U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 220558 
Storm Prediction Center ac 220557 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1257 am CDT sun Jul 22 2018 

Valid 221200z - 231200z 

..there is a slight risk of severe thunderstorms over parts of the 
southeast and northern/Central Plains... 

Severe storms, primarily capable of damaging winds and large hail, 
will be possible across parts of the southeast and northern/Central 
Plains this afternoon and evening. 

A highly amplified mid-level trough will persist across the eastern 
US today, as several vorticity maxima rotate around its periphery 
from the Mississippi Valley to the southeast and then northward 
along the Atlantic coast. To the west, a Stout Ridge will remain 
anchored over New Mexico, with enhanced westerly flow relegated to 
the northern rockies and plains. 

..southeast US... 
While mid-level lapse rates will be less impressive than the prior 
day, surface heating near/ahead of a weak east/west-oriented front 
will combine with rich boundary-layer moisture (e.G., Dew points in 
the mid/upper 70s) to produce moderate/strong buoyancy across far 
southern Alabama/Georgia and the Florida Panhandle today. As a lobe of ascent 
(related to an embedded impulse currently over the mid Mississippi 
valley) progresses towards the southeast, associated 
cooling/moistening aloft and modest low-level convergence are 
expected to support scattered to numerous thunderstorms by 
afternoon, primarily from southern Alabama eastward to the Atlantic 
coast. Effective shear around 40-45 kt should support several 
rotating updrafts/supercell structures initially, with a resultant 
threat of large hail and damaging gusts. Over time, however, shear 
vectors parallel to the initiating boundary will likely transition 
storms to more linear modes, with an increasing southward 
propagation component as cold pools mature. Damaging winds should 
then become the main hazard as cells advance south across the 
Florida Peninsula into the early evening. 

..northern/Central Plains... 
The progressive nature of a 500mb shortwave trough over southern 
Saskatchewan and the northern High Plains will shift the main 
convective focus eastward across the Dakotas and Nebraska today. 
Surface heating may be stunted some by cloud debris and ongoing 
precipitation from the prior overnight period, but pockets of 
insolation and dew points in the mid/upper 60s should support the 
development of at least moderate mixed-layer buoyancy, especially 
across South Dakota and Nebraska. Meanwhile, ahead of the surface 
trough, south/southeasterly winds at the surface will veer to 30-45 
kt of westerly flow at 500mb. Within a resultant environment 
characterized by 30-40kt effective shear, a mixture of multicells 
and supercells, capable of large hail and damaging winds, are 
forecast to materialize by mid/late afternoon. These cells will 
gradually shift southeast through the evening, although they should 
be fairly confined in longitudinal progression due to increasing 
convective inhibition to the east. Most of the threat will likely 
have diminished by late evening/early overnight. 

.Picca/Bentley.. 07/22/2018 


Mesoscale Discussion

acus11 kwns 220908 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 220908 

Mesoscale discussion 1130 
National Weather Service Storm Prediction Center Norman OK 
0408 am CDT sun Jul 22 2018 

Areas affected...eastern Long Island to Block Island 

Concerning...severe potential...watch unlikely 

Valid 220908z - 221145z 

Probability of watch issuance...5 percent 

Summary...a marginal risk for a brief tornado threat exists across 
mainly eastern Long Island, New York to Block Island, Rhode Island 
through the early morning. This threat could be a brief tornado 
forming over land or a waterspout moving inland. 

Discussion...trends in surface analyses indicated the western 
portion of a warm front had advanced into western Long Island where 
surface dew points were in the lower 70s f south of this boundary. 
This boundary extended to the east-southeast, south of eastern Long 
Island. Modifying the 00z okx sounding, warmer surface 
temperatures/dew points in the lower 70s f (i.E. Those that are 
found in the warm sector) are needed to weaken inhibition for 
convection to realize surface-based parcels/weak instability. 
Meanwhile, mosaic radar imagery showed a 40-50 mile wide corridor of 
showers and embedded storms extending from eastern Long Island to 
approximately 180 miles east/southeast of Acy, with this activity 
attendant to a confluence zone, which was moving slowly to the east, 
while individual cells were tracking to the north/northwest. Radar 
imagery has been indicating periodic, weak low-level rotational 
Couplets embedded in this convective band, and the okx VAD modified 
with a storm motion of 35-40 kt to the north-northwest results in 
0-2-km and 0-3-km srh around 300 m2/s2. The favorable low-level 
shear environment will support additional low-level rotational 
Couplets with a brief tornado threat as storms encounter the slow 
northward-moving warm front, where shear should be enhanced, and 
then weaken north of the front. Overall weak instability will limit 
both the coverage of this severe-weather threat and the need for a 

.Peters/Edwards.. 07/22/2018 

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


Latitude...Lon 40687303 41037299 41197235 41217195 41287152 41177128 
40957139 40637287 40687303