Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 161042 CCA
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
441 AM MDT Fri Apr 16 2021

.DISCUSSION...(Today Through Thursday)
Issued at 257 AM MDT Fri Apr 16 2021

Broad area of mostly light precip remains over the area tonight,
with snow over the Black Hills/foothills/wrn plains and rain farther
east across the plains. WV imagery reveals a fairly complex midlevel
pattern, with several vort maxima across the cntrl Rockies/Plains.
However, coupled with IR, satellite imagery indicates that the low
in the vicinity of nern CO/swrn NE is accompanied by the most
significant ascent at this hour.

This upper low will continue to steadily shift ewd thru the day,
reaching ern NE/KS by this evening. As it does so, secondary trof
axis/deformation zone will cross the nrn Plains from NW to SE.
Northerly boundary layer flow is expected to increase this aftn as
enhanced low-level RH lingers across the area. With daytime heating
later this morning into the aftn, should see stratus evolve into a
stratocu or cu field as lapse rates steepen. This will lead to the
development of snow showers across much of the area.

Localized fgen over the srn Black Hills will serve as a focus for
snow shower activity there. Given thermodynamic profiles, showers
could be rather intense at times this aftn, supporting high snowfall
rates and perhaps requiring a brief advisory due to potential travel
impacts. However, with temps expected to reach the mid 30s prior to
the onset of snowfall, suspect that any impacts will be rather short-
lived. Froude numbers become more favorable for upslope enhancement
over the nrn hills by mid-aftn, continuing into the evening.
However, as drier mid-upper level air arrives in the wake of the
secondary trof axis noted above, any upslope snow should begin to
taper off around sunset. Given diurnal timing and rather short
duration, not expecting significant impacts with this activity,
either. Could see one or two additional light showers (rain or snow)
on Saturday, particularly over the hills where recent precip will
support higher low-level RH. That said, drier columns should limit
precip overall.

Shortwave ridging will lead to warmer temps late Saturday into
Sunday. However, next trof rapidly approaches Sunday, with an
accompanying cold front likely entering the area by midday.
Positioning of associated mid-to-upper-level forcing is not ideal
for heavy precip across much of the area, with an elongated midlevel
shear axis and trailing concentrated vort max largely passing
north/east of wrn SD and a secondary vort max supporting appreciable
Q-vector conv over wrn/cntrl WY before it digs sewd. Thus, primary
focus for precip across our area will likely be the nrn hills, where
Froude numbers of 1-1.5, strong N/NE low-level flow, and deep DGZs
could support a period of noteworthy upslope snow.

Relatively active upper-level pattern remains in place thru much of
next week, with near-to-below temps and occasional chances for
precip.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 439 AM MDT Fri Apr 16 2021

Widespread IFR conditions in light snow and rain will gradually
improve to MVFR conditions from north to south later this
afternoon. Further improvement to VFR is expected later this
evening.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...Sherburn
AVIATION...10


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