Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210854

Area Forecast Discussion
National Weather Service Des Moines IA
354 AM CDT Sat Oct 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Sat Oct 21 2017

An intense 65+ kt low level jet is currently in place from Kansas
through eastern Nebraska and into southwest Iowa. This has led to
another round of strong theta-e advection into the area and an
increase in convective instability. A cluster of thunderstorms has
developed in response over southwest Iowa and is lifting north
northeast at around 40 mph. A few vigorous updrafts have occurred
but thus far they have had difficulty sustaining with the effective
deep layer shear less than 25 kts. Small hail remains possible with
these storms and it remains possible that these updrafts may sustain
just long enough for marginally severe hail to occur. This cluster
will continue to lift rapidly and should be into Minnesota by 9-10

A slight risk for severe weather still exists across much of central
Iowa for later this afternoon and into the evening as a cold front
enters the state concurrent as a vigorous short wave arrives. An
elevated mixed layer (EML) will exist much of the day and will limit
thunderstorm activity for much of the day after the morning activity
exits the region. Sounding profiles indicate a saturated layer just
below the EML that may result in a stratus deck persisting much of
the day. This is further supported by increasing status around 2800
ft over eastern Nebraska and into western Iowa and ahead of the
approaching boundary. The clouds may limit instability and could
dampen the severe weather threat.  That said, considering the
strength of the kinematic forcing arriving with the upper short
wave, the upper level divergence and the arrival of the boundary,
the EML should erode and convection should initiate.  Damaging winds
and large hail will be the primary threat with these storms though
an isolated tornado could occur.  There are a few things that are
not favorable for QLCS generation and this may limit the damaging
wind and tornado threat. The 0-3 km shear is generally 25 kts or
less. The 0-3 km theta-e differences are at 10 K or less which
diminishes a strong cold pool threat. Finally storm motions are
nearly parallel to the boundary. PWAT values will be nearly 225
percent of normal at 1.5 inches and warm cloud depths over 12 kft
along with the parallel storm motion to the boundary may lead to
localized heavy rainfall. The convection will gradually diminish
overnight though a few showers may linger late over the far east.
Temperatures will cool to near 40 over the west by Sunday morning
while hovering near 50 east.

.LONG TERM.../Sunday through Friday/
Issued at 340 AM CDT Sat Oct 21 2017

On Sunday the upper trough finally pushes east of the region by
the afternoon hours ending the precip over the east and ushering
in what will be the first of several shots of cool air into Iowa.

On the heels of the firs trough, a second trough spills out of the
Northern Plains into the Upper Midwest and deepens as it plunges
towards the Southern Plains and eventually moves off into the Great
Lakes.  This will bring the second shot of cool air down over the
region by Tuesday, dropping highs into the upper 40s northeast to
mid 50s southwest.  The Euro and Canadian are fairly consistent with
dropping a lobe of vorticity across the eastern portion of the state
which may trigger some showers across our far east and especially
east of the forecast area.

Wednesday into Thursday, short wave ridging moves across western and
southern part of the Upper Midwest as a surface low develops over
the Northern Plains and lifts northeast with time. This will draw
a little warmer air back into the state, especially west and
south so highs will not be as cold Wednesday and Thursday.

Beyond Thursday models vary on the evolution of a deep trough out of
the Pacific Northwest.  The GFS and Canadian keep this deep trough
open and progressive while the Euro tries to develop a closed low
over the Southwest U.S.  As a result the Euro draws up more moisture
with the trough and puts us in a southerly flow while the other two
models drop the trough through us putting us in a deep northwest
flow.  Obviously this will have a huge impact in temps.  At this
point the Euro is the odd model out and I have opted to go more in
line with the GFS and Canadian.  There is some very cold air with
this late week trough and if the GFS verifies, we will have highs in
the 40s and 50s with lows Friday morning in the lower to mid 30s. It
is quite possible that any precip associated with the passage of
this trough may be in the form of a rain/snow mix at least briefly.

Going into next weekend, the Euro and the GFS both drop another wave
across the Upper Midwest while the Euro finally lifts that closed
low over the Southwest U.S. up into MN, no matter the solution, next
weekend looks cold and at least a chance for precip.  The pattern
has definitely turned to fall by mid to late week and beyond.


.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

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.




SHORT TERM...Donavon
AVIATION...Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.