Area forecast discussion
National Weather Service Sioux Falls South Dakota
1112 PM CDT Friday may 17 2013
Short term...(this evening through Saturday night)
issued at 241 PM CDT Friday may 17 2013
Overall while thunderstorms will likely develop tonight and tomorrow
during the day most of the County Warning Area will likely be fairly quiet. A warm
front over the far northeastern part of the County Warning Area...around Lincoln
and Lyon counties in Minnesota could spark a quick shower or
thunderstorm this afternoon. Instability and shear are pretty
marginal so expecting non severe thunderstorms...although some small
hail would be possible. The other area of concern will be north
central Nebraska into central South Dakota. The latest NAM and rap
suggest that this development would track mainly northeast while the
hrrr pushes convection through the County Warning Area from about 2z through 8z. The
hrrr seems a bit overdone pushing things that quickly east as the
better updrafts would likely be confined to area farther west where
better convergence exists. If an outflow boundary could race
eastward some spotty showers could develop. But for now not going to
bank on that. Will keep the highest probabilities to the west and
north with only small probability of precipitation towards Interstate 29. Lows mainly in the
60s with clouds and a decent southerly wind.
Saturday looks like a reloading day and for now will only have probability of precipitation
from about the James River Valley west. Capping looks to be too
strong for development in most locations but if something could get
going cape values around 2000 to 3000 j/kg and 0 to 6 km shear
values around 30 knots suggest that something organized would be
likely. For now will look for just a couple of thunderstorms to
develop mainly west of the James River Valley then a more organized
area of thunderstorms...some of which could be severe as well...to
move northeast from Nebraska with the best chance from about 3z
through 9z. Believe that the main threat at this time will be strong
straight line winds although some severe hail would also be
possible. Ahead of the convection a humid...marginally breezy day is
in store with highs in the 80s while lows Saturday night will only
fall into the lower to middle 60s.
Long term...(sunday through friday)
issued at 241 PM CDT Friday may 17 2013
Sunday morning...severe/strong convection should be exiting to the
north and east along with main forcing from upper short wave and
low level jet. However...airmass remains convectively unstable and
surface trough lingers across the western forecast area from Huron and south
through eastern Charles Mix County. Likely probability of precipitation look warranted in
the morning out west and north of fsd as this surface boundary
continues to provide a source of lift in the low levels and as the
upper vorticity maximum lifts north. However...farther east/southeast
toward fsd/sux and into northwest Iowa...forecast soundings show
subsidence with weak ridging aloft creating a thermal cap through
Sunday mid-day...and have kept chance probability of precipitation there accordingly. This
downward motion looks short-lived however as main upper low and
surface low lift north toward the forecast area for Sunday
afternoon through Sunday night. GFS/NAM both show MUCAPE values in
the 2000-2500 j/kg range along and ahead of the main surface
trough Sunday afternoon...which will stretch north/S across southeast
South Dakota/NE Nebraska then and provide the low level focus for
convection. With deep layer shear increasing on the east side of
the upper low /however still looking moderate at 35-40kt at this time in
the fsd area/...should see another round of organized convection
with some severe storms Sunday afternoon/Sunday evening along and
east of surface trough. Main threat with these storms looks to be
large hail...transitioning to more of a damaging wind threat
farther east into Iowa/Minnesota where the stronger deep layer shear will
exist.
Upper low becomes a closed core core low and looks to be located
over the tri-state area by 12z Monday. This upper low will slowly
lift northeast over southern Minnesota by 00z Tuesday...and leave unsettled
weather over the Sioux fall forecast area. Model soundings showing
thin cap profiles...and with strong daytime heating...will likely
see scattered to numerous convective coverage through the day. At this time
it looks like the threat for tornadoes associated with cold core
lows will remain farther east into southern/central Minnesota...but will
monitor mesoscale trends as the time approaches.
The remainder of the week looks cool with 850 mb temperatures hovering in the
single digits above zero celsius. The upper low shears out a bit but
remains over the upper Midwest/Great Lakes region through Wednesday
with an upper trough axis over the northern plains. This upper
trough will bring continued unsettled weather with daily instability
showers through Wednesday. Upper ridging and gradual warming ensue
for Thursday and Friday as a full latitude trough deepens across the
Pacific northwest.
&&
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1112 PM CDT Friday may 17 2013
VFR expected through the majority of the taf period. Thunderstorm
chances return in the final few hours of the taf period.
Line of thunderstorms and rain was over Hon with -ra lingering for a while after with
no significant restriction to visibility in -ra. After the rain
clears nsw through most of the rest of the Hon taf period.
South/southeast wind will remain at or above 10kts at fsd/sux through the
night so have removed previous mention of fog. Wind wont shift much
in direction through the day on Saturday but speed will increase with
afternoon gusts expected.
In the latter part of the taf period in the 02-06z time frame...severe
thunderstorms will be possible mostly in vicinity of fsd/sux with damaging
hail and wind the main threats. Will add some thunderstorms and rain at that time but
confidence is only medium as far as timing and location.
&&
Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&
$$
Short term...08
long term...
aviation...Sally
