Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 181457
SPC MCD 181456

Mesoscale Discussion 0277
NWS Storm Prediction Center Norman OK
0956 AM CDT Wed Apr 18 2018

Areas affected...portions of northern and northeast IA

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 IA while the left exit region of a
100-kt 300mb jet is placed over eastern IA.  Strong DCVA (170m 12
hour height fall per Omaha, NE 12Z RAOB 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 IA 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 for graphic product...


LAT...LON   43649405 43619142 43449111 43159119 43059146 43169388
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