Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190748
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
148 AM CST Sun Feb 19 2017

.UPDATE...
Issued at 147 AM CST Sun Feb 19 2017

A large dense fog bank has developed across north central Iowa
overnight. Observational trends and high-resolution models
indicate that this fog bank will persist through sunrise, possibly
eroding around its edges or advecting slowly northward. Have
issued a dense fog advisory through 15Z and will monitor short
term trends for potential additional updates.

&&

.SHORT TERM.../Tonight through Sunday/
Issued at 352 PM CST Sat Feb 18 2017

SFC High pressure passing through the state of Iowa into Sunday
morning. Thermal ridge continuing to remain well-entrenched over
the intermountain west. 850mb temps will raise another 2C or so
to +9C to +12C for Sunday afternoon. With WAA returning have
bumped temps up to near-record territory.

Left fog out of forecast mention for DMX CWA. Short-term/hi-res
models are having a terrible time handling sfc dewpoints.
Initializing several degrees too moist across the northern tier.
Current fcst output has sfc dwpt depressions of 2 or more
degrees...suggesting any fog that forms should be patchy at best.

.LONG TERM.../Sunday night through Saturday/
Issued at 352 PM CST Sat Feb 18 2017

Near record warmth continues through the middle of the week, then
it appears winter will be blasting back at us by the end of the
week. Two main rounds for precip... Monday-Tuesday and Friday-
Saturday.

...Monday rainfall chances...
Confidence: Low-Medium
21z Sat Water vapor imagery picking up on a well-defined upper
low spinning off the Baja of California...and another sizable
upper low spinning in the Pacific Ocean about 1000 miles of the
Oregon coast. The more southern low is slated to track across the
SW CONUS...reaching the Midwest by Monday. Meanwhile, an impulse
is expected to eject off the northern low and interact with the
southern trough...though this interaction seems to not yield much
impact. Overall, models coming into line with PWATs of 1.25 inches
or so over Iowa... solidly in the +4 to +5 std dev range.
Lowering confidence is the degree to how well- realized/efficient
this moisture will be with this system. In keeping with the pvs
model suites, the 12z Sat GFS is more progressive, and the 12z Sat
ECMWF lags behind. Fcst soundings do not have prolonged, highly
saturated columns/high warm cloud depths. Nudged QPF up slightly,
but not by much, due to uncertainty. Instability...especially by
21z Mon, should be sufficient for thunder at least in the south.

...Tuesday-Thursday warmth...
Confidence: Medium-High
Precipitation from the Monday system quickly exits the CWA to the
east Tuesday morning and warm air settles back in. Tuesday and
Wednesday 850mb temps look to hit the +8C to +12C range
again...suggesting near-record high temperatures for both
days...and this wording has been added to the forecast. There
should be minimal cloud cover both days and decently light winds
under a relaxed pressure gradient.

Friday and beyond wintry weather...
Confidence: Low
Winter looks to return with a punch by next weekend. Long-range
models have been suggesting a deep low (with hints of
"bombogenesis") over the Midwest. Timing and location of the low
has been erratic at best...leading to low confidence in specific
placement of any precipitation/p-type changes. Confidence at this
time is higher at the return of colder air- at least for a few
days. The origin of this precipitation is the northern upper level
low mentioned earlier propagating through the country. The ECMWF
has overall been more consistent and deeper with its placement of
the low... The GFS has been faster and weaker...and the 12z Sat
GEM looks closer to the ECMWF than the GFS. Thus, initial lean is
towards the stronger ECMWF. Since so much variability, not worth
getting into specific details, but it does appear this may be a
more "classic" system, with rain through the leading edge/warm
sector, and a transition to snow on the backside, with very strong
and gusty winds to follow. 850mb temps by Saturday afternoon may
be near -10C.

&&

.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1142 PM CST Sat Feb 18 2017

LIFR visibility and even ceilings look to periodically impact FOD,
MCW, and ALO over the next couple of hours and possibly through
12z. Hires models keep fog mentioned through 14z across the
north to northeast but with the cirrus shield moving over the
state, low confidence the fog will last through that time frame.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CST this morning for IAZ004>007-
015>017-023>028-034>039-046>050.

&&

$$

UPDATE...Lee
SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Podrazik


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