Posted: 5:37 am Wednesday, May 23rd, 2018
By Kirk Mellish
A fire hose of moisture is a nice metaphor but I don’t mean to exaggerate. This is NOT some flooding everywhere disaster movie scenario.
It’s merely to point out the deep fetch of moisture coming into the Southeast U.S. with a direct connection to the the Gulf, Caribbean and points even further south which can plainly be seen in satellite imagery and computer models.
There will be decent dry breaks around the Atlanta Metro area the next 5-10 days. Rain odds will vary from 30-70%. Remember thunderstorm forecasts, unlike others, sometimes have to be updated about every 3 hours.
We don’t know yet if a tropical depression or storm will eventually form in the Gulf but probably (NHC says 60% odds). However, it doesn’t matter because the moisture connection is there with or without that outcome and it looks to be prolonged, 5-10 days. Rainfall rest of the month expected to be 200-500% of normal in parts of Georgia.
Tropical storms have occurred in May before the official June start of the season a handful of times, in fact twice this decade in time for Memorial Day weekend in the SE… Beryl in 2012 and Bonnie in 2016.
The earliest tropical storm on record to hit West of Florida was Arlene on May 30th 1959 which hit Louisiana with 45 mph winds.
You see the high water content of the air here connected well to the South, that’s the pipeline (WxBell graphic):
That means graduation, beach, pool, lake, BBQ and concert activities will be challenged all across the Southern and SE U.S. from the mountains to the Gulf coast and Atlantic coast the next 5 days or more. There will be a random sun-deluge mix with the luck of the draw determining who gets what at any give time on any given day.
I am glad I didn’t schedule a 3 or 4 day vacation on the Gulf or in Florida. Of course I work most holidays so that would be unlikely. A lot of gusty winds and big waves for the Gulf coast beaches as well.
The microphysics and raindrop generation are different in a tropical environment where high humidity extends from the surface to high up in the atmosphere. This allows for big fat rain drops and/or high density rainfall that can lead to big amounts in a short period of time (excessive rainfall) which leads to standing water on roads or flash flooding of low spots and creeks.
This is known as high precipitation efficiency. There are scientific papers you can google for more info.
Also you don’t always get lightning so beware you can go from a bit of sun or just cloudy to a frog strangler in the blink of an eye with no warning.
This system or series of mid to upper level low pressure areas sometimes with a surface low reflection means a long spell of unsettled weather with more than usual clouds for this time of year and greater than normal rain odds rest of this month.
If a tropical storm forms it would be Alberto. Lets just hope it doesn’t turn into the kind of Alberto the region had in early July 1994 (NOAA/NCEP graphic):
The reason systems or patterns like this get stuck is there are weak steering currents aloft and no mechanism to move the system or pattern away as the main jet stream has migrated up toward Canada and a ridge develops above the Gulf low. “A ridge over troubled waters” if you will. Sometimes weather patterns get stuck or “trapped” and have to wait a long time for something to move them, or for them to just “rain themselves out” or just weaken and fizzle out and dissipate.
That doesn’t happen all at once or suddenly, but gradually over time and in fits and starts. It holds temperatures lower than normal:
The CFSv2 and EPS models both hold back the heat through most of June!
RAINFALL AMOUNTS FROM THIS EVENING TO THURSDAY ESTIMATED AVERAGE:
RAINFALL ESTIMATED AVERAGE AMOUNTS THURSDAY EVENING TO FRIDAY EVENING:
RAINFALL AMOUNTS ESTIMATED AVERAGE FRIDAY EVENING SATURDAY EVENING:
The worst rain may end up being in LA/MS/AL and to a bit lesser extent FL/GA, details yet to be determined. The maps above WILL CHANGE each day, watch for updates and listen to my reports on the radio for the latest.
But all models agree on the trend, all signs say the same thing.
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