Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 211730
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1230 PM CDT Wed Jun 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 356 AM CDT Wed Jun 21 2017

Nearing 09z, a few showers have popped up over northwest Iowa and
these should track to the southeast early this morning. These
showers along with a thunderstorm complex over central South Dakota
are located within a zone of 850mb warm air advection. The
convection over South Dakota is expected to reach the northwest part
of our CWA around 8am this morning, though it is in question how
well this holds together. The CAMs have not been handling the
current trends exceptionally well and show an overall downward
trend. Given the warm air advection regime and a surface front near
the Missouri border early this morning that will lift northward
through the day, will continue with PoPs and the showers and storms
progressing through the state this morning and early afternoon. As
the warm front lifts northward today, a moisture rich airmass will
also advance northward with dewpoints in the middle 60s to near 70
degrees. This warm front will also divide central Iowa between above
normal to near normal highs. Raised temperatures a few degrees from
initial guidance over the south given how much we warmed up on
Tuesday. Further, as this warm front lifts across the state, will
once again see breezy conditions develop.

Attention turns to late this afternoon and evening as a weak surface
low along with a weak 500mb vorticity approaches Iowa. Warm
temperatures just above the 850mb level will limit thunderstorm
development to late this afternoon (e.g. 21 or 22z) when this
erodes. With the triple point likely over northeastern Nebraska and
dryline/trough axis stretch southwestward from this low, this will
be the area for initiation. No question there will be instability
available late this afternoon and evening and sufficient shear above
40 knots. Hail and wind appear to be the primary concerns with a non-
zero tornado threat. Initially, more discrete cells may favor hail
with freezing level around 14000 feet and -20C level is around 24000
feet. Localized interactions with the warm front or near the triple
point may be able to favor higher SRH values, but overall SRH values
are relatively low. As storms move eastward, hail and wind will
become the main mode. DCAPE values certainly increase this evening
above 1000 J/kg. Storms will exit from west to east overnight into
the very early morning hours.

.LONG TERM.../Thursday through Tuesday/
Issued at 356 AM CDT Wed Jun 21 2017

Regime changes will occur through the period with summer like
convection and humidity followed by cooler and less active weather
from the weekend into early next week. This will be driven by
fast zonal flow at onset eventually amplifying as the current
northern Canadian province short wave reaches the Upper MS Valley
by Thu Night. This will push a front through Iowa late Thu and Thu
Night ending our quick return to warm, humid air with below
normal temperature and humidity levels from the weekend into early
next week.

As the period begins Thu morning, weak remnants of a Wed Night
MCS may be just lingered over northern Iowa before a typical
nocturnal dissipation into the morning. Attention will then turn
to peak heating re-development along a cold front somewhere
across northern Iowa. Have also boosted highs farther south into
the warm sector to the warm end of the guidance suite based on
what occurred Tuesday. MLCAPEs look to be around 2500 j/kg with
minimal MLCIN and sufficient synoptic scale forcing with the
approaching wave to overcome whatever there is. 0-6km shear is in
the 30-40kt range so expect a burst of severe weather potential in
the 21-03z time range. This CAPE/shear parameter space would
support supercells and convection allowing model 2-5km updraft
helicities suggest that potential as well so there is a window
where all modes of severe weather would be possible. Instability
wanes after that time however with increasing MLCIN so expect the
discrete nature of cells to decrease and merge into a weakening
MCS into the evening with the loss of insolation. 03Z SREF
Calibrated Severe Thunderstorm probabilities suggest this as well
so suspect that the south and east extent of the current Day 2
outlook may be trimmed over time with central and northern Iowa
more likely. The heavy rain potential Thursday Night seems to have
diminished as well. With the stronger Upper MS Valley short wave
the front is now more progressive and oriented SW-NE vs west-east
and slower moving. The airmass will still support locally heavy
rain with anomalously high, but not extreme precipitable waters
and warm cloud depths, but with the mean wind now less parallel to
the front limited duration of efficient rains and high rates
should be preclude higher amounts. Model QPF has recognized this
too with lower amounts than shown yesterday at this time.

The work week should end with fair and cooler weather for Friday
behind the front. Much like its previous runs, the 00z ECMWF
continues to suggest a stronger wave along the baroclinic zone
into Saturday so have low PoPs for elevated, relatively stable
showers. The broad long wave trough will remain established
through the remainder of the period with spotty, intermittent low
PoPs for either weak elevated convection due to short waves
embedded in the persistent NW flow or diurnal weak surface based
showers during the afternoon and early evening. This will keep
temperatures below normal with a return to seasonal values not
occurring until the pattern finally becomes progressive enough
that warm advection returns by Tuesday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1230 PM CDT Wed Jun 21 2017

There will be scattered convection later this afternoon into
tonight and this will provide the main concern in regards to
restrictions to ceilings and visibilities. Otherwise, widespread
VFR conditions are forecast for the duration of the period with
south to southwest winds.

&&

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

&&

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Cogil



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