Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211011

Area Forecast Discussion
National Weather Service Des Moines IA
411 AM CST Wed Nov 21 2018

.DISCUSSION.../Today through Tuesday/
Issued at 410 AM CST Wed Nov 21 2018

Several forecast challenges through the period. Stratus over the
northeast one third of Iowa has been slowly sagging south this
morning. Areas of fog have developed ahead of the stratus where the
wind has become light. A few areas have had visibilities drop to
less than one mile at times. These lower visibilities should remain
localized through fog with visibilities of 3 to 5 miles will be
common. The stratus descent into central Iowa will slow this morning
before retreating back to the north as the low level steering flow
switches from northerly to southeasterly. High temperatures today will
range from the low 40s central and south to around 30 far northeast.
The far northeast may have cloud cover linger much of the day and
will impact temperatures.

Warm advection will commence tonight with a steep inversion
developing around 925 mb. Low level saturation below the inversion
is expected and should result in areas of stratus redeveloping.
Areas of fog may also develop and have add at least patch mention in
the forecast though it is possible that the fog may become more
widespread than currently forecast. The other concern overnight is
drizzle development. Expect lift within the low level saturated
layer due to the warm advection which may result in drizzle
formation. The shallow saturated layer may be a limiting factor in
areal coverage and intensity. If drizzle does develop, there could
be some impacts with freezing drizzle with travel over the northern
half of Iowa. The primary challenge on Thursday will be how long the
stratus will linger through the day. With the current setup, the
stratus may remain over northern Iowa well into the afternoon.

A short wave trough will enter the region late Thursday night and
into Friday and will bring a wave of precipitation to the state. Low
level theta-e advection should push temperatures above freezing
across the entire area prior to precipitation arriving. Therefore,
expect rain as the primary weather type though a brief mix early
could occur over the far northeast near Mason City before quickly
switching to rain. Saturday remains the transition day and should be
the most pleasant day in the 7 day forecast. High temperatures will
be in the low 40s north to low 50s south along with light west to
southwest winds.

The forecast story for the 7 day remains the storms system expected
to arrive Saturday night into Sunday. While there is descent
agreement temporally and spatially with the upper level system as it
dives southeast across the from Montana into eastern Colorado
Saturday then deepens and lifts northeast into northern Missouri by
Sunday, there still remains a lot of variables that need to be
addressed. One, it remains a troublesome that there is a lack of
continuity with the upper low to the north which will have impacts
on how this system evolves. It does appear the sub-tropical high
building into the southern Gulf may keep this system from dipping
much further south than currently progged. That said, there remains
more wiggle room to the north and west, which tends to be the bias
for solutions at this time range. In addition, as previously
mentioned, these deepening lows tend to slow 6-12 hrs from current
timing this far out. Therefore, can speculate that the surface low
pressure may track a bit farther northwest than currently progged
and be more delayed. This system still has the potential to be a
significant winter storm with wind and snow to the northwest of the
surface low and could include wind gusts in excess of 40 mph. At
this point with low to medium confidence in the exact low track, it
would be hasty to try to release snowfall estimates at this point.
The primary message that everyone should be aware of is that there
may be significant travel impacts due to winter weather by Sunday.
If that is all not enough good news, the strong cold push is still
on track following this system for early next week.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1126 PM CST Tue Nov 20 2018

Not much change versus previous TAF period. MVFR stratus will
continue to ooze into the northern sites, KMCW/KALO/KFOD overnight
and linger into Wednesday. Suggestions that at least FEW/SCT
ceilings under 3000ft may stick around much of the day as the weak
front washes out and southeasterly/southerly winds return. Hints
of drizzle as well across northern sites late in the TAF period,
but have left out at the moment due to some lingering uncertainty.




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