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fxus63 kdmx 151746 

Area forecast discussion
National Weather Service Des Moines Iowa
1246 PM CDT Mon Jul 15 2019

..updated aviation discussion...

Discussion.../today through Sunday/
issued at 354 am CDT Mon Jul 15 2019

The overall forecast thinking remains the same as last night,
generally hot with only occasional rain/thunderstorm chances. A
few light sprinkles have affected northern Iowa overnight but
these should fade out after sunrise this morning. From today
through Tuesday the remnants of tropical cyclone Barry will slide
by to our southeast, over Missouri and Illinois, pinching off the
large southwestern U.S. Pressure ridge and allowing the northern
stream zonal flow aloft to creep southward toward Iowa. With
thunderstorms developing daily/nightly in the broad, weak surface
baroclinic zone beneath the southern periphery of these westerlies
aloft, expect some chances for these storms to reach northern Iowa
at times, from later today through Tuesday at least. There may be
some marginal severe weather threat given decent magnitude of
instability, but flow/shear will be weak and nothing organized is
anticipated in our forecast area during this time. Meanwhile
temperatures will be moderated by both the aforementioned retreat
of the thermal ridge and associated warmer temperatures aloft, as
well as by intermittent cloud cover from convective debris and
relatively light surface flow limiting mixing. This results in
high temperatures only in the upper 80s to lower 90s today and

Between Tuesday and Wednesday night, with the 500 mb zonal flow
intruding southward toward Iowa in wake of barrys remnants as
discussed above, it appears two subtle shortwave impulses will
move through this flow and generate thunderstorms near the
Iowa/Minnesota border region. The timing of these impulses and
resulting storms remains unclear, however, the storms may once
again carry an attendant severe weather risk. It is also
difficult to determine how far south they may reach into Iowa
given the light to modest surface flow and possibly substantial
instability pool/gradient from north to south which the storms and
their outflow may feed into to fuel a more southward propagation.
This possibility around Wednesday/Wednesday night provides the
only realistic chance for rain in central/southern Iowa this week.
It also makes forecasting high temperatures for Wednesday
problematic due to the uncertainty in where the convective
boundary/cold pool will end up. However, the most likely scenario
remains that the storms will remain confined to our northern areas
while the central/south remain dry and hot south of the boundary,
and have maintained this in the forecast for now.

From Thursday through Saturday the southwestern/southern U.S.
Thermal ridge will build more earnestly, at least temporarily
winning the battle and pushing the northern stream zonal steering
flow further north, back across Minnesota and the Dakotas. This
will result in dry and even hotter weather across Iowa, and the
highest confidence portion of the forecast. Daily high
temperatures will climb well into the 90s on each of these three
days, and it is possible we could even see a few 100 degree
readings. This will be primarily dependent on the degree of low-
level mixing and moisture advection, but in any event a prolonged
period of potentially dangerous heat is still on tap toward the
end of this week, and headlines may become warranted at some point
in the next few days.

From the latter part of next weekend into the early part of the
following week there is more uncertainty as long-range models have
been flopping around since last night. Whereas 24 hours ago the
GFS and ec both had a large heat dome centered right over
Missouri/Iowa during this time, now they are all over the place
with the GFS showing this ridge way down over The Four Corners
region with cool and dry northwesterly flow over Iowa, the ec
maintaining the large, stronger ridge but shifting it far enough
westward that Iowa lies beneath the proverbial ring of fire and is
socked by round after round of thunderstorms, and the Gem now kind
of in the middle, keeping the ring of fire just to our north but
the ridge close enough nearby to leave US much warmer than the
cool GFS. In other words, we will need to wait a bit longer to get
a better handle on this portion of the extended forecast.


Aviation.../for the 18z tafs through 18z Tuesday afternoon/
issued at 1234 PM CDT Mon Jul 15 2019

Expecting VFR conditions through tomorrow with winds the main
forecast concern this afternoon and then again late in the period.
There is the potential for fog tonight across southern Iowa and
could see some MVFR visibility at dsm and otm, but had low
confidence to keep mention as possible cloud cover from Barry
remnants could hinder the fog development.


Dmx watches/warnings/advisories...none.


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