Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191029

Area Forecast Discussion
National Weather Service Sioux Falls SD
429 AM CST Sun Feb 19 2017

.SHORT TERM...(Today and Tonight)
Issued at 429 AM CST Sun Feb 19 2017

Today is likely to be one of the warmest days we`ve experienced in
some time. With daybreak temperatures starting out 5 to 10 degrees
above our normal daily highs, it won`t take long to really feel the
warmth today.  Breezy winds should promote mixing up to 900 mb,
again pushing highs well into the 60s.  70 degree readings seem
possible as well especially along the Missouri River valley region.
At the surface, mid-40 dewpoints have already reached southern
Nebraska with near 50 dewpoints anticipated by this afternoon.

By this evening, low level moisture transport really increases as
the LLJ intensifies. Boundary layer dewpoints surge into the lower
50s, which in many winter months would mean widespread fog. However,
this has been anything other than a typical February, and we`ll
rather end up with low stratus.

Theta-e advection increases through the night, but the best synoptic
forcing won`t reach the region until after midnight and closer to
daybreak Monday. Soundings again look marginal for any thunder
potential, but wouldn`t rule out an isolated rumble as a few showers
begin to develop into Monday morning.

The other story tonight will be extreme warmth over the region.
Cloud cover, persistent southerly winds, and high dew points will
hold overnight temperatures in the middle to lower 50s.  Basic
climatology searches looking at peak hourly temperatures during
February reveal that we`ll likely set new records for the warmest
hourly readings on record for the month of February.

.LONG TERM...(Monday through Saturday)
Issued at 429 AM CST Sun Feb 19 2017

Band of showers will likely start the day fairly close to I-29 and
eastward, shearing north and east fairly quickly during the morning
with punch of strong meridional jet across the area. Coverage of
precipitation should continue to improve as the area lifts
northeast. Not more than isolated rumbles of thunder are expected
with instability profiles fairly small and thin. Behind the band,
wind fields should veer and result in reduction of QG lift forcing
back toward the location of approaching lower- to mid-level boundary.
Instability component for additional precipitation beyond the early
morning band is fairly dismal, as prospect for significant
destabilization along the frontal boundary is fairly small at this
point, and have only mentioned an isolated thunder threat with the
fairly low chance PoP.  Should have quite a bit of cloudiness ahead
of the boundary, but a window to thin somewhat near and after
passage when airmass will still be at its warmest. Therefore, with
the very mild start propped up by the high surface dewpoint, should
have a chance to mix into the lower to mid 60s many areas,
especially with increasing westerly winds behind the front.

Weak surface ridge slips across the area during the evening and
early overnight, with drier air allowing for temps to drop a bit
more toward midnight, and likely keeping temps from remaining high
enough to set an all-time monthly high minimum temperature (44
degrees). Ironically, temps could easily begin to rise again later
in the night, especially at elevation, as southwesterly low-level jet
becomes established.

Tuesday still on track to be the warmest day of this exceptionally
mild pattern.  Very little on the negative side to potential warming
considering lack of clouds, flow direction with a westerly
component, and warm start to the day. Numerous records will
certainly fall, some by potentially large margins.  while a frontal
boundary will push across the area later Tuesday night or Wednesday
as a trough slings along the International Border.  Not a great deal
of cooling initially, so Wednesday will remain quite mild in the
upper 50s to mid 60s.

Confluence of northern/southern streams evolves going through
Wednesday night and Thursday, with approach of the southern stream
wave the most significant element of the forecast package despite
being in the day 4-6 period. Operational 00z GFS is an weaker, more
open, and more southern outlier within a pretty diverse ensemble
set, and support remains for a stronger closed system in
EC/Canadian. Would be premature to slide into a GFS fairly null
solution at this point, and other than a subtle southward shift,
have not made too many sensible weather changes during the Thursday
night through Friday night period. Confidence in system existence
remains high, details at this point quite a bit lower.

Precipitation which breaks out Thursday will likely be battling a
drier east to northeast lower level fetch, setting up potential
issues with wet bulbing and timing of rain to snow transition on the
front side of the system.  Period of overnight Thursday night into
Friday morning could potentially have most impactful weather in
terms of wind and snow accumulation.  Best to keep a watchful eye on
the progress of the system which is still well off the West Coast
this morning as it approaches the region late week.

One thing for sure, much more normal February temperatures will
follow for the weekend, likely the first below normal period in two


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1052 PM CST Sat Feb 18 2017

VFR conditions will prevail through the period for most areas,
though far eastern portions of the forecast area near the Highway
71 corridor may see patchy MVFR-IFR visibility in fog through
early Sunday morning. Stratus will begin expanding northward
toward the area Sunday night, but at this time it appears as
though sub-VFR ceilings should remain south of the area until
after 20/06Z.

Light southeast to south winds tonight will increase from the
south on Sunday, with occasional gusts to near 25kt possible
Sunday afternoon/evening.




LONG TERM...Chapman
AVIATION...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.