Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 130442

Area Forecast Discussion
National Weather Service Des Moines IA
1142 PM CDT Sat Aug 12 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 300 PM CDT Sat Aug 12 2017

Little change to forecast expected through Sunday. Subjective
surface analysis at 13z shows expansive ridge of high pressure from
the surface to H700.  Aloft H500 wave also visible on the water
vapor imagery continues to move slowly toward the region. Lower
levels show lee side development taking place over South Dakota at
the surface with an H850 trough beginning to take shape over the
Western Plains at H850.  Overall trends remain the same...with
initiation to take place west of our area later today and slowly
move to the east northeast tonight...tracking mainly with the low to
mid level warm air advection/thetae advective processes. Both the
Euro and the GFS suggest that the better forcing will again remain
north of the region...with northern Iowa seeing better forcing than
the remainder of the forecast area. Along with the chances for
precipitation Sunday during the morning/afternoon hours
cover should hinder afternoon warming north areas while the south
with some afternoon sunshine will again drift into the lower 80s
by late afternoon. Instability remains rather overall
either isolated to scattered thunder is anticipated over the north
Sunday. Lows tonight will be similar across the east...where
generally clear skies and light winds will promote decent
radiational cooling with the high still influencing the region...
while increasing cloud cover west will retard cooling to some
extent. Typically warmer conditions will be found in the metro.
Highs Sunday will struggle to reach the lower 70s north and reach
the lower 80s south.

.LONG TERM.../Sunday night through Saturday/
Issued at 300 PM CDT Sat Aug 12 2017

Somewhat active more summerlike weather pattern anticipated during
the period...though models are not in great agreement with the
details at this time. Main concerns currently are the extent of
forcing and placement of best forcing. Overall both the GFS/Euro
continue to trend of lingering showers/possible thunder over the
western/northern sections through early Monday morning...weakening
with time. The Euro suggests a weak boundary will be the focus of
additional development Monday afternoon over the west/south with the
GFS mainly dry. Some of the uncertainty is due to the very weak
waves tracking east through the flow. With the main system passing
east by Sunday night...prefer the drier solution for
least through 00z Tuesday. The GFS also develops return flow by 00z
Tuesday and with a weak sfc boundary forming the better push of
isentropic lift appears to be focused more over Nebraska into
western Iowa than the remainder of the forecast area. Overall the
best chances in the 7 day period for precipitation over a widespread
area is from late Tuesday night through Wednesday.  However the
pattern still suggests both a southern stream/northern stream wave
propagating through the flow Tuesday night into Thursday. As has
been the case for many instances this summer...moisture has been
limited after the southern stream wave has developed keeping the
better moisture channel either east or south of the region. Despite
this will maintain higher PoP Tuesday night into Wednesday night
with precipitation ending on Thursday. There remain some significant
differences regarding the expected wind fields/strength between the
GFS and Euro for the midweek system.  The GFS is much stronger with
H850/H300 winds by 00z Thursday over the east half of the area while
the Euro model higher amplitude winds south into Missouri. Given the
lack of consensus in forcing...the risk for severe storms is not a
certainty though increased instability by midweek may be enough to
support stronger storms either later Wednesday or into the overnight
hours. Still a few days to examine future model packages. For the
remainder of the forecast period there is an additional wave/cool
front that moves east into the region by Friday into Friday night.
With additional upper level support and some intermodel consistency...
showers and thunderstorms will be a possibility Friday into Friday
evening. Highs through the period will be warming from Monday
through Wednesday...then steady off in the mid 80s over the south
with 70s to lower 80s over the north. Lows should remain in the
60s to lower 70s.


.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1136 PM CDT Sat Aug 12 2017

Only aviation forecast concern through the next 24 hours will be
the potential for showers and thunderstorms in northern Iowa.
Chances for TSRA activity will steadily increase through the day,
and is mentioned in the KMCW and KFOD taf after 00z. The potential
for earlier timing will need to be resolved with future forecast
issuances. Otherwise VFR conditions will prevail for the current
taf period.




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