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Area Forecast Discussion (AFD)

Issued by Louisville, KY (LMK)

                            
204 
FXUS63 KLMK 191310
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
910 AM EDT Thu Jul 19 2018

.Forecast Update...
Issued at 909 AM EDT Thu Jul 19 2018

No major forecast updates for the near-term this morning. Have only 
tweaked hourly temps and dewpoints. Some mid and high clouds have 
been spreading over parts of the area. Rest of the day looks partly 
sunny and dry with seasonable temperatures.

&&

.Short Term...(Today and tonight)
Issued at 310 AM EDT Thu Jul 19 2018

Skies were mostly clear across the region this morning. Temperatures 
were generally in the lower to middle 60s with a few upper 50s seen 
over in eastern KY.  For the remainder of the overnight period, 
quiet weather is expected to continue.  Lows should bottom out in 
the lower to mid 60s in most areas.

High pressure over the Great Lakes will push into western New 
England today.  This will keep our weather quiet and dry.  Winds 
will start off light and variable before shifting to the southeast 
during the day.  Some upstream high cloudiness will roll into the 
region late in the day.  Highs look to average out in the 85-90 
degree range.

For tonight, high cloudiness will continue to stream in from the 
northwest.  Quasi-stationary boundary to our south is expected to 
surge northward in response to a stronger southwest flow developing 
ahead of an approaching mid-level trough axis.  Most CAM guidance 
suggests a belt of isentropic lift pushing through the area 
coincident with the surface boundary riding northward overnight. 
This should result in some elevated convection firing after 
midnight.  Overall coverage looks to be in the 30-40% range and thus 
plan on capping PoPs at those values for now.  Lows tonight will be 
a bit milder with reading in the upper 60s to around 70 in most 
locations.

.Long Term...(Friday through Wednesday)
Issued at 300 AM EDT Thu Jul 19 2018

...Confidence Growing For Friday Severe Weather Episode...

A significant severe weather episode could unfold on Friday as the 
combination of strong instability and anomalously strong shear 
create an environment capable of initial supercell modes that would 
ultimately (and perhaps quickly) congeal into an organized wind 
producing line of storms. At this point, the greatest overlap of 
strong shear and instability is expected to reside over the western 
two-thirds of our CWA where SPC has decided to upgrade to an 
Enhanced Risk for Day 2. More details below.

An anomalously strong shortwave trough will move from the upper 
Midwest to the southern Great Lakes on Friday, with the associated 
surface low dragging a cold front into our area by late Friday 
night. Ahead of this front, a severe weather environment should set 
up on Friday as the exit region of a 60-70 knot mid/upper level jet 
will overspread the Ohio Valley through the day. Also of note will 
be an unseasonably strong low level jet response around 30-40 knots 
over southern IN and northern KY. This setup will create a very 
strong and notably veered vertical wind profile supportive of 
organized convection, including supercells and a tornado threat. 0-1 
km SRH values should be over 200 M2/S2 for a time over much of the 
area late afternoon/evening.

In addition to the shear and exit region forcing, expect the 
environment to become strongly unstable in the afternoon. The 
combination of heating and low level moisture return beneath 
steepening mid level lapse rates will yield SB CAPE values north of 
2500 J/KG, and perhaps as high as 4000 J/KG across SW Indiana and 
western KY. As the previous discussion mentioned, forecast soundings 
for Friday afternoon/evening look pretty impressive with a "fat" 
CAPE profile given the steep mid level lapse rates. With the veering 
profile mentioned above, hodographs also take on an ominous long and 
looping look.

There is still some question as to how much any elevated morning 
convection will limit the ability to destabilize later in the day. 
However, models all suggest redevelopment by the afternoon/evening 
and at this point it seems more of a "when" question rather than an 
"if" question. Still thinking that early storm modes could be 
discrete and supercellular, but do anticipate that a transition to 
an organized cold pool driven wind producer is likely. Given this 
scenario, would expect an initial tornado and large hail threat, 
followed by a sizable wind threat into the evening hours. An 
isolated tornado threat would also continue into the evening. Did 
also want to mention an isolated flooding threat given high PWATs 
and potential for some repeated storm occurrences over the same 
area. 

Still some timing and storm mode questions to answer, but overall 
confidence is increasing in potential for some significant severe 
weather. Stay up to date of the coming forecasts.

Saturday - Wednesday...

A closed upper low should slowly wobble southward from the southern 
Great Lakes to the Gulf Coast state through Monday. As this occurs, 
look for cloudy and cooler conditions and scattered to numerous 
showers each day. Some storms are also possible, but mainly each 
afternoon and early evening. Highs each day Saturday and Sunday 
should be in upper 70s and low 80s. By Monday, temps should be a bit 
warmer in the low to mid 80s.

Isolated to widely scattered and mainly diurnally driven showers and 
storm chances should continue through mid week with temps warming 
back into the mid and upper 80s for highs.

&&

.Aviation...(12Z TAF Issuance)
Issued at 647 AM EDT Thu Jul 19 2018

High pressure over the eastern Great Lakes is forecast to slide 
eastward into western New England today.  VFR conditions are 
expected at the terminals today.  Winds will start off light and 
variable and then pick up steadily out of the southeast.  High 
clouds will stream in from the northwest today but will not have any 
impacts on aviation.

Tonight, clouds will continue to increase.  Some scattered 
convection will be possible late in the period as isentropic lift 
increases over the area.   For now, will continue a PROB30 group at 
KSDF after 20/06Z.  More substantial convection will be possible at 
the end of the current forecast period.


&&

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

$$

Update...EBW
Short Term...MJ
Long Term....BJS
Aviation.....MJ