FXUS63 KDMX 210443

Area Forecast Discussion
National Weather Service Des Moines IA
1143 PM CDT Fri Sep 20 2019

.DISCUSSION.../Tonight through Friday/
Issued at 403 PM CDT Fri Sep 20 2019

Main concerns for this forecast period surround precipitation
chances, severe storm chances and flash flood chances through
Sunday. Little changes in the overall setup for the expected
upcoming storm system, so have mainly fine-tuned details and
opted to match up to surrounding offices with a Flash Flood
Watch across the far south/southeast for Saturday night into

Large upper trough still moving through the northern Rockies with
the surface trough extending from south central Canada southward
through the central Dakotas back into northeastern Colorado.
Strong southerly flow off the Gulf has been in place all day more
oriented toward the Plains today, and will veer into Iowa late
tonight and remain through Saturday into early Sunday. This will
allow abundant moisture to surge northward with moisture from
Imelda beginning to push toward Missouri and portions of Iowa
through early Sunday. This will allow for much above normal
moisture values across the southern portion of the state with
PWATS approaching 2 inches or higher at times across southern and
eastern Iowa.

With WAA and theta-e advection push tonight, expect a few rounds
of showers and storms to move northward across the state mainly
after midnight then continued chances for more elevated activity
through early/mid afternoon with additional pushes of WAA/moisture
advection. Could see a strong to potentially severe storm with
this activity which would mainly be a hail or wind gust potential.
More widespread storms are expected to develop by early Saturday
evening into Saturday night as the main upper level trough ejects
from the Rockies in two different pieces. This will result in the
cold front dropping southward through Iowa Saturday night and
lingering across the far south into Sunday. Strong deep layer
shear in place late Saturday through late evening Saturday night
will present a severe threat, especially across south central into
southwest Iowa. Instability will be decent, however with cloud
cover and a very saturated profile, CAPE expected to top out
around 2k J/KG which could limit the overall severe potential
somewhat given the strong shear in place. With the abundant
moisture throughout the entire column and some training of storms
near and head of the boundary and continued redevelopment across
northern MO into southern IA late Saturday night into Sunday, the
potential for heavy rain continues. Additionally warm cloud depths
are close to 4km and should see efficient rainfall with close 2
inch per hour rates possible at times. FFG is higher across the
south with 1 hr values over 2 inches and some 3-6 hr values close
to 3-4 inches. However with some storm potential through the 24
hours prior to the onset of the heavy rainfall and the potential
rainfall rates and expected duration of the rainfall still have
the potential for flash flooding across the far south/southeast.
Thus the issuance of the Flash Flood Watch.

As the front moves south of the area late Sunday into Sunday night
rain should diminish across the state with much drier air moving
in and temperatures falling back to near the seasonal values for
highs Sunday into Monday. Dry conditions expected into Monday with
some storm chances returning mid-week next week.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1143 PM CDT Fri Sep 20 2019

Few changes for the going forecast with warm air advection
showers/-tsra increasing overnight and peaking between 11-18z.
There should be lesser activity from 18-20z then coverage of
showers/storms will increase once again. Cigs still expected to
lower to MVFR/IFR for much of the period only improving to
VFR/MVFR from 23z to 02z then dropping again to IFR/MVFR as
another round of heavier storms south site at KOTM. /rev


Issued at 403 PM CDT Fri Sep 20 2019

See more detailed discussion in the forecast discussion above,
but overall expect higher rain rates with the thunderstorm
potential Saturday night through Sunday across far southern Iowa.
Could see rates pushing 2 inches per hour at times with some
storms moving over areas repeatedly and a prolonged duration of
heavy rainfall. Therefore even with higher flash flood guidance
currently, expect a decent chance of flash flood potential across
the far south/southeast and therefore have issued a Flash Flood
Watch for that area.


Flash Flood Watch from Saturday evening through Sunday evening
for IAZ074-075-083>086-092>097.




NWS DMX Office Area Forecast Discussion