020 
fxus64 kjan 201000 
afdjan 


Area forecast discussion 
National Weather Service Jackson MS 
459 am CDT Sat Mar 20 2010 


Short term...rain chances will be on the increase today as a deep 
upper low system approaches from the west. We will see increasing 
cloud cover as wind flow backs and increases across the area and the 
cold front moves near the MS river around sunset. Vertical totals 
and k index values suggest enough instability for scattered 
thunderstorms this afternoon and tonight but air mass 
destabilization will be limited and severe convection is not 
expected. An 850 mb thetae minimum is indicated over the region 
through the early afternoon and most model precipitation occurs along and 
behind the cold front with weak 850 mb thetae ridging coincident 
with the frontal zone. Higher cape values only around 200 are 
forecast over southwestern zones this afternoon and evening and these 
decrease tonight. The cold front will pivot across the region 
tonight as the surface low hangs over Arkansas... resulting in 
widespread rainfall across the area. 


Significant cold air advection will develop in the wake of the cold 
front. The cold core associated with the low will rapidly cool the 
air column from the west toward morning late tonight. All but a 
shallow layer near the ground cools below freezing across most of 
the forecast area by noon Sunday. Have added a rain/snow mix to northwestern 
zones just before sunrise tonight and maintained the possibility of 
a light rain/snow mix for most areas north of I-20 for Sunday and Sunday 
night. Snow will be hard pressed to stick anywhere considering warm 
soil conditions currently in place and the fact that surface 
temperatures will remain above freezing through the short term. 
Developing isentropic lift in the wake of the cold core late Sunday 
into Sunday night will maintain good precipitation chances through the rest 
of the weekend. A lake Wind Advisory may be needed for late tonight 
into Sunday. Have refrained from issuing one this morning because of 
model discrepancies concerning the strength of the surface pressure 
gradient that will develop in the wake of the front. 


Did not stray from MOS guidance in the short term...the exception 
being today where the warmer NAM numbers were favored over the GFS. 


Long term...Monday through Friday...the sref...Canadian...UKMET and 
the GFS as well as GFS ensembles were in general agreement with the 
upper pattern. The upper low will lift out on Monday. This will 
move the light showers out of the region by Monday evening. Flat 
upper ridging develops by Monday night with surface ridging into the 
area by early Tuesday. Forecast soundings show clouds clearing 
with the possibility of some fog developing Monday night. Weak 
high pressure continues Tuesday before shifting east of the area 
Tuesday evening. The first in a series of shortwaves lifts into the 
area in developing SW flow Wednesday. Moisture will be on the 
increase by Wednesday night in developing southerly flow ahead of a cold 
front that will shift into the arklamiss Thursday night. There will 
be enough instability lapse rate wise and some cape for showers and 
mostly elevated thunderstorms with this system for Wednesday night 
through Friday. The rains will end from the west by Friday night. 


Went close to mostly ensemble guidance as the upper flow flattens out 
for the period until the system for Thursday into Friday. Made some 
downward adjustments to MOS probability of precipitation for Wednesday as the system 
approaches. Also increased probability of precipitation slightly for Thursday night into 
Friday. Otherwise went close to mex probability of precipitation for the rest of the period. 
/17/ 


&& 


Aviation...VFR conditions will persist across the area today. 
Clouds will be on the increase with broken-overcast ceilings from 4-6k feet this 
afternoon. Patchy fog can be expected this morning...especially over 
southeastern zones...occasionally lowering visibilities below 1/2 mile. Fog will 
clear shortly after sunrise. Southerly winds will increase between 
10-15 knots...with a few higher gusts around 20 knots possible 
across the Delta Region...kglh and kgwo. By late Saturday 
afternoon...as a storm system approaches from the west...some 
scattered showers...and perhaps an isolated thunderstorms...could 
affect far western portions of the area...kglh and kgwo. However... 
any prevailing shower activity that's expected will affect taf sites 
late Saturday night into early Sunday morning. /Ac/03 


&& 


Preliminary point temps/pops... 
Jackson 72 39 46 34 / 23 100 71 43 
Meridian 73 47 49 34 / 18 100 69 42 
Vicksburg 71 38 44 38 / 32 100 70 40 
Hattiesburg 73 48 51 38 / 15 100 41 29 
Natchez 71 36 45 35 / 31 100 59 31 
Greenville 71 40 45 39 / 38 100 77 48 
Greenwood 71 42 46 36 / 26 100 80 48 


&& 


Jan watches/warnings/advisories... 
MS...none. 
La...none. 
Arkansas...none. 
&& 


$$ 
03/17/ac