Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150347

Area Forecast Discussion
National Weather Service Des Moines IA
947 PM CST Wed Feb 14 2018

...Discussion on Near-Term Trends...

Issued at 947 PM CST Wed Feb 14 2018

No headlines planned at this time for development of dense fog
later tonight. GOES-East Nighttime Microphysics has shown the
western edge of the dense fog over eastern Iowa dissipating over
the last hour and a half. Most of the dense fog is now east of the
state over northern Illinois and southern Wisconsin. Over central
Iowa, mid and high level clouds obscure GOES-East`s view of any
low level water clouds. However, 03z surface observations have
visibilities ranging from 4 to 7 miles with isolated pockets of 3
miles. So far this evening, winds remain elevated from the south
between 8 and 14 knots, which will allow for a continued influx of
moisture into the state. These winds will eventually diminish
some toward 9 or 10z. In addition to the moisture advection,
temperatures ranging from the middle 30s to low 40s at mid-evening
will continue to allow for snowmelt further increasing the
availability of near surface moisture. Still think there will be
dense fog forming over the southeast part of the forecast area
toward 10 or 11z. CONSShort, RAP, and GLAMP support this idea
though differ on how much of the area will be impacted. Therefore,
will hold off on any advisory for dense fog later tonight.
Forecast overall remains on track including weather grid which
shows the patchy to areas of fog now as well as a possible area of
dense fog later tonight.


.SHORT TERM.../Tonight through Thursday/
Issued at 345 PM CST Wed Feb 14 2018

Main concern tonight will be fog and reduction of visibility.
Stratus and fog has been advecting northward all day long from
Missouri and has crossed into Southern Iowa.  With shallow moisture
in place up to about 2Kft and a fairly strong south to southwest
flow in place, we will see this moisture continue to advect into
Iowa.  Models are quite variable as to the extent of the dense fog
and as such and in collaboration with neighbors...we will hold off
on a dense fog advisory for now but I would expect with sunset, it
will not take long for fog to develop.  The most likely area to see
dense fog will be the southern third or so of the state and
especially east of I-35 although several models hint at a corridor
from Atlantic to Des Moines then just north of the I-80 corridor
will see quarter mile visibilities or less.

For Thursday a cold front will push across the state by early to mid
afternoon switching winds to the northwest and helping to lift the
low clouds and fog.  Thursday afternoon the winds will increase and
by evening a wind advisory may need to be considered.  This will be
discussed in the next section.

.LONG TERM.../Thursday night through Wednesday/
Issued at 345 PM CST Wed Feb 14 2018

Thursday night an upper trough/shortwave swings through the region
with a vorticity axis through MN and northern IA.  Moisture is
decent though not great and the forcing with this outruns the wave
so while precip chances are not great...there is at least a slight
chance for some very light snow or flurries across the far north for
a brief period of time late afternoon/early evening time frame.
Strong cold advection will occur late Thursday and Thursday night
and winds in the mixed layer, which will be quite deep, will be
30+kts.  While it is not quite advisory criteria, there will likely
be areas that have near or just touching advisory criteria winds.
this will need to be monitored for a potential advisory as the details
become a little more clear.

Friday, high pressure slides over the state and it will be cold but
with lighter winds so it won`t feel as bad as if there were some
wind with the cold.

High pressure will shift to the southeast on Saturday allowing winds
to switch around to a southerly direction and temps will respond by
climbing back into the 30s and 40s.  Another shortwave will pass
across the state.  The euro has been consistent spreading precip
across the forecast area while other models confine the precip to
the north.  A vort lobe slides down the wave and will likely be
responsible for precip further south.  The problem is moisture. We
are lacking it and as such it may be something along the line of
sprinkles south and flurries north.  There is better moisture and
some forcing northeast so I did put chance pops MCW and ALO but we
will see what future model runs do.

A stronger system impacts the area Sunday/Monday but temps make this
system interesting.  Depending on how fast the front moves thorugh,
southern or southeast Iowa could be quite warm and precip may be
rain with snow across the north/northwest.  QPF does not look to be
a lot at this point so we are not looking at a lot of snow at this
point but this will bear watching because it`s a pretty strong
surface low moving across the state.  Going forward... a deep trough
sets up over the southwest U.S. setting us up for numerous
disturbances to come across the state.  Temps will not be all that
mild though with a high sitting off to our northwest, low level cold
air will move into the state.


.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 556 PM CST Wed Feb 14 2018

Primary concern in the near term will be fog development and low
level wind shear. Low level wind shear may impact the northern
terminals this evening with wind speeds in the 40 to near 50 knot
range at 2000 feet with surface winds between 10 and 15 knots.
Otherwise, fog over eastern Iowa early this evening is expected to
develop westward. Given the good amount of snowmelt and therefore
available near surface moisture, will likely see IFR or LIFR
conditions develop at the southern terminals later tonight into
tomorrow morning. There may also be a window for similar
conditions at KALO this evening as well. Conditions will improve
into through Thursday morning with VFR conditions returning for a
period Thursday afternoon. With the focus of the TAFs on the fog,
lines were left off for MVFR ceilings arriving late Thursday
afternoon over the northern terminals. In addition, winds from the
northwest will begin to increase toward the end of the TAF
period into Thursday night.




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