138 
fxus63 kpah 082322 
afdpah 


Area forecast discussion 
National Weather Service Paducah Kentucky 
522 PM CST Thursday Jan 8 2009 


Update... 
updated for 00z aviation package. 


&& 


Short term forecast discussion... /tonight through Sat night/ 
this time period will be highlighted with with rapid baroclinic 
development of low pressure and significant warm air advection 
starting late tonight and continuing with the frontal passage late 
Friday night. 


There is a measurable time lag between the much slower deterministic 
12z operational GFS and GFS ensemble precipitation...and the slightly faster 
mean sref precipitation...and more intense and rapid 4km nmm/12k NAM WRF for 
the onset of precipitation very late tomorrow into tomorrow night. 


At this time...I am more inclined to utilize the more dynamic WRF 
guidance for the onset and maintenance of precipitation late Friday 
into Friday night...with the main warm conveyor belt/mean isentropic 
lift oriented from southwest to northeast across most of the weather forecast office pah 
forecast area. The time period from 09z Saturday through 15z 
Saturday will be the most interesting...as the sharpest lift 
associated with the approaching trough axis will be pushing through 
Missouri at that time. Even at this point there are some timing 
differences...with the deterministic GFS actually accelerating a 
littler faster than the NAM. Decided to blend the NAM/GFS in 
timing and slow the eastward progression of the system...as both 
models appear to show some intensification during the 09-15z 
Saturday time frame. 


Although it may be short-lived...actually introduced a small 
chance for thunder in the 06-12z Saturday time frame ahead of the 
cold front. This thunderstorm potential is a little further north 
than suggested by the Storm Prediction Center day 2 outlook...but am inclined to 
support the potential for elevated instability...at least through 
western Kentucky/southeastern Illinois/small part of southeastern MO. 


As the system moves southeastward during the day on Saturday...there is some 
secondary lift/instability with a vorticity /noted in the NAM and GFS/ to 
support at least a small chance for light mixed /mainly rain-snow 
mix/ precipitation into the afternoon hours...mainly across western 
Kentucky. After that time...the moisture and instability rapidly move away 
from the area. 


Hourly temperatures were adjusted to keep in line with previous 
forecast shift assessment of non-diurnal temperature trends late 
tonight to account for return southerly flow...and for falling 
temperatures following the frontal passage on Saturday. 




Long term forecast discussion... /Sunday through Thursday/ 
the new extended suite delays the onset of the coldest air until 
Wednesday-Thursday time frame...so that's one adjustment we made. Basically 
after a transitionary shift in the upper flow pattern on Sunday...we 
see our 1st cool off occur on Monday. This will happen as the now 
northwesterly flow pattern helps a strong clipper/wave dive down and impact US 
with smallish probability of precipitation Monday into Monday night. The models disagree on the 
lower tropospheric thermal profile with the GFS hinting at some 
warming into semo making for a rain/snow chance while the colder air lodged 
over our northern-eastern 2/3 counties supports all snow. The end result could 
be a light dusting from a few hundredths of liquid equivalent. It is 
behind this system that we get our 1st good plunge of Arctic air on 
the order of a composite -10 to -15c at 850 mb rotating over the 
lower Ohio Valley. This is a compromise European model (ecmwf) approach as the dgex 
suggests about 10c colder while the GFS suggests 5-10c warmer. 


We think airmass modification is going to be hard to come by on 
Wednesday so we keep highs down there well below MOS...as do most our 
neighbors. Wednesday night we get the 2ndary/reinforcing blast of 
Arctic air that may take lows into the single digits Wednesday night and 
Thursday night whilst highs struggle to reach 20s both days. Any way 
you cut it/even with the warmer models we're looking at a 
significantly lower than normal thermal setup with the only real 
question of how early in the week does it start...the remainder of 
the week being very cold. As a result you'll see our forecast 
temperatures/dew points at or below almost all MOS guidance although we noted the 
mex MOS was starting to pick up on the strength of the cold air 
today at least vs yesterday. Also modified the diurnal curves 
accordingly pretty much throughout the extended. 


&& 


Aviation... 
VFR conditions expected through taf period. Light and variable 
winds will become southeast to south by 12z...then increase to 10 to 
15 kts with gusts to around 20 kts by late morning. 


&& 


Pah watches/warnings/advisories... 
Kentucky...none. 
MO...none. 
Illinois...none. 
In...none. 
&& 


$$