U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 230600 
Storm Prediction Center ac 230559 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1259 am CDT Mon Jul 23 2018 

Valid 231200z - 241200z 

..there is a slight risk of severe thunderstorms across 
southeastern Louisiana... 

A few severe storms will be possible across southern Louisiana and 
vicinity today. Elsewhere, strong to marginally severe storms will 
be possible across the Florida Peninsula, mid-Atlantic, and eastern 
Colorado, primarily this afternoon and evening. 

A rather persistent mid-level pattern will feature an amplified 
trough across most of the eastern half of the US today. Around its 
periphery, a series of vorticity maxima will transit from the 
central Gulf Coast southeast to the Florida Peninsula and then 
northward across the mid-Atlantic. To the west, an expansive ridge 
will remain anchored over New Mexico, relegating stronger westerly 
flow to areas from the Pacific northwest to the northern plains. 

Along the southwestern fringe of the aforementioned 500mb trough, 
confluence from the surface through 850 mb is forecast to support 
increasing convective coverage in a northwest/southeast-oriented 
band over southern Louisiana and adjacent coastal waters today. 
Although 700-500mb lapse rates will not be particularly impressive, 
rich boundary-layer Theta-E ahead of the surface front will 
contribute to at least 2500-3000 j/kg of MLCAPE by afternoon. 
Additionally, continued anomalously strong north/northwesterly flow 
will offer substantial effective shear for updraft organization, 
further bolstering vertical accelerations. Storm coverage along the 
initiating boundary, combined with motion somewhat parallel to the 
boundary, should favor clusters of multicells/supercells that slowly 
advance southward. Damaging winds will likely evolve as the primary 
threat due to this cluster Mode, but isolated large hail and perhaps 
even a brief tornado may be possible, given ample boundary-layer 

..Florida Peninsula... 
Any severe potential today will be largely regulated by ongoing 
convection and an adjoining outflow boundary near Tampa Bay as of 
06z. The ongoing southward progression of this outflow over the 
peninsula may shunt the potential for strong convection farther 
south today, as areas farther north remain relatively more stable 
and new initiation is focused along the outflow boundary. Although 
mid-level flow and resultant shear will weaken with southward extent 
(where convection is more likely considering the current evolution), 
damaging downburst winds may be possible given favorably high pws 
and sufficient surface-based buoyancy. 

A channel of enhanced southerly 500mb flow will remain situated over 
much of the region today, continuing to transport plentiful 
tropospheric moisture northward across parts of the East Coast. 
Although mid-level lapse rates will likely remain weak across much 
of the region, diurnal heating amidst a moist/confluent regime will 
support clusters/bands of strong convection from the Carolinas 
northward to Pennsylvania. Downward momentum Transfer of 25-35kt 
850-700mb flow could support a few stronger/damaging gusts in the 
strongest cores this afternoon and evening. 

Despite a lack of stronger forcing for ascent, convection is still 
expected to develop along the higher terrain and foothills of 
Colorado today, aided by easterly upslope flow and surface dew 
points in the upper 50s to lower 60s. Prior convection across the 
region will further dampen mid-level lapse rates, but sufficient 
heating should yield at least modest surface-based buoyancy over 
eastern Colorado by afternoon. Veering flow with height will likely 
support primarily multicellular convection, although a few embedded 
supercells may materialize as well. Storms will pose a threat of 
isolated damaging gusts and large hail as they propagate 
south/southeastward through the evening. 

.Picca/Bentley.. 07/23/2018 


Mesoscale Discussion

acus11 kwns 230732 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 230731 

Mesoscale discussion 1142 
National Weather Service Storm Prediction Center Norman OK 
0231 am CDT Mon Jul 23 2018 

Areas affected...portions of near-coastal west-central/northwestern 

Concerning...severe potential...watch unlikely 

Valid 230731z - 231000z 

Probability of watch issuance...5 percent 

Summary...isolated gusts near severe limits may occur over the 
discussion area from the eastern edge of a complex of thunderstorms 
initially over the Gulf. Magnitude and coverage of severe threat 
appear too low for a watch. 

Discussion...a loosely organized, bowing mesoscale convective system that developed over the 
Florida Panhandle is moving southeastward to south-southeastward about 30 
kt across waters west of coastal Citrus/Hernando/Pasco counties as 
of 715z. The eastern portion of this complex will straddle an 
outflow boundary from an older, decayed mesoscale convective system. The older boundary was 
analyzed from between vrb-sua across Northern Lake Okeechobee to 
near pgd, just east and north of vnc, near srq, and roughly 10-15 nm 
offshore Pinellas County. That portion of this boundary near the 
Gulf Coast has stalled, and may be retreating slowly eastward toward 
areas between srq-pie. Northeast of the boundary, relatively 
cool/stable outflow air will reduce the potential for damaging gusts 
to reach the surface, with eastward extent. Within and west of its 
narrow, mesobeta-scale baroclinic zone, the air mass is richly 
moist, unstable, and weakly convergent, acting as a potential focus 
for maintenance of the eastern part of the newer mesoscale convective system as the latter 
obliquely approaches the coast. Modified raobs and rap soundings 
suggest dew points mid-70s to 80 f and seasonably steep 
low/middle-level lapse rates supporting MLCAPE 2000-3000 j/kg along 
and southwest of the boundary, within minimal cinh, while cinh 
increases to its northeast and inland. 

Northwesterly 35-45-kt effective-shear vectors also are evident over 
the tbw region and northwestward, and forced ascent related to 
cold-pool processes may maintain the mesoscale convective system longer than progged by 
cams. However, both deep shear and low-level lift decrease markedly 
with southward extent toward fmy, away from the boundary and from 
the stronger winds aloft. As such, the most probable area of the 
coastline to be affected by marginal severe-wind potential the next 
few hours is over the discussion area. 

.Edwards/Peters.. 07/23/2018 

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


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