Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221743

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Thu Jun 22 2017

...18z TAF Update...

Issued at 951 AM CDT Thu Jun 22 2017

Updated forecast to reflect ongoing convection and anticipated
trends. 2 areas remain favorable for showers and thunderstorms
throughout the morning. Elevated showers and thunderstorms will
continue along and ahead of 30-35 low level jet axis extending
from near Des Moines to Waterloo. Convection will become focused
east of Des Moines by late morning as low level jet veers.

Showers and thunderstorms are more numerous over north central
Iowa ahead of a cold front that will surge southeast late this
afternoon and tonight. Ongoing thunderstorms will move east into
northeast Iowa this morning. Additional thunderstorm development
along this front is likely later this afternoon and tonight.
Strong to severe thunderstorms will be possible with 0-3km bulk
shear 30-40 knots and steepening lapse rates.


.SHORT TERM.../Today through Tonight/
Issued at 351 AM CDT Thu Jun 22 2017

Scattered thunderstorms have blossomed over north central and
northeast Iowa in an area of 850mb warm air advection and on the
cusp of the low level jet early this morning. Recent GOES-16 IR
imagery shows this activity maintaining or slightly strengthening.
Another area of thunderstorms, albeit smaller, developed around
7z over northeast Nebraska and pushed into northwest Iowa, though
has since weakened. Convective allowing models show the
thunderstorms over north central and northeast Iowa continuing to
push off to the east this morning. However, additional development
back to the west of these storms is possible so have PoPs
lingering over north central Iowa this morning.

Otherwise, main feature today will be a cold front that as of 8z was
located near a line from Sauk Center, MN to Mitchell, SD to
Valentine, NE. As a shortwave trough digs out of the Dakotas towards
Iowa, this will help the front advance today. Temperature will peak
around 90 degrees over the southern half of the state, but lingering
clouds and storms will limit the warmup over the far north. Winds
from the south will remain breezy as they keep the supply of middle
and upper 60s dewpoints over areas south of the front. By mid to
late afternoon, this front will be the focus for thunderstorm
development likely over southern Minnesota, northwest Iowa, and
northeast Nebraska, as inhibition will be overcome by convergence
along the front. SBCAPEs ahead of the front will range from 2000 to
3000 J/kg with deep layer shear maximized around the front. Primary
concerns will be damaging wind gusts and hail. With mid-level dry
air shown in forecast soundings and DCAPE values above 1000 J/kg,
this will favor entrainment of dry air into the downdraft. This can
bring strong to severe wind gusts to the surface. Latest HRRR/WRF
cores/RAP/NAMNest show these thunderstorms congealing into a line
and moving southeastward through the overnight hours. Most of the
storms will have exited the forecast area by sunrise Friday. Behind
the front Thursday night, winds from the northwest will begin to
advect cooler and drier air into the state.

.LONG TERM.../Friday through Wednesday/
Issued at 351 AM CDT Thu Jun 22 2017

There will be little sensible weather of note through at least
early next week with the primary message cooler, below normal
temperatures. The vort lobe/PV trough axis currently extending
westward from the Manitoba upper low should be on the doorstep of
the IA/MN border at onset /12z/ with much of its attendant
synoptic scale forcing and associated cold front already into MO
and IL. This will lead to an extended period of cooler weather and
little in the way of precipitation. Highs from Friday through the
weekend look to be 5-15 degrees below normal with low humidities.
Dewpoints well below seasonal values in the 40s will be common by
Saturday and Sunday. The forecast at this time yesterday,
especially the ECMWF, suggested a weak wave may ride the MO Valley
baroclinic zone with some elevated showers, but that potential
appears to have pushed farther to the south and west leaving our
only precip chances from NE Iowa into the Upper MS Valley Saturday
as a weak wave drops into the long wave trough.

Otherwise Iowa looks to be in dry, subsident NW flow with little
in the way of forcing or moisture until warm advection returns Tue
Night into Wednesday. Chances for showers and thunderstorms will
return by that time as heights lower through the central CONUS
along with more seasonal temperature and humidity levels.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1234 PM CDT Thu Jun 22 2017

While storms have moved away from most TAF sites, KDSM will
continue to battle scattered storms early in the 18z issuance
period. Then all TAF sites will battle storms this evening with cold
frontal passage. Overall, VFR conditions expected, but may see
conditions fall to MVFR at times, especially with stronger storms
and frontal passage itself. Hints at some fog post frontal
passage, but confidence in winds going light enough not there to
introduce any at KFOD/KMCW/KALO, so have held off.




SHORT TERM...Ansorge
AVIATION...Curtis is the U.S. government's official web portal to all federal, state and local government web resources and services.