Flash Flood Warning 98, NWS State College, PA

CENTRAL PENNSYLVANIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
1 AM EDT WED JUL 8 1998

...WILL SEND WORK ZONES TO PIT/PHL BY 3AM...UNDER PHLADMCTP HEADER...

...DISCUSSION...

VERY INTERESTING SITUATION SETTING UP FOR WHAT LOOKS LIKE THE
SUSQUEHANNA VALLEY. MESO ANAL SHOWS COOL AIR DAMMING SETUP WITH WEAK
SFC BOUNDARY SNAKING THRU NERN PA. TDS HAVE COME UP MARKEDLY SINCE
LAST NIGHT WHICH WAS WELL ADVERTISED IN MODELS BY THE FCST SURGE IN
PRECIPITABLE WATER AND THETA-E.

LOOKING AT MESO DATA AS WELL AS SYNOPTIC MODEL OUTPUT SUGGESTS A N-S
BOUNDARY EXTENDING PRETTY MUCH ALONG THE SUSQ VALLEY. LOW LEVELS
OVER ERN PA HAVE CONTINUED TO BE COOLED BY SFC HIGH ANCHORED OVER
SRN NEW ENG WHILE MOISTURE HAS SURGED INTO MUCH OF THE REST OF THE
STATE ON SLY FLOW. WITH 850 JET FCST TO INTENSIFY FROM THE SW AND
CONTINUE TO FEED OVER THIS BOUNDARY...THINK THE ERN ARES WILL BE THE
FOCUS FOR POTENTIAL HEAVY RAINS. RADAR MAY ALREADY BE GIVING HINTS
OF THIS WITH RW/TRW STARTING TO FILL IN OVER AREA SOUTH OF MDT.

MANY INTERESTING THINGS GOING ON AT THE SYNOPTIC SCALE. MOST
PREVALENT BOUNDARY EXTENDS E-W OUT THRU OHIO VALLEY AND MIDWEST. ALL
MODELS AGREE IN FORMING WEAK SFC LOW ALONG THIS BOUNDARY AND MOVING
IT TO ERN OHIO OR WRN PA BY 12Z. THEN THEY SLOW IT DOWN AND SLIDE IT
SE TO MID ATL COAST. THIS HAS TO BE THE RESULT OF CONVECTION
DOMINATING THE LOW LEVEL MASS FIELDS BECAUSE UPPER PATTERN DOES NOT
SUPPORT THIS KIND OF SFC DEVELOPMENT...ALBEIT WEAK. LAST NIGHTS
MODELS PAINTED A SHARPENING COLD FRONT DEVELOPING OUT OF THIS PATTERN
WHILE TONIGHT THE PICTURE GETS MORE UNFOCUSED WITH TIME.

WHAT WILL PROBABLY PAY TONIGHT WILL BE TO LOOK HARD AT THE BIGGER
WELL HANDLED FEATURES. ETA IS CAPTURING COOL AIR DAMMING BEST OF ALL
MODELS WHICH SHUD BE EXPECTED. ALSO SEEN IS STRENGTHENING LOW LEVEL
SWLY JET WHICH ALL MODELS FOCUS UP INTO ERN PA. AVN IS SLOWEST...NGM
WEAKEST AND ETA MOST ROBUST. VERY INTERESTING TO NOTE WHAT ETA DOES
TO LOW LEVEL THERMAL FIELD. 850MB 14C ISOTHERM IS MADE TO DROP
SOUTHWEST IN THE FACE OF INTENSIFYING SLY FLOW OVER ERN PA
OVERNIGHT. THIS RESULTS IN MARKED TIGHTENING OF ISOTHERMS OVER THIS
AREA BY MORNING. HOWEVER LOOKING AT MODEL GENERATED FRONTOGENESIS
SHOWS FRONTOLYSIS OVER THIS STRENGTHENING GRADIENT IMPLYING THE
THERMAL COLLAPSE IS STRICTLY DUE TO INTENSE UPWARD VERTICAL MOTION
GENERATED BY THE STRONG WARM ADVECTION. A CHECK OF THE AGEOSTROPHIC
FLOW THRU THE REGION SHOWS FLOW FROM WARM TO COLD WHICH SUPPORTS THE
FRONTOLYTIC FORCING IDEA.

WHAT COMPLICATES MATTERS IS THAT NO MODEL HAS A HANDLE ON QPF. OVER
NRN HUNTINGDON COUNTY EARLIER CONVECTION DROPPED NEARLY 3.5 IN OF
RAIN IN JUST 45 MINUTES...A TESTAMENT TO JUST HOW EFFICIENT THE
STORMS CAN BE WITH SUCH DEEP MOISTURE IN PLACE. ANOTHER COMPLICATION
IS CONVECTION FIRING SOUTHWEST OF THE REGION. THIS IS SEPARATED FROM
WHERE MODELS WANT TO CREATE BEST LOW LEVEL FORCING AND WARM
ADVECTION SO HAVE TO WONDER IF THIS WILL ROB LOCAL CONVECTION OR
JUST BE IRRELEVANT. MORE QUESTIONS THAN ANSWERS.

OVERALL THINK SEVERE THREAT WILL BE DIMINISHED TODAY GIVEN EXPECTED
SFC PATTERN DEVELOPMENT. IN FACT IT WILL LIKELY END UP BEING KIND OF
DRAB AND COOL OVER MOST OF THE AREA AS THE GEOSTROPHIC FLOW
MAINTAINS AND ERLY COMPONENT.

DRY AIR IS MADE TO RETURN FOR THURSDAY WITH ETA SHOWING SOME MEASURE
OF MOISTURE REMAINING TRAPPED NEAR A LOW LEVEL INVERSION. IT DOESNT
COOL MUCH ALOFT SO POTENTIAL FOR COLD ADVECTION STRATOCU WILL PROB
BE LIMITED.

LA CORTE