U.S. Severe Weather Forecast (Convective Outlook)

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swody1 
Storm Prediction Center ac 200558 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1158 PM CST Thu Jan 19 2017 


Valid 201200z - 211200z 


..there is a slight risk of severe thunderstorms across parts of 
la/MS/al and the western Florida Panhandle... 


..there is a marginal risk of severe thunderstorms across parts of 
the lower Sabine/MS river valleys into the Gulf Coast states... 


..there is a marginal risk of severe thunderstorms across parts of 
coastal Southern California... 


... 
Severe storms may develop Friday night into early Saturday across 
the lower Mississippi Valley and adjacent Gulf coastal areas, with a 
few tornadoes, large hail, and damaging winds all possible. Strong 
thunderstorms with at least some severe-weather potential are also 
possible near the Southern California coast Friday afternoon. 


The potential for higher severe hail/tornado probabilities (and a 
corresponding enhanced risk) exists for late in the day 1 period 
across parts of la/MS/al, but a possible upgrade has been deferred 
to a later outlook due to: 
1. Considerable uncertainty in convective coverage/Mode across the 
warm sector late Friday into early Saturday per latest guidance, 
2. Relatively high potential for spatial error in the placement of 
higher probabilities, and 
3. The opportunity for later near-term guidance and observations to 
provide additional details on the mesoscale evolution of this event. 


A detailed discussion follows below. 


... 
A large-scale upper trough over the western Continental U.S. Will acquire an 
increasingly negative tilt as it shifts northeastward across the 
plains Friday. One upper jet initially across the Southern Plains 
will develop eastward across the lower to mid MS valley, Tennessee/Ohio 
valleys, and adjacent Gulf Coast through early Saturday. A separate 
upper jet is forecast to move from the eastern Pacific across California and 
parts of the southwest through the period. 


A shortwave trough embedded within the synoptic-scale 
mid/upper-level trough will likely move from northern Mexico across 
Texas and into the lower MS valley from Friday evening through 12z 
Saturday (end of day 1 period). An elevated mixed layer with steep 
mid-level lapse rates originating from the higher terrain of 
northern Mexico appears likely to shift northeastward into much of 
the Southern Plains and parts of the lower MS valley in tandem with 
the embedded shortwave trough. 


At low levels, Gulf moisture will stream northward across the lower 
MS valley and adjacent areas ahead of the approaching shortwave 
trough Friday night into early Saturday. A southerly/southwesterly 
low-level jet is forecast to strengthen to 40-50 kt across parts of 
la/MS/al, mainly after 21/03z. Higher surface dewpoints/moisture 
will likely be slower to advance northward across the same area due 
to a convectively reinforced boundary currently located over the 
northern Gulf of Mexico from prior thunderstorms. The primary 
surface cyclone is expected to remain well displaced to the 
northwest (across the central/northern plains) from the warm sector 
across the lower MS valley and Gulf Coast states. 


..East Texas/lower MS valley and adjacent Gulf Coast states... 
While isolated thunderstorms may occur near to just offshore parts 
of coastal Texas/la/MS during the day Friday along a remnant boundary, 
the better opportunity for scattered to numerous thunderstorms 
appears to be Friday evening after 21/00z. This later development 
would be tied to increasing large-scale ascent associated with an 
embedded shortwave trough over the Southern Plains approaching the 
region, as well as low-level warm air advection/ascent with a 
strengthening low-level jet. 


Considerable uncertainty remains regarding convective coverage and 
Mode late in the period. Regardless, forecast soundings across 
la/MS/al show slowly steepening mid-level lapse rates, strengthening 
low- to mid-level winds, and increasing low-level moisture Friday 
night into early Saturday morning that will support supercell 
structures. The threat for large hail, some potentially greater than 
2 inches in diameter, will exist with any initially discrete 
supercell storms that can form this environment, although the better 
mid-level lapse rates will be arriving toward 06-12z Saturday. In 
addition, damaging winds and a few tornadoes will also be possible 
across this region, particularly southern la/MS/al where higher 
surface dewpoints may support surface-based convection. 


..coastal Southern California... 
Numerous showers and isolated thunderstorms may develop Friday 
afternoon across coastal Southern California and adjacent eastern Pacific 
tied to ascent with the nose of an upper-level jet moving over this 
region. Forecast soundings indicate weak instability may combine 
with marginal bulk shear values to support some organization with 
this convective activity. Locally strong to damaging winds appear to 
be the primary threat. 


.Gleason/picca.. 01/20/2017 


$$ 




Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 200011 
gaz000-alz000-200215- 


Mesoscale discussion 0077 
National Weather Service Storm Prediction Center Norman OK 
0611 PM CST Thu Jan 19 2017 


Areas affected...eastern Alabama through west central Georgia 


Concerning...severe potential...watch unlikely 


Valid 200011z - 200215z 


Probability of watch issuance...20 percent 


Summary...thunderstorms will continue to pose a marginal risk for 
isolated damaging wind and a brief tornado through 02z as they move 
through the remainder of eastern Alabama into far western Georgia. 
Overall threat does not appear sufficient for a ww. 


Discussion...a line of thunderstorms with embedded bowing segments 
and meso-vortices extends from northeast through south-central Georgia. 
The line is moving east at 15-20 kt, while individual elements 
within the line move more rapidly northeast. Objective analysis 
indicates a marginally unstable warm sector with MLCAPE below 500 
j/kg. The weak thermodynamic environment should remain the primary 
limiting factor for a more robust severe threat especially as storms 
continue east into Georgia as evidenced by the 00z Atlanta raob. Based on 
the vwp from Montgomery al, low-level hodographs are not 
particularly large with unidirectional deep-layer wind profiles. 
However, effective bulk shear up to 40 kt is sufficient for storms 
to organize with embedded bowing segments and weak supercell 
structures. Some increase in the low-level jet is expected this 
evening in response to the ejecting upper trough. However, storms 
will begin to move east of the more unstable portion of warm sector 
beyond 02z, suggesting an overall diminishing trend by that time. 


.Dial/Edwards.. 01/20/2017 


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


Attention...WFO...ffc...bmx...mob... 


Latitude...Lon 32008649 32748614 33288591 33698578 34038556 33748505 
33268476 32478474 31878549 32008649