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fxus63 kdmx 201140 

Area forecast discussion
National Weather Service Des Moines Iowa
640 am CDT Thu Jul 20 2017

..updated aviation discussion...

Short term.../today through tonight/
issued at 413 am CDT Thu Jul 20 2017

Continued extreme heat and humidity was the main concern during
the short term. Mixing kept overnight temperatures across central
and southern Iowa slightly warmer than the forecast low as most
stations remained around 80 to 84 as of 3 a.M. This provides a
much warmer start than previously anticipated and thus nudged up
maximum temperatures slightly across the headlined area. This
resulted in going 100f at Des Moines with heat index values over
110f by this afternoon. Leaned closer the nam12 for maximum
temperatures today as the GFS/European model (ecmwf) are too cool at the
surface...especially since these two models suggest 850mb
temperatures range from +24c to +27c along and south of Highway

The next concern is with convection late tonight into early Friday
morning along the surface boundary to remain extended across
northern Iowa into east-southeast Iowa. Boundary does look to sag
southward somewhat today but should push back northward with the
shortwave riding the upper level ridge tonight. 20.00z GFS has the
front lighting up with storms by around 03-06z Friday over the
northern half of the forecast area but the NAM and European model (ecmwf) are
slightly further north with the boundary. Fairly strong and deep
moisture convergence along the boundary with a good swath of Theta-east
advection along and north of the boundary. Precipitable water
values range over 2 inches and warm layer cloud depths range from
4000-4500m across northern Iowa. Corfidi vectors coincide with the
east-southeast storm motion, so the potential for training is highly
likely. Confidence is high with regard to the heavy rain potential,
but still quite a bit of uncertainty where the boundary will stall
and allow for storms to evolve overnight. NAM would suggest more
into southern Minnesota while the GFS remains the further south
model run. Arw-east is more in agreement with the GFS but the nmm-
east is similar to the NAM. Regardless, enough confidence to bump up
pops overnight across the far north.

Long term.../Friday through Wednesday/
issued at 413 am CDT Thu Jul 20 2017

Thunderstorms may be ongoing over northern Iowa to begin the
period Friday morning. The surface boundary will have settled into
central Iowa by this time, however, low pressure will be moving into
northeast Nebraska and will begin to lift the boundary back to the
north. Storms may be able to maintain through morning across
northern Iowa as the low level jet remains focused into that
portion of the state and a band of strong Theta-E advection
continues. Central and southern Iowa will remain hot and humid.
Expect stronger mixing Friday than today and which should mix dew
points down to the upper 60s to lower 70s. This should allow a few
locations including Des Moines to approach 100 degrees with heat
indicies near 110. Due to the expected conditions today and Friday
along with lingering heat Saturday, have upgraded the Interstate
80 corridor including the Des Moines Metro to an excessive heat

Another strong complex of storms should develop over southern
Minnesota by Friday afternoon in vicinity of the boundary. These
storms may impact northern Iowa and will bring the threat for
severe weather and locally heavy rainfall late afternoon and into
the evening. Thunderstorms may reach central Iowa Friday night
but will have to overcome the warm dome in place. The warm air
will be slow to erode and will eventually lead to storm
deterioration as they move into a region of strong stability.
Saturday is expected to be mostly dry and again hot despite weak
northerly flow. The weak flow may lead to pooling dewpoints in the
mid 70s to remain much of the day while highs reach the mid to
upper central and south.

Sunday will be cooler with high temperatures dropping into the
80s. The high temperatures will remain in the 80s through much of
next week though a warming trend will begin toward the end of the
period as high pressure begins to shift to the east. Thunderstorm
chances will increase with the arrival of the return flow on


Aviation.../for the 12z tafs through 12z Friday morning/
issued at 633 am CDT Thu Jul 20 2017

MVFR ceilings over mcw look to dissipate shortly past 12z and thus
left out mention in taf attm. Otherwise, stagnant airmass remains
in place today and tonight providing VFR conditions throughout the
period. Storms are possible across northern Iowa past 06z Friday
and have thunderstorms in the vicinity mentioned at fod/mcw.


Dmx watches/warnings/advisories...
excessive heat warning until 8 PM CDT Saturday for iaz057>062-

Heat advisory until 8 PM CDT Saturday for iaz033>035-044>050.


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