Area Forecast Discussion
Issued by NWS Des Moines, IA

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591
FXUS63 KDMX 181121
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
521 AM CST Thu Jan 18 2018

.SHORT TERM.../Today through Tonight/
Issued at 407 AM CST Thu Jan 18 2018

Temps will be the only concern in this period with little
sensible weather. Water vapor imagery nicely shows a potent PV
anomaly/short wave tracking along the MN/Canada border toward the
Great Lakes this morning. There is a well defined lobe of
kinematic forcing/DPVA from SD into WI but this is quite moisture
starved and will result in little more than high cloudiness north
this morning and then additional mostly clear skies into tonight.

Temps have been fairly steady overnight in the persistent SW
surface flow, even with clear skies. 09Z readings were 15 to 25
degrees warmer than the same time last night: a quite welcome
change. Confidence in temps today and tonight is not great because
of the airmass change and snow cover effects. How temps reacted
yesterday vs today`s soundings would suggest highs mainly in the
30s.

.LONG TERM.../Friday through Wednesday/
Issued at 407 AM CST Thu Jan 18 2018

The main weather feature in this period remains the potent storm
affecting the Midwest early next week with temp trends to end the
work week a secondary concern. Conditions will become even more
favorable for warming into Friday with a dry airmass and stronger
SW surface flow, 40+ kts off the deck. This will of course be in
competition with eroding snow pack so confidence is not the
greatest. How things turn out today will eventually provide a good
hint at Friday`s outcome, but in the mean time have gone with a
bit more aggressive mixing than depicted in the soundings and
highs in the 40s. The snow would melt faster if dewpoints are not
so low.

Looking ahead into the weekend, the models have changed little
with their depiction of the big picture showing a closed upper low
passing through Iowa with the associated deepening surface low
track somewhere between Ames and Ottumwa, which is fairly good
agreement for this time range. There are some timing differences
however with the CMC/ECMWF/EPS solutions about 6 hours slower than
the GFS/GEFS, with the former somewhat preferred. There is steady
and somewhat elevated confidence for this time range with the
following scenario. It would favor drizzle or freezing drizzle
starting Saturday Night and continuing into Sunday morning as the
inverted trough enters the state with forecast soundings showing
only shallow moisture with little to no ice introduction above.
This insufficient moisture depth will keep precip rates low, but
could still lead to some minor icing central and north with ice to
liquid ratios at or above 1 to due to small drop size. This
process may continue into Sunday, but temps should warm
sufficiently through the day for only drizzle or light rain by
midday. Either Sunday evening or Sunday Night into early Monday
the upper system and deformation zone will mature spreading deeper
moisture, ice introduction and thus snow across at least parts of
northwest Iowa. The south and west edge of the snow is still in
question depending on the aggressiveness of the dry slot and
deformation placement, but it appears as though at least a small
portion of our NW forecast area will receive wind blown snow to
some degree. The above broader trends should be the point of
emphasis at this point rather than snow amounts at any particular
location until confidence increases in the surface low and dry
slot tracks, which will affect temps at the surface and aloft and
adequate moisture aloft. The models remain in good agreement with
strong, deep QG forcing and frontogenesis producing a significant
heavy snow band and potentially blizzard conditions, but the
heaviest snow axis could very well end up north and west of our
forecast area as well.

The system should exit by Tuesday with fair weather and seasonal
temperatures into midweek. The GFS and ECMWF now depict a bit
stronger ridging aloft upstream by Wed so PoPs have been removed
from the forecast with little in the way of forcing and moisture
phased for any chances of note.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 521 AM CST Thu Jan 18 2018

Confidence is high that VFR conditions will persist through the
period. A band of high cloudiness will pass through this morning,
especially north and east, but otherwise skies should be mostly
clear.

&&

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

&&

$$

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



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