Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 302354

National Weather Service Des Moines IA
654 PM CDT Fri Sep 30 2016

...Updated Aviation Discussion...

.SHORT TERM.../This evening through Saturday/
Issued at 347 PM CDT Fri Sep 30 2016

Iowa will be stuck between a low over the Ohio Valley and a large
ridge over the Plains getting pinched back to the West. The net
result for the state will be a large stratus shield that will
rotate Westward dropping ceilings tonight. The clouds will hold
temps up tonight so lows were nudged up a degree or two. The flow
will also advect dewpoints that will be in the mid 50s so I am
expecting patchy fog with visibilities generally in the one to 3
mile range but low lying areas could see lower visibilities at
times. There has also been patchy drizzle noted on regional radars
and that will...with time...likely work into Eastern locations so
I did put in some patchy drizzle with the fog from KMCW to KOTM
late in the period.

.LONG TERM.../Saturday night through Friday/
Issued at 347 PM CDT Fri Sep 30 2016

The situation has changed little over the past few days with
minimal effects from the current OH Valley low over the weekend
and then a large system into the middle of next week. The current
OH valley upper low will drift northward into the LP of MI late
tomorrow. The forcing remains neutral or subsident over a good
depth, but moisture does increase in the lowest 3km east. This may
lead to sprinkles or light rain there until the moisture decreases
somewhat by the afternoon. Expect plenty of clouds and possibly
some nighttime fog through the weekend as low level moisture
lingers in the surface ridge.

The pattern will finally become more progressive into next week
however with this upper low and a western CONUS trough on the
move. This will initially bring upper level ridging and some
mild warming Monday into Tuesday. The models are in general
agreement with the large scale forcing into midweek, but do differ
with regard to the speed of the frontal passage. Both the GFS and
ECMWF suggest theta-e advection and mid level DPVA will overspread
Iowa west to east late Tue followed by more mechanically forced
but still deep lift with diffluence and DPVA into Wed which has
resulted in PoPs increasing to likelies during this transition. As
mentioned before the low level convergence contributions are
uncertain due to timing differences with the ECMWF suggesting a
more diurnally favorable frontal passage. The shallow and deep
shear remains quite high, but instability is low so the severe
weather potential would be conditional on whether the shear and
instability could be phased during peak heating. Even so local
studies have shown that even weak convection can produce weak,
subtle QLCS tornadoes when low level shear is extreme. Potential
is low, but something to monitor in the coming days.

The instability will likely diminish further into Wed Night but
PoPs continue as models suggest another surge of mid level
kinematic forcing through the base of the trough into the mid MS
Valley. The forecast turns dry Fri behind the long wave trough
with only slight cooling, and temperatures still near seasonal


.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 649 PM CDT Fri Sep 30 2016

MVFR stratus deck looks to settle into the state later tonight,
if not already in place across the eastern fringes of the forecast
area. OTM/ALO will likely remain under at least MVFR stratus for
the entire forecast period. Brief break anticipated at DSM/FOD
before the MVFR ceilings move back in later tonight. Have mention
of IFR ceilings and MVFR visibility towards the early morning
hours across all TAF sites and expect this low stratus deck to
persist through much of the morning Saturday.





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