Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 282040

Area Forecast Discussion
National Weather Service Des Moines IA
340 PM CDT Tue Mar 28 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 340 PM CDT Tue Mar 28 2017

An upper level closed low within a deep trough will shift East
across the Southern Plains beginning tonight through beyond the
Wednesday forecast period.  At the surface, a low will travel from
Texas into Oklahoma with a warm front extending into Missouri by the
end of the forecast period.  For Iowa, an inverted trough will nose
into the West and South as this low travels East.  Soundings
indicate some dry air in the mid levels to overcome but low levels
will be saturated enough to keep the stratus going across the
Southern half or so of Iowa.  Across the Northern third, where skies
are currently clear or clearing, mid/high clouds will move back in
this evening.

As for precip...given the dry air to overcome with no real forcing
arriving in the CWA until around 12Z...I have a hard time believing
that we will see much precip before then.  However, in that 09Z to
12Z period, as the forcing comes in and the trough advances, the
South and West should quickly saturate and some rain is expected
there by 12Z.  By 18Z an area of theta-e advection works into the
South and by then we will have deep saturation and pretty strong
forcing in place and it should be raining across much of that area,
and moderately at times.  It is unclear how far North the saturation
will occur but if the entire CWA isn`t saturated by 17-18Z, it
shouldn`t take much into the early afternoon to accomplish this. We
will then see a fairly steady rain through the remainder of the
period.  Instability works into the Southern quarter or so of the
forecast area in the mid-afternoon and evening so I did add some
slight chance thunder wording to the forecast there.

I went cooler on MaxT for Wednesday then we had going.  With clouds
and precip occurring we will not see much of a diurnal trend in
temps.  Add to that an Easterly wind at 10 to 20 mph by late morning
and it will be a pretty cold, wet day.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 340 PM CDT Tue Mar 28 2017

20z water vapor imagery shows a longwave trough over Arizona and New
Mexico providing strong lifting for extensive thunderstorm
development over the southern Plains this afternoon. By Wednesday
evening, this trough will have swung eastward over the southern
Plains with rain scattered to widespread across central Iowa in
the warm conveyor belt and deformation zone. Over southern Iowa,
elevated instability will be rather limited, but may be able to
get a few peals of thunder, especially early in the evening.
Farther north, forecast soundings over the far north indicate
thermal profiles just above freezing, though isolated pockets of
brief rain/snow mix may occur where heavier precipitation falls.
As the low pressure passes near or north of St. Louis and into the
Ohio Valley on Thursday, rain will end by midday over western
Iowa and by mid-afternoon over the eastern part of the CWA per a
GFS/ECMWF/CMC solution. The NAM is slower and holds precipitation
into Friday, which seems rather unlikely. Have placed temperatures
on the cooler side of MOS guidance due to the clouds and
rainfall on Thursday

Behind the departing trough, ridging builds into the region.
Temperatures from the end of this week into early next week will be
at or slightly above normal. A new trough of low pressure will enter
the western US and dig into the southwestern US, which will bring
chances of mainly rain later Saturday and moreso Sunday. As this
system passes south of Iowa on Monday, most of the rain will be
focused south of the state, though it may not be an entirely dry
day. Another system in relatively fast moving northwest flow will
bring another chance of precipitation Tuesday.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1208 PM CDT Tue Mar 28 2017

Aviation forecast issues will revolve around cigs through 12Z.
Status is seen moving into the West already and should cloud the
North over with the Southern locations just remaining with MVFR
cigs. As the next system moves in it initially fights dry air but
between 12Z and 15Z all but the far North saturate quickly and
vsbys should drop as precip develops. Winds will also increase out
of the East to 12-17kts sustained.




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