SPC - No watches are valid as of Sat Oct 21 13:58:01 UTC 2017

No watches are valid as of Sat Oct 21 13:58:01 UTC 2017.


SPC - No MDs are in effect as of Sat Oct 21 13:58:01 UTC 2017

No Mesoscale Discussions are in effect as of Sat Oct 21 13:58:01 UTC 2017.


SPC Oct 21, 2017 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0742 AM CDT Sat Oct 21 2017

Valid 211300Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PORTIONS
OF SOUTHEASTERN KANSAS AND EXTREME SOUTHWESTERN MISSOURI ACROSS
OKLAHOMA...AND NEAR THE RED RIVER IN NORTH-CENTRAL/NORTHWEST
TEXAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
NORTH TEXAS TO THE SOUTHERN MINNESOTA/WISCONSIN BORDER AREA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING TO WESTERN WISCONSIN AND THE EDWARDS
PLATEAU REGION IN TEXAS...

...SUMMARY...
Scattered severe thunderstorms are expected especially across
portions of the south-central Plains late this afternoon and
evening.  Severe hail, damaging winds, and a few tornadoes are
possible.

...Synopsis...
The main mid/upper-level feature for this forecast is a full-
latitude, synoptic-scale trough, evident in moisture-channel imagery
from southern SK southward across eastern MT, WY, western CO, and
western NM.  Although several embedded vorticity maxima are
apparent, this trough should maintain coherent structure as it
shifts eastward across the Great Plains through the period.  By 12Z,
the trough should extend from western MN to eastern NE to
west-central/southwest TX.

At the surface, 11Z analysis showed a pronounced cold front from the
eastern Dakotas southwestward across central NE, western KS and
southwestern CO.  A dryline was drawn from eastern Chihuahua
northward across far west TX to near ROW then northeastward over the
western OK Panhandle and the KS/CO border region south of the front.
The dryline will mix and advect eastward today, reaching a late-
afternoon position over southwestern OK and northwest/west-central
TX.  By 00Z, the cold front should arc from northern MN to
northwestern MO, southwestern OK and southeastern NM, while
overtaking the dryline across the southwestern OK/northwest TX area.
By 12Z, the front should reach western Lake Superior, southern WI,
the MO/AR Ozarks, and south-central TX.

...Southern Plains to KS...
Thunderstorms are forecast to develop initially along the cold front
between northeastern KS and northwestern OK in the 19-22Z time
frame.  A short time window exists for supercellular structures,
before frontal forcing and a substantial component of flow aloft
parallel to the front contribute to quick quasi-linear evolution. 
Accordingly, hail threat with this activity should peak relatively
early in the convective cycle before transitioning to mainly wind
with isolated, marginal hail possible.  Farther south, initially
separate dryline or proximal warm-sector initiation will be either
concurrent with the frontal regime, or with a relative delay of an
hour or two, given the stronger capping (but also stronger surface
heating and boundary-layer lapse rates) over southwestern OK and
northwest TX.  This early-stage activity will offer an enhanced
significant/damaging hail and slight tornado risk.

The prefrontal moist sector should be characterized by steep
midlevel lapse rates, strong surface heating, and dewpoints in upper
60s F, yielding 3000-3500 J/kg MLCAPE from northwest TX to
central/northern OK, and around 2000 J/kg over northeastern KS. 
Forecast soundings also suggest 35-45 kt effective-shear magnitudes
and 150-250 J/kg effective SRH, the latter increasing around 00Z as
the boundary layer decouples and the LLJ strengthens.  However, the
increase in inflow-sector SRH will occur while storm modes get
messier, casting uncertainty on the longevity of any supercell
tornado potential.  However, damaging bow echoes are a distinct
possibility starting within 2-4 hours after storm initiation.  As
such, the enhanced wind probabilities are displaced eastward from,
but somewhat overlapping, those for hail.

The various convective regimes should merge as the front overtakes
the dryline throughout the evening, resulting in a lengthening/
backbuilding QLCS that sweeps eastward/southeastward across the
outlook area.  During that phase, an enhanced damaging-wind risk
exists, with isolated hail possible.  This segment of the squall
line will encounter a warm sector with relatively large values of
low-level shear and helicity, related to hodograph enlargement by
the southern sector of a 45-60-kt LLJ.  As such, embedded LEWP/bow
configurations with mesovortices, and related potential for
short-lived tornadoes and locally enhanced damaging-wind swaths,
will exist for a few hours.  Extent of backbuilding across west-
central TX and the Edwards Plateau region is uncertain, but the
severe threat appears lesser and more conditional than farther
northeast, given:
1.  Later development with respect to optimal boundary-layer
instability, and
2.  Potential for frontal undercutting of convection.

Overall, the severe threat will diminish overnight and with eastward
extent as the convection encounters progressively more static
stability near the surface -- though continuing boundary-layer
theta-e advection and moisture transport should make that process
gradual as well.

...Missouri Valley to upper Mississippi Valley...
Buoyancy is expected to decrease gradually with northeastward extent
from eastern KS, corresponding to lesser magnitudes of midlevel
lapse rates, low-level moisture and diabatic surface heating. 
However, frontal forcing still should encounter sufficient low-level
theta-e to support upscale evolution into a strong/severe squall
line with thunderstorm gusts the main severe threat and isolated to
widely scattered, marginally severe hail also possible.  

Peak prefrontal MLCAPE should reach near 2000 J/kg over northwestern
MO, grading to around 1000 J/kg over the upper Mississippi Valley. 
Midlevel winds, and the mean-wind vector, should be aligned only
slightly rightward of the axis of convective-scale lift, indicating
quickly quasilinear evolution and dominance of that mode, amidst
35-40 kt effective-shear magnitudes.  The severe potential should
wane through the evening hours, amidst diminishing instability
within and increasing elevation of the buoyant inflow layer.

..Edwards/Dean.. 10/21/2017

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