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Area forecast discussion 
National Weather Service New Orleans la 
755 PM CDT Sat Mar 20 2010 


Update... 
sounding discussion... 


The atmosphere has become very moist with a few areas of dry air 
in the middle levels...precipitable water value is 1.19 inches. 
Scattered showers began to develop during the afternoon producing 
rainfall amounts near one tenth of an inch...currently a large 
area of showers continue to move inland from the Gulf producing 
rainfall rates near one inch per hour. Directional shear is 
present with winds out of the east at the surface shifting 
southward then southwest aloft...helicity value is 228. 


&& 


Previous discussion... /issued 305 PM CDT Sat Mar 20 2010/ 


Synopsis... 
low pressure over southwest Arkansas with cold front southward to 
just west of Lake Charles. Second low over the western Gulf of 
Mexico southeast of Brownsville. Isolated marine layer showers 
have developed over southeast Louisiana. Solid band of showers 
along cold front...but lightning is almost non-existent at this 
time. Temperatures have generally plateaued around 70 degrees. 


Short term... 
stability parameters lacking for any significant threat of severe 
weather tonight. May be able to mix down some wind gusts around 40 
miles per hour..but that appears to be about it. 


Frontal system will move through most or all of the County Warning Area overnight. 
Temperatures fall rapidly behind the front...and should be near 40 
degrees within 3 hours of frontal passage. Once the main rain 
area moves out...could be some patches of light rain or 
drizzle...mainly north of Interstate 12...so will be carrying 
small probability of precipitation for Sunday and Sunday night. GFS model soundings show 
at least some potential for a few snow flurries in the McComb area 
Sunday evening...but threat of measurable snow is small enough to 
not carry in zones this run. 


Winds will be an issue...but mainly in the marine area as noted in 
the marine portion of the discussion. Do not feel that winds of 
advisory criteria...sustained 25 miles per hour or gusts of 40 miles per hour...will be 
achieved for any significant period of time over land and will not 
issue Wind Advisory for Sunday at this time. 


Have used met 3 hourly temperatures for trends tonight and Sunday. 
In areas where cloud cover is thickest...there will be little 
temperature recovery during the daytime hours on Sunday. High 
temperatures Sunday to be 15 degrees or more cooler than today. 
Temperatures Sunday night may approach freezing over southwest 
Mississippi...but we have not had a prolonged period of warm 
weather...so frost or freeze products not likely to be issued. 
Moderating trend begins Monday...with temperatures Tuesday 
returning to levels similar to today. 35 


Long term... 
next southern stream system rolls through the area about Thursday 
night or Friday morning...depending on your model of choice. The 
system will be weaker than the one moving through tonight...and 
there will be no cold air behind the system. Due to timing 
issues...will just carry chance probability of precipitation for now centered around 
Thursday afternoon and evening. Temperatures for the extended 
period will be near to slightly above normal. 


Aviation... 
some shallow marine layer showers developing this afternoon for 
minimal impacts on visibility...tempo 5sm -shra...at times through frontal passage 
at kbtr and kmsy. Organized convection along the front on the wane 
with moisture inflow becoming impeded by marine layer stability 
downstream. A period of showers with MVFR ceilings and visibility within 
1-2 hours of frontal passage. Still looking at frontal passage kbtr/kmcb 
00z...kmsy 02z...kgpt 04z. Frontal passage should impact 
configuration patterns with prevailing south shifting hard west to 
northwest. Low level wind shear Post-frontal until airmass deepens 
after 06z. Due to strong cold air advection next 18-24 hours...MVFR 
ceilings likely to prevail through 22/00z or longer...occasioned 
with -dz. Model soundings also supportive of light snow flurries at 
kmcb after 22/00z Sunday night but appears a low confidence outcome 
for mention in tafs at this time. 24/rr 


Marine... 
cold front off the Texas coast and moving southeast 29kt to move east of 
atchfalaya basin around 00z and east of MS coastal waters shortly 
after 06z...given current forward progress. Thunderstorms are 
confined to immediate wind shift over the coastal waters with dry 
air aloft to facilitate gale force gusts with frontal passage in downdrafts. 
As cold air advection and dry air mixes Post-frontal overnight and 
Sunday...frequent gale force gusts seem likely. Will maintain gale 
watch at this time and hold off upgrade to warning until late 
evening update package...which should still precede onset by a few 
hours. Conditions to improve steadily Sunday night into Monday as 
high pressure ridge settles over north Gulf. Next cold frontal 
passage appears to be struggling in model run consistency...now 
delayed until next Friday as pattern changes and flow across the 
lower latitudes becomes detached from upper latitude flow. Timing of 
future frontal passages may be suspect later in the week...but 
steady modification of air mass and perhaps a stretch of sea fog 
nights are possible from Wednesday Onward as a warming trend sets 
in. 24/rr 


&& 


Preliminary point temps/pops... 
mcb 39 50 34 55 / 80 30 30 10 
btr 40 54 36 58 / 80 20 20 0 
asd 43 54 36 58 / 80 20 20 0 
msy 42 54 39 57 / 80 10 10 0 
gpt 46 56 38 57 / 80 20 20 10 
pql 52 56 36 57 / 80 20 20 10 


&& 


Lix watches/warnings/advisories... 
la...none. 
GM...Small Craft Advisory until 4 am CDT Sunday for gmz530-550-555- 
570-575. 


Small Craft Advisory from 4 PM Sunday to 7 am CDT Monday for 
gmz530-550-555-570-575. 


Gale watch from 4 am CDT Sunday through Sunday afternoon for 
gmz550-555-570-575. 


MS...none. 
GM...Small Craft Advisory until 4 am CDT Sunday for gmz550-555-570- 
575. 


Small Craft Advisory from 4 PM Sunday to 7 am CDT Monday for 
gmz550-555-570-575. 


Gale watch from 4 am CDT Sunday through Sunday afternoon for 
gmz555-570-575. 


&& 


$$