U.S. Severe Weather Forecast (Convective Outlook)

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acus01 kwns 221248 
swody1 
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


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acus11 kwns 220803 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 220802 
mnz000-iaz000-nez000-sdz000-221400- 


Mesoscale discussion 0038 
National Weather Service Storm Prediction Center Norman OK 
0202 am CST Mon Jan 22 2018 


Areas affected...portions of eastern NE...far southeast 
South Dakota...northwest Iowa and far southwest Minnesota 


Concerning...blizzard 


Valid 220802z - 221400z 


Summary...moderate to heavy snow with blizzard conditions will 
continue across central NE and shift north and east through the 
early morning hours. Snowfall rates and blizzard conditions are 
expected to increase across the mesoscale discussion area over the next few hours. 


Discussion...a potent midlevel low over Kansas will continue to eject 
east/northeast early this morning. Latest water-vapor loop shows the 
dry slot associated with this system shifting east/northeast across 
far southeast NE and eastern Kansas with stronger forcing for ascent 
lifting east/northeast across the Central Plains. Currently, the 
heaviest snow is falling in the deformation region along a band of 
midlevel frontogenesis across portions of western into central NE. 
Snow rates will increase to the east and north as the system 
continues to lift northeast through the early morning hours. Snow 
rates will approach 1 inch per hour, with locally higher rates at 
times. 


Furthermore, strong northerly flow on the back side of a deepening 
surface low over north-central Kansas will likewise increase from 
southwest to northeast across the mesoscale discussion area through the early 
morning. Sustained winds around 20-30 mph with higher gusts will 
result in dangerous blizzard conditions with very low visibility. 


.Leitman.. 01/22/2018 


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


Attention...WFO...mpx...dmx...fsd...oax...Gid...lbf... 


Latitude...Lon 40319819 40259898 40459961 40969971 41749949 42519887 
43309771 43779664 44119546 44309454 44289409 44169376 
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