U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 011256 
Storm Prediction Center ac 011254 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0654 am CST sun Feb 01 2015 

Valid 011300z - 021200z 

..there is a marginal risk of severe thunderstorms from parts of southeastern Texas eastward to 
parts of western Alabama... 

A few marginally severe thunderstorms may occur this afternoon and 
early evening from portions of southeast Texas to the lower 
Mississippi Valley region. 

Morning water vapor imagery indicates a midlevel shortwave trough 
amplifying over the Central Plains. Associated surface low 
pressure...analyzed along the Kansas/MO border...will develop eastward toward 
the Ohio Valley and central Appalachians in tandem with the 
progression of the shortwave trough. In the wake of a cold front 
trailing SW of the deepening surface cyclone...sprawling surface high 
pressure will reinforce Continental polar air over the Great Plains. 
Meanwhile...ahead of the cold front...poleward mass fluxes will 
encourage the inland advance of partially modified Gulf moisture. By 
early Monday...the cold front is forecast to extend from portions of 
the central Appalachians to the eastern Gulf Coast. 

..portions of southeast Texas to the lower MS valley region... 
Surface observations indicate dewpoints from the upper 50s to around 60f 
extending inland from the Texas coast to the dfw metroplex...in advance 
of the cold front. This front is analyzed from the Kansas/MO-area low 
pressure southwestward to the Texas S plains. As mass fields respond to surface 
cyclogenesis occurring well north of the region...similar dewpoints will 
overspread areas farther east toward the lower MS valley ahead of the 
front...where 12z radiosonde observations indicate a dry/cool boundary layer presently 
in place. 

A steady supply of moisture east of a northwestern-Mexico midlevel 
cyclone...coupled with modest isentropic ascent in the entrance 
region of a surface-cyclone-preceding low level jet...will maintain clouds and 
areas of precipitation across the warm/moist sector. This will stunt 
diurnal destabilization. However...as the strongest low-level mass 
fluxes become concentrated over the Ohio and Tennessee valleys...breaks in 
cloud coverage/decreasing cloud-opacity may promote marginal 
pre-frontal destabilization. A corridor of 200-500-j/kg MLCAPE will 
likely materialize ahead of the front by mid-afternoon...from southeastern 
Texas to central MS. This will be sufficient for the development of 
isolated to scattered thunderstorms in vicinity of the front during the 
afternoon...with this activity spreading eastward into the evening. 
Additional convection may mature within the 
pre-frontal/isentropic-ascent regime throughout the day from central 
la to central/S-cntrl/ern MS...with this activity similarly shifting 

The presence of 55-60-kt 500 mb wswlys will offer sufficient deep shear 
for organized convective structures. Small bowing segments may yield 
a few instances of marginally severe wind gusts. Low-level flow will be 
slightly backed relative to the flow aloft in proximity to a 
low-level confluence axis extending south-southwest of the cyclone to the central 
Gulf Coast this afternoon. Pre-frontal convection evolving in this 
regime may occasionally exhibit mixed convective modes...perhaps 
including semi-discrete supercells. This would particularly be the 
case across portions of central/srn/ern MS and adjacent sections of 
northestern la and western Alabama. Curved...elongated low-level hodographs 
supporting 100-250 m2/s2 of 0-1-km srh suggest that a brief/weak 
tornado cannot be ruled out in these areas. 

However...the dearth of buoyancy will be a mitigating factor for 
more robust severe-thunderstorm potential...especially given incomplete 
modification of the air mass overlying the Gulf of Mexico and the 
presence of only modest midlevel lapse rates. Furthermore...the 
spatial displacement between the stronger deep ascent/surface 
cyclogenesis and greater boundary-layer Theta-E will be a mitigating 
factor for a more substantial severe-thunderstorm potential. As such...only 
marginal-svr-tstm delineation is in effect. Any severe-thunderstorm risk will 
diminish as convection spreads east of western Alabama this evening...and 
nocturnal cooling breeds an increase in boundary-layer static 

..extreme eastern NC including the Outer Banks... 
Strengthening low-level slys ahead of the shortwave trough will 
Foster an influx of low-level moisture from the southwestern North Atlantic 
late tonight...along with some uptick in convection. Despite strong 
flow just above the surface...forecast soundings indicate a paucity of 
buoyancy owing to statically stable thermodynamic profiles in the 
low levels. The nocturnally enhanced stable layer will minimize the 
opportunity for downdrafts to manifest strong winds at the surface. 
Therefore...no severe probabilities have been included. 

.Cohen/carlaw/Mead.. 02/01/2015 

Mesoscale Discussion

acus11 kwns 011343 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 011343 

Mesoscale discussion 0049 
National Weather Service Storm Prediction Center Norman OK 
0743 am CST sun Feb 01 2015 

Areas affected...portions of northern Illinois...northern Indiana...Southern Lower 
Michigan...northwestern Ohio 

Concerning...heavy snow 

Valid 011343z - 011915z 

Summary...areas of heavy snow with rates to 1 inch per hour will 
spread across the region through the day. 

Discussion...surface pressure falls of 2-3 mb per 2 hours are analyzed 
across the middle-MS valley to portions of northern IL/Indiana...reflecting 
the low-level warm air advection regime in place ahead of surface low pressure analyzed 
near Kansas City. Related isentropic ascent is supporting widespread 
precipitation in this regime...with moderate to occasionally heavy 
snow being reported across parts of northern Illinois. As the surface low develops 
eastward/east-northeastward while deepening and low-level frontogenesis 
correspondingly becomes enhanced...precipitation will further 
increase in coverage and intensity across the discussion 
area...where wet-bulb zero heights will remain sufficiently low/at 
the ground in support of snow per the 12z ilx radiosonde observation and rap forecast 
soundings. The stronger warm air advection and associated low level jet-enhanced ascent will 
shift eastward through the day. As such...snowfall rates to 1 inch per 
hour will gradually spread eastward as well -- reaching northwestern Ohio 
around/after 18z. Also of note...strengthening northeasterly winds to the northwest 
of the surface cyclone will combine with lingering light to moderate 
snow across northestern Illinois to encourage blowing/drifting of snow and very 
low visibilities by this afternoon. This includes the Chicago area 
and surrounding communities. 

.Cohen/carlaw.. 02/01/2015 

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


Latitude...Lon 41988761 41538935 40778913 40438808 40588673 40838506 
41118383 41828431 42068552 41988761