Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 071019
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
619 AM EDT Tue Jul 7 2015
Issued at 618 AM EDT Tue Jul 7 2015
Updated the forecast this morning, mainly to up pops given the
convection that has developed out to the west. This initial round
of storms will push through the region during the morning hours,
bringing mainly a heavy rain threat. It remains to be seen how much
recovery will take place in the wake of this activity this
afternoon, but think central and southern KY stand the best chance
to see 1000-2000 J/kg of MLCAPE build by the afternoon
hours. This may result in a stronger storm or two in this area this
afternoon into the evening, but still think any organized severe
threat is quite low given the reasons mentioned in the short term
discussion. The main threat today will likely be torrential rainfall
and perhaps some localized flooding. Updated products already out.
.SHORT TERM (Now through Wednesday)...
Issued at 315 AM EDT Mon Jul 6 2015
The synoptic pattern early this morning features generally zonal
flow across the Ohio Valley. This regime will allow for a shortwave
trough and associated surface cold front to affect the region
through the period, bringing rounds of showers and thunderstorms.
All is quiet across the region early this morning, but this will
likely change as we head towards dawn and beyond. Convection has
already begun to blossom over the past 30 minutes across southeast
MO and southwest KY. This is due to increasing isentropic ascent on
the nose of a 20-30 knot low-level jet. As we head through the
morning hours, this jet will veer into the Ohio Valley, which should
push a round of showers and thunderstorms across central KY and
southern IN mainly during the morning hours into the early afternoon.
The question then becomes whether or not we are able to sufficiently
destabilize in the wake of this morning activity. The latest
guidance continues to suggest that we should at least marginally
recover to see MLCAPE values of 1000-2000 J/kg, with the highest
values across the southwest CWA. The slab-like ascent along the
front and any lingering outflow boundaries should be the focus for
renewed convective development this afternoon into this evening.
The latest hi-res guidance suggests that outflow from the convection
this afternoon into this evening should help push the effective
boundary further to the south tonight, so have raised pops in the
south overnight while lowering them in the north. Any organized
severe weather today seems rather unlikely given 0-6km shear vectors
of only 10-20 knots, along with weak mid-level lapse rates.
However, there is some decent flow of 30-40 knots in the 850mb layer
which may help to foster some isolated instances of stronger wind
gusts as precipitation loading helps to mix these winds down.
Speaking of precipitation, PWATs will climb in excess of 2 inches
today as the front approaches, which will lead to very efficient
rain producers. Debated a Flash Flood Watch with neighboring
offices, but decided to hold off for now. The first reason is that
we have had a few days to dry out, which has pushed flash flood
guidance values up considerably from where they were a couple of
days ago. Additionally, there is some uncertainty as to how the
expected morning convection will play out, as it certainly could
affect more robust storm development this afternoon. It seems most
likely that southern IN and northern KY would stand the highest
chance of seeing the heavier rainfall, but storm outflows may stall
tonight over south-central KY which may focus some heavy rainfall
there too. Think the threat will be generally confined to where
storms locally train, thus do not think it is prudent at this time
to put out a watch given the uncertainties of where (and if) this
training might occur.
Otherwise, convection will push northeast on Wednesday as ridging
across the Southeast begins to strengthen. This will help push the
effective boundary north through the day, so the higher precip
chances will be across the south in the morning shifting to the
north in the afternoon. There will likely be a good temperature
gradient that sets up along this boundary, with south-central KY
climbing into the mid/upper 80s and northern KY/southern IN staying
confined in upper 70s and lower 80s.
.LONG TERM (Wednesday Night through Monday)...
Issued at 303 AM EDT Tue Jul 7 2015
An upper ridge will slowly retrograde across the southern United
States over the course of the long term forecast period. While the
ridge will generally suppress convection, we will be on the edge of
the ridge much of the time, so there will still be the question of
whether or not we will continue to get some thunderstorm activity.
Overall, the best chance for storms for the bulk of the long term in
the LMK CWA will be from southern Indiana into the northern Blue
Grass where the ridge`s influence will be the weakest. Then by
Sunday/Monday we`ll get into northwest flow as the ridge moves into
the southern Plains and southern Rockies. This will lead to better
areawide thunderstorm chances as waves drop down the back side of
Canadian troffing digging into the eastern U.S.
Wednesday night through Thursday surface low pressure will advance
from Missouri to Ohio. Showers and thunderstorms can be expected
with this feature, especially in southern Indiana Wednesday night as
the low moves by. Scattered convection will be possible over much of
the region Thursday as the low`s trailing front swings through.
MCS development is expected over the Midwest Thursday night. At this
time it appears that the bulk of the rain will remain to our
northwest. Nevertheless, we`ll have to account for the possibility
of some shower activity making it into the LMK CWA late Thursday
night into Friday morning as any remnants from the overnight
convection make it this far east before the upper ridge squelches
On Friday the deterministic model runs bring a fair amount of QPF
into the region. With the upper ridge strengthening overhead,
though, the feeling right now is that this QPF is overdone.
Supporting this idea is the fact that the ensemble means are quite a
bit drier. There could be some left-over boundaries from the
previous night`s MCS to our west (especially if any of that activity
does indeed make it here), so will go ahead and include a PoP in the
forecast but will keep it small and will restrict it to southern
Indiana/northern Blue Grass away from the heart of the upper ridge.
After a mostly dry weekend, PoPs will ramp back up for Monday as the
ridge moves away.
High temperatures will gradually climb from the 80s to finish out
the work week into the 90s for the weekend as the ridge asserts
itself. Heat index readings will peak around 100 Saturday afternoon.
Morning lows will generally be around 70, though by Sunday morning
urban cores may not fall much below the mid 70s.
.AVIATION (06Z TAF Update)...
Issued at 1251 AM EDT Tue Jul 7 2015
Generally VFR conditions are expected the rest of the overnight into
today outside of thunderstorms, but incoming convection will
likely bring brief restrictions to all sites through the period.
The latest guidance suggests there may be two rounds of convection
today, the first which will come towards dawn and last through the
mid morning. It appears KSDF has the best chance of seeing this
activity, but it could get into KLEX and KBWG as well.
After that pushes through, there may be a bit of a lull late this
morning into the early afternoon before yet another round of
convection fires along an approaching cold front. Have tried to
time the best chances for convection, but admittedly a thunderstorm
will be possible at all sites through any point of the day today
into this evening. Outside of convection, SW winds will gust in
excess of 20 knots at times, but should die down this evening once
the sun sets.