U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 231259 
Storm Prediction Center ac 231257 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0757 am CDT Mon Jul 23 2018 

Valid 231300z - 241200z 

..there is a slight risk of severe thunderstorms over parts of 
southeastern Louisiana... 

A few severe storms will be possible across southern Louisiana and 
vicinity today. Elsewhere, strong to marginally severe storms will 
be possible over parts of the Florida Peninsula, mid-Atlantic and 
southern/central rockies areas, primarily this afternoon and 

The basic upper-air pattern will change little over the Continental U.S. This 
period, with a large/blocky anticyclone centered over The Four 
Corners area and covering much of the south-central and southwestern 
parts of the country, as well as northern mx. Zonal to west- 
northwesterly flow will extend from the Pacific northwest across the 
northern Great Plains, becoming more difluent and northwesterly over 
the upper Mississippi Valley and Central Plains. A broad, weakening 
synoptic cyclone is evident in moisture-channel imagery over the 
eastern U.S., And contains numerous mesoscale and smaller vorticity 
lobes. The cyclone will remain essentially in place but gradually 
devolve to an open-wave trough through the period, with associated 
mid/upper-level speed maxima over the lower Mississippi Valley and 
mid-Atlantic regions. 

At the surface, a broad pressure weakness, with several subtle/ 
embedded lows, will persist under the deteriorating upper vortex/ 
trough over the eastern U.S. A trough and weakly baroclinic 
frontal/wind-shift line -- the remnants of the synoptic cold front 
related to the eastern cyclone -- will shift slowly southward across 
the Gulf coastal states, including TX, and extend northwestward into 
central nm. A weak cold front, analyzed initially from northern Minnesota 
across central NE to northeastern co, should move over parts of the 
upper Great Lakes, eastern Iowa and northern Kansas overnight, while 
returning northward as a warm front across the Central High plains. 

Widely scattered to scattered thunderstorms are expected to form 
episodically, from midday through afternoon, along a low-level 
convergence zone extending from southeastern/south-central la 
southeastward over the Gulf. Damaging gusts and large hail will be 
the main concerns, and a tornado cannot be ruled out. 

Low/middle-level lapse rates and boundary-layer moisture each should 
increase with proximity to the Gulf Coast. Preconvective surface 
dew points in the mid 70s to around 80 f will remain common over 
south-central/southeastern la. Those factors, along with diurnal 
heating, will contribute to MLCAPE commonly 2500-4000 j/kg along and 
south of the primary confluence/convective axis. Although the 
boundary-layer flow will be substantially westerly, directional 
shear still will be favorable, given a dominant northerly component 
of mid/upper winds and mean flow, leading to 30-40 kt effective- 
shear magnitudes. Organized multicells and heavy-precip supercells 
are possible in this regime, with new cells forming along a slowly 
southwestward-shifting boundary, while older storms move southward 
to southeastward and offshore. 

Scattered multicell thunderstorms should form today over the 
south-central parts of the peninsula, offering isolated damaging to 
severe gusts. The composite outflows from two evening/overnight 
mcss have settled southward across central FL, leaving behind an air 
mass with somewhat shallower and most stable moist layer. The 
Post-convective boundary layer will not be entirely hostile to 
thunderstorms, but the air mass farther south should be more 
thermodynamically favorable, with higher Theta-E and mixing ratios. 
Parts of southern Florida should heat strongly today with dew points 70s 
to near 80 f. This should boost MLCAPE values -- already around 
2500 j/kg in the 12z mfl raob, into the 3000-4000 j/kg range. The 
main threat will come from water-loaded downbursts. Weak vertical 
shear will limit overall organization of convection. 

A large area of the east, from portions of Georgia and the Carolinas to 
the Canadian border, will experience widely to scattered 
thunderstorms today. Isolated damaging gusts cannot be ruled out 
almost anywhere in that swath, within the broad, elongated and 
favorably moist fetch of southerly to southeasterly deep-layer flow. 
As such, the outlined marginal-severe area is necessarily fuzzy and 
not hard-edged. It represents a forecast relative maximum in 
marginal severe probabilities, based on what should be a loosely 
demarcated area of juxtaposition between stronger winds aloft and 
most-favorable boundary-layer destabilization. Mid/upper lapse 
rates should be only slightly greater than moist adiabatic, with 
pockets of sustained surface heating and 60s to 70s f surface dew 
points leading to MLCAPE 500-1000 j/kg (locally higher). Specific 
foci are nebulous and likely to be localized in nature (e.G, outflow 
and differential-heating boundaries, subtle low-level convergence 
axes) amidst weak cinh on the broader scale. 

..co/northeastern nm... 
After morning clouds break up, thunderstorms will develop this 
afternoon over the Front Range, Sangre de Cristos, and parts of the 
north-Central High plains across WY, as upslope lift and heating of 
higher terrain preferentially remove mlcinh. Isolated severe gusts 
and hail are expected as this activity aggregates and moves eastward 
to southeastward across the foothills and portions of the adjoining 
High Plains from eastern Wyoming and southwestern South Dakota to northeastern nm. 
Foregoing boundary layers will be characterized by favorably steep 
lapse rates from strong heating/mixing, and 50s to lower 60s f 
surface dew points, contributing to MLCAPE commonly in the 1000-1500 
j/kg range. Though low-level flow should be modest, its easterly 
component will contribute to strong veering with height, with 
resulting effective-shear magnitudes of 35-45 kt possible, as well 
as favorable storm-relative low-level winds. Convection should 
weaken this evening as the boundary layer stabilizes diabatically. 

.Edwards/Peters.. 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... 


Latitude...Lon 28818267 28668249 28418239 28018225 27598237 26968238 
27608280 27858283 28248279 28648265 28738266 28818267