Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180912
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
412 AM CDT Tue Apr 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 412 AM CDT Tue Apr 18 2017

A strong PV anomaly located over north central South Dakota will
continue to move rapidly east today. One area of thunderstorms has
developed immediately in advance of the anomaly and in the region
of strongest kinematic/thermodynamic forcing associated with the
system. Another area of thunderstorms has developed into northern
Nebraska in the vicinity of theta-e advection.

The kinematic forcing will move rapidly away from Iowa along with
the upper level anomaly. The theta-e advection will move into the
state and will generate a fairly narrow ribbon of saturation ahead
of a cold front sagging into the state. Showers and a few
thunderstorms will be possible along this ribbon though
instability will be generally decreasing through the morning and
into the early afternoon. The instability should increase again by
late afternoon over central and southern Iowa ahead of the cold
front that will be slowing down and eventually stalling. A few
additional showers and storms will be possible across this region
through midnight. Trends have been farther north on where this
boundary stalls which would have impacts into Tuesday.

The low level jet will increase late with theta-e advection
lifting north and back into the state by late tonight.
Thunderstorms will redevelop late tonight. Storm motions will be
parallel to the boundary which could bring heavy rain potential
along with isolated severe storms through 7 am.

.LONG TERM.../Wednesday through Monday/
Issued at 412 AM CDT Tue Apr 18 2017

The main forecast concern was focused on severe weather potential
Wednesday into Wednesday night. Leaned closer to the 18.00z ECMWF
for convective and temperature trends during this time period even
though the GFS/Canadian are in pretty good agreement with the
latest ECMWF surface low track and timing across the region. The
NAM remains the outlier as it seems too far south with the low
track Wednesday. Overall, the most significant change was to
maximum temperatures on Wednesday with the warm front looking to
surge further north than previously anticipated. Thus, placing
more of the CWA within the warm sector and a bit higher severe
potential.

Wednesday into Wednesday night...ongoing convection Wednesday
morning across the south along some mid-level theta-e advection
and a weak 500mb shortwave. This convection is expected to
transition northward as the warm front surges north through the
day. The surface low looks to be centered over south-central to
southeast Nebraska by around 18z before shifting northeast into
west-central Iowa by 00z Thursday. The aforementioned convection
looks to persist over northern Iowa during much of peak heating
and potential keep the warm front from surging any further north
than Highway 20. However, the 18.00z ECMWF has the warm front
possibly as far north as the Minnesota border and the 18.06z NAM
is now further north with the front. In the past, models have had
a tendency to under forecast the northern extent of the warm front
in these types of scenarios. Thus, higher confidence towards the
ECMWF solution and went warmer for maximum temperatures Wednesday
across much of the forecast area.

The warm sector will remain dry during the peak heating allowing
for some significant instability to build into central to southern
Iowa prior to the cold front to pass across the state in the
evening. The most likely scenario is for strong to severe storms
to develop along and ahead of the cold front as it sweeps across
the state during the evening. There is the potential for storms to
initiate near the warm front in the afternoon hours, leading to a
better tornadic threat in this location, especially if the
morning storms leave any outflow boundaries to help initiate
convection. By around 21z Wednesday, low level helicity ranges
from 150-250 m2/s2, 0-3km CAPE is sufficient with values from
50-150 J/kg, LCL heights around 500m, fairly steep lapse rates,
and storm motion vector nearly parallel to the front. Needless to
say, there are plenty of severe weather parameters that suggest
tornadoes are possible Wednesday afternoon into early evening,
especially in the vicinity of the warm front. The warm front is
likely set up between Interstate 80 to Highway 20 corridors.
With the cold front pushing across the state late in the day,
there is still plenty of instability and deep layer shear
available to help sustain convection. Damaging winds and large
hail are more likely along the cold front Wednesday evening.

The other threat Wedensday into Wednesday night is the threat of
heavy rain. Precipitable water values range from 1.00 to 1.50"
which is roughly 100-200 percent of normal for this time of year.
Storm motion and Corfidi vectors are nearly identical, so
training storms are possible across northern Iowa. Warm layer
cloud depths are in the 10kft to 11kft across northern Iowa and
with strong moisture advection into this area, some localized
heavy rain amounts over 2 inches are possible and could lead to
some isolated flash flooding.

Thursday through Saturday...Large surface high pressure builds into
the region by Thursday and looks to dominate the weather pattern
into the weekend.  The extended models continue to trend the next
surface low pressure system further south across Missouri, leaving
more of the forecast area dry Saturday.  More significantly are the
colder temperatures during this period with the potential for frost
Friday morning across the north as the surface high centers itself
directly overhead.

Sunday and Monday...next rain/storm potential looks to be early
next week with plenty of discrepancies in timing and location.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1153 PM CDT Mon Apr 17 2017

Will continue to see widespread VFR conditions overnight although
mid to high ceilings will increase from west to east toward
daybreak. Surface winds are expected to increase from the
southeast during the night. A surface front will settle into the
state on Tuesday with scattered showers and a few thunderstorms
but coverage will decrease as forcing pulls away from the state.
Have included showers in the north from mid morning to mid
afternoon but with little restriction. There may be some localized
MVFR ceilings with the showers near the boundary, however most
locations will continue to see VFR conditions on Tuesday into
Tuesday night. Winds will become south the southwest and gusty by
mid to late morning. However, the winds will subside into late
afternoon Tuesday.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Cogil



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