Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 272017

Area Forecast Discussion
National Weather Service Des Moines IA
317 PM CDT Tue Jun 27 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 158 PM CDT Tue Jun 27 2017

The primary concern through the period will be convective trends
into Wednesday as an MCS is expected to develop across the High
Plains and reach Iowa during the early morning hours Wednesday.
There will be a pronounced regime change heading into this with a
return to more seasonal temperature and humidity levels in just a
short amount of time. Dewpoints may rise 20+F in 12-14 hours in
many locations.

Sharply increasing warm/theta-e advection will advance through
Iowa this evening, but initially be moisture starved with little
sensible weather. However kinematic forcing ahead the current
WY/MT short wave will work with persistent Plains return flow to
moisten our upstream airmass and feed MCS development. MUCAPEs
should be 1500-3000 j/kg into early tomorrow morning west. This
diminishes by 09z however which supports the west to east gradient
noted in the SPC Day 1 outlook and 3hr Calibrated Severe Probs
per the 09z SREF. 35-50kts of inflow per 300-305K isent surfaces
will support the MCS in some form into the night however, likely
based on shallow convergence and outflow with some suggestions
that the synoptic warm front will try to surge to northern Iowa or
farther north.

It will not take much to get re-generation by tomorrow afternoon
however with most models suggesting uncapped 2000-3000 j/kg
MLCAPEs and 35-45kt of 0-6km shear favorably oriented to the
approaching boundary for discrete cells or at least small
clusters. This will support all modes of severe weather, most
likely over central and southeast sections, and fully expect
current SPC Day 2 Slight Risk to be upgraded once confidence in
location can be better pinned down after overnight outflow shows
its hand. One interesting note is that the convection allowing
model ARW cores are less aggressive than one might expect per the
aforementioned shear and instability based on updraft helicity vs
NMM cores, which are often on the higher end anyways. Thus there
is still some uncertainty with the tornado potential versus more
likely damaging wind and hail. If local outflow does not favorably
augment 0-1 SRH the general synoptic regime, broad SW low level
flow, would seem to be less favorable for tornadoes.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 158 PM CDT Tue Jun 27 2017

The potential for additional strong to severe thunderstorms
Wednesday night through Thursday is the focus of the extended

Thunderstorm development Wednesday night will be ongoing near a
cold front extended from northest to southwest Iowa. Ahead of the
front, an unstable airmass is expected with forecast models
progging MU CAPE values over 2500 j/kg along with a strengthening
and veering low level jet near 30-50 knots. Numerous strong to
severe thunderstorms will develop along this boundary during the
evening and overnight hours aided by the passage of a shortwave in
zonal flow aloft. Damaging winds, large hail, heavy rain and a
few tornadoes within a few hours of initiation will be possible.
The surface front looks to become parallel to the zonal flow
overnight Wednesday into Thursday morning shifting the focus for
showers and thunderstorms and heavy rain to southern Iowa.

Surface low deepens on Thursday across Kansas in response to a
deepening upper trough over the Dakotas with favorable moisture
transport over east west oriented surface boundary. Another round
of strong to severe thunderstorms will develop with shortwave
kicking ahead of a deepening upper trough. Along with heavy rain,
all modes of severe weather will be possible including tornadoes
with thunderstorm development near the front.

Thunderstorms will end west to east across the state Friday with
boundary exiting into the Mid-Mississippi Valley. The upper trough
digs over the Midwest Saturday. Northwest winds will increase
with windy conditions expected from strong mixing. A few showers
and thunderstorms will be possible over northern Iowa Saturday
afternoon and evening aided by daytime heating and another
shortwave rotating around the base of the upper low.

Next week, the pattern shifts to northwest flow aloft over Iowa
with an upper ridge building over the Rockies. Could see another
round of showers and thunderstorms on Monday before warmer and
drier weather returns.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1237 PM CDT Tue Jun 27 2017

There is a high confidence in VFR conditions into the evening with
mostly clear skies. Thunderstorms are expected to develop across
Nebraska later this evening however, reaching Iowa overnight while
dissipating. Confidence is sufficient for a period of thunder
mention west while only VCSH wording farther east. MVFR or less
conditions and additional thunder periods are certainly possible,
but confidence is not sufficient until trends suggest otherwise as
the event nears.




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