Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 290825
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
325 AM CDT Sat Apr 29 2017

.SHORT TERM.../Today through Tonight/
Issued at 325 AM CDT Sat Apr 29 2017

Multiple waves of precipitation will spread across the area from
southwest to northeast this weekend. Regional radar imagery depicts
the next swath of showers and thunderstorms currently across
southern Kansas and Missouri, which will advect up across at least
the southern two thirds of Iowa late this morning and into the
afternoon, similar to what occurred yesterday but with more QPF
today given the higher PWATs and generally greater forcing for
ascent. After a relative lull late today through this evening,
during which time light showers or drizzle may still occur, the next
wave of showers and thunderstorms will surge across the area
overnight tonight. Have added as much timing detail as feasible to
the POP forecast to reflect this scenario. As far as the possibility
of thunderstorms in our forecast area, it should be virtually nil
today, but tonight especially after midnight elevated instability
will be increasing and have maintained thunder chances roughly along
and south of Highway 30. Severe weather is not expected. All in all
we will see a continuation of cool temperatures with increasingly
grey and soggy weather through the short term forecast period.


.LONG TERM.../Sunday through Friday/
Issued at 325 AM CDT Sat Apr 29 2017

Sunday through Monday Night...Confidence: Medium

Wet weather to continue through early Tuesday. Though there remains
consistency regarding the evolution of the approaching storm in
terms of its occlusion with potential thunder in advance of the
system Sunday morning...then followed by cooler temperatures with
deformation precipitation Sunday afternoon and night...there remain
some differences in the details of the thermal characteristics from
model to model due to strength/track considerations. This will have
some impacts on total precipitation amounts as well as some impact
on precipitation type and temperatures. The Euro takes the track of
the low first into SW Iowa then north northeast to North Central
Iowa by 06z Monday...then northeast into NE Iowa by 12z Monday.  The
GFS has the low more elongated from SW to NE Iowa by 06z Monday with
a slightly deeper low by 18z Sunday over western Wisconsin.  Though
some differences continue in the expected track of both models...the
general thermal characteristics are similar in each model and to
previous model packages.  Between 06z and 12z Sunday the far western
to northwest portions of our forecast will be cooling to -2 to -3.5C
with 1000-500mb thickness of 538-539dm.  GFS Bufr soundings suggest
that temperatures will still be a bit too warm near Carroll with a
nearly 1400 ft melting level. Farther northwest near Cherokee the
melting level is about 600 feet. Though not likely...if there is any
more significant mixing along with the overnight precipitation...a
small stripe of snow may occur over the west which ends up
accumulating for a bit on mainly grassy areas. Otherwise without any
prolonged snowfall...accumulations are likely to be insignificant.
Sunday will overall be quite breezy and also quite chilly over the
north with highs in the lower 40s with mid 50s in the southeast.
East to northeast winds are likely to increase to 20 to 25 mph
during the morning with gusts over 30 mph on Sunday morning. Colder
air will move into the area Sunday night but recent rainfall and
stronger winds will prevent any possible frost despite lows in the
mid 30s north to lower 40s southeast. Highs Monday will be quite
forgettable with lower 40s north to around 50 central and southeast.
 Though about 8 to 10 degrees above the record low max for the
date...it will still be rather cold compared to our normal highs
which would normally be nearly 20 degrees warmer than the current
forecast. Light rain showers will linger as the low pulls out of the
area through Monday night northeast. Otherwise look for a quieter
period for the rest of the extended.

Monday Night through Friday...Confidence:  Medium

Model consensus is in good agreement regarding the departure of the
large...slow moving storm that will exit by Monday night.
Temperatures will present some challenge for Monday night. Depending
on the timing of any clearing...there may be an area over the
central to northwest where enough clearing takes place that lows dip
into the lower 30s.  Despite this possibility...winds may be just
strong enough along and along with recent moderate rainfall to
minimize the chances for frost or a widespread freeze.  This time
period will need to be monitored over the next couple of days.
Following the storm weak ridge of high pressure and slightly warmer
H850 temperatures will pass through the region Tuesday...most likely
with a mixture of sun and some clouds through the day with the
clouds due to the recent rainfall. A rather weakly amplified
secondary short wave will drop southeast out of Canada Tuesday night
into Wednesday night with cloud cover and a small chance of light
showers over portions of the forecast area.  Currently the GFS is
showing a more pronounced solution earlier on Wednesday with some
weak instability along with the passage of the upper level system.
The Euro is weaker still and slower with the main area of light
precipitation impacting western sections of the forecast area into
late Wednesday afternoon/night.  With low confidence in timing and
placement...will keep overall chances in the slight category for now
with no mention of thunder at this time.  The general trend for the
remainder of the forecast period from Thursday through Saturday is
for a significant relaxation of the H500 pattern with a ridge
beginning to build across the western CONUS.  This should result in
lesser chances for precipitation and more zonal flow.  Not only do
H500 heights increase but H850 temperatures also look to be on the
rebound by late in the period. Early on...H850 temperatures of 3 to
6 C are forecast Thursday/Friday with significant warming into the
remainder of the weekend.  At this time the GFS appears to be
somewhat faster in moderating temperatures by weeks end. For now
will hold onto the slightly slower warming trend into Friday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1200 AM CDT Sat Apr 29 2017

Little change to ongoing forecast except delayed the precipitation
onset, especially for the northern TAF sites. Once the rain sets
in, have MVFR ceilings and visibility through the rest of the TAF
period.


&&

.HYDROLOGY...
Issued at 325 AM CDT Sat Apr 29 2017

No significant changes from previous discussion. Currently some
minor to significant within-bank rises and possibly minor flooding
are expected on area rivers over the next 3 to 5 days due to the
upcoming multi-day rain event beginning today and ending Monday
evening. The greatest rises are expected across central and south
portions where the higher QPF exists.

Presently the best chances of seeing minor flooding are along
portions of the North Raccoon River...Raccoon River and the Des
Moines River from its confluence with the Raccoon River downstream
to Red Rock Reservoir. Even in these areas confidence is presently
not high enough to issue a river Flood Warning or even a river Flood
Watch. Over the next several days a continued monitoring of QPF
trends will be needed for any possible changes.

&&

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

$$

SHORT TERM...Lee
LONG TERM...REV
AVIATION...Podrazik
HYDROLOGY...REV



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