373 
FXUS63 KFSD 191138
AFDFSD

AREA FORECAST DISCUSSION
National Weather Service Sioux Falls SD
638 AM CDT Tue Oct 19 2021

.SHORT TERM...(Today through Wednesday Night) 
Issued at 409 AM CDT Tue Oct 19 2021

Early this morning, a sfc trough lies draped from near the Hudson
Bay to the central Dakotas to along the Colorado Front Range. 
This feature progresses slowly east today as a strong Colorado low
ejects into the High Plains. Out ahead in the Central Plains, WAA
persists, building a stout mid level inversion with 850mb 
temperatures in the top 10% of the NAEFS climatology for us to mix
toward. With this, expect mostly sunny skies and breezy south 
afternoon winds gusting 20-25 mph. Somewhat lighter winds and dew
points rising well into the 40s this afternoon should negate fire
danger concerns. Take advantage of one more warm day with highs 
5-15 degrees above normal values for mid October. Winds turning 
more northeast or east along and behind the approaching sfc trough
will make for slightly cooler weather along and northwest of a 
Brookings to Mitchell and Gregory line.

This evening, the sfc low tracks into northeast Nebraska, 
prompting a narrow axis of moisture return with the lingering LLJ 
and draping a warm front northeastward into southeast SD and 
southwest MN. A substantial temperature gradient develops  
overnight, with lows ranging from the 40s (near the upper closed 
low in central SD and behind the warm front) to the 50s downstream
across the warm sector in northwest IA.

The main concern after midnight through Wednesday morning will be 
potential for isolated strong to severe thunderstorms near the sfc 
low. The NAM and GFS both paint a narrow area of up to 1500 J/kg 
MUCAPE with 500mb and 250mb speed maxes nosing in. The
HRRR/NAM/GFS show potential for a few elevated supercells to 
develop near the SD/NE border, tracking NE into southeast SD and 
perhaps portions of northwest IA and southwest MN. Given 
steepening mid level lapse rates, agree with the SPC Day 1/2 
Outlooks that large hail would be the most likely threat with 
morning storms. Additionally, expect scattered showers and storms 
associated with the TROWAL to spread into central SD and southeast
SD as the upper close low shift east Wednesday morning.

On Wednesday, expect cloudy skies, breezy winds, and a much cooler 
day with temperatures only reaching the upper 40s to upper 50s. 
Drier air aloft wrapping in, mainly south of I-90, leaves major 
uncertainty in rainfall totals. In fact, much of the region could 
see more dry time than rainy on Wednesday. This uncertainty is 
focused on both 1) how widespread of an area will be impacted by 
entraining dry mid level air and 2) whether additional afternoon 
shower and storm development occurs south of I-90 (as suggested by
the 06z HRRR). 

Rain departs eastward Wednesday evening, but did extend rain chances 
a bit further west during this period as cold, dry air advecting in 
will take some time to erode the deeply saturated mid-low levels. 
Stratus will linger overnight as well, keeping temperatures a touch 
milder than further northwest. Lows dip into the 30s to lower 40s 
Wednesday night with gusty northwest winds slowly weakening as the 
pressure gradient relaxes.

.LONG TERM...(Thursday through Monday) 
Issued at 409 AM CDT Tue Oct 19 2021

For Thursday and Friday, cool autumn air prevails in northwest 
flow aloft with near to below normal temperatures. In the  
mornings, temperatures will drop well into the 30s, likely below 
freezing for portions of the region north of I-90. How far sub- 
freezing air spreads will be highly dependent on cloud cover from
weak shortwaves tracking through. Dry low levels will likely 
preclude any rain chances. In general, drier air should prevail by Friday
afternoon, resulting in sunny skies and temperatures in the 50s.

Model solutions begin to diverse Saturday as the GFS and Canadian
build a stronger ridge into the northern plains while the Euro is
weaker and slower. Temperatures will remain in the 50s due to the
ESE winds advecting in cooler air from high pressure to the east.
850 mb winds will increase Saturday afternoon, mixing down gusty 
winds 20-30 mph. Clouds will also increase in coverage throughout
the day due to mid to upper level moisture moving in aloft. 

These conditions will last into Sunday as an upper wave moves into 
the forecast area. This wave will generate a Colorado Low that will 
move NE towards the CWA. Stronger warm and moisture advection will 
further increase clouds and precipitation chances throughout  
Sunday. Beyond the weekend, deterministic models hint at a much 
stronger longwave trough progressing into the plains by the middle
of next week, but confidence is low in any details at this range.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 618 AM CDT Tue Oct 19 2021

VFR conditions will begin the TAF period. Wind gusts will increase 
to around 20 knots this afternoon. Clouds will increase in coverage 
and likely meet MVFR criteria at KHON and KFSD at the end of the 
period. Scattered showers and storms will begin to spread into the 
region at the end of the period. More likely affecting KHON and KFSD 
than KSUX.  

&&

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

$$

SHORT TERM...BP
LONG TERM...Meyers/BP
AVIATION...Meyers