Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 292021
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
321 PM CDT SUN MAY 29 2016

.SHORT TERM.../Tonight/
Issued at 320 PM CDT Sun May 29 2016

Current convection north will diminish toward sunset as instability
wanes. Surface trough and mid level shortwave will also move east
with time. In its wake...drier air is moving southeast into northern
sections. Have lowered mins to mid to upper 50s across the north
while the southwest third will see warm air advection return in
response to return flow. Old stationary boundary over Missouri
/Kansas will drift northeast overnight as low level jet begins to
impinge on the surface/boundary aloft. Area of mid level
destabilization will occur to the west of the region with increasing
chances for showers/thunder toward 12z west. Source regions include
High Plains in western South Dakota with secondary source region
over Kansas. The majority of the first batch of convection will
originate from the Dakotas. Have raised PoP to chance category
mainly west of I35 by 12z. Winds will remain light overnight
becoming more west southwest by morning.

.LONG TERM.../Monday through Sunday/
Issued at 320 PM CDT Sun May 29 2016

The upper level synoptic pattern at the start of the period will
be characterized by zonal split flow with weak perturbations
propagating across Iowa. A number of 29.12 NCAR ensemble members
and a few other CAMs do develop a weak multi-cell cluster across
central South Dakota tonight in response to a subtle H500
shortwave, and while timing and placement continuity is poor, most
solutions dissipate the convection as it shifts away from the
30-40 kt LLJ. Kept slight chance POPs through the morning hours as
weak theta-e advection overspreads the state, but the better
threat for storms will be during the peak heating hours across
western and southern Iowa. MLCAPE values across these regions will
exceed 1500-2000 J/kg, but given the weak shear profiles and lack
of baroclinic forcing, the threat for severe weather will be
limited and localized (will have to monitor to see if an outflow
boundary from the morning convection provides this needed focusing
mechanism). NAM soundings in particular for tomorrow afternoon
depict steep low-level lapse rates and high LCLs, hinting at
possible wind threat.

The threat for thunderstorms will persist into the overnight on
Monday, lessening towards morning. A cold front sweeping through the
state will provide a better chance of organized and severe
convection Tuesday afternoon and evening, but given the weak,
unidirectional flow ahead of the boundary, hail and wind would be
the main threats. The upper trough pivots through on Wednesday,
which may spark a few showers in the east before departing.
GFS/EC are in good agreement on a stout blocking longwave ridging
pattern setting up across the western CONUS for the end of the
week and weekend. This will bring a period of drier weather,
interspersed by a few chances for showers and storms as weak,
moisture-deprived waves work down the east flank of the ridge.
H850 temps rise to +12 to +19 C on Friday and nudged up highs into
the low 80s, closer to the raw model guidance.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1258 PM CDT Sun May 29 2016

Little change to previous update. Iso thunder over the
north/northeast through afternoon at KMCW/KALO. Winds have
increased as well with gusts near 20kts at these sites from WNW.
Otherwise...expect warm air advection/convection to increase
aft 06z at most sites. Coverage may be limited enough...so have
covered for now with VCSH. Cigs to lower to bkn060-110 aft 06z as
clouds spread over the region. Through mid morning expect coverage
to decrease...only to gen up again aft 18z. /rev


&&

.DMX Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...REV
LONG TERM...Skow
AVIATION...REV



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