Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271759
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1256 PM CDT Thu Apr 27 2017

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CDT Thu Apr 27 2017

A large stratus shield covers the region early this morning,
limiting radiation, but moderate northwest breezes and cold air
advection have still helped temperatures fall into the 30s across
our forecast area. The cloud cover and wind are preventing frost
from forming and have removed it from the weather grids for the
morning hours. Later this morning and especially in the afternoon,
the trough responsible for current cloud enhancement will move off
to the east and be replaced by a weak high pressure ridge, with
forecast soundings indicating that this subsidence and diurnal
mixing will allow the clouds to break up and largely dissipate
during the afternoon. Have maintained this trend in the cloud
forecast, but slowed it during the morning hours.

The forecast for tonight is somewhat more complicated. The
aforementioned high pressure ridge will retreat slightly into
southern Minnesota and far northern Iowa but hold there, even as a
surface low pressure area rapidly develops and moves in from the
southwest. Prognostic models are unanimous in generating
precipitation within the broad warm air/moisture advection zone on
the northern periphery of the approaching low, but this will likely
have a sharp northern cutoff as dry air entrainment out of the high
prevents precipitation across much of the area. As a result, POPs
remain fairly robust across southern Iowa overnight but have been
decreased central and north where the dry air will be more difficult
to overcome. Also, across northern Iowa conditions will be ideal for
radiative cooling and frost formation overnight, at least until
clouds encroach from south to north late. Forecast low temperatures
north of about Highway 18 are sub-freezing and widespread frost is
expected. However, with vegetative growth in that area marginal for
impacts and it being 24-plus hours out, have not issued a headline
at this time and will leave it to the day shift to reassess and make
a determination as to whether one will be necessary or not.

.LONG TERM.../Friday through Wednesday/
Issued at 346 AM CDT Thu Apr 27 2017

Main forecast concerns remain similar as in previous forecasts with
some precip chances Friday and then the main larger system for the
weekend into early next week. Upper level energy to lift
northeastward through the region Friday with broad upper level
troughiness in place across the western/central US. SFC low situated
across the nrn TX/OK will have a lobe stretching NE toward MO with a
warm front aloft moving northward. WAA and moisture advection aloft,
will lead to an overrunning type situation with precip chances
spreading northward across the state Friday. Expect one main band of
showers lifting northward and then otherwise precipitation likely to
taper off from south to north behind the main band. Expect band of
precip to dissipate across northern IA by late afternoon/early
evening as upper level energy shifts eastward.

SFC low to then redevelop/strengthen across the southern plains as
the main upper level trough digs through the southern Rockies Friday
night. This should result in majority of precip chances pushing
further south closer to the main sfc warm front across MO, however
may see some lingering drizzle/light rain across the far south so
have some small pops. Otherwise increasingly better chances for
widespread precipitation begin to spread across the state Saturday
and continue into Monday. This occurs as the longwave trough lifts
ENE through the southerly Plains late Saturday and into nrn MO by
late Sunday, before lifting into WI by late Monday. System becomes
cutoff across the southwest, therefore allowing it to slowly migrate
ENE through the central US this weekend. This will allow for an
extended period of widespread precipitation chances. Initially will
have WAA push Saturday into Sunday, but colder air wrapping into the
backside of the system to start to allow a mix of rain/snow across
the west/north by mainly Sunday into Monday morning as the
deformation zone lifts northeastward through the region.

Have gone with temps below guidance grids for Sunday through Monday,
with much of the northwest likely struggling to reach the upper 30s
to around 40. Ground temps likely to keep snow from accumulating much
Sunday, but overnight into early Monday as the column cools could
see more of a transition to all snow and snow then could maybe see
some light accumulations in the far north/northwest by mid-day
Monday. Precipitation to taper off across the area through the day
Monday, with a cooler northwest flow pattern remaining in place next
week. A few upper level impulses set to drop southeastward through
the state next week, but overall timing is very uncertain. Have some
chances for showers, but overall not confident in them at this
point. Should see a gradual warming trend back toward the upper
50s/lower 60s toward the middle of next week, but overall
temperatures still to remain below the seasonal averages through the
period.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1256 PM CDT Thu Apr 27 2017

Stratus continues to hang tough over much of the state however it is
slowly lifting Northeast and eroding from the SW.  We should see
some clearing over Southern TAF sites briefly before mid clouds roll
in from the next system.  Aft 06Z the airmass should saturate
significantly across the South bringing MVFR/IFR conditions back to
TAF locations South of highway 20.  Precip should develop from KDSM
Southward aft 12Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Lee
LONG TERM...Beerends
AVIATION...FAB


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