Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251141
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
641 AM CDT Tue Jul 25 2017

.SHORT TERM.../Today through Tonight/
Issued at 338 AM CDT Tue Jul 25 2017

Surface ridge continues to slide east early this morning with
increasing warm advection and as southwest surface winds strengthen.
Low level jet is focused into western Minnesota this morning where
bulk of convection is currently found.  While these storm should
remain north of the state through mid morning, an isolated storm
cannot be ruled out in far northern Iowa where theta-e advection
will be fairly decent.  However, much of today will see Iowa within
the warm sector of the system with increasing southerly flow through
the day.  Thermal ridging from the Plains will edge into the western
half of Iowa with temperatures climbing into the lower 90s in the
west.  Dewpoints will also be higher as moisture transport continues
into the state and anticipate Heat Index values climbing to 100 to
105 in the southwest quarter of the forecast area. This is not quite
advisory criteria and will be today only.  In addition, current
readings and dewpoints are quite seasonable, therefore no headlines
are necessary at this point.

For tonight, main axis of instability will be from western Iowa into
western Minnesota and far eastern SD.  Focus for convection will be
near surface low and boundary from South Dakota into Minnesota.
Farther south into Iowa, convective inhibition and lack of
substantial forcing will limit thunderstorms chances for much of the
night.  Eventually, the convection to the north and west will
approach the forecast area toward daybreak on Wednesday.  Therefore,
have included pops in the far north and west after midnight.
Otherwise, temperatures will remain quite warm overnight with decent
south winds and relatively high dewpoints over the state.

.LONG TERM.../Wednesday through Monday/
Issued at 338 AM CDT Tue Jul 25 2017

Synopsis...
Yet another large-scale ridge sets up over the southern third of the
CONUS... however... the core of this ridge is much further east
compared to the setup from last week that brought the excessive heat
to Iowa. In fact, for the duration of this long term fcst period
through next Tuesday or so, Iowa has a large northerly component to
the flow from 850mb. Subsequently, long-range models in great
agreement with bringing 850mb temps into the +12C to +15C range for
most of this long-term fcst period...a stark contrast to the +22C to
+25C temperatures from last week. Wednesday PM into Thursday morning
could feature heavy rainfall across much of the CWA. SFC high
pressure then propagates across the upper Midwest, generally keeping
us dry into the weekend.

Wednesday PM - Thursday AM Heavy Rainfall potential
Confidence: Medium-High

Bottom line up front...
Heavy rainfall is possible across much of the central to
especially the eastern/northeastern portions of the DMX CWA.
Flash flood/flood headlines may certainly need to be issued for
our eastern/NE counties this afternoon, to begin sometime
Wednesday PM.

Synoptics...
06z Tue water vapor imagery picking up on a shortwave centered
near the Four Corners region. This wave is expected to ride along
the above-mentioned ridge through the plains and towards Iowa. A
sfc reflection is slated to push through Iowa, making it towards
the eastern part of the state by 06z Thu. Slight disagreement in
precise track as ECMWF/GEM remain strong outliers to the
northeast, and the NAM being much more intense with the system.
However, trends are the ECMWF pushing further SW and the GFS
pushing slightly NE, so perhaps there is some convergence in
solution. Overall, am favoring the 00z Tue GFS. Given how last
week went cannot discard the intensity of the NAM.

Mesoscale...
The past several model runs have across the board shown a slug of
PWATs over 2.5 inches to push through Iowa, which would be near-
record for this time of year. 0-6km MUCape values reach the 1000
to 1500 J/KG range, with widespread mixing ratios of 8 g/kg to
accompany warm cloud depths over 4500m. In short, somewhere is
going to get some heavy rainfall, with higher totals over 2 inches
seeming very plausible. As of right now, the focus is over
central to eastern/NE Iowa. Obviously NE Iowa is very saturated
and cannot take high amounts of rain attm, so flash flood/flood
potential will need to be watched very closely.

The KDSM NAM fcst soundings completely saturate the profile by 00z
Thu. Resultingly, low to mid-level lapse rates are not that
impressive, and with these deep warm cloud depths, think severe hail
will be tough to come by. Overall damaging wind setup not very
impressive either, though cannot be ruled out as the primary threat.
Tornado threat early on, primarily across southern Iowa, cannot be
ruled out either. As night sets in, nocturnal LLJ kicks up into
Iowa, transitioning any remaining severe threat into a heavy
rainfall threat.

Thursday and beyond...
Confidence: Medium-High

By mid-morning Thursday, much of the precipitation should be
southeast of the area. With high pressure and large- scale
subsidence riding in quickly upon the heels of this system, am
expecting sunshine by Thursday afternoon and dry conditions with
near-normal temperatures into the weekend. Perhaps by early next
week, we will see return flow & warming conditions and
thunderstorm chances return.

&&

.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 641 AM CDT Tue Jul 25 2017

Surface winds will increase out of the south today with some gusts
by midday at most locations. Widespread VFR conditions will exist
for the duration of the forecast period with unrestricted
visibilities.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Cogil


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