Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 080454

Area Forecast Discussion
National Weather Service Des Moines IA
1154 PM CDT Fri May 7 2021

.DISCUSSION.../Tonight through Friday/
Issued at 305 PM CDT Fri May 7 2021

The primary weather highlights haven`t changed much from this
time yesterday and include:

* Showers and thunderstorms this weekend, especially central and
  south with appreciable rainfall possible
* Normal to below normal temperatures continuing through much of the period

Skies were mostly clear over the forecast area heading into the
evening, but the baroclinic zone that will bring appreciable
rainfall to the south half of the forecast area this weekend was
already showing its hand this afternoon with elevated showers and
isolated thunderstorms across eastern KS. This is expected to be fairly
steady-state into the night, but mid and high level cloudiness is
still expected to develop into Iowa SW-NE overnight with this
warm advection and some kinematic contributions from the
developing western long wave trough and much lower amplitude wave
moving out of New Mexico. The ridge along the Missouri River at
19z is expected to reach or just pass the Upper MS Valley by
sunrise leading to some concerns about frost northeast. There are
some positives such as projected lows in the mid 30s and dry
ground aiding vegetative cooling, however there are negatives as
well such as drier air in the heart of the surface ridge, the
potential for high cloudiness late, and the fact that the soils
and vegetation in the area of concern have seen full May sunshine
today. Thus with marginal parameters, have included a bit of
frost in the forecast far northeast, but the confidence and areal
coverage does not appear to warrant a headline at this time.

The weather is expected to then turn active late Saturday into
early Sunday. Components of the current western CONUS trough will
advance into the central and northern Plains increasing our
kinematic support with the aforementioned baroclinic zone also
nearing or reaching Iowa. The Euro suite of guidance continues to
suggest farther south into Missouri, however the GFS suite as well
as recent domestic model runs including higher resolution CAMs
are now highlighting stronger theta-e advection/convergence and
frontogenetical forcing farther north into the state. HREF
PMM/LPMM output suggests one or two west to east bands
highlighting this forcing across the southern three tiers of
counties with narrow 1-2" bands possible. This looks to be fed by
strong 305K inflow and moisture transport into the effective
instability base near 1.5km with flow from 50-60kts upstream
going down to around 30 into IA, with warm cloud depths of 3.5km
promoting more efficient rainfall than we have seen for awhile.
The forcing will be weaker, but lighter rains are also expected

The potential is low, but a strong storm with hail in the
elevated instability can`t be ruled out. There is a narrow few
hour window where elevated CAPEs approach 1000 j/kg south with
30-40kt effective shear. Any potential looks brief and transient
however during the early morning hours Sunday. Much of the support
exits by 12z Sunday leaving behind only chances for rain in cool
NE winds. In fact, the entire weekend will be cool with highs
staying in the 50s.

The outlook for next week calls for dry and cool conditions
initially with short wave ridging and weak surface flow. The
models then all agree on bringing a weak short wave through the
MO Valley midweek, but the timing and strength details are not in
good agreement leading to low confidence with both Wednesday
and/or Thursday possibilities, so unfortunately this leads to a
few days of low end PoPs. Whatever does occur looks light and
insignificant. Ridging approaches to end the period next Friday
with little to no precip, but it does look to at least develop
some return flow and get our temperatures back closer to normal.


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1139 PM CDT Fri May 7 2021

VFR conditions expected overnight although mid/high level clouds
will continue to increase from west to east towards daybreak.
Expect light east winds overnight, which will increase to 10-15
knots gusting to 25 knots by late morning/midday. CIGs will
continue lowering through the day Saturday but should remain VFR
until around 00Z where MVFR CIGs become likely at KFOD, KDSM, and
KOTM associated with the incoming showers. Occasional IFR CIGs and
VSBYs will be possible towards the end of period, especially with
any heavier pockets of rainfall. Although instability looks
relatively weak, some embedded thunderstorms will also be possible
after 21Z, however due to low confidence will leave out explicit
mention for now until timing/location can be better resolved.




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