Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201209

Area Forecast Discussion
National Weather Service Des Moines IA
705 AM CDT Sun Aug 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 351 AM CDT Sun Aug 20 2017

Ongoing convection moving into southwest Iowa in conjunction with a
shortwave aloft and warm advection...weak forcing and a frontal
boundary drifting across the area will continue across southern Iowa
this morning.  Moderate rainfall is expected with the storms along
with marginally strong wind gusts to 40 mph. The warm front should
shift east out of the area by 18Z giving us a brief break in
precip...or at least a more isolated coverage of precip.

Later today and tonight another shortwave will push across the state
and the surface low over the Plains will move little.  However a
cold front will nudge into the state and this boundary will be. the
focus for storms tonight. During the evening showers and a few
storms may try to percolate but overnight when the second, more
northern wave pushes across and interacts with the frontal boundary
is when precip will really pick up again. Additional storms will
develop especially after 06Z across northern Iowa and by 12Z the
front will sag towards the Highway 30 corridor and so storms will
likely make it to the highway 30 corridor by early Monday.

Looking at the severe potential...storms across the south this
morning may increase in intensity once the sun come up and lapse
rates steepen a bit.  Instability and shear will be sufficient for
at least some marginal hail but strong wind gusts with the storms.
Late this afternoon and tonight, hail will be a better risk along
the cold frontal boundary sagging south but strong and severe wind
gusts will also be possible.  There is some indication that ahead of
the boundary will be pockets of strong warm advection tonight.  If
that happens then severe storms may have a little more widespread
coverage. This will need to be assessed as things unfold.

.LONG TERM.../Monday through Saturday/
Issued at 351 AM CDT Sun Aug 20 2017

The primary focus of the long term forecast period covers the
Eclipse and potential for severe weather and heavy rain from
Monday through Monday night. Thereafter Iowa will reside beneath
northwesterly flow aloft, resulting in mostly dry, quiet, and
relatively cool weather from Tuesday afternoon through at least
Thursday. By next Friday and Saturday a more zonal flow pattern is
foreseen, indicating slowly moderating temperatures and increasing
thunderstorm chances at the end of the week.

At the beginning of the long term forecast period, on Monday
morning, a large slug of monsoonal moisture will be found around
New Mexico and the Four Corners region, while a northern stream
trough swings into southern Canada and impinges on preceding zonal
flow over Iowa. Beneath this zonal flow a quasi-stationary frontal
boundary will provide focus for thunderstorm development on Sunday
night, as discussed in the short term section above, with some
convection lingering into Monday morning. The most likely area
for overnight/early morning storms will probably be across the
northern half of Iowa, however, outflow boundaries moving south
may reach as far as the Missouri border and spark additional
storms in the pre-dawn hours Monday. In any event, as warm
air/moisture advection kicks in from the south/southwest later
Monday morning this activity should shove off to the north and
east, with warm and muggy conditions surging into about the
southern half of Iowa, reinforcing the effective frontal boundary
somewhere across the state. On Monday evening peak heating should
produce additional thunderstorms, especially given expected
destabilization, within a broad area of forcing for vertical
ascent on the leading flank of the approaching northern stream
trough. This scenario will favor growth of thunderstorms into one
or more convective complexes by Monday night, moving
east/southeast out of our area by Tuesday morning.

The instability/shear/forcing parameter space is favorable for
severe weather, especially Monday evening into Monday night, as
is well outlined by current SPC outlooks. It is also worth noting
that the high atmospheric moisture content associated with the
monsoonal airmass moving into our area from the southwest,
combined with steering flow roughly parallel to the quasi-
stationary frontal boundary, will promote a heavy rain threat near
the boundary on Monday night. Due to dry weather in recent weeks
our tolerance for heavy rain should be relatively high, but if it
occurs over a short duration and/or in urban areas then there
would be a higher probability of negative impacts.

In terms of the forecast for eclipse viewing on Monday, it will
largely center around two factors. The first is the extent to
which Monday morning convection will surge southward and persist,
cultivating low-level cloud debris. The second is the development
of a broad cirrus shield on the leading flank of the approaching
mid-level trough, spreading from west to east across the region
during the day. Even if morning convective debris clouds can
move/mix out by midday, the cirrus may prove problematic to
eclipse viewing depending on its thickness and opacity. All in all
there is still a window of opportunity for decent viewing, but
the probability of obscuration currently appears greater than the
probability of relative clearing.


.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 705 AM CDT Sun Aug 20 2017

First round of convection will move east of the southern TAF
locations by 17Z.  Brief MVFR cigs/vsbys behind line of
showers/storms will give way to VFR conditions.  Should be a
relative lull through much of the day before the next round develops
northwest invof a cold front as an upper level feature passes east.
Storms will again bring local MVFR conditions along with moderate to
heavy rain and frequent lightning.  Some risk of hail and
strong/damaging wind.





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