U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221248 
Storm Prediction Center ac 221247 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0647 am CST Mon Jan 22 2018 

Valid 221300z - 231200z 

..there is a marginal risk of severe thunderstorms over parts of 
the central/eastern Gulf Coast region... 

..there is a marginal risk of severe thunderstorms over parts of 
central/northern Illinois... 

Isolated severe thunderstorms may impact parts of the central and 
eastern Gulf Coast region and Illinois today. 

Split flow in mid/upper levels will continue across the central U.S. 
Today around a strong cyclone, currently centered over central Kansas 
based on moisture-channel imagery and available upper-air data. The 
trailing trough, now extending south-southwestward across west- 
central TX, will remain positively tilted as the 500-mb low moves to 
southeastern Iowa by 00z. The low aloft will accelerate east- 
northeastward overnight toward Southern Lake Huron, devolving into 
an open-wave trough near the end of the period. 

At the surface, 11z chart analysis showed the associated surface low 
over extreme northwestern MO, with occluded front eastward to a 
triple point over extreme southeastern Iowa. A warm/stationary front 
(moving northward slowly in some segments, stationary in others) was 
drawn from there across the southern tip of Lake Michigan across 
Western Lake Erie. A cold front arched from the triple point across 
eastern AR, western la, and the Texas coastal Bend. The occluded low 
will become increasingly vertically stacked with its mid/upper-level 
counterpart as the latter approaches today, while a new low forms on 
the triple point across northern Illinois by evening. The cold front is 
forecast to sweep eastward/southeastward by 00z to near the Illinois/in 
border, western TN, and southeastern la. By 12z, the newer low 
should occlude over lower mi, with cold front from southern on 
across central PA, western VA, GA, and the east-central/south- 
central Gulf. 

..Gulf Coast states... 
Scattered convection, with sporadic/isolated severe gusts possible 
and a marginal tornado risk, should cross the outlook area through 
the afternoon and perhaps into early evening. 

The southern section of a long, prefrontal convective plume 
currently extends from southwestern la across western portions of 
MS/TN. Infrared imagery shows extensive cloud Covert streaming off this 
activity to the east and northeast, which will restrict the pace and 
magnitude of diabatic surface heating and related destabilization 
today over much of the south. Still, a partially modified former 
Continental/polar air mass, now with surface dew points 60s f, is 
evident in a wedge-shaped swath of the preconvective sector across 
parts of southern la and southwestern MS, and just offshore from the 
MS/Alabama coasts. 

The return flow of that air should (barely) keep pace with the 
advancement of the main convective plume through most of today into 
this evening across most of the outlook area. Meanwhile, some areas 
near the coast may experience enough cloud breaks and insolation to 
allow erosion of weak mlcinh evident in morning Jan/lix raobs, with 
500-1000 j/kg MLCAPE possible, decreasing northward/inland under 
thicker/longer-duration cloud cover. 40-50 kt effective-shear 
vectors are expected, along with a broad swath of 30-55 kt 850-mb 
flow atop surface southerlies that should maintain enlarged low- 
level hodographs. Forecast vertical shear should support occasional 
Bow/lewp development and clustered convective structures with 
embedded/short-lived supercells possible. 

Counteracting factors aside from messy convective Mode include: 
1. Lack of greater lapse rates aloft (limiting buoyancy), and 
2. Considerable horizontal displacement of large-scale midlevel 
lift from the region as the cyclone aloft, and related DCVA, eject 
east-northeastward and gain latitude away from the Gulf Coast. 

An arc of convection, much of which may not extend deep enough to 
produce lightning, may develop this afternoon over parts of central/ 
northern Illinois and move eastward to northeastward, offering the 
potential for damaging gusts and perhaps a tornado. 

This outlook area is conditional, predicated on development of both 
favorable low/middle-level instability behind the morning convective 
plume, and enough sustained lift for thunderstorms to take advantage 
of that destabilization. Though neither is certain, the 
destabilization appears more probable, with a combination of strong 
cooling aloft, surface heating and residual moisture potentially 
yielding pockets of 300-600 j/kg MLCAPE. Forecast soundings in the 
narrow unstable corridor also show erratically shaped hodographs 
derived from minimal directional change with height but gradual 
strengthening, yielding 30-45 kt effective-shear magnitudes. 

Convective timing and coverage are the main concerns preventing 
greater unconditional severe probabilities at this time. In 
concordance with the concerns about sufficient lift, considerable 
differences exist among progs as to development/extent of any 
convection developing ahead of the surface low -- with more-recent, 
shorter-fused, convection-allowing guidance (e.G., Hrrr and esrl 
hrrr) being the most noticeably and consistently parsimonious. 

.Edwards/Broyles.. 01/22/2018 


Mesoscale Discussion

acus11 kwns 221521 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 221520 

Mesoscale discussion 0039 
National Weather Service Storm Prediction Center Norman OK 
0920 am CST Mon Jan 22 2018 

Areas affected...portions of eastern NE...far southeastern 
South Dakota...northern/western Iowa...southern Minnesota...and western WI 

Concerning...heavy snow 

Valid 221520z - 222115z 

Summary...heavy snow, with rates around 1-2 inches per hour, will 
spread northeastward through this afternoon. Blizzard conditions 
will continue across parts of eastern NE into far southeastern SD, 
western/northern IA, and southern Minnesota for at least a few more hours. 

Discussion...a mature mid/upper-level cyclone centered over eastern 
NE/Kansas this morning will move east-northeastward across Iowa/MO and 
towards Illinois by later today. A zone of strong low/mid-level 
frontogenetic forcing on the northern extent of this cyclone is 
supporting a band of moderate to heavy snow extending from eastern 
NE into western/northern Iowa and southern Minnesota. This heavy snow band 
will develop northeastward into more of southern Minnesota and eventually 
parts of western WI by this afternoon. Snowfall rates of 1-2 inches 
per hour will be common within this band, with locally higher rates 
possible within where localized convective enhancement occurs. 
Sporadic lightning strikes may continue given the very strong 
large-scale forcing and steepened mid-level lapse rates. 

Additionally, a strong surface pressure gradient exists on the 
northern side of a low centered over Iowa as of 15z, which is 
supporting blizzard conditions across parts of eastern NE, far 
southeastern SD, western/northern IA, and southern Minnesota. Here, 
frequent northerly/northeasterly gusts of 35-50 mph are resulting in 
blowing snow with substantially reduced visibilities (1/4 of a mile 
or less). These blizzard conditions are expected persist for at 
least the next few hours. 

.Gleason.. 01/22/2018 

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


Latitude...Lon 44029142 43629235 43279342 42579519 41119622 40939779 
41659826 42779779 44239582 45189339 45459190 45279101 
44379085 44029142