Alert archive / Louisiana / Mesoscale Discussions in 2017

Mesoscale Discussion 1799

December 20, 201720 parishes in Louisiana; 6 counties in Mississippi

The Storm Prediction Center issued this mesoscale discussion at Dec 20, 2017, 1:55 AM CST for 20 parishes in Louisiana; 6 counties in Mississippi. It was in force for 3 hr.

The area

The outline as issued.

The facts

Issued: Dec 20, 2017, 1:55 AM CST (07:55 UTC)
Expired: Dec 20, 2017, 4:30 AM CST
In force: 3 hr
Issued by: the Storm Prediction Center
Chance of a watch: 20%

All 26 counties

Acadia Parish, LA, Allen Parish, LA, Avoyelles Parish, LA, Beauregard Parish, LA, Calcasieu Parish, LA, Caldwell Parish, LA, Catahoula Parish, LA, Concordia Parish, LA, East Carroll Parish, LA, Evangeline Parish, LA, Franklin Parish, LA, Grant Parish, LA, Jefferson Davis Parish, LA, LaSalle Parish, LA, Madison Parish, LA, Rapides Parish, LA, Richland Parish, LA, Saint Landry Parish, LA, Tensas Parish, LA, West Carroll Parish, LA, Adams County, MS, Claiborne County, MS, Issaquena County, MS, Jefferson County, MS, Warren County, MS, Wilkinson County, MS

The radar

The Ft Polk radar (KPOE), 73 km from the middle of the area, began a volume at 07:55 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.

The nearest sounding before it

Lake Charles Mun, LA (KLCH), launched 00:00Z, 7.9 hours before the alert went out and 162 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.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 888 J/kg
Mixed-layer CIN: -6 J/kg
Mixed-layer LCL: 503 m above ground
0-6 km bulk shear: 53 kt
0-1 km storm-relative helicity: 118 m²/s²
0-3 km storm-relative helicity: 152 m²/s²
Significant tornado parameter: 0.7
Supercell composite: 3.7

Open this sounding in Storm Lab

The text

As transmitted.

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SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Dec 20, 6:18 AM CST	Mesoscale Discussion	SPC	Saint Helena, Tangipahoa, Washington	2 hr

Nearby

Mesoscale Discussions in Louisiana, 2017 Mesoscale Discussions in Mississippi, 2017 Every mesoscale discussion in 2017

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.