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ALL SPC Mesoscale Analysis Curently in Effect
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No watches are valid as of Thu Apr 26 14:00:04 UTC 2018.No Mesoscale Discussions are in effect as of Thu Apr 26 14:00:04 UTC 2018.SPC 1300Z Day 1 Outlook
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0757 AM CDT Thu Apr 26 2018

Valid 261300Z - 271200Z


Isolated severe thunderstorms are possible in parts of the Southeast
this afternoon and this evening.

In mid/upper levels, a highly amplified synoptic pattern will
persist through the period, characterized by mean ridging over the
West Coast States and western Great Basin, and broadly cyclonic flow
from the Rockies eastward.  A cyclone now over eastern Lake Ontario
will move eastward over New England then northeastward over the Gulf
of St. Lawrence, absorbing a shortwave remnant of the formerly
closed low that has affected the Southeast in recent days.  

Farther southwest, the main upper-air influence on this forecast is
a strong shortwave trough located from southern IL across AR to
western LA, with embedded closed 500-mb low over eastern AR
approaching MEM.  The low is expected to follow a gently curving
path generally eastward across northern parts of MS/AL/GA through
00Z, before weakening and ejecting northeastward across the western
Carolinas overnight. 

Upstream, a somewhat weaker, northwest-flow shortwave trough was
evident in moisture-channel imagery over northwestern MN, the
Dakotas, eastern/southern WY, and northwestern CO.  The primary
associated vorticity lobe -- initially over southeastern WY -- is
forecast to dig southeastward to the TX/OK Panhandles by 00Z, then
over central/east TX by 12Z.  Destabilization aloft associated with
this feature will contribute to some weak-moisture thunder potential
this afternoon and evening, mainly over portions of OK.  Meanwhile,
a long-lived closed cyclone over the northeastern Pacific will move
slowly eastward toward northern CA and OR, but its core region will
not move ashore until near the end of the day-2 period.  However,
associated height falls will start spreading across parts of OR and
northern CA today and this evening, including that general-thunder
outlook area. 

At the surface, 11Z analysis showed an occluded low over northern
MS, with trough extending eastward to a weaker low over central SC. 
An occluded front extended southeastward past MEI to an ill-defined
triple point in rain-cooled air just behind the leading edge of a
convective band.  The warm/marine front was better-defined from that
edge over southwestern AL, southeastward across the extreme western
FL Panhandle, just offshore from AAF, to the southwestern coast of
the FL Peninsula.  The cold front extended from southeastern MS
southwestward across southeastern LA and the northwestern Gulf.  The
primary/initial low is expected to move eastward across northern
AL/GA today, while the SC low stays weak most of the day, before
gradually strengthening tonight and moving northward over NC, with
the approach of the mid/upper-level perturbation.  The cold front
will move eastward over most of AL/GA through the period, reaching
the central/eastern Carolinas, northwestern FL and the central Gulf
by 12Z.  The warm front should move northeastward/inland over the
eastern Gulf coastal plain and become more diffuse today, as
heating/mixing occur on both sides.  

Two main convective regimes are expected this period, with marginal
severe potential accompanying parts of each:

1.  Main frontal/convective band:  An ongoing swath of precip, with
isolated to widely scattered embedded and episodic thunderstorms,
was evident from northern AL southwestward across southeastern LA. 
As the related zone of lift shifts eastward, it will encounter a
diurnally destabilizing boundary layer across parts of eastern AL
and central/western GA today.  Isolated damaging gusts and large
hail are possible.  Moist advection will offset vertical mixing
enough to maintain generally 50s F surface dew points, representing
climatologically low boundary-layer moisture and theta-e for this
time of year.  [For example, the 12Z TLH RAOB contained 0.8-inch PW,
about 0.2 below the daily moving average, and morning surface temps
in the upper 40s to mid 50s are common over the outlook area.] 
Still, forecast soundings suggest enough heating/moisture to offset
modest lapse rates aloft for 300-500 J/kg MLCAPE, amidst 35-45 kt
effective-shear magnitudes, and enough low-level SRH to support
occasional storm-scale rotation (supercell or bow modes) amidst more
predominant multicell clustering.  Activity should shift
northeastward across the outlook area through the afternoon before
weakening, with additional development possible this evening in a
warm-frontogenetic zone over NC.  

2.  "Cold-core" regime:  Widely scattered thunderstorms should form
during the day across northern parts of MS/AL and perhaps
southern/middle TN, as cooling aloft near the core region of the
mid/upper-level cyclone juxtaposes with some diabatic boundary-layer
destabilization behind (northwest and west of) the leading
convective band.  Weaker low-level moisture and vertical shear in
this regime should preclude organized severe, though isolated small
hail and strong gusts are possible.  Enough airmass recovery in the
low levels may occur late this afternoon or evening over eastern AL
and northwestern/western GA to support marginal hail/gust potential
for a few hours until boundary-layer cooling/stabilization becomes
too great for strong-severe storms.

..Edwards/Peters.. 04/26/2018



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