Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 200322

Area Forecast Discussion
National Weather Service Sioux Falls SD
1022 PM CDT Sun Mar 19 2017

.SHORT TERM...(This evening through Monday)
Issued at 328 PM CDT Sun Mar 19 2017

High and mid-level clouds will be on the increase tonight into
Monday as surface high pressure noses into the Northern Plains. Not
much spread in guidance for tonight, so sided with consensus.  Light
northerly flow will transition to northeast on Monday. With
increasing clouds and low level cold air advection, have sided
toward the cooler end on guidance temperatures for highs on Monday.
Some of the guidance have been slightly warmer along the Missouri
River Valley, but will likely be dependent on how thick the cloud
cover be across the area on Monday.

.LONG TERM...(Monday night through Sunday)
Issued at 328 PM CDT Sun Mar 19 2017

Monday night and Tuesday continue to be interesting. Mid level
moisture is abundant across the area in the 700-500mb layer as a
short wave moves eastward. The short wave is easiest followed via
the 500-300mb QG forcing field and is coupled with a baroclinic zone
in the mid levels moving southward. In addition, mid level
convergence maximized near 700mb is abundant along our southern
border with Nebraska by 12Z Tuesday which helps to develop some
strong frontogenesis in the 750-650mb layer. The question is the
amount of low level moisture as a large anticyclone based in south
central Canada advects dry air from the northeast into this area. So
there appears to be a decent battle shaping up on whether the low
levels can saturate under the strong mid level dynamics. The further
north you go, generally along and north of I 90, one runs into
increasingly dry air below 700mb and thus only slight chance
or chance pops are warranted in these locations with very light
amounts of QPF. Heading toward Nebraska, the various models show
some brief saturation in the 925-850mb layer for several hours late
Monday night and Tuesday morning right when the wave is on top of
them. The best chance for accumulating snowfall centered at 12Z
Tuesday will be right in our MO River valley zones from Chamberlain
SD to Sioux City/Storm Lake IA. These locations will likely receive
a quick 0.5 to 1.5 inches of snow based on a blend of the NAM12 and
GFS20 snow ratios. The superblend snow ratios continue to be way too
low as it is influenced too much by the GEM Global snow ratio which
is basically zero in our forecast area. That is not a realistic
scenario looking at the thermal profiles. The key to getting more
snow in our southern zones are surface temperatures. Right now, we
have lows in the MO River valley of 32 to 35 degrees which seems
reasonable given the amount of thick cloud cover. However if we wet
bulb downward as we saturate the low levels, the surface
temperatures will be colder which of course will allow more snowfall
stick to the surface before melting later on Tuesday. So this is
something to certainly watch for. The system exits Tuesday afternoon
drying things out, with seasonal highs mainly in the 40s.

Some drying from northeast to southwest takes place Tuesday night,
however our western zones will likely never lose their cloud cover.
Then another weak short wave moves eastward late Tuesday night and
Wednesday coupled with strong low and mid level thetae advection as
the high pressure center moves eastward. The low level jet
increases, however low level moisture fields are still rather dry.
The GFS has picked up on this and produces only meager amounts of
QPF, while the ECMWF and GEM Global are a bit more robust. Due to
the relatively dry low level air, not convinced we will see much in
the way of precip, either light rain or snow, so left primarily only
chance pops going with most of the QPF confined to our far western
zones where the deepest moisture resides. Perhaps another half inch
or inch of snow may fall west of the James late Tuesday night and
Wednesday morning, but with seasonal highs in the lower to mid 40s,
any snowfall will likely not stick around for long. Wednesday
probably will not be the most pleasant day in the world however,
with cloud cover, and brisk southeast winds of 20 to 30 mph coupled
with chilly air.

The next focus is the strong storm system which moves across the
plains Thursday night and Friday. Strong thetae advection continues
on Thursday with a breezy southerly flow of air. High pops are
warranted however rain amounts will likely be pretty light as the
mid levels from 700-500mb are dry. In fact the precip type for
Thursday is kind of tricky. One could argue a case for just drizzle,
but moisture is saturated through about 800mb and coupled with warm
air advection, there could be some bonafide light rain in there. As
far as the track and timing of the storm, the 12Z GFS and ECMWF have
come together in much better agreement. They track the upper low
across the central plains with surface low pressure following suit.
However a strong inverted trough develops Thursday night and extends
northward right into our forecast area creating strong low level
convergence which moves slowly eastward. The GFS is extremely
bullish on QPF late Thursday night and Friday morning just north of
the inverted trough passage, centered along an axis from Yankton SD,
to Sioux Falls and southwest MN. That said, decent amounts of
rain fall throughout the entire forecast area. The ECMWF is a
smidge further north and west with the maximized QPF. Interestingly,
the GEM Global cuts off our forecast area by Friday with the
northern stream taking over, as it is further south with the passage
of the upper low. This solution generally was not followed due to
the agreement of the ECMWF and GFS, but it did pull pops down on
Friday which are probably not high enough at this point. Convection
wise, the GFS still paints elevated TSRA possibilities along and
south of the inverted trough Thursday night, possibly lingering in
our far southeast zones on Friday. At this time TSRA chances could
occur through much of our forecast area going with the 850-800
lifted index values and elevated CAPE Thursday night. What is a bit
bothersome is with a 70 knot 500mb jet, some of these storms if
they occur could produce brief wind issues if the storms hookup with
the boundary layer. However, the soundings are pretty through the
lowest 2km so severe type winds may not reach the surface.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1014 PM CDT Sun Mar 19 2017

VFR conditions through the period at all TAF sites. Winds will
gradually shift toward the northeast. Mid level clouds at or
around 9 thousand feet will gradually build in as well.




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