Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180846
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
346 AM CDT Tue Jul 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CDT Tue Jul 18 2017

Thunderstorms developed along a cold front in central MN then back
built into Sioux Falls area in a corridor of theta-e advection and a
strengthening low level jet.  AS the theta-e advection shifts off to
the east, the storms are falling apart, as expected.  Later this
morning a shortwave will lift out of eastern NE across NW IA into
MN.  There may be some storms redeveloping as this lifts off by 18Z.
The main show however will be along that cold front that will slowly
sag south into Northern IA later today likely to make it as far
south as the highway 20 corridor by 12Z Wednesday.  This will be
coincident with a weak trough and embedded shortwaves traveling
across the state.  Given the amount of instability, convergence
along the front and deeper shear across northern Iowa, storms north
later this afternoon through the evening will be strong to severe
with damaging wind and large hail the main threats.  After 06Z we
lose our forcing despite the boundary being around.  We may see
continued convection into the wee hours of the morning but the trend
will generally be for storms to weaken/shift east.

Temperatures will again be warm today across the south where little
in the way of convection is expected.  In keeping with the trend of
the past few days, I went above guidance for highs. However,
soundings show some respectable mixing so dewpoints may not be as
high as forecast and thus heat indices maybe a little too robust
this afternoon but certainly at times a few spots may see around 100
heat index.

.LONG TERM.../Wednesday through Monday/
Issued at 346 AM CDT Tue Jul 18 2017

How extreme the heat and humidity will be Wednesday through
Saturday remains the primary forecast challenge as there are
several moving pieces yet that could impact the forecast. A
frontal boundary is expected to be in the vicinity of
central/northern Iowa through the period with periodic chances for
thunderstorms north of the boundary. These thunderstorms may have
a couple impacts including outflow pushing the boundary further
south and/or convective debris inhibiting peak heating. Outflow
from nocturnal convection would also drop overnight lows further
than current forecasts.

The current forecast dewpoints in the low to mid 70s is likely on
the high side with the exception in the immediate vicinity and
north of the boundary. The lack of widespread rainfall over a
large portion of central and southern Iowa will minimize
evapotranspiration from crops. Thursday and Friday in particular
also have the potential to be good mixing days south of the
boundary and very well could mix surface dew points into the 60s.
If this mixing does occur and dew points drop, a few locations
could reach 100 Thursday and Friday. A review of Des Moines 100
degree days from 1934-2013, only 16 of the 160 days had dew points
of 70 or greater at the time of max temperature. Another common
factor for 100 degree days at Des Moines is that they are
typically preceded by a dry period which fits the current
scenario.

By Saturday, the boundary will begin to make a southward push as
the subtropical ridge collapses as a strong short wave trough
enters the Northern Plains. At this time, will leave the current
headlines as a watch. Uncertainty of boundary position Wednesday
due to overnight convection and how progressive the boundary on
Saturday is, along with other factors does put into question the
duration of this event. The hottest days will be Thursday and
Friday but at this time, heat index values should remain below
110.

While heat is a concern central and south, heavy rainfall is
possible over the north with PWATs approaching 2 inches at times
and the threat for several rounds of storms, especially near the
Minnesota border.

The upper flow becomes more northwesterly following the boundary
passage on Saturday with relief from the very warm weather
gradually arriving as high temperatures fall back into the 80s.
Unfortunately if the dry areas of central and southern Iowa miss
out on the rain this week, chances next week are not promising.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1207 AM CDT Tue Jul 18 2017

Mainly VFR conditions through much of the period. Low chance that
current convection over far NW IA reaches KFOD. Better chances for
storms over northern Iowa by Tuesday evening. Expect south to
southeast winds at less than 10 kts overnight then becoming breezy
from the south to southwest on Tuesday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Watch from Wednesday afternoon through Saturday
evening for IAZ033-034-044>050-057>062-070>075-081>086-092>097.

&&

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Donavon



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