Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 231745

Area Forecast Discussion
National Weather Service Des Moines IA
1245 PM CDT Sat Jun 23 2018

...Updated for 18z Aviation discussion...

.SHORT TERM.../Today through Tonight/
Issued at 335 AM CDT Sat Jun 23 2018

Seems we just cant shake this active pattern.  Even though it will
be relatively quiet today compared to the past few days, there still
will be a series of weaker shortwaves moving across the state today
and tonight across central into southern Iowa.  A stronger shortwave
pivots across the Northern Plains by tonight as an upper low develops
across Wyoming. Tonight as this Wyoming low continues to deepen,
the upper flow will buckle over us and lift northward, shifting
these weaker disturbances northward.

At the surface, low pressure will begin to develop over the
Central Plains bringing at least a weak southerly low level flow
back to southern/western Iowa in the afternoon. This afternoon and
evening, forcing and theta-e advection increases across the west
and eventually north along with a modest increase in instability.
What will be lacking is shear. However the combination of forcing
with the amount of instability present will be sufficient to
produce showers and a few thunderstorms...especially north and
west this afternoon and evening. Therefore, I did have to add at
least slight chance PoPs to the central and south during this time
period with chance PoPs west and north.

Temps will moderate some today due to surface winds becoming
southerly but they will be light and may be hampered by any
showers/storms that may develop. Stronger warm advection won`t
arrive until later in the weekend.

.LONG TERM.../Sunday through Friday/
Issued at 335 AM CDT Sat Jun 23 2018

.../Sunday through Tuesday Night/ Confidence: Medium to High

Main challenges will be extent of convection...rainfall and
temperatures. Active weather pattern will begin again after a day
or so of relative quiet today. Subjective surface analysis at 03z
Saturday showed weak synoptic features over the midwest with
boundaries over Iowa associated with upper level departing over
Indiana. Back to the west a warm front is already in the making
over eastern Nebraska back into a low over Saskatchewan. A
secondary low over the Rockies and the Canadian low will begin to
phase in the next 48 hours as another piece of energy off the PAC
NW coast moves east southeast. By 12z Sunday a more pronounced
H500 feature will be moving into western KS with stronger warm air
advection/H850 flow aloft across KS/OK and into Nebraska. This
lift over the warm front to our west will result in increasing
thunder chances over the region especially during the afternoon
hours across the western/northern region Sunday. There may be a
decaying area of showers/storms over northern Iowa early on Sunday
with another round later in the afternoon north. As the stronger
upper level forcing arrives Sunday night, storms should increase
over the west/south from 03 through 12z Monday. Some uncertainty
remains at this time regarding the preferred location of what may
end up a more pronounced MCS, but most of the models are in
agreement that the higher risk of heavier rainfall would be over
eastern Nebraska south into Kansas and possibly into western and
southern Iowa. Highs Sunday are likely to be tempered by the
clouds and potential for showers/thunder once again with afternoon
readings in the lower to mid 80s. Moisture will be on the
increase again with dewpoints climbing into the lower to mid
60s...summertime humidity will be back. The rainfall forecast will
be somewhat complex over the period due to mesoscale forcing
expected. Currently we remain in moist air aloft with H850
dewpoints near 10C over Iowa. The deeper 15C dewpoints have been
forced south into TX/LA/AR/MS/AL for now but are likely to be
pulled northward by Sunday into Monday. GFS PWAT analysis
currently shows 1 to 1.5 inches through 00z Sunday. As the
stronger wave approaches Sunday night/Monday PWATs are forecast to
rise to 1.75 to near 2 inches over much of the region with
concurrent warm cloud depths to 4000m. Moisture will be ample to
support rainfall to several inches in some of the heavier
storms...but with some potential for an MCS to form southwest/west
of the region Sunday night...the heaviest rainfall in excess of 4
or more inches may be limited to those areas in KS, NE or MO. As
the system crosses the area Monday, more significant mechanical
forcing from 18z through 06z Tuesday should result in a period of
stronger thunderstorms from central through eastern Iowa. Already
SWODY3 has outlooked the region for a slight risk for Monday.
Depending on daytime recovery and preferential heating during the
afternoon...the current outline may shift east of I35 over time.
The models differ on placement of the line of storms ...but
generally have some consensus across the eastern portion of the
forecast area at this time. Depending on instability
parameters...the storms may be strong enough for all modes of
severe weather and potentially some locally heavy rainfall as
well. All of the models lift the H500 upper level low into
southern MN by 12z Tuesday with some residual energy rotating
across the region Tuesday afternoon. This is likely to create more
rounds of scattered thunderstorms with the final push of
shortwave energy moving through during the day. The period of
heaviest rainfall will be from Sunday night through Monday
afternoon across the region when 1 to 2 inches with locally higher
amounts are likely. Tonight gridded flash flood guidance (ffg)
averages about 1.5 to 2 inches per 1 to 3 hours over most of the
region except for the south 2 tiers which can handle about 2.5 to
3 inches in 1 to 3 hours. With a break for now...ffg values
should increase somewhat through Sunday which would help a bit
should a period of heavy rainfall occur. Will need to monitor the
region...especially Sunday evening through Monday for any
headlines but for now the main hydro issues will be the current
river flooding occurring across the region. Highs Monday will be
difficult due to clouds...and periods of storms and will likely be
in the 70s to lower 80s. By later on Tuesday some recovery in
temperatures is possible as the main area of forcing and
thunderstorms moves east. Tuesday night should see some clearing and
an end to the organized rainfall for the time being.

.../Wednesday through Friday/ Confidence: Medium

By Wednesday a ridge of high pressure will be building east...and
though flow aloft will remain relatively zonal H850 temperatures
will be on the increase into the upper teens to lower 20s by 00z
Thursday across the region. This should result in highs in the 80s
to lower 90s. There is a weak signal for some thunderstorms riding
over the ridge Wednesday night with another push of warm air
advection across Iowa into 12z Thursday. Forecast uncertainties
increase for Thursday into Friday as the Euro quickly builds a H500
ridge over southern MO with +22C to +26C across the area by
afternoon. The GFS is a full 5C cooler with additional showers
possible. The Euro continues the heat through Friday while the GFS
finally brings the hotter air into the region by then.  With some
uncertainty...both high and low temperatures will be more uncertain.
Trends suggest that upper 80s to mid 90s are possible both days with
an outside risk of upper 90s southwest. Late into the period and
toward the upcoming weekend leading to the weather
is expected to return to the Northern Plains and Upper Mississippi
River Valley.


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1242 PM CDT Sat Jun 23 2018

Have VFR conditions going at all sites throughout TAF period.
Cloud bases will be decently high, with winds under 10 kts

Most impactful weather will come at the end of this TAF period as
showers and thunderstorms move into Iowa from west to east. Storms
should be near most sites except for KOTM by 15-18z time period.
Left precip mention out at this point and will let next TAF
updates provide more finely tuned, more accurate info.




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