U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201301 
Storm Prediction Center ac 201300 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0700 am CST Tue Feb 20 2018 

Valid 201300z - 211200z 

..there is a marginal risk of severe thunderstorms across the 
Arkansas-la-tex and Ozarks... 

Thunderstorms are expected across a broad part of the south-central 
U.S. Into the Midwest, with the possibility of a few strong to 
locally severe storms mainly late this afternoon into evening across 
The Ark-la-tex vicinity and Ozarks. 

A highly amplified large-scale pattern will exist over the Continental U.S. 
Through the period, highlighted by a persistent longwave trough over 
the western states and an upper ridge centered over the coastal 
southeast states and nearby Atlantic Ocean. 

..arklatex and Ozarks... 
Multiple corridors of scattered thunderstorms will continue to move 
northeastward today across the Southern Plains/Ozarks to middle 
Mississippi Valley, both ahead of and behind a 
southeastward-accelerating strong cold front. This front should 
roughly extend from near the St Louis area southwestward to near 
Fort Smith, Arkansas and farther southwestward toward the 
Dallas-Fort Worth metroplex vicinity by early evening (00z). 

The large-scale pattern, with forcing for ascent largely maximized 
on the cool side of the southeastward-advancing front, typically 
supports a most-common hazard of locally heavy/persistent rainfall 
on the warm side of the boundary and wintry mixed precipitation on 
the cold side, with relatively limited or at least somewhat 
uncertain warm-sector severe potential otherwise. The aforementioned 
early-day scattered convection/cloud cover ahead of the front 
further casts uncertainty on the scope of today's severe risk. 

That said, some strong to locally severe storms are conceivable 
especially later this afternoon through early evening presuming some 
pre-frontal cloud breaks and modest destabilization. This currently 
appears plausible from the dfw metroplex vicinity and nearby parts 
of North Texas northeastward into Arkansas and southern Missouri. 
Presuming at least modest pre-frontal destabilization, there will be 
the potential for aggregating/organizing northeastward-moving 
clusters and bows amidst very strong southwesterly winds (50 kt 
within the lowest 1-2 km agl) just above the boundary layer. This 
could offer some potential for localized wind damage and possibly 
even a tornado mainly late this afternoon into early/mid evening. 

.Guyer.. 02/20/2018 


Mesoscale Discussion

acus11 kwns 201453 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201452 

Mesoscale discussion 0075 
National Weather Service Storm Prediction Center Norman OK 
0852 am CST Tue Feb 20 2018 

Areas affected...north-central/central Texas...far south-central OK 

Concerning...severe potential...watch unlikely 

Valid 201452z - 201645z 

Probability of watch issuance...20 percent 

Summary...isolated hail and damaging wind gusts are possible as a 
convective line moves north-northeastward across north-central Texas. 

Discussion...regional radar imagery shows a pre-frontal convective 
line extending from Montague County in far north-central Texas 
southwestward to San Saba County in central Texas. Given the lack of 
surface response as this line moves through as well as low-level 
stability noted on regional 12z soundings, this line predominantly 
consists of elevated thunderstorms. Updraft strength within this 
line has varied over the last hour but current radar imagery shows a 
few fairly strong updrafts with at least 30 dbz reaching above 9 
km/30kft. Additionally, vertical veering VAD wind profiles from fws 
and grk are supportive of rotating updrafts, which is verified by 
modest mid-level rotation observed by fws within a few cells. 

These kinematic and thermodynamic conditions will likely persist as 
the band moves north-northeastward, supporting the potential for 
isolated hail. This hail is generally expected to be sub-severe 
although some instances of hail around 1" are possible. 
Additionally, despite the low-level inversion and resulting 
low-level stability, strong low-level wind fields (i.E. 50 kt at 1km 
from both grk and fws) may support downdrafts strong enough to 
occasionally reach the surface. Consequently, isolated damaging wind 
gusts are also possible. Even so, instances of severe weather are 
expected to be isolated, precluding the need for a watch. 

.Mosier/Weiss.. 02/20/2018 

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


Latitude...Lon 33269806 34219757 34449708 34409669 34169646 33659634 
32939645 32289660 31689678 30949716 30529821 30719891 
31539886 32479845 33269806