U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201929 
Storm Prediction Center ac 201927 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0127 PM CST Sat Jan 20 2018 

Valid 202000z - 211200z 

..no thunderstorm areas forecast... 

thunderstorms appear unlikely through tonight across the contiguous 
United States. 


No changes to 1630z outlook. 

.Darrow.. 01/20/2018 

Previous discussion... /issued 1028 am CST Sat Jan 20 2018/ 

..synopsis and discussion... 
A vigorous southern-stream shortwave trough will move across the 
southwest and Great Basin to The Four Corners by early Sunday 
morning as a Pacific cold front likewise moves eastward across these 
same areas. Strong forcing for ascent will modestly steepen 
mid-level lapse rates preceding this shortwave trough across parts 
of Arizona/Utah into western Colorado/nm this afternoon and evening. Some 
mid-level moisture will accompany this shortwave trough, and very 
isolated lightning strikes cannot be ruled out across this region. 
However, overall coverage is still expected to remain too low to 
include a general thunderstorm area due to meager instability. 

A band of showers extends north-south across the western Gulf of 
Mexico to the central Texas coast this morning in a zone of low-level 
moisture return and warm air advection. While a lightning strike or 
two cannot be completely ruled out with this activity along the 
immediate Texas coast, low-level flow is forecast to slowly weaken 
across this region by this afternoon/evening. This diminishing 
low-level flow and warm air advection coupled with no obvious 
large-scale forcing mechanism for ascent suggest thunderstorm 
potential should remain low. 

A weak mid/upper level trough over la/MS this morning will continue 
to fill/weaken through the period as it moves slowly eastward across 
the southeast. While some low-topped showers have formed across la 
in association with this mid/upper-level trough, both lift and 
instability should remain too limited to support thunderstorms. 


Mesoscale Discussion

acus11 kwns 180310 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 180309 

Mesoscale discussion 0031 
National Weather Service Storm Prediction Center Norman OK 
0909 PM CST Wed Jan 17 2018 

Areas affected...the Outer Banks area of North Carolina into the 
Tidewater area of southeast Virginia 

Concerning...heavy snow 

Valid 180309z - 180715z 

Summary...heavy snow rates in excess of 1 inch per hour, including 
occasional heavier convective bursts, may develop across the North 
Carolina Outer Banks area, possibly into portions of the Virginia 
Tidewater, during the 11 PM to 4 am time frame. 

Discussion...a significant mid-level trough is forecast to continue 
pivoting east of the southern Appalachians during the next few 
hours. As it does, models suggest that it may take on more of a 
neutral to negative tilt, accompanied by an increasingly better 
defined lower/mid tropospheric cyclonic circulation center across 
the Piedmont through coastal areas. This appears likely to 
contribute to a consolidating area of upward vertical motion, with 
considerable strengthening by the 05-08z time frame, centered across 
the Outer Banks area of North Carolina. 

Aided by dynamic cooling, cold advection and melting precipitation, 
thermodynamic profiles near immediate coastal areas are expected to 
cool entirely below freezing, supporting snow or a changeover to 
snow. Rapid refresh forecast soundings indicate that lift may 
become maximized within the dendritic growth layer for several 
hours, supporting snow rates in excess of 1 inch per hour, in the 
presence of precipitable water between .5 and .75 inches. At least 
some convective enhancement appears possible leading to occasional 
heavier bursts of snow, before diminishing/spreading offshore after 

It is possible similar snow rates could impact portions of the 
southeast Virginia Tidewater, but the most prolonged/heaviest snow 
appears most likely to the south, across the Outer Banks area 
near/north of Cape Hatteras. 

.Kerr.. 01/18/2018 

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


Latitude...Lon 36377656 36697547 34977525 35127595 35307683 36377656