Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 211621

Area Forecast Discussion
National Weather Service Louisville KY
1221 PM EDT Fri Jul 21 2017

...Updated Forecast Discussion...

.Forecast Update...
Issued at 1220 PM EDT Fri Jul 21 2017

GOES 16 visible satellite imagery shows very clearly the outflow
boundary from the morning MCS over Indiana and Ohio now draped west
to east from near Owensboro to Danville. There is a subtle surface
wind convergence but overall the cumulus is more agitated along this
boundary. As we continue to heat into the 90s, the strong
destabilization should bring 2500 to 3000 MLCAPE across central
Kentucky. The surface boundary and plenty of instability may be
enough to initiate additional convection. Hi-res models have
struggled with this activity all morning but the recent run of the
WRF-ARW seems to at least have some idea of an arching line of
isolated showers developing between 2 and 5 pm EDT.

Issued at 812 AM EDT Fri Jul 21 2017

Updated precipitation chances across far southeast Indiana and the
northern Bluegrass of Kentucky. MCS outflow from western Ohio is
initiating new convection on the southwest flank and this should
continue move east/southeast this morning. Just brief heavy rain and
strong wind gusts are possible with this as it moves through.

This may complicate the forecast for later this afternoon as it
could lay up an outflow boundary across portions of the area. Thick
high cirrus should thin or dissipate by noon giving way to strong
heating and destabilization. This strong heating interacting with the
old boundary may be enough to spark new convection, which is
suggested by some of the hi-res models.

Otherwise, temperature and heat index forecast looks good at the
moment. No changes to the Heat Advisory at this time.


.Short Term (Now through Saturday)...
Issued at 255 AM EDT Fri Jul 21 2017

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

Heat and humidity will be the main impact through Saturday, but
perhaps more challenging is the extent of our thunderstorm chances
each afternoon.

Hot and humid air mass remains in place over the Ohio Valley, with
convection ongoing over north-central Illinois. While none of the
synoptic or hi-res models seem to have a very good handle on this
convection, still at least moderately confident that this activity
will push ESE along the I-74 corridor across Indiana, and into far
northern Kentucky. That in itself would be hard pressed to get any
storms in our area, but our better chance would come from any
outflow generated by these storms. Will carry a 20-25% POP roughly
along and north of I-64 late this afternoon and evening, but in
reality the extent of storms will be focused very tightly along the
effective boundary as 700mb temps at 11-12C will require better
forcing to overcome. Main threats will be locally heavy rainfall and
isolated strong wind gusts from pulse type storms.

Heat and humidity will be the biggest story, as temps are largely a
persistence forecast, only with even higher dewpoints into the mid
70s. Still on track to hit heat index values of 105 to 108 along and
west of I-65 both today and Saturday, with 100 to 103 fairly
commonplace farther east. Current advisory structure looks good for
now, though convection could limit the northward extent.

Saturday looks to be a carbon copy of today, except perhaps a bit
more unsettled as the ridging starts to break down and retreat
westward. Best precip chances will be from southeast Indiana into
the Bluegrass region, with the same pulse storm threats on the table

.Long Term (Saturday Night through Thursday)...
Issued at 255 AM EDT Fri Jul 21 2017

Upper ridging will break down during the latter half of the weekend,
and a fairly sharp northern stream disturbance will push into the
Great Lakes, opening up into a broad low-amplitude eastern CONUS
trof. Sunday and Monday will be fairly unsettled as a cold front
makes a slow trek SE across the Ohio Valley, supporting POPs up to
50% on Sunday and even a 60 POP on Monday across south central
Kentucky. Just enough deep-layer shear over southern Indiana and
north-central Kentucky to support some organization into multicell
clusters, but otherwise the main hazards will be pulse storms
producing locally torrential rainfall and strong wind gusts.

Canadian high pressure on Tuesday and Wednesday will bring slightly
cooler temps, but more notably a significant respite from the
oppressive humidity. More muggy air and hit-and-miss rain chances
will return on Thursday.


.Aviation...(12Z TAF Issuance)
Issued at 625 AM EDT Fri Jul 21 2017

LEX currently has a 5SM vsby, and MVFR fog is likely to persist
through 12z. Meanwhile, SDF/BWG will likely hover in the 6-8SM
range. The rest of the day will feature VFR weather with southwest
winds increasing to 5-8 kts.

Strong thunderstorms are ongoing across central Indiana this
morning, pushing southeast in the direction of KCVG. The bulk of the
convection is likely to stay north of the terminals today, but
forecast confidence is lower than normal today. Kept the dry
forecast at all TAF sites, which is the most likely scenario. As
they move into SW Ohio and far northern KY, the storms may send
outflow toward the I-64 corridor. In a hot and very humid airmass,
any boundary could be enough to kick up a few showers/storms. Some
recent hi-res guidance suggests LEX could get clipped by early
afternoon. But confidence remains too low to include in the TAF.


IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for INZ076>079-

KY...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for KYZ023>034-



Short Term........RAS
Long Term.........RAS
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