Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
735 PM CDT Monday Oct 20 2014
Short term...(this evening through tuesday)
issued at 405 PM CDT Monday Oct 20 2014
Quiet weather ahead for tonight and Tuesday with the only real
issues fire weather Tuesday afternoon. A ridge of high pressure will
move across the area with southeast winds developing in its wake.
These southeast winds will become a bit breezy on
Tuesday...especially west of Interstate 29. Couple this with fairly
low dew points and a very highs fire danger will exist as relative
humidities fall into the lower 30s. Lows tonight will range from the
upper 30s in the far eastern County Warning Area to the middle 40s in central South Dakota. Highs
on Tuesday will range from the middle 60s in far eastern County Warning Area to the middle
70s in central South Dakota as warm air surges northward behind the exiting
Long term...(tuesday night through monday)
issued at 405 PM CDT Monday Oct 20 2014
Strong southerly flow will continue Tuesday night...but a small
window for a more impactful decoupling will occur for parts of northwest
Iowa where drier air mass in place. Overall...temperatures will be
milder...but may see largely level to slowly falling readings
after quicker initial fall...with clouds increasing west and
gradient increasing east. Strong upper level wave will begin to
push through the northern rockies Tuesday night...with qg forcing
and increase in middle level Theta-E advection pushing into the County Warning Area
late night. Initial atmosphere is quite dry as first pulse of middle
level instability moves in later Tuesday night and early Thursday.
Likely that start of the day will not bring more than thickening
middle clouds...but enough elevated instability that isolated light
showers would not be out of the question...and perhaps even a
rumble or two of thunder.
Better threat for precipitation will come as main belt of deeper
dynamic forcing near trough axis swings out into the County Warning Area during
the afternoon and evening. Trend has been for a little quicker
progression on almost all solutions. Deeper convergence and
marginal instability suggests potential for a decent coverage
of precipitation...perhaps a quarter to third inch in places mainly near
and east of I 29...as well as at least an isolated thunder threat.
With increased clouds and precipitation...temperatures will be somewhat
cooler on Wednesday...mainly lower to middle 60s in the central
County Warning Area...to near 70 in the far west.
Quicker exit means practically no threat for rainfall by
Thursday...and only carried a 20 pop early for the extreme east.
Some significant difference in dealing with lower level
wind/moisture fields between models...with NAM failing to shift
low level winds at all to a westerly direction with trough
passage...while GFS/ECMWF/Canadian give a decent period of
westerly component and a much drier airmass. Raw mixed layer dew
point temperatures are as much as 15 to 20 degrees lower...and would seem
to be a bit preferred in this case...especially west of I 29.
A bit warmer again as a result with more sunshine...middle 60s east to
lower 70s west.
For the extended range...largely a quiet time...with a weak cold
front expected to pass in the Friday night window. Friday becomes
a pre frontal day with a decent potential for westerly mixed
component to wind. Have pushed temperatures several degrees upward on
Friday...with lower to middle 70s shared by all. While Saturday will
continue dry...high pressure ridge settling in from the north will
cool things off just a touch. Light winds and dry air to
start...especially for eastern County Warning Area...will favor quite large
diurnal ranges...and have stretched out a bit over initialization
values. Potential by the end of the period that some impact of the
Hawaiian tropical system will quickly move in western flow into
the vicinity. Structure of troughing is up in the air as it moves
through the region around Sunday night/Monday...so idea of lower
precipitation chances fits at this time. Temperatures cooling back below
normal a bit on Monday.
Aviation...(for the 00z tafs through 00z Tuesday evening)
issued at 732 PM CDT Monday Oct 20 2014
VFR will prevail tonight and Tuesday. One fly in the ointment
tonight is the probability of MVFR dust due to harvesting
generally east of I 29. Tis the season. Will just monitor the kfsd
and ksux taf sites if any of this drains into those taf sites.