Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 200451

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Fri Apr 19 2019

...Updated 06z Aviation...

.DISCUSSION.../Tonight through Friday/
Issued at 320 PM CDT Fri Apr 19 2019

The forecast period can be effectively split into three periods
with three different sensible weather/threat regimes. This weekend
will be mostly sunny and mild with an enhanced fire danger. Sunday
night into Tuesday will be cloudy with intermittent showers and
thunderstorms. The middle to latter part of next week will be
generally dry, quiet, and moderate in temperature.

...This weekend...from this afternoon through Sunday evening...
This afternoon we have seen generally north winds of 10 to 20
mph, with occasional gusts over 25 mph, and RH falling into the
30s at times. This has lead to an enhanced danger of spreading
fires, a large number of which have been observed across the area.
A cool surface high pressure ridge will slide across Iowa
tonight, causing winds to diminish quickly after sunset and be
nearly calm at times overnight, but then on Saturday as the ridge
moves away southerly winds will increase once again, with speeds
and gusts on Saturday afternoon only slightly less than those
observed today. Meanwhile a deep thermal ridge will move over the
region this weekend, especially from Saturday afternoon through
Sunday morning, leading to notably warmer weather and causing RH
to fall further. On Saturday afternoon RH is forecast to be in the
25-30 percent range, so there may be infrequent observations
right at Red Flag criteria - 25 percent with gusts over 25 mph -
but these should be sporadic and short-lived. However,
widespread/frequent near-Red Flag conditions are likely and
planned burns should take this into account. We will be
highlighting this concern in our messaging and fire weather
products from now through the weekend. Aside from the fire weather
concerns it will be an absolutely gorgeous spring weekend, long-
awaited by many, with mostly sunny skies, southerly breezes, and
temperatures peaking in the 70s on Saturday and mid 70s to lower
80s on Sunday.

...From Sunday night through Tuesday...
A series of 500 mb shortwave troughs ejecting out of the rockies
will push the thermal ridge off to our east and bring more active
southwesterly flow to the region, also pushing a surface cool
front across Iowa. There are differences among the prognostic
models in the timing and orientation of the shortwaves within the
series, causing some uncertainty in timing/extent of resulting
precipitation, but it is clear that intermittent showers and
thunderstorms will affect much of Iowa. The most likely areas
appear to be across the northwest half of the state, with the most
likely time frame late Sunday night. Instability and shear are
limited and severe weather and heavy rainfall are not anticipated,
so any impact will be minimal other than providing some water for
awakening vegetation and making for gray, damp weather at times
early in the week.

...In the latter half of next work week...
Iowa will reside beneath the southern periphery of brisk west
northwesterly steering flow spreading across the Upper Midwest.
This will keep us generally dry and quiet, with partly cloudy
skies and slowly moderating temperatures. No precipitation is
forecast during this time, though it is possible we could see a
couple of low rain chances across northern Iowa later in the week.
At any rate no hazardous weather is expected while high
temperatures climb from the 60s on Wednesday into the low to mid
70s by Thursday and Friday, at least across central and southern


.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1150 PM CDT Fri Apr 19 2019

VFR conditions through the period. Calm winds overnight. Wind
gusts around 10 knots Saturday afternoon.




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