Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131740

Area Forecast Discussion
National Weather Service Des Moines IA
1240 PM CDT Thu Jul 13 2017

Issued at 606 AM CDT Thu Jul 13 2017

Rainfall rates have diminished over southern Iowa and upstream
into SE Nebraska so the watch has been cancelled. Inflow along the
310K effective inflow actually strengthened over the past hour,
up to 40kts, but has also continued to veer pushing the
convergence max farther south into NW Missouri. This is expected
to weaken as well with the typical diurnal LLJ cycle precluding
any precip sufficient for flash flooding.


.SHORT TERM.../Today through Tonight/
Issued at 328 AM CDT Thu Jul 13 2017

There are several elements of concern into tonight ranging from
convective trends to temps and finally fog potential into early
Friday morning. Arc of convection across southern NE, SW Iowa and
into northern MO has moved little over the past several hours and
seems to be anchored in persistent area of 0-2km moisture
convergence, or around 310K isentropically, north of the surface
front from NE Kansas into the KC metro area. This orientation has
kept much of the heavier amounts west and south of the forecast area
this morning. However development should persist upstream with the
latest objective analysis showing 2+ precipitable water values and
impressive warm cloud depths to 4.5km. 30kts of inflow should
persist along the 310K surface for the next several hours until it
veers and weakens toward sunrise. This typical nocturnal progression
is also support by most convection allowing models.  The likelihood
of flash flooding is not high considering the recent dry conditions,
but confidence and is not there to cancel altogether either
considering the environment and current radar, so will leave 7
counties in the Flash Flood Watch west but cancel eastern sections
which will have less heavy rain potential and more significant
drought conditions.

Both temp and humidity levels should moderate today behind the front
with little if any precipitation anticipated by later in the day.
Highs should drop back into the mid 70s to mid 80s with somewhat
more tolerable afternoon dewpoints in the 60s rather than 70s.
Current northern Plains high pressure should reach MN and IA
overnight and most models suggest the current status shield ahead
of the high over MN and WI should dissipate and/or turn to
cellular CU into the afternoon. This lingering low level moisture
may also lead to some fog by morning into the ridge axis so have
added patchy fog wording to many areas.

.LONG TERM.../Friday through Wednesday/
Issued at 328 AM CDT Thu Jul 13 2017

Near or below normal temperatures along with northerly winds will
prevail with plenty of sunshine on Friday as surface high pressure
approaches from Minnesota. By middle of July standards, it will be
rather pleasant as dewpoints will be within a few degrees of 60.
Farther west, the upper level ridge and heat will build over the
western United States. As the high pressure passes overhead late
Friday into Saturday, winds will be light and become from the south.
There may even be some patchy fog Saturday morning. Forecast
soundings show nocturnal inversion along with some near surface
moistening. At this time, have not included fog in the official
forecast, but best areas may be roughly south and east of a line
from Mason City to Dension. MET/MAV guidance also hinting at perhaps
some denser fog in the Nishnabotna River Valley as well.

The surface high pressure will slide southeast away from Iowa on
Saturday as shortwave trough dives out of Canada into the western
Great Lakes with a slow moving front approaching late in the day.
This front will bring some clouds along with isolated thunderstorms
late in the afternoon if not in the evening. The best chance for any
storms will be over the northeast part of the state including the
northeast part of our forecast area as that is where the better mid-
level support and forcing will be found. This front will slowly move
southward through the forecast area through Sunday with the best
chances for any storms over eastern Iowa.

As the shortwave trough carves out over the Appalachians, the upper
level ridge of high pressure will build eastward over the Midwest.
This will bring 90 degree temperatures back into Iowa early next
week with the warmest days looking like Tuesday and Wednesday. The
exception to this may be across northern Iowa. The 00z ECMWF has
a 594dm high farther north compared to the 00z GFS. With these
differences in the models, the GFS brings rain and storms across
northern Iowa, which would also spell not as hot conditions.
Conversely, the the ECMWF places the rain and storms over central
Minnesota with hot weather persisting. For time being, have
blended in some chance PoPs across the north and have temperatures
a split between the ECMWF and GFS over northern Iowa.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1240 PM CDT Thu Jul 13 2017

MVFR ceilings will persist at MCW for the next hour or two before
breaking up, and may affect FOD/ALO even more briefly before
rising. Otherwise, VFR conditions will prevail for the remainder
of the TAF period, with winds diminishing and skies clearing
overnight. There is some potential for fog development during the
early morning hours Friday, especially at FOD/MCW, but confidence
is too low to include in the TAFs at this time.





LONG TERM...Ansorge
AVIATION...Lee is the U.S. government's official web portal to all federal, state and local government web resources and services.