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WPC Met Watch

Mesoscale Precipitation Discussion: #1084
(Issued at 159 PM EDT Tue Sep 19 2023 )

MPD Selection

Graphic for MPD #1084

Mesoscale Precipitation Discussion 1084
NWS Weather Prediction Center College Park MD
159 PM EDT Tue Sep 19 2023

Areas affected...central Iowa

Concerning...Heavy rainfall...Flash flooding possible

Valid 191758Z - 192200Z

Summary...A relatively focused band of storms persists just north
of Des Moines and may continue for another 2-4 hours or so. 
Localized instances of 2+ inch/hr rain rates could result in
excessive runoff during that timeframe.

Discussion...A relatively focused band of storms has persisted
from just northeast of Omaha to just north of Des Moines most of
the morning.  These storms have outlasted several models/CAMs
representations, and appear to be supported by 1) modest
convergence on the northwestern edge of 20-30 knot 850mb flow and
2) very steep lapse rates aloft (exceeding 8C/km per SPC
mesoanalysis) to support robust updraft strength.  PW values of
1-1.2 inch aren't particularly high, although the east-west
orientation of convection is continuing to favor convective
training and spots of 2 inch/hr rain rates (estimated per MRMS)
between Des Moines and Ames recently.  With additional
precipitation over west-central Iowa expected to traverse this
region, it appears that at least an isolated flash flood risk will
exist over the next couple hours especially as storms gain
mesoscale organization.

The temporal extent of this isolated flash flood risk is unclear.
The presence of an upstream mid-level wave over western Iowa
should continue to aid ongoing convection across the state through
the afternoon, while any low-level boundaries resulting from
ongoing storms should also help to maintain updrafts.  12Z HREF
guidance suggests continued potential for thunderstorm activity
throughout peak-heating hours along with a continued west-to-east
orientation favoring localized training.  Flash flood potential
may not become widespread, but a conditional risk should exist
through at least 22Z.




LAT...LON   42639459 42639449 42409282 41829184 41019176
            40939345 41159579 42379595

Last Updated: 159 PM EDT Tue Sep 19 2023

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