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Area forecast discussion
National Weather Service Sioux Falls South Dakota
627 PM CDT sun Aug 25 2019

Short term...(this evening through monday)
issued at 438 PM CDT sun Aug 25 2019

Main concern through this entire forecast cycle comes in the first
12 hours, with the threat for severe storms across parts of
southeast South Dakota.

So far this afternoon, stratus has been a tough customer, with self-
destructive sunshine filling in any meaningful gaps. The break to
sunshine has remained west of the area through mid-afternoon, and as
a result, surface temps have remained in the mid 60s to lower 70s,
rendering the deeper air mass to an unusable state. To the west, the
heating has allowed a narrow zone of MLCAPE from 1500-2000 j/kg to
develop across the western to central parts of the state. In this
zone and within the convergence along a cold front, have seen a
couple of storms develop across central ND, and even a few along the
edge of the Black Hills as the leading lift with a strong jet streak
punches into northwest South Dakota.

Earlier cams as typical just a bit slow with a rough consensus of
initiation for convection along or just Post-frontal from Corson to
Pennington counties by around 22z. There remains a significant
disparity in evolution tonight within the spectrum of solutions,
highlighted by the tardiness of the 18z NAM which keeps virtually
all precip west of I-29 through 12z Monday, then hammers I-29 east
with heavy rainfall during the morning. This seems to be a
convective feedback from later in the night development in the left
jet exit region, which seems to be a bit further eastward and
quicker when comparing to satellite this afternoon. The next effect
on timing after viewing the various options was an overall slower
solution than the last day or two, bringing leading convection into
the area after 02-03z, then another surge pushing through the area
later in the overnight, reaching I-29 by around 09-10z.

In terms of severity, have been gradually walking back the eastern
edge of the threat area through the day with the overall increased
stability of the air mass. While areas west of the James River are
likely close enough to reap benefit from the more unstable air west,
remains to be seen just how far eastward this can translate into
areas where the instability is increasingly high-based. While deeper
shear is quite marginal currently this far east, as the mid-level
wind maximum plows east/southeast through the overnight, will see up
to 40-45 kts of effective shear, more than enough to allow
organization of storms. There would be some support for initial
supercells near the Missouri River, then perhaps a transition to
shorter bowing segments overnight pushing east. For now, would
expect any larger hail threat to be mainly James Valley west during
the late evening and early overnight, with a wind threat extending
another County or so eastward, with by far the greatest severe
threat for areas near Brule-Charles Mix-Gregory counties.

Thunderstorms of a non-severe variety will continue, but probably
starting to lose thundery coverage as we go into the morning hours
with progression to a more stable airmass. Still plenty of dynamic
support for precipitation in the left exit region, so frontal zone
remains a good focus for precip. Going before the low-level boundary
exits northwest Iowa by early afternoon. Elsewhere, as long as
subsidence does not overwhelm, could see at least isolated to
scattered showers/storms redevelop during the mid to late afternoon
over parts of southeast South Dakota near/north of I-90 as secondary troughing
swings through.

As outlined by wpc, much of the area will have a marginal threat for
excessive rainfall, mainly from later tonight through early Monday.
Marginally high precipitable water, deep warm cloud layer, and
possibility of training storms is the primary reason for the threat.

Long term...(monday night through sunday)
issued at 438 PM CDT sun Aug 25 2019

Amplifying trough will dig into the northern Great Lakes through
Wednesday, pushing cooler air into the region. Secondary frontal
passage looks to occur on Tuesday morning. Added in just a small
area for isolated level pops across far east central South Dakota and northern
portions of southwest Minnesota on Tuesday afternoon, as will have a couple
of waves of showers/thunder rotate through the area as the cold core
aloft slides into Minnesota. The first would be closer to early afternoon
hours with pv lobe, and the second more the heating of the cold core
as another wave digs into western Minnesota late day. Winds will not take
too long to increase, likely getting to full strength around the
passage of the pv feature midday, with gusts 30 to 35 mph common.

More quiet conditions on the way from Tuesday night into Thursday.
Will begin the period with a sharp cool down as winds remain up
somewhat Tuesday night before a mixy and windy Wednesday. Winds
should finally diminish under the ridge axis Wednesday night, and
could even be a little non-diurnal as sharp warming and return flow
develops through the overnight. Thursday remains the warmest day of
the forecast period, with a good prefrontal westerly component, and
temps aloft favorable of at least a low of lower to mid 80s ahead of
the southward sinking boundary.

As the front settles southward on Thursday night into Friday, will
have a wave in the northwest flow ripple through, with potential for
enough frontogenetic forcing to generate a few showers mainly south
of I-90 from later Thursday night through Friday evening. Not a lot
of instability, even aloft, so likely more showers than thunder.
With the general north to northeast low level flow and clouds, will
again be looking at cooler temps in the upper 60s to lower 70s for
most, potentially even cooler where any rain band sits. Should also
see lows in the upper 40s to lower 50s, for a good early fall feel.

&&

Aviation...(for the 00z tafs through 00z Monday evening)
issued at 621 PM CDT sun Aug 25 2019

MVFR ceilings across the area may lower through the overnight as a
cold front works through the area. Convection may also be possible
through the night, but timing is a bit uncertain. At this point,
convection is not timed out but will be added to the tafs as
confience increases. Stratus will be stubborn throughout much of
the day on Monday, with increasing northwest winds.

&&

Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&

$$

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