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    Day 2 Outlook >
Feb 23, 2017 1300 UTC Day 1 Convective Outlook
Updated: Thu Feb 23 12:55:29 UTC 2017 (Print Version | 20170223 1300Z Day 1 shapefile | 20170223 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 231255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0655 AM CST Thu Feb 23 2017

   Valid 231300Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE AFTERNOON TO
   EARLY EVENING OVER PART OF NORTH-CENTRAL KANSAS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   NORTHERN KANSAS AND SOUTHERN NEBRASKA THIS AFTERNOON/EVENING TO
   EASTERN IOWA OVERNIGHT...

   ...SUMMARY...
   Thunderstorms may produce isolated severe hail across portions of
   southern Nebraska, northern Kansas, northern Missouri, and Iowa from
   this afternoon into tonight.

   ...Synopsis...
   The upper-air pattern will feature a broad area of cyclonic flow
   forecast to shift eastward from the western to central U.S. through
   the period.  Within that regime, a strong shortwave trough and
   intermittently closed mid/upper low were noted over the Great Basin,
   and will move eastward across the central Rockies through the
   period.  By 12Z, the primary 500-mb low and vorticity max should
   reach central/eastern NE, with shortwave trough southwestward across
   western KS and eastern NM.  In the southern stream, a cyclone now
   centered near coastal southeastern FL should move eastward over the
   northern Bahamas, then weaken and eject northeastward across open
   Atlantic waters.

   At the surface,  a cold front extended from Lower MI southwestward
   across central IL and central MO, becoming quasistationary over the
   KS/OK border region, then arching northwestward through an elongated
   area of low pressure from southeastern through central/northwestern
   CO.  The western segment of the front should move northward through
   the period as a warm front across KS and western/central MO. 
   Through mid-afternoon, the CO low should consolidate/re-develop
   southeastward along the warm front into southwestern KS, before
   strengthening and shifting northeastward across south-central KS by
   00Z.  A cold front behind that low will move southeastward through
   the southern High Plains and most of OK overnight.

   Around 00Z, a sharply defined surface ridge still will extend from
   the Carolinas southwestward across southern AL, the FL Panhandle and
   the northern/central Gulf, with offshore flow over the northeastern
   Gulf in the wake of the FL/Bahamas low.  Associated anticyclonic
   trajectories and immaturity of Gulf air-mass modification will
   continue to hamper moisture return in the Plains surface warm sector
   through this period.  Moisture will increase farther east across the
   MS Valley late in the period and into day 2, when the deep-layer
   wave ejects northeastward and severe potential increases markedly
   across parts of the Midwest.

   ...Central Plains to eastern IA...
   It still appears that EML-related capping and a lack of more robust
   boundary-layer moisture will preclude diurnal, surface-based
   thunderstorms on the surface warm front or a dryline/trough
   extending southward from the low.  Instead, widely scattered to
   scattered thunderstorms are expected to develop poleward of the
   surface low and warm front from late afternoon into evening, moving
   northeastward across the outlook area with the potential for
   isolated large hail.  As the surface low and mid/upper-level wave
   advance eastward this evening and overnight, additional convection
   should form north of the surface warm front over northern MO and IA,
   likewise moving northeastward and offering the risk of isolated
   severe hail.

   Afternoon development over parts of northern KS and southern NE will
   be related to a spatial juxtaposition of:
   1.  Frontogenetic/cyclogenetic forcing above the surface,
   2.  Large-scale DCVA/cooling aloft ahead of the mid/upper shortwave
   trough,
   3.  The western fringe of at least marginally supportive low-level
   moisture mainly atop the boundary layer,
   4.  Supplementary lift beneath a coupled upper-jet structure,
   characterized by the left-exit region of a cyclonically curved
   member over the Southwest and southern Plains, and the
   right-entrance region of a jet max over the upper MS Valley and
   Great Lakes.

   By 21-03Z, a narrow corridor of 6-8 degree C 850-mb dew points
   should appear north of the surface front and northeast of the low,
   expanding both laterally in north-south width and extending westward
   to the north of the surface low.  This should suffice for storms to
   form amidst strong deep-layer lift, and in a favorable kinematic
   environment with strengthening deep shear.  This regime will shift
   eastward through the evening and overnight, amidst increasing
   elevated moisture return, a widening moist plume above the surface,
   and overlying steep preconvective lapse rates, all in support of
   hail potential.

   Forecast soundings from several progs reasonably show 55-65-kt
   effective-shear magnitudes, and in a small area of north-central KS,
   a brief spatiotemporal "sweet spot" of 500-800 J/kg MUCAPE around
   00Z.  These conditions may support an isolated, fairly long-lived,
   severe supercell or two, perhaps even with damaging-wind risk in
   addition to a hail swath.  A storm in that environment briefly may
   become nearly surface-based, considering
   1.  Effective-inflow parcels near the surface in some forecast
   soundings, and
   2.  The potential for internal storm dynamic processes to force
   stronger/deeper convective-scale lift than enabled by parcel theory
   alone.
   That early in the convective period, capping will limit storm
   coverage enough that any such supercell(s) should be isolated in
   nature.  For now, the low-probability but potentially higher-yield
   mesoscale threat still is not certain, but hail probabilities have
   been bumped up a notch, with marginal wind probabilities introduced
   to account for that conditional threat.

   ..Edwards.. 02/23/2017

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

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

        
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