The last few runs of the HRRR computer weather model, coupled with the 00z HRW- suite of models, and the 00z NAM-3km computer weather model have me a bit more concerned with the threat for severe weather between 2am and 6am.
Originally, the thought was 6am would be the beginning of the threat for sever weather as we waited for the secondary push of warm, humid, Gulf of Mexico air. Latest model guidance – as well as the radar – show that push of air is already surging north across parts of the coastal counties of Mississippi.
Down along the Mississippi coast, and into the parishes of Louisiana, surface dewpoints are already in the upper 60s and low 70s. This is very moist air that wasn’t anticipated to arrive for another few hours.
As this warm air surges north, on the backside of it, showers and storms should develop in the new, very warm, sector.
The question is: When? And will they be rooted at the surface.
Some data
Model guidance seems to think by 2AM or 3AM, showers and storms will develop on the backside of the warm air flowing inland. These storms, given the environment, will have a tendency to rotate and become well-organized.
Here is a look at the 3z HRRR Updraft Helicity Streaks
This is the computer model picking out where the strongest storms will track, and how strongly rotating the updraft may be. Given the environment, these storms will be capable of very heavy rain, frequent lightning, wind gusts up to 60mph, hail up to the size of quarters, and depending on how things look at the surface, the potential to produce a tornado.
Here is a snapshot sounding form the 00z NAM-3km computer weather model showing a robust environment from about 925mb up. But from 925mb (about 1500ft) down, though, there may not be the right environment for tornado development.
Seriously. the area may get really lucky and from 1500ft down, it might be a bit too cool (relatively speaking) to produce a tornado.
If you run the numbers on the near-storm environment, the Karrie Meter runs out at a 4.81 with a lower-than-average historical analog tornado number.
But it will needed to be monitored overnight.
Bottom Line
some things with the forecast are changing, but none of them are set in stone. As you saw, the threat for a tornado is pretty conditional, and may cling to what the air is doing in the first 1500ft up – is it warmer or cooler than it should be? – even when the rest of the atmosphere is ready to go, so to speak with severe weather.
Keep your NOAA Weather Radio handy, have an app that will ping you when a new watch / warning is issued, and make sure you have a severe weather plan ready.
More details to come…
