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Area forecast discussion 
National Weather Service New Orleans la 
1011 am CDT sun Mar 21 2010 


Update... 
updated grids to account for current conditions. Any precipitation 
should be limited to extreme northern portions of the County Warning Area. Current 
brief breaks in cloud cover over areas south of Lake Pontchartrain 
should end as any sunshine on the cold unstable airmass will 
produce cloud cover. Those areas may see 8-10 degrees of 
temperature recovery today...but most areas will see less than 
that. Products already updated. On the marine side...small craft 
advisories after the Gale Warning have been extended through the 
daytime hours on Monday. 
&& 


Previous discussion... /issued 341 am CDT sun Mar 21 2010/ 


Short term... 


A cold currently passing through the forecast area will sweep 
toward the Florida Panhandle by daybreak. A band of moderate to 
locally heavy rainfall will continue to affect the Mississippi 
coast through 7 am before pulling into lower Alabama. In the wake 
of the front...a sharp drop in temperatures and strong northwest 
winds of 20 to 25 miles per hour with higher gusts has been noted through the 
night. Strong cold air advection and gusty winds are expected to 
continue to through the day. At this time...it appears that winds 
should remain below Wind Advisory criteria today. Cloud cover 
should also linger through the day and into the overnight 
hours...as an upper level low currently in northern Texas transits 
through the lower Mississippi Valley. Fortunately...there is very 
little moisture available to the system...as drier air off the 
Mexican plateau has wrapped into the system. With this in 
mind...have lowered probability of precipitation to slight chance for far northern 
zones and kept the remainder of the forecast area dry. If there 
are any showers that develop in southwest Mississippi...they will 
be very light. 


Skies should begin to slowly clear late tonight...as the upper 
level vortex pulls into the southeast. Subsident flow on the 
backside of the vortex...along with decent dry air advection 
tomorrow morning will allow for more rapid clearing through the 
day on Monday. However...strong cold air advection will remain in 
play...with 1000-500mb thicknesses still in the lower 540dm range. 
Temperatures are expected to be a good 20 degrees below normal 
once again on Monday. 


Upper level and surface based ridging will take hold by 
Tuesday...allowing for a quick moderation in the air mass to take 
place. 1000-500 mb thicknesses will rise a good 1500 
meters...allowing for a return to more normal daytime temperatures 
for Tuesday and Wednesday. However...with very dry air in 
place...there will be a larger than normal diurnal curve. As a 
result...overnight lows will remain around 10 degrees below 
normal. 


Long term... 


The ridge axis will shift toward the southeastern states Wednesday 
night...as a fast moving...but fairly shallow short wave trough 
slides into the plains. This system will be much weaker than the 
system currently affecting the southern portion of the 
country. A lack of strong forcing aloft across the region will 
limit the precipitation coverage...and have went with chance probability of precipitation for 
Thursday. Some isolated convection may develop as the weak cold 
front slips through on Thursday...but any convective activity 
should remain weak. This is due to a combination of weak dynamic 
forcing and shear in the area...weak lapse rates...and little in 
the way of thermodynamic support. The front will quickly slide 
through on Thursday...with rapidly improving conditions expected 
Thursday night. With a Pacific air mass following the 
front...little in the way of cold air advection is expected. 
However...some weak upper level riding will build in for Friday 
and Saturday. This will bring clear skies...dry conditions...and 
near normal temperatures to the area. A secondary front is 
expected to slide through on Sunday...as another fast moving short 
wave trough slides through the Midwest. However...this system will 
have little moisture to work with. 


Aviation...biggest issue will be ceilings today and some MVFR visbies 
this morning. At 8z cold front was already past new and asd and 
should move through gpt by 10z and pql around 11z. Winds will 
quickly veer around to the west-northwest (280/290) and initially pick up to 
sustained 20kts W/gusts. Winds will then ease somewhat until shortly 
after sunrise. Mixing will start to increase as strong cold air advection moves in 
and all terminals could see gusts to around 30 kts around late 
morning through early afternoon. As for ceilings once the front moves through 
ceilings will drop to IFR heights around 600-700 feet. Ceilings will begin to 
improve to MVFR after sunrise to around 1500-2k feet by 15z over much 
of the area. The exceptions will be the MS coast which may wait 
until closer to midday. Ceilings will remain in MVFR range through the 
rest of the day mainly around 2k-2500 feet this afternoon visbies over much 
of the area should be back in MVFR status shortly after frontal 
passage as winds increases. Of course the lone exception will be mcb 
which should be back to p6sm around 13/14z. /Cab/ 


