U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 181301 
Storm Prediction Center ac 181300 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0700 am CST sun Nov 18 2018 

Valid 181300z - 191200z 

..no severe thunderstorm areas forecast... 

a few thunderstorms will be possible over coastal south Texas, but 
no severe weather is expected. 

In mid/upper levels, high-amplitude synoptic-scale ridging will move 
inland from the Pacific this period, while a mean trough resides 
over northeastern/central North America, southwestward from a large 
/complex cyclone covering Hudson Bay and the eastern Canadian Arctic 
islands. Embedded in the associated cyclonic-flow field, is a very 
elongated, positively tilted shortwave trough -- now apparent in 
moisture-channel imagery from the upper Great Lakes across the 
Central High plains to southern Nevada. This perturbation and its 
associated vorticity field are forecast to become even more zonally 
deformed through the period, reaching PA, southern in, southern MO, 
western OK, central portions of nm/AZ, and into the mean ridge over 
the Sierra, by the end of the period. 

At the surface, a weak frontal-wave low was drawn at 11z over 
western AR, with cold front southwestward across east-central/ 
south-central Texas to the Guadalupe Mountains vicinity. The front is 
forecast to move erratically eastward/southeastward, reaching 
western TN, central la, and the northeastern Gulf off the Texas coast, 
by 12z. 

..TX coastal plain... 
Isolated thunderstorms have been observed near I-10 east of Sat the 
past couple hours, and with overall convective coverage currently 
increasing, the potential exists for a few more thunderstorms today 
before the causative processes weaken. The modified 12z crp 
sounding shows sufficiently deep but meager buoyancy, with weak 
lapse rates leading to small cape density and MLCAPE/MUCAPE under 
700 j/kg. Though radar cross-sections of most cells so far have 
indicated otherwise, some parcels that reach level of free convection also may survive 
into the layers colder than -20c where icing processes supporting 
lightning production occur. The overland part of the warm- 
advection/moisture-transport regime will become increasingly 
elevated upon being undercut by the cold front, weakening as 
low-level winds above the surface lessen. Deep shear would be 
favorable for at least a conditional severe threat with effective- 
shear magnitudes around 40-50 kt. However, weaknesses of deep-layer 
forcing, lapse rates and buoyancy combine to suggest severe 
potential is too small for any unconditional probability areas. 

.Edwards.. 11/18/2018 


Mesoscale Discussion

acus11 kwns 160857 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 160856 

Mesoscale discussion 1652 
National Weather Service Storm Prediction Center Norman OK 
0256 am CST Fri Nov 16 2018 

Areas affected...portions of the mid-Atlantic and New England 

Concerning...heavy snow 

Valid 160856z - 161500z 

Summary...one area of heavy snow with rates in excess of 1 inch per 
hour will develop northeastward across northern New England this 
morning. Another area of moderate to locally heavy snow, with rates 
up to 1 inch per hour, will move across eastern PA into southern New 

Discussion...within large-scale upper troughing encompassing much of 
the central and eastern conus, a compact shortwave trough will 
continue moving quickly northeastward over the mid-Atlantic and New 
England regions this morning. A deepening surface low associated 
with this shortwave trough will likewise develop northeastward along 
the coast of southern New England. A band of moderate to heavy snow 
is occurring over western/northern NY, attendant to strong forcing 
for ascent and a mid-level deformation zone immediately ahead of the 
shortwave trough. Snowfall rates in excess of 1 inch per hour will 
likely continue for at least the next several hours as this band 
develops into northern New England. 

Across parts of central/eastern PA, northern NJ, southern NY, and 
southern New England, a mix of snow, sleet, and freezing rain have 
all been observed recently as strong low-level warm air advection 
has allowed for at least some melting of hydrometeors. However, a 
quick transition back to mainly snow is likely from west to east 
this morning as cold mid/upper-level temperatures embedded within 
the core of the shortwave trough overspread these areas. Forecast 
soundings from the NAM and rap both suggest that there will be 
sufficient wrap-around moisture remaining within the dendritic 
growth zone and steepening mid-level lapse rates to support moderate 
to locally heavy snow for a couple of hours. Snowfall rates up to 1 
inch per hour may occur. This scenario is well supported by recent 
high-resolution guidance, including most href members. A rapid 
decrease in snowfall intensity/coverage will likely be noted from 
west to east through the morning, as strong subsidence and mid-level 
drying behind the shortwave trough passage suppress precipitation 

.Gleason.. 11/16/2018 

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


Latitude...Lon 43667018 42917138 42457230 41937334 40907478 40407593 
40217652 40257704 41037762 41687752 43387669 44457594 
45047489 45087164 45387140 45447091 45837051 46087035 
46257028 46417018 46566969 46536828 46286771 45706771 
45216802 44626873 43996958 43667018