Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181745
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1245 PM CDT SUN SEP 18 2016

.SHORT TERM...(Today and Tonight)
Issued at 346 AM CDT Sun Sep 18 2016

Decently strong low-level jet has become established with higher
elevation locations remaining in contact with stronger winds and
warmer temperatures thus far overnight. This low-level jet will be
in place across mainly the mid and eastern CWA roughly along and
east of the James valley for a quick surfacing by mid-morning in
most locations along with a stronger overall surface gradient
resulting in some breezy southerly winds. Even with some degree of
periodically thicker high clouds filtering in from the west as
shortwave zips along the International border, it will be quite warm
with highs expected mainly in the 80s.

As the day develops, battle wages between the overmixed GFS/RAP on
far too much dewpoint lowering with mid to upper 30s from the James
valley westward this afternoon, and the equally unlikely NAM with
near 70 dewpoints across northwest Iowa.  Expect actual conditions
will reflect somewhat between, but shade toward lower values west
and higher values east. Cold front will slide into the far western
CWA by early to mid afternoon, and push into northwest Iowa and
southwest Minnesota this evening.  A few models have latched on to
an unrealistic degree of almost uncapped instability toward evening
with some spotty convection along the frontal zone.  Do not agree
with this assessment with main deeper lift forcing remaining north,
and have not introduced any mentionable precip chance for the
evening.  Any mixing of northwest winds behind boundary will be lost
with diurnal cycle, with the night trending toward lighter winds
but still seasonably mild temps in the 50s.

Later Sunday night, enhanced mid-level frontogenesis along the
southern flank of stout zonal flow should bring an increase to mid-
level cloudiness once again across the northern CWA along the
highway 14 corridor. While there could be some fairly decent lift
near this area, not a great deal of elevated instability and lower
levels are quite dry, so expect any sprinkles or light showers would
likely hover just to the north of the CWA.

.LONG TERM...(Monday through Saturday)
Issued at 346 AM CDT Sun Sep 18 2016

Quasi-zonal upper flow will be found across the northern half of the
U.S. Monday and Tuesday. A fast moving mid level short wave,
maximized in the 700-500mb layer by Monday morning, will pass along
our northern zones before rapidly departing by midday. This wave is
easily followed using 700-500mb qg forcing. The models continue to
show an extremely dry profile below about 675mb, even in our north.
Therefore although cannot rule out a morning sprinkle along Highway
14, certainly left measurable pops out at this time. Behind the
departing weak cold front, our CWA will see northerly to northwest
winds with highs about a category or two cooler when compared to
Sunday.

A second wave will move eastward late Monday night and Tuesday
morning. This wave is stronger, with most of the forcing in northern
SD and southern ND. However it too is very dry below about 650mb in
our forecast area. So although there will be an abundance of mid
level clouds late Monday night and Tuesday morning, continued to
leave the forecast area dry. Because of these clouds, warmed up our
lows a bit in our far northern zones Monday night to the lower 50s.
This will be followed by another unseasonably warm day on Tuesday
with highs ranging from the upper 70s in southwest MN to near the
upper 80s in south central SD.

Moisture will increase on Tuesday, while Tuesday night there is
still strong frontal dynamics in play with plenty of ascent aided by
strong warm air advection. A warm front lifts northward across the
forecast area, maximized in southwest MN and southward through the
Iowa Great Lakes area overnight through early Wednesday. Instability
is abundant, with NAM/GFS/ECMWF ML Cape consensus of 1000 to 2000
J/Kg. Wind shear is strong, with a stout 40 knot low level jet
feeding into our eastern zones, and 0-3km helicity values of 200 to
300. In fact even the 0-1km shear is over 200 but that shear will
likely not be tapped with the thermal inversion in place. Will
mention an isolated severe threat for our eastern zones in the
forthcoming HWO. With humid air in place and southeast surface
winds, lows Tuesday night will be very warm for this time of year,
with 60s common.

The various medium range models are in good agreement with the next
short wave passage on Wednesday. First, would not be surprised if a
few thunderstorms were lingering early Wednesday in our eastern
zones from the aforementioned warm front. But then the short wave
will takeover giving more chances for storms, especially east of the
James River valley. At this early time, it appears much of the
afternoon focus will be maximized in northwest Iowa. Severe chances
are difficult to ascertain at this time in our forecast area
Wednesday afternoon and evening due to the uncertainty of the
associated frontal timing with the short wave passage. That said, am
concerned about a heavy rainfall threat in our southern zones due to
the short wave with sub tropical connections interacting with any
lingering boundary from Tuesday night`s activity.

Further out Thursday through Saturday, the models are in much better
agreement in the handling of the very large upper trough passing
across the intermountain west and eventually into the plains this
next weekend. There is strong consensus in another stout warm front
lifting northward across our area late Thursday night and Friday
morning. Cooler weather will follow into the weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1242 PM CDT Sun Sep 18 2016

VFR conditions for the majority of the region through the TAF
period. After midnight, winds will become light and variable, with
patchy fog possible mainly across northwest Iowa. It looks like
the most favorable fog area will be east of KSUX, so will leave
mention out of the TAF for now.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Chapman
LONG TERM...MJ
AVIATION...



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