Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 250024

Area Forecast Discussion
National Weather Service Des Moines IA
724 PM CDT TUE MAY 24 2016

.SHORT TERM.../Tonight/
Issued at 311 PM CDT Tue May 24 2016

Main challenge overnight tonight will be convective trends and
coverage. Old MCS over Missouri continues to move southeast this
afternoon with little remaining impact on our area. Mesoanalyses
shows little if any convergence across our forecast area now...with
increasing effective bulk shear to 30-35kts at 20z. Even with
uncapped atmosphere now...little other than instability expected to
be the main forcing for the remainder of the afternoon. Possible to
see some iso thunder from 21-01z over the east central to northeast
as evident by cluster of cumulus across east central Iowa. This may
be tied to a small remnant vort lobe from the overnight MCV that
extends northwest from the more pronounced MCV over Missouri. In any
event...any convection that fires should be short lived though brief
downpours could accompany the thunderstorms. Attention then turns to
the overnight as another weak wave now over eastern Colorado moves
northeast. Pulse of warm air advection increases isentropic lift
with thunder breaking out toward 06z west...then filling in across
the area by 12z. Hires models continue to show redevelopment across
Nebraska and after 04z concurrent with increasing low level jet. Two
areas of warm air advection will force one area of storms into
northern/northwest sections and another toward 12z across the
southwest. QPF amounts should exceed a half inch over the west with
some locally higher totals possible exceeding 1.5 inches. PWATs
remain in the 1 to 1.5 inch range. We continue outlooked for
excessive rainfall over the south/southwest. A lot depends on how
quickly any secondary convection can move east southeast from
Nebraska to Iowa. Confidence remains low on placement due to models
suggesting the better forcing will move into northern Iowa by 12z.
Will need to monitor realtime trends overnight tonight given current
uncertainties. Lows will be warm as boundary layer dewpoints have
risen into the lower to mid 60s and overnight lows will be similar.

.LONG TERM.../Wednesday through Tuesday/
Issued at 311 PM CDT Tue May 24 2016

An active weather pattern will persist into next week with
generally southwesterly flow aloft and extended thunderstorm
chances. Little has changed in overall thinking since yesterday,
however, a few details are starting to coalesce regarding timing
and hazards.

A swath of showers and storms associated with a warm air advection
regime will be ongoing across parts of our forecast area on
Wednesday morning and clear out to the northeast and east during
the afternoon. In its wake some destabilization will
occur, however there is considerable uncertainty in the amount of
clearing/insolation and neutral flow aloft and a lack of low level
focus mechanisms may inhibit convective initiation for much of the
afternoon. Meanwhile a weak shortwave will be moving over the
western plains, around Nebraska and South Dakota, where a
lingering synoptic boundary will enhance thunderstorm development
in the late afternoon and evening. This activity may subsequently
advect eastward across portions of our area, and chance POPs have
been maintained through the evening and nighttime hours.
Sufficient instability and deep layer shear exist to support some
severe weather threat with any storms that do develop/move across

From late Wednesday night through Thursday morning weak ridging
will move overhead and we should see a brief respite from
precipitation, but this will allow the atmosphere to destabilize
even more substantially by Thursday afternoon with forecast
soundings indicating uncapped surface based CAPE in the
neighborhood of 4000-5000 J/kg. Meanwhile somewhat stronger wind
fields aloft will result in 0-6 km shear values roughly around 40
kt by the afternoon. As a result vigorous thunderstorm development
is expected during the peak heating hours somewhere across the
region. The uncertainty lies in where this will occur. It may
occur along the aforementioned synoptic boundary, which most
prognostic models are predicting will stretch from southwestern
Minnesota down into eastern and southern Nebraska. However, the
ECMWF has been consistently predicting a more southern solution,
possibly due to effects of the Wednesday night convection and
resulting outflow reinforcement, with the front stretching across
central Iowa by Friday afternoon. Whether storms initiate over our
forecast area or farther northwest, it is likely that they will
initially be supercellular with all modes of severe weather
possible, then may congeal into a forward propagating MCS with
strong winds becoming more likely. The SPC outlooks cover this
well, with an expansive Slight Risk area due to uncertainty in the
location of the synoptic boundary and associated convective

From Thursday night through Saturday a 500 mb low pressure center
will gradually approach from the southwest and eventually swing
across Nebraska and Iowa. This will result in even higher POPs
with several rounds of showers and thunderstorms expected. The
details of timing and location become murkier during this time
frame, mainly due to variables associated with the extent of
earlier rounds of storms on Thursday/Thursday night. This
precludes categorical POPs at this time, though it is likely that
these will be introduced over the next couple of days as each
successive round of convection is better resolved.

Besides the timing of convective trends and potential for severe
weather, there is also a threat of heavy rainfall and associated
flooding over the next several days. PWAT values should be
seasonably high, around 1.23-1.6 inches at times, with surges of
moisture transport and a possibility of training storms given the
orientation of steering flow and expected boundaries. However,
between the degree of uncertainty in these details, the likely
cross-basin storm motion, and the fact that most of the forecast
area has been fairly dry recently and can take a decent amount of
water, at this time it appears likely that any flooding will be
localized and probably due to intense rainfall rates and focused
on urban or high topography areas.


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 724 PM CDT Tue May 24 2016

MVFR ceilings are possible overnight at FOD and have mention by
10z Wednesday. Lower confidence in mentioning at other TAF sites
are the storms look slightly delayed and mainly high based. Will
reassess at the 06z but also delayed the onset of storms/rain
overnight towards Wednesday morning.


.DMX Watches/Warnings/Advisories...None.



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