Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 180318
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1018 PM CDT Tue Apr 17 2018

.UPDATE...
Issued at 1017 PM CDT Tue Apr 17 2018

A vigorous PV Anomaly is racing eastward and is now entering
western Nebraska. The associated surface low pressure system is
currently entering central Kansas with an inverted trough
extending northward into Nebraska. This entire system will
continue to move east overnight. This area of low pressure is
drawing strong low level moisture advection northward from the
Gulf of Mexico in addition to some mid level Pacific moisture
being ingested from the southwest. A 70 kt low level jet will
develop around 06z tonight over eastern Kansas and will push
strong theta-e advection into southern Iowa and will help generate
good instability across the area. MUCAPEs in excess of 1000 J/kg
are possible initially before gradually weakening as it lifts to
the northeast. Expect a region of showers and thunderstorms to
develop in response. With this instability and mid level lapse
rates in excess of 8C/km along with a relatively cool atmosphere,
hail is likely to develop with a locally severe storm possible.

The precipitation type over northern Iowa will be mainly snow and
with the convection, thundersnow is certainly a possibility along
with snowfall rates of 1-2 inches per hour possible. Farther south
somewhere along the Highway 20 to Highway 30 corridor, a wintry
mix is possible and could begin with a period freezing rain
considering it will be dark and road temperatures will be near
their minimums and in the upper 20s to around 30 along and north
of Highway 30. Overall, generally expect a broad area of
precipitation initially with the convective wave then this system
likely will pivot somewhere over north central into northeast Iowa
and lead to a concentrated area of heavier amounts where good
dendritic growth maintains for a prolonged period.

&&

.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 00Z TAFS through 00Z Wednesday evening/
Issued at 638 PM CDT Tue Apr 17 2018

VFR conditions this evening the conditions will deteriorate after
06z as the next system impacts the area. Widespread IFR to LIFT
conditions are expected across most site by Wednesday morning due
to low cigs and developing snow. The precipitation will have the
most significant impact through 20z before diminishing. A few
thunderstorms will be possible late tonight into Wednesday
morning. Expect gusty northwest winds to develop on Wednesday as
well.


&&

.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.

&&

$$

UPDATE...Donavon
SHORT TERM...Ansorge
LONG TERM...Cogil
AVIATION...Donavon



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