U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 181258 
Storm Prediction Center ac 181256 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0656 am CST Tue Dec 18 2018 

Valid 181300z - 191200z 

..no severe thunderstorm areas forecast... 

severe thunderstorms are not forecast today or tonight across the 
contiguous United States. 

The mid/upper level pattern day-1 will feature a progressive 
shortwave train. A leading, negatively tilted but weak shortwave 
trough -- now located from the Central High plains east- 
southeastward across portions of OK/Arkansas -- will eject east- 
northeastward up the Ohio Valley and weaken through tomorrow 
morning. A strong upstream shortwave trough -- now apparent in 
moisture-channel imagery over parts of nm and the Sonora/Chihuahua 
border area -- will move eastward to southern OK, central Texas and 
coahuila by 12z. A temporary/embedded 500-mb cyclone is forming 
over southern nm at this time, and will progress across west-central 
Texas overnight before devolving back to an open wave. 

Meanwhile, a currently low-amplitude shortwave trough now 
approaching the coastal Pacific northwest will move inland today, 
its southern rim occupied by a 150-170-kt 250-mb jet Max. Portions 
of the two features discussed in this paragraph will phase/merge in 
the next couple days as part of the deep synoptic trough development 
pertinent to day-3 severe-weather potential in Florida. 

..western Pacific northwest... 
Isolated thunderstorms are expected to form episodically within a 
broader onshore-flow/convergence regime containing widely scattered 
to scattered shallower convective showers. This activity will occur 
in the zone of large-scale upward vertical velocity/cooling aloft near the mid/upper- 
level trough. Lapse rates in the lightning-crucial -20 to -30 deg c 
layer will steepen considerably, accommodating roughly the top 1/4- 
1/3 of a buoyant profile containing 200-400 j/kg MUCAPE. As such, 
potential for lightning over and just inland from the coast should 
increase from morning through afternoon prior to trough passage, and 
diminish thereafter as temperatures warm again in upper levels. 

..central/East Texas tonight... 
Offshore boundary-layer flow is still apparent across about the 
eastern 1/2 of the Gulf, and the return flow over the western Gulf 
is far from mature. Still, surface isodrosothermal analysis and 
available raobs show strong modification is underway, and will 
continue through the remainder of this period and beyond. While 
likely not supporting inland penetration of increasing open-water 
SBCAPE through tonight, the elevated warm-advection/moisture- 
transport regime beneath cooling mid/upper-level air ahead of the 
trough should result in steepening lapse rates aloft. As low-level 
Theta-E increases above the surface, marginally favorable buoyancy 
for thunderstorms should develop over the outlook area tonight, 
along with increasing convective coverage. During the 06-12z time 
frame, widely scattered thunderstorms cumulatively are possible as 
parcels are lifted isentropically to level of free convection. 

..west TX, afternoon/early evening... 
Isolated thunder cannot be ruled out over parts of the Permian Basin 
region of West Texas in the immediate cold-core area of the cyclone 
aloft, where mid/upper-level lapse rates will be maximized. 
Forecast soundings show variable moisture, and up to about 250 j/kg 
MUCAPE rooted in the 600-700-mb layer under the most aggressive 
scenarios. At this time potential appears too conditional/isolated 
for a general-thunder area. 

.Edwards.. 12/18/2018 


Mesoscale Discussion

acus11 kwns 170704 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 170703 

Mesoscale discussion 1714 
National Weather Service Storm Prediction Center Norman OK 
0103 am CST Mon Dec 17 2018 

Areas affected...southern and eastern Maine 

Concerning...winter mixed precipitation 

Valid 170703z - 171300z 

Summary...snow, perhaps mixed with freezing rain or sleet, will 
remain possible through the overnight hours. 

Discussion...frontogenesis in the 850-700-millibar layer within the 
warm-conveyor belt has resulted in a band of heavy precipitation 
along the southeast coast of Maine. This band will continue to pivot 
northwest through the overnight hours. In the vicinity of this band, 
surface temperatures have fallen below 32f, despite rap forecasts 
suggesting temperatures would remain near or above 32f. Modifying 
the rap soundings for the observed colder surface temperatures, wet 
snow is likely, but given the strength of the warm-air advection 
within the warm-conveyor belt, temperatures within the 
850-700-millibar layer may approach, or even exceed, 0c. As such, 
precipitation may mix with, or change to, freezing rain or sleet 

.Marsh/Goss.. 12/17/2018 

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


Latitude...Lon 44197086 45246936 45826711 44936676 44246739 43256962 
43007082 44197086