Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141143
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
643 AM CDT Sat Oct 14 2017

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

.SHORT TERM.../Today through Tonight/
Issued at 405 AM CDT Sat Oct 14 2017

Surface boundary has slipped south of the state with generally
northeast flow and temperatures ranging from the middle 40s to the
upper 50s in many places. Radar around 09z shows isolated showers
with perhaps a rumble of thunder southeast of the Des Moines metro.
Another area of thunderstorms has developed in the last few hours
over south central Nebraska aided by mid-level warm air advection.
Convective allowing models several runs ago struggled with this area
of convection initially having very little in this area. More recent
runs of the HRRR/ESRL HRRR and RAP and to a lesser extent the high
res windows of the WRF have caught onto this trend. 7z SPC
mesoanalysis shows weak MUCAPE, but with 50 to 60 knots of 0-6km
bulk shear where the storms have formed. Storms may struggle in the
next few hours as they move to the northeast, but better MUCAPE
currently resides over central Iowa with similar levels of deep
layer shear. Ultimately, expect these showers and storms to move
into Iowa and our forecast area in the next 2 to 3 hours (roughly 12-
13z).

Upper level trough will move from the Rockies this morning to Iowa
by late today with Iowa under the influence of the right entrance
region of a 250mb 150 knot jet. This trough will gradually become
more neutrally tilted with surface low pressure currently over
eastern Colorado moving to the northeast deepening as it reaches
northeast Iowa by 00z. This will allow the boundary south of the
state to advance northward into at least southeastern part of Iowa
as a warm front and be the focus for showers and storms. PoPs today
were a blend of the time lagged HRRR and RAP along with CONSShort.
North of the front, these showers and storms will be more elevated
with surface based storms possible along and south of the front.
Forecast soundings over southern Iowa have MLCAPEs between 1500 and
2000 J/kg late this afternoon and early this evening with deep layer
shear supportive of supercells. 0-1km SRH values around 200 m2/s2
are also supportive of rotating storms and isolated tornadoes seem
possible initially with favorable LCLs. There is also plenty of CAPE
in the hail growth zone for severe hail. While downdraft CAPE values
are not that impressive, winds aloft are strong on the order of 50
knots at 850mb toward 00z. So some of those strong gusts could be
entrained in the downdraft of a thunderstorm, especially as the mode
is expected to go from initially supercellular to more linear. Also
of note are the precipitable water values, which are robust for the
middle of October between 1.5 and 2 inches. This is 2 to 3 standard
deviation above normal so locally heavy rainfall is possible. Did
blend in some of the WPC QPF into the initially Superblend guidance
to yield some higher values.

As the surface low moves toward the Great Lakes tonight, a cold
front will sweep through the state and thus bring an end to the
rain. Winds will also becoming strong and gusty from the northwest
bringing in cooler and drier air.

.LONG TERM.../Sunday through Friday/
Issued at 405 AM CDT Sat Oct 14 2017

The only sensible weather concern through the period will be
frost potential toward daybreak Monday. Our mid amplitude pattern
will remain in place through next week with the polar jet
generally staying to our north with Midwest heights eventually building
again. The primary short wave driving our active weather in the
short term should be just exiting the area at onset with its
rise/fall couplet past. Brisk northwest winds will be in place
with plenty of sunshine later Sunday. A secondary short wave will
drop into the departing trough but any adequate phasing of lift
and moisture should remain to our north and east.

The surface high behind this long wave trough should stay to our
south by 12z Monday, but winds will still be fairly light. With
clear skies, a cooler airmass settling in and dewpoints hovering
near the freezing mark, there may be some patchy frost north and
in more favored locations such as river valleys into Monday
morning.

The airmass will remain quite dry into the work week with abundant
sunshine and temperatures just above normal keeping our only
frost/freeze window early Monday. There will be a hint of a weak
front/trough passage Wed into Thu, but this will be followed by
upper level ridging and stronger return flow to end the period.
The next chance of precipitation does not look to arrive until
next weekend, beyond the valid period, and even that is somewhat
uncertain as the GFS pushes a more progressive system through
while the ECMWF and its ensemble suggest a four corners cutoff or
at least a slowing long wave trough passage.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 643 AM CDT Sat Oct 14 2017

A prolonged period of impacts at the terminals is expected today
and tonight. IFR ceiling and/or visibility restrictions will
prevail much of the time with low clouds and scattered showers
and storms occurring. Ceilings may lift out of IFR this afternoon
at KOTM, but confidence was too low to include a mention at this
point. Otherwise, a cold front will sweep through the state with
strong, gusty winds from the northwest developing and persisting
into Sunday. In addition, winds around 2000 feet will be between
40 and 50 knots fostering low level wind shear conditions. Drier
air will arrive tonight with all but KMCW and KALO returning to
VFR conditions.

&&

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

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Ansorge


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