754 
FXUS63 KFSD 091032
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
432 AM CST Thu Mar 9 2017

.SHORT TERM...(Today and Tonight)
Issued at 431 AM CST Thu Mar 9 2017

Narrow and intense frontogenetic band of snow is now starting to 
weaken across southwest Minnesota, but some degree of frontogenetic 
forcing in the 850-700 hPa will continue through much of the day 
across the northern CWA as jet entrance region drags away and next 
upstream jet begins to push into SD this afternoon. Complicating 
factor will be an increasing push of cold and drier air which will 
ooze southward at lower levels, forced onward by support of jet 
secondary circulation. While there will be some degree of increased 
lift forcing midday into the afternoon, especially as weak PV 
advection slides through by mid afternoon, the undercutting drier 
air is likely to have some say on any precipitation coverage. Temps 
are nicely in the dendritic range, so could end up more of a 
scattered flurry or nothing set up, with the best chance for a light 
dusting of snowfall near the northern edge of the CWA along highway 
14.

Temperatures a challenge with impact of overnight narrow snowfall 
band, and the increasing cold advection which looks to drop 925 hPa 
temperatures a few degrees C through the day, especially set up to 
offset additional mixing during the afternoon. Likely to not break 
30 north of highway 14 with clouds, snow cover and cold advection, 
while a bit more sunshine for a while today, and warmer mixing 
potential to the lower to mid 40s in the Missouri valley.

Tonight, the dry air advection will really gain a foothold through 
much of the CWA as arctic ridge settles into the eastern Dakotas and 
Minnesota. There remains some threat of light snow, albeit minimal, 
across south central SD, both during the early evening as lower 
level thermal gradient slides past, and also very late at night as 
mid-level frontogenesis begins to increase once again. While there 
will be some cloudiness that holds on thicker for areas west of I-
29, especially late night, enough clearing and cold advection that 
temperatures will likely reach zero or a bit below toward KBKX/KMML 
with mainly single digits above to lower teens heading southward.

.LONG TERM...(Friday through Wednesday)
Issued at 431 AM CST Thu Mar 9 2017

On Friday, 1045 to 1050mb high continues to filter down into the 
northern plains from south central Canada. This high is advecting a 
ton of low level dry air below 700mb into much of our forecast area, 
with a little deeper moisture present in our far western zones on 
the periphery of the high. Therefore actual measurable pops, even in 
our far west, may be pretty tough to realize, and kept much of 
our forecast area dry. But mid and high level clouds will become 
abundant keeping sunshine sparse to none. Concerning temperatures, 
the trend keeps cooling off readings as anticipated, and now the 
superblend highs do not look too bad given the depth of the mixed 
layer. Therefore expecting a lot of 20s for highs, except for upper 
teens around Brookings and Marshall. 

On Friday night, short wave energy moves down the northwest flow 
aloft. The wave is not particularly robust in terms of PV. But the 
forecast area is placed squarely in the right rear quad of a strong 
jet streak enhancing broad scale ascent. In the mid levels, 
frontogenesis is strong, but is maximized in our eastern zones where 
deep layer moisture is actually the weakest, still battling some low 
level dry air from the northeast. However one thing that is 
agreeable on all model solutions is the very strong 700mb front with 
favorable temperatures of -10 to -18C squarely placed on our 
forecast area. In addition, there is some weak instability primarily 
in the James River valley and down toward Sioux City between 700 and 
500mb, coincident with deep layer moisture. This area is on the 
eastern edge of unstable EPV to their west. Therefore considering 
the low level drier air in our far eastern zones, and the subtle 
instability from Huron SD to Sioux City area, it appears the 
heaviest snow will fall in that area of 3 to 4 inches. Pockets of 4 
to 6 inches cannot be ruled in that stripe if the dendritic layer 
becomes deeper in depth than anticipated. In fact on Saturday 
morning, the dendritic layer is quite deep, from 1 to 4km, and is 
helping to linger light snowfall through a large portion of the 
forecast area. However by Saturday morning we have lost the 
short wave and and our jet dynamics. Needless to say, temperatures 
will be well below normal Friday night and Saturday, however there 
will not be a lot of wind. Similar to Friday, highs will be in the 
20s once again on Saturday. 

The next system impacts our weather on Sunday and Sunday night. This 
time we will not waste an entire period trying to saturate up the 
low levels as residual moisture is still shown in the profiles. 
The exact placement of the heaviest precip potential is not 
certain due to the usual discrepancies between the deterministic 
models, but high pops are warranted already on the Sunday day period 
as some warm air advection moves in just ahead of the short wave. 
With deep layer moisture coincident with the warm air advection, and 
still favorable mid level temperatures of -12 to -18C, efficient 
snowfall is quite probable. Then the short wave moves through Sunday 
night continuing the snow threat with cold air advection, and 700mb 
temperatures that are little changed. Snow accumulations of 2 to 4 
inches are already projected for Sunday day, and that does not 
include what should fall Sunday night, which may be another inch or 
two. One of the bigger stories into Monday is how the cold air 
lingers, and with the cold air advection, bumped super blend wind 
speeds higher as they should be a bit breezy. As cold high pressure 
moves eastward, moderating temperatures will occur by mid week on 
Wednesday and Thursday, and they are projected to be dry days.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1104 PM CST Wed Mar 8 2017

A narrow band of light snow will bring MVFR-IFR conditions to 
areas near and north of Highway 14 into Thursday morning. This 
will impact KHON at the start of the period, but KHON is largely 
expected to be on the edge of the sub-VFR conditions. 

Areas along/south of I-90 should see VFR conditions prevail through
this TAF period. 

&&

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

$$

SHORT TERM...Chapman
LONG TERM...MJ
AVIATION...JH