Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 232342
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
642 PM CDT Sun Apr 23 2017

.SHORT TERM...(This evening through Monday)
Issued at 427 PM CDT Sun Apr 23 2017

Breezy southerly winds will continue tonight with a tight pressure
gradient in place and winds off the surface averaging 35 knots near
925mb. This will moderate lows greatly when compared to the last
couple of nights. Consensus blended lows actually did not look that
bad, with a lot of upper 40s to lower 50s in place. Then on Monday,
we are still dealing with a short wave which will move from the
western Dakotas and western Nebraska early in the morning, to just
east of I 29 by 00Z Tuesday when looking at the 700mb wave. The QG
forcing in the mid levels is not maximized anywhere during the day
on Monday, but elongated broadly along the mid level troughing. Due
to a decided lack of low level moisture, rainfall chances will be
rather skittish through the day, with TSRA chances very limited. Did
use the ECMWF in the blend mix for pops and qpf as it brought a
stripe of very light chances for rain up through the I 29 corridor
in the late morning, then east of I 29 during the afternoon. This is
due to warm air advection ahead of the wind shift. But again, pops
are not worth anything more than low end chances to slight chances
at any given location. Instability is limited. There is some
instability in the mid levels between 700-500mb but the accompanying
elevated MU CAPEs are rather thin. Therefore limited small TSRA
chances to the late afternoon hours.

Temperatures on Monday are a bit of a challenge. Not totally buying
the consensus highs, or even the bias corrected high temperature
values. As the cold front moves through our western zones in the
afternoon, there is some cold air advection to contend with which is
being temporarily balanced by a westerly mixing wind. However am
concerned that the cold air advection will win out by mid afternoon,
so opted to use a blend of ARW and NMM high temperature values along
with the superblend. This had the affect of lowering highs in our
western zones into the lower 60s. In addition, there will be a
decent amount of mid and high clouds around on Monday limiting
sunshine. Even the dense cirrus on Sunday limited our highs a few
degrees from reaching their potential, so cloud cover on Monday is a
concern. But Monday will still be a mild day, with plenty of wind
ahead of the cold front in the 20 to 35 mph range. Use raw model
values for the wind speeds, blended winds were too low.

.LONG TERM...(Monday night through Sunday)
Issued at 427 PM CDT Sun Apr 23 2017

Tail end of upper forcing lingers across northwest Iowa and
southwest Minnesota early Monday evening, and with just a touch of
instability possible, if any showers managed to form or persist
into early evening, they could present a small threat for thunder.
For the most part, this chance will end fairly quickly, leaving the
advancement of cooler air behind the front as the main feature on
Monday night into Tuesday.

Another system in the zonal flow will spread frontal forcing back
into the south central SD locations by late night, with light
precipitation toward daybreak.  Precipitation should be fairly
widespread through the day on Tuesday, suggesting with the continued
low-level cold advection that temps will maximize earlier in the day
especially the southeast half of the area. There continue to be
hints of near zero to weakly negative EPV above the mid-level
frontal surface, which would bring some slantwise convective
processes into the mix, and perhaps a couple rumbles of thunder
along the main frontal axis near/south of a KYKN/KMJQ line.
Likely that there will be a band of slightly higher QPF from mid
morning until early evening Tuesday where perhaps a inch or little
more rain is possible near/south of this delineation.

Bulk of the precipitation will dwindle during the evening as the mid-
level wave pulls northeast and frontal boundary collapses to the
southeast. Secondary wave pushing northeast should ignite some
renewed precipitation, but mainly southeast of the CWA across
eastern Nebraska toward central Iowa. However, temps start to get a
little dicey as to getting some mix or transition to light snowfall
on the back edge of the band. Temps will need to be watched, as even
with continued northerly winds and even cloudiness, may get readings
down toward 30 degrees west of I-29.

Wednesday should be drier, but combination of lingering troughing
aloft, cyclogenesis to the east wrapping moisture back into the
area, and diurnal destabilization could bring back threat for
isolated to scattered short showers around the midday and afternoon
hours. Much below normal with highs in the 40s for most, and wont
even feel that warm with northerly winds 15 to 30 mph. As closed
wave pulls away and high pressure noses in at lower levels,
Wednesday night will bring another threat for sub-freezing temps,
even with threat for wrap-around clouds lingering near/east of I-29.

Thursday should be dry between systems with influence of weak upper
ridging. A bit of recovery in temps into the upper 40s to mid 50s
with less wind, but still below normal.

Beyond Thursday, models continue to knock heads against each other
in terms of how to handle energy in the split flow. A series of
waves will move across the area, with GFS continuing to dump a
strong piece of energy northeast into the region on Friday, which is
strongly against ECMWF (which holds in drying from high pressure
into Friday which saving back more energy for a weekend system), and
several of the GFS ensemble members. Not a great deal of confidence
in precipitation timing, but all in all, should be periods of precip
with no indications of any appreciable warming through the weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 615 PM CDT Sun Apr 23 2017

VFR conditions expected through the TAF period. Low-level wind
shear up to and around 45 knots possible in the overnight hours.
Surface winds to 25-30 knots will strengthen again ahead of a
cold front that will be moving across the area by midday. This
will also bring low ceilings in the afternoon hours.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...Chapman
AVIATION...Dux/05



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