Marine Weather and Tide Forecast for Prairieville, LA

FXUS64 KLIX 091002 AFDLIX

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.
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