Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201150

Area Forecast Discussion
National Weather Service Des Moines IA
650 AM CDT Sat Aug 20 2016

...Updated Aviation Discussion for 12z TAFs...

.SHORT TERM.../Today/
Issued at 347 AM CDT Sat Aug 20 2016

Primary forecast challenges are precipitation chances today along with
much cooler temperatures.

Forecaster Confidence = Medium/High

Potent upper level trough will continue to pivot through the upper
Midwest today. Large scale synoptic forcing will peak early this
morning with Q-vector divergence /and implied subsidence/ moving
into the state after 18z.  As such...the best potential for showers
will occur early this morning over eastern sections of the forecast
area. However...deep layer cold air advection will persist for much
of the day with H7 temps falling to near 0C by 00Z Sunday. These cool
temperatures aloft will likely keep a broken/overcast stratocu deck
in place for much of the day and also lead to areas of shallow
instability with models indicating about 250 J/kg of SBCAPE. As such
kept a slight chance of showers and maybe an isolated lightning strike
along the northern and eastern fringes of the area.  Several CAM solutions
such as the experimental HRRR suggest that light showers or sprinkles
would be even more isolated shower wording could be
expanded in later forecasts.

The other story will be the much cooler temperatures today. The
aforementioned cloud cover and H8 temps falling to 10C will result in
high temperatures in the upper 60s north to mid 70s south.  In addition
a deepening sfc low over WI will produce a modest sfc pressure gradient
over Iowa. This gradient combined with steep low level lapse rates will
result in breezy northwest winds of 15 to 30 mph by afternoon. All in will feel like an early touch of autumn.

.LONG TERM.../Tonight through Friday/
Issued at 347 AM CDT Sat Aug 20 2016

The regime change and trough passage with anomalous heights and
wind speeds will be well underway by the beginning of the period
with only one real window of sensible weather concerns into the
middle of next week. Strong subsidence will be in place by this
evening and really only rebound to neutral lift by late Sunday.
This will result in cooler drier weather with highs only in the
70s Sunday. Monday should remain dry but the main weather feature
should be increasing southerly winds between departing high
pressure and our next system Tuesday into Wednesday. Forecast
soundings show mixing to at least 1km by afternoon with wind
speeds suggesting gusts possibly into the mid 30s /kts/. Have not
gone quite that high for a start, but have raised wind gusts from
previous forecasts. This return flow will boost highs back to
normal for Monday and Tuesday, and even seasonal humidity levels
by Tuesday, but it will be short lived.

The attention will then turn to another iteration of the fairly
progressive long wave pattern with a system not too dissimilar to
what occurred Friday. A long wave trough will advance out of the
northern Plains pushing another cold front into Iowa Wednesday. PoPs
will gradually increase late Tuesday and Tuesday Night with the
threat seemingly driven by peak heating convection to our west late
Tuesday moving into Iowa Wednesday with modest but large scale
synoptic scale kinematic forcing and steady low level southwest flow
and moisture transport. Instability will increase with MLCapes 2-3k
j/kg but shear will be lacking through much of the event. This
should limit the severe potential somewhat and mainly restrict it to
daytime heating. Another similar cycle may occur Wednesday, mainly
affecting southeast Iowa as the front exits during peak heating.
This may be the best instability/shear phasing as the shear
increases through the Mississippi Valley by then but it will be
short lived as the front exits. There may also be a period of
heavy rains, especially Tuesday Night, with fairly weak mean wind
parallel to the front, minimal Corfidi vectors and elevated
precipitable waters and warm cloud depths. Much of the precip
should be over later Wednesday Night with the following Canadian
mean trough returning cooler, drier air to Iowa and surface high
pressure back into the Missouri Valley Thursday into Friday.


.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 642 AM CDT Sat Aug 20 2016

A low pressure system and associated cold front continues to make
its way across Iowa. Expect scattered showers through the morning
hours becoming more isolated by afternoon. Cigs will be a
challenge today as heights have fluctuated between LIFR/IFR and
VFR. Believe the lowest cigs will be this morning...and have
trended cig heights higher into the afternoon as some drier air
is advected into the area. Winds across most terminals have
already shifted to the west/northwest with speeds expected to
increase into the 15 to 30 kt range by afternoon. Expect
conditions transitioning to VFR overnight with diminishing winds.




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