U.S. Severe Weather Forecast (Sääkatsaus)

Day Three

acus01 kwns 191255 
Storm Prediction Center ac 191254 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0754 am CDT Mon Mar 19 2018 

Valid 191300z - 201200z 

..there is an enhanced risk of severe thunderstorms over middle 
Tennessee...central/northern Alabama and northwestern Georgia... 

..there is a slight risk of severe thunderstorms surrounding the 
enhanced risk and extending across much of Georgia to the Florida 

..there is a marginal risk of severe thunderstorms surrounding the 
slight risk... 

Tornadoes (some strong), very large hail, and damaging winds are 
expected from eastern parts of the mid-south across the Tennessee 
Valley region toward the southwestern Appalachians. 

Downstream from a split-flow regime over the northeastern Pacific, a 
generally low-amplitude/progressive pattern aloft is forecast for 
most of the nation, trending toward another eastern mean trough by 
the end of the period. The main mid/upper-level shortwave 
influences in this process will be: 
1. A compact cyclone now located over southern Kansas and northern OK, 
with vorticity Banner trailing westward over the south-central 
rockies. The associated 500-mb low is expected to weaken slightly 
as it moves across the lower Ohio Valley through the period, but the 
trailing trough should phase with the next feature... 
2. A strongly stretched, weak trough now over the Sierra Nevada and 
lower Colorado River valley, forecast to move rapidly eastward to a 
position from the mid-south across the arklatex to central Texas by 
3. 2-3 small/weak perturbations now over the northern plains should 
get caught in strengthening northwesterly flow ND accelerate into 
the larger vorticity field related to the merger of features 1 and 
All this should yield broadly cyclonic flow from the Southern Plains 
across much of the southeast, and the northern Gulf, by the end of 
the period. 

At the surface, 11z analysis showed an occluded surface low over the 
Kansas/OK border region between pnc-cfv, with occluded front to near 
dqu, and cold front arching southwestward over East Texas and 
south-central Texas. Another frontal zone -- quasistationary in some 
segments and warm in others -- was analyzed from the arklatex across 
northern la, central MS, south-central al, and south-central GA, to 
a weak low near ssi, then northeastward over Atlantic waters. 

The primary surface low should move across the mid south by 00z, 
perhaps discretely redeveloping eastward in the process, before 
curving northeastward toward eastern Kentucky around 12z. The cold front 
will sweep eastward/southeastward, reaching northern/central MS, 
central la and the Texas coast around 00z, then shifting eastward to a 
newer frontal-wave low forecast to develop over eastern NC near the 
end of the period. The warm front should move rapidly northward 
today over MS/Alabama and into middle Tennessee. Farther eastward from the low, 
the relatively low-Theta-E/damming air mass north of the warm front 
over northern Georgia and much of SC will be slow to erode today and this 
evening. Still, the warm frontal zone should shift northward 
erratically and probably diffusely across northern Georgia and SC 

In the main threat area, scattered thunderstorms -- including 
supercells -- should develop mid/late this afternoon across parts of 
western/middle TN, northeastern MS and northern Alabama. Tornadoes, 
large/damaging hail, and severe wind gusts all are possible with 
this activity. The most favorable combination of ingredients still 
appears to be over the enhanced-risk area. Enough uncertainties 
remain to preclude more tightly focused/higher probabilities at this 
time, though an upgrade of part of this area may be needed as 
mesoscale diagnostic tendencies and 12z guidance inform update 

An ongoing/weakening mesoscale convective system moving southeastward into more stable air 
over northern Florida should dissipate through the remainder of the 
morning, but has produced an outflow boundary south of the 
aforementioned warm front, from the Northern Peninsula northwestward 
over south-central Alabama. This boundary will erode gradually from west 
to east and retreat northward today into this evening, perhaps 
merging with the synoptic front. The boundary and frontal zone will 
be important sources of vorticity, even as they retreat 

An arc of enhanced upward vertical velocity now located near the surface occluded front 
is expected to shift eastward today and meet the returning warm 
sector across western/middle Tennessee and northern MS mid-late afternoon. 
As this occurs, thunderstorms will develop in an air mass 
destabilizing from both diabatic surface heating and warm advection, 
with surface dew points 60s f and strong deep shear. Substantially 
southerly surface flow component will be maintained amidst pressure 
falls ahead of the low, and height falls aloft, contributing to 
enlarged hodographs and effective srh 200-400 j/kg in the warm 
sector and potentially exceeding 400 j/kg in the warm-frontal zone. 
With MLCAPE 2500-3000 j/kg in the eastern MS/western Alabama part of the 
pre-cold-frontal air mass to 500-1000 j/kg in the warm-frontal zone 
over TN, sufficient instability will exist to support sustained 
supercells, some producing tornadoes. Supercells may be discrete 
and/or at least loosely embedded in a southwest-northeast primary 
convective band. Also in that environment, beneath steep 
low/middle-level lapse rates (associated with eastward transport of 
a residual eml), and sufficient shear for optimal updraft tilt and 
precipitation size sorting around mesocyclones, large hail is likely 
as well -- including significant/damaging stones above 2 inches in 

Separate areas of thunderstorms may develop across southern al, 
central/southern Georgia and the Florida Panhandle/northern Florida region from 
this afternoon through overnight, in a somewhat nebulously forced -- 
but also weakly capped and favorably moist/unstable regime -- 
related to both the retreating/modifying outflow boundary and the 
damming front. Supercells with damaging gusts, at least isolated 
large hail, and a few tornadoes, are possible in this area as well. 

.Edwards/Mosier.. 03/19/2018 


Mesoscale Discussion

acus11 kwns 190927 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 190926 

Mesoscale discussion 0142 
National Weather Service Storm Prediction Center Norman OK 
0426 am CDT Mon Mar 19 2018 

Areas affected...far southeast Kansas...southwest MO...western/central 
Arkansas...far eastern OK 

Concerning...severe potential...watch unlikely 

Valid 190926z - 191130z 

Probability of watch issuance...20 percent 

Summary...isolated hail possible across the region for the next 
several hours. 

Discussion...arcing band of elevated convection, extending from far 
southeast Kansas across western Arkansas and into far northeast TX, continues 
to show occasionally strong updrafts as it quickly moves 
northeastward. This general northeastward progress is expected to 
continue as the strong forcing for ascent along the leading edge of 
the mid-level jet streak continues northeastward. Downstream 
airmass, particularly along the northern extent (i.E. Southern mo), 
will gradually become more hostile to convection as cooler 
temperatures and less favorable low/mid-level moisture result in 
less instability. Vertical shear also decreases farther north. 
Despite these negative factors, the strength of the forcing for 
ascent will likely allow for storm persistence and some isolated 
hail will remain possible. Farther south (across ar), prospects for 
storm persistence and occasional hail are relatively higher given 
the more favorable instability and vertical shear. 

.Mosier/Edwards.. 03/19/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 37009503 37359562 38119500 38079307 36639193 34759220 
33699343 33799474 34869482 35939469 37009503