Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191722
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1222 PM CDT MON SEP 19 2016

.SHORT TERM...(Today and Tonight)
Issued at 353 AM CDT Mon Sep 19 2016

Strong zonal jet across North Dakota and northern Minnesota this
morning. Intense mid-level frontogenesis on southern flank of the
jet will continue to enhance development of mid-level cloudiness as
far south as I-90 through the early to mid morning hours. Profiles
remain very dry below cloud base, but would not be surprised to get
a few sprinkles down to around the Highway 14 corridor through early
morning, but and measurable precipitation threat likely remaining
north of the CWA.  Moisture band should thin with weakening
frontogenetic forcing by mid to late morning, leaving a very
pleasant day with much less wind and temperatures in the upper 70s
to lower 80s.

Another quick moving impulse will ride the flat ridge into the
northern plains tonight. Frontogenesis increases, along with the
gradual veering of the low-level jet and advancement of gradient in
the elevated lapse rate.  As a result, this time around there is
just a bit more instability pushing in aloft with the wave, but a
continuation of the very dry air below cloud base. Feel fairly safe
with just keeping a mention of a few sprinkles again toward the
Highway 14 corridor. Temps likely to exhibit somewhat a non-diurnal
trend with increase in clouds again after 06z and increasing
southeasterly flow on back side of retreating ridge overnight.

.LONG TERM...(Tuesday through Sunday)
Issued at 353 AM CDT Mon Sep 19 2016

As a mid level short wave passes rapidly eastward, kept isolated
sprinkles going through mid morning Tuesday along Highway 14 in our
extreme northern zones. Mid level clouds will likely be abundant
generally along and north I 90, but it continues to look very dry in
the lowest 3km, even in our far north. Further west, stratus could
edge northward close to Gregory county Tuesday morning along the
strongest axis of return flow. But at this time, it appears that
this fetch of moisture will stay just to the south and west of our
forecast area. Tuesday afternoon will be a bit breezy with southerly
winds mainly along and east of I 29. Concerning temperatures, bias
corrected values look too cool, therefore used non bias corrected
values including raw model values as well as MOS. This warms
temperatures into the 80s which fit the 900-850mb thermal profile
and what should be plenty of afternoon sunshine.

The models continue to show a warm lifting northward across the CWA
Tuesday night. The warm frontal dynamics are maximized in our
eastern zones, most notably in southwest MN and the IA Great Lakes
area, where a 40 knot low level jet in the central plains collides
with only 10 to 20 knots in our northeast zones. Therefore high pops
are warranted for thunderstorms in these locations. ML CAPE is still
averaging around 1500 J/Kg in these areas along with strong wind
shear. Now some of this wind shear in the lowest kilometer will not
be tapped due to a developing inversion. But the wind shear is
fairly burly above 1km, so certainly cannot rule out some isolated
severe storms in the aforementioned locations. Further south and
west, conditions become increasingly capped with little in the way
of a focusing mechanism. Lows will be very mild Tuesday night for
this time of year, with widespread 60s.

On Wednesday, all models are more aggressive in bringing southward
the surface wind shift, as Canadian high pressure begins to filter
southward. In fact by afternoon, the wind shift should be through
our northern zones. Our southern zones will see a short wave passage
with tropical and sub tropical origins, which is really the remnants
of Paine, Wednesday afternoon and night. At this time, primarily
concerned about a potential heavy rain threat in northwest Iowa,
especially Wednesday night. Northwest Iowa is squarely on the nose
of a strong low level jet in the central plains of 30 to 40 knots,
and a profile which is currently exhibiting thin CAPE. Something to
watch for in future forecasts. In addition, the aforementioned
surface boundary will continue to sag southward providing a low
level focus mechanism for storms. For temperatures, blended quite a
bit of the ECMWF in for highs on Wednesday, as it seemed to capture
the north to south thermal gradient which will likely develop on
Wednesday along the frontal boundary.

By Thursday and Thursday night, a well advertised very large upper
low will trudge slowly eastward into the intermountain west. As it
does, building heights will be found on Thursday behind the
aforementioned short wave departure. So only skittish pops are
warranted on Thursday with highs in the upper 60s and 70s. Still
looks like another warm front lifting northward across our forecast
area Thursday night and Friday morning which will once again provide
higher end chance pops. Because of the warm front, Friday looks to
be another day of a strong thermal gradient across our forecast
area, with highs in the lower 70s north, to lower 80s south.

Further out, consensus model output shows a cold front moving across
our forecast area Saturday night as the upper low gets closer to our
area. This could very well keep the weather unsettled Saturday and
Saturday night. That said, the deterministic ECMWF is a fair amount
slower then the GFS, therefore it is very difficult to ascertain at
this early time frame any sort of a severe weather threat, or if
there will be one at all.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1213 PM CDT Mon Sep 19 2016

VFR conditions will prevail through the TAF period. There could be
a few isolated showers near KHON site between 07Z and 11Z, but
ceilings and visibility will remain within the VFR parameters.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...MJ
AVIATION...JM5



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