U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 190555 
Storm Prediction Center ac 190554 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1154 PM CST Wed Jan 18 2017 

Valid 191200z - 201200z 

..there is a marginal risk of severe thunderstorms across portions 
of the lower Mississippi Valley and central Gulf coastal region... 

A marginal severe threat, with isolated strong to locally damaging 
winds and perhaps a tornado, will be possible across parts of the 
lower Mississippi Valley and central Gulf Coast states on Thursday. 

An upper trough/low centered over the Central Plains Thursday 
morning will move slowly northeastward to the upper Midwest as an 
embedded shortwave trough moves from Texas to the lower MS valley 
through the day. A low-level jet is forecast to strengthen across 
the lower MS valley by 12z Thursday, and showers and thunderstorms 
should be ongoing across la/MS in association with low-level warm 
air advection and ascent at the beginning of the period. A weak 
surface low over the mid-south should develop northward to the mid 
MS valley/lower Ohio Valley by Thursday evening as a trailing cold 
front becomes increasingly ill-defined across the lower MS valley. A 
warm front will lift slowly northward across the Tennessee Valley/mid-south 
in conjunction with the surface low, and a moist low-level airmass 
characterized by low 60s to low 70s surface dewpoints will be in 
place across much of the lower MS valley into the central Gulf Coast 
region. A shortwave trough will move across Southern California Thursday 
morning, as a large-scale trough develops eastward over the western 
Continental U.S.. 

..lower MS valley and central Gulf Coast region... 
The effect of early-day precipitation and clouds across la/MS will 
likely serve to limit substantial diurnal heating downstream across 
the central Gulf Coast region through the afternoon. 00z soundings 
from lch, shv, and Jan all show the presence of poor low- to 
mid-level lapse rates (generally 5.5-6.0 degrees c/km), and the 
Prospect for steepening of these lapse rates appears very low given 
the sub-tropical origin of the upstream airmass. NAM/rap forecast 
soundings suggest that even with modest diurnal heating, MLCAPE 
should struggle to exceed 500 j/kg on a widespread basis across the 
warm sector Thursday afternoon. 

A gradual intensification of early morning convection as it moves 
eastward across la/MS/al and the Florida Panhandle/far western Georgia appears 
to be the most probable convective scenario for Thursday 
afternoon/evening. Effective bulk shear values on the order of 35-45 
kt and modestly enlarged low-level hodographs will generally support 
rotating updrafts with this activity even though instability will 
likely remain weak. Primary severe threats should be isolated strong 
to locally damaging winds as convection attempts to form into short 
line segments, and perhaps a brief embedded tornado or two. 

A more organized line of thunderstorms may develop from the 
early-day convection, but this does not appear overly likely at this 
time given the limited thermodynamic environment. Additionally, the 
cold front is forecast to weaken through the day, and flow aloft 
should remain largely parallel to the front and forecast morning 
convection. The marginal severe threat should slowly diminish 
through the evening across al, far western GA, and the Florida Panhandle 
with the loss of daytime heating and related instability. 

..coastal Southern California... 
A line of low-topped thunderstorms associated with a shortwave 
trough may be approaching the Southern California coast at 12z Thursday. 
While isolated lightning strikes may occur with this activity, 
forecast instability and low-level flow both appear too weak to 
support the introduction of low wind probabilities. 

.Gleason/picca.. 01/19/2017 


Mesoscale Discussion

acus11 kwns 190556 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 190556 

Mesoscale discussion 0073 
National Weather Service Storm Prediction Center Norman OK 
1156 PM CST Wed Jan 18 2017 

Areas affected...southwest through central Louisiana into southwest 

Concerning...severe potential...watch unlikely 

Valid 190556z - 190800z 

Probability of watch issuance...20 percent 

Summary...threat for a couple instances of damaging wind and a brief 
tornado will exist into the early morning from southwest through 
central Louisiana into southwest Mississippi. Threat does not appear 
sufficient for a ww, but area will continue to be monitored. 

Discussion...some increase in thunderstorm organization and coverage 
has been recently observed over southern la into the northwest Gulf. 
Uptick in convection appears to be in association to a modest 
increase in the southwesterly low-level jet which will persist into 
the morning hours in response to an upstream impulse. The marginal 
thermodynamic environment with weak lapse rates and MLCAPE below 500 
j/kg will remain the primary limiting factor. Moreover, while some 
increase in vertical shear will occur, low-level hodograph size is 
not expected to become exceptionally large. Nevertheless, the 
kinematic environment will be sufficient for storms to develop at 
least marginal supercell structures, especially as they move 
northeast and interact with the stationary boundary across 
south-central la into southwest MS. 

.Dial/Edwards.. 01/19/2017 

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


Latitude...Lon 30429318 31799182 32469055 31839014 31189065 30629162 
29719304 30429318