Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191148
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
648 AM CDT Wed Apr 19 2017

.SHORT TERM.../Today through Tonight/
Issued at 406 AM CDT Wed Apr 19 2017

Sfc boundary has stalled near the IA/MO border and southeast into
northern Kansas while cyclogenesis continues over eastern Colorado.
The low level jet has increased to near 60 kts through Kansas and is
bringing strong theta-e advection northward into portions of
Nebraska and southern Iowa. MUCAPEs have increased to near 1000 J/kg
through that region and has led to elevated thunderstorm development
that has spread into southwest Iowa. These storms will continue to
lift northeast this morning with much of the activity into northern
Iowa by noon. A couple strong updrafts are embedded with these
storms currently and a few have cloud tops exceeding 40 kft and GOES-
R IR data showing cloud tops near -65C. These storms will carry an
attendant quarter sized hail threat along with locally heavy
rainfall as they lift north.

The warm front will lift north this morning and be near Highway 34
in southern Iowa by noon then be near the Interstate 80 to Highway
30 corridor by mid to late afternoon. The presence of an elevated
mixed layer(EML) should inhibit thunderstorm initiation along the
warm front through much of the afternoon. Surface based storm
activation likely will be over far west central and southwest Iowa
late this afternoon in vicinity of the surface low and attendant
cold front and in a region where dynamic cooling aloft associated
with an approaching short wave will help erode the EML. Deep layer
shear of 40 to 50 kts will supercell development upon initiation.
Golf ball sized hail or larger and damaging wind gusts in excess of
60 mph will be possible with these storms. In addition a tornado
threat will develop along the line but especially near and just east
of the surface low and in the vicinity of the warm front. Storm
relative helicity values should be enhanced through this region to
over 300 m2/s2 thanks to differential heating with clouds and cooler
temperatures north of the warm front and good heating to the south.
This will also be a region of best backed winds to the southeast.

The storms may gradually transition into line segments with stronger
HP structures embedded along the line as it translates east. Low
level shear profiles will increase after 00z and will keep an
attendant tornado threat along the line. 0-1 km shear values
increase significantly to near 50 kts towards 03z over eastern Iowa.
The threat may become more QLCS as storm tops lower with loss of
some instability but storm motions become more oriented along with
the 0-3 km shear vector. A damaging wind threat will persist through
this time as well though the large hail threat will gradually
diminish.

Locally heavy rainfall will be possible through the period. A flash
flood watch is still in effect across southern Iowa though the
threat in that area appears to be diminished.  Will keep it going
for now to ensure the morning activity moves through then reevaluate
again.

.LONG TERM.../Thursday through Tuesday/
Issued at 406 AM CDT Wed Apr 19 2017

The main forecast concern over the extended was on temperatures
through the weekend and then precipitation chances early next
week.

Thursday through Sunday...CAA looks to develop throughout the day
Thursday as surface high pressure builds into the region.  Decent
mixing to around 900-850mb will cause for a fairly breezy day.
lingering mid-level cloud cover should inhibit the deeper mixing, at
least across central to northeast portions of the forecast area. The
southwest looks to clear out during the afternoon and mixing might
be a bit stronger in this location. However, the surface pressure
gradient will not be as strong in this location as the high begins
to nudge into western Iowa late Thursday afternoon.  Regardless,
confident enough to nudge up winds 2-4 knots during the day Thursday
across the entire forecast area.

Models continue to track the next upper low over the Southern Plains
and Ozarks Friday into Saturday and with the strong surface high
pressure over the region, the majority of the state will be dry
Friday night into Saturday. Temperatures look to be actually at or
slightly below normal Thursday into Saturday. Temperatures finally
rebound on Sunday as the surface high pressure moves over the Great
Lakes and allows for weak return flow to develop Sunday afternoon.

Monday into Tuesday...The GFS and ECMWF are in good agreement for a
surface low pressure system to move through the region Monday into
Tuesday. The GFS is slightly further south and a bit slower than the
ECMWF as it tracks the low through southern Minnesota/far northern
Iowa Monday night.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 648 AM CDT Wed Apr 19 2017

Showers and thunderstorms will move north through the area this
morning. A warm front will lift north into southern and central
Iowa through this afternoon with areas of MVFR to local IFR cigs
north of the boundary. Additional storms will fire along a cold
front over western Iowa this afternoon. This boundary will bring a
switch to westerly winds overnight with fropa. Additional lowering
cigs will arrive over northern Iowa late tonight with the
potential for IFR cigs.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for IAZ057>062-070>075-
081>086-092>097.

&&

$$

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



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