Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 140530

Area Forecast Discussion
National Weather Service Des Moines IA
1130 PM CST Tue Feb 13 2018

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Wednesday/
Issued at 303 PM CST Tue Feb 13 2018

Confidence: Medium

Main challenge will be the chances for overnight fog across the
region and this lowers the confidence in the forecast. Competing
factors may limit overall coverage to low lying areas and river
valleys. Temperatures have warmed faster across the south today than
earlier anticipated with above freezing readings from west central
through southeast this afternoon. Southwest winds of 10 to 15 mph
likely through late afternoon then dropping off to 5 to 10 mph
overnight...but steady. Currently there is a narrow ribbon of
slightly higher moisture tracking into southeast Nebraska/western
Iowa and this will move over northern/western/northern Iowa
overnight. Overnight thickness values warm then steady off somewhat
through morning. This may help lessen radiative processes. That and
winds generally just above a preferred level for extensive fog
formation should limit the coverage.  SREF ensembles are the most
aggressive solution for thicker fog potential tonight/early
Wednesday morning and are highlighting fog southwest/west and
northern areas. Evening and overnight crew will need to monitor
trends and make adjustments as necessary. Given the lack of a
boundary to focus the moisture...and a lack of snow cover over most
of Missouri/Kansas...the airmass advecting north into Iowa isn`t
really experiencing a large increase in moisture just yet. Normally
would wish to see an area of mid 30 dewpoints moving north into the
region to enhance fog potential.  For tomorrow the hires models
suggest some clouds over the south as stratus in MO may clip the
area in the morning into the afternoon hours. Given the more rapid
recovery today in afternoon highs south...have raised temperatures a
few degrees tomorrow over the south as well. Otherwise overnight
tonight into Wednesday winds will remain from the south southwest
increasing some by late morning into the afternoon hours.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 303 PM CST Tue Feb 13 2018

In general a large trough digs into the southwest United States
putting the Upper Midwest in a southwest flow.  This will mean
warmer temps overall with the exception of late Thursday/Friday when
a cold front drops through the region.  The forecast concern becomes
just how warm we will get.  The two factors hampering the warm up
are the deep snow pack over the state and the resulting clouds/fog
from the snowmelt that does occur.  Soundings show a very strong and
pronounced warm layer aloft but with little mixing expected that
very warm air will not make it to the surface.  This has been
largely accounted for with the previous forecasts and highs in the
upper 30s to mid 49s looks reasonable.  This will be enough to melt
snow during the daytime so with sunset and falling temps, we should
see the development of fog Wednesday night into Thursday.  Took a
look at drizzle potential but the lack of depth of moisture and the
absence of any weak forcing indicates a better potential for fog
than any precip.

On Thursday a surface low pushes across the region as Canadian high
pressure begins to move in.  There is little moisture to work with
and forcing is weak and spotty at best so don`t really see a precip
potential with the frontal boundary crossing.  Just some clouds, a
wind shift and colder air to move in late Thursday and Friday.

The aforementioned Canadian high is pretty progressive and by
Saturday we are back in return or southerly flow so warmer temps
return for Saturday and Sunday.  There is some model discrepancy by
this time with respect to the handling of a shortwave passing
through the region.  It appears that we will still be lacking
moisture so I do not see much of a chance for precip but the Euro
and Canadian both have some qpf over Iowa.  Model blends and
surrounding offices kept the slight chance of PoPs in the forecast
due to the model discrepancy and by now we are 5 days out so
confidence in any one solution is not high.

Sunday into Sunday night another surface low and frontal boundary
begin to lift out of the Plains and into MN dragging a cold front
behind.  On Sunday, the precip should be north and west of Iowa but
as the front drags through Sunday and Monday, precipitation chances
do increase and chance PoPs are warranted, especially for Monday.
The predominant ptype will be snow as cold air pushes in pretty fast
but some locations in the southeast may be rain of a mix by later in
the day Monday where some warm air will still be in place.  Models
are not in good agreement at this point but they all do slow the
front across eastern Iowa so light snow may linger over much of the
CWA Tuesday, with cooler temps moving back in next week.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1130 PM CST Tue Feb 13 2018

Primary concerns through this TAF period will be fog development
tonight, low clouds arriving tomorrow, and then fog development
again toward the end of this period. Confidence is increasing
that most of the patchy fog will be confined to northern Iowa with
perhaps some development over western Iowa where dewpoint
depressions are generally less than 5 degrees. Within these areas,
MVFR visibilities will be possible.

An area of stratus clouds has been progressing northeastward and
is currently nearing the I-70 corridor in Missouri. This should
reach southern Iowa mid to late morning and may produce a brief
window of MVFR ceilings before lifting.

Another round of fog is expected to develop Thursday evening.
While MVFR visibilities are mentioned at this point, further
reduction in visibility will be possible with future TAFs.

One last note that low level wind shear mention was left out.
While flow is not purely unidirectional, wind speed difference
between the surface and 2000 foot wind remains below 30 knots.




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