Severe Thunderstorm Warning 55, NWS Jackson, MS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JACKSON MS
220 PM CST FRI FEB 18 2000

SEVERE TSTORM POTENTIAL THE MAIN FORECAST CHALLENGE TONIGHT...THEN
THINGS SETTLE DOWN SIGNIFICANTLY AFTER 09Z SATURDAY WITH TEMPERATURES
COOLING DOWN SIGNIFICANTLY ALL OF THE FORECAST AREA BY 00Z SUNDAY.
INITIAL LOOK AT VARIOUS PARAMETERS ON GRIB/SPECTRAL MODELS INDICATE
THE ETA HAS BEST HANDLE ON CURRENT COLD FRONT POSITION/SPEED AS
NGM/AVN A LITTLE TOO FAST AND HAVE FRONT OUTRUNNING THE QPF FIELDS
WHICH DOESN/T LOOK LIKELY WITH KANSAS SHORTWAVE/MISSOURI SURFACE LOW
RACING EAST ON HEELS OF 130-150KT 250 MB JET MAX OVER OHIO/TN
VALLEYS. ETA WILL BE USED WITH MODIFICATIONS AS IT UNDERDOES THE
DRY MID LEVELS TONIGHT AND IS TOO COOL AT THE SURFACE...THUS
UNDERDOING MID LEVEL LAPSE RATES AND INSTABILITY. OTHERWISE...REST
IS FINE.

TONIGHT...BEST CHANCE FOR SEVERE CONVECTION TO BE NORTH AND WEST OF
A NATCHEZ MS-JACKSON MS-COLUMBUS MS LINE WHERE BEST UPPER LEVEL
SUPPORT/1000-850MB MOISTURE CONVERGENCE/ AND SHEAR WILL BE OVER
THESE AREAS AS BOTH RR ENTRANCE REGIONS OF UPPER/LOWER JETS MOVE
ACROSS NORTHERN MS. WITH RECORD TO NEAR RECORD HIGHS TODAY (WHICH
ETA UNDERESTIMATED)...MODIFICATION OF JAN 12Z SOUNDING SHOW THE
FOLLOWING ENVIRONMENTAL PARAMETERS: 700-500MB LAPSE RATES OF NEAR
6.5 DEG C/KM...LIFTED INDICES AROUND -5...CAPES AROUND 2500
J/KG...0-3KM STORM RELATIVE HELICITIES 300-450 AND WET BULB ZEROES
AT AROUND 8000 KFT DUE TO DRY AIR AT 700 MB WHICH WILL LEND TO GOOD
EVAPORATIVE COOLING. THUS...SPC/S LOCATION OF SLIGHT RISK OVER NW
HALF OF FORECAST AREA LOOKS GOOD. WIDELY SCATTERED SHOWERS ALSO
FORMING ON DIFFERENTIAL HEATING BOUNDARY WELL AHEAD OF MAIN SQUALL
LINE OVER CENTRAL AR INTO NW LOUISIANA. THIS WILL HAVE TO BE WATCH
CLOSELY FOR POSSIBLE...RAPID SUPERCELL DEVELOPMENT NORTH OF I-20 IF
MESO-ETA MOISTURE CONVERGENCE PROGS PAN OUT AS THESE STORMS WOULD
HAVE THE ENVIRONMENT FOR THEMSELVES. AS FRONT MOVES THROUGH CENTRAL
AND SOUTH MISSISSIPPI AFTER 04-06Z...STORMS SHOULD WEAKEN AS THEY
PROPAGATE AWAY FROM UPPER SUPPORT. LOOK AT RADAR HAS 1ST PD POP
TREND TO BE LIKELY NW ZONES...TO GOOD CHANCE CENTRAL TO SCATTERED/
WIDELY SCATTERED SE ZONES. MAIN THREAT WILL BE HAIL AND DAMAGING
WINDS WITH MAIN SQUALL DO TO  AFOREMENTIONED PARAMETERS. TORNADO
WATCH #39 IN EFFECT UNTIL 7 PM  LOCAL TIME NORTHWEST OF A CROSSETT
AR-GREENWOOD MS-GRENADA MS LINE.

SATURDAY THROUGH SUNDAY...WIDELY SCATTERED SHOWERS WITH ISOLATED
THUNDER MAY LINGER IN THE EXTREME SOUTHEAST EARLY...WITH MOST
LOCATIONS SEEING PARTIAL CLEARING AND MUCH COOLER TEMPERATURES BY
AFTERNOON. 290-300K ISENTROPIC PROGS AND PROGGED SOUNDINGS INDICATE
ENOUGH DRYING/MIXING TO BREAK OUT EARLY MORNING STRATUS BY 15-18Z AS
STRONG SURFACE HIGH BUILDS INTO SOUTHERN PLAINS. LOW LEVEL CAA TO BE
PRETTY SIGNIFICANT WITH 15-20 DEG COOLER TEMPS FOR THIS WEEKEND WITH
NORTH FLOW AROUND 10 KTS AND PLENTY OF SUN. WE WELCOME BACK FEBRUARY.
SUNDAY MAY BE A FEW DEGREES WARMER...BUT NOT MUCH. GENERALLY TAKING
BLEND ON FAN/FWC MOS FOR BOTH LOWS/HIGHS.

IN EXTENDED...UPPER RIDGE DEVELOPS OVER SOUTHERN US WITH NWLY/DRY
FLOW FOR THE LOWER MISSISSIPPI VALLEY. RETURN FLOW BEHIND EXITING
SURFACE HIGH SHOULD RESULT IN TEMPERATURES REBOUNDING BACK TOWARDS
70 DEGREES WITH SCATTERED PRECIPITATION WEDNESDAY AS NEXT SYSTEM
DEVELOPS OVER SRN PLAINS AND COLD FRONT APPROACHES FROM THE WEST.

QPF DISCUSSION: APPROACHING COLD FRONT WILL MOVE RAPIDLY THROUGH THE
FA BY 12Z ON SATURDAY. MOST OF THE RAINFALL WILL ACCOMPANY THE
FRONT...THEREFORE...QPF NUMBERS WILL BE HIGHEST IN THE NORTHWEST
EARLY AND TREND TOWARD THE SOUTHEAST. MOST AMOUNTS WILL BE 1/2 TO 1
INCH.

PRELIM NUMBERS
JAN 048/057 035/062 5100
MEI 052/058 032/063 4200

.JAN...
MS...NONE.
AR...NONE.
LA...NONE.
05/02