Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 171043
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
643 AM EDT FRI MAY 17 2013
...Updated Aviation Discussion...
.Short Term (Today - Saturday)...
Issued at 330 AM EDT Fri May 17 2013
Early this morning an upper level low was slowly spinning eastward
into the mid Mississippi River Valley. At the sfc, a frontal
boundary was stalled over northern IL/IN. The Ohio Valley was
sitting in between these weather systems in a very moist environment
with subtle vort waves pushing eastward from the upper low kicking
off morning showers over southwest IN and east central KY. Expect
this light activity to continue during the pre-dawn hours. Also,
light fog has formed over portions over southern Indiana and
northern KY where it rained yesterday. Expect light fog to continue
throughout the pre-dawn hours. Think that increasing winds aloft
and a mixy atmosphere may prevent widespread dense fog. However,
will continue to monitor.
Around and after sunrise, the better bands of showers over western
KY/TN will push northeast into our area and overspread the region.
T-storms will become more common over the area by mid morning and
last through the afternoon. By late morning or early afternoon,
high-res models indicate that t-storms will erupt ahead of the main
line of convection pushing northeast through the morning hours.
This will likely happen in a moist convergence zone as it did
yesterday morning/early afternoon. With the upper level low pushing
into the Ohio Valley this afternoon/evening, felt confident enough
to go with 60% POPs later in the day. Storm strength for today
still looks marginal with the best storms containing small hail
and/or gusty winds up to around 40 mph.
Tonight the forecast gets a little tricky with some of the models
pushing the upper low farther to the south and east. However, the
majority of models and ensembles indicate the upper low will linger
over the Ohio Valley providing bands of showers with embedded
t-storms throughout the night, therefore, will go with 40% POPs for
much of the night.
Saturday the upper low will slowly start to exit the area but still
provide plenty of convection over the area before it does so.
Soundings for Fri night/Sat indicate a very warm, moist environment
with PWATs reaching the 1.5-1.7 inch range. Thus, would expect some
heavy rainers Fri night/Sat which may lead to some minor flooding
issues if t-storms and/or steady moderate rains train over the same
Temperatures should remain in the mid to upper 70s for highs today
and Saturday given the anticipated rains and cloud cover. Lows
tonight will bottom out in the low to mid 60s.
.Long Term (Saturday Night - Thursday)...
Issued at 307 AM EDT Fri May 17 2013
Saturday Night - Sunday Evening...
The weak upper level low will continue to gradually move east to
around the I-75 corridor by early Sunday morning. This will keep
scattered convection going through the late evening and overnight
across eastern portions of the CWA, although with lower overall
coverage than during the daylight hours. West of the I-65 corridor,
expect just isolated coverage of showers or storms, likely becoming
compltetly dry in the pre-dawn hours of Sunday. Mostly cloudy skies
should rule across all but the far western CWA through the night,
and therefore will keep low temperatures in the low to mid 60s.
By Sunday, the upper low should move to the central Appalachian
spine with an upper level ridge axis building in across western
Kentucky. Upper low will still have influence over eastern portions
of the CWA and expect a diurnally driven uptick in convection during
the afternoon hours. Coverage east of I-65 should generally be
30-50%. Should see a bit of a warmup on Sunday across western
portions of the CWA where low to mid 80s will be possible. Will keep
temps in the upper 70s to the east closer to the upper low. Any
lingering isolated convection in the east may last until the late
evening on Sunday night.
Sunday Night - Tuesday...
The beginning of the new week looks to offer the best chance at
mostly dry conditions as the Ohio Valley will be under the influence
of an upper level ridge. This ridge will work to block upstream
flow, resulting in a closed upper low over the Dakotas and a
generally slower arrival of the next chance of significant
precipitation. Will continue to go dry for the early Monday morning
to Monday night time frame as an inversion associated with the upper
level ridging should limit convective potential. Models have
inconsistently depicted some convection firing along a weak warm
front on Monday afternoon, however placement has been erratic and
with the expected inversion in place will continue to leave forecast
dry. Will continue to monitor trends. Monday will continue the
warming trend with highs solidly in the mid 80s. Monday night should
be more mild with lows only in the upper 60s as southerly flow takes
Tuesday will bring a return to scattered shower and storm chances as
the central CONUS trough begins to make progress eastward. However,
the overall trend is slower making Tuesday an overall nicer forecast
than previous solutions. Best chances for mainly afternoon/evening
storms will be over the northwestern CWA closest to the cold front
associated with the system. Only expecting coverage in the 20-40%
range under the WAA regime. Highs on Tuesday should again be in the
Tuesday Night - Thursday...
A more active period of weather looks to take place through mid week
as the associated central CONUS trough begins to open up and eject
to the eastern CONUS. It will drag the surface low through the Great
Lakes, and its associated cold front through the CWA sometime
Wednesday night. Expect numerous showers and storms ahead of this
front from late Tuesday night through at least Wednesday evening.
Have continued 60% chances during this time frame. May have to watch
for stronger storms as stronger westerlies combine with what is
expected to be a moderately unstable airmass. Highs Wednesday will
be in the upper 70s.
Models diverge in solutions by Thursday with how progressive the
frontal boundary is. At this point, prefer the less agressive
solution which keeps the slower front near the region, and scattered
showers and storms still possible.
.Aviation (12Z TAF Issuance)...
Issued at 640 AM EDT Fri May 17 2013
A complicated TAF forecast is in store with fog, low cig, and
t-storm issues. First of all, it looks like SDF/LEX will stay in
the VFR cat with some very light br observed this morning.
Convection ahead of an approaching upper low is pushing northeast
out of western TN/KY this morning and will make it into the TAF
sites at roughly 12-13Z at BWG...15-16Z at SDF...and 1530-1630Z at
LEX respectively. Although precip may begin as showers at BWG, it
will likely transition to some t-storm activity by mid morning with
on and off t-storms expected throughout the day due to the
approaching upper low and a good moist/unstable pool of air south of
a sfc boundary located over northern IL/IN.
After 3Z tonight, we`ll likely see some break in convection but
waves of showers and possibly an isld t-storm will again be possible
tonight as the remnant upper trough pushes into the Ohio Valley. As
the upper trough draws near, observed IFR-MVFR conditions will
overspread the area. Will go conservative with just MVFR conditions
for late tonight for this TAF issuance, but that may need to be
adjusted downward in coming TAF issuances.
Winds should remain out of the SSE likely topping 7kts this