Flood Warning expires at 10:04 PM on 4/24, issued at 10:04 PM Blackshear, GA | Bristol, GA | Mershon, GA | Millwood, GA

A Discussion & Explanation of the Nor'easter & Isolated Supercell Storm: Maps, Radar, Photos

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Updated: 5/06/2013 12:47 am
Wow....what a siege of wet & stormy weather.  The good news is the worst is over. 
There will be scattered showers & storms Tue. While there may be some brief heavy rain, total additional rain should average a half inch or less & not add to flooding woes.

So Mon. will be a beautiful day with lots of sun...a nice, cool start & pleasantly warm afternoon temps.  An upper level disturbance (rotating around a large cut-off low just north of the First Coast [see map below]) will help trigger the showers & isolated t'storms Tue. then the rest of the week will be dry & much warmer.  Highs Thu., Fri. & into the weekend will soar well into the 80s & few inland spots could hit 90 degrees.  The next significant front looks to arrive about Sunday with another shot at showers & storms.

Now for the set-up for this past several days of rain & storms:

** First of all....we had a weak upper level low moving in from the Gulf of Mexico.  The upper low helped develop a surface low across S. Fl. that then moved into the W. Atlantic.  This is what helped produce the very heavy rain & Nor'easter conditions Thu.-Fri.

** Secondly....& a good part of the reason we had the severe storm that moved from Duval into St. Johns Co. Sat. evening....was a large & strong upper level low.  It appears that the weak upper low over the Western Atlantic along with southeast surface winds was pulling warm, moist air off the Atlantic.  As the much stronger & large upper low approached from the west, winds were veering to the southwest inland.  This set up a "covergence boundary" where the southeast winds met the southwest winds.  It's along this boundary where the severe storm with at least supercell characteristics of not a bonified supercell anchored itself beginning near & west of Folkston then drifting south/southeast into Western Duval Co (fed by the warm, moist air off the ocean).  It's in W/SW Duval County where the cell began to intensify & was dropping extremely heavy rain causing flooding.  Luckily this cell stayed far enough north & northeast of the Black Creek basin to not aggravate flooding along the already over-flowing creek.  The storm continued southeast but then became more or a so-called right-turner.  Realize that all the other cells of showers & storms were moving north or northeast so the single severe storm was what is termed as moving "deviant to the mean flow".  The storm was the only one in the area moving south or southeast.  Deviant-moving storms often become severe & this one was no exception with hail, high winds & a possible tornado in Southern St. Johns Co.  The storm is turning to the right -- or south/southeast -- because of strong updrafts that repeatedly were developing adjacent to downdrafts on the backside of the storm.  These updrafts vs. downdrafts (also known as the rear flank downdraft) create small scale low pressure & since air flows from high to low pressure, the storm turns to the southeast (right) into the area of lower pressure. Check out the radar images below from "RadarScope" which shows the progression of the storm from Duval into St. Johns Co.  In third image you can see a thin red line "feeding" from left to right (west to east) into the storm which is an indication of inflow.  The images with pink pixels is where there was hail & damaging winds. The 2nd to last image is a velocity image & shows rotation -- a possible tornado -- near I-95 close to Elkton northeast of Hastings & W/SW of St. Augustine.  It's in this area where a car was blown off a highway (click ** here ** for the drivers' story)...several barns were destroyed...a roof of a building was taken off...power lines were snapped & numerous trees were toppled. 

The photos below -- from Action News videographer Mark Dougherty -- show a destoryed barn & a tree that literally split a home in half.

Other pictures below are from Brian Damico - lightning, World Golf Village - check out the classic cloud to clear air lightning!...Lauren Kruta - the text book shelf cloud near the Duval/St. Johns Co. line on the leading edge of the severe storm moving southeast Sat. evening...Jules Dakota - flooding on George Wood Lane off 103rd & Tampico...Donna Yost-Williams - Broward River above avg. high tide...Janet Sparks - S. Fork of Black Creek (where flooding was worse than "Debby" last year but N. Fork including Middleburg did not seriously flood)..."Tktwins" - Jags fans came out in the rain Sat. to watch practice.

Click ** here ** for an excellent discussion of the heavy rain & storms from our Jax N.W.S.  The images below are courtesy the N.W.S. showing measured 3-day rain totals followed by doppler radar estimated rainfall.

So here's to some sun & a nice, drier start to the week!..........

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