Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 262348

Area Forecast Discussion
National Weather Service Des Moines IA
648 PM CDT THU MAY 26 2016

.SHORT TERM.../Tonight/
Issued at 348 PM CDT Thu May 26 2016

Main concern is convection and potential MCS overnight. Models
continue to hint that warm front will drift far enough north into
Iowa to keep south primed for severe most of evening and potentially
overnight as the front lifts north and warm air advection increases
along/above the boundary. Watch out now covers immediate threat.
Potentially seeing a watch east of that area later...will need to
monitor. Other concern will be locally heavy rainfall. Though
gridded FFG about 2 inches/hr...may reach that mark in some cases if
MCS congeals over west/southwest this evening and then becomes
rooted into the warm front. This would amplify rainfall rates and
trends overnight and require updates to qpf/grids/potentially
headlines. At this time have rainfall amounts of .25 to 1.0 through
12z. Min temperatures look to remain warm but difficult to pin down
in some cases due to mins reaching the Tw with rainfall.  Other less
affected areas will remain warmer. Heavy rain threat mainly after 05-
06z time frame and thereafter...prior to that main concern will be
severe chances with some potential for locally heavy rainfall and
minor runoff issues as well.

.LONG TERM.../Friday through Thursday/
Issued at 348 PM CDT Thu May 26 2016

The main concern through the period will obviously be convective
trends, which look to gradually decrease in intensity into the
weekend. At onset the current Four Corners long wave trough will
be on the move crossing the Plains later in the day tomorrow.
Remnants of our current Missouri Valley MCS will likely still be
ongoing to some degree across Iowa and expect a typical diurnal
fade with loss of nocturnal warm advection. Later in the day the
surface trough associated with the aforementioned long wave trough
should be oriented SSW-NNE into NW Iowa providing the necessary
convergence trigger for peak heating convection. Model instability
projections vary from 1500-3500 j/kg with the Nam on the higher
end of that range. 0-6km shear does not appear to be as robust as
what is currently occurring throughout the region, but at 35-45kts
still certainly sufficient to produce organized convection and
possibly supercells given the degree of instability. This
convection should then persist to some degree into the night
driven by weakening but persistent warm advection and moisture
transport. Expect yet another cycle of this daytime peak
heating/nocturnal warm advection cycle into Saturday and Saturday
Night but both instability and shear will continue to weaken by
that time so any severe potential should be low. By Sunday the
only lingering forcing and trigger mechanism appears to be the
upper low itself exiting and followed by weak northwestern flow.

This will not dry us out too much however. Heights will rise for
a bit with weakening flow and diminishing shear and the airmass
will become somewhat less unstable with low level moisture
slightly diminishing, but the northern stream will remain active
to some degree with Iowa on the southern edge of a fairly strong
wave into the middle of next week. Details are hard to come by at
this point, but most solutions can justify a daily mention of
chances through the remainder of the extended forecast.


.AVIATION.../For the 00Z TAFS through 00Z Friday evening/
Issued at 648 PM CDT Thu May 26 2016

The primary concern will be convective trends with widespread
showers and thunderstorms over the southern half of Iowa and
spotty associated ceilings down to MVFR or IFR. There is plenty of
additional convection upstream into Nebraska and Missouri so have
a fairly high confidence that thunder will persist into the
evening, but not so much with ceiling trends at any particular
location. Higher confidence that ceilings will trend downward
into the night however with rain and nocturnal cooling so have
hit harder during that period. Precipitation chances persist into
Friday but with no timing or location confidence have just
mentioned extended VCSH wording. Similar thinking with trend back
up to VFR later in the day.


Issued at 348 PM CDT Thu May 26 2016

Three day total QPF is in the 1.25-2.25 range with highest amounts
over the southeast half of the forecast area where increasing
precipitable water, warm cloud depths and persistent moisture
transport are highlighted. These would just be average amounts
however with locally higher amounts possible, especially over the
next 12-36 hours when convection is expected to be more robust.
RFC contingency forecasts suggest within bank rises at most
locations, although a few spots may just eclipse flood stage. The
most likely candidates for this would be locations that saw
flooding during the previous episode, portions of the upper Des
Moines and North Raccoon River basins.


.DMX Watches/Warnings/Advisories...


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