Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220458

Area Forecast Discussion
National Weather Service Des Moines IA
1158 PM CDT Wed Mar 21 2018

.SHORT TERM.../Tonight through Thursday/
Issued at 355 PM CDT Wed Mar 21 2018

Bottom Line up Front...
Though light precipitation is expected tonight into tomorrow
morning across central to southern Iowa, no significantly
hazardous impacts expected in the short term.

20z GOES-R water vapor imagery picking up on a huge upper low
spinning off the British Columbia coastline. This low is not slated
to impact Iowa until next weekend, 10 days from now. Additionally, a
deep low pressure will continue to impact the northeastern CONUS. In
the meantime, a weak, broad sfc high currently centered over NE KS
is expected to propagate eastward overnight though tomorrow morning,
making it to the Mississippi River by 15z Thu. A pocket of 850mb-
750mb moisture is being advected southeastward from the Dakotas,
slated to make it into Iowa by 09z Thu. Concurrently with the
moisture advection, a band of low-level frontogenetical forcing,
noticeable from 925mb up to 750mb, will establish itself from
around Denison through Ottumwa. KOTM BUFKIT soundings have from
the sfc to around 3000ft above freezing, which should be enough to
support melting and a very light rain/snow mix. With winds
expected to be in the 5 to 10 mph range, there should be no impact
from this precipitation. Enjoy the sunshine tomorrow while it
lasts, because starting Friday, it may be cloudy across Iowa
until at least next Thursday...

.LONG TERM.../Thursday night through Wednesday/
Issued at 355 PM CDT Wed Mar 21 2018

Bottom Line up Front...
This weekend`s system is expected to bring widespread impacts
across Iowa, with light to moderate snowfall accumulation across
northern Iowa, and moderate to heavy rainfall across southern
Iowa. Strong winds, approaching Wind Advisory criteria are also
likely with this system.

Models now in very good agreement with featuring low pressure
pushing off the Rockies by Friday morning and propagating across
northern Missouri Friday night into Saturday afternoon. Looking back
in time, the ECMWF has been far more consistent with both the
location and the depiction of the feature itself, so will lean
towards it. Confidence is high enough to use the model consensus
track... again, with a nudge towards the ECMWF. Confidence in
widespread QPF amounts of at least inch or so is high, so sfc
temperatures will be the key with this system. At this point,
Highway 20 and northward is more or less the cutoff for most of the
rain/snow line, with higher amounts of snow to the northeast and
little to no snow to the south.

Fascinating is the moisture sources, plural, for this system.
Northern Iowa will be impacted by polar moisture, and in the warm
sector of the low, into northern Missouri, will be gulf moisture.
So, if the track of this low shifts northward 50 miles, southern
Iowa could receive significant rain. This possible northward shift
in the track would also push the heaviest snow band well into
Minnesota. Rather intuitively, if the track of the low shifts
south, then the snow would drop down to Highway 30 or further.

High confidence in easterly winds of at least 15 to 25 mph, gusting
to 35 mph with this system. If the low deepens some, these winds
would need to be bumped up. Needless to say, with the strong winds,
any periods of snow would feature visibility reduced to 1 mile or
possibly less.

Precip should depart to the east by around 00z Sun. With weak
thermal ridging remaining in place, supported by the aforementioned
British Columbia low, temperatures should warm up fairly quickly
behind this system, as highs make it into the 40s and 50s by Monday.
Early indications are the low-level moisture is effectively going
to stagnate through at least Thursday of next week. With this
British Columbia finally making it into the Midwest by next
weekend, perhaps, any windows of sunshine on Thursday of Friday
will be very short-lived.


.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1157 PM CDT Wed Mar 21 2018

Skies varied from clear to patchy VFR cloudiness at 05z, but
additional VFR ceilings were beginning to spread east into SW Iowa
from the KOMA area. Although some light rain or light snow will
spread across southern Iowa later today with this, both ceilings
and visibilities are expected to remain VFR. The precip should
then exit to the south and east by late afternoon with continued
VFR following into the evening.





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