Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271742

Area Forecast Discussion
National Weather Service Des Moines IA
1242 PM CDT Sun May 27 2018

.SHORT TERM.../Today through Tonight/
Issued at 333 AM CDT Sun May 27 2018

Hot weather will continue today as the upper level ridge persists
over the region. 850mb temperatures will be a degree or two higher
than Saturday and the NAEFS 850mb temperatures across central Iowa
will be at the 99th or higher percentile with the GEFS Extreme
Forecast Index for temperature at the 99.5th percentile. What this
all means is record hot weather for this time of year across central
Iowa. Looking at the high temperature 10th percentile from all
available guidance for today would favor records being broken at Des
Moines and Waterloo by one to a few degrees and the current forecast
has both cities breaking their record highs today by 3 or more
degrees. While high temperatures will be in record territory,
dewpoints will once again mix down from the low levels aloft into
the upper 50s to low 60s. This will keep maximum heat index values
in the middle 90s to near 100 degrees. This includes the Des Moines
metro and this is where a heat advisory will be issued for today.
While not meeting our criteria of a heat index temperature of 105
degrees or higher, aggravating factors today are the Heat Health
System still pinging in this dry tropical airmass and the likely
higher than typical number of people outdoors on this Memorial Day
Sunday. The western forecast area was also considered as well for a
heat advisory, but there is concern for cumulus and isolated
thunderstorm development toward mid-afternoon that adds a bit more
uncertainty in this area.

Convective allowing models (CAMs) such as the HRRR, ESRL HRRR, and
high resolution windows of the WRF NMM and ARW cores show this
possibility. RAP and HRRR soundings indicate the air temperature
nearing or exceeding the convective temperature in these areas
perhaps aided by weak surface convergence. A factor against
thunderstorm development will be the subsidence from the mid-level
high pressure moving overhead. For now, have slight chance PoPs
between roughly Highway 71 and Hwy 169. This spatial area is likely
too large, but difference in the CAMs prevents a more targeted area.
Not surprising with high pressure overhead, shear is almost
nonexistent. Therefore, convection will be unorganized, but could
pose a threat of gusty winds with downdraft CAPE above 1500 J/kg.

Any convection that does form will quickly dissipate after sunset
with skies remaining mainly clear overnight. Lows will be quite warm
for late May and within a few degrees of the record high low
temperatures by Monday morning.

.LONG TERM.../Monday through Saturday/
Issued at 333 AM CDT Sun May 27 2018

Upper ridging to remain in place aloft with high pressure situated
to the east of the state Monday. The upper ridge will continue to
nudge eastward into Tuesday as an upper trough moves into the
central US with southwesterly flow aloft by Tuesday. Still expecting
Monday to be hot and mainly dry across the forecast area. NAM hints
at some possibility of late afternoon convection developing in the
far north/northwest in the vicinity of a baroclinic zone aloft. For
now have left the forecast dry, and will have to monitor for the
possibility of an isolated storm. Otherwise surface flow to shift
around to the south in advance of an approaching area of low
pressure off to the west that is slated to impact the state Tuesday
into Wednesday. H85 temps aloft in the low 20s Celsius, therefore
max temps likely to push up into the mid 90s again which would be
another day for possible record high temperatures across the entire
forecast area. Lows to remain mild Monday night, with slightly
cooler temps expected into Tuesday as cloud cover increases with
some precipitation chances as well. With the approaching area of low
pressure and southwest flow aloft, moisture advection expected back
into the state for mid-week with thunderstorm chances spreading
across the area. Better chances will come Wednesday as the upper
trough moves lifts from NE into the Dakotas/MN.

Upper ridging then to build back into the central US to end the
week. A shortwave trough will move through the ridge by late
Thursday and could bring another chance for showers and
thunderstorms Thursday night. Otherwise mainly dry conditions
expected Thursday and again Friday into Friday evening. Upper
ridging then to quickly break down with southwesterly flow across
the Plains states into Saturday and Sunday with continued chances
for storms. EC showing the thermal ridge building into Iowa for
Saturday though especially across the southern portion of the state,
with warmer low level temps in place for much the latter portion of
the week. Therefore did nudge guidance temps up a bit into the end
of the week and into Saturday. A cold front then to move through the
state associated with the upper trough lifting from the Plains
states into south central Canada. This should allow for temps to
return to more seasonal values for Sunday into early next week.
Overall a length warm period expected through the entire forecast
period as temperatures are expected to remain above to well above
average through the week into next Saturday.


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1242 PM CDT Sun May 27 2018

VFR conditions expected throughout the TAF period. The only
caveat would be possible isolated thunderstorm development this
afternoon, however with low confidence in storm development have
left this out of TAFs. Otherwise, expecting high clouds and winds
remaining relatively light.


Issued at 333 AM CDT Sun May 27 2018

            |        Sunday        |        Monday        |  Period
            |  Forecast   Record/  |  Forecast   Record/  |    of
City        |    High      Year    |    High      Year    |  Record
Des Moines        98      91/2012        94      92/1895      1878-
Waterloo          97      93/1900        96      93/2006      1895-


Heat Advisory until 8 PM CDT this evening for IAZ059-060.



SHORT TERM...Ansorge
LONG TERM...Beerends
CLIMATE...Ansorge/Beerends is the U.S. government's official web portal to all federal, state and local government web resources and services.