Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 212320

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
520 PM MDT Thu Jun 21 2018

.DISCUSSION...(This Evening Through Thursday)
Issued at 213 PM MDT Thu Jun 21 2018

Water vapor imagery this afternoon indicates a closed low centered
over the Midwest and a progressive series of shortwaves from the
Pacific Northwest into the Northern Rockies, with general shortwave
ridging in-between. Cloud cover over the western periphery of the
Midwestern low is extending across central SD, while areas of
showers and thunderstorms are evident over the Northern Rockies in
advance of the aforementioned shortwaves. Meanwhile, a field of
cumulus continues to build over the Black Hills.

With weak ridging building into the area, generally dry conditions
are expected today, particularly when compared to the past few days
and the next few days. Some afternoon thunderstorms will be possible
over the Black Hills today, where saturated soils and upslope flow
may locally overcome modest capping aloft. A vort max shifting
eastward into the Intermountain West has led to the development of
thunderstorms from central MT southward into western WY this
afternoon. With CAPE of 1000-1500 J/kg and 0-6 km shear vector
magnitudes of 40-50 kt, some strong storms will be possible across
northeast WY this evening before convection moves into a more
stable environment over western SD and weakens overnight.

PWATs and low-level theta-E will increase Friday as better forcing
aloft approaches. Fairly widespread thunderstorms are expected,
particularly over the Black Hills and western SD. Modest shear west
and low-level capping east should limit overall severe potential,
but one or two strong to severe storms cannot be ruled out.

Weak shortwave ridging will nose into the area early Saturday behind
a cold front, which could offer a brief reprieve from rain and
storms. However, with anomalously high moisture content and general
troughing to our west from late Saturday into early next week, it
appears we will again be dealing with several rounds of rain through
at least Monday. Storm-total rainfall through Monday will generally
range from one to two inches across the southern half of our area,
with up to an inch for our northern zones. Locally higher amounts
are possible due to thunderstorms. This rainfall will exacerbate
high river levels and soil moisture content across the area and
could potentially contribute to a flooding threat this weekend
into early next week. The time period of primary concern may be
late Sunday into Monday, when persistent low-level upslope flow
develops near the Black Hills as a strengthening mid-level low
digs south of the area.


.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 517 PM MDT Thu Jun 21 2018

VFR conditions expected most places. Sct SHRA and ISOLD
thunderstorms are expected across northeast Wyoming and far
western South Dakota tonight, with more widespread activity Friday
afternoon. Local MVFR-IFR conds possible in heavier cells.




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