Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 292334

Area Forecast Discussion
National Weather Service Sioux Falls SD
634 PM CDT Sat Apr 29 2017

.SHORT TERM...(This evening through Sunday)
Issued at 405 PM CDT Sat Apr 29 2017

The upper level low presently situated over New Mexico/Colorado will
lift into the Central Plains overnight. Out ahead of the feature,
midlevel theta e advection/isentropic lift will spread northward
into our area, and with that showers will begin to develop over the
lower MO River Valley and lower IA zones in the evening, spreading
northward into parts of southeastern SD and southwestern MN by early
Sunday morning. The better focus for precipitation however will be
over northwestern IA and extreme southeastern SD as 850 mb
frontogenesis backs into the area late at night. Temperatures
aloft will generally be supportive of snow tonight, so
precipitation type will come down to temperatures in the lowest
levels which will remain at 1 to 3 degrees C. That being said,
cannot rule out a few snowflakes mixing in with the rain in our
western and northern areas overnight. Overall, low temperatures
will range from the mid to upper 30s.

On Sunday the upper level low lifts northeastward, being situated
over eastern KS/eastern NE by late in the day. The isentropic lift
becomes even more pronounced during the day, as rain, moderate at
times, overspreads almost the entire CWA by afternoon, with daytime
QPF amounts running around an inch through the lower MO River
Valley. The higher pops slowly trend to the north and west through
the day as drier midlevel air begins to work out of southeastern IA
by late afternoon. Models continue to depict very weak elevated
instability in the afternoon, so left in some isolated thunder over
portions of northwestern IA. With the surface low lifting into IA
during the day, northeasterly winds will really pick up in the
tightening gradient, gusting 25 to 35 kts by late morning. With the
showers, temperatures will move little in the east with afternoon
temperatures remaining in the mid and upper 30s. Back to the far
west where showers will be less, readings may make it to near 50
through the Chamberlain to Huron corridor.

.LONG TERM...(Sunday night through Saturday)
Issued at 405 PM CDT Sat Apr 29 2017

Relied heavily on a blend of the 12Z GFS and 12Z ECMWF for the
timing and placement of the strong upper and surface lows ejecting
northeastward Sunday night and Monday. Both models take the 500mb
low from near Topeka KS 00Z Monday, and move it to northeast WI by
00Z Tuesday. The QG forcing follows suit in bringing the most upper
level forcing just to the southeast of our forecast area Sunday
night. In addition, PV using the 1.5 pressure surface is strong, and
gets down to an average of 500mb both on the front side of the upper
low just east of our forecast area Sunday evening, and also just
west of I 29 coincident with the trowal described below. Lastly, the
jet is positioned Sunday evening in showing a very strong left exit
region along and just west of I 29 when moisture is the deepest. At
the surface, low pressure near 993mb is elongated from northeast KS
to south central IA at 00Z Monday, then consolidates in northeast WI
by 00Z Tuesday as a burly 987mb low. The NAM was a slow outlier,
while the GEM Global was also a bit slow, but not as bad as the NAM.

You can tell from the thermal profile that the system is totally
wrapped up, with warm air in the 925-850mb layer arching back
westward in southern MN through the night Sunday night. Mid level
frontogenesis is initially moderately strong in our southeast zones
early Sunday evening, but then dissipates as the occlusion takes
over. The various models do not show as much elevated instability
late Sunday and Sunday evening anymore, even in our southeast zones.
Elevated MU cape is struggling to get much over 0 j/kg even per the
more unstable NAM, therefore left out the mention for isolated TSRA
this go around.

In terms of snow, the coldest air in the 925-850mb layer exists from
east central SD to southeast SD Sunday night, then slowly translates
eastward to east of I 29 through the day on Monday. Instability is
not overly abundant as EPV* in the mid layers shows a fairly stable
layer above the max frontogenesis. However, there is strong
trowaling generally from near Brookings to Tyndall SD Sunday night
before it too dissipates on Monday as everything moves eastward. In
addition, we have the aforementioned strong PV forcing in this same
area. Therefore have a swath of 3 to 6 inches of snow along either
side of this axis, with the most snow of 5 to 6 inches north of I 90
in east central SD, and into Lyon and Pipestone counties MN. This is
the area that shows the coldest air aloft coincident with decent
amounts of QPF. 850mb temperatures are close to -4C, while 925mb
temperatures are very close to freezing. When one takes into account
a possibility of subtle wet bulb cooling, a watch is certainly
warranted at this time north of I 90. In terms of liquid amounts,
still have around 0.4 tenths of liquid near Chamberlain, to just
under two inches around Sioux City where some frontogenetic forcing
earlier in the event aids in tallying up the cold rainfall.
Initially, believe I had too much snow in our far eastern zones
around Windom. Tailed them off to two inches or slightly less
because areas east of I 29 actually lose their saturation in the
dendritic layer Sunday night. In addition, we lose our higher
terrain enhancement in the eastern sections of southwest MN that the
Buffalo ridge is likely to have in aiding their snowfall from
Ivanhoe to Worthington MN. Finally in our far east, another thing
that may hold snow amounts down Sunday night is I am concerned that
the warm air aloft from 925-850mb could back in a bit further
westward into our far eastern zones.

In terms of wind, boosted superblend wind speeds by adding consmos
to them. Looks like a solid 20 to 35 mph wind event Sunday night and

Small pops are warranted in the middle of the week as weak upper air
disturbances move through a general northwest flow aloft in the
northern plains. But should not be any big deal, especially since it
will be liquid. A warming trend will take hold with perhaps some
strong upper ridging by next weekend.


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 634 PM CDT Sat Apr 29 2017

VFR conditions to start the period will gradually deteriorate from
south to north tonight and Sunday, as a storm system lifts through
the central Plains. Rain will accompany lowering ceilings, with a
potential for MVFR-locally IFR visibility as well.

In addition to the lowering ceilings and/or visibility, northeast
to north winds will further increase, with wind gusts at or above
likely for most areas by Sunday afternoon.


SD...Winter Storm Watch from Sunday evening through Monday evening
     for SDZ039-040-054>056.

MN...Winter Storm Watch from Sunday evening through Monday evening
     for MNZ071-097.



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