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SPC Apr 8, 2020 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0259 PM CDT Wed Apr 08 2020

Valid 082000Z - 091200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
INTO TONIGHT FROM THE MIDDLE MISSISSIPPI VALLEY INTO THE
OHIO/TENNESSEE VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS
AFTERNOON/EVENING ACROSS PARTS OF CENTRAL AND SOUTH CENTRAL TX...

...SUMMARY...
Scattered to numerous severe thunderstorms capable of producing
large hail and damaging winds, some significant, are expected from
mid/late afternoon across the middle Mississippi Valley eastward
into the Ohio and Tennessee Valleys through late tonight.

...20Z Update...

...Mid MS/TN/OH Valleys...
Only minor changes to the outlook over this area, including trimming
up the northwestern extent based on the position of the cold front
and also slightly extending 15% wind/5% tornado probabilities across
southern IN.

19Z surface analysis placed a low near the southern IA/IL border,
with a cold front extending southeastward through northeast KS.
Strong heating has contributed to deep boundary-layer mixing ahead
of the front across much of northern MO and northern/central IL
(sampled well by the 18Z ILX sounding). Dewpoints are currently in
the upper 40s/low 50s across this region. 

Strong forcing for ascent both along the front and ahead of the
approaching shortwave trough are still expected to increasing
thunderstorm coverage over the next few hours. An initially cellular
mode will quickly transition to a more linear mode with one or more
bowing segments then progresses southeastward. Strong wind gusts
remain the primary severe threat with large to very large hail also
anticipated. A brief embedded/QLCS tornado or two is also possible
once the bowing line segment(s) mature and reach the better
low-level moisture farther southeast over southern IL, southern IN,
southwest MO, and western TN/KY. 

...South-Central TX...
Severe thunderstorms are still expected across the area this
afternoon and evening. Near-term severe threat was recently address
via MCD #311.

...Southeast...
Ongoing thunderstorms across central MS and AL are expected to
persist for at least the next several hours with an attendant threat
for large hail and damaging wind gusts. This severe thunderstorm
threat was recently addressed by the issuance of Severe Thunderstorm
Watch 88.

..Mosier.. 04/08/2020

.PREV DISCUSSION... /ISSUED 1129 AM CDT Wed Apr 08 2020/

...Overview...
An active severe-weather day is expected across a relatively broad
region encompassing the Middle Mississippi Valley, Lower Ohio
Valley, Tennessee Valley to the south-central Appalachians. Aided by
a notably amplifying trough over the Upper Midwest, generally
strengthening west-northwesterly flow aloft in conjunction with an
observed eastward-advecting plume of steep mid-level lapse rates,
particularly across Iowa/Missouri and likely into southern portions
of Illinois/Indiana and parts of Kentucky/Tennessee, will support a
broad corridor of large hail/damaging wind potential this afternoon
and tonight. 

...MS/OH/TN Valleys this afternoon through tonight...
The region will be decidedly influenced by the southeastward-digging
upper trough/jet streak over the Upper Midwest that will spread
toward the Lake Michigan vicinity by evening, with an overlap of
considerably strengthening winds aloft with a plume of steep
mid-level lapse rates atop relatively rich early-April
boundary-layer moisture. A cold front will accelerate/steadily
progress southeastward across the middle Mississippi Valley and
Tennessee/Lower Ohio Valley through tonight. Ahead of this front,
relatively rich low-level moisture with boundary-layer dewpoints in
the upper 60s to lower 70s will continue to spread northeastward
from the northwest Gulf coast to the Lower Ohio Valley today.

In particular, strong/severe thunderstorm development is anticipated
by early-mid afternoon initially across east-central/southeast Iowa
as height falls with the northern stream trough (and a lead speed
max) begin to interact with the more marginal moisture on the
northwest edge of the unstable warm sector. The initial storms will
likely evolve into supercells capable of producing very large hail
into Illinois. Convection will subsequently spread
east-southeastward through the evening along the instability
gradient across central/southern portions of Illinois/Indiana, and
likely grow upscale into a larger cluster with increasing damaging
wind potential.  

Farther south, additional storm development is also likely along and
ahead of the cold front, starting late this afternoon in the
vicinity of southeast Missouri. Large buoyancy (MLCAPE of 3000 J/kg
or larger) is likely from southeast Missouri eastward as
boundary-layer dewpoints near 70 F spread northeastward beneath 8-9
C/km mid-level lapse rates. Deep-layer vertical shear will initially
be a little weaker in this corridor between the amplifying northern
stream and a southern stream from Texas to the Southeast, but the
large buoyancy/steep lapse rates and increasing deep-layer
flow/shear by this evening will favor a mix of supercells and
growing clusters capable of producing very large hail and damaging
winds. 

Like the initial convection farther north in Illinois/Indiana,
upscale growth into an extensive QLCS appears likely this evening
across the Tennessee and Lower Ohio Valley regions, with an
increasing threat for fairly widespread damaging winds. A couple of
tornadoes may also occur with the initial discrete cells or with
embedded circulations in the later QLCS as low-level shear increases
this evening/overnight and moderate-strong buoyancy is maintained
through the late-night hours.

