Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 271811

Area Forecast Discussion
National Weather Service Sioux Falls SD
111 PM CDT Thu Jul 27 2017

.SHORT TERM...(Today and Tonight)
Issued at 331 AM CDT Thu Jul 27 2017

High pressure will be in control of this area today and tonight. It
is centered across northern MN and and western Ontario, and will
drift slowly eastward toward Lake Superior by 12Z Friday. A short
wave, currently shown on water vapor imagery in northwest Nevada and
southeast Oregon will move eastward during this time frame, but
still too far west of this forecast area tonight to provide any
precip chances. Only an increase in thicker high clouds across our
western zones. Went with bias corrected highs today which were
warmer then non bias corrected readings. For lows tonight, trended
toward the guidance values which cooled off the eastern half of our
forecast area along and east of I 29, due to a light east to
northeast wind draining into those locations. On top of that,
manually reduced some lows in the favored cool spots.

.LONG TERM...(Friday through Wednesday)
Issued at 331 AM CDT Thu Jul 27 2017

The longer range period has not changed much on the larger scale,
continuing to indicate that ridge axis will retrograde and amplify
somewhat toward the intermountain west, keeping the region in a
varying strength of northwest flow.  Typically, timing of what would
be weaker impulses is tenuous at best, but in this case, probably
would only be the difference between obviously dry and perhaps the
smallest of chances for precipitation.  Models are generally on the
same page relative to the larger scale set up, but differ rather
grandly on details, as expected.

A fairly decent wave currently in Nevada will be ride the ridge and
start the dive southeast across eastern SD on Friday morning.  While
there is a great deal of upper-level moisture, and some mid-level
moisture favoring areas west of I-29, the lower levels remain quite
dry with impact of the sluggish surface ridge across Minnesota.
While would not be surprised to see a couple of sprinkles or even a
rogue light shower in parts of eastern SD through midday, too low a
probability to mention. Nevertheless, there should be a greater
degree of cloudiness with the wave, and that along with light flow
is likely to keep from reaching a full warming potential, with highs
in the upper 70s to mid 80s.  Ever so small a chance that a shower
or weakening thunderstorm wanders into areas near/west of the
Missouri River in south central SD Friday night from origins in
upslope flow across the west, but running out of this low-level
return flow in any attempt to move too far eastward.

Saturday, not much change to the larger pattern with surface ridge
stubbornly wedging back through SW MN and NW IA, with weakish return
flow through eastern SD, but again with more influence of flow out
of eastern surface ridge. By Saturday night, another ill-timed wave
will dig through the region, and have only a small chance for
thunderstorms in the overnight hours with less than stellar
convergence, and stable air to the east.

Wave passing through will tend to reinforce the low-level ridge to
the east, and the net effect is that 925-850 hPa temps change by
about 2 degrees C at most over the following 3-4 days, with some
slight warming around Tue.  Next best chance for precipitation, and
that may be generous, would come later Tuesday into Wednesday, as a
little stronger wave could approach in northwest flow and drive a
little more convergent boundary across the area. ECMWF about a day
slower with wave, so no confidence in timing to make any adjustments
to grids at that range.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 106 PM CDT Thu Jul 27 2017

VFR conditions expected through the period. Low VFR ceilings due
to scattered Cu expected to remain through early evening.




LONG TERM...Chapman
AVIATION...05 is the U.S. government's official web portal to all federal, state and local government web resources and services.