Severe Thunderstorm Warning 244, NWS Twin Cities, MN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
900 PM CDT SAT SEP 18 1999

RADAR LOOP SHOWS WELL DEFINED FINE LINE ASSOCIATED WITH COLD FRONT
FROM JUST SOUTH OF REDWOOD FALLS NORTHEAST TO MPX SITE ON NORTH TO
CAMBRIDGE. PRECIPITATION ACTIVITY LAGS WELL BEHIND FRONTAL BOUNDARY
OR ACROSS CENTRAL MINNESOTA BACK INTO DAKOTAS.

PRECIPITATION LINES UP WELL WITH RIGHT REAR QUAD JET DYNAMICS OF 80
KT SPEED CORE JUST NORTH OF INTERNATIONAL FALLS MN. ALONG STREAM
SPEED SHEAR OF 40 KNOTS BACK INTO DAKOTAS IS FAIRLY IMPRESSIVE FOR A
SPEED MAX OF THIS SCALE ETA/NGM AND AVN MODELS ARE IN REMARKABLE
AGREEMENT ON 30H LEVEL JET FEATURES.  PRECIPITATION IS ALSO
OCCURRING IN AN AREA OF DEEPER MOISTURE AS REFLECTED IN LOOP OF
MOISTURE CHANNEL IMAGERY.

THE SHAPE OF UPSTREAM RIDGE HOLDS THE KEY TO WEATHER DEVELOPMENTS
ACROSS MPX COUNTY WARNING AREA OVER NEXT 24 TO 36 HOURS.
SUPERCRITICAL CURVATURE IS DIAGNOSED WITH UPSTREAM RIDGE. THUS...FOR
GRADIENT WIND RELATIONSHIP TO HOLD UNDER THIS FLOW PATTERN...PARCELS
MUST FIRST FLOW ACROSS HEIGHT CONTOURS TOWARDS LOWER PRESSURE JUST
EAST OF THE RIDGE AXIS (AND ACCELERATE AS THEY DO SO)...THEN BLAST
BACK TOWARD HIGHER PRESSURES AS CORIOLIS TERM TAKES OVER AFTER
HIGHER VELOCITIES ARE ACHIEVED. (I.E. FLOW COMPENSATION). THE END
RESULT OF ALL THIS IS A SHEAR AXIS WILL FORM IN THE UPPER TROPOSPHERE. WITH
THESE DYNAMIC DEVELOPMENTS THE ABOVE MENTIONED JET MAX WILL BE
PRESSED SLOWLY SOUTH AND EAST WITH TIME. THIS WILL MEAN RIGHT REAR
QUAD INDUCED UPPER LEVEL DIVERGENCE WILL PUSH SOUTH AND EAST ACROSS
THE ENTIRE MPX CWA BY SOMETIME SUNDAY EVENING.

ISENTROPIC LIFT MECHANISM EXPECTED TO GRADUALLY SLIDE SOUTH AND EAST
AS COLD DOME PUSHES IN. ATMOSPHERE LIKELY TO MOISTEN WITH TIME (JUST
AS BUFKIT ETA PROFILES SHOW) AND HIGHER POPS ARE NEEDED. ZONES
UPDATED EARLIER TO REFLECT THIS THINKING.

.MSP...NONE.
WET

.MSP...NONE
JVM