Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181745
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1145 AM CST Sat Nov 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 319 AM CST Sat Nov 18 2017

Upper system approaching the state currently with robust mid level
forcing.  The subsequent lift has led to generally widespread light
rain across the forecast area early this morning.  This
precipitation will continue through mid morning although the entire
area will become more confined to the southeast portions of the
forecast area.  Sufficient cold air and the far northern edge may
produce a brief period of rain/snow mix or outright snow, however
the accumulation threat is minimal if not outright non-existent. The
forcing will move quickly east by midday with good subsidence
spreading statewide by this afternoon.  The strong cold advection
will help to continue the brisk northwest winds through much of the
day and the subsidence will help to lead to clearing in most
locations by afternoon.

Little sensible weather is expected overnight with surface ridging
sliding across the Plains and toward the Missouri River Valley by
12Z Sunday.  Winds will slacken considerably this evening under
clear skies and temperatures are expected to fall nicely into
tonight.

.LONG TERM.../Sunday through Friday/
Issued at 319 AM CST Sat Nov 18 2017

Bottom Line Up Front...
The curtain will rise on a new weather pattern beginning on
Sunday afternoon. Into the work-week, abundant blue skies &
sunshine will be present, along with mostly above normal
temperatures. In fact, near-record highs cannot be ruled out for
Friday.

Sunday and Monday...
08z Sat Water vapor imagery picking up on an upper low spinning
over northern Manitoba. By 12z Sun, this low may be in the exact
same location as it presently is. Continuing at 12z Sun, the heart
of the associated trough will likely be near the Great Lakes
Region, and thermal ridging will be building over the
intermountain west... gradually beginning to crash down over the
Midwest. Across the DMX CWA, 850mb temps will start Sunday around
-6C to -4C.

With the ridge crashing over Iowa, 850mb temps will be around +5C to
+9C by 12z Mon. The GOES-16 Upper-level water vapor IR band is
picking up on a pocket of upper-level clouds currently located over
the Pacific NW. These clouds should be in place Monday and Tuesday,
preventing pure radiational cooling overnight and complete blue
skies during these afternoons. Regardless, high confidence in
highs in the 50s for Monday.

Tuesday through Thursday...
A fairly well-defined shortwave/impulse will race across Canada
and phase with the above-mentioned upper low... which should be
barely further east by Tuesday morning. This wave will re-ignite a
longwave trough to dig across the Midwest Tuesday through
Thursday... entrenching the region in NW flow/CAA. As quick as
850mb temps went up for Monday, they will fall by Tuesday, as they
should be in the - 6C to -10C range by 18z Tue...with CAA
continuing into Wednesday morning. Absolutely no low-level
moisture returns remain in place...meaning partly cloudy to mostly
clear/sunny conditions will persist through Thursday, with only
scatter mid to upper level clouds to speak of.

Similar to the pattern from Sunday into Monday, a thermal ridge will
be building over the intermountain west. This ridge should crash
over Iowa by thursday into Friday, bringing another notable warm-
up to Iowa.

Friday and Beyond...
The thermal ridge slated to crash over Iowa is notably stronger
than the one on this Sunday/Monday. By 00z Fri, 850mb analysis
pins +15C temps all the way into eastern Montana. The ECMWF is
definitely the model of choice in tracking this warm-up, as it has
maintained far greater consistency than the GFS. Since the 12z
Wed output (280 hour) The 12z ECWMF had WAA and 850mb temps very
similar to the +10C to +15C range they currently show. Conversely,
the 12Z Wed GFS had strong CAA and 850mb temps of -12C to -16C.
Since 12z Wed, the GFS has been slowly catching up to the ECMWF.
If this solution holds, current forecast temps for Friday
afternoon are 5 to 10 degrees too cool. The record high at Des
Moines for Friday is 67 (2011)... Cannot rule out temperatures
near that value at this time.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1145 AM CST Sat Nov 18 2017

Low level stratus moving east of the forecast area with VFR
conditions moving into central Iowa. There is a patch of MVFR
stratus over southwest MN moving southeast. This stratus may
remain intact long enough to reach KFOD and KMCW. Have not
included MVFR conditions at these sites yet but may have to add.
Gusty northwest winds today with gusts over 30 kts possible before
diminishing overnight.

&&

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

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Donavon


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