Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241136

Area Forecast Discussion
National Weather Service Sioux Falls SD
636 AM CDT TUE MAY 24 2016

.SHORT TERM...(Today and Tonight)
Issued at 432 AM CDT Tue May 24 2016

Should be on the quiet side during the daylight hours today, with
areas of fog in areas east of the James River Valley early this
morning in the wake of MCS pulling east through central Iowa. Most
areas will see mostly sunny skies today, though will see some early
morning stratus associated with the aformentioned fog, and a cirrus
shield expanding northward from convection in northern Kansas will
brush through northwest Iowa this morning as well.

This line of convection has pushed main surface boundary well south
of the forecast area this morning, with another boundary/moisture
discontinuity located from north central Nebraska through northeast
South Dakota into the Red River Valley. The latter boundary weakens
as it pushes east through the day, but the greatest model consensus
still depicts the drier low level air behind the boundary expanding
over most of eastern South Dakota/western Minnesota by mid to late
afternoon. With shortwave mid-upper level ridge moving northeast
across eastern South Dakota through 25/00Z, have favored high-res
model solutions which keep any convective redevelopment suppressed
to our south and west in Nebraska and western South Dakota into the
evening. Expected sunshine and fair mixing in the drier air mass
should allow most areas to climb into the lower 80s for highs today.

Concerns for tonight hinge on timing of mid-upper level wave and how
quickly it can drag the low level warm front back into our forecast
area. Wave begins to impact the forecast area during the evening,
but currently appears as though warm front and associated low level
jet convergence will remain focused in Nebraska until after 06Z. As
a result, have trimmed pops back through the evening, but quickly
ramp pops up into high likely range around 06Z as the wave swings
through and elevated warm front attempts to lift toward the Missouri
River Valley later tonight. NAM forecast soundings show decent mid
level lapse rates and modest effective shear in the late afternoon
and evening, and if storms are able to develop prior to 06Z, could
see some hail threat from what should be largely elevated storms.
Greater threat should remain south of our forecast area, however,
closer to the low level boundaries. Strong moisture push from the
south through the evening brings precipitable water values back to
near 1.5 inches, and as low level jet turns more southwesterly in
response to the passing wave, we could see threat of locally heavy
rain become primary concern overnight.

.LONG TERM...(Wednesday through Monday)
Issued at 432 AM CDT Tue May 24 2016

Wednesday is a difficult forecast as there is likely to be ongoing
convection to start.  Corfidi vectors suggest convection should work
east through the morning hours.  As the atmosphere warms in the low
levels, forecast soundings suggest the potential of convection
redeveloping east of the James river with strong enough heating.
Despite searching, could not find a strong trigger mechanism, which
may limit the coverage of any convection that is able to get going.
As such, kept pops mainly in the low chance range.

Cold front is is expected to work across the western half of the
forecast area Wednesday night.  Upper low is passing to the north
across North Dakota, and front across the forecast area is really
suffering from a lack of support aloft.  As such, appears as though
the front will struggle to move completely through the forecast
area.  Have highest pops in the north, but am not convinced large
scale convection will build far enough south into the forecast area.

Airmass ahead of the orphaned boundary becomes unstable throughout
the day on Thursday.  Very steep lapse rates from 700-600 mb leads
to 1500-2500 J/kg of CAPE.  In addition, there is nice directional
and speed shear, potentially leading to bulk shear of 25-30 m/s if
convection is able to become surface based.  The question is whether
convection will be able to become surface based with elevated
inversion.  Model sounding appear to artificially erode the cap too
quick. There is southwest flow aloft, but not a strong wave moving
through the area until Thursday night into Friday. Have continued to
raise pops with this feature. With a fair amount of clouds on Friday
into Saturday, favored cooler guidance values.

Not as much convection is expected through the latter half of the
weekend; however, Sunday night appears to be interesting with
potential low level jet developing across the plains.  Southwest
flow remains aloft across the area with troughiness over the west
coast, so there is the potential of a shortwave or two lifting
through the region. Have maintained chance pops Sunday night and
beyond, but at this time, is too far out to discern the details.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 635 AM CDT Tue May 24 2016

Areas of IFR-LIFR ceiling/visibility mainly north of I-90 and
along/east of the James River Valley early this morning expected
to diminish by 14Z as daytime heating begins to heat the boundary
layer airmass. After 14Z-15Z, VFR conditions expected to dominate
through this evening with light and variable winds. Isolated
thunderstorms could develop during the evening, but confidence in
this development remains low and have not included mention in TAFs
at this time. The chance of thunderstorms does increase across
the region after 04Z-06Z. Brief periods of MVFR or lower ceilings
and/or visibility are possible with these thunderstorms, however
with uncertainty in timing/location, will hold at lower end of VFR
range for now.


.FSD Watches/Warnings/Advisories...


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