Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182041
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
341 PM CDT Tue Apr 18 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 339 PM CDT Tue Apr 18 2017

A cold front over northwest Iowa will move to the southeast the
remainder of this afternoon and evening with plenty of clouds
accompanying it. A few showers or thunderstorms may form this
evening as this front losing momentum over southern Iowa and stalls.
More widespread showers and thunderstorms should develop late this
evening as the low level jet increases with moisture transport into
the boundary. Precipitable water values over southern Iowa will
remain in the 1 to 1.5 inch range, which is well above normal for
mid-April. In the mid-levels, winds are parallel to the boundary and
could lead to storms traversing the same area. High resolution
models indicate that these storms may produce heavy rainfall with an
inch in 3 hours. For rain amounts, have tried to incorporate some
higher totals across the south using some of this guidance along
with initial guidance and WPC. With relatively low flash flood
guidance values and antecedent wet soils, have issued a flash flood
watch for southern Iowa, including the Des Moines metro. See the
hydrology discussion below for a few additional details regarding
the hydrology concerns from now through late this week.

The stalled front will lift northward as a warm front during
Wednesday with showers and thunderstorms primarily on the `cold`
side of it. This warm front will lift to between Interstate 80 and
Highway 20 over central Iowa, with a cool and wet day over northern
Iowa. South of the warm front, temperatures will be quite balmy for
mid-April and have bumped up highs a degree or two from initial
guidance. Main concern though will be severe potential tomorrow
afternoon into the evening. Setting the stage in the afternoon will
be temperatures 10 degrees above normal, dewpoints in the low 60s,
MUCAPE values of around 2000 J/kg, and 0-1km SRH values around 100
m2/s2 as surface low pressure moves from south central Nebraska at
midday to west central Iowa by evening. 21z NAM sounding near the
warm front (in this case MIW) shows backed low level flow and a 0-
1km SRH of around 400 m2/s2. Storms that do initiate along the
boundary will be supercellular with the possibility of all modes of
severe convective weather including tornadoes. Delaying development
will be a warm nose around 800mb that will keep the atmosphere
capped in the warm sector until late afternoon. This is corroborated
by high resolution convective allowing models. As a cold front
enters the state from Nebraska late in the day, more storms will
initiate along the front with primary threats being large hail and
damaging wind gusts. Locally heavy rainfall will be possible with
storms tomorrow afternoon as well.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 339 PM CDT Tue Apr 18 2017

The primary concern will be at onset with strong to severe
thunderstorms likely either ongoing or soon to be developing. Models
are in better agreement bringing the triple point from west central
to northeast Iowa 00-06z with the trailing cold front not exiting
the forecast area until 09z or so. The remnants of the potent short
wave/PV anomaly now moving through CA should reach NE/SD into the
Siouxland area by 00z providing more than sufficient forcing along
and ahead of the surface low for convection to develop, especially
along the warm front. This should provide for two areas of
development: warm frontal and then convergence along the cold front.
It is somewhat interesting however that the convection allowing
models /CAMs/ seem to form a SW-NE line of convection without too
much regard for surface features and low level convergence, possibly
suggesting a synoptic, elevated forcing mechanism, at least
initially. MLCAPE/CINH forecasts do suggest a surface based
potential for uncapped CAPEs to 1500 j/kg toward the warm front
however so if the convection does form aloft initially it may not
last long. With 40-60kts of deep shear across Iowa supercells are
likely, which is re-reinforced by elevated updraft helicities in
many CAMs. Low level shear/SRH and LCLs are quite high and low
respectively toward the warm front so this will be a zone of
elevated tornado potential for any supercells with favorable
tracks. North of the front large hail would be the predominate
threat with large hail and wind farther south along the cold
front. Would not be surprised to see the current SPC Day 2 Slight
outlook expanded and/or increased to Enhanced or more by early
tomorrow. There will be some heavy rain threat as well, but this
will be diminishing. The moisture transport, precipitable water
values and warm cloud depths will all be quite favorable for
efficient rainfall, but storm motion should be fast as well.

The system should exit overnight leading to a period of relatively
weak and inactive flow through the weekend. There will be another
potent system with significant forcing and precip dropping from the
NW CONUS through the southern Plains and mid MS Valley early in the
weekend, but present indications are that the track should remain
just south of IA with only token, low end chances toward the MO
border. The GFS/ECMWF deterministic runs and ensembles are in good
agreement for this time range with another northern Plains/Upper MS
Valley system Monday into Tuesday. This will bring precip chances to
northern IA, but the return flow time will not be great so it
appears to be somewhat moisture starved at this point. There is a
great deal of 1-3km warm/theta-e advection but much of that will go
toward saturation north of IA.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1250 PM CDT Tue Apr 18 2017

Two separate cloud bands are moving across central Iowa. The
first is dwindling as it exits the forecast area with mainly VFR
ceilings. The second cloud band along the cold front extends from
southeast of KMCW to southeast of KFOD, which will bring MVFR
ceilings to the northern sites this afternoon. As the cold front
stalls out across the southern part of the state tonight, showers
and thunderstorms are expected, though timing is a challenge. This
front will lift north tomorrow towards Highway 20 with MVFR
ceilings accompanying the front. Breaks in the clouds over
southern Iowa are expected such as at KOTM with VFR ceilings
towards midday.

&&

.HYDROLOGY...
Issued at 339 PM CDT Tue Apr 18 2017

Recent rains over this past weekend brought heavy rainfall to parts
of southern Iowa with some areas receiving between 1 to 2 inches
with a few isolated places approaching 2.5 inches. Latest flash
flood guidance values for southern Iowa are generally between 2"/3hr
to 2.5"/6hrs. Model guidance the last few days has shown the
potential for 1 to 1.5 inches during the 19/00z to 19/12z timeframe
as a cold front stalls near the Missouri border. Additional showers
and thunderstorms are expected later Wednesday and Wednesday night
as a cold front moves through the state. More recently, high
resolution convective allowing models show the potential for heavy
rainfall such as the HRRR, RAP13, and HiRes NAM cores. 12z NAM/GFS
soundings for southern Iowa locations show precipitable water values
between 1 and 1.5 inches from late tonight through Wednesday
evening. While flash flooding will be the concern from later tonight
through Wednesday evening, area rivers may also swell and leave
their banks starting as early as later Wednesday into late this
week.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 1 AM CDT Wednesday through late Wednesday
night for IAZ057>062-070>075-081>086-092>097.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Ansorge
HYDROLOGY...Ansorge



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