Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151150
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
650 AM CDT Fri Sep 15 2017

...Updated for 12z Aviation Discussion Below


.SHORT TERM.../Today through Tonight/
Issued at 354 AM CDT Fri Sep 15 2017

A couple forecast challenges in the short term: 1)showers/tstms
this morning; 2)high temperature trends today; 3)Thunderstorm
potential tonight.

Today...Forecast Confidence:  Medium

00Z upper air analysis and water vapor imagery indicated weak upper
ridging across the central CONUS with a series upstream troughs over
the western states. At the sfc...MSAS analysis indicated a broad
area of low pressure extending from eastern CO northward thru NE and
into southern MN.  Nocturnal LLJ has developed once again overnight
with 09z radar mosaic showing a recent emergence of showers/tstms
over far western IA.

First concern this morning is the area of scattered showers/tstms
over western Iowa. This activity is being driven by isentropic
ascent and moisture convergence along a plume of higher low/mid
level moisture. Expect this activity to gradually spread north and
east thru the morning hours until the LLJ begins to wane. Any precip
should remain very light...generally a tenth of an inch or less. In
the wake of this morning activity a lull in precip should occur as
forcing diminishes and a stout cap develops as H7 temps rise to near
+10C. With breezy south/southeast winds and 850mb temps in the low
20s Celsius, high temperatures should again rise into the upper 80s
to lower 90s. Like yesterday...a few CAM solutions do show an
isolated shower/tstm developing in the late afternoon during peak
heating, but coverage should be very minimal.

Tonight...Forecast Confidence:  Medium

A stronger upper shortwave is expected to pivot northward across
NE/SD during the evening hours. Upper forcing associated with this
feature and convergence along a developing cold front should lead to
scattered thunderstorm development initially over the NE/SD/MN tri-
state area.  A broken line of storms should then slide
east/southeastward into northwest IA around 03Z.  Severe parameters
are not overly favorable...but modest deep layer shear /20-30kts/
and MUCAPE values near 1000 J/kg could yield a few strong to severe
storms /mainly gusty winds/ per the SPC Marginal outlook.  Its still
unclear how far south these storms will progress by 12Z
Saturday...with a number of models showing them reaching
southwest/south-central IA while others keep a majority of the
activity across northern IA. Will keep the highest chance for pcpn
in northwest sections of the forecast area...with a slight chance in
the Des Moines metro area. With the thicker cloud cover and a
persistent southerly breeze expecting mild overnight lows in the 60s.

.LONG TERM.../Saturday through Thursday/
Issued at 354 AM CDT Fri Sep 15 2017

Large upper low currently spinning over NE Utah will continue to
push east northeast slowly into the weekend, with the associated
elongated surface trough slowly marching eastward across the central
US and to be oriented from NE MN southwestward through western IA
and into central KS by early Saturday. May see have some ongoing
showers/storms across the west/northwest early in the period, with
that activity likely diminishing through the morning hours as the
LLJ dissipates and focus switches to strong cold front expected to
push into western IA by Saturday evening. Main upper trough and
associated energy rounding the base of the rough to push into the
Dakotas by early Saturday evening, with storms expected to fire in
vicinity of the front during the late afternoon/early evening hours.
As the front continues to push east southeast through the state,
expect line of storms to progress across the forecast area as well.
Instability still looking ok for possible strong to severe storms,
but overall shear still somewhat marginal. Decent instability with
better shear still just behind the front which NAM trying to show
more post-frontal storms moving through, so still will have a severe
threat with any development just behind the front as well.
Temperatures to also take a dive behind the front, with the NAM
soundings showing a close to 10 degree Celsius drop with H85 temps
falling to around +11C for Sunday. Front to stall out across
southern IA/northern MO with continued chances for storms into
Sunday.

Broad upper level troughing to continue across the western US with
energy ejecting through the region with a southwest upper level flow
pattern expected through much of next week. Shortwave energy to lift
through the region Monday with the stalled cold front now expected
to lift back northward Monday as a warm front with another chance
for some showers/storms across much of the area. Another large
trough to dig into the intermountain west toward the middle to end
of next week. A series of energy will move through the trough, with
one lobe lifting into central Canada toward Wed/Thu while the other
lobe digs into the Pacific NW. A cold front and sfc low associated
with the first wave will move through the state late next week
bringing another chance for precipitation to the state. As the upper
level trough across the western US continues to deepen into the end
of next week/weekend an active pattern looks to continue across the
region. After temperatures cool for Sunday into Monday a general
warming trend is expected into Tuesday and Wednesday with another
slight cool down behind the mid-week cold front for Thursday into
Friday. Temperatures through the entire period will stay near or
above the seasonal averages for mid September.

&&

.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 643 AM CDT Fri Sep 15 2017

Scattered showers and thunderstorms have developed over western
IA this morning. As the storms drift northeastward there is some
potential they could impact FOD/MCW later this morning with a
brief period of lower CIGs/VSBYs. Any activity is expected to
remain west of DSM. Strong winds just off the sfc are leading to
marginal LLWS at FOD/MCW as well. After the morning activity...VFR
conditions are expected for the remainder of the day. South winds
will again range from 15 to 25 KTS. Another round of
showers/tstms is possible tonight...but due to uncertainty in
timing and coverage decided to leave explicit mention out of TAFs
for now.

&&

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

$$

SHORT TERM...Fowle
LONG TERM...Beerends
AVIATION...Fowle



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