Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 301700
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
100 PM EDT Thu Mar 30 2017

.Forecast Update...
Issued at 1055 AM EDT Thu Mar 30 2017

Surface warm front continues to work northward through the area this
morning.  A few showers and storms were noted to the northeast of
the region and continue to move off to the northeast.  Despite the
widespread mid-high level cloudiness, warm advection continues and
has allowed temperatures to warm into the upper 60s and lower 70s
across the southern half of KY.  The Bluegrass remains in the lower
60s, but temperatures are expected to rise over there shortly.

High resolution GOES-R imagery shows the back edge of the cirrus
shield moving through western KY.  Based on current movement, it
should enter our western CWA by 17Z and approaching the I-65
corridor by 18Z.  In addition, we are seeing some thinning of the
cirrus as well across central TN and across the TN Plateau.  This
may work into southern KY this afternoon.

In terms of the latest guidance, the short term NAM and HRRR
generally keep the atmosphere relatively capped through the early
afternoon.  After 18Z or so, destabilization looks to occur in
earnest through the afternoon hours.  Strong forcing in the dry slot
ahead of the upper level low will likely spur rapid convective
development.  Model proximity soundings continue to show
unidirectional shear profiles, so discrete convection looks likely
across W KY, southern IL and SW IN.  Cellular splits are possible in
this environment.  Severe threats look to be large hail and damaging
winds.

This activity looks to move eastward and congeal into a
squall line.  The squall line looks to move into the western parts
of the CWA late this afternoon, crossing the I-65 corridor by mid-
evening and then move into east-central KY late tonight.  The
threats with this line still look to be damaging winds with some
large hail here and there.  Some isolated tornadoes will be possible
within the line.  Tornado threat looks to be low given the
relatively unidirectional wind profiles.  However, this risk could
become higher should a more backed surface flow develop in advance
of the line.

For now, have not modified temperatures all that much.  Highs in the
75-80 degree range look attainable, especially if this cirrus shield
continues to thin and clear out.

&&

.Short Term (Now through Friday)...
Issued at 258 AM EDT Thu Mar 30 2017

Bottom line first: Confidence is high that there will be
thunderstorms in the region this evening. However, widespread cloud
cover here and convection along the Gulf Coast raise uncertainty
about the storms` degree of severity.

Details:
Low pressure near Kansas City this morning with a warm front
reaching ESE into the Tennessee Valley will move to central Illinois
by this evening and then to south of Detroit by Friday morning. Its
attendant warm front will surge northward early in the period,
placing us in the warm sector today and this evening.

Storms to our west overnight have struggled to organize and have
produced only sporadic severe weather reports. The storms are
expected to further weaken as they approach southwest Indiana/west
central Kentucky this morning.

The HRRRX has been verifying very nicely with the overnight
convection to our west and south. The RAP, 3km NAM, and NAM12 have
been doing a decent job as well. Leaning on those models, it appears
that the main event will take place when storms develop to our
southwest and surge northeastward into central Kentucky and southern
Indiana late this afternoon through the evening hours, moving off to
the east after midnight. This is not far off from what the forecast
data were showing yesterday, lending some additional confidence to
this general idea.

Model QPF fields and reflectivity progs suggest some discrete storms
may initially fire before the main squall line moves through later.
This would be similar to what happened on the 27th when cells fired
out ahead of the main line. Sometimes when this happens the initial
cells can steal some of the potential intensity from the incoming
line if they are widespread enough. Right now, though, it looks like
they will remain widely enough scattered to not rob much from the
incoming line, as was seen on the 27th.

Another factor working against us will be potential cloud cover
limiting instability. Atmospheric cross sections do show quite a bit
of 950-700mb RH this afternoon.

Also, models and current radar trends suggest that there may be
robust convection along the central Gulf coast which may help to rob
us of some available moisture.

Looks like the best chance of severe will be in the late afternoon
and evening hours with steep mid-level lapse rates, approach of the
cold front, weak capping, and best shot at decent instability
(models suggest LI around -4 and CAPE around 1000). There will be
some weak theta-e ridging around that time as well. The best chance
of a tornado looks to be along and west of I-65 south of the Ohio
River in the early evening where 0-1km helicity will be on the order
of 100-150 m2/s2. STP is low, suggesting any spin-up would be
relatively weak and short-lived.

Winds will be gusty this afternoon, somewhat dependent on cloud
cover, possibly into the 30-35mph range.

Normally it would be wise to go well above guidance for highs this
afternoon given the synoptic set-up, but the expected clouds should
help to keep temps from attaining their full potential. Will still
go on the warm side of the guidance envelope, though.

On Friday plenty of clouds will sweep in from the northwest behind
the departing system, along with scattered light rain showers. High
temperatures on Friday will probably be about 20 degrees cooler than
today.

.Long Term (Friday Night through Wednesday)...
Issued at 304 AM EDT Thu Mar 30 2017

High pressure nosing in from Canada will provide us with a dry
weekend, though it may take some time for clouds to scour out on
Saturday. More sunshine expected Sunday. Lows Saturday morning and
Sunday morning are still progged in the 40s, so we should stay out
of a frost/freeze situation...though the NAM suggests some spots
could dip into the mid/upper 30s.

Models are still having a hard time figuring out the system ejecting
out of the Red River/lower Mississippi Valley Monday/Monday night,
but the general idea is still that any severe storms will remain to
our south with more general shower/storm activity in the Ohio Valley.

The next system will then approach from the Plains around the end of
this forecast period.

&&

.Aviation...(18Z TAF Issuance)
Issued at 1258 PM EDT Thu Mar 30 2017

A strong low pressure system will track northeast from central IL to
the Great Lakes region today into tomorrow. For the next few hours,
VFR conditions with just a few cu and cirrus are expected to
prevail. Winds will be gusty out of the south with the strong
pressure gradient.

Scattered storms will begin to develop this afternoon out ahead of
the main area of storms that is expected to develop this evening.
The scattered storms in the afternoon could affect BWG and SDF, so
will carry VCTS there this afternoon. A more widespread area of
storms and/or a squall line will develop this evening and move
across central KY, mainly after 0Z. These have the potential to
affect the terminals for a number of hours this evening. The storms
will last the latest at LEX with some showers lingering after the
storms.

By early tomorrow morning, ceilings will lower, likely to fuel
alternate by 12Z. Some light fog may affect SDF and LEX as well.
Winds will shift to west-southwesterly tomorrow.

&&

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

$$

Update...MJ
Short Term........13
Long Term.........13
Aviation...EER



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