Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251750

National Weather Service Des Moines IA
1250 PM CDT Tue Oct 25 2016

.SHORT TERM.../Today/
Issued at 327 AM CDT Tue Oct 25 2016

Precipitation chances will gradually increase today ahead of an
approaching storm system. The best rain chances will be across the
north today, where a band of warm air advection and frontogenetical
forcing aloft should generate intermittent light showers. Radar
imagery currently depicts this zone across our north, but as yet
there is too much dry air in the column and no rain is reaching the
surface. Later today more broad and stronger forcing for vertical
ascent will move in from the west, in conjunction with the mid/upper
level trough, but most associated precipitation will reach our
forecast area tonight. Meanwhile, temperatures will be held in
check today by thickening clouds, east-southeasterly low-level
flow and occasional showers. Have maintained high temperatures
only in the mid to upper 50s across most of the service area.

.LONG TERM.../Tonight/...Confidence High
Issued at 327 AM CDT Tue Oct 25 2016

Short wave currently over Nevada will cruise over the H500 ridge now
cross the Plains and amplify this evening near Iowa as it begins
to encounter the backside of the east coast H500 trough.
Sufficient height falls and increasing moisture transport as well as
warmer air will aid in thunderstorm development overnight.  Though
there are some differences in the placement of the surface low and
track of the storm...high confidence for rain...nearing 100% tonight
along with thunderstorms as well. There is some concern and
increasing of a severe weather threat into the evening and
possible overnight hours. With the surface low near the I80
corridor...0-6km bulk shear will be 45 to 50kts...surface warm
front aligning east to west over the corridor as well. Despite the
overnight occurrence there will be MuCape of 400-800 j/kg and
0-3km SRH values of 400-500m2/s2 will be common along the boundary
as well. Will need to watch model trends through the day today for
potential late evening severe threat. Lows will be quite warm
south of the boundary with showers and thunderstorms most areas
and potential for 1 to 1.5inches of rainfall across the north...
lesser amounts south.

Wednesday through Friday...Confidence Medium to High

Exiting system Wednesday will bring in clouds and cooler flow as the
storm quickly moves east. Highs may struggle to reach the lower 50s
north while the south may see lower 60s early in the day. High
pressure will settle into the region overnight with clouds
continuing. This will keep temperatures from falling too far...but
still cool.  A rapid return to southwest flow will occur on Thursday
as west northwest flow and adiabatic warming returns. Highs will
quickly lift into the 50s to 60s from east to west. A ridge aloft
will amplify through Friday with afternoon highs Friday in the mid
70s or higher.  Some differences remain in model output but strong
southwest flow...good mixing...and H850 temperatures in the upper
teens to lower 20s should lift highs well into the 70s most areas.
Will probably need some upward adjustments going forward if the
trends continue.

Saturday Through Sunday...Confidence Medium

Differences on timing and trends creep into the end of the period
due to fast flow...potential model biases and stronger systems. The
GFS is attempting to bring cooler air into the region faster than
the Euro by Saturday into Sunday so highs both days are possibly too
cool. Despite this time the extended looks generally dry
through Sunday as ridging will be the predominant feature here with
the storm track slightly north of the region over the weekend.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1242 PM CDT Tue Oct 25 2016

VFR conditions will persist the remainder of the afternoon with
ceilings around 6KFT. The ceiling will continue to lower into
tonight with showers and thunderstorms spreading across all of
central Iowa. Expect widespread IFR conditions after midnight with
ceilings crashing as moisture continues to stream northward into
the state. Winds will remain strong from the south until frontal
passage toward daybreak with northwest winds behind the front.
Poor aviation conditions will continue into Wednesday morning
although ceilings will be begin to lift toward midday as drier air
pushes into the state.





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