Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 170442

Area Forecast Discussion
National Weather Service Des Moines IA
1142 PM CDT Wed Aug 16 2017

...Updated for 06z Aviation Discussion...

Issued at 706 PM CDT Wed Aug 16 2017

Biggest changes were to the PoPs, wind, and wind gusts for tonight
through tomorrow. Subjective analysis at 6:30pm shows low
pressure is over southwestern Minnesota near Worthington with a
cold front extending southward nearing Estherville, Fort Dodge,
Des Moines, and Osceola. 23z objective SPC mesoanalysis shows
MLCAPE ahead of the front is between 1000 and 1500 J/kg with
effective bulk shear generally 25 to 35 knots. Overall, the best
shear has stayed ahead of the best instability and forcing.
GOES-16 low level water vapor imagery shows plenty of dry air over
the cold front, though moisture is streaming northward into south
central Iowa from convection that is crossing from Kansas into

Looking at current radar and satellite trends, HRRR and ESRL HRRR
seem to have the best handling of the situation with dry slot
pushing into the western part of the state. Post-editing from the
HRRRs accounted for showers that have been trying to develop
along the front and may move into northern Iowa this evening. PoPs
were greatly lowered over the southwest forecast area through the
metro this evening. Still have some PoPs in the east and south,
especially southeast, where showers and storms may come out of
Missouri and clip the southeast forecast area.

Sustained winds and corresponding wind gusts were increased
slightly overnight as low pressure deepens a bit as it moves
eastward over Minnesota. Winds were also increased moreso tomorrow
as low pressure moves into the Great Lakes and with cold
advection prevailing for a good part of the day. Wind gusts over
will average between 20 and 30 mph with the highest gusts over
northern Iowa.


.SHORT TERM.../Tonight through Thursday/
Issued at 243 PM CDT Wed Aug 16 2017

Surface low pressure was located across northwest Iowa at 19Z with a
cold front extending southward into southwest Iowa. Convection thus
far today has been relatively light. Although instability exists,
lack of coherent forcing has limited the convection.

Expect the cold front to help provide some coherent forcing for
convection this evening and tonight as it slides eastward across
Iowa. CAMs keep most of the convection to the south in Missouri and
southward, however cannot rule out an isolated severe storm across
southern Iowa. Damaging wind and large hail would be the primary

Wrap-around moisture and instability behind the system on Thursday
will keep chances of showers and possibly thunderstorms mainly
across the northeastern CWA.

.LONG TERM.../Thursday night through Wednesday/
Issued at 243 PM CDT Wed Aug 16 2017

Thursday Night through Sunday Night:

Confidence: Medium

Main concern will be next upstream short-wave/frontal boundary that
is expected to arrive Friday into Friday night. Quiet weather will
continue into the night with lows in the 50s to lower 60s. Bulk
shear will be increasing through the morning/afternoon
potentially a few storms may be strong to severe and we continue
outlooked for marginal. Highs will be similar to the past few days
with upper 70s to mid 80s in the south. There are some minor
differences between the GFS/Euro as for timing and best forcing. The
Euro is a bit farther north with the better forcing Friday into
Friday evening.  After a quick exit warm air advection will return
for late Saturday into Sunday as another system begins to move into
the Great Plains. Both the GFS/Euro are hinting at increased warm
air advection into the northwest Sunday...which may lead to late day
thunder/storms over the far northwest. As a warm front lifts
northeast into Minnesota Sunday night convection will be favored
over the north/west during the nighttime hours.

Monday through Wednesday

Confidence:  Low to Medium

Though the pattern is active through the period...timing differences
based on model choice and consensus continue to lead to low
confidence. Models diverging Monday with the GFS farther south
with the main forcing compared to the European model. The GFS
develops an MCS over eastern Nebraska by 06z Monday and slowly
decays the activity through 18z...taking the complex over western
Iowa/northwest Missouri through 18z. Previous model packages were
in better agreement with timing/placement of features. Given the
interest in the forecast for Monday...too early to have a
definitive answer regarding cloud cover/rain chances given the
current information. H850 temperatures continue to be warm both
Sunday into Monday and if sunshine is present...should allow highs
to reach well into the 80s both days...especially across the
south. Models continue to focus more of the thunderstorms through
the overnight hours Monday night into Tuesday with some lingering
storms in the south later in the afternoon. There remain some
timing difference later Tuesday night into Wednesday.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1142 PM CDT Wed Aug 16 2017

Low clouds have begun to wrap low pressure over south central
Minnesota into northwest Iowa with MVFR ceilings moving over the
northern terminals tonight. Ceilings are expected to drop into IFR
at KMCW toward daybreak and this is also the terminal with the
best chance of spotty showers tomorrow morning. IFR conditions are
possible at KFOD and KALO, but still uncertain if those low of
ceilings will be that far south. Ceilings will improve into VFR
toward midday Thursday and into the afternoon hours. For the
southern terminals, expect VFR ceilings. Otherwise, winds from
the west will become from the northwest and turn gusty tomorrow
morning, especially over northern Iowa. Winds will decrease
around sunset Thursday.




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