Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201743
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Tue Jun 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 355 AM CDT Tue Jun 20 2017

08z water vapor imagery shows shortwave trough moving southeastward
through northern Minnesota with scattered showers over central
Minnesota. At this time, these showers look to track just north of
our forecast area skirting northeastern portions of the state this
morning. Along with this shortwave, a front will drop through the
state causing winds to shift from the south to the north and
northwest later today. Winds behind the front will be breezy with
gusts of 20 to 25 mph possible. With convergence along the front and
temperatures peaking in the upper 80s near the convective
temperature, a few thunderstorms will develop. Several of the CAMs
(WRF cores and HRRRs) show this possibility, mainly over our far
southeast counties of central Iowa. Have introduced PoPs over this
area from initial guidance and may need to watch for the possibility
of development a bit farther west than forecast. Forecast soundings
for KOTM show DCAPE of 800-1000 J/kg so may have a few strong gusts
with the strongest of the storms. This front will push just south of
the state this evening and with it the storms as well. However, the
front should begin to return northward and with weak warm air
advection, may have a few isolated showers or storms that are able
to form.

.LONG TERM.../Wednesday through Monday/
Issued at 355 AM CDT Tue Jun 20 2017

The primary concern through the period will be precip trends and
thunderstorms in particular as the pattern again turns more active
into midweek. Our current relatively dry and inactive NW flow
will become more zonal into Wednesday starting another round of
weak warm advection with a surge of weak-moderate theta-e
advection and frontogenetical forcing during the day, mainly
north. This will also usher in increased instability with
attention turning to peak heating development as lowering upstream
heights gradually push a weak front and low level convergence
zone into the Plains by 00z. Models depict a narrow ribbon of
MLCAPEs pushing 3000 j/kg just ahead of the front into the
Siouxland area with eroding CIN so convergence should be
sufficient to trigger some storms. 0-6km shear is not extreme, but
30-40kts would be sufficient for organization and supercell
potential west per the Day 2 outlook before insolation wanes. The
convection will likely continue into the night, fed by 30-50kts of
inflow per model 305-310K isent surfaces near the effective
inflow layer. These same isent surfaces suggest the baroclinic
zone and best convergence would be over northern Iowa. With lesser
coverage farther south.

Heights will continue to dip slightly into Thu in response to the
upper low advancing through Ontario. This will lay out the front
west to east across Iowa as it drifts southward through the state
providing a nice convergence/baroclinic zone into Thu Night. This
will likely drive a repeat of peak heating storms followed by
nocturnal convection into the night. The instability looks a
little weaker with recovery also possibly tempered by lingering
precip and debris clouds through the day, but the shear will again
be sufficient for organization, especially north.  With
instability fading into the night the southern extent of the Day 3
outlook may be a bit overdone. While the effective inflow looks
weaker into the night, what there is will be favorably oriented
opposite the mean wind to reduce Corfidi vectors such that
individual cells will be slow moving with heavy rains becoming
more likely versus the previous night. The airmass will be more
favorable as well with precipitable waters to around 2 inches and
warm cloud depths 4+km. Could easily see regenerating storms train
west to east along the boundary through the night.

The front should push south and east into Friday with a cooler,
drier airmass to end the work week. PoPs do return into Saturday
and Saturday Night however with the ECMWF depicting a stronger
wave into the MS Valley as our flow amplifies once again. The
thunder and severe threat will be relatively low with weak
instability. This pattern will keep precip chances fairly low for
the start of next week with seasonally cool and less humid
conditions.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1243 PM CDT Tue Jun 20 2017

VFR conditions will prevail through the next 24 hours. The TAFs
are split out only for changes in wind direction, gradually
veering from northwest this afternoon to southeast by midday
Wednesday. Shower and thunderstorm chances will increase during
the day Wednesday, but are most likely after the end of the
current TAF period.

&&

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

&&

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Lee



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