Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180524
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1224 AM CDT Sun Aug 18 2019

.UPDATE...
Issued at 933 PM CDT Sat Aug 17 2019

Concerns are increasing regarding convective severe and heavy rain
impacts into the night. 00z model trends suggest the moderate to
extreme MUCAPE axis currently just to our west will shift eastward
into at least western Iowa overnight, coincident with the
maturing 40kt low level jet and strong effective shear. This
should have no problem sustaining convection into the night, and
most convection allowing guidance suggests this as well with
several depicting an increasing damaging wind threat across
northern IA during the early morning hours on the south end of a
spiraling MCV moving across MN. There will be some nocturnal QLCS
potential as well with persistently high and favorably oriented
0-3km shear vectors. Would not be surprised at all to see a Severe
Thunderstorm Watch issued by the early morning hours.

The wind threat should not last too long, but the guidance suite
also suggests the south end may lay out west to east easily
realizing the moisture transport and creating a favorable
situation for heavy rains. The environmental looks good
considering 4km warm cloud depths and precipitable water values
approaching two inches. Many CAM solutions are showing one to
three inch rainfall amounts over three hours. Will increase heavy
rain messaging, but hold off on a Flash Flood Watch for now
however considering our antecedent moisture conditions and the
assumption that the MCS will be somewhat progressive with the
strength of the low level jet producing fairly strong, southerly
Corfidi vectors.

&&

.SHORT TERM.../Tonight through Sunday/
Issued at 322 PM CDT Sat Aug 17 2019

Forecast challenges continue to be thunderstorm chances over the
next 24 hrs and severe weather potential. An initial band of theta-e
advection has led to an area of modest thunderstorms to develop over
west central Iowa while additional activity has developed over east
central Iowa. At this point would expect the activity over western
Iowa to maintain and continue to move east/northeast this
afternoon. Once that activity moves through, do expect a lull
until later this evening as warming forms a cap near 750mb which
should limit additional development until the low level jet
increases. The low level jet will increase by mid to late evening
and will bring another strong surge of theta-e advection into
southwest Iowa. By this time, there should be ongoing storms over
southern Nebraska into northern Kansas which should translate to
the southeast and miss most of Iowa. A few additional storms may
develop into southwest Iowa then spread east/northeast across the
southern portion of the state. Another area of storms is already
firing over western South Dakota in vicinity of an advancing cold
front. These storms should reach northwest Iowa around midnight
then move across portions of northern Iowa. The movement of these
two systems could lead to a split across central and western Iowa
where little to no precipitation occurs after this afternoons
convection passes.

The greatest severe weather threat should coincide with the northern
storms late tonight with damaging winds and an isolated tornado or
two possible. The tornado threat would be fueled by sufficient 0-3
km CAPE and effective SRH > 200 m2/s2. That said, both 0-1 km shear
and 0-3 km shear are not real favorable for QLCS tornadoes.

The cold front will reach central Iowa by Sunday morning. Any
afternoon thunderstorms will be conditional placement of the
boundary, which now looks to be lingering over southern Iowa. Near
unidirectional flow aloft and sufficient CAPE for updrafts
accelerations for an attendant damaging wind or large hail threat.
High pressure arrives Sunday night with temperatures dipping into
the mid 50s north. The arrival of high pressure and light winds
along with recent rainfall may lead to fog development by early
Monday morning.

.LONG TERM.../Sunday night through Saturday/
Issued at 322 PM CDT Sat Aug 17 2019

High pressure will pass through the region on Monday morning and be
moving east through the day as upper level high pressure builds over
the southern Plains with Iowa on the northern periphery of this
feature. As the high moves away, return surface flow will become
from the southeast ahead of a shortwave trough moving overtop the
upper level high. While 850mb temperatures change little from Sunday
afternoon into Monday afternoon over central and eastern Iowa,
western Iowa should see temperatures several degrees higher than
Sunday. Compared to previous forecasts, trends continue to decrease
temperatures and therefore the early week heat that has been
messaged may not come to fruition. The strongest Q-vector
convergence will pass north of the state with the low level jet
strongest over Minnesota. With low level warm air advection, may be
able to get a few storms to develop late in the night over western
Iowa. Main concern with these would be hail given strong MLCAPE and
instability forecast within the -10 to -30C layer along with steep
mid-level lapse rates around 8.5C/km. Any storms that persist are
expected to pass through central Iowa in the morning to early
afternoon hours. It should be noted that the southern extent of
storm development may be inhibited by rising heights. Another
shortwave will round the top of the upper level high/ridge Tuesday
night into Wednesday. This will bring another chance for
thunderstorms, perhaps in the way of a mesoscale convective system.
The Canadian is a northern outlier in precipitation location
compared to the GFS, ECMWF, and the National Blend of Models. So,
favored location for rainfall will be over southern Iowa with
highest QPF focused over southwest Iowa. Drier and cooler air will
arrive on Thursday into the end of the week. By next weekend, all
models signal a slow moving trough/cut off low approaching from the
west. This results in differences in timing and placement as this
feature moves eastward, but could bring additional storm chances
late in the week or early next weekend with overall confidence low
on details.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1223 AM CDT Sun Aug 18 2019

Main concern will be strong convection through 12z...lingering
into 15z southeast. Expecting possible winds to 45kts with
passage of convection at KFOD/KMCW...with lesser wind at
KDSM/KALO. Low visby and MVFR/IFR cigs will also accompany the
passage of storms over northern sites. Aft 15z will see
improvement in cigs with return to mainly VFR conditions aft
12-20z KFOD/KMCW/KALO and aft 13z-15z south sites KDSM/KOTM.

&&

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

$$

UPDATE...Small
SHORT TERM...Donavon
LONG TERM...Ansorge
AVIATION...REV



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