Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 221133
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
633 AM CDT Tue May 22 2018

.SHORT TERM...(Today and Tonight)
Issued at 433 AM CDT Tue May 22 2018

With late exit of clouds, recent rainfall and weak flow, fog
developed overnight and has become quite dense from parts of east
central SD through southwest MN and most of northwest IA.  Weak
southeasterly gradient working to advect more significant fog area
northwest into I-29 corridor. Fog should reach its worst in coverage
and visibility between 10z-12z, then gradually dissipate through mid
morning. Advisory currently issued through 14z.

Otherwise, the other feature early this morning to watch is a weak
vorticity axis just south of the Missouri River across northeast
Nebraska which has started to spark up with isolated thunderstorms,
and activity both likely to increase slightly in coverage and spread
slowly to the east/northeast with fairly uncapped elevated
instability within initiation of deeper layer warm advection.  While
even some of the hi-res solutions indicate a diminishing coverage
threat later morning into midday, a few solutions indicate that
there will continue to be a threat of at least isolated storms right
through the day which spreads ever so slowly northward, following
the larger scale treatment of the vorticity axis through the day,
along with profiles which are basically uncapped to lift. Do have
most solutions indicating building heights in the mid-levels which
is typically not favorable for maintenance of convective threat, but
have held on to isolated level pops mainly south of I-90 through the
day.

By later afternoon, pretty solid consensus that will start to see
some more near surface-based elements in proximity to the surface
warm front and along the elevated axis just north.  Scattered
convection should continue to become more widespread through the
evening as boundary lifts northward, positioning itself in
diffluence near ridge axis with an increase in theta-e advection
feeding in with slight increase in the low-level jet. Mid-level flow
with proximity to building ridge axis is quite weak tonight, and as
a result hard to picture more than an isolated severe threat
tonight, with much more concern toward potential locally heavy
rainfall as band of storms advances northeastward across southwest
MN, but motion of cells is more toward the east/southeast.  A couple
of storms may be able to produce some marginally large hail tonight,
but mainly looking again at some areas of heavy rainfall.

.LONG TERM...(Wednesday through Monday)
Issued at 433 AM CDT Tue May 22 2018

Better clustering of convection likely to remain around southwest MN
into the early morning hours, before further amplification of the
upper ridge stomps down convective threat during the afternoon. Will
continue to carry a slight chance across areas north of a
Chamberlain to Storm Lake line with threat for a few boundaries and
diurnal heating into the 80s driving a healthier surface-based
instability. However, forcing for lift is largely absent.  Will
begin to feel a bit humid as dewpoints climb further into the 60s.

On Wednesday night, upper low across the northern Rockies will begin
to wobble into the ridge over the plains, with low-level jet likely
initiating convection over western SD, which could leak edge of
organized storms into area mainly from the James Valley and westward
later Wednesday evening, but focus for anything else largely lost
with synoptic boundary largely pushing well north of the area. Deep
layer shear remains fairly weak, and should only get some weakening
showers/storms perhaps toward I-29 toward daybreak, In general
should just be a warm and humid night with lows in the 60s.

Thursday has the potential to be a very warm, if not just plain hot,
day. Would not be shocked to get temps into the lower to mid 90s in
south central SD, and upper 80s and near 90 most other locations
with lack of morning convection to contaminate skies.  Warming temps
aloft also making things a bit more cappy, and more of a southwest
flow aloft should give rise to a steeper mid-level lapse rate
resulting in late day MLCAPE up to 2500 J/kg.  With wave pushing
closer and knocking down ridge, shear does begin to improve,
although winds are still somewhat weak.  Still have some concern
about the degree of convective threat from Thursday evening into
overnight Thursday. Storms are most likely to fire around early
evening near the location of a pre-frontal trough across southwest
MN into eastern SD.  These types of features will often exhibit more
shear due to winds backing slightly, especially aided by diurnal
cycle. Given the moderate to strong potential instability, the
combination could result in a full range of severe weather types
should storms initiate.

Final push to frontal boundary as wave passes north on Friday, and
may again bring a small chance for storms to southwest MN/northwest
IA later in the day, as atmosphere again primes to a fairly unstable
profile ahead of the front. So much will depend on convective events
prior to this day, and boundary could even be well off this timing
by this point. On the large scale, we are in the warm wedge south of
the upper system, and easily will see temps into the upper 80s and
lower 90s.

Models indicating a strong difference in handling this upper wave as
it moves downstream of the ridge axis by Saturday, with several
solutions stronger and more digging, which brings a threat for
thunderstorms once again mainly east of I-29 by late Saturday. Other
solutions rebound the ridge strongly and keep things dry. It will be
warm in any scenario, with temps in the 80s and lower 90s once again
with more of a westerly component.

The rest of the Memorial Day weekend should bring a minimal threat
for storms, with continued very warm temperatures as a strong ridge
builds across the northern plains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 630 AM CDT Tue May 22 2018

Dense fog continues to impact Sioux Falls this morning, and with a
light northeasterly wind persisting, this fog may hold over the
airport for the next hour before slowly mixing out.

Convection will stay near but perhaps not impact Sioux City
directly this morning, as better focus has aligned itself along a
linear band south of Sioux City. Will continue to hold a vicinity
thunder mention for a couple hours.

Otherwise, once fog slowly burns off, VFR skies will prevail
through the day. Our attention then turns to convective chances
later this evening and overnight. Overall, confidence too low to
be specific as there could be substancial differences in
initiation. There is also sufficient time to refine the convective
forecast over the next 12 hours to provide more detail in later
tafs.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Dense Fog Advisory until 9 AM CDT this morning for SDZ039-040-
     055-056-062.

MN...Dense Fog Advisory until 9 AM CDT this morning for MNZ071-072-
     080-081-089-090-097-098.

IA...Dense Fog Advisory until 9 AM CDT this morning for IAZ001>003-
     012>014-021-022.

NE...None.
&&

$$

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


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