U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211254 
Storm Prediction Center ac 211252 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0752 am CDT Tuesday Apr 21 2015 

Valid 211300z - 221200z 

..there is a marginal risk of severe thunderstorms West Texas to southern portions KS/MO... 

a few thunderstorms may produce hail and/or gusts near severe limits 
this afternoon and evening over some of southern Kansas and 
Missouri. Isolated strong to severe hail and thunderstorm gusts are 
also possible this afternoon through tonight over the southern High 
Plains and into western Oklahoma. 

Upper-air pattern is characterized by split flow over western North 
America and large area of cyclonic flow east of rockies. Latter flow 
field is related to middle/upper-level low initially located over western 
ls. Cyclone is forecast to move slowly east-northeastward across ls and parts of ont 
to its NE through remainder period...orbited by several shortwave 
perturbations and accompanying vorticity maxima. High-amplitude 
ridging now over inland Pacific northwest and western Canada is forecast to shift eastward 
as trough now over northestern Pacific moves ashore British Columbia and amplifies southeastward over 
Pacific northwest coast. As this occurs...weak southern-stream low -- now near California 
Channel Islands -- will move slowly inland/northeastward and reach southern Sierra 
by 12z. Foregoing shortwave ridge -- currently located from northern Arizona 
southeastward across Chihuahua -- will move northeastward to southern High Plains by end of 
period. Vorticity maximum associated with weak shortwave trough -- 
evident in moisture-channel imagery over Pacific near 28n125w -- will 
pivot cyclonically around southern semicircle of California perturbation then 
move over northwestern mex after 00z...reaching nm/mex border around 12z. 

At surface...leading cold front has moved offshore middle Atlantic and extends 
southwestward over NC Outer Banks region to northern Florida and N-central/W-central 
Gulf S bro. Cold front should move southeastward on Florida Peninsula while 
weakening/decelerating. Gulf portion of cold front likewise will 
weaken...with western segment soon returning northward across S Texas as diffuse 
warm front. Secondary cold front -- now developing over northern plains 
-- is expected to strengthen and move southeastward to northern MO...southern Nebraska and 
central Wyoming by 00z...reaching Kentucky...southern MO/northern Arkansas Ozarks...and western OK 
by 12z. Overnight...weak surface low is expected to develop over Texas 
Panhandle...in zone of warm frontogenesis extending east-southeastward across southern 
OK. This low may link with western segment of secondary cold front by 
12z. As moisture returns northward through Texas...dryline should develop 
from S-north across West Texas. 

..KS/MO area...Aftn/early evening... 
diabatic surface heating...steep midlevel lapse rates...and precipitable water 
increasing into marginal .5-inch to .75-inch range will support 
development of weakly inhibited MLCAPE in 250-500 j/kg range this 
afternoon ahead of secondary cold front...near prefrontal 
trough/confluence line. Though buoyancy will be limited by lack of 
greater moisture...favorable deep shear and elongated/nearly 
straight-line hodographs may support brief severe potential with any 
sustained thunderstorms. Onset of nocturnal low level jet and impingement of combined 
cold-frontal zone and warm-frontogenetic regime may help to continue 
some thunderstorms on western limb of this process into evening over southern 
Kansas...perhaps merging into initially separate nocturnal convective 
regime described below. 

..srn High Plains...Late afternoon through overnight hours... 
isolated thunderstorms may form over higher terrain of eastern nm and move eastward 
into increasing moisture...and/or in vicinity of sharpening dryline. Any 
activity that does develop will occur atop well-mixed subcloud 
layers that will support maintenance to surface of any hail/gusts 
generated aloft..including some downdraft acceleration. Despite 
increase in precipitable water...support for convection will be marginal with only 40s 
to lower 50s surface dew points realized before dusk. Lack of more 
robust moisture and of substantial upper-level support will limit 
organized severe potential. 

Late in period...probably after 06z...thunderstorms rooted above surface should 
develop around eastern Texas Panhandle or northwestern OK. This area will reside 
near juxtaposition of several important sources of lift at various 
scales--cold front...elevated warm front...and northern rim of 30-40 knots 
low level jet associated with substantial low-level warm air advection and increasing 
moisture. This activity may continue southeastward over OK into day-2 period 
and influence development/evolution of more widespread/sfc-based/svr 
mesoscale convective system discussed in that outlook. 

.Edwards/picca.. 04/21/2015 

Mesoscale Discussion

acus11 kwns 210633 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210633 

Mesoscale discussion 0411 
National Weather Service Storm Prediction Center Norman OK 
0133 am CDT Tuesday Apr 21 2015 

Areas affected...northeast NC/far southeast Virginia 

Concerning...Tornado Watch 93... 

Valid 210633z - 210800z 

The severe weather threat for Tornado Watch 93 continues. 

Summary...an isolated severe risk continues overnight mainly across 
far northeast NC/extreme southeast Virginia. Tornado Watch 93 continues 
until 09z. 

Discussion...ahead of a cold front...a quasi-linear band of 
convection has evolved early in the overnight across across 
northeast/east-central NC...with the strongest part of this 
convective line near/immediately east of I-95 in areas just 
northeast of Rocky Mount as of 0615z. The preceding air mass remains 
modestly unstable/weakly inhibited amid modest boundary layer 
moisture with surface dewpoints in the lower 60s f. 

Low-level winds are generally weak/southerly...while strong /40+ knots/ 
winds just above the surface have tended to veer and are now more 
uniform southwesterly per regional WSR-88D vwp data. With aid of 
modest low-level storm-relative inflow...a quasi-linear Mode should 
remain prevalent as storms spread east-northeastward overnight. 
Particularly given a trend of weakening convergence near the 
southwest portion of the line...the strongest/most-organized storms 
including small bowing segments are likely to continue to across far 
northeast NC and possibly extreme southeast Virginia...particularly as 
storms interact with immediately preceding outflow. Locally damaging 
winds and a brief tornado risk remain possible. 

.Guyer.. 04/21/2015 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 35817865 36497722 36897682 36737609 36147619 35707772