Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131757
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1257 PM CDT Fri Apr 13 2018

...Updated for 18z Aviation Discussion...

.SHORT TERM.../Today through Saturday night/
Issued at 343 AM CDT Fri Apr 13 2018

Active 48 hours coming up today into the weekend. Little changes
still in the overall large features, with a large deep upper level
trough spinning over the four corners region. The associated surface
low centered across central/western KS with a warm front extending
ENE through SE NE/NE KS and into northern MO. This front will lift
northward into southern Iowa and become stretched across the I-80
corridor by mid/late day as the surface low moves into far SE NE. An
area of overrunning showers and thunderstorms present across the
northern forecast area currently and are anticipated to continue
through the day. This activity to remain elevated and of not much
concern regarding severe chances initially, but may intensify into
the daylight hours with a few strong to severe storms possible. By
this afternoon a greater chance for severe storms especially into
the southern half of the forecast area where activity is expected to
develop along and south of the west to eastward stretching warm
front, and then again into late afternoon/early evening with
additional thunderstorm activity expected to develop along a an
eastward progressing dry line feature extending from the surface
low. This feature will rotate E-ENE through the southern forecast
area into this evening. Instability will however wane into the
evening hours limiting the severe threat into the late evening and
overnight hours. Some concern with the stronger CAP in place and the
cloud cover through the day, however expect some peeks of sun in the
southwest and in the area along/south of the warm front. Strong EML
will also aid in the severe potential with instability into the 1000-
2000 J/kg range and strong deep layer shear expected. Still
expecting potential for all modes of severe weather this afternoon
into early evening. Instability then wanes late tonight with the
severe threat diminishing late tonight.

Focus will then shift toward the winter portion of this system as
the surface pressure gradient tightens as the sfc low slides across
northern MO tonight and into IL Saturday. Gusty winds to really
increase tonight across the north and continue into Saturday. Have
sustained winds into advisory criteria north, and may not even be
strong enough with a 15-20 mb surface pressure spread across the
state. Additionally winds aloft int the lower levels around 50 to 60
kts across northern into NW Iowa. Mixed layer winds pushing 30 to 45
kts by late tonight and through the day Saturday. With colder air
moving in and the column cooling and precipitation expected to
change to snow in the north/northwest by early Saturday and continue
through the day. Have upgraded the far NW (where most confident in
higher snow amounts) to a Blizzard Warning for now. Moisture amounts
to lead to higher snow totals, with initially lower snow ratios, but
these should increase as colder air moves in. Also instability
present, so expect some enhanced snow rates at times and potential
for heavier snow in the far northwest where nearly 8 inches of snow
will be possible. Further south and east, snow amounts more
questionable given when the column cools enough for ice introduction
and changeover to snow occurs. Therefore have lighter amounts from 2
to 5 inches from along Highway 30 northward, with the highest
amounts expected toward the MN border. Left the Winter Storm Watch
in place for areas outside of the far NW where less confident in
snow amounts and some areas more borderline for winds and blowing
snow, meaning the rain may changeover to snow later than the time of
strongest winds. Therefore will allow for later shifts to further
evaluate these areas and have greater certainty of where additional
counties in the warning may be needed or placement of an advisory as
well.

.LONG TERM.../Sunday through Thursday/
Issued at 343 AM CDT Fri Apr 13 2018

On Sunday the upper low will be exiting to the east keeping the
threat of at least light snow going through much of the day with
snow changing to rain across the south in the afternoon.  By late in
the day the precip will be mainly confined to the east.  Cold temps
can be expected with highs on Sunday in the 30s and lows at night in
the teens north to mid 20s south.

Monday through Tuesday upper ridging will shift into the region so
dry conditions are expected with temps moderating some...especially
Tuesday.  Highs on Monday will still be cold north especially in
areas where snowpack will likely still exist.  Mid 30s north to mid
40s south is all we can expect on Monday.  By Tuesday the upper
ridge shifts east putting us in a more west to southwest flow aloft
while at the surface a low developing over the Plains will have a
warm front extending into southwest and southern Iowa.  Temps will
be in the mid 40s or so north, largely hampered by residual snow
pack but across the south temps will be in the upper 50s to lower
60s.   The aforementioned low is part of a larger system that will
shift across Iowa Tuesday night and Wednesday.  A wintry mix is
expected Tuesday night north with rain south changing to all rain on
Wednesday.

Upper ridging then moves into the region again for Thursday and
Friday with temps returning to the 40s north and 50s south.  These
highs are about 10 degrees below normal for this time of year so
definitely below normal through the week, except for Tuesday. At the
end of the extended into next weekend will come the next big storm
system.  At this time there are large disagreements between the
models with the GFS and Canadian further south with the low and
trough while the Euro tracks the low further north and east.  This
is beyond the range of this forecast package but will bear watching.
For now we just have rain mentioned west on Friday.  May need to
consider thunder if the Euro is correct or snow if the GFS is
correct.  The crazy cold spring pattern continues for the
foreseeable future.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1255 PM CDT Fri Apr 13 2018

TAFs will be a mess throughout this period. High confidence in IFR
conditions. Will need to watch KFOD and KMCW in particular after
12z for LIFR potential for reduced vsbys due to blowing snow.

Confidence medium to high on thunder... KDSM and KOTM in
particular will need to have thunder modified for this evening as
strong to severe thunderstorms possible. Next TAF update will
feature more precise timing and location on thunder, along with
possible BLSN mention at KFOD and KMCW.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday morning through late Saturday
night for IAZ023-024-033>035-044-045.

Winter Storm Watch from Saturday morning through Sunday morning
for IAZ006-007-016-017-025.

Blizzard Warning from 7 AM Saturday to 1 AM CDT Sunday for
IAZ004-005-015.

&&

$$

SHORT TERM...Beerends
LONG TERM...FAB
AVIATION...Kotenberg



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