Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172330
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
530 PM CST Tue Jan 17 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 318 PM CST Tue Jan 17 2017

A welcome reprieve from the past few days, the main forecast
attention now turns to fog and stratus trends rather than freezing
rain. An impressive short wave/PV anomaly shows up nicely in the
water vapor imagery this afternoon and any lingering mid level
forcing should continue to push east away from Iowa this evening.
The following low level cyclonic flow will weaken as well but the
associated stratus and fog has been slow to dissipate, often
slower than what models anticipated. The more recent HRRR and RAP
reflect reality better and suggest the stratus will be slow to
depart, and even if it does more fog should develop with weak
return flow over the cold, icy ground in the ridge. Thus have
added fog back into the forecast tonight, which could end up being
dense in spots. The subsidence behind the system will likely
reduce the depth of the low level moisture layer sufficiently to
stop any drizzle however.

The pattern and return flow into Wednesday will hopefully be
progressive enough to scour out the low level moisture into
Wednesday leading to some sunshine and melting. The new ice and
old snow cover will reduce confidence in the degree of mixing and
high temperatures however. Morning snow depth measurements still
show several inches of icy snow across the north so have depicted
somewhat of a north/south gradient just north of Highway 20
separating 30s and 40s. There is certainly the potential for 50s
south with aggressive mixing but have not gone that highs as of
yet with concern about lingering low level moisture and icing.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 318 PM CST Tue Jan 17 2017

An active weather pattern is still expected for the long term
period with five upper-level shortwaves or closed lows progged to
impact the CWA. Above normal temperatures are expected during the
period. Highs are generally expected to range from the middle 30s
to lower 50s, and lows are generally expected to range from the
middle 20s to lower 40s.

First feature will begin affecting the CWA during the day on
Thursday as an upper-level low across the southwestern U.S. drifts
northeastward toward the CWA. Ample low-level moisture is
expected from this time period through the rest of the long term
period. Forcing with this feature will be relatively light so have
gone mainly slight chance POPs.

This feature will be followed quickly by a shortwave associated
with the upper-level low, and will traverse the CWA on Friday.
Forcing and moisture will be more plentiful with this feature.
There may be some light freezing rain across the northern CWA
Thursday night into Friday before low-level temperatures warm
enough for all liquid precipitation.

Another shortwave quickly rotates around the upper-level low
Friday night into Saturday, and will be associated with similar
amounts of moisture and forcing. Either rain or snow, or a
rain/snow mix are expected with this system with the best chances
across the northwestern half of the CWA. Saturday looks to be the
warmest day during the long term period. Saturday morning lows
will range from the lower 30s across the north to the lower 40s
across the southeast. Highs will range from the lower 40s across
the northwest to possibly the middle 50s across the southeast.

The fourth feature will be a shortwave rotating around a northern
U.S. to southern U.S.shortwave from Sunday into Monday night.
Mainly rain is expected, however either snow or a rain/snow mix is
possible on the northwest edge of the precip shield Sunday night
and Monday night.

Fifth and final feature is an upper-level low progged to affect
the CWA during the middle of next week. Precip chances are
currently expected to increase during the day on Tuesday with
mainly rain except for snow or rain/snow mix across the northern
CWA on Tuesday night.

&&

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 530 PM CST Tue Jan 17 2017

IFR or lower conditions will persist for most or all of the night
across the forecast area. Satellite imagery indicates a back
clearing edge progressing across western Iowa that may or may not
stall out overnight with a surface ridge building into the area.
However, even if skies do briefly clear across the north, low
dewpoint depressions and saturated ground should quickly lead to
fog and stratus redevelopment. Significant improvement is not
expected until after sunrise Wednesday as winds come around to SSW
and we burn off/mix out lingering fog and stratus. Thereafter, VFR
conditions are forecast for tomorrow.

&&

.HYDROLOGY...
Issued at 318 PM CST Tue Jan 17 2017

Stream flows across the southeast three-quarters of Iowa is now
generally above or much above normal for this time of year.
Precipitation from the Sunday-Monday storm system has led to
within-bank rises on streams, especially across the south where
precipitation was seasonally heavy /i.e., 1 or more inches/ and
significant within-bank rises resulted. Most of the precipitation
became runoff because the frozen ground allowed for little if any
infiltration.

Although some break-up ice jams have already been reported with
no flooding, additional break-up ice jams may occur within the
next day or so due to additional runoff from the earlier
precipitation. At worst, isolated minor flooding is expected, with
most of the rises remaining within bank.

&&

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

&&

$$

SHORT TERM...Small
LONG TERM...Zogg
AVIATION...Lee
HYDROLOGY...Zogg


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