Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180919

Area Forecast Discussion
National Weather Service Sioux Falls SD
419 AM CDT Thu May 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 409 AM CDT Thu May 18 2017

Cooler and drier conditions are expected today, with increasing rain
chances tonight.

Early this morning (08Z) the water vapor imagery shows the center of
the strong low pressure system east of the CWA, near
Minnesota/Wisconsin border. A few spotty/light rain showers
associated with this low will remain, mainly east of I-29, before
exiting the forecast area by mid- morning. There will be a break
in showers, as drier air aloft associated with a weak upper level
ridge moves across the region. Northerly flow/cold advection will
continue to spread southeast into the area. This will lead into a
cool Thursday under mostly cloudy skies. Below normal temperatures
are expected with highs from the mid 50s across southwest
Minnesota to lower 60s elsewhere. It will be a breezy day, mainly
in the morning, with north-northeasterly winds of 15 to 20 mph.

Wet weather conditions will gradually increase later this evening,
as a deep mid-level low pressure tracks northward across the area
from the Central Plains. As the system lifts northeast, and the
surface front sets up across the High Plains, deeper saturation
will be in place thanks to decent forcing and strong isentropic
lift on the 295K layer. With that said, increased rain chances
across the Missouri River Valley and portions of northwest Iowa
after 03Z, then becoming more widespread northward by 12Z. Most of
the precipitation will occur south of the I-90 corridor, as rain
may struggle with drier air aloft along and north of the Highway
14 in the overnight hours. There is little to no instability in
place as soundings suggest; therefore did not mention thunder.

Wet conditions and north-northeast surface flow will bring cooler
low temperatures tonight with readings in the lower 40s.

.LONG TERM...(Friday through Wednesday)
Issued at 409 AM CDT Thu May 18 2017

Friday will be another wet day for much of this forecast area,
especially in the morning hours. Synoptically, upper level low
pressure is centered over Colorado early Friday morning, then
broadens over the western high plains by evening. The models have
clearly been showing a short wave moving northward for quite some
time now, attendant to the east side of the upper low and moving
into the forecast area. At the surface, a cold northeast wind will
be in place due to high pressure in southern Canada, with the
surface low well to the south in the central and southern plains.
Superblend pops were too low Friday morning along an east to west
axis near Interstate 90, so raised them substantially, giving
values closer to 90 percent. That said, overall QPF amounts did not
look too bad, averaging a third of an inch near I 29, with lighter
amounts in our northern and southern zones. The wave continues to
hit an air mass of drier air aloft as it moves northward, therefore
only chance pops are warranted during the afternoon hours across our
north for light rain.

As the upper low moves a bit further eastward Friday evening, the
next impulse organizes with a subtle inverted trough forming and
extending northward by late afternoon into our southern zones.
Therefore kept likely pops going Friday afternoon south of I 90 in
response to this, extending the likely pops to include I 90 by
Friday evening. With the upper low over the Dakotas and Nebraska by
12Z Saturday, surface low pressure follows suit, deepens a little
bit, and moves northeastward through central Iowa. Trowaling looks
to be to the west of our forecast area on Saturday. However this
forecast area exhibits a strong baroclinic zone from 850-700mb
behind the low along with a 100 knot jet streak moving northward
just east of I 29. In addition, the upper low is progged to have a
decent PV surge off the 1.5 pressure surface, with values near 650mb
moving northeastward near Interstate 29. Therefore shower activity
will likely continue on Saturday, decreasing through the afternoon
hours from south to north.

In terms of temperatures, superblend temperatures did not look too
bad on Friday given the 925-850mb thermal regime. Anything but
pleasant for this time of year, highs look to be in the mid 40s to
around 50, and coupled with a brisk northeast wind, it certainly
will not feel like mid May. On Saturday, superblend temperatures
looked too mild. In fact superblend was one of the warmest guidance
readings out there with the Canadian, ECMWF, and american bias
corrected values all considerably chillier. Given the 850mb
temperatures on Saturday and considering the showers and strato
cumulus, used the weighted model guidance which cooled off readings
a couple of categories. Therefore Saturday will not be much warmer
than Friday, with temperatures banking on an afternoon which becomes
increasingly dry.

Finally, Sunday looks drier and warmer, with the next impulse moving
southeastward on Monday along the southwest side of the
aforementioned upper low which is in southeast Canada by Monday.
Shower activity on Monday will likely be aided by heating in the
late morning and afternoon with a deep mixed layer. There could even
be a few instability showers around again on Tuesday behind the
departing upper wave centered across Wisconsin and Illinois.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1031 PM CDT Wed May 17 2017

Scattered showers will continue to swirl around an upper level low
tracking across the region, though slowly pull away by 09Z. Will
see low clouds begin to lift at KHON after 09Z, though MVFR
stratus now looks to persist at KFSD and KSUX into Thursday
afternoon. Northerly winds will gust to around 25 kts overnight,
then become northeasterly and taper down just a bit on Thursday.




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