Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 241129
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
629 AM CDT Thu May 24 2018

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

.SHORT TERM.../Today through Tonight/
Issued at 325 AM CDT Thu May 24 2018

8z GOES-East Nighttime Microphysics shows high clouds streaming into
Iowa while convective, colder IR cloud tops remain over north
central Nebraska into central South Dakota. Radar trends have shown
the convection west of us weakening and most CAMs show this
dissipating or staying northwest of our forecast area. Some of the
CAMs glance western Iowa with a few developing showers or storms
this morning, but forcing is weak with a greater chance for
thunderstorms later in the afternoon into tonight. Before then, it
will be another hot day as there is little change in the way of
850mb temperatures from Wednesday afternoon to Thursday afternoon.
This will translate to highs in the middle to upper 80s, which is at
least 10 degrees above normal across central Iowa. As q-vector
convergence begins to arrive late in the afternoon, will see showers
and storms develop over central Minnesota into northwest and western
Iowa. This area will have MLCAPEs between 1500 and 2000 J/kg.
However, deep layer shear and mid-level lapse rates will be weak to
modest at best with these parameters better west of our forecast
area. So isolated severe hail and damaging wind gusts will be the
concerns. As for heavy rainfall threat within 850-300mb flow of 15
to 20 knots, precipitable water values will be around 1.5 inches,
which is in the 175% of normal. Further, the NAEFS has specific
humidities in the 99th percentile. This could certainly lead to some
locally heavy rainfall, but don`t see a widespread flash flood risk
at this point. By Friday morning, the showers and storms will be
pushing east from central Iowa into eastern Iowa.

.LONG TERM.../Friday through Wednesday/
Issued at 325 AM CDT Thu May 24 2018

Mean ridging across the central United States will be the big
forecast driver over the next several days.  That being said, the
beginning of the longer term on Friday morning will see a weakness
in the upper ridging with a shortwave passing across the upper
Midwest.  Isolated to scattered convection will linger into Friday
morning across central Iowa as the low level jet veers into the
state but will be decreasing in coverage and intensity with
weakening moisture transport.  The bulk of forcing with the
shortwave will remain well north of Iowa with the boundary washing
out across the state late Friday into early Saturday.  However,
sufficient convergence along the weakening boundary should produce
some scattered convection in the weakly capped environment on Friday
afternoon/evening over the southeast half of the forecast area.
Shear will be paltry although strong CAPE sufficient of large hail
production may briefly lead to an isolated severe storm or two. A
strong downburst may also occur with the heavy rain cores as well.
This convection should weaken quickly into Friday evening with
stabilization of the boundary layer after sunset as dynamic forcing
will be nearly nil.

The holiday weekend is shaping up to be quite hot with upper ridging
re-establishing itself in the region in the wake of the shortwave.
Thermal ridging in the Plains will extend eastward into the state
and with generally sunny conditions, temperatures into the lower 90s
seem likely on Saturday through Monday and possibly Tuesday.
Dewpoints are expected to remain mainly in the mid 60s which should
keep heat indicies below 100 but still unseasonably warm for this
time of year. Will likely highlight the expected heat in a Situation
Report and HWO with no headlines expected, especially given
overnight lows dropping into the upper 60s to around 70 which
does give some break to the daytime heat.

The GFS does continue its proclivity towards more widespread
convection through the weekend into early next week despite weak to
nil forcing and prefer the Euro which keeps the bulk of convection
associated with deeper forcing toward later Tuesday and into the
middle of next week.

Of additional concern, will be continuing to monitor the dry
conditions in south central/southeast Iowa as rainfall deficits
continue to mount. Many of these locations have seen significant
deficits since 2016 and the early growing season amounts so far
this year in these location remain below normal.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 629 AM CDT Thu May 24 2018

Primary concern will be scattered thunderstorms expected to
impact the terminals in the latter half of this TAF period. At
this point, have continued VFR at all terminals with mention of
VCTS at FOD, MCW, and DSM with much uncertainty around timing
owing to low confidence. Thunderstorms may impact ALO and OTM
late in the period, but not enough confidence to include at this
time. Otherwise, winds will become breezy from the south through
the day today.

&&

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

$$

SHORT TERM...Ansorge
LONG TERM...Cogil
AVIATION...Ansorge



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