Area Forecast Discussion
Issued by NWS Des Moines, IA

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533
FXUS63 KDMX 172119
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
419 PM CDT Tue Apr 17 2018

***Spring Storm With Wintry Precipitation Shifting Southward***

.SHORT TERM.../Tonight through Wednesday/
Issued at 415 PM CDT Tue Apr 17 2018

This evening will start off quiet as low pressure approaches the
state from Kansas. However, wintry precipitation will spread over
central Iowa late this evening and through the overnight hours. As
has been the case over the last 24 hours or so, the preponderance of
model guidance has continued to shift southward and this southward
shift continued with the 12z runs today. Consensus of model guidance
shows the low pressure tracking from near Topeka, KS to central
Illinois from midnight tonight through noon Wednesday. This shifts
the heaviest QPF into our northern forecast area and now brushes the
I-80 counties with wintry precipitation as well.

As for the details, soundings show initially dry air in the low and
middle levels. This will allow for a period of rain or freezing rain
depending the surface temperatures initially. However, saturation
and cooling occurs rather quickly in both the NAM and the GFS
between roughly midnight and 6am that much of the precipitation
over the north will change to snow. Strong frontogenesis in the
850-700mb layer and down right impressive omega values will aid in
dynamic cooling for a period of heavy snow. Cross sections along
the Highway 20 corridor at KFOD and KALO show this strong lift and
saturation occurring over a 3 to 6 hour window centered on
roughly 3am to mid-morning across northern Iowa. Rates will
exceed 1 inch per hour and perhaps reach 2 inch per hour at times.
This is shown in the SREF probabilities from the 9 and 15z runs
over northern Iowa, though these will likely be a bit farther
south than shown. With the southward shift in the low pressure and
QPF, will add a row of counties to the warning (Highway 20 from
Waterloo to Fort Dodge and Pocahontas County) and a row of
counties for the advisory (I-80 counties). With any of this
precipitation late tonight into early tomorrow morning, thunder
will be possible with steep mid-level lapse rates supporting
elevated instability. In addition, soundings still show
instability in the low-levels tomorrow afternoon pointing to the
potential of horizontal convective rolls that may enhance snow
rates at times. When the event wraps up Wednesday afternoon or
evening, will see an area of 6-8 inches over northern Iowa, though
wouldn`t be surprised that a few places overachieve toward 12
inches due to the convective, dynamic nature of this event. Closer
to Highway 30 and I-80, still not out of the question for higher
snow amounts if this system continues to track south.

Farther south where boundary layer temperatures are warmer and there
is not as much ice introduction, will see a period of freezing rain
from south of Highway 20 to between Highway 30 and I-80. Much of
this freezing rain will occur in the pre-dawn hours Wednesday,
though may linger in some areas through midday. Overall, icing
amounts won`t be that great with the highest amounts largely
confined to elevated surfaces. This was the reasoning to adding
the I-80 counties and also takes into account the uncertainty in
case slightly higher snow amounts shift south.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 415 PM CDT Tue Apr 17 2018

Will begin to see some change in the upper pattern into the longer
term with a much needed break from the active and record cold seen
over the first half of the month.  The mean position of the upper
jet will be displaced farther north into the weekend and early next
week allowing for some recovery in temperatures to normal or
even above normal.

Will see the departure of winter storm Wednesday evening with any
lingering pcpn ending.  Surface ridging will build toward the state
late overnight with increasing subsidence and thermal trof lingering
in the state.  Temperatures will remain unseasonable cold from
tomorrow night into early Friday with the thermal trof lingering
near the state.  However, warm advection is forecast to slowly begin
on Friday as the ridge slides just east of the forecast area.  The
ridge will linger in that position for much of the weekend until the
upper low to the south of Iowa passes.  The dry air and subsidence
associated with the high will keep the threat of precipitation with
the upper low mostly to the south of Iowa although a few sprinkles
may catch far western and southern portions of the forecast area on
Friday night into Saturday morning.  More seasonable temperatures
will be in place on Sunday into Monday as warm advection continues
and the bulk of snow cover should be melted away by then.  A
boundary does arrive toward the end of the period with some threat
of showers into Tuesday, although with limited moisture, coverage
and intensity look rather sparse at this time.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1214 PM CDT Tue Apr 17 2018

High confidence in VFR conditions through at least early this
evening and then confidence decreases as a spring storm with
wintry precipitation moves into the state. Will see conditions
dropping into IFR at all sites but OTM a few hours either side of
12z. Did include a period of LIFR visibilities at FOD and ALO
Wednesday morning where confidence is highest in a period of
heavy snow, though fine tuning of timing and overall impacts at
all terminals is expected. Finally, still going over the
preponderance of incoming data so not included in this set of
TAFs, but wintry mix may slip a bit farther south into the DSM
terminal.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 AM Wednesday to 1 AM CDT Thursday
for IAZ047>050-059>062.

Winter Weather Advisory from 1 AM to 7 PM CDT Wednesday for
IAZ033-034-044>046-057-058.

Winter Storm Warning from 1 AM to 7 PM CDT Wednesday for IAZ004-
005-015-023-024-035.

Winter Storm Warning from 4 AM Wednesday to 1 AM CDT Thursday
for IAZ006-007-016-017-025>028-036>039.

&&

$$

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



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