371 
FXUS63 KLMK 260446
AFDLMK
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1245 AM EDT FRI APR 26 2013

...Updated Aviation Discussion...

.Short Term (Now through Friday Night)...
Issued at 300 PM EDT Thu Apr 25 2013

Latest surface analysis shows a weak cold front stretching northeast
to southwest from Ohio, Indiana and back into west-central
Kentucky.  Some high-res guidance continues to show a shower or two
sparking along this front, but due to the lack of low-level moisture
(dewpoints in the lower 30s), think that conditions will remain dry
for the remainder of the afternoon.

Focus then shifts to frost potential tonight.  An area of surface
high pressure will shift east across the Ohio Valley overnight,
settling across southern Ohio by 12Z Friday morning.  This feature
will cause all winds to go light for most of the overnight hours.
However, as the high continues to shift east, some light return flow
may develop across western Kentucky towards dawn.  Additionally,
some upper-level clouds will be pushing in ahead of the next
approaching system, which will quickly limit radiational cooling
across the west.  For this reason, will hold off on a frost advisory
for the southwestern FA, where the combination of clouds and light
return flow should limit frost potential.  Confidence is low on the
western periphery of the advisory, as the clouds and/or winds coming
in quicker/slower than expected may affect temperatures.  However,
do have high confidence in another widespread frost event across the
Northern Bluegrass, where clouds and winds will be less of a
factor.  Lows will range from the lower 40s southwest near Bowling
Green to the low and mid 30s northeast across the Northern Bluegrass.

The next upper level system will remain south of the stronger
westerlies, meaning it will slowly meander into the Ohio Valley late
Friday into Saturday morning.  The latest trends in guidance
continue to exhibit a northern shift, which seems logical given the
system is now being sampled by the upper-air network.  Therefore,
have raised pops across the region Saturday morning.  Still think it
may take awhile for the column to saturate, but isentropic ascent
ahead of this upper-level low looks like it'll be enough to pull
moisture far enough north to warrant at least slight chances across
the whole area by 12Z Saturday.  However, the bulk of the
precipitation looks to come after 12Z, which is detailed in the long
term section below.

Given the expected mid and upper level clouds on Friday and Friday
night, look for highs in the mid 60s, with lows Friday night in the
mid to upper 40s.

.Long Term (Saturday - Thursday)...
Issued at 330 PM EDT Thu Apr 25 2013

Saturday through Monday Night...

Latest suite of forecast data continues to be in fairly good
agreement with the synoptic weather pattern in this portion of the
forecast period.  The upper low ejecting out of the southwestern US
will move slowly across the Plains over the next 24-48 hours and
then roll through the Ohio Valley just in time for the weekend.
This feature will bring plenty of clouds, periods of showers, and
much cooler than normal temperatures to the region for Saturday and
into Sunday.  At this time, the best coverage of precipitation still
looks to be mainly across Kentucky...and in particular the southern
half of the state.  The models have been fluctuating a bit with
their QPF fields but recently have come into better agreement in the
last 12 hours.  Still plan on running chance PoPs across southern
Indiana and northern Kentucky for Saturday with likely to
categorical PoPs (70-80%) across the southern half of the state.
The high PoPs will continue into Saturday night as well.  Forecast
model soundings still do not look all that impressive with respect
to instability.  Plenty of moisture and upper level support look to
produce plenty of rain showers with perhaps an embedded thunderstorm
or two possible...mainly across southern KY.  With all the
cloudiness and rainfall, have lowered temperatures a bit with this
forecast and will go with highs in the upper 50s in the north with
upper 50s to near 60 in the south.  Lows Saturday night will cool
back into the lower-middle 50s.

Plenty of cloudiness and shower activity will likely persist into
Sunday as well as the upper low and associated surface low take a
slower track through the Ohio Valley.  Thus, have increased
cloudiness and rain chances into Sunday as well.  Overall, it
appears that rainfall will probably not be as extensive on Sunday
with better chances in the east and lesser chances in the west.
Highs may be a touch warmer than Saturday with readings in the lower
to middle 60s in the north and middle to upper 60s in the south.
The upper low is forecast to pull out Sunday night, so we should see
a diminishing trend in the showers by then with lows cooling into
the lower 50s.

A rather zonal flow pattern will set up for Monday and we should see
a weak ridge of high pressure build into the region.  Looks like
we'll see a partly to mostly sunny day on Monday with temperatures
moderating into the lower 70s and lows Monday night cooling into the
lower 50s.

Tuesday through Thursday...

The long term guidance continues to suggest that the upper level
pattern will become quite amplified through much of next week.
Early in the period, a new upper trough is forecast to develop
across the western US and then push into the Plains.  High level
blocking pattern looks to set up with more of an omega type pattern
setting up over the eastern US.  The GFS ensembles show 500 hPa
positive height anomalies of over 2.5 standard deviations over
Ontario by Wednesday and much of the Ohio Valley will be centered
under the omega ridge axis.  This should result in continued dry
conditions with temperatures warming to above normal levels.  Highs
Tuesday-Wednesday look to warm into the mid-upper 70s with a shot of
80 degrees across the far south.  Overnight lows will generally be
in the mid-upper 50s.

Model spread rapidly starts to increase by Thursday as the guidance
starts to diverge a bit on how the upper air pattern will evolve.
The strong positive height anomaly over Ontario and Quebec looks to
continue and this will cause quite a bit of blocking over the
eastern third of the US. As the upper trough across the Plains
attempts to barrel eastward, the block will likely result in the
upper trough closing off and turning into a large 500 hPa gyre
across the northern Plains. It does appear that some sort of cold
front or possibly a large occluded front will push through late
Thursday and into Friday bringing a round of showers and storms to
the region.  Temperatures look to remain mild with readings in the
low 70s and overnight lows in the 50s.

Beyond the period, the general model consensus is for the upper
level blocking to continue resulting in a large cut off low
developing and dropping southward into the Midwest and possibly down
into the central Plains.  Several vorticity maxima rotating around
the cut off could bring episodic rounds of convection to the area,
but timing these disturbances is impossible at this point.  With the
region being on the east side of the gyre, temperatures look to
remain below seasonal normals through next weekend.

&&

.Aviation (06Z TAF Issuance)...
Issued at 1245 AM EDT Fri Apr 26 2013

High pressure moving from Ohio to New York today will keep winds
light and variable with well VFR flying conditions.

An upper level disturbance approaching from the west will spread mid
clouds into the region late this afternoon into tonight.  Some light
rain may begin to affect BWG by the end of the TAF period, but
should hold off until after this TAF period at SDF/LEX.  Conditions
will still be well VFR with light winds.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...FROST ADVISORY UNTIL 9 AM EDT THIS MORNING FOR INZ076>079-084-
     090>092.

KY...FROST ADVISORY UNTIL 9 AM EDT /8 AM CDT/ THIS MORNING FOR
KYZ025-
     028>043-045>049-053>057-064>067-077-078-082.

$$


Short Term.......KJD
Long Term........MJ
Aviation.........13