Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211802 RRA
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1256 PM CDT Thu Jul 21 2016

.SHORT TERM.../Today/
Issued at 328 AM CDT Thu Jul 21 2016

A complicated forecast scenario is shaping up for today. Currently a
large swath of castellanus is blossoming from southern Minnesota
into northern and northeastern Iowa, with regional radar indicating
some light showers in far northeastern Iowa and adjacent states.
Meanwhile a large MCS is rolling across northern Minnesota. The main
push is west to east, however, the southern flank of the line has
thrown out an outflow boundary that is progressing south
southeastward at a good clip toward the Twin Cities. Showers and
thunderstorms continue to percolate on top of the shallow cold pool
behind this boundary. The model suite is not picking up well on
either of these features and solutions for convection today are all
over the place. However, the showers over northeastern Iowa are
largely being driven by a band of nocturnal warm air advection that
is expected to mix out after sunrise. Also, the outflow boundary
over Minnesota may or may not make it down into our northern
counties later this morning, but even if it does it will be far from
its parent system and forcing and should likewise wash out in the
mid to late morning hours. Therefore, have introduced some isolated
thunderstorms into our northern/northeastern counties early this
morning but ended them by noon. Later in the day, toward this
evening, we may see some redevelopment along any lingering outflow
boundaries in conjunction with peak heating and weaker capping in
the northeast, and have reintroduced low POPs accordingly.

Aside from precipitation/thunderstorm chances, the more hazardous
weather concern is the continuation of excessive heat today. Barring
the outflow boundary/storms from Minnesota making it further south
and lasting longer than expected, see no reason to deviate
substantially from previous issuances in terms of temperatures and
dewpoints. Mid-level ridging and temperatures aloft will continue to
be substantial and low-level/surface winds will have more of a
southwesterly component today, so expect temperatures to be higher
than yesterday. Even at this hour dewpoints across the area are in
the 75-77 range and will increase during the day, approaching 80
degrees at times. This results in forecast maximum heat index values
of 110 to 115 degrees across virtually the entire forecast area,
thus no changes have been made to the ongoing excessive heat warning.

.LONG TERM.../Tonight through Wednesday/
Issued at 328 AM CDT Thu Jul 21 2016

Somewhat of an uncertain forecast in the near term tonight into
Friday, with the upper ridge over the area and ring of fire
scenario in place. Very warm air aloft with the thermal ridge
building across the CWA today into Friday. Also a weak boundary to
drop into the CWA Friday and stall out before lifting back
northward Friday night into Saturday as a warm front. Weak
impulses topping the ridge could set off convection across the CWA
or near the area tonight into Friday. H7 temps around +13C to +14C
which should keep much of the area capped to daytime
convection...so therefore will keep this evening dry. Late tonight
a few models develop an MCS across WI closer to the edge of the
cap and along the nose of the theta-e ridge/push of moisture
transport, and try to back this MCS into Iowa into Friday morning.
Have included some slight chance pops for this scenario, if MCS
were to ride the edge of the cap, with uncertainty high in this
scenario as well. This could allow storms to linger through the
day across the south Friday, which would keep temps cooler and
heat indices down. For now have included the slight chance pops
but kept temps warm leaning toward the dry scenario. This would
keep advisory heat index values across much of the CWA and near
warning criteria in the south. Will continue the heat warning into
Friday, even with more advisory criteria conditions given it will
be the third day straight of oppressive humidity and heat.
Additionally with the boundary across the CWA moisture likely to
pool with dewpoints pushing close to 80 and overnight lows to be
quite warm with the increasing moisture.

More certain is the chance for precipitation late Friday night
into Saturday with the push of theta-e advection across the
west/northwest late Friday night, and then spreading across Iowa
Saturday as a stronger shortwave trough moves through the central
US dampening the central US upper level ridge. As this wave pushes
east into Sunday, the upper ridge to build back into the central
US briefly with more of a flattened ridge pattern/near zonal flow
into the next week. It does set up the pattern for additional
precipitation chances into the middle of next week, with more
seasonal temperatures by late in the weekend and early next week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1256 PM CDT Thu Jul 21 2016

VFR conditions forecast with a potential for brief MVFR cigs through
00Z across the Eastern TAF`s as they lie on the periphery of a high
pressure system extending from OK.  There is also a potential for
fog or haze with the Eastern TAF`s later tonight for the same
reason.  After 12Z a weak frontal boundary will sink across central
Iowa with weak shortwaves dropping across the state coincident with
the surface feature.  I expect to see a few more clouds...perhaps a
mid level cloud deck and possibly some isolated TSRA.  This would be
similar to what is over central/Eastern Nebraska at 18Z Friday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 7 PM CDT Saturday for Adair-Adams-
Appanoose-Boone-Clarke-Dallas-Davis-Decatur-Guthrie-Jasper-Lucas-
Madison-Mahaska-Marion-Marshall-Monroe-Polk-Poweshiek-Ringgold-
Story-Tama-Taylor-Union-Wapello-Warren-Wayne.

Excessive Heat Warning until 7 PM CDT Friday for Audubon-Black
Hawk-Bremer-Butler-Calhoun-Carroll-Cass-Cerro Gordo-Crawford-
Emmet-Franklin-Greene-Grundy-Hamilton-Hancock-Hardin-Humboldt-
Kossuth-Palo Alto-Pocahontas-Sac-Webster-Winnebago-Worth-Wright.

&&

$$

SHORT TERM...Lee
LONG TERM...Beerends
AVIATION...FAB


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