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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
issued by National Weather Service North Platte NE
535 PM MDT Wednesday Apr 1 2015

Short term...tonight through Thursday
issued at 209 PM MDT Wednesday Apr 1 2015

Strong winds continue this afternoon in the wake of a cold front
that moved through the County Warning Area overnight. Pressure rises are beginning
to decrease as the surface low kicks northeast into southern winds should slowly begin to decrease late this
afternoon and early evening. Relative humidity recovery will be slow this evening
as dry air advects into the area out of southeast Montana where
dewpoints in the single digits and low teens are being reported.
However diurnal cooling and the decreasing winds will be enough to
substantially improve fire weather danger after dark.

A shortwave trough and attendant surface low tracks eastward along the
Canadian border Thursday. Northwesterly 850mb winds increase to near
50 kts on the back side of the system. Expecting another windy day
especially in northwest South Dakota where gusts in excess of 40 miles per hour
at times are likely. The northwesterly flow will also bring in
another reinforcing shot of cooler air. This will limit highs to the
low to middle 50s with cooler readings in the Black Hills and northwest
Wyo. Trimmed back probability of precipitation for Friday afternoon as most model guidance
has trended on the dry side. Most of the County Warning Area will be dry as precipitation
remains closer to the aforementioned shortwave along the Canadian
border. Another area of precipitation will likely develop just south
of the County Warning Area as another shortwave ejects out of eastern Idaho and into
southeast Wyo. Left slight chance probability of precipitation in the grids for northeast
Wyo and extreme southwest South Dakota during the afternoon...can't rule out
that a shower or two will clip the area.

Long term...Thursday night through Wednesday
issued at 209 PM MDT Wednesday Apr 1 2015

Forecast concerns through this period lie mainly with low
precipitation chances through much of next week as the weather
pattern becomes more active.

Models are in decent agreement through the weekend...but begin to
diverge early next week leading to a high degree of uncertainty in
the forecast for next week.

Beginning Thursday night...a shortwave trough will be moving
across the northern plains which will bring some cooler air
southward. This air will be drier as well with good subsidence
seen from the models so not expecting any precipitation as it
passes through. With the colder airmass in place and as winds
diminish overnight...will continue with lows in the low to middle 20s
across much of the forecast area. On Friday...the colder air will
push eastward allowing warm air advection across eastern Wyoming
and western South Dakota. 850mb temperatures increase from below
0c across the entire forecast area Thursday night to a range of 0c
to 5c /east to west/. With this warmer air moving in...looking for
highs Friday near normal...generally in the 50s with the exception
of the Black Hills which will be a bit cooler. Do have a slight
chance for rain showers Friday afternoon over the northern part of
the forecast area as a weak shortwave trough moves out of Montana
and into northwest South Dakota. Cross sections show a decent
layer of dry air below the saturated layer...but also shows decent
lift in the saturated layer so felt there could be some light rain
showers with light quantitative precipitation forecast/S of a few hundredths in those areas.

After Friday afternoon...ridging builds into the region for the
weekend with dry and warmer air moving into the area. Very little
in terms of moisture will be seen through any levels so
anticipating little cloud cover from Friday night into Sunday.
With the warm air-mass in place over the weekend...temperatures
will be near or above normal. A cold front moves through Saturday
night...and some area may see higher winds to keep the boundary
layer mixed to keep temperatures from getting too cold overnight
with lows forecasted in the 30s. The colder air will lag a bit
behind the initial cold front which will move south across South
Dakota through the day Sunday. Concurrently southerly flow will
persist into Wyoming which will keep warmer temperatures over the
southwest part of the forecast area. the colder air
moves in through the day...slightly cooler temperatures are
expected over the northern parts of the forecast area Sunday...with
highs in the 50s...southern South Dakota should see highs in the
middle to upper 60s prior to frontal passage as warm air and more
sunshine is expected.

Again...the model divergence begins Sunday night as a fairly
strong shortwave trough moves onshore over the Pacific northwest.
The GFS and Gem push out an initial wave into the plains through
the day Monday...then keep a the main shortwave off-shore. The
European model (ecmwf) keeps the majority of the system off shore while bringing
much weaker energy into the region early in the week and is very
slow to start transitioning the the main system east across the
Continental U.S..the GFS/Gem solutions would bring a higher chance for
precipitation through the early parts of the week. The models are
in decent agreement with the main system moving into the central
part of the Continental U.S. For the middle part of the week. With the
discrepancies in model solutions...a general blend of the
solutions was used...with slight chances for precipitation through
much of next week. Either way...cooler air should be in place
which would support a changeover to rain/snow or snow in the
evening and overnight periods for the Black Hills and possibly
northwest South Dakota most nights. Daytime temperatures look to
rise into the 40s and any precipitation would change over
to rain during the day.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 529 PM MDT Wednesday Apr 1 2015

VFR conditions will continue across western South Dakota and
northeast Wyoming through the current taf period. Strong northwest
winds currently gusting in excess of 30 knots will begin to
decrease over the next couple hours...dropping to around 10 knots
for the overnight hours. The winds will restrengthen after 14z
Thursday with northwesterly gusts near or exceeding 30 knots


Fire weather...
issued at 209 PM MDT Wednesday Apr 1 2015

Critical fire weather conditions continue across much of western
South Dakota and northeast Wyoming this afternoon. Relative humidity
values are generally around 20 percent...and may drop a little
further in the next couple hours before beginning to recover early
this evening. Strong northwest winds gusting above 40 miles per hour have been
ongoing for several hours now...but should begin a downward trend in
the next hour or two as a deepening area of surface low pressure moves
off to the northeast. Wind speeds will more rapidly decrease after
sunset as the boundary layer decouples.

High fire danger will be once again be a concern on Thursday as
winds increase and become strong out of the northwest by late
morning. However cooler conditions should keep relative humidity values a little
higher than today...generally ranging from around 20 percent in
south central South Dakota to near 30 percent in the northwest.


Unr watches/warnings/advisories...
South flag warning until 9 PM MDT this evening for sdz260>266.

Wind Advisory until 7 PM MDT /8 PM CDT/ this evening for sdz001-
002-012>014-025-026-030>032-043-046-072>074. flag warning until 9 PM MDT this evening for wyz259-297>299.



Short term...Martin
long term...Brooks
fire weather...Martin

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