U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 120057 
Storm Prediction Center ac 120056 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0656 PM CST Tue Dec 11 2018 

Valid 120100z - 121200z 

..no severe thunderstorm areas forecast... 

isolated thunderstorms remain possible this evening through tonight 
across western Washington and northwest Oregon. 

..western Washington/northwest or... 
Trends in satellite imagery and objective analyses indicated a 
change in air mass will occur across western parts of Washington/or this 
evening into tonight. A cold front (now moving into far western wa) 
will advance eastward, while cold low- to mid-tropospheric air 
(500-mb temperatures at or below -30 c) spreads across the 
Post-frontal environment. These cold midlevel temperatures are 
associated with a Pacific shortwave trough/exit region of an 
accompanying 100+ kt 500-mb jet shifting inland this forecast 
period. Lightning data from today through this outlook issuance 
indicated sporadic thunderstorms within an area of Post-frontal 

Steepening lapse rates are already evident in northwest WA, given 
the 00z uil sounding representing the leading edge of the 
Post-frontal environment, with buoyancy deep enough for charge 
separation. Sufficient destabilization, given steepening lapse 
rates, will allow for low-topped cellular convection that spreads 
inland to possibly produce isolated thunderstorms this evening into 
tonight. Weak instability/shear will preclude a severe-weather 

.Peters.. 12/12/2018 


Mesoscale Discussion

acus11 kwns 091734 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 091734 

Mesoscale discussion 1705 
National Weather Service Storm Prediction Center Norman OK 
1134 am CST sun Dec 09 2018 

Areas affected...portions of northern NC into southern WV and 
central/southern Virginia 

Concerning...heavy snow 

Valid 091734z - 092230z 

Summary...heavy snow will continue for a few more hours across parts 
of northwest NC and the higher terrain of southern WV and and 
western Virginia. Heavier snowfall also will continue to spread northeast 
into parts of central Virginia into early evening. A mix of snow and 
freezing rain and/or sleet also is possible across north-central NC 
into south-central Virginia the next few hours. 

Discussion...expansive area of moderate to heavy snowfall continues 
across parts of NC, WV and Virginia early this afternoon. The surface low 
is currently located just offshore the SC coast, and will continue 
to shift northeast near the NC coast through this evening. Midlevel 
moisture wrapping around the low level cyclone and forcing for 
ascent ahead of the approaching shortwave trough (currently over the 
Tennessee valley) will maintain wintry precip into the afternoon. A dry 
slot noted in WV imagery will continue to stream across NC and into 
eastern Virginia and some warming will occur over this area. As a result, 
snow may mix with freezing rain and/or sleet at times across 
north-central NC into south-central Virginia. 

Otherwise, cold air damming with a well-established cold conveyor is 
evident in observations across the Piedmont. Moderate to heavy snow 
will persist in this regime across the higher terrain and spread 
northeast into parts of central Virginia through the afternoon. Snowfall 
rates will generally be around 1 inch per hour, though heavier 
bursts approaching 2 inches per hour are possible. Precipitation 
will gradually wane from southwest (across nc) to northeast (wv and 
parts of va) late this afternoon into this evening. 

.Leitman.. 12/09/2018 

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


Latitude...Lon 35917922 35728036 35938125 36128151 36458188 36858190 
37438177 37808142 38068060 38447893 38507830 38447756 
38297706 38097673 37747670 37307681 37017694 36487754