Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
505 PM CST Wednesday Dec 17 2014
Short term...(this evening through thursday)
issued at 317 PM CST Wednesday Dec 17 2014
High pressure continues to slowly shift off to the east tonight.
Expect clouds to increase from west to east through the night. Given
the light winds...pretty good radiational cooling expected before
the cloud cover arrives...thus low temperature forecast dependent on
timing. Looks like best area to see readings drop into the single
digits to around zero is southwest Minnesota where clouds should be
last to arrive. In other areas would expect a fall this
evening...with slowly rising temperatures after clouds arrive. Upper
wave will cross the region tomorrow. Lift is pretty weak
though...with the better forcing staying to our south and north. In
general...the lower and middle levels become pretty saturated as the
wave crosses. Although will be fighting a dry layer. Question
becomes whether this can be overcome and result in some light
For now will carry a slight chance for light snow in our south from
northeast Nebraska into adjacent areas of northwest Iowa...as this
is where saturation and lift looks greatest. Would not be surprised
to see a few flurries further north as well...just about anywhere in
the County Warning Area. But not expecting anything that would cause any issues.
There is some question about saturation into the ice growth region
and resulting drizzle potential. Most solutions suggest at least
initially we are saturated deep enough to support flurries...with
the moisture depth decreasing in the afternoon. However at this time
feel like we will not be quite saturated enough...and lift will be
weak enough...to prevent any freezing drizzle threat. But will have
to monitor trends...as can not completely rule it out tomorrow.
Slightly warmer conditions expected tomorrow with middle 20s to around
30 expected...but snow cover and weak mixing should prevent US from
getting any warmer.
Long term...(thursday night through wednesday)
issued at 317 PM CST Wednesday Dec 17 2014
The primary concern in the Middle Range is the development of low level
stratus and any precipitation that may develop within this layer.
With middle level warming arriving Thursday night into
Saturday...southerly winds tap into some increased low level
moisture which should become trapped under the temperature
inversion. While some decease in cloud coverage will be
possible...think that the bulk of the forecast area will remain
blanketed for much of this time frame. Again south central South Dakota will
see the best potential for some clearing...but will hedge towards
overcast skies elsewhere. Without much lift or forcing...measurable
precipitation does not seem likely. Model soundings suggest that the
cloud layer depth is marginal for some patchy drizzle...with the
best potential seen in the NAM. The GFS is a bit too shallow with
this layer so will leave out any mention for now. Temperatures
continue to moderate both days...with highs in the upper 20s to middle
30s Friday and a few degrees warmer on Saturday. If drizzle were to
develop...much of it would fall in temperatures warm enough for
freezing drizzle concerns. Certainly something to keep monitoring in
future model runs...but with low confidence in the low level
moisture depth profiles...will leave out mention for now.
Still looking at some fairly significant model differences for the
first half of next week as troughing over the northern plains ramps
up. The differences in depth and track of this trough is still
uncertain...beginning with some modest lift arriving Sunday night
with some modest warm air advection. Temperature profiles are tricky
at this point given the somewhat large pattern differences...which
places some concern with precipitation type overnight. Will leave in
mention of rain and snow...but warmer profiles could spell rain or
possibly even freezing rain with any light precipitation that does
fall. The European model (ecmwf) continues to dig the trough furthest
south...although the GFS is starting to fall in better alignment
with this solution. The Gem has more of an open wave feature...but
does eventually deepen the low to our northeast over Lake Superior.
The European model (ecmwf) slow the upper low and deepens it over our eastern half
before it slowly wobbles across the Great Lakes...while the GFS is a
little further to the east.
Will go with a rain or a rain/snow mix on Monday...which will switch
over to all snow Monday afternoon/evening. As the upper low heads
east. The European model (ecmwf) paints a decent amount of quantitative precipitation forecast with less seen on the
GFS and much less at the tail end of the Gem. Will need to watch
this system in the future as it could have some significant impacts
to Holiday travel with multiple precipitation types possible and
plenty of wind to go along with it.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 501 PM CST Wednesday Dec 17 2014
IFR stratus deck remains west of the terminals this evening.
Southeast winds within the cloud bearing layer should keep this
stratus away from the terminals for much of the overnight hours.
However by midnight...winds aloft will turn southwesterly which
will allow this stratus to advect northeast.
Current indications point to scattered development ahead of the
stratus prior to daybreak...with IFR ceilings moving in through
Thursday morning. At the surface...winds will light for the next
few hours...before turning southeasterly and increasing in speeds
Will need to watch for a bit of very light snow or perhaps even
freezing drizzle in Sioux City. Will not include in this taf...but
may hint at reduced visibilities in later tafs.