U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 161950 
Storm Prediction Center ac 161948 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0148 PM CST Sat Dec 16 2017 

Valid 162000z - 171200z 

..no severe thunderstorm areas forecast... 

isolated to scattered thunderstorms should occur from parts of south 
Texas to western Mississippi, with the possibility of a few strong 
storms near the Texas coast late tonight. 

..20z update... 
No changes have been made to the ongoing forecast. Forecast 
soundings from recent rap/hrrr runs continue to suggest the better 
potential for surface-based thunderstorms will remain just offshore 
the middle/Upper Texas Gulf Coast late tonight. Greater low-level 
moisture characterized by mid to upper 60s dewpoints remains more 
than 100 miles off the middle Texas Gulf Coast per 19z observations, 
and some uncertainty remains with the inland extent of mid 60s 
dewpoints late tonight. For now, appreciable surface-based 
thunderstorm and associated strong/gusty wind and isolated tornado 
risk over land still appears too low across any portion of the Texas 
Gulf Coast to include severe probabilities with this update. 

.Gleason.. 12/16/2017 

Previous discussion... /issued 1028 am CST Sat Dec 16 2017/ 

..coastal Texas to far southern Louisiana... 
In response to an amplifying/kicker shortwave trough along the California 
coast, a closed upper low over northern Mexico at midday will become 
more progressive and eventually take on a more negative tilt as it 
accelerates northeastward over the eastern half of Texas toward eastern 
OK/Ark-la-tex tonight. Weak near-coastal cyclogenesis is expected 
along the middle/Upper Texas coast this evening with gradual air mass 
modification occurring over the western Gulf of Mexico. 

As this occurs, convection is likely to increase this evening with 
widely scattered thunderstorms spreading northeastward through the 
overnight. While deep-layer/low-level shear (accentuated by a 40-55 
kt low-level jet) will steadily increase this evening, the 
likelihood of a surface-based-convection-conducive air mass remains 
questionable over inland areas. 

Consistent with late-morning satellite imagery and inland/maritime 
observations, latest short-term guidance continues to indicate that 
the semi-moisture rich air mass will likely remain just offshore 
through tonight. This should limit the potential for surface-based 
convection over inland areas with various rap/12z NAM forecast 
soundings depicting the persistence of a shallow near-surface stable 
layer with surface dewpoints generally no higher than 63-64f inland. 
While the potential for a tornado and/or damaging winds should 
remain very low inland, a few supercells and/or semi-organized bows 
may be noted across the offshore waters tonight off the middle/Upper 
Texas coast toward coastal southwest la. 


Mesoscale Discussion

acus11 kwns 152243 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 152242 

Mesoscale discussion 1797 
National Weather Service Storm Prediction Center Norman OK 
0442 PM CST Fri Dec 15 2017 

Areas affected...portions of western New York and far northwestern 

Concerning...heavy snow 

Valid 152242z - 160245z 

Summary...a squall of heavy snow will continue to move ashore areas 
near Eastern Lake Erie this evening. Thereafter, lake effect snow 
bands will develop through the overnight. Snowfall rates upwards of 
two inches per hour will be possible. 

Discussion...along the cyclonic flank of a mid-level jet, broad 
ascent has fostered the development of a lake-enhanced snow squall 
over Eastern Lake Erie. This band should gradually move ashore over 
the next hour or so, aided by sufficient low-level instability and 
convergence. Kbuf dual-pol data suggest some enhancement of Crystal 
growth around -12 to -18 c, with subtle increases in kdp (upwards of 
0.4-0.6 deg/km) noted around 5000-6000 ft above radar level. 
Therefore, heavy snow is likely as the band continues to move 
onshore, and snowfall rates upwards of 2 inches per hour appear 
possible. Following the squall, lake-effect bands will likely 
organize later this evening, with continued localized heavy snow 

.Picca.. 12/15/2017 

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


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