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fxus63 kdmx 182337 
afddmx

Area forecast discussion
National Weather Service Des Moines Iowa
635 PM CDT Mon Jun 18 2018

..updated for hydrology discussion...

Discussion.../tonight through Monday/
issued at 412 PM CDT Mon Jun 18 2018

Thunderstorms have been developing over northern Iowa along the
meandering front and can already see the upscale growth back to
the southwest toward the Omaha area. Airmass in this area is
characterized by MLCAPE uninhibited with 1500+j/kg with shear up
towards 30 knots near the boundary. More impressive is the
precipitable water values of nearly 2 inches. The storm that is
east of Webster City brought around 1.5 inches in 30 minutes to
Fonda. So heavy rain and hail will be the primary concerns through
early this evening. South of this boundary, plenty of cumulus
have formed within the warm sector with heat indices in the upper
90s to around 103. Heat advisory will be allowed to expire at 7pm
or be canceled early if thunderstorms move over a significant
portion of the advisory.

As for tonight, convection is expected to continue into the evening
with more uncertainty after midnight, but confidence high in heavy
rainfall. Cams like the hrrr, namnest, nam12, rap, and nbm show
it continuing after this time with less activity in the esrl hrrr
and WRF cores. While the mean flow would suggest storms moving off
the boundary, the set up still has moisture transport into the
state and this boundary along with a shortwave trough moving
overhead. With the atmosphere moisture-laden with precipitable
water (pw) values around 2 inches tonight (00z naefs shows this as
97.5th percentile or higher for pw), held on to high pops
throughout the night. 12z href probability matched mean (pmm)
showing widespread 2 to 3 inches with the localized pmm having
maxima of around 5 inches of rainfall through tomorrow morning
along and south of Highway 20 to near I-80 primarily west of I-35
to our western forecast border. Deterministic models such as the
NAM, GFS, CMC regional, and cams show varying location, but
similar message of 1 to 2 inches over a wide area with maxima of 4
to 6 inches possible. Quantitative precipitation forecast was therefore a blend of the wpc and
nbm, which lined up well with much of the model guidance for where
heavy rainfall is expected. Amounts are generally 1 up to 2
inches in much of the watch area, but higher potential is
certainly there given ensemble/deterministic guidance. Given the
recent stretch of wet weather and in spite of the relatively dry,
hot weekend, have issued a Flash Flood Watch from the Highway 3
counties down to the Highway 34 counties tonight through noon
tomorrow. Area may be too broad, but unable to focus it any
further than this and don't want to leave future shifts having to
extend to new areas (hopefully!).

The boundary will push into southern Iowa by later Tuesday with a
relatively lull from the rain by the afternoon over a majority of
the area as the shortwave departs. However, the next in the
series of shortwaves will arrive Tuesday night with precipitable
water values still around 2 inches and the mean flow a bit more
favorable along the boundary for training storms. Likely much of
the activity will be toward or after midnight Tuesday into
Wednesday morning and new flash flood watches will likely be
needed. As a more substantial, slow moving, shortwave trough
begins to move into the area Wednesday into late Friday as shown
by the GFS and European model (ecmwf) and the front meanders into northern
Missouri, expect more rounds of storms with locally heavy
rainfall. With placement of the boundary and lift, this will focus
much of the heavier rainfall over central to southern Iowa
Wednesday through Friday. Throughout much of this period,
precipitable water values will remain in the 1.5 to approaching 2
inches so locally heavy rainfall is possible. For the Tuesday
night through Friday night period, widespread rainfall of 2 to 3
inches is likely over central to southern Iowa. Of course, locally
higher amounts are expected. High temperatures Tuesday through
Friday will be more seasonable ranging from the low 70s to the
lows 80s in most areas.

As the shortwave trough moves into the Great Lakes Saturday, a
break from the rain is becoming more likely on this day. However,
another shortwave trough will pass over the state Saturday night
into Sunday with additional storms possible.

&&

Aviation.../for the 00z tafs through 00z Tuesday evening/
issued at 635 PM CDT Mon Jun 18 2018

Stalled/slow moving frontal boundary will be the focus for storms
with heavy rainfall. MVFR or IFR conditions likely near to and
north of the boundary with low cigs and reduced vsbys in storms.

&&

Hydrology...
issued at 508 PM CDT Mon Jun 18 2018

Last week saw heavy rain fall over parts of central Iowa with some
areas receiving between 2 to 4 inches with a few localized places
upwards of 6 to 7 inches. While the weekend was relatively dry
and hot, antecedent soil conditions are or will soon be primed
for flash flooding. In addition, while there are no rivers above
flood stage at this moment, flows remain above base flow so
capacity is reduced along with soils favoring quicker runoff.

Already this afternoon we have seen locally heavy rainfall in a
narrow band just north of Highway 20 from near Storm Lake to near
I-35 with 0.5 to 1 inch total so far. Fonda, Iowa recorded 1.5
inches in 30 minutes with other personal weather stations showing
0.75 to 1.25 inches as well as these storms moved through earlier
this afternoon. Wpc continues to highlight parts of our forecast
area for excessive rainfall the rest of today through Wednesday.
Our official 5 day rain forecast along with wpc 5 day quantitative precipitation forecast
forecast has broad area of 3 to 5 inches over central to southern
Iowa, which agrees or is slightly higher than the deterministic
models. However, higher end potential is expected and wpc shows
locally 7 inches over the next 5 days. While not inside our
forecast area at this time with this set of model runs, there are
10 inch bullseye from some deterministic models, which lends to
the idea of what is in the realm of possibility for 5 day rain
totals across some part of the region within this moisture-laden
atmosphere.

As for rivers, rfc ensemble forecasts today based off of 72 hour,
maximum future rainfall would have numerous gauges into moderate
to perhaps major flood stage. However, 72 hour, best estimate
future rainfall would take only the Des Moines River at southeast
6th street and the north Raccoon river at Perry into minor flood
stage by middle to late in this week. A better handle on river
flooding will obviously be known after the rain falls in the
various basins and is routed into the rivers.

&&

Dmx watches/warnings/advisories...
heat advisory until 7 PM CDT this evening for iaz049-050-057>062-
070>075-081>086-092>097.

Flash Flood Watch through Tuesday afternoon for iaz023>028-
033>039-044>050-057>062-070>075.

&&

$$

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