Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181148

Area Forecast Discussion
National Weather Service Des Moines IA
648 AM CDT Sun Mar 18 2018

...Update on Dense Fog Advisory, Near Term Weather Trends, and
12z Aviation Discussion...

Issued at 648 AM CDT Sun Mar 18 2018

No changes planned at this time to the advisory. Visibilities
remain at one half to one quarter of a mile at Ottumwa and
Oskaloosa at 6:30a with Grinnell bouncing between one quarter mile
and 2 miles. Pella and Marshalltown have seen improvements over
the last few hours from one quarter to one half mile to 2 to 4
miles likely due to the cloud cover moving over these areas.
Tempting to cancel the western counties, but visibilities are
still reduced in these areas (though not dense fog criteria) and
with the clouds being fickle in their movement decided not to
cancel early in case the clouds move out and visibilities
drop again. In addition, with temperatures near or below freezing
there is likely areas of freezing fog that may create some slick
spots with road and bridge temperatures generally at or below
freezing. Otherwise, routine update to the forecast with
slight adjustment downward with temperature/dewpoints over far
eastern areas early this morning.


.SHORT TERM.../Today through Tonight/
Issued at 404 AM CDT Sun Mar 18 2018

Plenty of low clouds shroud western and a good part of central Iowa
while much of eastern Iowa is clear early this morning per GOES-East
Nighttime Microphysics. With high pressure over the state, winds are
light and areas that are clear have had temperatures plummet with
most guidance not resolving the near term trends such as at Waterloo
and Ottumwa. Have manually adjusted temperatures downward to provide
a more realistic trend in these areas through early this morning.

Otherwise, the clear skies over eastern and parts of central Iowa
along with light winds and ample low level moisture from recent
precipitation and locally enhanced by lingering snowpack has allowed
for areas of dense fog to form. At 3am, surface observations under
one quarter mile include Ottumwa, Oskaloosa, and Pella. GOES-East
Night Fog showed these fog patches quite well and for these areas
issued a Dense Fog Advisory a few hours ago. However, clouds have
moved over the western counties of the advisory already so will need
to monitor for possible cancellation if cloud trends and noted
visibility improvements hold/continue to improve. There is also
light fog generally 3 to 5 miles under the clouds over south central
and central Iowa as well. The visibility reductions due to the fog
should improve this morning as clouds spread back across all of
central Iowa.

NAM and RAP cross sections over central Iowa show the saturated low
levels persisting through today. Other than perhaps far eastern
sections of the forecast area such as Waterloo and Ottumwa this
morning, expect little in the way of sunshine today. This led to
favoring high temperatures more toward the cooler NBM or NAM/ECMWF
MOS guidance versus the warmer MAV MOS.

Next shortwave trough that will start to influence our weather
Sunday night is currently over the Intermountain West per GOES-East
Clean IR channel. It will move quickly eastward today with surface
low pressure developing east of the Rockies and moving onto the
central Plains. The low will remain south of the area with the best
Q-vector convergence skirting southwest Iowa tonight mainly after
midnight. In addition, surface winds will be from the northeast with
low and mid-level winds from the east or northeast, which will bring
drier air into the atmospheric column. This will limit the northern
and northeastern reach of the precipitation primarily to southwest
Iowa or south of the state.

.LONG TERM.../Monday through Saturday/
Issued at 404 AM CDT Sun Mar 18 2018

Still expecting the upper low to be skirting the area to the south
Monday with a small chance of some showers in the morning in the far
south. Drier northeasterly flow should really limit the northward
potential so majority of the area dry Monday morning with the entire
area dry Monday afternoon. Broader open wave dropping southeastward
through the northern Rockies and through the central US Monday will
begin to move eastward through Iowa mainly late Monday night into
Tuesday. The wave deepens some as it moves through the state, and
could bring some precipitation chances into the far northwest by
late Monday evening, and then spread across the forecast area late
Monday night into Tuesday. Expect a longer duration light
precipitation event, with an initial mix of rain/snow. Sfc temps
then cool enough to expect a changeover to all snow by late Monday
night into Tuesday morning. Precipitation to begin to diminish
across the area Tuesday afternoon as the wave begins to shear apart.
Still expect a narrower band of light snow from north to south
across the central portion of the forecast area before diminishing
through the afternoon hours. Have cooled temps in that area as
expect even with temps slightly above freezing that warmer air is
shallow enough to allow for more of a snow potential than rain at
this time. Overall only have light snow amounts with this system
with around half an inch from Audubon northeastward toward Mason
City, and around 1-2 inches in the far northwest near Estherville.

Expect fairly quiet conditions then into Wednesday and generally
Thursday. Upper ridging aloft flattens out with an upper trough
digging into the western US by Thursday into Friday. Energy to eject
eastward and move atop the flattened ridge. A developing surface low
will be situated to the west of the region with return flow setting
up for Thursday night into Friday. This will bring a return of
warmer air and moisture to the state. A baroclinic zone to setup
across the state into Friday with waa/moisture advection and precip
chances expected. Could see some wintry precip mix in at times late
Thursday night into early Friday and expect mainly rain chances by
Friday afternoon into Friday evening.  This system quickly shoves
east and should allow for mainly dry conditions Saturday, with
another push of energy toward late Saturday into Sunday. This is
quite different from the larger system expected to linger longer
across the area in previous model runs. Therefore stuck with some
small chance pops across the area Saturday for now to see if the
more progressive solution continues in later runs.

Temperatures to be back and forth through the period with some
warmer temps into Monday with a cooldown again for Tuesday. Warming
back up through the end of the week into Saturday, then cooler temps
again just beyond the current forecast period for Sunday. Overall
though temperatures expected to remain near or slightly below the
seasonal averages for mid March.


.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 648 AM CDT Sun Mar 18 2018

Medium to low confidence in this period with low clouds and/or
fog causing restrictions. There will be some improvement with
regard to visibility restrictions this morning, but IFR or MVFR
ceilings will persist through this afternoon if not through the
evening. Conditions may return to VFR by evening to ALO and then
should gradually improve overnight at the remainder of the sites.


Dense Fog Advisory until 10 AM CDT this morning for IAZ049-050-



SHORT TERM...Ansorge
LONG TERM...Beerends
AVIATION...Ansorge is the U.S. government's official web portal to all federal, state and local government web resources and services.