SPC AC 181247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Sun Mar 18 2018

   Valid 181300Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH-CENTRAL
   TEXAS AND THE RED RIVER AREA TO SOUTHWESTERN GEORGIA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK AND EXTENDING TO CENTRAL AND SOUTHERN KANSAS...

   ...SUMMARY...
   Afternoon thunderstorms with very large, damaging hail and a tornado
   risk are possible over North Texas and far southern Oklahoma.  
   Severe gusts and isolated large hail may occur from there through
   parts of the Southeast.  Isolated severe storms also may form this
   evening and tonight over central and southern Kansas and parts of
   Oklahoma.

   ...Synopsis...
   A complex, generally progressive upper-air pattern with lowering
   amplitude on the synoptic scale is expected through the period, as
   the western-U.S. cyclone breaks down into several distinct
   shortwaves, each moving different directions.  The most relevant of
   those, for this forecast, is evident in moisture-channel imagery
   over AZ.  This perturbation should assume negative tilt by 00Z, when
   it is located from central CO to eastern NM, with a 500-mb low
   forming in a vorticity maximum near the southern Sangre de Cristos. 
   By 12Z, a compact cyclone should form around that low near the
   eastern KS/OK border, with associated shortwave trough extending to
   northeast TX.

   At the surface, troughing with weak/embedded lows, now over
   northern/eastern CO, will consolidate into a strong cyclone over
   east-central/southeastern CO by 00Z.  This cyclone then should pivot
   southeastward across southwestern/south-central KS, becoming nearly
   stacked under the 500-mb low between ICT-CFV by 12Z.  A dryline
   initially just west of a DRT-SWW line will shift eastward to the
   Hill Country and areas between ABI-SPS today.  Meanwhile a nearly
   stationary frontal zone across northern LA and north-central/
   northwest TX will merge gradually with a marine frontal zone now
   located over south-central/southeast TX, south of which dew points
   upper 60s to near 70 F are common.  The combined frontal zone will
   be the primary focus for the severe threat this afternoon/evening,
   east of its intersection with the dryline.

   ...North-central TX, Red River region to Arklatex...
   A few supercells are expected to develop this afternoon near the
   triple point and along/east of the dryline, right-moving generally
   eastward (with isolated northeastward left-splitters also possible).
    All forms of severe are possible.  Multi-county swaths of damaging
   hail have occurred here with similar scenarios during recent years,
   and such potential exists again today.

   Near and south of the warm front, steep low/middle-level lapse
   rates, and adequate water content with mean mixing ratios 11-13 g/kg
   occupying the inflow layer, will combine with areas of diabatic
   surface heating to yield MLCAPE 1500-2500 J/kg, locally near 3000
   J/kg.  SBCAPE will drop sharply with northward extent away from the
   warm front, near the Red River, though elevated CAPE appears
   sufficient to sustain some threat into extreme southern OK.  A
   substantial portion of the buoyancy, as well as mesocyclonic
   augmentation to lift aloft, will reside in optimal hail-growth
   layers.  Accordingly, and in this case justifiably, hail-model and
   analog applications to forecast soundings each suggest
   significant/2-inch hail is probable, and hailstones of 3+ inches in
   diameter possible, with any mature, relatively discrete supercells. 
   Lower LCL and higher SRH/0-1-km shear in the frontal zone between
   I-20 and the Red River also indicate at least slight tornado
   potential, though still-uncertain small-scale factors (such as
   storm-spacing/interference considerations and downdraft
   characteristics) will regulate the longevity and intensity of
   tornado risk as hodographs enlarge late afternoon into evening. 
   Some of the activity should weaken as it moves eastward over and
   part the Arklatex region this evening, however potential exists for
   some upscale growth and movement into the rest of the slight-risk
   area as discussed below.

   If diagnostic/mesoscale trends, as well as subsequent progs using
   morning RAOB data, continue to affirm the relatively tight
   afternoon/early-evening focus that a consensus of overnight progs
   have exhibited, a corridor of greater hail probabilities may be
   needed in a succeeding outlook.

   ....Lower Mississippi Valley and Southeast...
   Widely scattered to scattered thunderstorms in clusters will offer
   the risk of damaging gusts and isolated large hail through tonight.

   The convective potential on this corridor appears rather
   poorly/broadly focused and inconsistently forecast by assorted
   models -- which is not surprising given the broad/diffuse nature of
   the baroclinicity, likelihood of weak CINH in much of the area, and
   subtle nature of the large-scale support aloft.  Still, a few areas
   of thunderstorms with severe wind/hail potential are expected to
   form through the period, initially in association with the area of
   UVV contributing to the precip/convection now over portions of
   MS/LA.  Mesoscale to localized boundaries -- including outflows,
   differential-heating zones and sea breezes -- also may aid
   development.  Patchwork fields of maximized diabatic
   destabilization/buoyancy are expected to develop under thin/open
   areas in the cloud cover, while the northward shift of the
   Gulf/marine air contributes to growing theta-e.  Additional air-mass
   recovery is expected behind the effects of precip swath as well,
   late this afternoon into evening, in the form of warm/moist
   advection.

   Lapse rates aloft and buoyancy will be on the low side east of the
   Mississippi River, with MLCAPE under 1000 J/kg in most areas except
   for portions of southern MS/AL/LA where somewhat larger midlevel
   lapse rates overlie 60s to near 70 F surface dew points.  40-50 kt
   effective-shear vectors suggest some potential for supercell and/or
   bowing organization.  Tonight, one or two clusters of thunderstorms
   evolving from southern Plains convection may enter the region as
   well, also with a risk of severe gusts and isolated large hail.

   ...KS/northern OK...
   Scattered thunderstorms may develop late this afternoon or early
   evening over parts of southwestern KS and/or the OK/northern TX
   Panhandles and move northeastward, offering the potential for hail. 
   Strong-severe gusts also are possible, though relative lack of both
   buoyancy and mid/upper flow (under the difluent region of the
   shortwave perturbation), along with a nocturnal stable layer near
   the surface, indicate the wind risk is too conditional/isolated for
   more than marginal probabilities at this time.  The convective
   potential will be supported by a narrow, southeast/northwest-
   oriented corridor of at least marginally favorable buoyancy wrapping
   into the sector near, east and southeast of the surface low, along
   with strong low-level convergence and maximized mid/upper-level lift
   ahead of the shortwave trough.

   The risk further southeast into central and eastern OK appears more
   uncertain and conditional, though any sustained convection this
   evening and tonight likewise will pose a risk of severe hail/gusts.

   ..Edwards/Mosier.. 03/18/2018

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