Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 151923
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
323 PM EDT Sat Mar 15 2014
.SHORT TERM (Now through Sunday Night)...
Issued at 320 PM EDT Sat Mar 15 2014
...Accumulating Snow Expected Sunday Afternoon and Night...
Certainly hope you had the opportunity to enjoy today`s weather,
because conditions will begin to deteriorate tonight. A low pressure
system will move from Texas eastward across the Gulf coast states
through the short-term period. Precipitation will spread
northeastward into central Kentucky late tonight and eventually the
entire forecast area on Sunday. Northeast winds will strengthen
tonight and be gusty through the day Sunday. Wind speeds of 15-20
mph will be common with gusts of 25-35 mph possible.
Temperatures still seem to be a struggle with this system, as the
latest guidance continues to vary on just how much cold air will
make it into our area and at what time, as a quasi-backdoor cold
front across Great Lakes eases in here tonight. Since we are not
starting off with a cold airmass in place, this temperature
transition is quite important and we will have to rely on a colder
airmass advecting in from the north-northeast and wetbulbing.
Temperatures Sunday morning should range from middle 30s far north
to middle 40s far south. Still looks like temperatures will hold
fairly steady through Sunday morning before beginning to cool in the
afternoon. This will coincide with the surface low shifting more to
the east and winds becoming more north-northeasterly. As mentioned
in the previous discussion, there is a warm layer aloft, though it
is not nearly as strong as we have seen in past systems. So, not
expecting complete melting in this layer, which would mean sleet
mixing with rain and snow during the transition to snow.
With the warmer temperatures across our south, precip will remain
rain through Sunday before a brief changeover to snow Sunday night.
Not expecting much accumulation here. Along the Interstate 64
corridor, rain will transition to a rain/sleet/snow mix during the
afternoon hours, then all snow early in the evening. Across our
northern most counties in Indiana, temperatures will hover around
the middle 30s for much of the morning, before cooling in the
afternoon. So, there could be a rain/sleet/snow mix in the morning
before transitioning over to all snow in the afternoon.
This system has bust potential. First, we are forecasting the
advection of colder temperatures into an area that has been well
above normal for roughly 5 out of the last 6 days (including today).
Ground and subsurface temperatures are running generally in the 40s
and 50s. This should cut into snow/sleet amounts, at least initially
unless precip rates are high. If precip rates are high, we could see
a quick accumulation then melting as the precip wanes. Second, the
system will be rather fast moving when considering when the snowfall
should be flying, diminishing the precip from northwest to southeast
starting late Sunday evening and continuing through early Monday
morning, as dry air builds in at mid levels.
As far as accumulations, have gone with a consensus, because the
placement of the deformation snowband still seems to baffle the
guidance. This consensus is more in line with the 09Z and 15Z SREF
probabilities. After coordinating this difficult forecast with
neighbors, have issued a Winter Weather Advisory for Sunday afternoon
and night. The southern portion of the Advisory may not pan out if a
northern track becomes apparent. The same is true for our northern
counties if a southern track becomes apparent. Or both our north and
south may not reach criteria if the frontogenetical forcing is
stronger, resulting in a sharp precip gradient.
So, have gone with 1-3 inches of snow and sleet accumulation
centered along a line from roughly Perry County Indiana
east-northeast to Harrison/Nicholas Counties in Kentucky, with the
higher range of accumulations being across the Bluegrass region as
the low begins to strengthen and push east. These accumulations are
more likely to occur on grassy areas and elevated surfaces. Light
accumulations on roadways with temperatures dropping into the 20s
and lower 30s by daybreak Monday possibly causing some slick
roadways might be the biggest impact.
.LONG TERM (Monday through Saturday)...
Issued at 320 PM EDT Sat Mar 15 2014
Long Term Synoptic Overview
In the long term, the hemispheric flow pattern looks to become a bit
more stable up over the pole as we slowly translate from a 4 wave
pattern to more of a 3 wave pattern. Strong negative height
anomalies look to be locked over Hudson Bay which will keep the very
cold air up across Canada perhaps spreading over into northern
Europe by late in the forecast period. The flow across the CONUS
will be progressive but with a distinct split flow pattern setting
up. The southern branch of the jet will continue to supply a
constant feed of moisture into the southern US, while the northern
branch will remain active sending numerous perturbations through
North America. Overall, this will lead to a rather active pattern
of weather across the Ohio Valley with a few cold frontal passages
bringing chances of rainfall...along with brief warmups ahead of the
fronts followed by brief cooldowns on the backside of the systems.
Model Preference & Confidence
In general, the deterministic solutions are in fair agreement
through the first half of the forecast period with an increase in
spread in the latter half. To maintain continuity, have generally
gone closer to the 00Z Euro Ensemble and 12Z GFS ensemble blend for
this forecast. Confidence in temperatures remains a little below
normal here as the models have been exhibiting a cold bias of late.
To make matters worse, the split flow pattern out west will likely
lead to lower forecast skill with waves moving through the split.
Thus, expect to see model jumpiness from run to run, but this is
mitigated to an extent by utilizing the models ensemble means.
Heavily leaned on the bias-corrected model data here which results
in going on the higher end of the guidance. With regards to
precipitation, average confidence here with two decent shots of
precipitation coming around midweek and then toward the weekend.
Sensible Weather Impacts
After a cool start to the week, we should see a milder pattern
re-emerge by Tuesday as return flow becomes more established across
the Ohio Valley. Generally dry and cool conditions will be seen
Monday with highs warming into the low-mid 40s in the north and
upper 40s across the south. After morning lows in the lower 30s,
Tuesday afternoon should see temperatures warming back into the
upper 50s to around 60. Temperatures here could still be a bit
underdone depending on how fast the return flow becomes
established. So we could end up adjusting upward in future
forecasts. Mild weather looks to continue into Wednesday before the
first frontal passage occurs. Both the GFS and Euro swing a front
through here on Wednesday. This feature should bring a band of
showers and possibly some thunderstorms into the region. This
feature will quickly move out due to the progressive flow aloft.
Highs Wednesday should still be in the upper 50s to the lower 60s,
but look to cool back into the upper 50s for Thursday. High
pressure will then build back in and we should see yet another
warmup commence for the end of the week with highs on Friday warming
back into the lower-middle 60s, with lows in the upper 30s to the
lower 40s. As this occurs, next frontal boundary looks to swing in
Friday night into Saturday bringing yet another round of showers and
possible thunderstorms into the region. Highs Saturday look to be a
little cooler than Friday with readings generally in the middle to
.AVIATION (18Z TAF Update)...
Updated at 130 PM EDT Sat Mar 15 2014
VFR conditions are expected through tonight. Conditions will
deteriorate from southwest to northeast starting around daybreak
Sunday as rain overspreads the area. Visibility and ceilings will be
reduced coincident with the steady rain through Sunday morning. For
SDF, rain is expected to transition to rain/sleet/snow mix in the
mid-to-late afternoon hours before changing over to all snow. Will
not include this mixture in the TAF at this time. Winds will become
northeasterly tonight and strengthen through the day Sunday. Steady
northeast winds of 12-20 knots and gusts of 20-27 knots are expected.
KY...WINTER WEATHER ADVISORY from 2 PM EDT /1 PM CDT/ Sunday to 8 AM
EDT /7 AM CDT/ Monday FOR KYZ023>043-045>049-053>057-
IN...WINTER WEATHER ADVISORY from 2 PM EDT /1 PM CDT/ Sunday to 8 AM
EDT /7 AM CDT/ Monday FOR INZ076>079-083-084-089>092.