Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 200925
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
425 AM CDT TUE SEP 20 2016

.SHORT TERM...(Today and Tonight)
Issued at 425 AM CDT Tue Sep 20 2016

Much in the weather picture early this morning appears to have
changed little over the last 24 hours. A strong zonal jet along the
International border, albeit a bit of a meridional shift with
heights nudging upward across the plains through today. Push of
steeper mid-level lapse rates and frontogenesis along advancing
thermal ribbon has generated a scattering of sprinkles and a few
legitimate light showers near and north of the highway 14 corridor
in the overnight hours, and zone will continue to slip eastward and
spread northeast this morning. Coarser model solutions appear to
have a better picture of this compared to high-res CAMS at least so
far.  Frontogenesis shifts east through the morning with rising
heights through at least early evening, and will see meager
precipitation come to a bit slower end by mid to late morning across
southwest Minnesota. Surface ridge will continue to retreat as
frontal boundary and low pressure limps into the far western CWA
during the late afternoon. Some decent moisture at low levels is
actually not too far away in Kansas and Missouri, and that along
with the local moisture source from recent rainfall will start to
increase dewpoint considerably during the midday on increased
southerly flow beneath what is a fairly strong warm layer aloft.
It will be another very warm day with developing sunshine
encouraging temps mainly in the 80s.

Doubt growing as to whether convection will be able to initiate to
any appreciable degree tonight.  Despite increase in low-level jet
tonight, the convergent focus is at best along the far
northeast/eastern border of the CWA by later evening and through the
overnight. Problem to initiation is that warm layer aloft which
practically appears to cap off much of the area through the later
afternoon and early evening. With main gradient in mid level temps
removed to the north/east of the area, hard to picture that with
warm temps aloft sitting across a stagnating boundary from south
central to east central SD that will be able to initiate much given
larger scale forcing is somewhat on vacation tonight. Have started
walking back the pops in coverage and extent, holding on to modest
chance pops from KMML to KMJQ areas.  In the event that storms do
develop, there is a risk with the potential instability to get a
marginally severe hail or wind producing storm given degree of the
elevated lapse rates, but again highly conditional on development at
all.

.LONG TERM...(Wednesday through Monday)
Issued at 425 AM CDT Tue Sep 20 2016

Rainfall chances look rather skittish on Wednesday during the day
period. Small pops are warranted in the morning in our far northeast
zones along the warm front. Then perhaps bending back into northwest
Iowa and extreme southeast SD in the afternoon just ahead of the
advancing short wave. This short wave has origins in the tropics and
sub tropics, so moisture with this wave is in no shortage. With the
warm front draped close to I 90, highs on Wednesday are quite
interesting. The various bias corrected and non bias corrected
readings have caught onto the fact that there will likely be a tight
thermal gradient from north to south. But blended in some ECMWF
values which warmed up northwest IA and locations over to Yankton
into the upper 80s. Conversely, the highway 14 corridor from Huron
to Marshall may be lucky to hit the mid 70s.

Wednesday night still looks like the period when the aforementioned
short wave will impact our southern zones the most. The surface warm
front stays nearly stationary, perhaps slipping just a bit further
south, with the greatest push of northerly air confined to our zones
in south central SD behind a subtle surface low pressure wave in
northeast Neb. At any rate, CAPES in our southern zones are thin,
with ML CAPE only averaging a few hundred J/Kg, coupled with a
strongly veering profile present and deep saturation. The ECMWF is a
tad further northward with the heavy rainfall when compared to the
GFS. The NAM is off on its own and really wasn`t used, keeping the
heavy rain bottled up north of the warm front in central MN. Not
ruling out that there may be heavy rain in that area, just do not
like its totally dry solution in our southern zones as the NAM is
capped in that area. Therefore blended the Ecmwf and GFS outputs for
precip as its difficult to ascertain exactly where the warm front
will be providing the greatest low level focus. For instance the
ECMWF is bit further north with the warm front then the GFS
Wednesday night, thus its placement of QPF a bit further north.
Still believe that areas of heavy rain are possible Wednesday night
due to the deep saturation in northwest IA and immediate adjacent
locations, and will mention that in the forthcoming HWO.

On Thursday, pops become more disorganized again as the short wave
departs. In fact not sure if any pops are needed in the afternoon.
Highs are somewhat similar to Wednesday except cooler. Continued to
add in some warmer ECMWF output which warms up our southern zones
into the 80s south of the warm front. Therefore highs range from
near 70 in our far north, to the lower 80s around Sioux City.

As the very large upper low continues to move slowly eastward into
the intermountain west by late this week, the models have greatly
backed off on the warm frontal passage for Thursday night and early
Friday. The boundary just kind of pin wheels back into our zones
along the Missouri River. Therefore kept the trend of lowering pops
Thursday night and Friday, with highs only in the lower 70s across
southwest MN, to lower 80s along the MO River valley zones.

The deterministic GFS and ECMWF are in reasonable agreement with the
track of the upper low through 12Z Sunday, then they begin to
severely diverge as the ECMWF keeps the low closed and tracking much
slower eastward Sunday and Monday. Consensus shows the cold frontal
passage in our forecast area Saturday night and early Sunday when
the next primary focus for storms will be.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1037 PM CDT Mon Sep 19 2016

VFR conditions expected through the TAF period. Isolated to
scattered showers may develop through central SD by 06Z, and could
threaten KHON between 08Z to 11Z, however confidence is low and
coverage would be too sparse to include in KHON TAF at this time.

&&

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

$$

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



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