Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
536 am CST Friday Jan 30 2015
Short term...(today and tonight)
issued at 327 am CST Friday Jan 30 2015
Unseasonably warm late January temperatures will be on tap today.
Surface high pressure currently centered over northeast Nebraska
will slip to the southeast...allowing return flow to develop this
morning. Winds turn southerly and increase into midday around 10 to
20 miles per hour with some higher gusts possible. The residual band of stratus
across our eastern half will continue to erode through middle
morning...with otherwise mostly sunny skies expected. Some high
clouds may lift into the southern forecast area later in the
day...but there should be ample sunshine and southerly flow for
warming well above normal. Highs will range from the middle 30s in our
far eastern counties to lower 50s in the Lower Brule area.
Mostly clear skies persist through much of the night...with high
clouds beginning to increase towards daybreak as a boundary sags
into the area. Breezy winds decrease this evening...but the
southerly flow should keep lows relatively mild in the 20s.
Long term...(saturday through thursday)
issued at 327 am CST Friday Jan 30 2015
Low level cooling will start slowly Saturday morning but steadily
increase during the day. The cooling will result in surface
temperatures leveling off around noon before dropping slowly.
Clouds will be steadily increasing...first from the south with the
system approaching the Central Plains...then overall as low level
moisture increases in the colder air.
Short wave coming up from the Southern Plains and northwesterly wave
approaching from the northern rockies makes the snowfall outlook
somewhat complicated. There is little doubt that the best snowfall
with the southern system will be south/southeast of the area...but 1
to 3 inches should still accumulate over part of northwest Iowa. The
bigger question is with the northwesterly system and the interaction
of the two systems and the tightening thermal gradient. Believe the
snowfall potential over our eastern forecast area may be underdone
for late Saturday into Sunday morning...but for now will keep total
amounts at an inch or less north of the sux to slb and south corner.
The problem with going 1 to 3 is it is uncertain whether the higher
amounts would be over the northeast corner of the area up toward mml
and mwm in southwest Minnesota...or further south near the Minnesota/Iowa border as
a result of the two systems interacting. This uncertainty...and
northwest Iowa being on the edge of the southern system...is also why
probability of precipitation will not yet be real high.
As whatever snow there is falls Saturday night and decreases
Sunday...cold high pressure will be approaching and will move over
the area Sunday night. The next rapidly moving wave in the strong
west northwesterly upper flow will combine with the developing warm
advection to bring some chance of light snow later Monday.
Another rapidly moving and stronger wave will approach Tuesday...
replace modest warming with another cooldown behind it...and bring
another snow threat. There is a timing problem with the GFS and ec
both now slowing the system to a peak snow producing time of Tuesday
night. The Canadian is faster and matches the current forecast and
extended guidance...which will be followed for now with a Tuesday
snow threat ending by Tuesday night. Another modest surge of Arctic
air...like the weekend surge not as cold as what we had early this
month...will spread in for Wednesday. This will be replaced by
warming just barely getting going by late Thursday...then picking up
steam after the forecast period.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 531 am CST Friday Jan 30 2015
Models keep the narrow band of MVFR stratus across southwest
Minnesota into the Brookings area nearly stationary and gradually
shrink the area through around 17z. Otherwise...expect VFR
conditions through the period. Breezy southerly winds today will
gradually shift northwesterly from west to east overnight...but
will remain around 7 knots or less overnight.