Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 160841

Area Forecast Discussion
National Weather Service Des Moines IA
341 AM CDT Sun Jul 16 2017

.SHORT TERM.../Today through Tonight/
Issued at 339 AM CDT Sun Jul 16 2017

The primary concern through the period will again be subtle
convective trends through peak heating. The current northern Great
Lakes short wave will continue to push a trough and trailing weak
cold front slowly south through Iowa today. Weak convergence
associated with the trough was just passing through a Carroll to
Ames to Waterloo line at 07z and is expected to just be exiting
southern IA into MO during peak heating of the afternoon or early
evening. Models suggest uncapped 2-3K j/kg MLCAPEs will be in place
by this time as 70 dewpoints continue to pool near the boundary. 0-
6km shear will be fairly weak, 30kts at best this far west, which
should limit our severe weather potential somewhat. However a strong
to severe storm cannot be ruled out considering the instability.
Could be a storm or two with hail and especially wind considering
the degree of dry air above 1.5km.

Any lingering convection should quickly exit by 00z or shortly
thereafter with precip ending. The Great Lakes surface high will
funnel some drier air into eastern sections but the airmass will
remain fairly muggy farther west. There is some fog potential, but
not great enough to mention as of yet.

.LONG TERM.../Monday through Saturday/
Issued at 339 AM CDT Sun Jul 16 2017

The overall pattern has not changed much in the long term but the
finer details will make the difference and models are still having a
tougher time resolving this.

On Monday the upper ridge axis shifts east putting western Iowa in a
deeper southerly flow allowing a low to develop over the northern
Plains.  Western Iowa will see highs in the mid 90`s while the rest
of the state is 10 to 15 degrees cooler than that.  Monday should
remain dry as well.  Late Monday night a shortwave flattens the
ridge some and pushes it east allowing the frontal boundary to
creep into south central MN then back towards Sioux Falls.
Thunderstorms will develop along/near the boundary and by 12Z some
of these may approach the far NW.

Tuesday morning that first wave passes east but the ridge does not
recover before another shortwave passes across southern MN/northern
IA and dropping the frontal boundary a little closer to IA and
perhaps into northern IA.  This will bring the focus for storms now
into IA and with the proximity of the boundary and decent
instability with increasing shear as the boundary sags further into
IA a few storms could be severe.  South of this boundary 850 temp s
are +20 and higher so temps will be hot both Tuesday and Wednesday
with highs in the mid 90s.  Cloud cover and morning convection is a
concern at this point so I am still not going with an advisory
because it appears we will see enough cloud cover to potentially
mess with highs.  Of course any storms/rainfall would alos impact

Wednesday is when there is bigger discrepancy with the models as the
Euro advances the shortwave east as does the Canadian but the
latter brings a second wave in during the day.  The GFS is just
slower in the progression of waves all together.  Either way by 12Z
Wednesday the frontal boundary moves little with still hot
conditions possible across the southern two thirds of Iowa.  We may
actually be warmer Wednesday because  we should have less storms
over IA in the morning and more of a late day/evening event.

This scenario plays itself over again for Thursday and it is not
until Friday when the boundary is finally south of the state that we
see somewhat diminished chances for storms.  However there is some
indication that the ridge will try to rebuild to the west putting
northern/eastern IA in northwest flow and susceptible to shortwaves
riding the flow and more precip chances.  Too bad the precip won`t
be more evenly distributed.  Once again northern/eastern IA has the
best chances while the parched sections of the south and west have
lesser chances.


.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1156 PM CDT Sat Jul 15 2017

VFR conditions through the period. Some low VFR clouds possible
with frontal passage late tonight into early Sunday. Westerly
winds becoming northerly then easterly through the period.




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