SPC AC 211247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Thu Apr 21 2022

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   NORTHWESTERN OKLAHOMA TO CENTRAL KANSAS REGION...

   ...SUMMARY...
   Large, damaging hail, severe gusts, and perhaps a tornado are
   possible with any thunderstorms that can develop over parts of the
   northwestern Oklahoma to central Kansas region.

   ...Synopsis...
   A substantial mid/upper-level pattern change is underway across the
   CONUS, increasingly emphasizing cyclonic flow across the West.  This
   will occur in response to a large synoptic-scale cyclone --
   initially centered approximately 500 nm west of the mouth of the
   Columbia River -- and the troughing that extends southward past 30N
   in the northeastern Pacific.  As several vorticity maxima orbit the
   mid/upper low, the cyclone will move inland over the Pacific
   Northwest tonight.  An intense, basal shortwave trough will move
   eastward and onshore central/southern CA tonight as well.

   By 12Z tomorrow, the resulting major 500-mb trough will extend from
   southern BC through the low, to coastal southern CA, and farther
   south, to the west of much of Baja.  In response to these
   developments, heights should rise over much of the Great Plains
   through the period, while falling west of the Rockies.  A very
   subtle shortwave trough -- evident in moisture-channel imagery over
   parts of southern NM, far west TX and Chihuahua, should move slowly
   northeastward today, its northern fringe reaching parts of
   southwestern KS and the TX/OK Panhandles around 00Z, in even weaker
   form.

   The 11Z surface analysis showed a low near AMA, with a weak cold
   front westward over northeastern NM and a wavy warm front over
   southwestern, central and northeastern OK, to northwestern AR.  The
   low is expected to drift northward to northeastward and weaken
   through the afternoon, while the warm front moves northward to
   southern KS.  As lee cyclogenesis occurs overnight in CO, the front
   will move northward across eastern CO and central/northern KS.  A
   dryline was analyzed near a CDS-HOB-INK line and southward over the
   TX Big Bend region.  This boundary will mix eastward through late
   afternoon, as far as southwestern KS, northwestern OK, to northwest
   TX, to near 6R6, before retreating rapidly northwestward across the
   southern High Plains and western KS overnight.

   ...Central/southern Plains to parts of MO/IA...
   Isolated to widely scattered, surface-based thunderstorms are
   possible late this afternoon and this evening over parts of western
   OK into central KS.  Any sustained convection could become
   supercellular, with up to significant (2+ inch diameter) hail sizes,
   damaging gusts, and a brief/conditional window of tornadic
   opportunity.  Severe potential in this regime is conditional, and
   the "slight" area has been pared down to where the parameter space
   is most favorable (below) and largest hail sizes possible, should
   storms form.

   In the area from northwestern OK to central KS, the already meager
   large-scale effect of the weak, approaching southern-stream
   shortwave trough may be offset by ambient synoptic height rises.  As
   such, little (if any) mid/upper support is expected, and surface-
   based convective potential will depend on sustained lift related to
   the front and dryline beneath a weakening EML inversion.  This may
   be sufficient for initiation anyway, given 3-4 hours of essentially
   no MLCINH present in unmodified soundings from multiple synoptic
   models and CAMs.  These progs, unsurprisingly in such a subtly
   balanced scenario, appear:
   1) Largely underdone on warm-sector surface heating and
   2) Very inconsistent on convective production/duration.

   The parameter space will be favorable, with surface dewpoints
   generally in the upper 50s to mid 60s F and steep mid/upper-level
   lapse rates supporting MLCAPE in the 2000-3000 J/kg range.  Low-
   level flow will veer well with height, along and within about 100 nm
   south of the front, contributing to 35-45-kt effective-shear
   magnitudes.  Hodographs should enlarge considerably during the early
   evening as boundary-layer moisture increases somewhat, with
   effective SRH as high as the 350-450 J/kg range.  A 1-2 hour early-
   evening time window will exist for large hodographs while effective-
   inflow parcels are still surface-based -- if a storm can develop and
   mature to take advantage.

   More-certain, denser-coverage, but mostly less-intense convection is
   expected tonight north of the front, in a broad, elevated low-level
   warm-advection regime with steep preconvective lapse rates aloft. 
   Isolated severe hail and perhaps a strong-severe gust may occur with
   that activity atop a boundary layer that is still somewhat dry as
   well as stable -- before convection/precip become overly expansive
   and messy in mode.  The greatest potential for large and perhaps
   briefly significant hail will be on the southern rim of this
   activity, closest to the surface warm front, where inflow of
   relatively high-theta-e air atop the frontal surface will be least
   impeded.

   ..Edwards/Dean.. 04/21/2022

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z