Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211134
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
634 AM CDT Thu Sep 21 2017

.SHORT TERM...(Today and Tonight)
Issued at 435 AM CDT Thu Sep 21 2017

Expecting another relatively quiet day, although will have to
monitor a couple of items for possible updates throughout the day.
Currently looking at some 4-7kft cloud cover just southeast of the
forecast area, on the southeast side of surface ridge which extends
from northeast Nebraska into eastern Minnesota at 08Z. Ridge axis
weakens through mid morning, allowing stronger southerly flow to
develop and begin an influx of low level moisture by late morning
into the afternoon. Anticipate a strong increase in surface dew
points, from early morning readings a few degrees either side of
50F, to upper 60s-lower 70s for areas east of the James Valley by
mid-late afternoon. Southerly winds aloft also push an elevated warm
front northward through the CWA today, crossing I-90 late morning,
and nearing the northeast corner of the CWA by mid-late afternoon.
Many forecast soundings develop a decent capping inversion with this
warm advection aloft, though some high-res solutions do develop some
spotty activity near the elevated warm front, mainly in portions of
southwest Minnesota after 20Z. Will carry a slight chance northeast
of the Buffalo Ridge for a couple of hours before the warm front
lifts out of the area. If storms do develop, cannot rule out an
isolated strong-severe storm given projected MLCAPE values near or
above 2500J/kg. However, shear is a bit on the low side at 25-30kt,
and mid-level lapse rates marginal as well, so would not expect much
more than an isolated/marginal hail threat. High temperatures will
be well above seasonal normals, with expected readings in the mid-
upper 80s. Still well below record highs, and also below raw GFS
values which appear over-mixed as per its typical bias.

This boundary lifts northeast of the forecast area prior to 00Z,
shifting focus for overnight development north and northeast of our
CWA, as developing 40-50kt low level jet impinges on the boundary
from northeast South Dakota into central-northern Minnesota. Should
be a warm/humid night, with modest southerly winds and dew points in
the 60s hold temperatures in the upper 60s-lower 70s for lows. This
could put some records in jeopardy for warmest low temperatures for
Friday, 9/22. Current records are 70 at KSUX, 71 at KFSD, 68 at KMHE
and 66 at KHON. The latter 2 may have toughest time holding on to a
record, with potential to drop below before the end of the day.

.LONG TERM...(Friday through Wednesday)
Issued at 435 AM CDT Thu Sep 21 2017

Despite the calendar flipping the page to Autumn on Friday, it will
feel much more like the dog days of summer with high humidity and
strong southerly winds gusting to 30 to 35 mph.  Will have to watch
for the potential of some low-level jet stratus early in the
morning, but strong advection likely to shove any development out
fairly early, and with a warm layer aloft, any diurnal cumulus
development below the cap should be fairly minor.  By late day, a
frontal boundary will push toward the James River valley, and will
again have to consider this when considering potential for late day
convection. Ahead of the front, temps in the upper 80s to lower 90s
will be common.

GFS raw profiles look unreasonably too-well mixed on Friday, failing
to keep much of any cap for many locations, and mixing temps well
into the 90s to around 100 while considerably drying out profiles.
Strong preference weighting toward thermodynamic profile of SREF/NAM
which maintain a modest cap, limiting the potential for warming
somewhat as well as trapping appreciable moisture below the
inversion for a uncomfortably muggy day. The exception to this will
be a narrow zone just ahead of the front which will be most likely
to develop a narrow corridor of deep mixing.  Likely that front will
remain weakly capped even by late afternoon, which again will bring
reliance on a deeper lift forcing to break.  A strong jet will be
surging northward through the western high plains in the late
afternoon, but orientation this time around does not take it far
eastward, and does not appear to strongly impact lift forcing as was
the case on Tuesday evening.  However, can still expect to find
enough support between the lesser convergence and weak support of
the jet to initiate an isolated storm toward early evening.  Deep
layer shear remains higher to the areas west of I-29 and especially
toward the James River, reaching 45-55 knots, while 0-3km shear is
likely 25-35 knots, sufficient to storm organization and even a
couple of supercells as the available CAPE is likely around 2000
J/kg. Slight risk from SPC is certainly warranted, and while storm
motion will be much more north than east, there should be risk for
large hail, damaging winds (DCAPE again will be formidable in narrow
zone near expected frontal initiation), and even a couple of
tornadoes (although this would appear a much lower but non-zero
threat). Timing is primarily a 23z-06z window, as eventually will
run into a higher degree of capping eastward which will begin to
take the edge off, and the orientation of shear vectors is not
especially supportive of longer-lived bowing structures. Eastward
progress of storms overnight will be slow, and in all likelihood not
more further than a MML-FSD-YKN line.

Story then turns mainly toward the potential for heavy rainfall
through early next week as the amplification of large scale flow
with stalling of relative ridge/trough locations due to persistent
tropical activity of Jose/Maria in the western Atlantic. This will
set up a largely meridional flow which will supply moisture into a
persistent boundary across the area. There will be periods of lift
enhancement as energy ejects out of the western trough every 24-36
hours.  Timing of these pulses of enhanced convection will be
difficult to determine, suggestive by guidance of current runs to be
around later Saturday, Sunday afternoon, and then late Monday with
the main wave.  Location of heavier rainfall will be very dependent
on boundary location, both surface and aloft (the latter will be
much more resistant to move until main wave comes out on Monday
night and Tuesday). With some uncertainty in the frontal position,
will be hard to diagnose severe potential for each weekend day
with instability decreasing some after Friday with loss of the
strong EML trending toward moist neutrality. There is enough weak
elevated instability that thunder may be present even into Monday
back to location of the 850 hPa boundary/theta-e advection well
back over the cool dome. Temperatures over the period will vary
considerably across the boundary, with differences likely larger
each day. Blended in CONSRaw guidance to sharpen differences
again, and this boosted lows a few degrees ahead of the front in
more persistent southerly winds. Likely a couple of record high
minimum temps could be set Friday into the weekend.

Over the weekend, precipitable water values (while typically
overdone by model forecasts) would still, when reduced to more
reasonable 1.5-1.75" amounts, place toward 99th percentile for KABR
and KOAX raobs. This certainly brings analogs of early fall heavy
rains into the mix, and looking at daily records suggests that a 3-
day rainfall total of record levels for the last half of September
would yield between 2.5 and 4.5 inches sampling several sites, well
within the superensemble of precipitation totals of the last several
days model runs. WPC QPF guidance right in that area as well. We
could be dealing with some record precipitation as well.

Main trough starts to eject out of the southwest by later Monday
into Tuesday, and this should finally give the boundary a healthy
kick to allow cooling and drying. Temps will recover somewhat from
dreary and damp mid 50s to lower 60s behind the front, gradually
moderating to the mid 60s to lower 70s by Thursday. A cool surface
ridge in the vicinity around Wed-Thu a.m., and if things time out
right, could have one of those night be a bit cooler with some 30s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 633 AM CDT Thu Sep 21 2017

Scattered to broken, mainly VFR cloud cover, will build northward
with increasing low level humidity today, mainly in areas east of
the James River Valley. This expected to lift northeast of the
forecast area by this evening, but a stray shower/thunderstorm
cannot be ruled out in southwest Minnesota before the clouds
depart. Increasing southerly flow will also be a concern through
this period. Frequent gusts 20-25kt likely to develop by late
morning today. The gusts will subside with sunset, but a 40+ kt
low level jet will bring areas of non-convective low level wind
shear and occasional surface gusts near 20kt after 03Z this
evening.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

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


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