U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 231958 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0258 PM CDT Wednesday Jul 23 2014 


Valid 232000z - 241200z 


..there is a slight risk of severe thunderstorms from eastern Washington into the northern 
plains... 


..there is a slight risk of severe thunderstorms over parts of OK/AR/TX... 


..There is a slight risk of severe thunderstorms from PA/New York into New England... 


... 
Severe storms with mainly large hail are likely from parts of 
Washington eastward into the northern Great Plains this afternoon 
into tonight. Storms with locally severe winds will occur through 
the evening from eastern New York into New England. Additional 
severe storms producing strong and potentially damaging winds are 
expected across parts of Arkansas and southeast Oklahoma into North 
Texas. 


..northern rockies... 


The previous forecast is generally on track. Only minor changes were 
made to the western edge of the sig hail area. Latest rap forecast 
soundings and hi-res hrrr suggests better sig hail threat will exist 
to the east from near hvr eastward into NE Montana. 


..se OK...Nrn/cntrl Arkansas into North Texas... 


The slight risk area has been expanded to encompass ongoing severe 
storm clusters from NE Arkansas into eastern OK...and to include the dfw 
metroplex. Several 60+ miles per hour gusts have already been reported with 
ongoing convection...and threat should increase and become better 
organized as line/bowing segment propagates generally S/SW into 
greater instability. A 30 percent wind area has been included where 
midlevel lapse rates are approaching 8 degree c/km and effective bulk 
shear values have increased to near 30 knots across parts of southeast OK/SW 
Arkansas. Here...strong downdrafts will have a better chance of producing 
potentially widespread strong/damaging winds. Hi-res guidance shows 
a strong line/bowing segment persisting into North Texas this 
evening before weakening after about 01-02z near/just south of the 
metroplex. For more details reference mesoscale discussion 1444. 


The western extent of the severe threat is a little more uncertain 
as some guidance suggests additional storms may develop into SW OK 
late this afternoon/early evening. Effective shear and surface dewpoints 
/while adequate/ diminish with westward extent however. As 
such...the 5 percent area has been expanded slightly westward to 
account for at least a small potential for isolated strong/severe 
storms into this evening. 


..New England... 


No changes needed. 


.Leitman.. 07/23/2014 


Previous discussion... /issued 1128 am CDT Wednesday Jul 23 2014/ 


..northern rockies into High Plains... 
A rather strong upper trough is digging into the Pacific northwest 
region. Increasing large scale forcing and destabilization will 
help to initiate scattered thunderstorms over the mountains of 
eastern Washington/Oregon into Idaho and western Montana late this afternoon and 
evening. Strengthening winds aloft and steep lapse rates will 
promote the risk of strong/severe storms capable of hail and 
damaging winds. During the evening...storms are expected to develop 
eastward into the plains of central/eastern Montana where stronger low 
level winds and moderate cape values will result in a greater risk 
of supercell storm structures. These may be capable of very large 
hail or isolated tornadoes...mainly after dark. 


12z model solutions are less bullish on evolution of storms into the 
Dakotas late tonight...so have reduced severe probabilities in this 
region. Consensus of models also suggests that initiation of storms 
over eastern Wyoming into western South Dakota will be very isolated this afternoon 
and evening...so have reduced severe probs in this area as well. 


... 
A remnant cluster of thunderstorms is tracking southeastward over 
southeast Kansas. Meanwhile scattered altocumulus castellanus/high-based storms are 
forming over central Arkansas. It seems likely that some combination of 
these two clusters of convection will spread south-southwestward 
into eastern OK and western Arkansas this afternoon. Dewpoints in the middle 
70s and strong heating will lead to extreme instability...along with 
30+ knots of effective shear. More recent model solutions show 
remarkable agreement that a bowing structure may form in this region 
and pose a risk of damaging winds this afternoon and evening. 
Storms may move into northeast Texas before weakening after dark. 


..New England... 
A band of thunderstorms is slowing increasing along/ahead of a cold 
front extending from Lake Erie into eastern Quebec. Strong heating 
ahead of the front and ample low level moisture will yield afternoon 
MLCAPE values of 1500-2000 j/kg. 12z observed and model forecast 
soundings suggest that winds aloft will not be particularly 
strong...and that middle level lapse rates will be only marginally 
steep today. This suggests that while convective coverage may be 
rather high...its unclear how many storms will become severe. 
Nevertheless...have maintained the slight risk area across the 
region today. 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 232306 
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Mesoscale discussion 1449 
National Weather Service Storm Prediction Center Norman OK 
0606 PM CDT Wednesday Jul 23 2014 


Areas affected...northestern Texas...southern Arkansas and northern la 


Concerning...Severe Thunderstorm Watch 431...434... 


Valid 232306z - 240030z 


The severe weather threat for Severe Thunderstorm Watch 
431...434...continues. 


Summary...storms will continue to pose a threat for strong to 
damaging wind gusts until middle evening from northestern Texas through southern Arkansas 
and northern la. A new ww will likely be required south of ww 434 to 
include northestern Texas and northwestern la. 


Discussion...a large mesoscale convective system with expansive trailing cold pool and 
stratiform region extends from east-central Arkansas southwestward through extreme northestern 
Texas into extreme southeastern OK moving south at 30-35 knots. The latest 
objective analysis indicates the storms are currently moving through 
axis of maximum cape. However...downstream warm sector remains 
moderately unstable with temperatures around 90f and 70f dewpoints. 
Given the moist surface layer...weak cap and strong convergence along 
the gust front...storms should continue developing southward next 3-4 
hours before the stabilizing boundary layer contributes to the 
ultimate demise of the mesoscale convective system later this evening. 


.Dial/Edwards.. 07/23/2014 


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


Attention...WFO...Jan...lzk...shv...fwd...oun... 


Latitude...Lon 33839600 33189427 33799253 34309161 33409152 32299316 
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