Flash Flood Warning 98, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
402 AM CST Mon Dec 9 2024

...New SHORT TERM, LONG TERM, AVIATION, MARINE...

.SHORT TERM...
(Today through Tuesday)
Issued at 358 AM CST Mon Dec 9 2024

The next disturbance, which looks to be the strongest, comes in
today just before noon. Again, there will be light to moderate
showers around before and after this moves through, but the heaviest
rain with this one should start just before noon today through about
7p. This is all due to the lift from the H3 jet moving over gently
rising isentropic surfaces(warm front). But just how gentle is this
slope? If we look at one sfc, we will use the 308PT sfc from the
NAM12 but any other model will show relatively similar solutions.
Draw a cross section through the warm front which means we draw a
line perpendicular to the warm frontal interface. We then see the
308PT sfc move from 168 miles south of Galliano to 57 miles south of
Galliano and rise from 775mb to 725mb. We need to look at a rise/run
for a solution that shows how "steep" this rise is, so our rise is
775-725=50mb and our run is 168-57=111miles which gives
50mb/111mile=.45mb/mile. Now we can compare this to other frontal
surfaces to see just how gentle or strong the slope is. If you were
walking along a sidewalk with this slope, you would probably not be
able to tell there was a slope. But if we give you a strong cold
frontal slope that could be roughly as high as a 1 to 0, you would
definitely know there was a slope which in this case is like
climbing a wall. This is set into perspective when we are trying to
see how much a sfc can lift and how rapidly it would do so. The
308PT sfc actually levels out at the warm front and the surfaces go
parallel to each other. This occurs around Franklinton LA as of this
morning around 230a. And this is also where we see the moderate to
heavy rain being produced from this location northward. This means
that the H3 jet is lifting the moisture up along these surfaces and
since they rise gently, the ascent is gentle and gradual. But it
brings a strong vertical depth of moisture up into these areas from
the gulf, so there is plenty of moisture to drop out as rain. The
warm front is along the line of moderate to heavy rainfall which
starts for our area in Pearl River Co. over to Franklinton to just
north of BTR then down to Lafayette and into eastern Cameron Parish
before diving into the gulf. This boundary will stay in this
position this morning, but models want to shove this back south late
today toward the coast, or at least flatten out the isentopic
surfaces down to the coast. This would simply help produce more rain
closer to the coast like what is occurring north of the coast now.
But with the help of the next distubance moving through the jet, it
will help enhance the rainfall over these surfaces, which brings
about the reason for the flood watch and higher amounts of rain
totals later today.

The warm front moves or expands northward again ahead of the next
cold front for Tue as another disturbance in the H3 jet flow moves
over and moderate to heavy rainfall begins again over areas that are
already saturated. This looks to start around 3a Tue through about
noon. It is about this time frame, noon, that we should see the cold
front start to enter the NW corner of the area and move all this
mess to the east exiting the area around or shortly after midnight
Tue night/Wed morning. And this cold front can do that because of
its strongly sloped surfaces(almost straight up vertically before
leveling out horizontally) acting like a bulldozer. But this can
also cause an abrupt lift to this moisture and it does which will be
able to add to the rainfall as well.

All of that to say we will be keeping headlines as they are with an
additional 1 to 3 inches possible today through Tue.

&&

.LONG TERM...
(Tuesday night through Sunday)
Issued at 358 AM CST Mon Dec 9 2024

Once the front moves through, strong winds will carry CAA and DAA
along with it bringing highs back into the 50s by Wed and Thu and
freezing temps for northshore areas for Wed night. A warming trend
should take hold by the end of this week as winds shift from
northerly to easterly then southerly as the sfc high moves in Thu
and exits east Fri. The next front looks to approach by the weekend.
We will need to get a bit closer to this timing to figure what this
one will do.

&&

.AVIATION...
(12Z TAFS)
Issued at 358 AM CST Mon Dec 9 2024

The mix of IFR, MVFR and even VFR cigs will remain through this taf
pack. Each site will be in and out of every level through the day
with the addition of -SHRA for the most part but there could be a
few vis restrictions to IFR with +SHRA. Can't rule out TSRA but
should not be as common.

&&

.MARINE...
Issued at 358 AM CST Mon Dec 9 2024

Headlines will be lowered this morning as southerly winds begin to
ease. A cold front will move through Tue evening bringing strong NW
winds around 25-35kt. There is the potential that a gale watch could
be issued for these winds late Tue night or early Wed. Winds will
ease through the day Wed and should be low enough Thu morning to
drop all headlines.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  71  61  69  37 /  80  80  70  30
BTR  76  65  74  41 /  80  70  70  30
ASD  72  62  74  41 / 100  70  80  50
MSY  73  64  74  45 /  90  70  90  50
GPT  69  61  72  41 /  90  80  90  70
PQL  74  62  76  42 /  90  80  90  80

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...Flood Watch through Tuesday morning for LAZ039-056>060-064>071-
     076>090.

GM...Small Craft Advisory until 9 AM CST this morning for GMZ570-572.

MS...Flood Watch through Tuesday morning for MSZ077-083>088.

GM...Small Craft Advisory until 9 AM CST this morning for GMZ572.

&&

SHORT TERM...TE
LONG TERM....TE
AVIATION...TE
MARINE...TE