Scientific Forecaster Discussion
Area forecast discussion for western South Dakota and northeastern Wyoming
National Weather Service Rapid City South Dakota
1142 am MDT Wednesday Jul 23 2014
issued at 1141 am MDT Wednesday Jul 23 2014
Upper ridge remains across the region with low level return flow
and thermal ridge bringing warmer air back into to the County Warning Area. 12z
models are trending drier for this afternoon and evening as more
significant upper wave activity stays north of the County Warning Area. Strong cap
in place will keep convection from forming across most of western South Dakota.
Will drop probability of precipitation this afternoon now...and this evening with the
later forecast issuance. NAM shows enough erosion of the cap
across the warmer NE Wyoming to will leave sl chance probability of precipitation this afternoon...and
over the Black Hills area. Hrrr is showing very isolated storms
developing over portions of NE Wyoming into the northwest South Dakota plains into the
Short term...today through Thursday night
issued at 230 am MDT Wednesday Jul 23 2014
08z surface analysis had Lee trough from central Montana into
southeast Wyoming. Weak outflow boundaries noted across southeast Montana
into central South Dakota. Cluster of severe convection heading southeast
across southern portions of the County Warning Area early this morning...spurred
on by weak shortwave over northeast Wyoming per water vapour loop and
30kt low level jet. Behind it...upper ridge building over Montana/Wyoming
with upstream waves over Idaho/western Montana. Short range guidance shows
organized thunderstorms and rain exiting County Warning Area by 12z...but could be a few lingering
Today...upper ridge amplifies into the northern plains with thermal
ridge slowly building. Lee trough shifts east a bit with low
pressure developing over central Wyoming in response to shortwave
rotating around ridge into Montana. Surface high over the Midwest
combined with Wyoming low results in somewhat tight pressure gradient and
strengthening south/southeast flow up to 25kts. Ahead of Lee
trough...2-4kj/kg MLCAPE develops by afternoon with mlcin
weakening...especially after 21z. Direct upstream middle-level
shortwaves hard to depict in guidance or to see upstream on water
vapour...but a couple of weak 700mb shortwaves forecast by models
to shift into eastern Wyoming late in the day. This should provide the
last nudge to allow thunderstorms and rain to erupt. 0-6km bulk shear 15-20m/S with
rather slow right-moving supercell movement...in the 5-10kt range.
0-6km mean wind 15-20kts. With slightly faster movement than
guidance showed yesterday...it appears flash flood potential not
as clearly defined...although precipitable waters do reach 150
percent of normal. Local flash flood composite parameter has
backed off noticeably. Thus will hold off on any water headlines
at this point and pass on concern to day shift. Temperatures will
be near guidance in the west and near bias corrected values in the
Tonight...low pressure consolidates over eastern Montana with warm front
across northern South Dakota. Activity will slowly shift east as 30-40kt low
level jet develops/intersects warm front...translating best
850-700mb Theta-E advection east of area after midnight.
Temperatures will stay up given persistent southerly flow.
Thursday...upper ridge flops over in response to shortwave/upper low
moving across southern BC/ab. Low pressure over eastern Montana will move
into central South Dakota. Ahead of it...strong thermal ridge peaks over County Warning Area
with dry line surging into southwest South Dakota from Wyoming. Trailing cool front
behind low will drop into County Warning Area in the afternoon. 1-2kj/kg Post
frontal MLCAPE develops over the northern half of County Warning Area. 2-3kj/kg
MLCAPE in warm sector likely...but mlcin will be much stronger
there. 0-6km shear sufficient for supercells Post frontal in the
north...with marginal values in the south...but deep mixing may
promote severe wind gusts with any convection that develops over
southwest South Dakota. Expect severe storms later Thursday afternoon in the
far north. Temperatures will be slightly below guidance Post frontal
and have adjusted near/above guidance south of dry line.
Thursday night...cool front surges through County Warning Area with breezy spots
behind it and convection moving east of the County Warning Area overnight.
Temperatures near guidance.
Long term...Friday through Tuesday
issued at 230 am MDT Wednesday Jul 23 2014
Upper-level ridge breaks down by the end of the week as a strong
shortwave trough slides southeast across south central Canada. This
will bring another chance of showers and storms to the area on
Friday into Saturday.
As the shortwave trough slides east...the upper ridge will retrograde
west...bringing predominantly northwest upper flow to the northern
plains. This will result in cooler temperatures for the rest of the
extended period...along with mainly dry conditions.
Aviation...for the 18z tafs through 18z Thursday
issued at 1115 am MDT Wednesday Jul 23 2014
VFR conditions are generally expected through the forecast period.
Isolated thunderstorms are possible late today across northeast Wyoming
and tonight across northeast Wyoming and much of western South Dakota.