071 
fxus63 klmk 091054 
afdlmk 


Area forecast discussion 
National Weather Service Louisville Kentucky 
554 am EST Friday Jan 9 2009 


Short term (today and tonight)... 


Surface high pressure and low amplitude upper level ridging will 
keep US dry today. Other than some middle level clouds moving in we 
should receive a good amount of sunshine. After a cold 
start...southerly flow and sunshine should allow temperatures to 
rebound into the 40s central and north...and to around 50 in the 
south. 


Tonight low pressure will move from the Red River valley to western 
Kentucky with a warm front extending to the east along the length of 
the commonwealth. Overrunning precipitation will break out along and 
north of the warm front...and then become more widespread as the low 
to our west draws closer. The heaviest rainfall amounts will be 
along and north of Interstate 64 where some spots could see as much 
as half an inch. An isolated rumble of thunder will be possible in 
our west and northwest late tonight near a low level jet coincident 
with some weak elevated instability...but chances are low enough to 
leave out of the grids for now. 


Low temperatures will likely occur late this evening around 03z or 
so and then become steady or rise overnight. 


Thanks to jkl for coordinate. 


Long term (saturday through thursday)... 


Saturday through Tuesday night... 


Very active weather pattern shaping up this weekend and into the 
first half of next week as active northwest flow sends a series of 
clippers through the region. 


First clipper system will be arriving Saturday into the region. 
Overall trend in the models in the last few days has been for a 
slightly more southerly path. That southern trend seems to have 
leveled out in the last two sets of model runs. Current model 
consensus suggests the clipper will swing just along the Ohio River 
during the day on Saturday. With the clipper going across our 
northern forecast area...decent southwesterly flow on the southeast side of the 
system will help warm temperatures up rather quickly Saturday...but as the 
system passes off to the east...brisk northwest flow will help swing temperatures 
back down toward freezing by Sat evening. Therefore...this forecast 
suggests another round of rain changing to snow for Saturday. Quantitative precipitation forecast 
amounts from the model have been on the uptick and with confidence 
increasing on the track...have increased probability of precipitation substantially with 
this forecast package. Will be running Cat probability of precipitation for Sat with rain in 
the morning and then a transition over to rain/snow during the 
afternoon with all snow Sat night. Highs for Sat will be tough to 
nail down. Have generally stayed close the bias corrected raw GFS 
numbers here which suggest upper 40s to near 50 across the south 
with upper 30s to the lower 40s across the north. Highs will likely 
be around middle-morning with a decent fall in temperatures during the 
afternoon. By Sat night...strong northwest flow aloft along with 
steepening lapse rates will promote scattered snow showers. Areal 
concentration is still tough to nail down...but best moisture will 
be generally east of I-65. Some very minor accumulations are 
possible. 


For sun through Tuesday night...we'll be in a lull between systems sun 
through early Monday as large 500 mb baggy trough will remain over the 
region keeping temperatures below normal across the region. Lows look to 
be in the lower to middle 20s with highs in the middle to upper 30s for 
Sunday...with highs on Monday in the upper 30s to around 40. 
Approaching clipper system may bring a few rain showers to the 
region as early as Monday afternoon...but feel that the bulk of 
precipitation will hold off until Monday night and Tuesday. 00z GFS 
and Euro solutions are in good agreement here with just some minor 
differences in timing. Again...based on the current forecast track of 
this clipper...this will be another rain to snow event...but the 
models are keeping a bit more moisture on the backside of this one 
for Monday night and Tuesday...which may bring another chance for some 
minor accumulations. Lows Monday night will be in the lower 20s 
with highs on Tuesday only warming into the lower to middle 20s. Much 
colder air will pour in for Tuesday night with lows dropping into 
the middle to upper teens. Another clipper will make a run for the 
region late Wednesday night night and will likely bring another chance for 
some light snow. 


Wednesday/Thursday and beyond... 


Did not make any real changes to this portion of the forecast period other 
than keeping a bit of light snow in for early Wednesday. Latest trends in 
the extended models including the ensembles are showing a slightly 
slower timing of the arrival of bitterly cold air late next week. 
Seems like the models are about 12-24 hours slower at pushing the 
colder air into the region. Comparing the 00z Euro and GFS...they 
are in pretty good agreement late in the period but the GFS is showing 
its cold bias in the east as it develops a much larger/baggier 
trough. The Euro is a bit sharper with its trough...but looks to 
expand/deepen it very late next week. Super large 1050+ high 
pressure cell looks to drop south across the plains and then settle 
somewhere in the southeast. Therefore...very cold and dry weather 
looks to be in store Wednesday/Thursday into next weekend. Temperatures will likely 
be some 25 to 30 degree below normal...which would equate to the 
coldest temperatures of the season thus far...and the coldest temperatures within 
the last 10+ years. Brrrr! 


Aviation (12z tafs) 


All three sites fell to 6sm visibility this morning but any drops below 
that into high end MVFR category will be very brief so will keep 
tafs VFR today into this evening. Middle level clouds will stream 
overhead today and this evening with southerly breezes of 8 to 10 
knots courtesy of weak upper ridging and a departing surface high. 


By midnight tonight low pressure will have advanced from the Red 
River valley into the middle Mississippi Valley...then to west 
central Kentucky by 12z...and northeast Kentucky by 18z Saturday. 
Showers will dramatically increase in coverage starting around 
midnight and will continue into Saturday morning. These showers will 
bring MVFR ceilings and probably at least occasional MVFR visibilities. As 
the low passes through...a cold frontal passage is expected at sdf around 15z 
Saturday. 


Even though it is still early...went ahead and included low level wind shear for 
several hours tonight as a low level jet Cranks up. 


&& 


Lmk watches/warnings/advisories... 
Kentucky...none. 
In...none. 
&& 


$$ 


Weather.Gov/Louisville 


Short term...13 
long term....mj 
aviation.....13 

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