as there
are hints that this front will also slow down. Will carry low POPs
on Thursday, especially in south central and east central Kentucky.
Better confidence that temps will run solidly below normal, and by
then the bigger deal will also be tallying up the early-week
rainfall. Widespread 3 to 5 inch totals are possible, depending on
how much the front meanders early in the week. Where the heaviest
rain falls, look for rises on some of the rivers. We'll keep a
Hydrologic Outlook in play just to provide a heads-up until the rain
starts to fall, and we can start to pin down which rivers will be on
the rise.
&&
.Aviation...(06Z TAF Issuance)
Issued at 158 AM EDT Sat Sep 22 2018
The surface cold front is just south of SDF as of this writing, and
terminal has observed a healthy 10-15 kt breeze out of the north
since about 0430z. HNB has seen similar breezes, but the boundary's
sluggish southward progress means LEX and BWG will continue to see
light and variable winds until fropa.
Decided to initialize BWG with VCSH followed by VCTS in a few hours.
Convection has lit up along the elevated front from northern
Arkansas through Tennessee and into far southern KY. Today, enhanced
moisture currently over Missouri and Arkansas is forecast to spread
northeast downstream of a shortwave trough. That moisture will get
squeezed over central KY, resulting in rounds of rain today into
tonight. The thunderstorm potential will be limited to areas near
and south of the surface cold front, affecting primarily BWG over
the next 24 hours. Ceilings will gradually lower to MVFR from mid
morning into early afternoon, and BWG could see IFR conditions by
late afternoon. Timing widespread moderate rains is still a bit
problematic with some question as to how far north it spreads, but
BWG is most likely to be impacted. HNB will likely be the driest TAF
site today with brief light showers possible.
&&
.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&
$$
Short Term...EBW
Long Term...RAS
Aviation...EBW