Severe Thunderstorm Warning 113, NWS Rapid City, SD

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
526 PM MDT Thu Jul 27 2017

.DISCUSSION...(This Evening Through Thursday)
Issued at 306 PM MDT Thu Jul 27 2017

Current surface analysis shows low pressure over eastern MT, with
frontal boundary stretching southeast into far western SD. Weak
high pressure is centered over northeast ND. Upper level analysis
shows weak ridging over the high plains, with trough off the
pacific NW coast. West-southwest flow over the western states is
bringing disturbances across the Rockies and into portions of the
Northern Plains. Skies are variably cloudy across the area with
temps mostly in the 80s over areas where it has been dry, cooler
over rainy areas. Moist atmosphere is in place over much of the
area, with monsoonal moisture aloft and dewpoints in the 50s and
lower 60s at the surface. Precipitable water values are above
average, some areas 150 percent or more of average. Winds are
south to southeast, breezy in some areas from northeast WY to
western SD.

Scattered showers and thunderstorms have developed over much of
northeast WY and western SD now. Some of the activity has produced
heavy rainfall, especially over portions of the southern Black
Hills, where up to 4 inches of rain was reported in spots earlier
today. Some heavy rain has also fallen over southern Butte County
into portions of far northeast WY. Activity will continue across
the area into tonight, with some areas of heavy rainfall. The best
instability remains over far northeast WY, where CAPE values are
near 1000 j/kg. The threat for a few strong to severe storms
should mostly remain limited to this area through early evening.
More isolated activity will likely linger into the overnight.

A similar setup is expected tomorrow, though the main area for
showers and storms will shift a bit further east to the Black Hills
area and the western SD plains, as the boundary nudges eastward.
Some heavy rainfall is likely again with the stronger activity. The
threat for strong to severe storms could be limited in coverage like
today, with the best chance looking to be across southwest SD at
this point. Highs will be in the 80s to around 90.

The weekend will be very warm and unsettled at times, though chances
for storms should mostly be limited to the afternoon and evening
hours. The better strong to severe storm potential looks to be on
Sunday at this point. Highs will be in the 80s and lower 90s.
Unsettled weather will likely continue into early next week, with
daily chances for showers and thunderstorms expected, as west-
northwest flow remains over the region. Temperatures will remain
near to slightly above average through next week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 515 PM MDT Thu Jul 27 2017

A cluster of TSRA across northeast WY into northwest SD will
propagate east-southeast tonight with local IFR conditions due to
locally heavy rain. VFR conditions will then prevail overnight
into Friday until additional TSRA develop Friday afternoon with
local MVFR conditions.

&&

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

DISCUSSION...26
AVIATION...Helgeson