Area Forecast Discussion
Issued by NWS Des Moines, IA

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602
FXUS63 KDMX 161753
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1153 AM CST Tue Jan 16 2018

...Update for Wind Chill Advisory and 18z Aviation Discussion...

.UPDATE...
Issued at 1153 AM CST Tue Jan 16 2018

Remaining part of the Wind Chill Advisory will be allowed to
expire as most observations show wind chills higher than -20F.

UPDATE Issued at 553 AM CST Tue Jan 16 2018

Recent observations here at the office and elsewhere with
visibility restrictions show that the snow has transitioned from
flurries to a bit greater intensity light snow event. Soundings
depict the column warming somewhat suggesting an ice crystal
change with the moisture now into the dendritic growth zone. The
lift remains quite weak however so still expect only a dusting, if
that. Thus will transition from non-measurable flurries to a
higher PoPs light snow event.

&&

.SHORT TERM.../Today through Tonight/
Issued at 338 AM CST Tue Jan 16 2018

The primary concerns through this period will be ongoing current
weather relating to temps/wind chills and light precip trends. Iowa
is still feeling the effects of the departing long wave trough with
low level cyclonic flow wrapping a last surge of moisture into Iowa.
Temperatures in this cloud layer will be quite favorable for ice
production, but forcing and lift will be negligible so have opted
for flurries vs higher PoPs and light snow wording into the morning.

Regarding temp and wind chill trends, the low level thermal trough
is actually to our south and west at 09z with warmer air beginning
to enter Iowa from the north and east. This isn`t helping sensible
weather too much however with widespread below zero temps still in
place here and wind chills in the 20s to lower 30s below zero as
Iowa remains in a decent MSLP gradient between the Lake Michigan low
and Dakotas high. The SW section of the warning may be under-
performing a bit but is not worth downgrading at this point as it
would still be well into advisory criteria and better blends into
adjacent office headlines. Thus will leave advisory unchanged
through 18z expiration. East and southeast sections may nudge above
criteria a few hours before that time, but feel day shift can
adjust/cancel as needed versus trying to get too detailed with
timing at this point.

Outside of the NAM MOS, which is too warm for some reason, other raw
and MOS guidance suggest highs today staying in the single digits
above zero outside of some 10+ locations east. Lows tonight should
be milder with warm advection returning. Lows may dip below zero
again north and a few spots could briefly touch advisory criteria
but considering where we`ve been and winds will be lighter do not
anticipate a headline extension at this time.

.LONG TERM.../Wednesday through Monday/
Issued at 338 AM CST Tue Jan 16 2018

Summary...Arctic air begins to get replaced as southerly winds
return and boost temps into the 30s to mid 40s by Friday. Dry
weather will persist through the remainder of the work week and
first half of the weekend. Winter weather increasingly likely to see
a return Sunday as a trough ejects out of the SW US.

Wednesday through Saturday...Generally a very quiet and much more
pleasant period than the previous few days. At the beginning of this
window the deep upper level trough across the Midwest will have
started its exit eastward, allowing periods of weak ridging and
zonal flow to reintroduce WAA/southerly winds. Prior to the warming,
Wed morning will remain cold around or just below zero with wind
chills in the -10 to -20 deg F range.  WAA will begin in earnest
aloft, with 900 to 850 mb temps will absolutely soar by the end of
the week, approaching 12 to 14 deg C, but unfortunately we will not
be tapping into any of it given lack of mixing during these times of
the year. One can wish. Surface temps will still be quite pleasant
over early in the week though with 30s to mid 40s expected by Friday
thanks to sustained SW winds since Wed. Solutions now in good
agreement in sliding a compact upper level trough north of the area
and across the Great Lakes Thursday. Previous GFS iterations had
tendencies to slide said trough further south across Iowa,
potentially providing a quick opportunity for precipitation.

Sunday through Monday...There exists a growing potential for
impactful weather within this window. Models have suggested as much
for the last couple of days, but varied significantly in both track
and timing. Over the last couple of cycles GFS/Euro have come into
fair agreement that a large upper level trough and surface low will
move out of the SW US and track through the area late Sun through
Mon. Given antecedent surface temperatures above freezing, track
uncertainty does raise questions as to how much of any precipitation
is rain or snow. It can probably be said that someplace will see an
opportunity for winter headlines, just cannot ascertain exactly
where yet. Even so, it is a near certainty that winds are underdone.
Most likely caused by a Canadian solution bringing down the blended
guidance. That may be significant in that they are most underdone in
the areas that could receive snow. Void of a large change in track,
winds are probably 5 to 10 kts under within a significant portion of
the 00z to 18z window Monday. Have avoided making any sweeping
changes though given the distance out, but will need to be monitored
as an opportunity to improve forecast and messaging over blended
guidance.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1153 AM CST Tue Jan 16 2018

Primary concern is timing of the return of VFR conditions to the
terminals this afternoon. Clouds are beginning to break, but MVFR
ceilings are still quite widespread. In addition, there are still
areas of blowing snow that are causing some high end MVFR
restrictions. As winds decrease this afternoon and clouds further
break, conditions will become VFR by late this afternoon. VFR
conditions will persist as winds from the northwest become from
the southwest later tonight through the end of the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until noon CST today for IAZ004>007-015>017-
023>025-033>035-044>046-057-058-070-071-081-082-092-093.

&&

$$

UPDATE...Ansorge
SHORT TERM...Small
LONG TERM...Curtis
AVIATION...Ansorge



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