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Area forecast discussion 
National Weather Service New Orleans la 
738 am CST Sat Nov 21 2009 


Update... 
sounding discussion... 


The atmosphere has become saturated from 815 mb to 580 
mb...precipitable water value is 1.10 inches. An altostratus cloud 
deck was present at launch due to quite a bit of stratiform rain 
moving across the County Warning Area along with a few embedded stronger showers. 
An inversion is present near 3000 feet while winds are out of the 
east at the surface shifting south then southwest aloft. 


&& 


Previous discussion... /issued 403 am CST Sat Nov 21 2009/ 


Short term...at 9z the surface low was located just east of the central 
Texas coast and still well SW of the County Warning Area while a large area of rain showers are 
making there way north onto the southwestern and S-central la coast. There is 
an area of thunderstorms and rain well south of the la coast and these were mainly 
moving to the east. The middle level disturbance was still back in eastern Texas 


Today through tomorrow...there is still some uncertainty in the 
track of the surface low. Even though the models are within the noise level 
the GFS is further south than the European model (ecmwf). Looking at current 
features...msas shows best pressure falls east to east-northeast of the surface low 
along with the strongest convection well into the Gulf moving east. 
The gfes and sref are also more in line with the GFS and with all of 
this I prefer the further south track. That said any deviation 
north(even by just 50 miles) would make a pretty good difference in 
strong/severe risk. With the track expected to remain just off the 
coast today the warm sector will remain over the coastal waters 
making today more of a isentropic lift/warm air advection rain showers day and this shows 
up really well in the 290-300k levels for our County Warning Area. With that said I 
expect the large area of rain showers just off to our SW to continue to 
expand to the NE through the morning. There will even be isolated to 
scattered thunderstorms and rain embedded in this activity thanks to elevated 
instability(showalters near -3). As for rainfall totals area wide 
1-2" look pretty good. The heaviest rain will likely occur along and 
south of a line from Morgan City to Lucedale this afternoon. This 
location lines up well with the lfq of a 100kt subtropical jet. 


Our surface low will continue to push east-northeast along/just off the coast and 
should be along the Alabama coast by tomorrow morning. This will lead to 
very good rain chances across the eastern half of the County Warning Area this evn with 
rain tapering off after midnight. Across the SW we could see rain 
come to an end quickly late this afternoon/early evn as we should begin 
to get dry slotted. By tomorrow morning light rain may still be 
occurring across the extreme eastern portions of the County Warning Area and possibly in 
our outer waters. Sunday may be a dreary cool day as the middle level 
disturbance will be slow to lift out of the lower MS valley twrds the 
Tennessee Valley. The ll thermal trough will remain west of the area so we 
expect to see a good bit of low clouds still. /Cab/ 


Long term...no big changes in the extended. There area some timing 
differences b/T the medium range models with the passage of the next 
frontal system but overall the models are in general agreement and 
your Turkey day is looking rather quiet along the north-central Gulf Coast 
and lower MS valley. With mainly just timing issues I will use a 
blend of the GFS and European model (ecmwf) with our next system 


The first half of the work week will begin quiet but another mostly 
dry frontal system will approach the area sometime Tuesday/Tuesday night/Wednesday 
morning. By Monday night another disturbance will drop into the 
central/northern plains with a surface low deepening just west of the middle/upper 
MS valley. As this system pushes east Tuesday it will drag a cold front 
across the Southern Plains and into the lower MS valley. Again there is 
some uncertainty as to when the front will push into the County Warning Area but it 
is looking like sometime Tuesday aftn/evn. Now moisture return will be 
quite limited ahead of this system thanks to strong high pressure 
remaining over the southeastern Continental U.S./Northern Gulf. With that look for isolated to 
widely scattered rain showers mainly with the front. As for temperatures look for 
seasonably cool temperatures with lows in the mainly in the 40s to middle 
50s(north to south) and highs in the upper 60s/lower 70s Monday and Tuesday 
and then in the 60s behind our front on Wednesday. 


Your Turkey day looks rather quiet and cool. Another surge of cooler 
Continental air will be on the way down and this should help keep 
highs in the upper 50s to lower 60s. Lows Thanksgiving morning will 
mainly be in the 40s thanks to the surface high still off to our northwest but 
if the cooler air is a little faster then upper 30s will be likely 
across the northern 3rd. 


High pressure will dominate at the surface into the weekend and this 
should help keep things on the cool and dry side. /Cab/ 


Aviation... 
current VFR conditions likely to persist through through at least 
sunrise...before lower conditions move in. Current forecast may 
actually be a tad on the pessimistic side as lower ceilings have not 
moved on shore yet. Once conditions do fall below VFR...they will 
remain at MVFR to IFR conditions through the remainder of the 
forecast period. Convection should be limited...and do not plan on 
mentioning thunder in terminals at this time. 35 


Marine... 
main concern remains winds today. Most of the coastal waters remain 
in exercise caution wind ranges...but a few sites over the western 
waters have reached 20 knots sustained. Spll1...south of Terrebonne 
Parish reporting 30 knots...but this is somewhat elevated with 
anemometer at 40 meters...so surface winds likely somewhat less. 
Gradient forecast to relax during the day...and this is shown in 
model wind grids. Will carry Small Craft Advisory in western waters...and exercise 
caution east and tidal lakes through 00z. Expect that the exercise 
caution will need to be carried into the overnight hours by the day 
shift. 


Once conditions relax overnight tonight...no flags expected before 
midweek. At that point...cold air advection behind next frontal 
system Tuesday night or Wednesday will likely allow exercise caution 
conditions to return. 35 


&& 


Preliminary point temps/pops... 
mcb 58 49 60 48 / 100 80 10 10 
btr 58 50 62 48 / 100 40 10 10 
msy 63 53 64 51 / 100 50 10 10 
gpt 60 52 65 51 / 100 90 10 10 


&& 


Lix watches/warnings/advisories... 
la...none. 
GM...Small Craft Advisory until 6 PM CST this evening for gmz550-570. 


MS...none. 
GM...Small Craft Advisory until 6 PM CST this evening for gmz570. 


&& 


$$ 

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