Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251147

Area Forecast Discussion
National Weather Service Des Moines IA
647 AM CDT Fri May 25 2018

...Updated for Short Term Trends and 12z Aviation Discussion...

Issued at 646 AM CDT Fri May 25 2018

Updated PoPs across far north central Iowa for the lone storm
that has been able to sustain itself early this morning. Also have
been watching a few of the latest CAMs coming in this morning and
seeing trends of convection perhaps being a bit farther southwest
than expected later today. Therefore, did slightly adjust PoPs a
bit farther southwest late today into this evening for the
convection that is expected to drop through the state. Otherwise,
remainder of the forecast remains on track.


.SHORT TERM.../Today through Tonight/
Issued at 356 AM CDT Fri May 25 2018

Regional radar composite shows an MCV over Kansas that kicked out a
cold pool that interacted with convection over southeast Nebraska
that brought about its demise. Farther east of the MCV over Kansas,
convection now crossing the Kansas/Missouri border near and south of
Kansas City has become reinvigorated with elevated convection
recently forming over northeastern Missouri. The HRRR along with 00z
HREF members NSSL WRF and WRF NMM show the convection near the KS/MO
border continuing to maintain or strengthen over Missouri with some
of the rain and storms spreading into our southeast forecast area
this morning. The bulk of the stronger storms should remain south of
the Iowa border however. Farther north over northern Iowa, GOES-East
imagery shows a few linear areas of relatively cooler cloud tops.
Some of the CAMs have been trying to indicate some light rain or
perhaps a storm, but 00z KOAX RAOB shows a good amount of mid-level
and low level dry air. Therefore, have maintained a dry forecast
this morning in this area.

The heat and humidity will continue today, though will be a bit
disrupted due to this morning`s convection, especially over eastern
Iowa. 850mb temperatures are similar to about a degree higher than
yesterday so highs today will be in the upper 80s to near 90 degrees
once again. While we heat up, the instability will also build with
MUCAPE between 1000 and 2000 J/kg by mid to late afternoon ahead of
a shortwave trough moving through the northern Plains. This trough
will help to better focus convection and deep layer shear will be
better than recent days at 25 to 30 knots. Convective initiation is
expected toward 3 or 4 pm over southern Minnesota to near the Iowa
border. Latest runs of the HRRR and ESRL HRRR show linear segments
trying to congeal into an MCS moving southeastward across the area,
especially eastern Iowa late this afternoon into this evening. The
00z HREF has spread in placements and timing ranging in area from
northwest Iowa to mainly east of I-35. As was noted in the past
discussion, the idea of an MCS seems plausible given RAP 0-3km theta-
e differences of 30 to 35K allowing for cold pool to develop and aid
in storm propagation and favor more damaging wind gusts with a
lesser threat of large hail. Most of the convection should exit the
forecast area near or shortly after midnight. After this time, cross
sections from the NAM, GFS, and ECMWF show pooling of low level
moisture possibly fostering patchy fog later tonight. The
temperature/dewpoint spread is around or less than 3 degrees, but
winds may be a knot or two too high to allow for much to develop.
Nonetheless, did go ahead and include patchy fog at this time if
winds are light enough.

.LONG TERM.../Saturday through Thursday/
Issued at 356 AM CDT Fri May 25 2018

Upper ridging across the central United States will be the major
player for much of the longer term, especially in regards to
temperatures.  The influence from the shortwave affecting Iowa today
into tonight will be waning quickly on Saturday with thermal ridging
in the Plains advecting east into the state during the day.  There
will be plenty of sunshine along with generally westerly flow at the
surface.  Deeper mixing by afternoon will push dewpoints lower as
well with temperatures responding nicely into the low to mid 90s in
most locations.  The lower dewpoints will keep heat index values in
check however, with values into the mid 90s in most locations by mid
afternoon.  Sunday will be much of a repeat if not slightly warmer
but with similar dewpoints.  Once again, the dewpoints will keep the
heat index values in the 90s.  These lower dewpoints will also help
overnight lows to drop into the 60s providing relief to the hot
daytime conditions, therefore heat headlines are not anticipated at
this point.  Will continue to mention the heat in the HWO and in a
Sit Rep along with the weather stories.

There is actually some cool advection from the southeast on Monday
into Tuesday which will moderate temperatures slightly on those
days.  Also by Tuesday, a trof across the western United States will
lift northeast with a surface boundary approaching from the west.
There will be increasing forcing, albeit modest but sufficient for
some low POPs at that time. The best threat for precipitation in the
forecast will be on Wednesday as forcing associated with the upper
system intensifies along with the surface boundary crossing the
state.  The push of cooler air on the backside of this system is
still weak with temperatures remaining above normal through the end
of the work week.


.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 646 AM CDT Fri May 25 2018

While VFR conditions prevail in this TAF package, there will be
opportunity for restrictions with challenges revolving around
timing and impact from thunderstorm chances primarily later today.
A storm over northwest Iowa and another cluster of storms over
northern Missouri early this morning have shown weakening trends.
Initially was planning on TSRA at OTM in the next few hours, but
have backed off to VCTS at this time. Otherwise, attention turns
to later this afternoon into this evening when thunderstorms will
move from northern Iowa southeastward. This will bring MVFR or
lower restrictions along with gusty winds, but model solutions
vary on timing so have a wide time window of VCTS. Patchy fog is
also possible to develop by sunrise Saturday morning, but spatial
uncertainty precludes mention.




SHORT TERM...Ansorge
AVIATION...Ansorge is the U.S. government's official web portal to all federal, state and local government web resources and services.