Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 260454
AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1150 PM CDT Tue Oct 25 2016
.SHORT TERM.../This evening through Wednesday/
Issued at 343 PM CDT Tue Oct 25 2016
Surface boundary has remained south of Iowa this afternoon with
strong southeast flow across the state. Main push of moisture has
been near 850mb where a few isolated showers have occurred in far
south central Iowa. Otherwise, band of frontogenetically forced
showers in far northeast Iowa has remained to the north of the
forecast area this afternoon with most locations staying dry.
Overall, progression of this system has slowed and overall
evolution of precipitation into tonight has been backed up about
3-5 hours across the forecast area. Of concern this evening, first
area would be the far southeast into east central portions where
theta-e advection may produce a few isolated showers this evening.
This should move east out of the CWA by mid to late evening.
Second concern is the rapidly increasing mid level forcing in
northern Iowa with approaching shortwave. Anticipate showers and
storms to develop by mid to late evening in northwest Iowa back
into eastern Nebraska. This area of precipitation will move east
through the night into central Iowa as system progresses through
the state. Instability may be sufficient for a few strong storms
later this evening in the west where some small hail may be
produced. The other threat for this system will be some heavier
rainfall which will be possible across northern Iowa during the
night. However, recent dryness should limit overall flooding
potential. Temperatures will remain relatively warm overnight with
cloudy skies and southerly flow for a good portion of the night.
.LONG TERM.../Wednesday night through Tuesday/
Issued at 343 PM CDT Tue Oct 25 2016
The main forecast concern over the extended was focused on
temperatures and winds Thursday night into Saturday as the
majority of the period looks to remain dry. Leaned closer to the
warmer ECMWF late in the week into the weekend.
Wednesday into Thursday...precipitation looks to be ending during
the morning hours Wednesday and even a bit quicker than previously
expected. Have dry conditions by 18z Wednesday with pops lingering
east of Interstate 35. Behind the trough, weak CAA develops
tomorrow afternoon resulting in a chilly time frame. Nudged up
wind speeds as the surface pressure gradient remains fairly tight
through 00z Thursday. Wednesday night will be the coldest night of
the extended as the surface high moves in over the state towards
Thursday morning. Some lingering cloud cover looks to get trapped
under the inversion across the central to northern portions of the
forecast area which should hold temperatures from dropping off too
drastically. However, with the light winds and strong subsidence
over the state, and if cloud deck does clear out, the current
minimum temperature forecast is likely at least 10 degrees too
warm. Lowered the minimum temperatures a 2-3 degrees across the
CWA Thursday morning.
Thursday night through Saturday...WAA develops overnight Thursday
night into Friday morning and likely to see a non-diurnal
temperature trend during the overnight hours. Raised minimum
temperatures a few degrees as temperatures look to warm overnight.
WAA continues to increase throughout the day Friday with the
strong mixing developing during the afternoon. Deep mixing to
around 900mb (and possibly higher) with winds atop the mixed layer
ranging from 35 to 45 knots by 21z Friday. Raised wind speeds and
gusts Friday afternoon and with the strong mixing, raised maximum
temperatures across central Iowa roughly 3-4 degrees. Both the
ECMWF and GFS have warmed to the upper 70s to around 80. Also with
the strong mixing, the current dew points forecast is likely
slightly too high and might need to be addressed in a future
forecast package. That being said, with the dry air mass, strong
winds, and much of the vegetation becoming dormant, fire weather
concerns certainly exists.
By Saturday, the next frontal boundary looks to be delayed
slightly, at least across southern Iowa until the evening. Thus,
raised maximum temperatures slightly across the southern half of
the state. The precipitation chances remain over far northern Iowa
into Minnesota Saturday night.
.AVIATION.../For the 06Z TAFS through 06Z Wednesday evening/
Issued at 1150 PM CDT Tue Oct 25 2016
A low pressure system will shift northeast across Iowa dragging a
cold front through the TAF locations. MVFR cigs with local IFR cigs
will be common through much of the forecast periods with gradual
improvement expected after 18Z. MVFR to local IFR vsbys can be
expected in TSRA and possibly some fog with improvement by 18Z.
Strong southerly surface winds will become NW as the low passes and
pulls a cold front through the TAF sites.