Area Forecast Discussion
Issued by NWS Des Moines, IA

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531
FXUS63 KDMX 222038
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
338 PM CDT Thu Sep 22 2016

.SHORT TERM.../This evening through Friday/
Issued at 307 PM CDT Thu Sep 22 2016

The situation is still conducive for heavy rains tonight but
hopefully not to the same extent as yesterday. Water vapor imagery
does not show the moisture fetch to the same degree as was shown
Wednesday, but there is a decided back edge to the vertical motion
passing through the Plains into western KS/NE which should aid lift
into the night. This has resulted in weak but broad mid level
kinematic forcing with a ribbon of somewhat favorable mid level temp
and theta-e lapse rates from KS into IA. Thermodynamically the
forcing is somewhat better with 850/700mb warm advection noted into
IA and 0-2km moisture pooling into our baroclinic zone as well. 20Z
surface analysis shows at least two boundaries, one tied into low
pressure near Lincoln between Red Oak and Atlantic, another farther
northeast through the Des Moines metro area more associated with
previous MCS outflow and likely another from Estherville to Grinnell
with seems to be immediately associated with the weak MCS.  This
will provide for ample isentropic lift into the night, but the flow
is weaker along the 305K inflow surface vs last night, down to 15-
30kts. RAP elevated instability layers suggest elevated CAPEs will
reach uncapped 1500 j/kg later tonight sustaining additional MCS
development. There could be a few strong to severe storms, but
overall the Marginal Risk seems fine due to reduced effective shear.


.LONG TERM.../Friday night through Thursday/
Issued at 307 PM CDT Thu Sep 22 2016

Expecting some early morning ongoing convection across the eastern
and northeastern areas to end by mid to late morning. Warm front
will finally retreat north into the afternoon hours with rather warm
and humid conditions across the area for the remainder of the day.
Some cloud cover extending across the northeast with light showers
then will give way to some sun in the afternoon.  As has been
advertised...highs will warm into the upper 80s to lower 90s over
the south with the upper 70s in the far north by late day Friday. No
rainfall is expected until the long wave trough approaches Saturday
morning with showers and thunderstorms moving into Central Iowa by
mid to late afternoon. Remnant tropical moisture from Paine will
probably still be available at this time across our area. With a
ribbon of higher precipitable water and decent convergence along the
boundary...will likely see a decent rain event with up to an inch
additional from Saturday evening through Sunday.  Once this feature
pulls east...the models are in some consensus for next week. Both
the Euro and GFS now suggest that the H500 long wave trough will
keep the moisture channel south and east of the region.  This should
allow for a period of little to no precipitation and help the river
situation across the area. Temperatures next week look cooler and
more seasonal for the first week of fall.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 139 PM CDT Thu Sep 22 2016

Conditions are generally improving with VFR in most locations and
only a few MVFR spots. Weak convection will also linger in the
KALO area through the afternoon. The question becomes convection
and cig/vsby trends into the night and there is low confidence
again in that regard. Until confidence increases have only
included VFR conditions and vicinity wording rather than prolonged
periods of thunder and/or category degradations.

&&

.HYDROLOGY...
Issued at 307 PM CDT Thu Sep 22 2016

Magnitude and placement of the heavy rains will be key tonight.  As
mentioned above the environment is not as favorable as last night
but still supports heave rain. The surface based potential is trying
to recover with uncapped 1500-2500 j/kg MLCAPEs now in place over SW
IA.  This recovery may be too late to get anything going, but will
likely aid slightly elevated convection just to the north and east
tonight. The latest objective analysis pegs 0-2km moisture
convergence along the Highway 20 to 30 corridors so this will be the
favored location, likely anchored along or just south of the current
Waterloo MCS which won`t go away. Precipitable waters are already
around 2 with warm cloud depths higher than yesterday to 4km. Thus
the moisture transport and instability components will not be as
good as yesterday, but efficient rains are still anticipated. This
will result in 1 rainfall amounts through the aforementioned
favored corridor with locally higher amounts from 2-3 possible,
thankfully south of where yesterday`s heavy rain occurred.  Storms
will not be fast moving with a mean of only 15-20kts and Corfidi
vectors slow to the south. Thus have kept the flash flood watch in
place through 18z Fri and expanded down to the full Highway 30
corridor from Guthrie County eastward. A small section of west
central IA counties has been canceled however, which does not seem
to be under the gun and didn`t receive much rain overnight.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for IAZ004>007-
015>017-023>028-034>039-046>050.

&&

$$

SHORT TERM...Small
LONG TERM...REV
AVIATION...Small
HYDROLOGY...Small



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