Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201128
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
628 AM CDT Mon Mar 20 2017

.SHORT TERM.../Today through Tonight/
Issued at 330 AM CDT Mon Mar 20 2017

The cold front/trough axis will be exiting the forecast area by
early this morning. Winds from the north and northeast averaging 10
to 15 mph will prevail and bring relatively cooler and drier air
into the state. Highs will still be mainly in the 50s and 60s, which
places readings around 10 degrees above normal. A mixture of
sunshine and clouds will fill the sky, but clouds will thicken late
in the day and overnight as a short wave approaches from the
northwest. Additional cold air will arrive overnight as the thermal
gradient becomes more perpendicular to the air stream flow.
Precipitation will try to develop toward Tuesday morning in response
to the aforementioned wave and an area of frontogenesis that will
move out of northeast Nebraska into western Iowa. The onset of the
precipitation may be a bit delayed as the dry air in the low levels
will need to be overcome first. Then, it is a question of
precipitation type. Have rain south of Highway 30 with a rain/snow
mix primarily north of Highway 30 until daybreak Tuesday with all
snow on the northern edge of the precipitation shield. With the
recent warm weather, frost depth is nil and ground temperatures are
above freezing across central Iowa so accumulations prior to
daybreak Tuesday are unlikely.

.LONG TERM.../Tuesday through Sunday/
Issued at 330 AM CDT Mon Mar 20 2017

The weather will be fairly active through the period with two
systems of note, the biggest of which will occur late this week. At
onset, an upper level ridge will be exiting the Rockies Tuesday with
weak short waves topping it into the MO Valley. Much of the forcing
will be mid level oriented NW-SE through central and southern IA
with the only 1-3km thermodynamic component far SW and mainly west
and south of the state. Much of IA will remain in weak cool/negative
theta-e advection and implied 1-3km QG subsidence. Temps will
initially be cool enough for a brief period of light snow/flurries
west central, but much of the event should be rain. The period of
moisture and forcing should be fairly transient and followed by dry
weather into Wednesday until the H85/H7 ridge passes and warm
advection returns once again.

Persistent warm/theta-e advection will then begin Wednesday Night
into Thursday with the max push eventually leading to the forecast
area getting fully into the warm sector by Thursday Night. This will
begin low end PoPs for light rain or light snow north Wednesday
Night and then expand greatly into Iowa by Thursday afternoon with
the strong thermodynamic surge. This could initially be either light
rain or light snow but should be rain heading into Thursday. Little
to no snow accums are anticipated.

The strong upper low and surface reflection should then move into
the Central Plains by midday Friday leading to the warmest day of
the period with highs in the 50s and 60s. There looks to be a strong
push of coupled thermodynamic and kinematic forcing with the vort
lobe ahead of the upper low Friday with a noted deformation zone
also maturing across the north and west during the day. This has led
to relatively high end PoPs with thunder and potentially severe
weather during peak heating Friday along and head of the frontal
boundary pushing south and east through IA. MLCAPEs do not look too
extreme, staying below 1000 j/kg, but the 0-6km shear will be very
strong while the aforementioned forcing lobe pushes through. The
instability/shear balance may be tipped too far toward shear with 0-
6km values possibly over 60kts, but this window will certainly need
to be monitored for southern IA as both shallow and deep shear
values would be high enough to support all modes of severe weather
if what instability there is can be realized.

Central IA should mainly be on the cool/deformation side of the
system by late Friday Night into early Saturday morning. This may
lead to a narrow swath of at least a rain/snow mix north, but
confidence into the weekend lessens as model agreement diminishes
in terms of deterministic and ensemble solutions. The 00z GFS and
ECMWF have come into better agreement with their companion
ensemble runs vs 12z, but they still differ as a whole with the
GFS suite farther south than the ECMWF. Thus have lingered chance
PoPs central and SE into Saturday and even low end south Saturday
Night lending some credence to the ECMWF, before leading to dry
weather to end the weekend.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 627 AM CDT Mon Mar 20 2017

VFR conditions with FEW to SCT high clouds over the state will
prevail through the TAF period. Winds from the north and north-
northeast may be a bit gusty at times, especially over northern
Iowa. Ceilings will begin to lower as precipitation advances
toward western Iowa near the end of the TAF period.

&&

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

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Ansorge



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