Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180850

Area Forecast Discussion
National Weather Service Des Moines IA
350 AM CDT Thu Aug 18 2016

.SHORT TERM.../Today/
Issued at 350 AM CDT Thu Aug 18 2016

Forecast confidence = Medium/High.

00Z upper air analysis and water vapor imagery indicated a series of
low amplitude shortwaves moving across portions of the upper Midwest.
One wave appeared to be triggering convection over far eastern Iowa
into Illinois with a stronger and more impressive wave over MT/ND.
At the surface...a weak low was analyzed over central portions of ND/SD
with some semblance of a weak sfc trough stretching from west to east across

Early this morning...mostly clear skies...light winds...and residual PBL
moisture is creating a favorable set-up for patchy radiational fog.
While visibility has dropped to at or below one quarter mile in
appears that it is too patchy and variable to issue any fog headline so
will continue to highlight impacts via an SPS.  Believe the fog should
gradually diminish after daybreak.

Otherwise...expecting a very warm and humid day under partly to mostly
sunny skies.  H8 temps are forecast to rise about 3C the warming
trend into the upper 80s to low 90s seems on track. With dewpoint values
in the 70s...heat indices will likely approach 100F in places but will
forgo any heat headlines at this time.  Also expecting much of the day to be
dry as warming mid-levels should keep the environment capped.  However...
a few convection allowing models /CAMs/ are suggesting the possibility of an
isolated storm or two during peak heating over far northern or western Iowa...
so will pass along this concern to future shifts for further monitoring.

.LONG TERM.../Tonight through Wednesday/
Issued at 350 AM CDT Thu Aug 18 2016

Main challenges for the period are lining up with greater consistency
in the past few model runs resulting in higher forecast confidence.
Late evening into the overnight hours MCS north into South Dakota/MN
may brush the northern counties toward 09-12z timeframe. Otherwise
much of the area will remain very warm and humid with light winds
across the region. Despite higher moisture and light winds...thickness
values remaining high tonight will likely reduce the risk of any
appreciable fog...if any. Friday will once again be very warm and
humid over the south half with highs moving into the mid to upper
80s over the region. Across the north cloud debris and increasing
forcing from the main short wave will keep highs cooler then the
south. By mid afternoon QG forcing will increase rapidly northwest
areas with thunderstorms quickly responding to the increased lift.
This wave will track southeast into southern Iowa by 06z with the
trailing shortwave catching up with the first. This will result in
continued shower and thunderstorm chances all night with widespread
rainfall. Continued with trends the past few packages of raising
PoP Friday evening and overnight as nearly a certainty to have
measurable precipitation over most of the region by 12z Friday.
Looking at chances for locally heavy rainfall once again near and
just along the boundary Friday evening. Subjective 00z H850 analysis
tonight indicates a subtle change to slightly less moisture over
the region...though still ample. Forecast warm cloud depths have
lessened by about 500m from previous nearly 4000m forecasts and
precipitable water is now running closer to 1.5 to 1.75
inches...slightly less as well. Despite the slightly weakening
trends...still expect some healthy 1 to 2 inch amounts possible in
stronger convective bands. The severe risk remains the same with a
slight risk splashed over the entire forecast area. Once again the
better 0-6km shear lags the boundary a bit but initial instability
will likely overcome any lack of instability...enough at least to
warrant a severe threat. Slight modifications in coverage may
still occur in the next 24 hours. Lows Friday night will quickly
responds to the cooler airmass heading southeast into the region.
Upper 50s north to the upper 60s southeast. By Saturday the system
pulls far enough southeast by afternoon that some sunshine should
be realized. Depending on the actual separation between several
H500 shortwaves tracking east southeast...partly cloudy conditions
may return soon enough for a pleasant afternoon. Still will have
to watch for potential stratus though...with the cooler air moving
into the region. Sunday will see cooler and pleasant weather with
a surface high gradually shifting east by Monday. The remainder of
the period shows increasing H500 heights as the upper level
pattern becomes zonal and warmer air builds toward Iowa from the
western Plains. Highs should rise into the 80s by Tuesday with
another system approaching on Wednesday. This looks to bring
another chance of thunderstorms by midweek with generally warm and
humid weather.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 112 AM CDT Thu Aug 18 2016

Clear skies...light winds...and residual boundary layer moisture
is leading to patchy fog as of 06z. The terminals most likely to
be impacted are KMCW/KALO/KOTM...although this fog could spread
to all terminals towards daybreak. Due to the shallow nature of
the fog...expect that vsbys will bounce around between VFR and
IFR...with even LIFR at times. Fog should dissipate by mid-morning
with VFR conditions expected the remainder of the day with just
some diurnal cumulus or high cirrus clouds. Light and variable
winds overnight will become southerly by Thursday afternoon.




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