Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181648
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1148 AM CDT Tue Jul 18 2017

.UPDATE...
Issued at 1148 AM CDT Tue Jul 18 2017

Have updated the forecast for the light showers still lingering
from the overnight MCS that pushed into SE NE/SW IA. Also adjusted
pops late this afternoon across the central/north limiting pops to
the far north by late afternoon and slowing the progress into
central IA this evening. Latest CAMS and new NAM suggest
convective initiation later in the day/evening. Clouds lingering
from this morning`s activity keeping temps down a bit up north may
also limit storm potential this afternoon.

&&

.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 12Z TAFS through 12Z Wednesday morning/
Issued at 707 AM CDT Tue Jul 18 2017

Mainly VFR conditions through much of the period. Storm chances
increase over northern Iowa by this evening with a potential for
brief MVFR cigs in storms.  Some indication that stratus may for
towards 12Z but this would depend on how much rainfall occurs. Sfc
wind S to SW at 10-15kts.


&&

.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.

&&

$$

UPDATE...Beerends
SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...FAB



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