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Area Forecast Discussion (AFD)

Issued by Sioux Falls, SD (FSD)

FXUS63 KFSD 162156

Area Forecast Discussion
National Weather Service Sioux Falls SD
356 PM CST Fri Nov 16 2018

.SHORT TERM...(This evening through Saturday)
Issued at 355 PM CST Fri Nov 16 2018

Current surface analysis shows the leading edge of the cold front on 
the northern fringe of our forecast area. The upper wave has moved 
into our western zones and is best followed by using 700-500mb QG 
forcing. The current band of snow extending from north central SD to 
the northern sections of southwest MN is indeed lining up right 
along the 750mb frontogenesis where moisture is maximized in the 
middle of the saturated layer through 6km. Checked for elevated 
unstable EPV* near 700-650mb and aside from perhaps right now, there 
is not a lot of banded instability this evening across our north and 
northeast zones where moisture is the deepest. What we do have 
aiding ascent is a jet streak over northern WI which is placing the 
forecast area in a right entrance region into the overnight hours, 
sluggish to leave our far eastern areas. 

This evening, the aforementioned QG forcing moves into southwest MN 
and exits through extreme northwest IA close to the IA Great Lakes 
region. Frontogenetic forcing in the 850-750mb layer remains very 
strong in these locations below the QG forcing, with saturated 
levels through 6km. Some of the newer cams such as the HRRR, as well 
as the RAP and NMM really crank up the QPF on the eastern edge of 
our forecast area this evening as the dynamics becomes coincident 
with the deep layer moisture. Therefore decided to add Murray and 
Jackson counties to the winter weather advisory and will have to 
keep an eye on Dickinson county in far northwest IA. One more item 
is how terrain may play into snow accumulations. With a north to 
northeast low level wind component, we do have an upslope flow along 
and north of the Buffalo Ridge in southwest MN. There could be a 
very narrow local enhancement of snowfall in that area and manually 
drew in this possibility of increased QPF, which is another reason 
to add Murray county MN. As the cold front moves through, winds will 
increase dramatically into the 20 to 30 mph, perhaps gusting as high 
as 35 mph, which could create localized blowing snow conditions 
during falling snow. In addition, lows will plummet into the teens, 
perhaps near 20 at Sioux City and Storm Lake. 

Cold air advection stratus may linger Saturday morning over many 
areas. Not sure if the stratus layer is deep enough to produce 
flurries, but it is -12 to -14C in the cloud layer so there could be 
a few flurries around. At this time due to the thin stratus deck, 
decided not to include a few flurries Saturday morning but it 
certainly could happen with no accumulation whatsoever. Temperatures 
will be so to climb with only upper teens to mid 20s, with morning 
wind chills a bit below zero north of I 90.

.LONG TERM...(Saturday night through Friday)
Issued at 355 PM CST Fri Nov 16 2018

On Saturday night, lows will be cold, but how cold is rather a 
question mark. The northeast half of our forecast area will likely 
have snow cover. However as the 1040mb high exits to the south, our 
winds begin to turn around to the southwest over locations which 
will have bare ground. Therefore temperatures may steady out after 
midnight. But still, lows in the single digits over southwest MN and 
far northwest IA are a good bet. With warm air advection continuing 
on Sunday, highs should warm into the 30s to lower 40s. 

To underscore the model variability we had yesterday, in which many 
of the deterministic models were showing a cold front passage Monday 
night. That is gone now. The cold front now comes through a whole 24 
hours faster on Sunday night. The cold front looks dry, but there 
should be some stratus coming southward behind it. In addition given 
some of the latest 925mb temperatures, our highs may be too warm on 
Monday. Lowered them a little bit from superblend values, but would 
not be surprised if we are about 5 degrees too warm given the 
current low level thermal values. 

Otherwise the next upper troughing makes it into our area toward the 
end of the extended forecast. There is a great deal of model 
discrepancy that far out with the ECMWF stronger with the wave as 
opposed to the GFS. The ECMWF is primarily the reason why we have 
chance pops for precipitation late Thursday night and Friday along 
and east of I 29. Before then, the ECMWF brings a very large 
Canadian high southeastward on Wednesday through the Great Lakes 
region which would bring a chillier easterly flow of air into our 
area for Wednesday. The GFS is not even close to that solution, so 
for now just went with the blend for Wednesday's highs.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1106 AM CST Fri Nov 16 2018

An area of light snow, with attendant MVFR to IFR conditions, 
will continue to spread southeastward through the afternoon and 
evening hours. Heaviest snow is still expected to impact mainly 
areas east of KHON to KFSD although periods of reduced visibility 
are still expected at both of those terminals while snow is 
falling. KSUX will mainly be south of the main swath of snow but 
may see a period of hit or miss showery precip later this evening.


MN...Winter Weather Advisory until 4 AM CST Saturday for MNZ071-072-