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Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
527 am MDT Tue Jul 16 2019

Discussion...(today through monday)
issued at 243 am MDT Tue Jul 16 2019

08z surface analysis had meso-high over the County Warning Area as outflow
boundaries from earlier convection has pushed surface boundary
into central NE stretching west into central Wyoming. Water vapour had
shortwave pushing east of the County Warning Area. Looking upstream, not much in
the way of energy over WY, so it should be a relatively quiet
early morning. Attention turns to potential for severe weather
today and tonight.

Today/tonight, active west-southwest flow aloft continues. Current
water vapour and guidance suggests a shortwave will come out of
southern Idaho and impact the County Warning Area late this afternoon/early evening and
then another across Montana/ND overnight. Weak low should develop along
aforementioned frontal boundary over central WY, which will gently
increase low level return flow. Most guidance suggests at least
2kj/kg MLCAPE will develop late afternoon over much of the County Warning Area with
perhaps more east of the Black Hills. Mlcin should keep things in
check until about 21z, save for the higher terrain in the early
afternoon. Hodographs lengthen in the afternoon and especially the
evening hours when modest low level jet develops. 0-6km shear
increases from about 35-40kts to 50-60kts. Coverage in the afternoon
will be modulated by the initial shortwave timing. Suspect a
supercell or two will develop after 21z with coverage increasing in
the evening hours with bowing segments possible. Low level shear
pumps up early in the evening, too, with isolated tornado threat
in a small diurnally driven window. Slight risk on target with
potential for very large hail with any supercells. Additional
shortwave energy will bring elevated convection to the north
overnight tonight. Temperatures will be near guidance.

Wednesday, early morning convection ends with marginal risk of
isolated severe storms in the southwest for the afternoon/evening
hours given moderate instability/shear. Temperatures will cool off,
but be near guidance.

Thursday/Friday, westerly flow should bring welcome drier weather to
the County Warning Area. Fast moving upper trough supportive of low pops for the
weekend. Temperatures will be seasonally warm.

&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 523 am MDT Tue Jul 16 2019

Isolated/scattered thunderstorms will develop across the area
this afternoon and evening. Severe storms are likely with large
hail, very gusty winds, and torrential rainfall expected. LIFR in
and near storms. Otherwise, VFR conditions expected.

&&

Hydrology...
issued at 243 am MDT Tue Jul 16 2019

Forecast supercell motion this afternoon/evening less than 10kts
over northwest SD, but 850-300mb mean wind 25kts. Pwats will be at
least 150% of normal. Additional storms with secondary wave overnight
night could bring storms that move, but might fall over the same
area. After collaboration with neighbors, confidence in location
isn't high enough to hoist Flash Flood Watch at this point.
Perhaps 12z cams will better define the threat and day shift and
reassess.

&&

Unr watches/warnings/advisories...
South Dakota...none.
Wyoming...none.
&&

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