Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 240540

Area Forecast Discussion
National Weather Service Des Moines IA
1140 PM CST Tue Jan 23 2018

.SHORT TERM.../Tonight through Wednesday/
Issued at 322 PM CST Tue Jan 23 2018

Little impactful weather is expected through the week with regard to
storms crossing the region though clouds/snow cover will drive
temperatures and some uncertainty remains regarding areas of clearing
tonight and Wednesday. Mid level water vapor channel shows an
upstream wave crossing the eastern Dakotas now with a reflection
in the height field aloft and a weak reflection a the surface.
Cloud cover associated with this feature will slide southeast
overnight tonight. Models not in great agreement with the handling
of the expansive stratus deck stretching from eastern Manitoba
south into Iowa and Missouri. Hires models continue to suggest
that that moisture supporting the deck will remain trapped
overnight into Wednesday and slowly drift east. Synoptic models
barely recognize the lingering effects of the stratus cloud cover
at this time. Supporting this thesis is the warm air advection
expected to overspread the region by 12z Wednesday which will
result in a strengthening inversion. 12z H850 temperatures over
the area vary little with -5C to -6C across the board. By 12z
Wednesday the airmass is expected to moderate about 2 to 3C aloft.
The area of clouds over the our western counties should be slow
to lift tonight while mid level clouds override it tonight. Will
keep clouds going through the overnight and lingering in the west
during the morning hours Wednesday...partially dissipating in the
late morning before another area of clouds streams southeast with
the next resurgence of warmer air moving into the region by Thursday.
Hires models also beginning to suggest some fog formation. Already
some light fog has developed over northeast Nebraska. This is
primarily due to some melting taking place west of the generation
area and then advecting east over the large snowpack. Some concerns
as to how far east this will advect/move overnight. The RAP/HRRR
expand fog along the Missouri River Valley southeast into the our
west. The ARW expands the area through most of the snowpack all
across northern Iowa. Arrival of mid to high level clouds may keep
the more widespread fog from developing...but given the potential
for advection overnight the oncoming shift may have to update the
forecast north/west for more widespread fog. Lows tonight will
remain in the teens and 20s with highs Wednesday in the 20s north
to the lower 40s south.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 322 PM CST Tue Jan 23 2018

Both the Euro/GFS remain in generally good agreement through the
weekend.  Modified zonal flow will continue to replace the higher
amplitude flow of early/mid January. Once again the main emphasis
will be for warming conditions with weak cold intrusions pushing
south following a cool front Friday evening.  Clouds remain the
limiting factor on higher confidence in the extended forecast.  With
increasing warm air advection across the deep snowpack over Nebraska,
northern Iowa and southern`s likely that the persistent
southwest flow over the longer fetch of snowpack will result in
an area of higher sfc dewpoints and greater potential for either
fog or stratus - especially Wednesday night into early Thursday.
Rather than try to define the exact area that may see fog today...will
wait to see where sfc wind influences are least and convergence
is maximized overnight as well as any influence from high level
clouds expected to move over the region. By Thursday the deepening
trough over the western US will build heights across the Plains
as H850 temperatures increase into the lower teens over the
western half of the forecast area. Both the GFS/Euro lower H850
temperatures Friday though with rather mild overnight lows anticipated
Thursday night...even modest insolation Friday will likely bring
highs back into the lower 50s south with upper 30s to lower 40s
over the north. The greatest threat for any precipitation will be
Friday afternoon/evening across the southeast as a wave will
develop along the cool front and possibly bring some precipitation
to those areas. GFS has backed off on chances with the 12z run
with the 12z Euro also showing a more southeasterly track of the
system. For now will leave a small chance of precipitation over
the southeast Friday night into Saturday morning. Into the
remainder of the weekend a stronger cold front will pass southeast
of the region Saturday night into Sunday. This quick Arctic
intrusion will lower highs Sunday but exit quickly with warming
returning Monday into Tuesday. After highs Sunday in the 20s and
30s with northwest flow conditions will improve by Tuesday with
southwest flow and highs back into the 40s north to lower 50s


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1140 PM CST Tue Jan 23 2018

Low stratus in place across much of the area with some clearing
in the northeast initially. Low clouds should move back in. Fog
will be possible in the far west, but more confined to areas where
the stratus deck has moved out of the area further west of the
forecast area. CIGS to be mainly IFR/MVFR and will begin to erode
through late morning/mid-day Wednesday. Winds to still shift
around to the west to south through the period. Chance for fog
again Wednesday night just beyond the current forecast period.





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