Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 170546
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
145 AM EDT FRI MAY 17 2013
Issued at 145 AM EDT Fri May 17 2013
Updated the forecast to remove thunderstorms until after sunrise.
Upstream obs just show rain showers moving northeast out of western
TN. Judging by what happened yesterday morning along with model
precip fields and soundings, t-storms will probably hold off until
Areas of fog still look like a good bet over portions of southern
IN/northern KY where it rained yesterday. Fog may become locally
dense in river valleys and typical fog prone locations. Frankfort
has reported 1/4 mile vsbys this morning.
.Short Term (Now - Friday Night)...
Issued at 312 PM EDT Thu May 16 2013
Convection continues to rumble across portions of the forecast area
this afternoon. The western batch across our west-central southern
Indiana counties continues to rain itself out. Some renewed
convection is trying to develop across southern IL and eastern MO.
Overall, the atmosphere is fairly worked over here, so we`re not
expecting much in the way of redevelopment this afternoon. Across
the Bluegrass region of central Kentucky, showers and thunderstorms
continue to push on off to the east. This activity will continue to
affect our northeastern quarter for the remainder of the afternoon
and should diminish during the evening hours. Elsewhere, partly to
mostly cloudy skies were noted across the region. Southwesterly
flow will continue across the region and an isolated shower or storm
is still possible across much of central and southern Kentucky this
afternoon. There is no clear focusing mechanism for convective
initiation other than differential heating and convective
overturning. So the coverage is expected to remain quite isolated
and random this afternoon. With all the clouds to the north and
some partial clearing across the south, we`ve got a pretty good
gradient of temperatures across the region. Temperatures range from
the mid-upper 60s in the north to the lower 80s across the south.
Not really expecting a massive change in temperatures this
afternoon. Though, some warming will be possible across southern
Indiana and portions of north-central Kentucky as the convection
comes to an end and we see a slight thinning of the cloud cover.
For tonight, the latest short term multi-model consensus continues
to be in agreement with convection ending during the evening hours
due to the loss of heating. Will probably have partly to mostly
cloudy skies across the region due to approaching mid-high level
clouds to the west/southwest and left over convective debris from
today`s convection. Surface winds should slacken off overnight and
we will probably see some patchy fog develop. Overall, we believe
that the best chances of fog will be in those areas that saw
rainfall during the daylight hours (southern Indiana and in KY along
and north of the I-64 corridor). Some of the high resolution WRF
models (3km NCAR WRF and 4km LMK WRF) do show a weak vort lifting
northward through western KY tonight. This may kick off another
round of showers/storms just to the west of CWA. For now, plan on
keeping things mostly dry tonight, but if convection does develop to
our west and south, some modifications to the ongoing forecast to
add in PoPs in our far west may be required with subsequent forecast
updates. Lows tonight are expected to drop into the lower 60s in
the west with upper 50s to around 60 in the east. These numbers are
very close to the short term multi-model consensus forecast.
For Friday, the latest data suggests that the upper low pinwheeling
across the Arklatex this afternoon will slide eastward into MS/TN/AL
during the day on Friday. Plenty of moisture, combined with daytime
heating should result in scattered convection developing across the
area once again. Unlike today, it appears that the most favored
region for development on Friday would be in our southern and
southwestern areas. For now will continue with the 40-50 percent
coverage as the high resolution WRF models show multi-cellular
convection developing during the afternoon hours. With regards to
severe weather, the threat continues to look rather low at this
point. Moisture is plentiful, but the shear and instability
parameters are only marginal. Still, any storm will be capable of
producing heavy rains, gusty winds and some small hail...along with
plenty of lightning.
Temperatures look to top out in the upper 70s...though if more
sunshine gets through the clouds, lower 80s would certainly be
possible. Convection looks to diminish in coverage Friday night due
to the loss of heating once again. However, with the upper low in
the vicinity, will continue to keep isolated showers/storms in the
forecast for now. Lows Friday night look to cool back into the
.Long Term (Saturday - Thursday)...
Issued at 314 PM EDT Thu May 16 2013
At this time, a weak closed low is over Oklahoma. This feature will
open up and weaken farther as it moves into slightly confluent flow
over the Lower Ohio Valley by early Saturday. With this weak wave
moving into a warm and humid environment characterized with PWATs
around 1.5 inches, scattered thunderstorms are expected Saturday and
Saturday night, and to a lesser extent on Sunday. With the polar jet
displaced well to the north, any storm movement will be slow,
bringing a possibility of localized heavy rain. Saturday may stay
mostly cloudy, restricting highs to the mid to upper 70s.
A significant trough will exit the Rockies early Sunday, assuming a
negative tilt. In response, ridging at 500mb will build over the
Mississippi Valley. Think that with building heights to our west on
Sunday, much of any scattered afternoon precipitation will stay
restricted to eastern Kentucky.
Expect a warming trend Sunday and Monday. Southerly flow will begin
Sunday afternoon and increase on Monday ahead of a surface trough
expected to develop across the Great Plains. Expect a dry Sunday
night through Tuesday morning time frame as weak subsidence
associated with this ridge will limit instability, despite ample
moisture. Temperatures in the lower to mid 80s on Sunday may rise
into the upper 80s Monday.
Both the ECMWF and GFS forecast a blocking pattern to begin to
develop across central North America beginning Monday. In response
to ridging across Manitoba, the trough exiting the Rockies will cut
off into a closed circulation centered somewhere over the Dakotas by
late Monday. At this time, this circulation will become vertically
stacked as any surface low will lie right underneath the 500mb
center of circulation.
A front will slowly trudge southeast towards the Ohio River late
Tuesday through early Wednesday. This boundary will provide the
focus for several rounds of convection from late Tuesday through
possibly as late as Thursday. Summer-like temperatures in the
mid-80s on Tuesday will gradually cool towards the upper 70s by
.Aviation (06Z TAF Issuance)...
Issued at 125 AM EDT Fri May 17 2013
A complicated TAF forecast is in store with fog and t-storm issues.
First of all, it looks like SDF/LEX will see some fog in the early
morning hours due to a relative stable layer that has trapped good
low level moisture from rains yesterday. Dewpt depressions have
already fallen to between 1-2 degrees at SDF/LEX and will continue
to close as the night wears on. Think that SDF will see MVFR vsbys
in light br while LEX (which had more rain today) should see IFR
vsbys in br.
As for convection chances, we should see a lull in activity during
the early morning hours. Convection ahead of an approaching upper
low is pushing northeast out of western TN this morning and will
make it into the TAF sites at roughly 10Z at BWG...14Z at SDF...and
1430Z 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.
Winds should remain out of the SSE likely topping 7kts this