U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211239 
Storm Prediction Center ac 211237 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0737 am CDT Wed Mar 21 2018 

Valid 211300z - 221200z 

..no severe thunderstorm areas forecast... 

a few thunderstorms are possible this afternoon and evening across 
parts of central/northern California and the Great Basin. 

Moisture-channel imagery and available upper-air raobs revealed 
broadly cyclonic mid/upper-level flow over much of the eastern U.S., 
With a low over northeastern Kentucky. As upstream speed/vorticity maxima 
move through the cyclone's southern semicircle, the vortex will 
expand somewhat and move eastward, its center crossing the Delmarva 
area around 00z. By 12z, the mid/upper-level cyclone center's path 
should curve back toward the ack/Cape Cod region, becoming more 
vertically stacked with the low-level vortex. At the surface, 11z 
analysis showed the main low over Atlantic waters, south of Long 
Island and east of Hampton Roads. This low is expected to move 
northeastward to just offshore from ack by 12z. The cold front was 
evident across the northern Bahamas and southern Straits of Florida, 
and will continue moving away from the Mainland. In its wake, a 
low-Theta-E, Continental/polar air mass will preclude thunder 
potential over the central and eastern parts of the U.S. 

Ridging aloft will build over The Rockies through day-2, as a 
high-amplitude trough strengthens offshore from the Pacific coast. 
A cyclone over the eastern Gulf of Alaska, centered just off the Alaska 
Panhandle, will migrate south-southeastward, while a series of 
mostly weak shortwaves in southwest flow cross California. As this occurs, 
midlevel lapse rates should steepen enough to support widely 
scattered to scattered showers and at least isolated thunderstorms, 
particularly over the Central Valley of California where pockets of diabatic 
heating also will increase low-level lapse rates. Forecast 
soundings in the southern Sacramento Valley area show 200-600 j/kg 
MLCAPE developing, amidst brief periods of enough low-level shear to 
enlarge 0-3-km hodographs and cause some concern for storm-scale 
rotation. Small hail cannot be ruled out, though a 
consistent/coherent focus for growth/maintenance of any 
supercellular structures appears lacking at this time. Given 
favorable mesoscale boundaries and destabilization trends, 
marginal/brief tornado risk may develop. Currently that threat 
appears too small, isolated and conditional for an outlook area. 

Very isolated lightning cannot be ruled out farther north across the 
interior Pacific northwest, wherever some weak cape can be generated 
that extends past about the -20 c isotherm. However, Theta-E 
appears too weak, coverage too low, and forcing too nebulous, to 
draw/extend a general-thunder area over that region at this time. 

.Edwards/Peters.. 03/21/2018 


Mesoscale Discussion

acus11 kwns 210156 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210155 

Mesoscale discussion 0164 
National Weather Service Storm Prediction Center Norman OK 
0855 PM CDT Tue Mar 20 2018 

Areas affected...portions of WV and western/central Virginia 

Concerning...heavy snow 

Valid 210155z - 210800z 

Summary...a band of moderate to heavy snow will develop 
northeastward across WV and western/central Virginia this evening 
and overnight. Snowfall rates around 1 inch per hour are likely, 
especially in the higher terrain. 

Discussion...a shortwave trough will pivot northeastward across the 
mid-Atlantic this evening/overnight, while a large-scale upper 
trough over the Ohio/Tennessee Valley transitions to a closed low. Forcing 
for ascent preceding the shortwave trough will encourage development 
and deepening of a surface low off the NC/Virginia coast through the early 
overnight hours. A separate area of weak low pressure will remain 
over WV/far southwestern Virginia. A front will extend between these two 
lows, with a cold low-level airmass in place across much of the 
mid-Atlantic to the north of the front. 

Moderate to heavy precipitation is ongoing this evening across 
southwestern Virginia into parts of WV in association with the previously 
mentioned shortwave trough. The 00z sounding from Roanoke, Virginia shows 
a modest (around 3.7 degrees c) low-level warm nose centered around 
820 mb. As mid-level temperatures cool with the shortwave trough 
passage, the low-level thermal profile is likewise forecast to 
quickly fall below freezing across WV and parts of western/central 
Virginia. Indeed, recent surface observations and mping reports across 
southwestern Virginia are indicating a fast transition from rain to snow, 
with some sleet mixed in at the leading of the precipitation shield. 

Continued east-northeasterly winds in the surface-850 mb layer will 
enhanced uplift along the Blue Ridge and Appalachian Mountains this 
evening and overnight. Snowfall rates of 1 inch per hour appear 
likely across much of the discussion area, especially in the higher 
terrain where this low-level uplift will be maximized. Most recent 
high-resolution guidance is in reasonable agreement in the placement 
of this moderate to heavy snowfall axis through about 08z. 
Precipitation may lessen/shift farther eastward after this time as 
the coastal low develops northeastward over the western Atlantic and 
the shortwave trough moves away from this region. 

.Gleason.. 03/21/2018 

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


Latitude...Lon 36998146 37498166 37918170 38398135 38708084 38878036 
38907998 39057954 39317912 39457873 39457833 39327795 
38877784 37997819 37257893 36777983 36608045 36658104