Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 280513
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
113 AM EDT Fri Apr 28 2017

...Updated Aviation Discussion...

.Short Term (Now through Friday Night)...
Issued at 230 PM EDT Thu Apr 27 2017

...Severe Storms Possible Friday Evening...

Cold front just about clear of our eastern forecast area this hour,
with just some low clouds across the region in its wake. A small
area of surface high pressure will cross over the Commonwealth this
evening, bringing light and variable winds before shifting into the
Appalachians by daybreak Friday. A warm front then will lift north
across the region during the day Friday. Guidance is showing this
passage dry during the day, just with a noticeable jump in
temperatures...as high temperatures tomorrow should be 10 to even 15
degrees warmer than today.

The next question will be the chance for convection Friday afternoon
and evening within the warm sector behind that departing front. The
NAM continues to hold to its thought of a dry forecast, with a cap
holding in place, whereas the GFS/Euro/GEM all call for some break
in the cap. There is quite a bit of instability above that cap, so
any breaks could bring some hail storms as the storms would be
strong enough to support good rotation. GFS soundings indicate the
best chance for this break to be west and north of Louisville,
though any storms that develop to our west would get shifted east
northeast in the mean flow and could have outflow deep enough to get
over that cap late Friday night. Have gone with scattered storms
along and west of a Morgantown to Shelbyville, KY and isolated
storms just east of that for the daytime hours. At night blended
guidance went with likelies along and north of the Ohio, and this
make sense with flow pattern and any upstream development.

.Long Term (Saturday through Thursday)...
Issued at 230 PM EDT Thu Apr 27 2017

...Severe Weather Potential Increasing for the Weekend...
...Heavy Rains Expected With These Storms Too...

The weather for this weekend still is a matter of potential, nothing
certain. The storms that develop Friday night will have a big impact
on what kind of weather we see Saturday. Current deterministic
models have the area drying out after the morning rains are over.
The question mark will be any remaining boundaries across the region
as well as cloud cover. We have temperatures shooting up into the
mid 80s for highs, but once again forecast soundings show a stronger
low-level inversion capping us. Shear profiles are impressive, so
any boundaries that could force enough lift in that environment
would produce more severe weather. As before, the best chance to
break this cap later in the afternoon looks to be along and north of
the I-64 corridor.

After daytime heating dies down, we should get another break in
widespread activity until a cold front starts advancing in from the
west. We should once again have good heating across the region ahead
of this front, and once again the GFS/Euro/GEM is at odds with the
NAM. The model consensus keeps us dry most of the day, whereas the
NAM has some development in peak heating along a faster advancing
front. With the ensemble guidance also leaning the consensus
direction, and given uncertainty in convection ahead of it, will not
make many changes to the blended model forecast here.

One other thing to note with these rounds of precipitation,
especially come Sunday, is that the available atmospheric moisture
will be pretty high for this time of year. Model soundings for 00Z 1
May at BNA for PW range from 1.6-1.8 inches, which would be above
the record value on SPC`s sounding climatology page...so a soaker is
possible. We still have time to determine if a flash flood threat is
possible, especially as we see how Friday night`s event unfolds.

Rains should come to an end Monday morning as cooler and drier air
filters into the region. We moderate a little Tuesday before the
forecast becomes a little uncertain again for midweek. The GFS/GEM
both have a frontal boundary stalling in the vicinity, whereas the
00Z Euro briefly had rain chances Tuesday night before high pressure
builds across the region for a dry Wed/Thu. Given lack of skill in
any one particular model at this time frame, will continue with the
blended forecast there.

&&

.Aviation...(06Z TAF Issuance)
Issued at 111 AM EDT Fri Apr 28 2017

The main challenge for this TAF forecast will be the timing of
convection this afternoon and tonight. For the remainder of the
predawn hours this morning, high pressure will keep the weather
quiet with winds light and variable to calm. Some light fog at BWG
is not completely out of the question, but will keep it out of the
forecast for now.

Winds will pick up today out of the south and become gusty by the
afternoon. As the atmosphere destabilizes this afternoon scattered
thunderstorms are expected to develop. The latest guidance has the
warm front with this system further to the north so thunderstorm
chances for SDF look a bit less than in previous guidance.
Therefore, will stick with PROB30 thunderstorm chances for all sites
this afternoon. There looks to then be a break in the showers before
a more widespread area of rain develops tonight and sinks southward.
This will affect SDF 06Z or after. Will carry showers and VCTS for
this activity. It should be after the forecast period for LEX and
BWG.

&&

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

$$

Short Term........RJS
Long Term.........RJS
Aviation...EER



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