Severe Storm Outlook Narrative (AC)
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ACUS48 KWNS 190841
SPC AC 190839

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0339 AM CDT Tue Sep 19 2017

Valid 221200Z - 271200Z

A persistent longwave trough over the West will be key to most
severe weather concerns for the extended forecast period.  This
trough will spread south-southwesterly mid/upper flow across the
Plains and into the Upper Mississippi Valley for much of the
forecast period, with a slowly moving surface boundary separating
cooler continental air to its north from modified tropical airmass
to its south.  Some severe risk will be likely on D4/Friday
afternoon and evening, where models focus convective potential in
portions of Minnesota and vicinity within a strongly sheared,
moderately unstable airmass.  Some difference in depiction of key
synoptic features preclude Slight/15% probabilities for this
outlook, although the region will be monitored.

From D5/Saturday and beyond, the aforementioned surface boundary
will migrate slowly southward across portions of the High Plains and
likely provide a focus for deep convection.  At least a Marginal
severe risk is likely with this activity, although subtle
differences in guidance continue at this time range and widespread
clouds/precipitation may limit the degree of destabilization in some
areas.  These factors preclude 15% or higher severe probabilities at
this time.

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