Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 142322
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
522 PM MDT Mon May 14 2018

.DISCUSSION...(This Evening Through Monday)
Issued at 208 PM MDT Mon May 14 2018

20z surface analysis had weak high over the CWA with several
boundaries, one across northern MT through northern MN. Another
from southwest WY into southeast MT. Water vapour loop had upper
low over NV and a weakening circulation over southeast MT. 500-
750J/kg MLCAPE with little MLCIN noted over much of the CWA per
SPC Mesoanalysis. 0.5km visible loop showing CU over much of the
CWA with two clusters of convection. One near the Black Hills and
one under the southeast MT 500mb circulation. 0-6km bulk shear
decreasing as weak winds aloft drift over the CWA. Outflow
dominant storms will be the main mode. Convection this
afternoon/evening should be die off after sunset with little upper
support and loss of solar insolation. Temperatures will be near
guidance.

Tuesday, southeast MT circulation dies in response to upper ridge
popping up in it`s place over southwest Canada into the northern
Plains as NV upper low dies off. Very little shear and forcing
for Tuesday with 500J/kg MLCAPE by afternoon. Might see some TSRA
rooted over the higher terrain, but should be weak. Temperatures
will be near guidance.

Wednesday, a new upper low moves onto the CA coast turning flow
southwest aloft over the CWA as aforementioned upper ridge dies
off. Return flow will support an uptick in boundary layer moisture
in the low levels and 1KJ/kg MLCAPE. Increasing flow aloft will
bring 15-20m/s 0-6km bulk shear. A few strong to severe storms
may develop in the afternoon/evening as lee trough slips into far
western SD, interacting with the higher terrain.

Thursday, NAM/GFS different on placement of robust low level
moisture return. Both show significant SBCAPE as well as sufficient
shear for supercells, but differ in placement of moist tongue and
frontal boundary which will slip south through the CWA. There is
some support for post frontal active convection. A day to be
watched.

Friday through Monday, upper low moves into the central CONUS and
becomes a disorganized large scale loose trough. CWA will see cooler
temperatures given cloud cover and colder 850-700mb temperatures.
Unsettled weather is the word.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 519 PM MDT Mon May 14 2018

Scattered showers and thunderstorms are expected this evening.
Strong thunderstorms with large hail and heavy rain are possible
early this evening. MVFR conditions are possible with the
heaviest precipitation, otherwise VFR conditions are expected
through the forecast period.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...10



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