Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131027
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
427 AM CST Fri Jan 13 2017

.SHORT TERM.../Today through Tonight/
Issued at 352 AM CST Fri Jan 13 2017

Radar echos beginning to increase from western Iowa and into
northeast Nebraska and South Dakota. High pressure centered over
southern Minnesota though continues to funnel very dry low level air
under these returns thus nothing currently reaching the ground in
Iowa as the hydrometeors evaporate during descent. A ribbon of
better moisture depth will develop this morning over the northwest
as the high pressure begins to shift east into Wisconsin and this
will allow cloud bases to lower and will have a period with some
light snow possible in that area. Otherwise, today will remain cold
with high temperatures near 10 above near the Minnesota border to
the low to mid 20s south.

Another wave of strong theta-e advection will lift into far southern
Iowa tonight. The NAM continues to be the most robust on moisture
return and eroding the dry layer near 700 mb. Have trended towards
this solution and kept pops into the southeast focused between 06-
12z. Some convective instability will move through Missouri
overnight and may clip into southeast Iowa as well with modest
MUCAPEs of 100-200 J/KG reaching Davis county much like Wednesday
night. This could occur if the dry pocket does erode which would
steepen lapse rates for better vertical ascent. Any precipitation
will be a wintry mix of of freezing rain, sleet and snow. This
precipitation may switch to light freezing drizzle late over the
south as a reinforcing push of mid-level dry air arrives.

.LONG TERM.../Saturday through Thursday/
Issued at 352 AM CST Fri Jan 13 2017

Adjustments to timing were made to the going forecast as we come
closer to the large winter storm expected to impact iowa late this
weekend. They main player in the delay of precip is a surface high
over Northern Iowa drifting slowly East. The high will keep
sufficiently dry air in place which will slow saturation and the
start of precip.

For Saturday and Saturday night high pressure and a zonal flow
will be in place and while temperatures won`t be especially
warm...they will be near normal for this time of year and the high
will provide sufficient drying that the precip potential from
tonight will have limited moisture to work with. As a result any
potential precip from the system will be South of the forecast
area. Temperatures will be near normal and we will see some sun.
This will be the best day of the weekend.

On Sunday the surface high will slide sufficiently East and a
large and pretty potent low will begin to shift out of the SW U.S.
buckling the flow aloft and allowing warmer and more moist air to
spread into Iowa. Precip will be slow to start but once we get
past early to mid morning, soundings show a fairly rapid
saturation of the airmass. Despite an impressive warm nose aloft,
surface temps will not warm much. As a result and snow that
develops aloft will melt as it falls through the warm layer.
Initially we may see snow but as the warm layer aloft increases in
depth there will be a shallow cold layer at the surface and
precip will fall as sleet or freezing rain. There will be quite a
bit of precip with this system and this far out the question
becomes one of when will the precip change over to best determine
how much snow, ice or liquid will occur. At this point Southern
Iowa looks to receive a dusting at best of snow but freezing rain
should be responsible for about a quarter inch of ice with the
bulk of that falling in the afternoon and evening. Further North
precip will be in the form of snow longer so I am expecting around
an inch of snow and less ice. However if warm air surges North
and the changeover occurs quicker then more ice could occur
North...mainly Sunday night into Monday. A winter storm watch was
adjusted for timing South to account for the slower saturation and
expanded North for the snow/ice combination and the potential for
a little more ice then what we have currently forecast.

On Monday the low pressure system begins to lift to the Northeast
and models at this point still vary in the location and timing of
how fast the low will lift out. We will however be in the warm
sector over much of the South so precip should change over to rain
across the South and East by early to mid morning. Depending on
where the low actually ends up...Northwest Iowa could see either a
little more ice or snow. At this point it appears that the
Northwest will be closest to the colder air spilling in with the
low so any rain would be brief. With the amount of qpf expected
and the warming...aside from everything else, we will also need to
keep an eye on rivers. Runoff from this event will be quite a bit
since the ground is still frozen and rivers will have water on top
of ice or ice breaking up and moving potentially creating ice jam
flooding. There is a lot going on from Sunday through Monday and
a lot to monitor. Travel will be probably be hazardous beginning
Sunday into Monday morning then the South should improve as we
change over to rain.

The low finally lifts out Tuesday with precip ending across the
far Northeast late in the day. The flow will return to a more
zonal pattern so there is not a lot of cold air behind the system.
As a result, once the system passes, we are looking at a dry
period with fairly mild temps through the end of the week. The
models are then hinting at a repeat next weekend as another low
digs into the SW U.S. beginning on day 7.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1100 PM CST Thu Jan 12 2017

VFR conditions will remain widespread overnight into Friday as
surface ridging passes from the north to the east. Ceilings will
lower with time but should remain above 3kft for the forecast
period.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday morning through Monday morning
for IAZ044>049-057>062-070>075-081>086-092>097.

Winter Storm Watch from Sunday afternoon through Monday morning
for IAZ004>007-015>017-023>028-033>039-050.

Wind Chill Advisory until 7 AM CST this morning for IAZ004>007-
015>017.

&&

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Cogil



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