Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 240541
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1141 PM CST Fri Feb 23 2018

.SHORT TERM.../Tonight through Saturday Night/
Issued at 335 PM CST Fri Feb 23 2018

The big concern will be yet another complex winter system in the
offing with lingering questions about precip type and associated
snow and icing accumulations. The parent system is still quite a
ways off, just rounding the base of the NV/UT long wave trough.
Warm/theta-e advection will organize over the southern Plains
tonight and eventually spread light precip into southern Iowa by
morning. The most likely precip type will be light freezing
rain/freezing drizzle initially with most models showing
saturation up to -10C. Some models do depict moisture to a slight
deeper extent however so there is some potential for inefficient
non-dendrite snow/sleet mixed in as well. Confidence in this
regard is low. By midday this 1-3km thermodynamic forcing will
strengthen further with deeper moisture entering NW Iowa as the
larger kinematic forcing reaches the Plains. This will be
coincident with inverted trough development into western Iowa
which will place much of southern and east central sections
sufficiently into the warm sector to flip precip from freezing
rain to rain as air and road temps warm. This may place <0.10 ice
accums into southern and west central Iowa by Noon so Winter
Weather Advisories have been issued at various start and end times
to account for this.

Heading into the afternoon a fairly narrow ribbon of icing may
continue SW-NE from roughly Audubon and Denison toward Charles
City with the main attention turning to snow trends farther north.
The typical deformation/frontogenic maturation will occur toward
00z and place the primary burst of snow across northern Iowa 21z-
03z. The best Fn response may be just outside of our forecast area
and is somewhat disjointed and the deformation zone residence
time is not great with fairly fast movement. Although the
Dendritic Growth Zone lift is strong for 4-6 hours, moisture in
that layer is somewhat questionable due to the dry slot so all of
these factors may limit overall snow accumulations and ratios
somewhat. Thus expect general three to six inch accumulations
roughly along and north of a Sac City to Hampton line. Low static
stabilities on the dry slot gradient may lead to brief convection
at times, with some models depicting token MUCAPEs so would not be
surprised to see isolated higher amounts where convection and/or
more efficient snow ratios can occur. The advisory segment is in
effect 18z-09z to account for mixed precip, snowfall and then
blowing snow.

The aforementioned instability may also lead to some degree of
weak convection and thunder in the warm sector with frontogenesis
also setting up farther south and east just ahead of the sharp
H85/H7 thermodynamic UVM/DVM couplet. The passage of this feature
should quickly reduce precip rates by 03z in both the cool and
warm sector. The system eventually deepens at a quicker rate
Saturday Night as the PV anomaly approaches the surface low and
low level baroclinic zone, which results in a brief surge of gusty
wind Saturday Night. This may result in a period of blowing snow
into the overnight hours north in the deeper accums.

.LONG TERM.../Sunday through Friday/
Issued at 335 PM CST Fri Feb 23 2018

Sunday through Friday... Southwest flow aloft will continue into
next week with building heights aloft. A progressive upper trough
emerges out of the Southern Rockies by Wednesday. The upper trough
will move through the plains Wednesday and into the Great Lakes and
Ohio Valley by Thursday. The 12 UTC GFS/ECMWF have significant
differences in handling the upper trough. The GFS is initially much
faster and then deepens the system into a closed upper low with a
negatively tilted shortwave over northeast Kansas by 00 UTC
Thursday. Meanwhile, the ECMWF is much slower and weaker and by 00
UTC has an upper, positively tilted shortwave over eastern New
Mexico. The GFS moves the upper low into the Ohio Valley by Thursday
while the ECMWF has a weaker open wave moving through the southern
plains. Needless to say, the differences in the sensible weather in
central Iowa is significant with these two solutions. The best
course of action is to forecast precipitation across Iowa Wed/Thu
time period which is supported by all solutions. The GFS QPF is
much more robust which makes sense. Thermodynamics are warm in all
cases and much of the precipitation would be liquid with this
system. If the upper trough closes off and moves overhead, there
could be more frozen precipitation. However, there will be no cold
air source in the vicinity by mid week other than the upper low.

Look for dry weather early in the week with a warming trend.
Temperatures should be well above normal with highs approaching 60
in southern Iowa on Tuesday.  Fairly high POPS with the storm, but
no significant cold air, the temperatures should only retreat back
to near normal for late February.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1138 PM CST Fri Feb 23 2018

Dense fog and LIFR cigs are developing across northern Iowa.
Expect the fog to persist into Saturday morning. Lowing cigs and
precipitation will lift out of Missouri by Saturday morning and
spread across the entire area. Expect a wintry mix at all
locations transitioning to rain at KDSM/KOTM by the afternoon and
snow over the northwest. Gusty west winds will arrive late. Much
of Saturday will have LIFR/IFR conditions.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 AM to noon CST Saturday for
IAZ060-070>075-081>086-092>097.

Winter Weather Advisory from 9 AM to 9 PM CST Saturday for
IAZ033-034-044>048-057>059.

Winter Weather Advisory from noon Saturday to 3 AM CST Sunday
for IAZ004>007-015>017-023>026-035>037.

Dense Fog Advisory until 6 AM CST Saturday for IAZ004>007-
015>017-023>025-033>035.

&&

$$

SHORT TERM...Small
LONG TERM...Johnson
AVIATION...Donavon



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