Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 300705

Area Forecast Discussion
National Weather Service Louisville KY
305 AM EDT Thu Mar 30 2017

.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.

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

.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 (06Z TAF Update)...
Issued at 1252 AM EDT Thu Mar 30 2017

Expect easterly flow to prevail through sunrise as broken high
clouds continue to move over central Kentucky. Winds will become
southerly after 12z and begin to increase into the 12-18 kt range
through the morning hours. A strong low pressure system near Kansas
City currently will track eastward during the day. Strong wind gusts
up to 30 kts are likely at all TAF sites from late morning into the
afternoon, ahead of any showers or thunderstorms.

Thunderstorm chances will increase by mid-afternoon at SDF and BWG
and mid-late afternoon at LEX. Some uncertainty in how the event
will evolve with the potential for storms to develop as soon as mid-
afternoon, or hold off until early-mid evening. As this point, the
best consensus puts the greatest chance of seeing stronger storms
from 23z to 03z across central Kentucky.

As storms end west to east toward the end of the current TAF period,
winds will shift to the southwest then eventually west while
ceilings lift back to 5kft or higher.




Short Term........13
Long Term.........13
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