Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171130
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
530 AM CST Wed Jan 17 2018

.SHORT TERM.../Today through Tonight/
Issued at 358 AM CST Wed Jan 17 2018

There is little weather of concern in this period beyond
temperature trends. The long wave trough will continue to depart
with warm advection in place throughout the day. Moisture will be
very limited however so there will be little in the way of cloud
cover. The warm advection will wane somewhat into tonight however
with the H85/H7 thermal ridge crossing the Plains into IA.
Attention will turn to the current compact short wave/PV anomaly
as it skirts the MN/Canada border later tonight. This will have
little effect this far south however keeping skies mostly clear in
our dry airmass.

Current 09z obs show temps ranging from just above zero to the
single digits below, but with the surface ridge moving through
winds are fairly light. A few spots are showing wind chills near
advisory criteria but this should be brief and not too impactful
considering the light wind so no additional wind chill advisories
this morning. Examination of soundings suggests highs today could
rebound more than recently with the warm advection pushing highs
closer to normal, into the lower to middle 20s.

.LONG TERM.../Thursday through Tuesday/
Issued at 358 AM CST Wed Jan 17 2018

The initial but secondary concern in this period will be temp
trends as we continue to warm, but the primary impact will be the
large storm anticipated for the weekend. At onset, the aforementioned
system will be passing through the Great Lakes with little impact
here besides ushering in additional warm advection in its wake.
Surface flow will change little through the work week with highs
pushing back to or even just above normal by Thursday. The bigger
question will be how warm can we get by Friday. The 00z NAM snow
depth initialization was fairly good, but it changes little into
Friday, which seems unlikely, especially south. Considering the
persistent warm advection and favorable trajectories it would seem
we could erode more of the inversion than depicted in soundings
by that time, especially in the lesser snow depths south and west.
Have not gone as aggressive of what MOS depicts, but have warmed
temps further through the 40s. It is often difficult to get beyond
mid 40s for highs with at least several inches of snow depth
keeping readings there north, but 45-50 would seem attainable
eroding at least a third of the inversion south and west. Would
not be surprised to see some 50s depending on the degree of snow
melt Thursday.

Looking into the weekend there continues to be high confidence
and good agreement in larger scale features and some of the
sensible weather. The ECMWF/EPS and GFS/GEFS all have similar
depictions of a deepening surface low advancing from the Plains
into Great Lakes Sunday into late Monday. There are some timing
and track differences at this time range of course which will
diminish confidence in details, but some higher confidence broader
things to key on at this point include strong winds as the system
exits, likely stronger than currently depicted in the forecast
until timing and location can be better defined, and the potential
for accumulating, wind blow snow over some part of northwest
Iowa.

Dropping down to medium confidence will be a period of convection
south and east in the warm sector, which has yet to be introduced
until timing confidence increases, and likely drizzle or lighter
precip in the shallow moisture along and south of the triple point
track Saturday Night. Confidence decreases even further with
regard to localized precip types early next week with surface
temps and the location and extent of the potent dry slot all still
in question. Both of these will have an appreciable impact on
precip type and magnitude and will not be better defined until the
coming days, if then. The current forecast only has rain or snow
depicted, but all kinds of mixed wintry precip such as drizzle,
freezing drizzle, and sleet are on the table wherever the
transition zone lays out depending on the dry slot and ice
introduction as well as warm layer and surface temps. Confidence
may remain low with this system even right up to game time because
of all these moving parts.

PoPs will decrease later Monday as the system exits and will be
followed by relatively benign NW flow aloft. The source region
will have sufficient westerly component to keep our temps close to
normal into early next week. There are some token low end PoPs to
end the period, but confidence is low in model agreement and
anything that does occur does not look too significant at this
time.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 529 AM CST Wed Jan 17 2018

There is high confidence in VFR conditions and essentially clear
skies through the period. Winds will change little in speed or
direction outside of minor peak heating speed increases, staying
south-southwesterly.

&&

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

&&

$$

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


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