U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 191248 
Storm Prediction Center ac 191246 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0746 am CDT Thu Apr 19 2018 

Valid 191300z - 201200z 

..there is a marginal risk of severe thunderstorms for parts of 
central and southern New Mexico and far West Texas... 

Isolated severe thunderstorms may impact parts of central and 
southern New Mexico and far West Texas late this afternoon and 
evening with a risk for downburst winds and hail. 

In mid/upper levels, a complex cyclone will shift eastward across 
the northeast and Canadian Maritimes through the period, while a 
progressive/split-flow pattern will dominate the western U.S. A 
large cyclone -- now apparent in moisture-channel imagery over much 
of California/Nevada -- will move eastward to Utah/Arizona and western parts of Colorado/nm 
by the end of the period, with the 500-mb low over Utah at 12z. 

At the surface, a large, Continental/polar anticyclone will spread 
across much of the central/eastern U.S. In the wake of a low-level 
cold front related to the departing northeastern cyclone aloft. 
This will preclude substantial moisture return or destabilization 
except for a narrow corridor over and near the mountains of northern 
Mexico, around the western part of the residual frontal zone now 
stalled across northern portions of coahuila and Chihuahua. 

..central/southern nm, far West Texas... 
Widely scattered thunderstorms are expected to develop over northern 
Chihuahua and then southern nm this afternoon into early evening, 
shifting eastward to northeastward over the outlook area. While 
storms are surface-based or very nearly so, isolated severe gusts or 
large hail may occur. 

The aforementioned ribbon of convectively favorable moisture return 
through northern mx should extend northward over the marginal-severe 
outlook area this afternoon and evening, juxtaposed with surface 
heating of the desert valleys and higher terrain. The air mass over 
the region is quite dry at this time; however, some lower-elevation 
surface dew points in the 30s to low 40s f should result from moist 
advection by mid/late afternoon. This process will start sooner in 
northern Chihuahua this afternoon, where stronger heating and 
somewhat higher precipitable water will combine with low-level convergence and 
orographic forcing to erode mlcinh and boost MLCAPE to near 1000 
j/kg. Buoyancy should diminish gradually northward across southern 
nm, while becoming less surface-based eastward over The Trans-Pecos 
and Big Bend regions. 

Storms will have little room to access favorable boundary-layer 
parcels before moving atop a more stable/lower-Theta-E air mass this 
evening, even though a general thunder risk may persist eastward 
toward the Pecos Valley. Within the narrow zone of favorable 
MLCAPE, however, well-mixed subcloud layers beneath that buoyancy 
will support maintenance of any hail to the surface, as well as some 
downburst potential. Deep shear should be increasingly favorable, 
with the juxtaposition of surface southeasterlies under 
strengthening mid/upper gradient winds ahead of the cyclone. 
Forecast soundings show effective-shear magnitudes 45-55 kt, along 
with sufficiently straight hodographs to support splitting cells 
with both right- and left-moving offspring. 

.Edwards/Goss.. 04/19/2018 


Mesoscale Discussion

acus11 kwns 181457 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 181456 

Mesoscale discussion 0277 
National Weather Service Storm Prediction Center Norman OK 
0956 am CDT Wed Apr 18 2018 

Areas affected...portions of northern and northeast Iowa 

Concerning...heavy snow 

Valid 181456z - 181800z 

Summary...heavy snowfall rates ranging from 1 to 1.5 inches per hour 
are expected through midday. 

Discussion...water-vapor imagery this morning shows an eastward 
migrating shortwave trough over Iowa while the left exit region of a 
100-kt 300mb jet is placed over eastern Iowa. Strong DCVA (170m 12 
hour height fall per Omaha, NE 12z radiosonde observation data) coupled with low- to 
mid-level frontogenesis is resulting a west-east corridor of snow 
from Sioux Falls to the MS river. Weak convection as evidenced by 
lightning is moving east on the leading edge of stronger ascent 
located within the warm conveyer. 

Model forecast soundings show weak buoyancy co-located with a 
dendritic growth layer for a few hours this morning. Expecting the 
heavy snowfall rates to continue through midday across northern and 
northeast Iowa before gradually diminishing this afternoon. Although 
the precipitation shield will spread east into southern WI, models 
suggest a gradual reduction in the spatiotemporal character of the 
heavy snowfall rates as this area moves east into WI. 

.Smith.. 04/18/2018 

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


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