Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210430
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1130 PM CDT Fri Oct 20 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 333 PM CDT Fri Oct 20 2017

Upper ridging that has been in place the past few days will continue
to push east through the OH Valley tonight with a broad trough in
place across the western/central US. This has set up southwest flow
aloft with moisture return in the lower levels from the Gulf. The
upper trough will elongate and push eastward through the central US
into Saturday/Saturday night, with an associated area of surface low
pressure centered across Manitoba by late Saturday night. An
associated strong cold front is expected to sweep through the state
by late Saturday into Saturday night. An initial shortwave
trough/impulse of energy will approach the state tonight with the
associated moisture advection and lift. This will allow for showers
and some thunderstorms to develop across the region and spread
across portions of western and central IA. CAMS not too robust with
this activity as overall theta-e advection somewhat weak and deep
moisture more limited. Into Saturday morning, weak lift and moisture
likely to keep more of the isolated to scattered shower/storm
development possible and allow for increased cloud cover. As the
main upper trough moves out into the central and northern plains, a
line of thunderstorms is expected to develop in vicinity of the
frontal boundary and spread across central IA as the front pushes
east. Frontal passage timing slowing a bit, but overall expected to
be situated across far western IA by the late afternoon hours at the
time of convective initiation. Strong CAP evident on soundings ahead
of the front, so have lowered pops some across the central/eastern
forecast area through the day Saturday with more of a focus on the
passage of the front. Steep mid level lapse rates will allow the CAP
to erode as the front approaches with sfc-based storms becoming
possible. Expect storms to merge into a line, with MUCAPE values
expected in the 1000-1500 J/kg range. Deep layer shear around 30kts
or slightly higher, with decent SRH values around 100-200. Therefore
this will suggest there is the potential for a damaging winds
threat, especially with fast storm motions. Additionally with a
squall line/line segments and the shear profile/SRH, brief
tornadoes/QLCS tornado activity cannot be ruled out. The front will
quickly progress through the forecast area with some lingering
showers expected behind the front, otherwise as this system moves
east, cooler and dry conditions are expected for Sunday with a west
to northwest upper level flow pattern taking shape.

.LONG TERM.../Monday through Friday/
Issued at 333 PM CDT Fri Oct 20 2017

Another broad upper trough will drop into the north central US into
Monday/Tuesday. This will bring another push of cold air into the
state with a chance for some weak showers Monday afternoon with the
frontal passage. This front will send temperatures closer to the
seasonal averages for the middle of next week. Another stronger
broad upper level trough will drop through the area Thursday into
Friday with another very strong cold front. Could see some showers
with this front and maybe even a little snow mixing in the backside
as showers taper off.  H85 temps will drop into the -8C to -10C
range across the state by late Thursday which should send lows into
the 30s and possibly even upper 20s in spots. This night is when the
greatest frost/freeze potential exists. The cold will continue into
Friday as highs top out in the mid 40s to around 50.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1127 PM CDT Fri Oct 20 2017

For tonight, introduced LLWS for most TAFs. Very strong winds
within 2000 ft of sfc near 40Kts according to model soundings.
Omaha 00Z RAOB supported this. Surface winds should be below
15kts.

Developed SC deck under cap after sunrise and widespread showers
and thunderstorms and frontal boundary moves across after 21z this
afternoon. MVFR CIGs look likely.

&&

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

$$

SHORT TERM...Beerends
LONG TERM...Beerends
AVIATION...Johnson


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