National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

000
FXUS63 KDMX 060753
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
253 AM CDT Thu May 6 2021

.DISCUSSION.../Today through Wednesday/
Issued at 254 AM CDT Thu May 6 2021

Forecast Highlights:
-- Below normal temperatures through much of the period
-- Gusty winds and low dewpoints foster a somewhat elevated fire
   weather day today
-- Showers and elevated storms later Saturday into Sunday; highest
   chances southern Iowa decreasing northward

Details: Broad troughing over central Canada and north central US is
shown this morning on GOES-East upper level water vapor imagery.
Embedded within this cyclonic flow, a shortwave trough is passing
south of the state. Dry air in the low levels below 750 or 800mb as
shown on the 00z DVN and OAX RAOBS has kept these as light showers
and overall not too widespread. Any lingering light rain will end by
sunrise this morning over central Iowa as the shortwave scoots into
the Ohio Valley. Otherwise, cold air advection will continue to
spread across the area this morning and will bring in slightly
cooler air today with highs a bit lower than yesterday. Deep
boundary layer mixing is expected per forecast soundings with winds
at the top of the mixed layer a little lighter than last night.
Still, wind gusts to around 30 mph are expected over northern Iowa
and between 20 and 30 mph elsewhere. The boundary layer mixing will
also result in lower dewpoints this afternoon and have trended the
forecast to the 10th percentile of all available guidance, which
brings dewpoints under 30 degrees in most areas. This results in RHs
by this afternoon into the middle to upper 20 percents and with fine
fuels not fully greened up over northern Iowa, it will be a somewhat
elevated fire weather day in that area.

Another potent PV anomaly will rotate around and drop into the
region tonight. NAM and GFS deterministic models and a few ensemble
members of the GEFS and CMC show light QPF reaching our far
northeast counties. Overall, anything will be sprinkles to at worst
(or welcome) light rain and have some token PoPs around midnight
tonight.

Mid-level ridging will try to push eastward on Friday into this
weekend and the broad troughing over the central US will budge
enough eastward for this to occur. This will result in temperatures
higher on Friday and closer to typical early May values. Meanwhile,
a trough will be entering western Canada and the Pacific Northwest
on Friday and pushing over the northern Rockies by late Saturday. A
surface low is expected to develop over the eastern Plains of
Colorado on Saturday with a warm front extending ahead of it, but
remaining well south of Iowa. Low level thermal lift will increase
over the region on Saturday with increasing QG convergence as well.
Surface based convection will remain south of the state, but
elevated instability will foster thunderstorms over southern Iowa
with showers farther north later Saturday afternoon into Sunday.
While shear will be between 30 and 40 knots, CAPE profiles from the
NAM and GFS are rather skinny so severe risk looks on the side.
Looking at ensemble guidance for rain placement, there is certainly
strong support to have the highest PoPs over southern Iowa tapering
to the north. In collaboration with northern neighbors, have also
lowered PoPs away from initial guidance with ensembles showing lower
support for PoPs in those areas. Further, 5/12z WPC cluster analysis
shows that the northern QPF solution is a result of 50% of the GFS
ensemble members and 30% of CMC ensemble members with hardly any
contribution from the ECMWF. So, out of 100 available ensemble
members from the GFS, CMC, and ECMWF, only about a quarter show the
northern solution. As for QPF amounts, the GFS and ECMWF ensemble
mean show a greater than 50% chance of at least a half an inch south
of I-80. Rain should end from northwest to southeast on Sunday as
the low pressure moves into the Ohio Valley.

While the western US trough will remain in some form into early next
week, surface high pressure dropping through the western Great Lakes
States will keep rain chances at bay over central Iowa. Confidence
is low on any rain chances toward midweek as the GFS is the slowest
of the global deterministic models, which it and about half of its
ensemble members are wetter in the WPC cluster analysis. With less
confidence, have left PoPs alone for the middle of next week. One
thing that has been interesting the last few nights is the lack of
opportunity to include thunder in the forecast. Sure, we have some
this weekend, but instability has been lacking, which given the
pattern is not surprising, but for May that is surprising.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1146 PM CDT Wed May 5 2021

Little change to previous forecast. Frontal passage and a 4 to 5
hour patch of MVFR cigs developing behind the boundary will begin
at 07Z north and exit 15-17z southeast. Winds will increase toward
12z with gusty northwest winds most of the period through 00z and
then decouple. As mentioned, cigs return to VFR as the patch of
lower clouds departs. Some afternoon cu likely to form with BKN070
to BKN100 during that time then fading away by 00z. /rev


&&

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

$$

DISCUSSION...Ansorge
AVIATION...REV