Marine...winds have been up through the night over the coastal 
waters and once the cold front pushed through the tidal lakes winds 
quickly ramped up to near gale force. A Gale Warning will go in 
effect at 9z for all of the coastal waters until 0z Monday. The tidal 
lakes currently will go under a scy but if wind gusts remain right 
around 34 kts a tad longer I will need to upgrade them to a Gale 
Warning as well. Nothing has changed in the thinking. Strong 
cold air advection...dry descending air...and decent surface pressure rises will create 
a windy day across the entire area. West winds of 20-30kt with 
frequent gusts of 40-45kts will be possible throughout the day. Seas 
will also build to around 5-7 possibly 8 feet in the inner waters and 
could be as high as 12 feet in the outer waters. Winds should begin to 
relax this evn but a scy will remain in effect through Sun night for 
the entire area including the lakes. This may even need to be 
extended into the day Monday for eastern portions of the coastal waters. 
High pressure should finally build in enough late Monday to allow US to 
drop any flags for the coastal waters. It looks like the next front 
will push through late Thursday now. There is still some model issues so it 
is too early to tell if we will need any headlines for the end of 
the work week. /Cab/ 


&& 


Preliminary point temps/pops... 
mcb 43 33 54 38 / 20 20 10 0 
btr 48 35 58 40 / 10 10 0 0 
asd 51 35 58 38 / 10 10 0 0 
msy 52 39 57 45 / 10 10 0 0 
gpt 52 37 58 40 / 10 10 10 0 
pql 53 37 58 38 / 10 10 10 0 


&& 


Lix watches/warnings/advisories... 
la...none. 
GM...Gale Warning until 6 PM Sunday for the following zones: coastal 
waters from Pascagoula MS to the Southwest Pass of the 
Mississippi River from 20 to 60 nm...coastal waters from 
Pascagoula MS to the Southwest Pass of the Mississippi 
River out 20 nm...coastal waters from the Southwest Pass of 
the Mississippi River to lower Atchafalaya River la from 20 
to 60 nm...and coastal waters from the Southwest Pass of 
the Mississippi River to lower Atchafalaya River la out 20 
nm. 


Small Craft Advisory from 6 PM Sunday to 7 PM Monday for the 
following zones: coastal waters from Pascagoula MS to the 
Southwest Pass of the Mississippi River from 20 to 60 
nm...coastal waters from Pascagoula MS to the Southwest 
Pass of the Mississippi River out 20 nm...coastal waters 
from the Southwest Pass of the Mississippi River to lower 
Atchafalaya River la from 20 to 60 nm...and coastal waters 
from the Southwest Pass of the Mississippi River to lower 
Atchafalaya River la out 20 nm. 


Small Craft Advisory until 7 PM Monday for the following zones: 
and Lake Pontchartrain and Lake Maurepas. 


MS...none. 
GM...Small Craft Advisory until 7 PM Monday for the following zones: 
and Lake Pontchartrain and Lake Maurepas. 


Gale Warning until 6 PM Sunday for the following zones: coastal 
waters from Pascagoula MS to the Southwest Pass of the 
Mississippi River from 20 to 60 nm...coastal waters from 
Pascagoula MS to the Southwest Pass of the Mississippi 
River out 20 nm...coastal waters from the Southwest Pass of 
the Mississippi River to lower Atchafalaya River la from 20 
to 60 nm...and coastal waters from the Southwest Pass of 
the Mississippi River to lower Atchafalaya River la out 20 
nm. 


Small Craft Advisory from 6 PM Sunday to 7 PM Monday for the 
following zones: coastal waters from Pascagoula MS to the 
Southwest Pass of the Mississippi River from 20 to 60 
nm...coastal waters from Pascagoula MS to the Southwest 
Pass of the Mississippi River out 20 nm...coastal waters 
from the Southwest Pass of the Mississippi River to lower 
Atchafalaya River la from 20 to 60 nm...and coastal waters 
from the Southwest Pass of the Mississippi River to lower 
Atchafalaya River la out 20 nm. 


&& 


$$ 

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