Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190335

Area Forecast Discussion
National Weather Service Des Moines IA
1035 PM CDT Fri May 18 2018

.DISCUSSION.../Tonight through Friday/
Issued at 335 PM CDT Fri May 18 2018

Severe weather potential for Saturday afternoon and evening over
the southern half of the CWA was the main focus of the forecast.
Active weather pattern continues throughout much of next week but
the only time period with any confidence of mentioning widespread
storms looks to be Tuesday night into Wednesday. Otherwise,
nothing significant to really hang your hat onto wrt to severe
storms as models remain in disagreement.

Tonight...Weak shortwave providing enough lift for elevated shower
activity in northeast Missouri into western Illinois may trickle
into southeast portions of the forecast area. However, showers are
high based and are weakening as they migrate north-northwest this
afternoon. Can`t rule out a sprinkle or two in the southeast, but
kept the forecast dry attm as low confidence with any measurable
precipitation. The dying convection in eastern Nebraska may
meander into portions of west-central to northwest Iowa later
tonight, but with the current radar trends and hires models
keeping the rain west of the CWA, kept the forecast dry tonight.

Saturday into Sunday...warm front bulges northward on Saturday and
looks to orient itself west to east along Highway 34 by around 00z
Sunday. The surface low is forecast to be located in northeast KS
and this is the area where initiation is likely. The better
instability builds into south-central to southwest Iowa where
MUCAPE values increase to 1500 to 2500 J/kg by 00z Sunday. Strong
mid-level lapse rates ranging from 7.5-8.5 C/km, decent
convergence along the boundary, and deep layer shear all look to
be at least favorable for large hail and damaging wind threat.
Certainly cannot completely rule out a low-end tornado threat if
any storm nears the boundary in southern Iowa, especially with
low level helicity values over 100-150m2/s2. However, low
confidence with any widespread tornadic activity. Storms look to
evolve into a MCS late Saturday night into Sunday, providing the
potential for widespread rainfall across the forecast area. Much
cooler on Sunday with the storm chances and cloud cover across the

Monday through Friday...departing vort max early Monday leaves
much of the state dry by the afternoon hours. Generally zonal to
northwest flow aloft is expected over the region through Thursday
before upper level ridge builds into the central CONUS. Still,
models are having a difficult time pinpointing the quick moving
shortwaves during this period with (as mentioned above) the only
one with moderate confidence is Tuesday night into Wendesday.
MUCAPE values do increase to over 3000 J/kg by 00z Wednesday with
decent 0- 3km and 0-6km shear should be enough to become
marginally outlooked for hail.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1035 PM CDT Fri May 18 2018

Still some concerns over patchy fog overnight from 10-14z across
the region. Will keep some mention at most sites from 09-14z. MVFR
and IFR conditions also possible early as stratus/br expands over
southeast Iowa and northern Iowa. Mixing will allow cigs to lift
by 15-16z most areas back to VFR. As system approaches Sat PM some
chance of VCTS/VCSH aft 19z west and 00z central. Better chances
thunder aft 00z all sites. /rev




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