Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211137

Area Forecast Discussion
National Weather Service Sioux Falls SD
637 AM CDT THU JUL 21 2016

.SHORT TERM...(Today and Tonight)
Issued at 349 AM CDT Thu Jul 21 2016

Another hot and humid day is in store for the region, but there are
a few question marks again today regard the heat.  We`re continuing
to watch a continually developing MCS across portions of northern
Minnesota this morning. This system is generally expected to
continue to track southeast, but there has been some development on
the western flank of the MCS. Several CAMs push this system through
eastern MN and western WI, but this system may drag it`s foot (in
the form of an advancing outflow boundary) across the far eastern
CWA through mid morning. Obviously, if this happens, it could stunt
the temperatures a few degrees in the far east and perhaps lower dew
points slightly, but again we should have plenty of recovery time.
Will track the progress of this system through the daybreak hours.

Of more certainty will be the potential for a few showers through
daybreak over the Buffalo ridge, as the LLJ continues to focus on
ACCAS field in the area. This activity should slowly slide eastward
as it develops.

Further west, it`s looking like a very hot day, with temperatures
soaring towards 100 degrees in areas generally west of the James
River. Heat index values again will push 100-115 across much of the
CWA as rather oppressive dew points in the upper 70s persist over
parts of the Tri-State area.

By this afternoon, we`ll watch a very poorly defined sfc boundary
settle southward towards the CWA, while a weak shortwave drifts
eastward towards central Nebraska. Aloft, temperatures will still be
very toasty, with 700 mb temperature AOA +15C. Aforementioned upper
low may be strong enough to produce a minor sfc low over south
central SD late this afternoon.  Increased convergence due to this
feature, along with weak convergence along the NE stretching
boundary may be just enough to spark a few late day storms.
Obviously should a storm overcome the cap, reasonable risk for
isolated severe given large scale instability.  Shear remains a bit
questionable, with majority of the shear focused in the top half of
the atmospheric column. Still, downburst winds and occasional hail
could be a risk.

Tonight we may see a few elevated showers or storms develop as the
frontal boundary settles southward into the MO River valley. The
main risk overnight should shift south of I-90.  Given steep lapse
rates and very warm air aloft, possible to see an isolated heat
burst, should a few storms percolate late tonight.

.LONG TERM...(Friday through Wednesday)
Issued at 349 AM CDT Thu Jul 21 2016

Friday appears to be warm and humid, albeit not as warm as yesterday
and today.  The elevated thunderstorm potential across northwest
Iowa appears to be fairly limited after 12z, and went with a dry
forecast during the daylight hours on Friday.  Increasing easterly
flow will hamper mixing slightly throughout the day.  With 850 hpa
temperatures in the low to mid 20s instead of the mid to upper
20s...should see temperatures mainly in the 90s.  Still expect dew
points in the 60s to lower 70s...resulting in heat indices near
100.  Considered lowering headline to an advisory across southwest
Minnesota, but through collaboration, decided conditions will be
close enough to leave as is right now.

Friday night strong 850 hpa theta-e advection develops as low level
jet advects warm air and moisture into the region.  As short wave
moves across the area, expect thunderstorms to develop.  These
storms look to be a bit more delayed as the low level jet increases
through the evening hours.  This may result in showers and storms
lingering into Saturday.

Right now, Saturday is a difficult animal to quantify.  The name has
the main short wave moving through during the morning hours, while
the GFS suggests additional waves moving through during the day. The
ECMWF is not very strong with any particular vort max.  With the
lack of agreement amongst the models, could not refine pops beyond 6
hour blocks.  Model soundings hint that capping inversion may be
strong enough during the day, with storm potential both saturday
morning and evening.  Agree with SPC`s outlook for a slight risk,
especially for the late afternoon and evening hours across the
northern half of the forecast area.

Sunday looks to be cooler and drier as the upper pattern shifts
towards a more progressive zonal-northwestern flow.  The flow looks
to be fairly dirty with several disturbances moving through in the
Tuesday-Thursday time frame.  Tried to glean something from MEN
guidance as to favored periods for precip, but guidance is really
flat so stuck close to guidance for now.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 634 AM CDT Thu Jul 21 2016

VFR conditions expected to continue through the daytime hours. A
weak frontal boundary will approach later today, perhaps bringing
an increase in mid-lvl cloud cover. Thunderstorm chances continue
to look on the low end later today, but can`t be ruled out.

Winds will turn easterly as a front moves through the region later
this evening...VFR conditions again anticipated into early Friday.


.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Warning until 7 PM CDT Friday for SDZ038>040-050-

MN...Excessive Heat Warning until 7 PM CDT Friday for MNZ071-072-080-

IA...Excessive Heat Warning until 7 PM CDT Friday for IAZ001>003-

NE...Excessive Heat Warning until 7 PM CDT Friday for NEZ013-014.



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