Area Forecast Discussion
Issued by NWS Louisville, KY

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584
FXUS63 KLMK 210542
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
142 AM EDT Fri Jul 21 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 900 PM EDT Thu Jul 20 2017

It was a hot one today, compared to other days this summer, but not
atypical summer heat. Humidity was the key factor today, with drier
air just not able to mix down today. Most places kept their
dewpoints in the 70s. Tonight, lows will only fall to the mid/upper
70s for most locations.

Most of the hi-res models this evening have a long east/west string
of shower and storm activity running from Iowa to Ohio, but nature
only is providing us with those storms over Iowa. A couple of the
individual NCAR ensemble members picked up on the "lull" area, and
those two members show some storms around KCVG by daybreak Friday
and then new development starting around lunchtime along the
leftover boundary. Thus think the focus for further storms will be
northeast of a Louisville to Richmond, KY line. Trended the pops for
tomorrow afternoon with more of a gradient to the southwest of this
line.

That unfortunately means the hottest day of the week, heat index
wise for most of the rest of the area. Still looking at peak heat
indices in the 105-110 degree range in our advisory area and 100-105
outside of it. Our Madison, IN to Eminence, KY areas have the best
chance to get a break from the heat with rains.

&&

.Short Term...(This evening through Friday night)
Issued at 255 PM EDT Thu Jul 20 2017

...Very Hot and Humid Through Saturday...

The main focus in the short term is on the high heat and humidity
across southern Indiana and central Kentucky.

As of mid afternoon, a surface analysis shows high pressure centered
over Tennessee with a surface boundary well north across the lower
Great Lakes. Satellite imagery shows the remnants of a morning MCS
spreading high level clouds across portions of the area, though this
hasn`t had too much of an effect on temperatures as readings have
easily climbed into the lower to middle 90s. Compared to yesterday,
dewpoints haven`t mixed out and are in the lower to middle 70s. This
is creating high heat indices in the 100 to 103 range, especially
for areas along and west of I-65.

The remainder of the evening will be hot and humid with temperatures
very slow to fall. Evening heat indices will remain in the upper 90s
to around 100. Overnight temperatures will be very warm, only
falling into the mid to upper 70s. Louisville metro may only briefly
drop below 80 for a couple of hours.

One of the hottest and most humid days of the summer so far is
expected tomorrow and again on Saturday. Overall, no large changes
in the forecast message as temperatures climb into the low to mid
90s. If dewpoints remain in the mid 70s, this will put the highest
afternoon heat indices in the 105 to 108 range for areas mainly
along and west of the I-65 corridor. In coordination with
surrounding offices, will add a few counties in southern Indiana and
northern Kentucky to the Heat Advisory where max heat indices will
top out in the 105 to 108 range. Also, have decided to go ahead and
extend the advisory through Saturday evening (8 PM EDT) as
conditions are expected to remain just as hot/humid Saturday.

Additionally, some relief from the heat and humidity Friday may
come with pop-up showers or thunderstorms. Hi-res and other meso
models show diurnally driven convection by mid afternoon mainly
across southern Indiana and north central Kentucky where the ridge
is slightly weaker and there is a weaker cap. Will maintain the
20 to 30 percent chances across the area. Given the very strong
afternoon destabilization, some locally strong storms may be
possible across portions of southern Indiana and north central
Kentucky including the Bluegrass region. This is where the deep
layer shear is slightly higher. Gusty winds would be the main
threat.

.Long Term...(Saturday through Thursday)
Issued at 255 PM EDT Thu Jul 20 2017

Toward the end of the weekend and into early next week, the upper
ridge is expected to break down and shift its center back to the
west. This should lower thickness and 850 mb temperatures and allow
upper troughing to spread across the lower Great Lakes and lower
Ohio Valley.

A cold front will pass through the area Sunday into Monday bringing
scattered to perhaps numerous showers and thunderstorms to the area.
Couldn`t rule out isolated stronger storms on Sunday, particularly
over southern Indiana and far northern Kentucky where there is
slightly better deep layer shear (30 kts). Otherwise, storm
organization should be weak across south-central Kentucky.

Slightly cooler but much drier Canadian high pressure is then
expected to build into the region Tuesday and Wednesday providing a
couple of drier and less humid days.

&&

.Aviation...(06Z TAF Issuance)
Issued at 140 AM EDT Fri Jul 21 2017

Visibilities at a couple of the TAF sites haven`t really improved
since the afternoon haze of a hot summer day. As of this writing,
SDF/LEX were both at 7SM. As surface temps slowly cool this morning,
we`ll likely see brief, high-end MVFR vsbys. Much like yesterday
morning, the bulk of the shallow fog will tend to be concentrated in
river valleys. Vsbys should improve quickly after 12z.

VFR is then expected for the rest of Friday. Early this morning, a
line of thunderstorms was focused along a frontal boundary across
eastern Iowa and northern Illinois. Recent model guidance suggests
thunderstorm activity will spread east-southeast along an KIND-KCVG
line through midday. This activity will remain north of the
terminals. However, outflow from those storms propagating to the
south may be able to generate additional convection across southern
Indiana and north-central KY Friday afternoon. Confidence was still
too low with this TAF issuance, so have held on to the dry forecast
for now. But if storms can develop in vicinity of a terminal
(generally after 19z), they could become strong with heavy rainfall
and gusty winds.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Heat Advisory from noon EDT /11 AM CDT/ today to 8 PM EDT /7 PM
     CDT/ Saturday for INZ076>079-083-084-089>092.

KY...Heat Advisory from noon EDT /11 AM CDT/ today to 8 PM EDT /7 PM
     CDT/ Saturday for KYZ023>034-038-045-053-061>063-070>074.

&&

$$

Update...RJS
Short Term...ZT
Long Term...ZT
Aviation...EBW



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