Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 170834

Area Forecast Discussion
National Weather Service Sioux Falls SD
334 AM CDT Thu May 17 2018

.SHORT TERM...(Today and Tonight)
Issued at 334 AM CDT Thu May 17 2018

Weak moisture transport in the neighborhood of 700-800 mb leading to
some scattered showers and thunderstorms across Central South Dakota
this morning.  Expect area to expand in the pre-dawn hours, and then
try to dwindle through the morning hours.  Received a report earlier
this morning of nickel size hail and gusty winds near Chamberlain.
With weak shear, expect these storms to be pulsy, but one could
briefly spike up to near severe limits. At this point, CAMs are
keeping spotty showers going into the afternoon hours, but am not
sure it will last that long with weak forcing and flow.

Cloud cover will obviously have an impact on highs today, but in
general, should be a warm day with 850 HPA temps in the teens
yielding widespread 80s for highs.

Tonight, elongated short wave train gradually approaches from the
west ejecting from a disorganized upper level trough across the
intermountain west. Models are a bit slower with the forcing, and
have held pops west of the James till after 06Z Friday.  With
blocking pattern to the east, this system is a relatively slow mover
with forcing largely lifting to the north.

Low level jet tonight transports a large amount of moisture
northward, setting the stage for what appears to be a wet weekend.

.LONG TERM...(Friday through Wednesday)
Issued at 334 AM CDT Thu May 17 2018

On Friday, shortwave train gradually lifts to the north and the
atmosphere reloads as another piece of the upper trough approaches
from the west.  With weak mid and upper level flow, may be hard to
get storm scale organization, but could still see pulsy strong
storms at times with CAPE values are mainly around 500-750 J/kg.
Environmental lapse rates are essentially moist adiabatic.  PWAT
values continue to run extremely high around 1.3-1.5", suggesting a
saturated environment. With the slow nature of storms and the
potential for training, expect possibility of locally heavy rainfall
on Friday night into Saturday as a slow moving cold front moves
across the area.

Upper wave pinwheels across central and eastern Nebraska on Saturday
leading to additional rounds of showers and thunderstorms. The upper
wave starts to move east into Iowa on Sunday, but is markedly slower
than the models suggested 24 hours ago.  Have therefore raised pops
for Sunday into the chance range.

Conditions look to dry out early next week but a more active pattern
appears to be shaping up as upper trough once again develops across
the west and shortwaves eject from the trough into the plains riding
the southwest flow aloft. Shear and instability appear to be
somewhat limited, so at this point, am not anticipating any severe
weather outbreaks.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1027 PM CDT Wed May 16 2018

VFR conditions should continue through most of the period. Some
high res guidance tries to renew some shower activity later
tonight west of the James River but appears too remote of a
possibility to include mention. Southerly winds will be breezier
on Thursday, gusting in the 15-25 kt range at times during the
afternoon and evening. Shower and thunderstorm activity will begin
to approach KHON from the west very late and just beyond the
current TAF period Thursday night.




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