Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 210415
SWOMCD
SPC MCD 210414
IAZ000-NEZ000-SDZ000-210545-

Mesoscale Discussion 1537
NWS Storm Prediction Center Norman OK
1114 PM CDT Sun Aug 20 2017

Areas affected...Northeast NE...West-Central/Central IA

Concerning...Severe potential...Watch possible

Valid 210414Z - 210545Z

Probability of Watch Issuance...40 percent

SUMMARY...Thunderstorms capable of large hail are anticipated across
the region. Trends will be monitored for possible watch issuance.

DISCUSSION...Regional radar imagery has shown an increase in
thunderstorm activity over the past hour amidst a strengthening
low-level jet. This low-level jet will contribute to persistent warm
air advection/isentropic ascent within the frontal zone draped
across the region. Additionally, the low-level airmass upstream is
very moist, evidenced by surface dewpoints in the upper 60s/low 70s
and a mean mixing ratio of 16.7 g/kg on the 00Z TOP sounding.
Continued isentropic ascent within this moist airmass will result in
increasing storm coverage. Based on the 00Z OAX and recent RAP
sounding, storm bases will likely be around 850 mb/1 km.

These elevated storms will occur in an environment characterized by
steep mid-level lapse rates (7.7 deg C per km on the 00Z OAX
sounding) and moderate low and mid-level shear. Latest OAX VWP
showed strong directional shear within the lowest 3 km. These
environmental factors support strong, organized updrafts capable of
producing severe hail and a watch may be needed to cover the
anticipated severe threat.

..Mosier/Goss.. 08/21/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...DMX...FSD...OAX...

LAT...LON   41009435 41549572 41769721 42319778 42829726 42829604
            42549480 41949340 41149345 41009435



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