Severe Thunderstorm Warning 151, NWS Grand Forks, ND

FORECAST DISCUSSION FOR EASTERN NORTH DAKOTA AND NORTHWEST MINNESOTA
NATIONAL WEATHER SERVICE EASTERN ND/GRAND FORKS
215 PM CDT THU JUL 15 1999

...MAIN FORECAST CHALLENGE LOCATION/STRENGTH OF CONVECTIVE ACTIVITY
OVER FA THIS EVENING...

MODELS INTIALIZED WELL THIS AM BUT BEGIN TO DIVERGE BY 18Z AT THE SFC.
ALL DO AGREE ON TAKING SFC LOW NE ACROSS NRN RRV BY EARLY EVENING WITH
LOW CONTINUIING TO TRACK INTO CENTRAL ONTARIO BY 12 FRIDAY. ETA DID
BEST WITH LOW LEVEL MOISTURE OVER FA...AND WITH PAST PERFORMANCE WILL
FOLLOW IT THIS PACKAGE.

.SHORT TERM...BROAD BAND OF SHOWERS AND THUNDERSTORMS CONTINUE TO PUSH
EAST ACROSS ND THIS AM AHEAD OF SFC LOW AND 5H VORT LOBE. ERN ZONES
HAVE HAD MORE SUN TODAY WHICH HAS PUSHED TEMPS UP INTO THE UPPER 80S TO
LOWER 90S WITH DEWPOINTS IN THE 70S MOST AREAS. LAPS SHOWS BEST
MOISTURE CONVERGENCE OVER FAR NW MN THIS AFTERNOON WITH 88D SHOWING
RAPID CONVECTIVE DEVELOPMENT THERE ATTM. HIGHEST CAPES FROM LAPS DATA
OVER SRN ZONES HOWEVER WITH ALL AREAS ALONG AND EAST OF RRV BECOMING
MORS UNSTABLE WITH TIME. BEST DIRECTIONAL SHEAR NOTED OVER FAR NW MN
PER ETA...BUT HIGHEST HELICITIES SHIFT NORTH INTO CANADA BY 0Z.
THINKING IS THAT BEST POTENTIAL FOR SVR THUNDERSTORMS TO BE OVER MN
ZONES INTO SE ND THIS EVENING WITH LARGE HAIL AND DAMAGING WINDS THE
MAIN THREAT WITH THE LOSS OF DIRECTIONAL SHEAR IN THE LOWER LEVELS.
ONGOING CONVECTIVE ACTIVITY OVER NORTHEASTERN ND HAS WORKED THE
ATMOSPHERE OVER ENOUGH THERE SO SEVERE THREAT LESSENED IN THIS AREA.
STILL EXPECT STRONG STORMS THIS AREA THIS EVENING/OVERNIGHT AS COLD
FRONT BEGINS TO MOVE IN.

TIMING OF FROPA OVER FA TONIGHT PROBLEMATIC AS MISSING ETA DATA FOR
THOSE TIMEFRAMES. ATTM EXPECT FRONT TO BE THROUGH ALL BUT EXTREME SE
ZONES BY 12Z. DRIER AND COOLER AIR MOVING IN BEHIND FRONT...BUT WITH
NGM UNDERPLAING DEWPOINTS OVER REGION WILL STAY ABOVE FWC NUMBERS.

.LONG TERM...HIGH PRESSURE BUILDS INTO FA BEHIND COLD FRONT WITH DRIER
AIR FILTERING IN. COOLING IN MID LEVELS COMBINED WITH SOME DAYTIME
HEATING SHOULD BE ENOUGH TO TRIGGER INSTABILITY SHOWERS OVER THE FA
WITH HIGHEST POPS OVER THE NORTHEAST. WILL GO WITH 20 POPS ERN ND AND
WEST CENTRAL MN...WITH 30 POPS ACROSS FAR NE. WILL LEAN CLOSER TO
COOLER FAN NUMBERS NORTH AS ETA/AVN SHOWING A SECONDARY COOL SURGE
THERE...AND SPLIT DIFFERENCE WITH FWC/FAN OVER THE SOUTH. SFC HIGH
DOMINATING REGION FOR THE START OF THE WEEKEND WITH DRY AND MILD
CONDITIONS PREVAILING.

.EXTENDED...MRF PRETTY CONSISTENT FROM RUN TO RUN IN BUILDING 5H RIDGE
OVER FA. WEAK S/W MOVING INTO THE RIDGE SATURDAY NIGHT WILL BRING A
SLIGHT CHANCE FOR THUNDERSTORMS TO FA THEN WITH A BETTER CHANCE IN
RETURN FLOW ON SUNDAY. COLD FRONT TO SAG SOUTH ACROSS REGION SUNDAY
NIGHT. WHILE WEAK S/W CONTINUE TO PASS OVER AREA...ONLY EXPECT SCT
SHOWERS AS SFC HIGH REMAINS IN PLACE. WILL CONTINUE TREND OF COOLER
THAN NORMAL HIGHS AND NEAR SEASONAL LOWS THROUGH EXTENDED.

THANKS FOR COORDINATION DLH.

.FGF...NONE.

RJT