Alert archive / Texas / Tornado Watches in 2020
Tornado Watch 520
The Storm Prediction Center issued this tornado watch at Dec 31, 2020, 7:40 AM CST for 13 counties in Texas. It was in force for 7 hr.
The area
The outline as issued.
The facts
- Issued
- Dec 31, 2020, 7:40 AM CST (13:40 UTC)
- Expired
- Dec 31, 2020, 3:00 PM CST
- In force
- 7 hr
- Issued by
- the Storm Prediction Center
- Largest hail expected
- 1 in
- Strongest gust expected
- 60 kt
Counties Brazoria County, TX, Chambers County, TX, Fort Bend County, TX, Galveston County, TX, Hardin County, TX, Harris County, TX, Jasper County, TX, Jefferson County, TX, Liberty County, TX, Matagorda County, TX, Newton County, TX, Orange County, TX, Tyler County, TX
The radar
The Houston radar (KHGX), 13 km from the middle of the area, began a volume at 13:42 UTC, the scan nearest the moment this went out.
Open the radar archive at this moment
The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.
What SPC expected
The probabilities SPC published with the watch, for the watch area over its life.
- Two or more tornadoes
- 50%
- One or more EF2 to EF5 tornadoes
- 20%
- Ten or more severe wind reports
- 40%
- One or more gusts of 65 knots or more
- 20%
- Ten or more severe hail reports
- 20%
- One or more hailstones of 2 inches or more
- 10%
- Six or more combined severe hail and wind reports
- 60%
Warnings issued inside it
4 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Dec 31, 10:03 AM CST | Severe Thunderstorm Warning | HGX | Brazoria | 29 min |
| Dec 31, 11:25 AM CST | Severe Thunderstorm Warning | LCH | Hardin | 50 min |
| Dec 31, 11:51 AM CST | Flash Flood Warning | HGX | Brazoria, Galveston | 114 min |
| Dec 31, 11:53 AM CST | Flash Flood Warning | LCH | Hardin, Jefferson | 3 hr |
Discussions around it
SPC mesoscale discussions over the same counties from four hours before the watch to its end.
The nearest sounding before it
Lake Charles Mun, LA (KLCH), launched 12:00Z, 1.7 hours before the alert went out and 189 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.
- Mixed-layer CAPE
- 238 J/kg
- Mixed-layer CIN
- -8 J/kg
- Mixed-layer LCL
- 492 m above ground
- 0-6 km bulk shear
- 58 kt
- 0-1 km storm-relative helicity
- 181 m²/s²
- 0-3 km storm-relative helicity
- 209 m²/s²
- Significant tornado parameter
- 0.2
- Supercell composite
- 1.9
The text
As transmitted.
940 WWUS20 KWNS 311336 SEL0 SPC WW 311336 TXZ000-CWZ000-312100- URGENT - IMMEDIATE BROADCAST REQUESTED Tornado Watch Number 520 NWS Storm Prediction Center Norman OK 740 AM CST Thu Dec 31 2020 The NWS Storm Prediction Center has issued a * Tornado Watch for portions of The upper Texas coastal plain Coastal Waters * Effective this Thursday morning and afternoon from 740 AM until 300 PM CST. * Primary threats include... A few tornadoes possible Scattered damaging wind gusts to 70 mph possible Isolated large hail events to 1 inch in diameter possible SUMMARY...Thunderstorms will increase in coverage and intensity through the morning while spreading inland across the upper Texas coast. The storm environment will gradually become more favorable for supercells inland, with an attendant threat for a few tornadoes and damaging winds through early to mid afternoon. The tornado watch area is approximately along and 40 statute miles north and south of a line from 25 miles west southwest of Angleton TX to 20 miles north northeast of Port Arthur TX. For a complete depiction of the watch see the associated watch outline update (WOUS64 KWNS WOU0). PRECAUTIONARY/PREPAREDNESS ACTIONS... REMEMBER...A Tornado Watch means conditions are favorable for tornadoes and severe thunderstorms in and close to the watch area. Persons in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings. && AVIATION...Tornadoes and a few severe thunderstorms with hail surface and aloft to 1 inch. Extreme turbulence and surface wind gusts to 60 knots. A few cumulonimbi with maximum tops to 400. Mean storm motion vector 21035. ...Thompson
SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Dec 30, 8:15 PM CST | Mesoscale Discussion | SPC | Brewster, Crane, Pecos, Presidio, Upton | 6 hr |
| Dec 31, 2:16 AM CST | Mesoscale Discussion | SPC | Brewster, Crane, Glasscock, Pecos, Reagan, Sterling and 2 more | 6 hr |
| Dec 31, 7:10 AM CST | Mesoscale Discussion | SPC | Brazoria, Chambers, Fort Bend, Galveston, Hardin, Harris and 3 more | 95 min |
| Dec 31, 9:09 AM CST | Mesoscale Discussion | SPC | Archer, Baylor, Borden, Callahan, Childress, Clay and 39 more | 6 hr |
| Dec 31, 11:21 AM CST | Mesoscale Discussion | SPC | Chambers, Galveston, Hardin, Harris, Jefferson, Liberty and 2 more | 114 min |
| Dec 31, 1:27 PM CST | Tornado Watch | SPC | Newton, Orange | 8 hr |
| Dec 31, 2:44 PM CST | Mesoscale Discussion | SPC | Newton, Orange | 2 hr |
| Dec 31, 2:59 PM CST | Mesoscale Discussion | SPC | Baylor, Callahan, Coke, Cottle, Fisher, Foard and 16 more | 4 hr |
Nearby
Sources
The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.