...Southern WV to the VA/NC border this afternoon/evening...
Outflow continues to surge southward late this morning while
weakening across North Carolina. Gradual air mass recovery will
occur near/south of a differential heating zone near the
Virginia/North Carolina border vicinity. Storms are likely to
initially develop/intensify across southern West Virginia and in
vicinity of the Appalachians spine. Ample heating, some moisture
recovery, and effective bulk shear of 35-40 kt in the presence of
straight hodographs will favor splitting cells capable of producing
damaging gusts and isolated large hail as storms spread
east-southeastward.

...Central MS/AL into southern GA this afternoon/evening...
Initially elevated storms continue to develop/slowly increase across
far southern Arkansas late this morning, a probable precursor for
additional clusters of thunderstorms that are expected to develop
and move east-southeastward this afternoon across north-central
portions of Mississippi/Alabama into southern Georgia. This
development will be along the eastern edge of the richer moisture
(dewpoints near 70 F) and within the right-entrance region of the
mid/upper jet, in conjunction with thinning clouds/daytime heating
and a weakening of convective inhibition. While mid-level lapse
rates are not overly steep, MLCAPE between 1500-2500 J/kg and long
straight hodographs with effective bulk shear of 40-50 kt will favor
splitting supercells capable of producing large hail and locally
damaging thunderstorm gusts.

...Central/south-central Texas this afternoon/evening...
Modest convergence in vicinity of an eastward-mixing dryline should
focus at least isolated thunderstorm development generally around
the I-35 corridor this afternoon as convective inhibition weakens
and buoyancy becomes very large. Upwards of 3500-4000 J/kg MLCAPE is
expected by mid/late afternoon in the presence of lower 70s F
surface dewpoints. Despite weak low-level flow/shear, effective bulk
shear around 50 kt and ample moisture/MLCAPE will support supercells
capable of producing isolated very large hail and locally damaging
winds this afternoon/evening.

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SPC Apr 8, 2020 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook
      
Day 2 Outlook Image
Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1230 PM CDT Wed Apr 08 2020

Valid 091200Z - 101200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF THE TX COASTAL PLAIN...

...SUMMARY...
Severe thunderstorms are possible across portions of central and
south Texas on Thursday, mainly during the afternoon and evening.
Very large hail and strong wind gusts are the primary threats.

...Synopsis...
Upper pattern will have two prominent features on Thursday, a large
upper low over southern CA and the southern Great Basin and a
deepening/mature mid-latitude cyclone over the Northeast States. The
southern CA upper low will be displaced southwest of the stronger
flow aloft, with little progression of this system anticipated
throughout the day. Maturing of the cyclone over the Northeast will
be supported by a pair of shortwave troughs, one moving through the
region during the first half of the period and the other moving
through early Friday morning. Very strong mid-level flow (i.e. 110+
kt at 500mb) will accompany these shortwaves, spreading
eastward/northeastward over the northern Mid-Atlantic states and
southern New England by Thursday afternoon. 

Despite these more prominent features, the primary severe weather
threat will likely be tied to a more subtle shortwave trough
ejecting out of northern Mexico and across TX. Ample low-level
moisture will be in place ahead of this shortwave, leading to
strong/severe thunderstorm development Thursday afternoon.

...Central/South TX...
A cold front will likely extend from northern LA southwestward into
the Edwards Plateau early Thursday morning. This front is expected
to slowly push southward throughout the day as temperatures and
low-level moisture gradually increase ahead of it. By the early
afternoon, temperatures will like be in the upper 80s and dewpoints
in the upper 60s/low 70s south of the front. Ascent along the front
will be augmented by the subtle shortwave trough mentioned in the
synopsis, leading to thunderstorm initiation by the early afternoon.

The warm and moist low-levels coupled with steep mid-level lapse
rates (generally steepest between 800 to 600 mb) will support strong
buoyancy and the potential for very strong updrafts. Winds up
through about 700 mb will be modest but strong westerly flow will
exist about 700 mb, supporting ample deep-layer shear for updraft
organization. Large to very large hail appears to be the main severe
threat, although the potential exists for upscale growth into an
organized convective line. As such, threat for strong wind gusts
exists from both water-loaded downdrafts within more cellular
activity and more organized updrafts/downdrafts within a convective
line. The tornado threat is non-zero, but weak low-level flow
suggest very low probability.  

...Mid-Atlantic...
Remnants of the overnight convective line are expected along the
front across the region early in the period. Much of the stronger
forcing for ascent will be displaced north but strong vertical shear
will be in place over the region and a few stronger storms are still
possible.

...Southern New England...
Secondary surface cyclogenesis will likely be ongoing early Thursday
morning across portions of southern New England, with the resulting
low continuing to deepen as it moves into Downeast Maine Thursday
evening. In addition to the mesoscale forcing provided by this
cyclogenesis, the first of the pair of shortwaves mentioned in the
synopsis will move into the region during the early afternoon. The
better low-level moisture and buoyancy will be displaced farther
south, but there is expected to be enough low-level moisture to
support modest elevated instability as mid-level temperatures cool
and lapse rates steepen. As such, thunderstorms are expected within
the warm conveyor as well as along the front. The strong flow aloft
will support fast storm motions as well as the potential for a few
more organized/stronger storms capable of hail and/or downdrafts
strong enough to penetrate the low-level stability to produce
damaging wind gusts at the surface.

..Mosier.. 04/08/2020

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