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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
829 am MDT Sat Sep 5 2015

Update...
issued at 826 am MDT Sat Sep 5 2015

14z surface analysis had frontal boundary from northwest Minnesota to the
Montana/Wyoming border with a low near kpir with trough into eastern Colorado.
Shortwave energy over southwest South Dakota/eastern Wyoming supporting band of
-shra/isolated -tsra over eastern reaches of the County Warning Area. Overnight
and new guidance in good agreement on taking majority of this
activity out of the County Warning Area by early afternoon. Attention then turns
to upper trough/low over ID/WA/OR. Likely to be a large dry slot
with this system...with best chances for thunderstorms and rain west/north of County Warning Area
late this afternoon/tonight.

Forecast updated to account for latest trends/timing.

&&

Short term...today through Sunday night
issued at 238 am MDT Sat Sep 5 2015

Lead impulse in the process of traversing the forecast area...supporting scattered
rain showers/isolated ts across portions of the forecast area. This system will shift
quickly NE this morning as the stronger/deeper Pacific northwest upper trough
ejects into the northern plains...supporting pressure falls and
hanging up the Lee side surface cyclone over the region. Positive Theta-E
advection regime this morning will support an advancing area of rain showers/ts
across the South Dakota plains...with the best chances for coverage over
scentral South Dakota this morning. Wake subsidence will quickly spread over
the region this afternoon...with dry conds expected most
places...save perhaps an isolated ts around Tripp County. Warm conds
expected today with 80s and 90s forecast...especially over the SW half
where sink SW flow will be in place. Attention then turns toward
the advancing Pacific northwest upper trough and its associated potent upper
jet streak. Main lobe of lsa and associated uvm will remain northwest of
the forecast area with this system tonight...with the associated dry slot
region over NE Wyoming/western South Dakota. This will highly limit precipitation
chances to perhaps the far northwest zones where a shower or storm may
clip this area. There will also be a slight chance for rain showers/ts
activity across scentral South Dakota after midnight as the ll Theta-E ridge
and enhanced lift from the right rear entrance region of the upper
jet streak will align with steep middle level lapse rates. This
activity may linger into Sunday morning. Lack of appreciable
cape/forcing/shear overlap will support benign ts across the
region...with perhaps a few gusty winds at most. Upper trough will
shift east into Sun morning...with a decent cold front expected
across the region. A period of gusty winds is expected just Post
frontal. However...lack of deep layer linkage with SW flow
remaining aloft will support a quick wane in winds...with flow
veering east-southeast by evening most places. Much cooler conds expected
Sunday with highs in the 60s and 70s. Another impulse will push
into the region Sunday night with steep middle level lapse rates and
positive Theta-E advection possibly supporting a few rain showers/ts over the
southern half of the forecast area. Did add a low pop for this.

Long term...Monday through Friday
issued at 238 am MDT Sat Sep 5 2015

Seasonable temperatures can be expected for Labor Day
and the first half of next week as zonal flow aloft develops across
the region. Will keep a chance for showers and thunderstorms across
south central South Dakota Monday into Monday night as a weak disturbance
passes through more favorable moisture in place over that area.

A mainly dry cold front is prognosticated to pass through the region
sometime on Wednesday...bringing below average temperatures for the latter
part of the week. An upper level disturbance could bring some
showers to portions of the area on Thursday...especially the Black
Hills area.

&&

Aviation...for the 12z tafs through 12z Sunday morning
issued at 545 am MDT Sat Sep 5 2015

VFR conditions generally expected across the area today
into tonight. Isolated to scattered showers and thunderstorms will
continue across western and south central South Dakota this morning. Brief
MVFR conditions are possible with any heavier showers. This
activity should move east of the forecast area by early afternoon.



&&

Fire weather...
issued at 238 am MDT Sat Sep 5 2015

Dry gusty SW flow will develop today across the northeast Wyoming
plains. Sustained winds of 15 to 25 miles per hour with higher gusts are
expected over much of Campbell and portions of Weston counties.
Relative humidity/S are forecast to drop to around 15 percent over southern
Campbell County this afternoon...supporting critical fire weather
conds with the gusty winds. Have issued a red flag warning for
this. Carried the red flag headlines through Sunday afternoon
given poor relative humidity recovery tonight/a frontal passage/and very dry
gusty SW flow expected once again Sunday.

&&

Unr watches/warnings/advisories...
South Dakota...none.
Wyoming...red flag warning from noon today to 8 PM MDT Sunday for wyz259.

&&

$$

Update...helgeson
short term...jc
long term...26
aviation...26
fire weather...jc

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