SPC AC 302046

   Day 1 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0346 PM CDT Tue Apr 30 2019

   Valid 302000Z - 011200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL
   AND EASTERN OK...FAR SOUTHWEST MO...AND FAR NORTHWEST AR...

   AMENDED TO UPGRADE TO MODERATE RISK

   ...SUMMARY...
   Severe thunderstorms are possible in a corridor across parts of the
   southern Plains northeastward toward Illinois/Indiana today and
   tonight.  This will include a risk for tornadoes, large hail, and
   locally damaging gusts.

   ...Amendment Discussion...
   An upgrade to Moderate is being issued based on recent data from 20Z
   OUN sounding and a recent special NSSL sounding in southwest OK.
   These soundings show an uncapped environment with strong low-level
   hodographs (lengthened by 40 to 50 kt within the 850 mb layer).
   Resulting 0-1 km SRH values are around 300 m2/s2. This environment
   downstream of developing storms in SW OK and Far NW TX as well as
   the expected increase in the low-level jet later increases
   confidence in higher tornado probabilities.

   Numerous tornadic supercells across northwest OK, southwest MO, and
   northwest AR also warrant the upgrade.

   ...Previous 20Z Update Discussion...

   ...TX/OK...
   Recent surface analysis places a boundary from low over southeast KS
   southwestward across central OK to near FDR in southwest OK. The
   boundary then extends more south-southwestward towards SANK and then
   westward through the Permian Basin. Portion of this boundary over
   central and southwest OK is moving gradually northward as a warm
   front. CHK has gone from 67/64 at 18Z to 77/70 at 19Z with a
   southeast wind gusting to 25 kt. In relation to this shift in the
   frontal position, severe probabilities where increased west of a
   line from TIK to DUC.

   18Z sounding at FWD showed a deep moist layer beneath a
   well-developed elevated mixed layer. Resultant capping is expected
   to weaken over the next few hours as the shortwave trough over the
   southern High Plains continues eastward. As it does, convective
   coverage will likely increase across western north TX and southwest
   OK. The beginnings of this scenario appears to be underway with
   developing storms near the boundary from CHK southwestward into far
   northwest TX.

   Severe probabilities were also increased across portions of the TX
   South Plains and east-central NM. In this area, cumulus has been
   building along a well-defined surface boundary and the potential for
   convective initiation has increased.  More details about this region
   are available in MCD 481.

   ...MO/IL...
   Thunderstorm coverage is expected to continue increasing near and
   south of the front extending from the central IL/IN border
   southwestward into northeast OK. Environment remains supportive of
   severe thunderstorms, particularly if cells continue to remain
   discrete.

   ..Mosier/Hart.. 04/30/2019

   .PREV DISCUSSION... /ISSUED 1105 AM CDT Tue Apr 30 2019/

   ...TX/OK/MO/AR...
   Water vapor imagery shows a strong upper trough moving across
   eastern NM this morning, with fast southwesterly flow aloft across
   all of the southern Plains.  Ample moisture and instability is
   present over TX/OK/AR with dewpoints in the 60s to lower 70s and
   near dry-adiabatic mid level lapse rates.  A surface boundary
   extends from southeast KS into central OK and northwest TX.  This
   boundary is expected to remain nearly stationary through the day, as
   the warm sector slowly warms into the mid/upper 70s.  The result
   will be a rather widespread area of substantial CAPE (MLCAPE over
   3000 J/kg) and minimal inhibition from southwest MO into southern OK
   and north TX.

   Meanwhile, model guidance is consistent in developing a strong
   southerly low-level jet over north TX and southeast OK by late
   afternoon. This will enhance the low-level shear profiles (SRH
   values of 200-350 ms/s2) and encourage supercell storm structures. 
   Given the relatively weak cap, 12z CAM solutions suggest numerous
   rotating storms will occur this afternoon and evening over the ENH
   risk area. These storms would pose a risk of tornadoes, large hail,
   and damaging winds.  If model solution are right in the widespread
   nature of storm development, destructive interference between storms
   may limit the potential of a more significant severe/tornado event. 
   Nevertheless, a strong tornado or two is possible today - mainly
   across south-central into eastern OK.

   It is also uncertain how far south storms will form into TX.  Only a
   few 12z CAMs show storms developing south of the DFW area, but
   forecast soundings suggest a conditional risk of hail and damaging
   winds with any storms that form in this area.  Therefore have nudged
   the severe risk areas slightly farther south.

   Later this evening and tonight, lines and clusters of storms will
   develop eastward into parts of southern MO and western AR, with a
   continued risk of damaging winds and a few tornadoes.

   ...NV/UT...
   Behind the primary upper trough over NM, a second progressive system
   is moving across northern CA and will dig into the Great Basin
   today.  Scattered thunderstorms are expected to form this afternoon
   over east-central NV and spread into UT. The strongest of these
   cells will pose a risk of gusty winds and hail.

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