Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 201743

Area Forecast Discussion
National Weather Service Louisville KY
143 PM EDT Mon Mar 20 2017

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1016 AM EDT Mon Mar 20 2017

Scattered showers and isolated thunderstorms have begun to develop
across all of east central KY down to the TN border. Did a quick
update to pops to expand them to the south for the next few hours.
Also brought the grids in line with current observations.

Issued at 902 AM EDT Mon Mar 20 2017

Scattered showers have begun to pop up across the Bluegrass this
morning. Some of the thunderstorms to the north nearer to the
Ohio border have been producing some pea size hail this morning and
this will be possible with any of the stronger cells moving across
the region this morning. Pops were updated over the next few hours
based on current radar trends as well as recent HRRR runs.
Generally, they were lowered west of I-65 today as most of  the
activity looks to stay farther east.

.Short Term (Now through Tuesday)...
Issued at 320 AM EDT Mon Mar 20 2017

Skies are mostly clear across much of central KY at this time but
the GOES low cloud/fog imagery is showing some clouds over west-
central and southwest IN into parts of west-central KY moving
southeast. Thus, clouds will increase this morning across southern
IN and parts of central KY.

At the same time, elevated convection has developed in eastern IA
and northern third of IL in an area of isentropic lift and exit
region of the low-level jet, just ahead of a surface frontal
boundary that extends from IA southwest through the central Plains.
High resolution model forecasts suggest the convection will move and
develop southeast early this morning across IL and IN and into OH.
Modest elevated instability should be present by daybreak over a
good part of southern IN and central KY. However, low-level forcing
should weaken so cells should weaken with time as they approach our
area. Or western flank cells may weaken with eastern flank cells
continuing in the better forcing as they eventually head toward
southeast IN, southern OH, and perhaps northern KY later this
morning. Will keep a chance of precip in the forecast for this
morning over the northern third of our forecast area.

For this afternoon, high resolution models show convective
redevelopment well to our north over northern half of IN into OH.
Over our area, elevated instability will remain, but model soundings
show enough capping to prevent much in the way of convective cell
development. Only caveat may be any residual boundary from the
morning convection that could create enough local forcing to
overcome the cap resulting in isolated convection. Otherwise, it
should remain dry in most areas.

As the frontal boundary slips southeast across central IN this
afternoon and evening, a thermal ridge axis will push northeast
across the lower OH Valley in advance of it with S to SW surface
winds. This often can mean that surface temps can overachieve ahead
of the front, although this depends on cloud cover. The GFS wants to
develop low clouds over the area during the day today, while other
models show a little less or are slower in doing so. This is why GFS
MAV MOS numbers are lower than the NAM MET values. The latest HRRR
is quite aggressive with surface temps but given current low cloud
trends, am a bit concerned this may be too much warming. Still, for
now, have upped temps from what we had previously. Western parts of
south-central KY (with least expected clouds) should rise into the
lower and mid 70s, and could go higher if enough sunshine prevails.

For tonight, the frontal zone is forecast to drop just to the south
of the OH River by 12z Tuesday. Ahead of the front, lapse rates
aloft are steep in model soundings, with low-level moisture present.
However, a capping inversion is present. Therefore, it may be tough
to get widespread precip going tonight, but steep lapse rates aloft
in a northwest flow pattern could easily spell isolated to scattered
elevated showers/thunderstorms, even with some small hail. Will need
to watch this. Lows tonight should vary from upper 40s across our
northern most southern IN counties to upper 50s near TN border.

On Tuesday, the front sags south to around the KY-TN line. Moderate
instability is forecast along and just ahead of it (even becoming
rooted in boundary layer), and models show more organized showers
and thunderstorms developing and moving ESE across western and
southern KY, with a few stronger cells expected, with scattered
lighter showers over our northern forecast area. Expect highs to be
near 60 in southern IN to the upper 60s in southern KY.

.Long Term (Tuesday Night through Sunday)...
Issued at 320 AM EDT Mon Mar 20 2017

The extended forecast starts out with the surface frontal zone
across the TN Valley within northwest flow aloft. Scattered to
numerous showers (and scattered thunderstorms downstate) are
expected Tuesday evening before precip clears central KY on
Wednesday as the front sags south. Behind the front, cool air will
be in place with a northeast surface breeze. Expect highs only in
the mid 40s to lower 50s Wednesday afternoon in our area.

For late this week, a ridge axis aloft will move east from the
Plains to the OH Valley which will cause a moderating trend. The old
frontal zone will lift back to the northeast from the northern Gulf
States to mid MS Valley but lose definition. A band of clouds and
perhaps isolated showers will accompany the warm advection zone
ahead of this residual boundary. Highs Thursday will climb back into
the mid 50s to lower 60s and in the 60s to around 70 Friday.

With good model agreement, the next weather system to affect our
area will be on Saturday as a large upper low moves into the central
Plains. Moisture advection ahead of it will result in numerous
showers and isolated thunderstorms Saturday and Saturday night over
the lower OH Valley, with scattered showers lingering into Sunday.
Highs should remain in the 60s on Saturday.


.Aviation (18Z TAF Update)...
Issued at 143 PM EDT Mon Mar 20 2017

A dissipating MCS that moved by to our north and northeast earlier
this morning emitted low level outflow, about 2000 feet deep, that
turned winds at SDF/LEX to the northeast. This slab of easterly flow
has slowly been mixing out and winds have been gradually veering
through east to southeast, and should end up coming in from the
south or SSE for most of the afternoon at speeds around 10 knots at
those two airports. BWG will remain unaffected by the outflow and
keep its SSW winds and very warm temperatures.

A cool front roughly along a DTX-STL-AMA line will sink southward.
By this evening the front, possibly augmented by outflow from
thunderstorm activity to our north, will cross the Ohio River and
move into Kentucky. Scattered showers and isolated thunderstorms
will probably accompany the boundary, primarily affecting SDF/LEX.

As the surface boundary continues to advance southward, approaching
the Tennessee line by Tuesday morning, it will introduce post-
frontal low level moisture resulting in low ceilings. Ceiling
heights around a thousand feet can be expected, with dips into IFR
possible especially at LEX toward morning. MVFR BR can be expected
as well.

Those low ceilings will gradually mix out over the course of the
morning and any vsby restrictions will disappear as north breezes
come in to clean things out.




Short Term.....TWF
Long Term......TWF
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