Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190447

Area Forecast Discussion
National Weather Service Des Moines IA
1147 PM CDT Sun Aug 18 2019

.DISCUSSION.../Tonight through Sunday/
Issued at 242 PM CDT Sun Aug 18 2019

Surface cold front is bisecting the state from roughly northeast
Iowa to southwest Iowa at mid-afternoon. There are patches of low
clouds, especially over west central and southwest Iowa, with the
southeast quadrant mainly clear. High resolution models are
largely quiet the remainder of the afternoon into this evening
outside of low chances for scattered showers or perhaps isolated
thunderstorms over our southeast forecast area. SPC mesoanalysis
shows the airmass is recovering with instability highest over the
southwest part of the state. However, forecast soundings show a
warming layer just above 850mb and SPC analysis shows this area is
largely capped, which will contribute to limited coverage this
afternoon/evening. Any lingering precipitation will end by
midnight if not sooner as an area of high pressure moves into the
state. Winds were lowered a few knots lower than previous forecast
so fog potential is increasing over southern Iowa tonight,
especially in low lying valleys.

As the high moves off to the east on Monday and the upper level
ridge over the Southern Plains builds northward, a shortwave trough
will top this ridge and move across the northern Plains. At the
surface, low pressure will move along the Canadian/North Dakota
border with a front extending southward. The low level jet will be
directed up the Missouri River Valley into the Red River Valley with
good theta-e advection ahead of this front. GFS and CMC show
convective development later Monday night passing over northern
Iowa, but the ECMWF is the farther south and has been the last few
runs moving a mesoscale convective system through Iowa into Tuesday
morning. While will continue with initial guidance for PoPs during
this period, future forecasts may be able to focus thunderstorm
chances over northern Iowa with non-ECMWF models showing rising
heights and NAM and GFS forecast soundings showing a capping
inversion between 850mb and 700mb. The uncertainty around the
convection and possibility of any lingering convective cloud
debris brings into question the high heat and humidity we have
been messaging. If convection does not occur as far south, heat
index values will be into the 90s across central Iowa and may bump
up to 100 degrees over central and southern Iowa.

The front will continue to move southeastward Tuesday night into
Wednesday with another chance for thunderstorms over the southern
two thirds of the state with highest chances over far southern Iowa.
Could be some locally heavy rainfall over southern Iowa with
precipitable water values greater than 1.5 inches, deep warm cloud
depths, and 850-300mb flow parallel to the boundary. As the front
moves farther south of the state Wednesday night, our region will
become under the influence of high pressure passing north of the
area. This will sponsor lower humidity and temperatures and a
pleasant end to the week for late August. Deterministic models show
a mid-level trough perhaps cutting off mid to late week east of the
Colorado Rockies and meandering eastward. This would have
thunderstorm chances perhaps nearby, but currently south of the area
in these models. The deterministic models` counterparts -- the 00z
ensembles -- as well as the 00z NAEFS do not show this mid-level
feature in the mean fields with only a handful of ensemble members
showing this at all. So, the end of this week will likely end up
drier as the high moves into the Ohio Valley. As return flow begins
and heights rise this weekend, will have temperatures climbing
upward with additional low chances for storms.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1147 PM CDT Sun Aug 18 2019

VFR conditions will prevail for most of the TAF period, with a
couple of exceptions. First, valley fog may impact FOD briefly
around sunrise Monday morning, but would be highly localized and
due to low confidence in affecting the runway/MTR have not
included in the TAF at this time. Second, a swath of VFR clouds
affecting DSM/OTM currently is expected to largely persist through
sunrise Monday and may lower to MVFR at times, but again,
confidence too low at DSM to include in current TAF. At OTM there
is a somewhat higher probability and also more chance of light fog
developing overnight, so have included a group for that terminal.





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