U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211258 
Storm Prediction Center ac 211257 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0657 am CST Thu Feb 21 2019 

Valid 211300z - 221200z 

..no severe thunderstorm areas forecast... 

scattered thunderstorms are possible across the southeast and 
southern mid-Atlantic states today. Isolated thunderstorms also may 
affect parts of Arizona and Southern California. 

In mid/upper levels, mean troughing will be maintained over the 
western conus, as a strong shortwave perturbation digs southeastward 
over California and northern baja. This will lead to both: 
1. An erratic, net southward shift of the cyclone now centered over 
central NV, with a 500-mb low probable over the yum/eed corridor of 
the lower Colorado River valley by 12z; 
2. Increasing heights within a low-amplitude, synoptic-scale 
ridging over the Ohio Valley and southeast -- also enabled by the 
eastward ejection of a trough now over northern on. 

A positively tilted shortwave trough in the southern stream -- now 
evident in moisture-channel imagery from southern baja southwestward 
over the Pacific -- will eject northeastward in confluent flow 
through the period, while weakening. Leading parts of the 
increasingly stretched/deformed vorticity Banner should reach West 
Texas by 00z, then extending from eastern OK to near drt by 12z. 

At the surface, 11z analysis showed an occluded low over 
northeastern on with triple point over southeastern on, and cold 
front southwestward across the central/southern Appalachians to 
southeastern MS. A weak low was drawn over the northwestern Gulf, 
offshore from Padre Island, with wavy/quasistationary/marine frontal 
zone across the mouth of the Mississippi River to southeastern Alabama 
and south-central GA, the offshore SC. The latter front will move 
northward as a warm front through the period, merging with the 
southern limb of the other frontal zone, itself having become nearly 
stationary over MS/al/GA. 

Scattered thunderstorms are expected to occur episodically through 
the period, in a slowly northward-shifting, west-southwest/east- 
northeast belt of elevated convection. This activity will be most 
concentrated near and north of the northern front, and well north of 
the marine/warm front. The most favorable deep shear will be north 
of the surface-based effective-inflow parcels, with winds aloft 
decreasing southward toward the Gulf coastal plain, where isolated 
to widely scattered also thunderstorms are possible. 

No organized severe is expected, with the bulk of the convection 
occurring in an environment of modest low/middle-level lapse rates 
and predominantly elevated buoyancy. Hail near severe limits from 
any of the convective areas, or a strong/damaging gust closer to the 
coast, cannot be ruled out. However, the potential appears too 
speculative, conditional and isolated to warrant a categorical 
outlook area at this juncture. 

..southern CA/AZ... 
Shortwave-encouraged amplification of the synoptic trough across 
this region will maintain and expand the area of cold air aloft via 
large-scale lift, related to midlevel DCVA and forcing by the 
left-exit region of a cyclonically curved jet streak digging 
southward down the West Coast. Associated steep low/middle-level 
lapse rates will support scattered to numerous showers and isolated 
thunderstorms, both: 
1. In portions of Southern California and baja where a moist marine 
boundary layer moves onshore and is lifted upslope; 
2. Over parts of AZ, where low/middle-level moisture will favor 
convective development embedded in a persistent/growing precip area, 
amidst weak cinh and strongly difluent flow aloft. 

.Edwards.. 02/21/2019 


Mesoscale Discussion

acus11 kwns 210315 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210315 

Mesoscale discussion 0118 
National Weather Service Storm Prediction Center Norman OK 
0915 PM CST Wed Feb 20 2019 

Areas affected...parts of PA/NY/NJ into New England 

Concerning...winter mixed precipitation 

Valid 210315z - 210815z 

Summary...a large area of wintry precipitation -- with mainly snow 
across far northeast NY, much of New Hampshire and VT, and most of ME, and 
transitioning to mainly freezing rain south of this area -- is 
expected to spread across the discussion area overnight. 

Discussion...a widespread band of primarily freezing rain continues 
to shift quickly eastward across central New York/eastern PA, where ice 
accumulation continues. This area of precipitation will spread into 
-- and across -- New England overnight, moving off the southern New 
England coast and diminishing over Maine beginning prior to sunrise. 

At this time, temperatures remain in the low to mid 20s across most 
of eastern New York and southern New England, and in the teens to low 20s 
over much of VT/NH/ME. Gradual warming in the 900 to 700 mb layer 
will continue as low-level warm advection increases and secondary 
surface low pressure develops offshore. 

This increasing warm advection should allow a transition from 
freezing rain to rain to occur across coastal southern New England 
overnight, and likewise a transition from light snow to freezing 
rain over the remainder of southern New England. Indeed, this 
transition from snow to freezing rain is already observed at some 
locales, and this trend will continue over the next few hours as 
warm advection above a shallow/cold boundary layer increases. 

The precipitation area is relatively fast-moving -- owing to the 
fast westerly flow aloft -- and as such, overall snowfall 
accumulations will be limited despite rates of 1" per hour briefly 
possible in a few areas. Meanwhile, freezing rain accumulations of 
over 0.05" to .10" per hour will occur for a couple of hours in some 
locations, potentially resulting in hazardous Road conditions in 
some areas overnight. 

.Goss.. 02/21/2019 

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


Latitude...Lon 41657170 41317340 40707456 40607590 40937692 41747730 
43107713 43907563 44667360 45097032 44546951 42787108 
41817083 41657170