766 
FXUS63 KFSD 041103
AFDFSD

AREA FORECAST DISCUSSION
National Weather Service Sioux Falls SD
603 AM CDT Sun Apr 4 2021

.SHORT TERM...(Today and Tonight)
Issued at 410 AM CDT Sun Apr 4 2021

Subtle wave and associated surface boundary will traverse the region 
today, with the boundary expected to stall over northwest Iowa this 
afternoon. Relatively weak surface gradient around the boundary 
should allow winds to remain light enough to hold shy of critical 
levels today. That said, despite a less favorable easterly flow in 
areas north of the boundary, have seen deeper than expected mixing 
of late regardless of wind direction. Thus have nudged winds upward 
in areas away from the boundary, and our western areas could see a 
few gusts approaching 20-25 mph at times this afternoon. 

Even with the boundary, the air mass remains extremely dry. We do 
have a projected increase in low level moisture along and south of 
the boundary through the day, and mid 40s dew points have worked 
their way into far southern Nebraska as of 08Z. However, with the 
very dry air aloft, think expected deeper mixing will overcome this 
boundary layer moisture advection, and have dropped afternoon dew 
points toward the lower end of guidance yet again. With temperatures 
likewise expected to mix deeper/warmer, yielding highs in the 80s 
most areas, expect to again see afternoon humidity levels fall below 
20 percent, perhaps below 15 percent in areas north of the boundary 
across our SD/MN counties. Given these very low humidity levels, 
will have to closely watch winds north of the boundary in case they 
begin climbing even higher than currently forecast.

Tonight, this boundary lifts back to the north in response to 
another approaching weak wave. Given the expected depth of dry air, 
have slowed the start time of our low pops across northwest Iowa, 
but will still carry a chance of showers or isolated storms from 
late evening through the overnight in response to the subtle wave 
and warm advection.

.LONG TERM...(Monday through Saturday)
Issued at 410 AM CDT Sun Apr 4 2021

The boundary lifts north Sunday night, placing our entire CWA in the 
warm sector ahead of yet another subtle wave brings a cool front 
into central South Dakota late in the day. This boundary looks a 
little slower in the latest model runs, allowing even warmer air to 
build northward into the forecast area for Monday. Winds are a much 
more favorable south-southwest most of the day, which should again 
promote deeper mixing than most models indicate, leading to a breezy 
day for most areas. In this case, we do see a deeper layer of low 
level moisture work into the forecast area from Sunday night through 
Monday. This should help keep humidity levels higher in areas east 
of the boundary on Monday, though still opted to hedge toward the 
drier/warmer end of guidance. This yields highs in the lower to mid 
80s in our eastern areas, with afternoon humidity levels from 30-40 
percent. Farther west, areas toward the approaching boundary are 
farther removed from the influx of deeper moisture, and also closer 
to a strong thermal ridge just ahead of the boundary. Low level 
temperatures support highs approaching 90 degrees in areas west of 
the James River, with the drier southwest-westerly flow allowing for 
humidity levels nearing 15-20 percent once again. At this time, 
winds appear to remain shy of critical levels, but this will have to 
be closely evaluated in later forecasts.

Monday night onward should see a pattern change away from our warm 
and dry conditions, as a cutoff upper low slowly works its way east 
across the Plains. Could see a few storms develop along the cool 
front as it pushes east Monday evening, and SPC has maintained a 
Marginal Risk for portions of the Highway 14 corridor. Instability 
would support isolated hail threat, while the dry mid-levels are 
supportive of downburst winds, though deep layer shear is rather low 
in our area, more supportive of organized storms farther to the 
northeast. 

The cutoff upper low, with its associated TroWAL wrapping around the 
north side, will then spread increasing rain chances across the 
forecast area from Tuesday through Wednesday, lingering across our 
eastern areas into Thursday. Still some uncertainty on how far 
north/south the system will track, so will cap pops in the likely 
range for now. Though model averages point to a decent potential for 
total rainfall amounts in excess of an inch across much of the CWA 
through the midweek period. 

This system works east of the area by Friday, with seasonable temps 
and limited rain chances into next weekend.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 603 AM CDT Sun Apr 4 2021

VFR conditions and light winds will prevail for most of the
forecast period, though the afternoon/early evening hours may
bring gusts of 15-20kt to some areas west of I-29, and toward 
KSLB.

Some indication that areas from KSUX-KSLB could see a period of
low level wind shear after 05/04Z as southwesterly low level jet
increases aloft. However, some uncertainty regarding where the
stronger winds aloft will be located, so will forgo mention of
LLWS in KSUX TAF at this time. A few isolated high-based showers
or storms will also be possible in northwest Iowa overnight, but
risk for any one point is too low to include in TAFs.

&&

.FIRE WEATHER...
Issued at 410 AM CDT Sun Apr 4 2021

Very warm and dry air mass will remain in place through Monday. 
Winds will be marginal today, perhaps gusting 20 to 25 mph across 
portions of southeast South Dakota this afternoon. However, 
humidity levels will be very low this afternoon, as low as 10 to 
20 percent, leading to continued elevated fire danger. 

Areas along and east of the I-29 corridor should see some 
increase in humidity levels by Monday, though southerly winds will
also be a bit more breezy, so we will see one more day of elevated
fire danger before a slow moving system brings widespread rain
chances across the region midweek.

&&

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

$$

SHORT TERM...JH
LONG TERM...JH
AVIATION...JH
FIRE WEATHER...JH