Happy new Year! At least it isn’t tornadoes, right?
As the next system swings through the Gulf Coast, there is the potential for heavy rain and flooding. And while severe weather can’t be ruled out, the main concern with the next system moving through the region is for localized flooding.
What follows is my stream of consciousness look at the setup, the data, and the forecast! Most of this discussion will be for southern Mississippi, but will be applicable to Louisiana, Mississippi and Alabama. AS well as Tennessee and Florida.
The Setup
A cut-off Low currently over the Baja of California will drift to the east during the next 48 hours and meet up with a trough that is set to dig south across the Southern Plains on Wednesday and Thursday. As this happens, parts of the Gulf Coast and lower Mississippi Valley will be in the right place to pick up copious amounts of rainfall – perhaps over an extended period of time.
The southwesterly flow aloft, on top of a generally southwesterly flow at the lower levels of the atmosphere should help to lower the overall threat for severe weather, but will, instead, increase the chance for extended heavy rainfall.
That, coupled with the transport of incredibly moist Gulf of Mexico air into the region, will set the stage for potential flooding.
The Data
No, no, no… Not that Data. Computer model data!
It is amazing what the forecast data is showing in terms of warm air advection (advection is a fun weather word for ‘the movement of air from one place to another’). Take a look at the two Skew-T charts below. They are nearly identical.
One of these is just off the coast of the border of Mexico and Texas. The other is just south of Jackson, Mississippi – 18 hours later. All of things aside, that is a good indicator that some pretty warm, moist air is about to invade the Gulf Coast.
The above map is a look at the surface Precipitable Water. It is a good indicator for which places will be the most moist. The places in purple, are the places where it is going to be the most humid.
Dave Chappelle fans may be looking at that map like…
ut you don’t, really. Because when warm, moist air like that settles across our area, it is only a matter of time before a cold front swings through. And when that cold front moves through it will lift all that warm, moist air up, squeeze out all of the extra moisture, and rain.
So the more moist it is to begin with, the more rain we end up with when it rains itself out.
And in this case we will have plenty of warm moist air and – bonus! – a lot of vertical motion in the atmosphere.
The above map shows – in shades of red – where the most vertical motion in the atmosphere will be on Thursday afternoon. So, not only is the atmosphere shoving a bunch of warm moist air into the area, but it is also moving that air vertically. That means even more rain.
The Forecast
The Weather Prediction Center has outlined sections of the Gulf Coast with a Marginal Risk and Slight Risk for flash flooding. And that threat stretches all the way up the the Tennessee Valley.
Along the Gulf Coast, it looks like between 1″ to 3″ will be likely everywhere from Lake Charles, Louisiana to Pensacola, Florida. And as far north as a line from Shreveport, Louisiana, to Memphis Tennessee to Asheville, North Carolina.
Some spots will have a chance to pick up between 3″ and 6″ of rain. That will be closer to places like Baton Rouge, McComb, Hattiesburg, Meridian, Birmingham, Huntsville and Knoxville.
It is also possible for areas near the places listed above to experience greater than 6″ of rain. But we are still trying to figure out where the stripe of highest rainfall will be.
The severe weather threat should remain pretty limited. Right now the SPC has the immediate Gulf Coast in Louisiana, Mississippi and Alabama under a Marginal Risk
The main concern would be for lightning, gusty wind and small hail. While a tornado can’t be ruled out, that isn’t a main concern.
The Timeline
This may be the most important part of the forecast. The “When will it hit my house” part!
Right now, model guidance is showing the first rain beginning around daybreak on Thursday morning for Louisiana and South Mississippi. And lingering around through Friday midday for those areas.
For Northern Mississippi and Alabama. Rain will begin a bit later on Thursday and linger through Saturday morning.
In a nutshell, be ready for an off-and-on rain starting at some point on Thursday. And once it starts, it will be around for about 18 to 36 hours, depending on where you live. The further south, the longer the rain will last.
