Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 281153
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
653 AM CDT Wed Jun 28 2017

.SHORT TERM.../Today through Tonight/
Issued at 435 AM CDT Wed Jun 28 2017

Strong theta-e advection is lifting across Iowa early this morning
as an intense 70 kt low level jet is focused into the state. In
addition, a strong short wave approaching from the west is bringing
good kinematic forcing and upper level divergence to the state.
Thunderstorms have developed in response to the forcing with a few
strong updrafts developing into central Iowa. These storms may
briefly produce quarter sized hail however the updrafts have been
having a difficult time maintaining despite good deep layer shear as
cold pool development appears to be cutting off the updrafts. Storms
will likely bubble through mid morning then there should be a
reprieve before redevelopment occurs this afternoon.

The airmass is expected to recover by the afternoon with a surface
boundary setting up somewhere across the central portion of Iowa.
Favor the CAM/RAP solutions that are a bit farther north with
initiation closer to the Highway 20 to 30 corridor then gradually
settling south through this evening. A ribbon of 2500-3500 J/kg
CAPES are expected along the boundary. A modest cap over the south
should keep the activity focused near the boundary. The near storm
environment will be supportive for organized updrafts and supercells
early with deep layer shear of 35-50 kts. The primary severe weather
threats are expected to be very large hail 2 inch plus in diameter
and damaging wind gusts. A few isolated tornadoes are possible
though a more southwest trajectory of the low level wind field may
limit the potential. Reference the latest SWODY1 for additional
details regarding today`s enhanced risk for severe weather across
central Iowa.

A band of very heavy rainfall is also possible somewhere across east
central Iowa given very moist PWATs approaching 2 inches advecting
into the state. Localized flash flooding may occur but with the
recent dry weather and lack of confidence where this band may set
up, will not issued headlines at this time. The activity will
continue to sag south overnight with the activity diminishing after
06z.

.LONG TERM.../Thursday through Tuesday/
Issued at 435 AM CDT Wed Jun 28 2017

Severe weather and heavy rain potential on Thursday and Thursday
night were the main focus of the extended period. Leaned toward a
blend of the NAM/ECMWF as the GFS seems too far north with the
surface boundary Thursday afternoon.

Thursday and Thursday Night...surface boundary looks to stall
across the state from west to east during the day tomorrow. The
main question is how far north does the boundary reach. The GFS
puts the boundary as far north as the Minnesota border while the
ECMWF and NAM are roughly between the Highway 20 to Highway 30
corridors by 00z Friday. Certainly another conditional event as
the timing of the morning convection will be critical to allow for
the atmosphere to recover/destabilize by the afternoon. With the
ECMWF, NAM and the hires ARW/NMM confining the Thursday morning
convection to southern Iowa and diminishing it by around 15z,
increased confidence with the severe weather potential Thursday
afternoon and evening.

Instability increases along and south of the aforementioned
boundary over central to southern Iowa. MUCAPE values increase
to 3000-4000 J/kg by 00z Friday in this location. Steep low level
lapse rates present by the afternoon but less impressive mid level
lapse rates Thursday afternoon. Still, very weak cap in place
with 700mb temps less than +10C and expect a good swath of storms
across central Iowa by the afternoon/evening hours Thursday. There
is fairly good veering winds through the first 8kft or so with
strong deep layer shear available as 0-6km bulk shear tops 50
knots, which is sufficient for long-lived updrafts. 0-1km and
0-3km shear values range from 20-25 knots and 30-35 knots
respectively and low level helicity values (0-0.5km) and (0-1km)
around 100 m2/s2 and about 200 m2/s2 respectively. Plenty of
severe weather parameters pointing toward all modes of severe
weather on the table thursday afternoon and evening and cannot
rule out the Enhanced Outlook to be extended further east to
encompass southern portions of the forecast area.

The other significant concern is heavy rain Thursday night. Warm
layer cloud depths as high as 4000m with over 2" of precipitable
water present by 00z Friday. Weak storm motion or at least the
potential for storms to backbuild, especially along and just
south of the boundary will lead to the potential for training to
occur. Rainfall rates over 2" inches per hour seem reasonable
and may see some isolated locations topping out with near 3 inches
of rain by the time convection ends early Friday morning. Something
to consider in future updates is a flash flood watch for portions
of the area.

Friday through Tuesday...Surface high pressure builds into the
region and provides a dry period until Sunday night. Another
boundary extends west to east across the state, possibly as far
north the Minnesota border late Sunday night into Monday. Surface
low tracks across Kansas and provides plenty of moisture transport
into the state Sunday night into Monday. Certainly confident with
the potential for heavy rain during this time with the
precipitable water values over 1.5" and warm layer cloud depths at
3500m.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 652 AM CDT Wed Jun 28 2017

Showers and thunderstorms will diminish this morning. Localized
wind gusts in excess of 40 kts will be possible vicinity of
KMCW/KALO through 14z. Additional storms are expected by mid
afternoon over central Iowa then gradually settle south through
the evening towards KOTM. The storms should have ended at most
sites by 03z though may linger over southern Iowa after that time.
Severe storms with strong wind gusts will be possible.

&&

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

$$

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



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