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  Tuesday April 13, 2021

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SPC - No watches are valid as of Tue Apr 13 15:31:02 UTC 2021

No watches are valid as of Tue Apr 13 15:31:02 UTC 2021.

No Mesoscale Discussions are in effect as of Tue Apr 13 15:31:02 UTC 2021.

SPC Apr 13, 2021 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
0757 AM CDT Tue Apr 13 2021

Valid 131300Z - 141200Z


Episodes of strong to severe thunderstorms are most probable today
and tonight from east Texas across parts of Louisiana.

A blockier mid/upper-tropospheric pattern will cover much of the
northern CONUS, related to two slow-moving, synoptic-scale cyclones
that will be connected by troughing, with a col located over the
northern High Plains.  The eastern vortex is centered over northern
MN, and zonally elongates across parts of the Dakotas, Lake
Superior, MN, WI, and Upper MI.  The 500-mb low will meander
erratically around the northeastern MN/western Lake Superior area
through most of the period before turning east-southeastward over WI
late tonight.  

To the west, moisture-channel imagery indicates mid/upper
cyclogenesis over the interior Northwest, within a pronounced, zonal
trough.  The resulting closed gyre will pivot south-southwestward
over OR, then southeastward to the northern Great Basin, with 500-mb
low over northern NV by 12Z tomorrow.  This vortex will be the
cyclonic member of a Rex pattern days 2-3, in combination with a
developing anticyclone over BC.  Between the lows, shortwave ridging
will develop/amplify tonight over the central High Plains.  South of
the twin cyclones, a belt of southwesterly to westerly flow aloft
will extend from CA and northwestern MX across the southern Plains
and Southeast.  A series of weak shortwaves will traverse this flow
field from the Southwest and northern MX to the Mississippi Delta

Surface analysis at 11Z depicted a low associated with deep-layer
cyclone over northern MN.  A cold front extended across northeastern
WI and central IL, becoming quasistationary to a weak low near TXK. 
From there the front extended southwestward across south-central TX
to near DRT.  A warm/marine front was drawn across south TX near
Eagle Pass and HOU to near LCH, then southeastward to near EYW.  The
northern frontal zone will become quasistationary across parts of
the Mid-South and Arklatex regions.  However, the boundary should
continue slow southward progress over central/southwest TX, behind a
frontal wave located over north-central/northeast TX. Extent of
inland progression of the marine front is uncertain, but will be
modulated by extensive convection to its north and northeast over
parts of LA and the Delta region today, and tonight over east TX and

...LA, Arklatex, central/east TX...
Severe potential appears over this swath in two main episodes, with
some spatial overlap possible in the middle across western LA.

1.  Midday into tonight, mainly LA:
Scattered, ultimately numerous thunderstorms are expected to develop
by midday and through the afternoon, aside from early convection now
underway across parts of southwestern LA.  Most of the activity will
form in a zone of elevated low-level warm advection and moisture
transport along and north of the marine front, though some may build
into the optimally well-modified maritime/tropical boundary layer of
the Gulf, near the LA Coast.  Large hail will be possible early in
the convective cycle, with relatively discrete cells, and isolated
significant/2+ hail cannot be ruled out.  The main factor precluding
an unconditional significant-hail outline, for now, is uncertainty
over coverage/duration of any such potential, given the expected
quick upscale clustering of convection that is forecast.  As that
occurs, damaging downdrafts will transition from an isolated to an
areal/organized severe concern, especially with forward propagation
into the ore-unstable Gulf air mass.  A tornado cannot be ruled out
near the coast as well, this threat being regulated by both local
storm mode and limited spatial coverage of a favorable parameter

The air mass along and south of the marine boundary, near the
immediate coast, will be most favorable, characterized by PW around
1.5-1.75 inches and upper 60s to low 70s F surface dew points,
beneath steep midlevel lapse rates.  This will contribute to MLCAPE
around 1500-2500 J/kg away from convection, and where a few hours of
diurnal heating can be realized.  However, weak low/middle-level
wind speeds will limit vertical shear and hodograph size, with
supercell modes probably transient, messy, and located along/north
of the boundary.  Buoyancy will become more elevated with northward
extent, but amidst somewhat greater (35-40 kt) effective-shear
vectors.  Most synoptic and convection-allowing guidance reasonably
indicates southward to southeastward forward propagation of
resulting convective complex across the coast and over the Gulf,
leaving an outflow-reinforced baroclinic zone to its west across
parts of east/southeast TX and perhaps extreme southwestern LA. 

2.  Evening and overnight, LA/TX/AR/OK area:
Widely scattered thunderstorms should develop this evening over
portions of east and perhaps central TX, to southern AR and central
LA.  A mix of multicellular and messy supercellular modes should
result, offering a threat for large hail and damaging to locally
severe gusts.  

This activity will be supported by both surface-based and elevated
destabilization occurring in the warm sector, around the western
fringe of the air mass processed by the earlier MCS, and ultimately,
atop the cold pool over LA as well.  Low-level shear and SRH will
increase somewhat overnight as a broad area of 25-35-kt 850-mb flow
develops over east TX and LA, and a tornado cannot be ruled out from
any relative discrete/surface-based cell that can sustain itself in
that environment before being assimilated into a convective cluster.

MLCAPE/MUCAPE around 1500-2500 J/kg is expected near the outflow
boundary/marine front, and over much of southeast into east-central
TX.  Buoyancy will diminish and become more elevated with northward
extent from there into parts of AR, north TX and southeastern OK. 
However, with stronger deep shear (e.g., effective-shear magnitudes
45-55 kt possible), isolated severe hail may occur in those areas,
especially early in the local convective cycle.  In such a setting,
the transition between marginal and no severe potential is
ill-defined, and not well-represented by a sharp categorical line on
a screen.

..Edwards/Broyles.. 04/13/2021

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