Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 221131
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
631 AM CDT THU SEP 22 2016

.SHORT TERM...(Today and Tonight)
Issued at 426 AM CDT Thu Sep 22 2016

Tricky short term forecast once again with unsettled weather pattern
continuing today and tonight. Surface low pressure over southwestern
Nebraska will very slowly lift northward over the next 24 hours. An
attendant frontal boundary extends across northeast Nebraska into
northwest Iowa while a fairly weak shortwave likewise lifting into
northeast Nebraska has sparked a broken like of garden variety
showers and thunderstorms. Further east, a large cluster of showers
and storms...with heavy rainfall continues to train near our eastern
forecast border in central IA and MN. With outflows extending from
this convection, there remains some concern that this heavy rainfall
will backbuild further into our area, especially with the arriving
shortwave. Not a great deal of rainfall has occurred in the flash
flood watch area so far, but with the wave quickly tracking into
this area and likely to exit before mid morning, think that the
flash flood threat for today is quickly diminishing. The exception
may be across our eastern tier of counties where backbuilding may
occur for the longest. Will leave these counties in the watch, and
drop the western two rows of counties.

Otherwise, low stratus will continue to blanket the region behind
the boundary, leading into a dreary fall day. While the stratus will
lift slightly through the day, the clouds and cool northeast flow
behind the boundary will high temperatures in the mid 60s to lower
70s. Ahead of the front in the warm sector, the upper 70s and lower
80s will accompany humid dewpoints within a few degrees of 70.
Similar to yesterday, there will be strong differential heating and
decent instability in the warm sector, but shear and forcing looks
weak. Given the capping in place, the severe threat is marginal at
best.  Cannot completely rule out a rogue severe storm with quarter
hail or a gust to 60 mph. Spotty showers, sprinkles, and
thunderstorms will remain possible midday today into the afternoon
behind the boundary, but should not pose a heavy rainfall or severe
threat. Will keep an isolated to scattered pop going into the
afternoon with the weak isentropic lift. Pops increase to scattered
areawide overnight as the boundary makes a northward push through
the overnight hours. Lows tonight will remain seasonally mild, only
dropping into the upper 50s to mid 60s.

.LONG TERM...(Friday through Wednesday)
Issued at 426 AM CDT Thu Sep 22 2016

Strong warm frontal boundary will be lined up along the southern
tier of the CWA at daybreak Friday, with an abundance of lower
clouds north. Wind fields within the sharp ridge aloft remain
substandard to support any broader organized convection, with an
arcing band of isolated to scattered elevated activity at best with
better coverage perhaps near stronger low level jet focused toward
south central SD.  Ahead of large scale trough pushing through the
Rockies, unidirectional deep southerly flow will work to shove the
frontal boundary and elevated lift zone northward through the
morning and early afternoon, gradually removing chances for
precipitation from south to north. Will be a process to erode the
widespread lower clouds, perhaps even with a little fog and drizzle
thrown in for good measure during the morning. South of the warm
front by mid afternoon, temps will again reach the 80s, warmest in
the Missouri valley.  Some question as to whether boundary will be
able to fully clear the far northeastern CWA in time to allow lower
clouds to erode and temps to warm a great degree, and have held on
to a few lower 70s around KMML to account for a bit more pessimistic
forecast.

Friday night into Saturday, large upper low pressure area will lift
gradually northward toward eastern Montana and western North Dakota.
A lobe of lift forcing will shear out across the plains, and work to
interact with a deep moisture trajectory streaming north through the
eastern plains.  Models have trended a bit quicker with the eastward
progression of this moisture conveyor and also with the frontal
boundary, so expect showers and thunderstorms to be in the James
valley by daybreak Sat, and somewhere near the I-29 corridor around
midday, with best chances for rain southwest MN and northwest IA
during the afternoon and early evening.  Not out of the question
with the strong mid level flow that a storm could be marginally
severe along the boundary working eastward, but instability looks
more representative of a heavy rainfall profile.

Continues to be quite entertaining observing differences in trough
evolution which begin by Saturday evening, and continue through the
remainder of the forecast period. GFS works quickly to evolve a
massive closed low over the northern plains by Sunday which wobbles
around and finally exits through Wisconsin on Wednesday. Ensembles
have no suggestion of a preferred solution. ECMWF/Canadian solutions
break off a fair amount of energy and dig into a home in and around
the Four Corners area through mid week, while much more progressive
with northern trough. Where the northern trough goes, there would be
a least a minimal mention of showers due to cyclonic cold core. At
this point, with confidence in any evolution at low-levels, have
kept forecast fairly close to initialization grids, with exception
of removing some of the pops in the James valley and west areas from
Monday through Wednesday.

The big story of the longer range - daytime temps after Saturday
could be some 20 degrees cooler than much of those from this week
and what will be around on Friday... namely in the 60s...with some
moderation by Wednesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 625 AM CDT Thu Sep 22 2016

Unsettled weather pattern will bring low stratus to much of the
region through the TAF period. Stratus blankets the area north of
a front and includes areas north and west of a yankton to Windom
line. Expect mainly ceilings around 300 to 1500 agl in this area,
with only slight improvement to mvfr during the daytime today.
Additionally, scattered showers and storms will be possible
throughout the forecast period. The best chances will be late this
afternoon into tonight. Have added vcts to KSUX TAf as this
location has the best chances for precipitation, but still
generally less than 40 percent.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...Flash Flood Watch through this evening for MNZ081-090.

IA...Flash Flood Watch through this evening for IAZ003-014-022.

NE...None.
&&

$$

SHORT TERM...
LONG TERM...Chapman
AVIATION...


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