U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 170600 
Storm Prediction Center ac 170558 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1158 PM CST Sat Dec 16 2017 

Valid 171200z - 181200z 

..there is a marginal risk of severe thunderstorms across parts of 
coastal southern Louisiana... 

A strong wind gust or an isolated/brief tornado may occur near -- 
but more likely off -- the Louisiana coast this morning. Otherwise, 
severe weather is not expected. 

A short-wave trough in the upper-level flow will move quickly 
eastward from the Canadian prairies and northern plains across the 
Great Lakes today, while farther south, a low closes off and moves 
very slowly eastward along the southwestern U.S./Mexico border. At 
smaller scales, a trough moving quickly across the Ozarks/lower 
Mississippi Valley region during the morning will weaken within a 
confluent flow field, shifting east-northeast across the Ohio Valley 
through the afternoon, and then off the Atlantic coast during the 

At the surface, high pressure will largely prevail over the U.S., 
With a weak low/inverted trough to cross the lower Mississippi 
Valley and central Gulf Coast region early in the period before 
weakening thereafter. 

..coastal Louisiana... 
Convection will be ongoing at the start of the period across the 
lower Mississippi Valley area, with thunderstorms largely confined 
to southern Louisiana and spreading across southern portions of the 
central Gulf Coast states and possibly/eventually the Florida 

With Richer Gulf air to remain confined to coastal areas, storms 
will largely be elevated above a shallow/stable boundary layer. 
However, given very favorable low-level shear, a brief tornado 
cannot be ruled out near the coast. A locally stronger wind gust or 
two may also occur within the organized convective band -- and thus 
will maintain low severe probability/marginal risk across a small 
portion of southern Louisiana during the morning hours. 

.Goss/leitman.. 12/17/2017 


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... 


Latitude...Lon 42118055 42997924 43347900 43417838 43297826 42687842 
42067905 41907963 41838010 41868039 41918050 42118055