Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 232018

Area Forecast Discussion
National Weather Service Des Moines IA
318 PM CDT Sun Jul 23 2017

.SHORT TERM.../Tonight through Monday/
Issued at 316 PM CDT Sun Jul 23 2017

Confidence: Medium to High

Secondary cold front moving into northern Iowa early this afternoon
and will progress south tonight.  Fair weather cumulus accompany the
boundary with the better convergence over Wisconsin where a few
isolated thunderstorms are occurring.  Main focus this period will
be cooler overnight lows...residual boundary layer moisture and fog
potential. Once sun sets temperatures will fall into the mid 50s to
lower 60s by 12z. Light winds with high pressure building into the
area will allow for strong radiational cooling.  Though dewpoints
are also falling behind the boundary...near to saturated boundary
layer conditions will develop over the north/northwest between 4 and
7 am. This should lead to pockets/areas of thick fog primarily in
this region.  For now will hold off on headlines...though evening
and overnight shifts may need to issue an advisory for fog in a
portion of west/northwest areas.  After the fog burns off Monday
morning...light winds and a comfortable day are expected.  Northeast
areas will recover to the upper 70s and the central/south to the
lower to mid 80s. Models indicate some clouds over the northeast
mainly fair weather cumulus by late morning into the afternoon.

.LONG TERM.../Monday night through Sunday/
Issued at 316 PM CDT Sun Jul 23 2017

Monday night and Tuesday we will be in a warm advection regime with
an upper level ridge of high pressure aloft and a surface high
slipping to the east bringing a southerly flow to the state. a warm
front will reside to the west, over eastern Nebraska so on Tuesday,
the warmest temps will be across the west, with highs in the mid
90`s. The GFS continues to model some precip developing in the
vicinity of the warm front and shift it into Iowa early Tuesday but
it is the only model doing this and with the boundary expected to be
west, I think any precip should remain west so I left this period

Tuesday night into Wednesday, a trough associated with a Canadian
low will flatten the ridge over the region, allowing the base of
this trough to sweep northern Iowa.  At the same time, a frontal
boundary will extend from northeast MN to roughly Sioux Falls and
gradually advance southward.  This will bring a chance for storms to
northern Iowa.  The front will advance across the forecast area
Wednesday bringing a chance for storms across central Iowa through
the day.  Shear will increase through the day into the evening on
Wednesday especially along the front so a few severe storms are

By 12Z Thursday the front will stretch across southern Iowa and slow
some keeping a chance for storms going across the far south through
early to mid morning before exiting to the south and east.

After Thursday, the upper ridge rebuilds across the west while a low
drops into the Great Lakes with a surface high over Wisconsin and
Minnesota nosing into IA.  The result will be dry conditions and
cooler temps with highs in the upper 70`s to lower 80`s northeast to
the mid 80`s southwest.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1249 PM CDT Sun Jul 23 2017

Main concern will be coverage and density of fog tonight through
14z Monday along with potential IFR/MVFR cigs with fog. Hires/guidance
hitting most of northern/northwest hard overnight. Despite
lowering dewpoints...cooler air will filter in behind a secondary
cold front and allow temps to drop through morning hours.
Expecting the most affected areas from KFOD to near KMCW with
lesser impacts for KALO/KDSM/KOTM. Possible 1/4sm at KFOD with
1-3sm KALO/KMCW from 11-13z. Aft 14z...fog should dissipate with
little impacts remainder of period. /rev




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