Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 140048

Area Forecast Discussion
National Weather Service Louisville KY
848 PM EDT Fri Apr 13 2018

.Forecast Update...
Issued at 845 PM EDT Fri Apr 13 2018

...Potential for Strong Storms & Localized Heavy Rain Threat
Saturday/Saturday Night...

Seeing pretty good hi resolution model agreement for a line of
showers and embedded thunderstorms pushing into our CWA in the pre-
dawn hours. Latest output indicates this activity should have a nice
cold pool push associated with it to the point that some gusty winds
around 30 mph would be possible. Suppose an isolated 40 mph gust
isn`t out of the question. A strong low level jet of 40-50 knots
will also be in place during this time, but think a stable layer
near the surface should limit some of the higher momentum air aloft
from reaching the surface. Updated the HWO to mention 30 to 40 mph
winds possible mainly along and west of I-65. In fact, the best
likelihood for any stronger wind gusts would be across southern KY
on either side of dawn.

Behind the first round of convection, hi res models keep storm
scale low level flow associated with the cold pool westerly for
much of the morning, before the synoptic scale southerly flow
takes back hold by late morning to around midday from west to
east. At this point, the atmosphere will try to destabilize once
again (from an advection standpoint), but will likely struggle
given expected thick cloud cover. Mentioned in the previous
forecast discussion, the ability to destabilize in the
afternoon/evening will ultimately decide our conditional severe
threat as we move into the later evening and early overnight
hours. After a lull in the afternoon shear profile, it appears to
pick up again in the evening and overnight hours as the low level
jet re-intensifies and the exit region of a mid level jet begin
to nose into the area. As depicted by the SPC outlook, greatest
threats for severe would be in our NW CWA nearest the triple point
(again conditional upon destabilization), and across our southern
CWA if instability axis is able to nose in.

Regardless of whether a severe threat materializes, we`ll have to
continue to monitor the potential for hydro concerns and any
potential headlines for Saturday night. A rich plume of 1.5-1.6"
PWAT air will spread into the region as a healthy LLJ responds
beneath the slowly progressing right entrance region of an upper
jet. Given the SSW-NNE orientation of the moisture plume combined
with the slow eastward progression of the upper forcing, some
training of heavy showers/storms could lead to localized amounts
higher than the 1.5 to 2.5 inches forecast for most spots. At this
point not enough confidence for widespread higher amounts, but will
still have to monitor latest data if potential for Flash Flooding
becomes more apparent.


.Short Term...(This evening through Saturday night)
Issued at 235 PM EDT Fri Apr 13 2018

...Strong Storms Possible Late Saturday Afternoon and Evening...
...Soaking Rains This Weekend...

Surface low pressure system is centered over southeast NE this hour,
with a warm front stretching east to near Chicago and a cold front
down to southwest OK. The surface low will not move much by daybreak
Saturday, but a line of storms is expected to form this afternoon
across Arkansas and push east northeast overnight. This line will
continue into western KY before daybreak Saturday. A low-level jet
forming ahead of this line could help to reinvigorate the cells, as
it gets to our zones west of I-65. Some brief gusty winds could mix
down with these cells.

As the low-level jet winds down and the line continues eastward, we
should see it weaken, with perhaps a brief period of either dryness
or very light rainfall. By late afternoon/early evening, models are
picking out a new low pressure developing east of the original low,
somewhere over southern IL. With the weakening of the earlier
convection, we could see some pockets of instability develop out
ahead of the cold front, still holding back along the MS River.
These cells will be the ones to watch for any chance for severe
weather. The better threat for severe would be closer to the new low
and its warm front, which looks to stay just north of our area. Wind
fields will still be a little weak over our area in the afternoon,
but as the evening goes on, they get more impressive as a new low-
level jet feeds into the surface low. Will have to watch for some
bowing segments heading north along the front for potential for
damaging wind gusts as well as isolated tornadoes, as low-level
helicities are pretty high.

The cold front behind these will slowly move east to the I-65
corridor by daybreak Sunday. Rainfall totals will be another
concern. Values will depend strongly on how convective the various
bands are...with multiple storms leading to better chances for
totals of 3-4 inches, whereas weaker more stratiform rain could make
it more like 1-2 inches. Current QPF splits the difference, ranging
from 2-3 inches across southern KY up to and ~2 inches in the I-64
corridor. Will continue the Hydrologic Outlook product for now,
talking about how the rivers will handle that much rain, but if it
looks like we`ll lean more toward the heavier rain solution, we may
need to bump up to a Flood Watch.

.Long Term...(Sunday through Friday)
Issued at 235 PM EDT Fri Apr 13 2018

Front continues to head slowly east Sunday, but the upper low will
be equally slow to cross the region through Monday. That will keep
some light showers in the forecast through Monday...with raw
conditions expected both Sun/Mon. We should see a warmup with the
sunshine Tue/Wed before another couple of systems move through here
late Wed and by the end of the work week, bringing yet another cool
down. Rainfall totals should not be nearly as much with these
systems, if at all, as flow aloft never gets a good tap of Gulf
moisture into our area.


.Aviation...(00Z TAF Issuance)
Issued at 723 PM EDT Fri Apr 13 2018

Low pressure crossing Missouri and Illinois will pull a cold front
into the Mississippi Valley by tomorrow evening and position a warm
front to our north across Indiana and Ohio. The TAF sites will be in
the warm sector of the storm with gusty south surface winds and a
low level jet producing LLWS tonight, especially west of LEX.

An upper level impulse over Arkansas this evening will head
northeast and bring showers and thunderstorms to the TAF sites
Saturday morning. We may get a break in the afternoon as the impulse
pulls off, but additional showers and thunderstorms will move in
Saturday evening ahead of the approaching cold front. The front
should be just about to SDF at the end of the SDF TAF period.




Short Term...RJS
Long Term...RJS
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