Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172342
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 PM CDT Mon Apr 17 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 329 PM CDT Mon Apr 17 2017

Primary concern through the short term period will be shower
and thunderstorm chances on Tuesday, but first quiet conditions will
prevail over central Iowa through the remainder of today. As of 19z
the upper level ridge axis was positioned up the Missouri River
valley, while an area of surface high pressure has drifted eastward
toward the Great Lakes Region. The next shortwave trough to impact
the area has quickly moved eastward out of the Pacific NW and into
northern MT.

Winds have already begun to switch around to out of the south in
response to deepening lee side cyclogenesis over the northern High
Plains. It will take a while for the southerly flow to usher in
better low level moisture in our area, as the winds right now are
just recycling dry air around the backside of the aforementioned
high. Moisture advection ramps up late tonight and into Tuesday
morning with the help of a strong LLJ. A surface low will push
through the Dakotas with a trailing cold front eventually moving
into Iowa Tuesday morning. Showers and storms will likely already be
ongoing in the vicinity of the front as it moves out of NE/SD and
into the CWA. Inherited POP forecast still looks good, just made a
few adjustments to slightly slow up the timing. Still some
uncertainty with respect to spatial extent and intensity of the
precipitation. This is a rather compact system, which keeps much of
the deep layer lift to the north into Minnesota. This might keep the
QPF scattered and restricted to areas along and north of the front,
an idea that has been supported by morning CAM runs. The front will
slowly push south through central Iowa eventually stall out late
Tuesday near the IA/MO border as it becomes oriented parallel to the
mean flow aloft.

.LONG TERM.../Tuesday night through Monday/
Issued at 329 PM CDT Mon Apr 17 2017

Stalled front over southern Iowa and northern Missouri Tuesday night
will be the focus for showers and a few stronger thunderstorms as
the low level jet ramps up with MUCAPE values between 1000 and 1500
J/kg. PWATs will be between 1 and 1.5 inches so locally heavy
rainfall will be a concern along with elevated storms that will
pose a hail and wind threat over southern Iowa. This stalled
boundary will lift north as a warm front back into Iowa towards
I-80 as surface low pressure moves from Colorado into eastern
Nebraska. This places southern Iowa in the warm sector
characterized by mid-afternoon dewpoints in the low and middle
60s, MUCAPE generally between 1000-2000 J/kg, and deep shear
supportive of supercells. This will be the area most prone to
severe weather Wednesday afternoon and evening, including all
modes of severe weather and localized heavy rain. Farther north,
it will be much cooler as temperatures struggle to reach the low
50s under the clouds and showers accompanied with elevated
thunderstorms.

As the low moves through Iowa and into the western Great Lakes,
showers will come to an end on Thursday with cooler and drier air
arriving on winds from the north and northwest. The next system will
drop out of the Pacific Northwest over the Rockies with an upper low
moving through Kansas and Missouri Saturday and Sunday. The CMC
continues to be an outlier with rain approaching the Minnesota
border. The GFS is farther south with rain reaching the southern
half or so of Iowa with the ECMWF just brushing far southern
Iowa with rain. Bottom line is that the best chance for rain will
be over southern Iowa from late Friday into late Saturday.
Temperatures will remain below normal for late this week and early
this weekend.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 642 PM CDT Mon Apr 17 2017

VFR conditions will be widespread across the forecast area
overnight with winds increasing from the southeast overnight.
Low level non convective wind shear appears likely over the
northwest overnight and have included in KFOD and KMCW. Scattered
showers will increase in coverage toward daybreak in the northwest
with this activity passing mostly across northern Iowa
through early afternoon. Forcing will diminish during the day with
coverage decreasing by afternoon. There may be some localized MVFR
ceilings will the precipitation, but mostly VFR conditions are
expected in Tuesday with gusty south to southwest winds ahead of
the boundary by midday.

&&

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

&&

$$

SHORT TERM...Martin
LONG TERM...Ansorge
AVIATION...Cogil



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