8 short years ago today, an intense pacific jet carved a deep progressive longwave trough over the High Plains

...following a sharp arctic blast across the midwest characterized by sub 490 dm heights over the Hudson Bay. The Pacific jet reached its full fury when the western ridge crashed, followed by the retreat of the PV over Hudson Bay. At H5, the jet streak reached speeds of over 110 kt as it crashed ashore in California. With very little impedement in the form of a SE ridge or a substantial PV over Hudson Bay anymore, a large low-amplitude longwave trough was carved, and with several days of moderation along with a strong surface low pressure, moisture streamed in. Strong speed and directional shear provided an adequate environment for strong, long track supercells capable of producing significant to violent tornadoes. SPC issued a High Risk for January 21, 1999, at 6Z.

Thanks to the SPC High Risk archive...

CONVECTIVE OUTLOOK...REF AFOS NMCGPH94O.

VALID 211200Z - 221200Z

THERE IS A HIGH RISK OF SVR TSTMS THIS AFTERNOON AND EVENING OVER
PARTS OF CENTRAL/EASTERN AR...NORTHEAST LA...NORTHERN MS...AND
WESTERN TN...TO THE RIGHT OF A LINE FROM ARG DYR 30 WNW MSL CBM 45
NE JAN MLU HOT ARG.

THERE IS A SLGT RISK OF SVR TSTMS TO THE RIGHT OF A LINE FROM GLS
GGG PGO HRO MDH OWB BNA BHM MOB.

GEN TSTMS ARE FCST TO THE RIGHT OF A LINE FROM 45 E PSX TYR MKO SZL
PIA MIE CRW PSK SPA ATL PNS.

GEN TSTMS ARE FCST TO THE RIGHT OF A LINE FROM 30 WSW P28 CSM TCC
LVS ALS COS GCK 30 WSW P28.

STRONG UPPER JET MAX CURRENTLY OVER THE WESTERN STATES IS FORECAST
TO MOVE SOUTHEASTWARD INTO THE SOUTHERN ROCKIES BY THIS
AFTERNOON...WITH LARGE UPPER TROUGH RAPIDLY AMPLIFYING OVER THE
PLAINS. NOSE OF STRONG MID LEVEL WINDS WILL SPREAD INTO CENTRAL TX
BY 00Z...WITH SURFACE LOW DEEPENING OVER OK. SURFACE DRYLINE WILL
EXTEND SOUTHWARD ACROSS EAST TX...WITH WARM/HUMID AIRMASS ALREADY
IN PLACE TO THE EAST.

ISOLATED STRONG/SEVERE THUNDERSTORMS MAY BE ONGOING AT THE
BEGINNING OF THE PERIOD OVER SOUTHERN IL/WESTERN KY...WITH LARGE
HAIL BEING THE MAIN THREAT. THESE STORMS SHOULD SPREAD EASTWARD
ACROSS THE OH VALLEY DURING THE DAY IN REGION OF MODERATELY STRONG
WARM ADVECTION. HOWEVER...WEAKENING INSTABILITY WILL LIMIT
EASTWARD EXTENT OF SEVERE THREAT.

AIRMASS ALONG AND EAST OF DRYLINE OVER AR/LA WILL BE MODERATELY
UNSTABLE BY MID AFTERNOON WITH SURFACE-BASED CAPE VALUES OF 1500-
2000 J/KG. ISOLATED THUNDERSTORMS ARE EXPECTED TO DEVELOP IN THIS
AREA AND SPREAD RAPIDLY EAST-NORTHEASTWARD ACROSS WESTERN
TN/NORTHERN MS DURING THE EVENING. MODEL FORECAST SOUNDINGS SHOW
VERY STEEP MID LEVEL LAPSE RATES AND DRY AIR ALOFT...FAVORABLE FOR
LARGE HAIL AND DAMAGING WINDS. ALSO...STORM-RELATIVE HELICITY
/300-500 M2/S2/...MAGNITUDE OF DEEP LAYER SHEAR /SFC-6KM SHEAR OF
45-55 KNOTS/...AND STRENGTH OF MID LEVEL STORM-RELATIVE WINDS /20-
30 KNOTS/ INDICATE POTENTIAL FOR A FEW TORNADIC SUPERCELLS ACROSS
THE HIGH RISK AREA. GIVEN THE STRENGTH OF FORECAST SHEAR
MAGNITUDES AND THERMODYNAMIC PROFILES...ISOLATED SIGNIFICANT
TORNADOES ARE POSSIBLE TODAY OVER PARTS OF SOUTHEAST AR...NORTHEAST
LA...SOUTHWEST TN...AND NORTHERN AL.

ACTIVITY WILL LIKELY PERSIST THROUGHOUT THE NIGHT...SPREADING INTO
PORTIONS OF MIDDLE TN AND AL BY 22/12Z. DAMAGING WINDS AND HAIL
SHOULD BE THE MAIN THREAT BY THAT TIME.

..HART.. 01/21/99


Nobody would've ever fathomed of the destruction that would follow. 84 tornadoes touched down on that fateful day, including 24 significant tornadoes and 10 tornadoes of F3 intensity or greater. Several counties in central AR were pummeled again and again by training tornadic supercells.

Image courtesy of JKTinMemphis:

http://www.easternuswx.com/bb/uploads/post-1877-1151183686.png

"Classic" string of pearls look, never before seen in January. It was by far the most powerful tornado outbreak in January ever, following the outbreak of January 17.

In all, 8 people were killed and hundreds injured in what would be the biggest January outbreak ever.

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500mb plots, geopotential height and wind speeds:

0Z Jan 21

http://img66.imageshack.us/img66/8075/1999012100z2zj.png




0Z Jan 22

http://img66.imageshack.us/img66/3277/1999012200z2tw.png