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Area forecast discussion 
National Weather Service Des Moines Iowa
1207 am CDT Thursday Aug 28 2014

issued at 939 PM CDT Wednesday Aug 27 2014

No changes to existing Flash Flood Watch headline. Current NE/Kansas
mesoscale convective system upstream is embedded within a broad area of weak/moderate
kinematic and thermodynamic forcing ahead of rockies long wave
trough in airmass very favorable for heavy efficient rainfall.
Strongest convection coincides nicely with 01z rap precipitation
potential placement maximum and this crosses MO river into western
forecast area 09-12z with mean wind...thus slowed pop advancement
somewhat in evening update. 1-2km moisture flow and associated
transport is not overly strong...but veers into western Iowa by
daybreak. With precipitable waters still in excess of 2 inches...k indices around
40...and warm cloud depths 4... stronger convection could
certainly still produce excessive rains.


Short term /tonight/...
issued at 345 PM CDT Wednesday Aug 27 2014

Timing and location of heavy rain was forecast concern tonight
with another round of convection anticipated to develop overnight.
Leaned toward a blend of the European model (ecmwf)/nam12 and the latest hrrr/hopwrf/4.0km
spcwrf for timing of the precipitation. Biggest change was slowed down
the onset roughly 2 to 4 hours from previous forecast package and
still may be too quick for redevelop tonight.

Low level jet increases over south-central Nebraska/northern Kansas with the
nose nudging into southwest to western portions of the Iowa past 06z
Thursday. The best Theta-E advection surges across southwest Iowa
northeastward into north-central Iowa by Thursday morning and the
best moisture convergence looks to be b/T Highway 20 to near I-80
corridors but certainly not limited to those locations. Precipitable
water values in the 2 to 2.25 inch range over western Iowa and warm
layer cloud depths around anticipating some efficient
rain producers overnight. Moisture transport erupts with the low level jet
increasing overnight and with weak corfidi vectors...suggesting
training potential as well. Leaned toward issuing a Flash Flood
Watch based on the latest hires model trends and 27.12z NAM/European model (ecmwf)
convective/quantitative precipitation forecast trends. It will not take much to get some flash flood
issues over the western portions of the forecast by early Thursday
morning. May need to extend/expand the watch further north...but ffg
is a bit higher along the Minnesota and had lesser confidence with
flash flood potential in this area.

Long term /Thursday through Wednesday/...
issued at 345 PM CDT Wednesday Aug 27 2014

Main concerns will be rainfall and threat for any flooding through
the extended period. Ongoing convection should be diminishing
through the day...but moderate rainfall is still expected as the
boundary lifts to the northeast and mesoscale convective system follows and lifts into
southern Minnesota. Models are now split on location of heavier rainfall
through tomorrow...but consensus is for a slightly slower evolution
and farther south and west maximum overall...which puts the rainfall for
Thursday over northern Iowa a bit longer into the morning and early
afternoon hours prior to chances diminishing during the afternoon.
Euro/GFS have come into line farther south with todays 12z package.
Isentropic lift and moisture transport focusing best lift over
west central/northwest Iowa to start the period at 12z Thursday
and gradually shifting the best lift toward the Iowa/Minnesota border by
18z. Somewhat complicated scenario both tonight and Thursday as
upper level ridge axis holding strong...and effectively will
create a stronger boundary aloft as strong push of thetae advection
advances north...then hitting a slowly retreating ridge axis. This
is part of the reason for the more southern focus as mass convergence
increases overnight/Thursday morning across the north. Once the
main forcing lifts northeast with the first strong push of warm
air...two more weak waves will lift northeast into the afternoon
and overnight hours with additional light to moderate rainfall
expected through Friday night. The highest rainfall totals will
occur through Thursday...but any additional rainfall will add to
the potential for a risk of some flooding...either river or
isolated flash flooding. Will be monitoring closely over the next
couple of days after tonights event unfolds. With clouds and
rainfall expected...highs will be held back to near 70 north to
the lower 80s across the south. Rain chances remain high through
Friday night with a much needed break arriving for most of
Saturday and a part of Sunday. Models differ on details again by
Sunday...with Euro being slightly slower in arrival of rather
robust late Summer system. Another period of moderate to possibly
heavy rain could occur Sunday evening with the passage of the next
system. Additionally...along with a short wave arriving in the
early evening...upper level wind fields and instability increase
enough to be concerned with more organized severe weather late
Sunday afternoon and evening. The more progressive nature of the
system should result in a quicker exit...with a return to drier
weather for Tuesday and Wednesday. For now will trend probability of precipitation down on
Tuesday...with a chance we could be totally dry by that time.
Highs through most of the period will be in the 70s to lower 80s.


issued at 1207 am CDT Thursday Aug 28 2014

Little change from previous thinking /00z/ with primary concern category
changes driven by expanding convection overnight. Although MVFR
ceilings have expanded somewhat they have yet to reach taf sites which
remain VFR at 06z. Expect ceilings and visibilities to lower to MVFR and
possibly IFR with precipitation onset by early this morning as it crosses
the MO river into Iowa...and could last for a prolonged period.
Conditions may return to VFR by Thursday afternoon...but potential for
at least light precipitation will linger through the entire period. Northern
sites /kmcw/ may also experience a return to lower ceilings/visibilities
during the evening.


issued at 345 PM CDT Wednesday Aug 27 2014

Rainfall rates of 1 to 2 inches per hour with locally higher
rates are possible over west-central to southwest Iowa late
tonight into Thursday. 1-hour ffg is around an inch and 3-hour ffg is
less than 2 inches in most ground is primed for flash
flooding overnight. With the widespread quantitative precipitation forecast amounts of 1 to 2
inches from 09z Thursday to 00z Friday...the Raccoon and smaller
river basins may see rises by Thursday into Friday and at this time mainly
reaching action stage or minor flood stage.

Hydrology concerns remain across the area through Monday. With
additional rain expected Thursday night through early Saturday
morning...and another round of possible moderate to heavy rainfall
over portions of the area Sunday afternoon/evening...additional
smaller stream and river responses are likely. Its too early to
gauge the overall magnitude of the impacts...with river levels
currently running well below flood stage at most locations. After
tomorrow...there will be better clarification on the impacts
expected over the weekend.


Dmx watches/warnings/advisories...
Flash Flood Watch from 3 am CDT early this morning through this
morning for Adair-Adams-Audubon-Calhoun-Carroll-Cass-Clarke-



short term...podrazik
long term...rev

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