Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 250124
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
924 PM EDT Tue Mar 24 2015
Issued at 923 PM EDT Tue Mar 24 2015
Made some additional tweaks to the forecast this evening. Have
upped pops substantially where rain continues to track across
portions of southern IN and northern KY. This should quickly push
east over the next few hours, giving way to a general lull before
the convection currently over MO arrives.
The main PV anomaly and associated surface low will pass across
portions of Illinois and northern IN. The latest SPC mesoanalysis
data shows current convection in MO is already outrunning the better
low-level instability, thus the noted weakening trend in the past
hour. With the better upper-level forcing and low-level convergence
passing to the north near the surface low center, don`t see much
reason this convection will remain vigorous into southern
IN/northern KY. Therefore, think a broken line of weakening showers
with some embedded rumbles of thunder seems probable over southern
IN and extreme northern KY. Further south, have trimmed back pops
as low-level convergence will be even weaker there and instability
will be waning. In fact, much of southern KY will likely remain dry
tonight into Wednesday morning.
Issued at 601 PM EDT Tue Mar 24 2015
Have updated the grids early this evening to account for ongoing
trends. Convection continues to blossom in two areas early this
evening, one across MO/OK along a synoptic cold front, and another
over portions of southeast MO and southern IL in response to
increasing isentropic ascent and elevated instability atop a cool
airmass in place over the Ohio Valley. There has recently been a
report of dime sized hail with this latter activity across southeast
MO. The latest hi-res guidance shows this main elevated instability
axis remaining mainly off to the west through much of this evening,
so convection should continue to spark over southeast MO and western
KY and weaken as it moves away from the instability over southern IN
and central KY.
However, the latest RAP depicts this elevated instability axis
working into the southern IN and central KY later on this evening
into tonight as the low-level jet veers in response to a PV anomaly
ejecting out of the Central Plains. Therefore, have introduced
thunder into the forecast for late this evening into the overnight
hours, mainly across southern IN and extreme northern KY. Cannot
completely rule out some small hail, but think the more robust
updrafts capable of hail will remain off to the northwest.
Will continue to keep an eye on the second area of convection
currently across MO as it will approach the region toward dawn.
However, with the surface low passing well to the north, the focused
convergence which will be needed to make up for the loss of
instability will be passing to the north as well. Will continue to
keep up with trends this evening as things unfold.
.SHORT TERM (Now through Wednesday Night)...
Issued at 225 PM EDT Tue Mar 24 2015
Surface front is now a very slow-moving warm front draped across
south-central Kentucky. Isentropic lift is triggered an area of
light rain affecting southern Indiana, and just edging into the I-64
corridor in Kentucky. Temps range from the lower/mid 40s along and
north of I-64 to the 60s in south central Kentucky.
Still looking for the front to lift northward overnight. Low-level
jetting will kick up south of the front, with 900 mb winds
increasing to 50-55 kt. Precip chances initially confined to the
north will spread over the entire area as the night progresses, in
response to the front trying to wave back to the south by morning.
Will keep POPs in the chance category and QPF fairly light. Temp
curve is non-diurnal as we expect only a slight drop this evening,
and then rising temps before daybreak as the low-level jet creates
more of a mixy boundary layer.
Even though a surface wave will scoot past to our north Wednesday
morning, the front is oriented parallel to the flow aloft and will
really be left behind. Mid-level subsidence should provide a break
from any precip, so hourly POPs will be slight chance at best for
much of the day. Temps will depend heavily on how much lingering
cloud cover we have, and the current forecast of lower 70s is a
fairly conservative one. Places that get a decent amount of sunshine
could punch well into the 70s Wednesday afternoon.
Another wave will take shape over the southern Plains and lift ENE
through the Ohio Valley Wednesday night. Expect widespread precip
with embedded thunder. Strong mid-level winds through a deep layer
will not need a lot of help to mix down, which supports the marginal
svr wx threat advertised by SPC. The QPF could be fairly substantial
in places, as our current grids show rainfall in excess of 1 inch
during the night over southern Indiana. Short-fuse nuisance flooding
is possible under the heavier pockets of rain.
.LONG TERM (Thursday through Tuesday)...
Issued at 245 PM EDT Tue Mar 24 2015
The focus in the long term is on precipitation/temperature trends
Thursday then the unseasonably cold period Friday into this upcoming
The synoptic pattern Thursday morning is expected to feature a
deepening and digging upper level trough through the Upper Midwest
into the Ohio Valley. At the surface, low pressure will likely be
tracking through southern Indiana into central Ohio with warmer,
moist air ahead. Highs Thursday will occur during the morning and
are are likely to be mild, in the mid 50s to low 60s. However, a
very strong cold front will cross through the forecast area over the
course of the day. 24.12z data still shows some minor timing
differences in how quickly the front passes, but overall, a
non-diurnal temperature trend is expected with readings falling into
the 40s by afternoon for much of the area.
Showers and elevated thunderstorms riding on the nose of the 850 mb
moisture transport will likely push across the area early Thursday
morning before moving off into Ohio. That leaves the post-frontal
rain showers and possibly a thunderstorm associated with the upper
level divergence within the right entrance region of the 130 kt jet.
There is some weak MUCAPE noted, on the order of 200-400 J/kg. The
upper level trough axis is expected to pass through Thursday night,
so expecting a quick shut off of precipitation west to east during
the evening. Rainfall amounts Thursday 1/3 to 1/2 of an inch look
Strong Canadian high pressure nosing down out of the northern Plains
will bring an anomalously cold air mass to the Ohio Valley for this
time of the year on Friday. With the deep upper trough overhead,
soundings show favorable, steep low level lapse rates of 7.5 to
8.5C/km with about 150-200mb of saturation. This could be enough to
spark scattered showers across the northern/eastern areas. Thermal
profiles would support a rain/snow/sleet pellet mix at times. Plan
on highs Friday to be stuck in the low to mid 40s with partly to
mostly cloudy skies and brisk northwest winds.
850 mb temps bottom out in the -8 to -11C range Friday night, or
about 3 standard deviations below normal. Not quite record territory
but lows Saturday morning likely to be in the low/mid 20s
regionwide. Would not be surprised if the colder, sheltered locations
fall into the mid/upper teens. Although the growing season hasn`t
fully taken off, those with agricultural interests or sensitive
vegetation should take note for the prolonged hard freezing
Below normal temperatures should continue through this upcoming
weekend as 850 mb temperatures climb slowly. The upper level flow
looks to remain northwesterly or zonal at best. A model consensus of
highs in the 40s Saturday then 50s Sunday look reasonable at this
time. 24.12z guidance showing another vigorous shortwave trough
coming down late Sunday or Monday that could reinforce colder
temperatures and bring a chance of precipitation to the area.
.AVIATION (00Z TAF Update)...
Issued at 706 PM EDT Tue Mar 24 2015
Generally VFR conditions are expected through the period. A band of
precipitation continues to push into portions of southern IN and
northern KY, which will likely affect KSDF and KLEX this evening.
May briefly see vsbys drop to high-end MVFR in this activity, but
cigs should remain VFR.
Otherwise, this precipitation will lift northeast later on this
evening, but another line of showers and perhaps even a rumble of
thunder will push in overnight into the pre-dawn hours Wednesday.
The bulk of the precipitation should remain to the north of KSDF and
KLEX, but there may be just enough coverage to include VCSH wording.
High-end MVFR cigs are briefly possible Wednesday morning behind
this activity, before daytime mixing quickly raises bases to VFR.
Otherwise, after the morning precip chance at KSDF and KLEX, the
bulk of the day looks dry before more showers/storms move in