Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180916

Area Forecast Discussion
National Weather Service Des Moines IA
416 AM CDT Thu May 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 414 AM CDT Thu May 18 2017

Main upper low that impacted the state Wednesday has now lifted into
northwest WI, with the sfc low in the same vicinity. Some wrap-
around cloud shield still in place across the CWA and expect it to
be for much of the day. Colder air moving into the state on the
backside of the system, with gusty westerly to northwesterly winds
expected into the morning hours as well. As the system continues to
lift northeastward away from the state, winds will start to diminish
and some breaks in the cloud cover are expected to develop
especially across the south. The north should be under that cloud
shield longer, with thicker low clouds expected. With the clouds and
cooler air in place, expect highs today to be much cooler with highs
only in the 50s north to around 70 far south.

Focus shifts to the next system to impact the state tonight, as
another upper low is spinning to the west across northern Utah. This
upper low will move east today, with an associated area of low
pressure forming along the front range of the Rockies. Southerly
flow ahead of this system will move back in throughout the lower
levels by this evening into the overnight hours. Theta-e advection
to increase, with moisture flowing northward from the Gulf into Iowa
by late tonight. Sfc flow still off the drier high pressure system
off to the north/east therefore very low level moisture expected to
be limited. However as a push of energy lifts northeastward through
the state ahead of the main upper level system, could still see some
chances for precipitation by late evening and into the overnight
hours as moisture advection becomes oriented into the state
throughout the lower/mid levels. Decent CAP in place, but steep
lapse rates aloft should allow for elevated convection through the
overnight hours. Some marginal shear and ok instability, so could
see an isolated strong to severe storm mainly across the far south.
Overall expect hail or secondarily wind to be the main severe
threats with any stronger storm that could develop.

.LONG TERM.../Friday through Wednesday/
Issued at 414 AM CDT Thu May 18 2017

08z Thu water vapor imagery picking up on deep well-defined upper
low spinning over the Four Corners region. Models starting to
cluster together in terms of tracking this low across the Rockies
and towards the upper Midwest... generally placing the closed
upper low over South Dakota by 12z Sat, and the associated sfc low
in southwestern Iowa at that time. The 00z NAM continues to be a
fast outlier, so have discarded that solution and gone with a
general ECMWF and GFS blend for track.

Ahead of the system, isentropic lift, well-phased with decent
moisture advection overspreads Iowa between 12z Fri and 18z Fri. PVA
also overspreading Iowa to help ascent. PWAT values roughly around 1
inch at this time. Though not the most efficient-looking sounding
profiles, have upped QPF slightly vs guidance. QPF may need to be
further increased for this Friday morning period... half an inch to
three-quarters of and inch of rain certainly plausible across
northern Iowa.

Saturday into Sunday...
Lull in heavier precip Friday afternoon, before another shot of
forcing/moisture arrives in southern Iowa Friday evening with the
approaching sfc low. The attendant warm front is slated to set up
over northern Missouri. Low-level frontogenetical forcing is very
strong through 00z Sat in northern Missouri as differential
heating enhances the boundary during the daytime. After 00z Sat,
however, this boundary really loses its gradient as it progresses
into Iowa...aiding to weaken/cap an strong to severe storms that
may be in northern Missouri Friday PM. Enough lift in this warm
advection sector though to keep precip going into Iowa as the
boundary makes it to around I-80 by 18z Sat.

Interestingly, the GFS in particular is excited about trailing a
weak cold front oriented N-S through Iowa with this low. The
location of the boundary during peak heating could be key...though
as of now, it appears the DVN CWA may be more under risk for some
strong storms...especially given the GFS is the slower of the
solutions. Regardless, it seems like we will get dry- slotted
behind this boundary. If faster track wins out, sun may make it
out Saturday, in which case temps will need to be warmed 5 to 10
degrees across central to southwestern Iowa. Wrap-around stratus
may impact Iowa for Sunday...suggesting temps would need to be
lowered a solid 5 to 10 degrees across Iowa. Have already nudged
down temps vs guidance to begin to implement this trend.

Next week...
Models starting to come into focus with rapidly propagating a
shortwave southward through Manitoba and wrapping it into the
backside of this parent upper low. Given the fast-moving nature of
this shortwave, models struggling with location and timing, showing
little run-to-run consistency. Therefore, have left widespread POPs
across Iowa Monday and Tuesday. MUCAPE values reach 1500 J/kg
Monday, so strong storms cannot be ruled out... though we may be
very much "gunked over" depending on how timing plays out. PWAT
values near 1 inch, so this weekend`s event definitely has
potential for more rainfall at first glance.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1136 PM CDT Wed May 17 2017

Main concerns this period will be stratus and lingering light
showers. Upper level low slowly lifting northeast. Area of showers
from central Iowa back to eastern South Dakota will rotate and
lift northeast through 12z affecting mainly KFOD...KMCW...and
KALO. Meanwhile MVFR to IFR stratus filling in behind the system.
Confidence on timing of bands of differing cig categories
low...but over all improvement expected south by 18z...and with
the north sites improving toward 00z...though still not a certainty.
Will need to monitor for in later TAF packages. Next upstream
system will begin to affect western areas with MVFR cigs late in




SHORT TERM...Beerends
LONG TERM...Kotenberg
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