Area Forecast Discussion
Issued by NWS Sioux Falls, SD

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
000
FXUS63 KFSD 221726
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1226 PM CDT MON AUG 22 2016

.SHORT TERM...(Today and Tonight)
Issued at 356 AM CDT Mon Aug 22 2016

Strong ridging aloft across the Northern Plains will keep conditions
fairly quiet through the next 24 hours, before upper wave near the
Four Corners area starts to make a move into the Central Plains
toward daybreak Tuesday. Starting to see a few mid-level clouds
bloom in southwest Minnesota and northwest Iowa in fairly distinct
theta-e advection zone.  Some high-resolution CAMS indicating a
minimal threat for development of precipitation (likely non-
measurable), but counting on fairly dry lower levels to keep out of
the forecast for early morning.  Otherwise, sunshine, very warm and
windy conditions will prevail today. 25 to 30 knots of wind atop the
mixed layer, so winds will likely settle into the 20-30 mph range
with some gusts to 35 mph by later morning and through at least mid
afternoon.  While overheated GFS/RAP are old news at this point of
summer, mixing does suggest that will see at least mid to upper 80s
east of I-29, with 90s west.  Good news for our comfort is that
humidity levels should remain in check, and the bad news for fire
weather conditions is that humidity levels will remain in check.
Lowest humidity levels will be west of the James River, where south
central SD locations could dip to 25 to 30 percent. Strong winds
will result in some high fire danger south of I-90, and will have to
watch for deeper mixing that could present some even higher danger
toward south central SD. Locations that have missed out on recent
rainfall are probably at a slightly enhanced risk over the areal
average.

Breezy southerly winds will remain a feature overnight, with winds
continuing especially strong near/downstream of the Buffalo ridge.
Another mild night, with some increase in higher level clouds from
the southwest ahead of approaching wave, with temps in the mid 60s
to around 70.

.LONG TERM...(Tuesday through Sunday)
Issued at 356 AM CDT Mon Aug 22 2016

The primary forecast concern for Tuesday through Wednesday night
will be the potential for severe weather and heavy rain from late
Tuesday afternoon into Tuesday night.

There is generally good model agreement on the evolution of the
front, instability, and shear through Wednesday. The NAM is a little
slower than other models with the front progression and generally
followed the faster GFS and ECMWF for frontal timing. All models do
agree a strong upper level wave will move from Nebraska into western
Iowa on Tuesday. A strong low level jet ahead of this wave will
continue to bring very moist and warm air northward with
precipitable water reaching 1.75 in by Tuesday afternoon around
Sioux City. It will also become increasingly unstable with 7 to 7.5
C/km lapse rates moving into western Iowa and eastern Nebraska. At
the same time, a weak warm front with a strong theta-E gradient will
move northeastward toward Highway 20. With afternoon heating, the
cap should weaken to less than 50 J/kg and am expecting convection
to develop sometime late Tuesday afternoon somewhere in northeast
Nebraska and spread toward the Sioux City area as the upper wave
reaches the mid-Missouri Valley. While the NAM Sioux City sounding
shows the 0-6 km bulk shear only around 30 kts, the 0-3 km storm
relative helicity is over 200 m2/s2 which is adequate for rotating
updrafts. This, combined with MLCAPEs around 2000 J/kg, should be
sufficient for organized multicells and perhaps a couple of
supercells. If supercells can develop, cannot rule out an isolated
tornado but 0-1 km shear is marginal so the greater threat will be
damaging winds and large hail. As storm progress into the evening,
they will become more widespread over portions of western Iowa. The
severe threat will lessen as instability decreases but Corfidi
vectors indicate the MCS could be slow moving resulting in the
potential for heavy rainfall primarily southeast of a Sioux City to
Spencer line. Rainfall around an inch with local amounts exceeding 2
inches will be possible although there is uncertainty if the
widespread heavy rainfall will get north of Hwy 20.

Otherwise, a cold front will move across southeast South Dakota and
southwestern Minnesota on Tuesday night. This area will be between
the two upper waves with the main wave along the US-Canadian border
and the southern stream wave over Nebraska and Iowa. With
temperatures warming into the 80s and lower 90s and dew points into
the 60s, a few thunderstorms should develop during the evening along
the front. Not as confident how widespread this convection will be
given there will be a lot of storms in Nebraska and Iowa and the
atmosphere will begin to stabilize after sunset. Since at least a
few thunderstorms will develop during the evening, kept
precipitation chances from 30 to 60 percent but did lower expected
QPF to be generally less than 0.5" although a few places may see up
to an inch of rain, especially in the evening, in the vicinity of
I29 and I90 in SD and MN. The severe threat will be much lower in
this area as shear remains less than 20 kts near the cold front
where storms will develop.

Once the front moves through, rain chances will gradually come to an
end by Wednesday morning. The atmosphere will be well mixed but
highs will be several degrees cooler than Tuesday - generally from
the upper 70s to lower 80s. Drier and cooler air will spread over
the area on Wednesday night with lows in the 50s.

In the extended, Thursday through Sunday, the primary focus will be
a good chance of rain Friday night into Saturday as another upper
wave moves across the area. There is still quite a bit of
uncertainty on the location of the wave. This wave could move a
little farther north than Tuesday`s and if moisture is able to
return quick enough, there is the potential for over a half inch
rain in parts of the area. Thursday and Friday both look cooler with
highs in the 70s. The weekend temperatures are much more uncertain
as the GFS keep warm southerly flow over the area through Sunday
while the ECMWF brings a front through by Saturday night. There also
could be a lot of cloud cover on Saturday if thunderstorms do
develop Friday night.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1226 PM CDT Mon Aug 22 2016

Southerly surface winds will occasionally gust 25-30kt into the
early evening. Gusts should ease a bit after sunset, but the low
level jet over the area will persist, leading to the development
of low level wind shear again from around 23/05z through shortly
after daybreak Tuesday. Approaching upper trough and cool front
will bring a chance of convection to the area later Tuesday, but
any threat of thunder is expected to hold off until after this TAF
period.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Chapman
LONG TERM...Schumacher
AVIATION...JH



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.