National Weather Service

FXUS61 KCTP 180646
AFDCTP
CENTRAL PENNSYLVANIA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE STATE COLLEGE PA
145 AM EST MON JAN 18 1999

...WORK ZONES OUT BY 330 AS PHLADMCTP...
...ZONES OUT BY 4 AM AS PHLZFPCTP...

DAYSHIFT ERECTED A FLASH FLOOD WATCH FOR SRN HALF OF OUR CWA AND 
THIS STILL LOOKS REASONABLE...NOT SO MUCH FOR EXCESSIVE QPF BUT FOR 
THE FACT THAT WE HAVE A FAIRLY COMPACT HARD SNOWPACK THAT IN MOST 
LOCATIONS WILL PROBABLY NOT ABSORB MUCH WATER. THIS POINTS TO A GOOD 
DEAL OF WHATEVER RAIN WE GET MANAGING TO RUN DIRECTLY OFF INTO THE 
MAINLY FROZEN WATERWAYS. WITH TEMPS EXPECTED TO JUMP INTO THE 40S 
ONCE AGAIN... POTENTIAL FOR ICE JAMS BREAKING UP AND CAUSING 
FLOODING EXISTS. GOOD NEWS IS THAT THERE ISN'T THE EXPECTATION OF 
REAL HIGH DEWPOINTS SURGING IN...SO RAPID MELTDOWN OF SNOWPACK 
DOESN'T LOOK LIKELY. STILL IT'S GOING TO BE A INTERESTING 12-24HRS.

FIRST SURGE OF PRECIP IS GOING THRU NOW ON THE NOSE OF A 50-60KT 
LLJ...EVIDENT IN MODEL OUTPUT AND 88D VWP'S. THIS FEATURE IS MADE TO 
WEAKEN AS IT SURGES THRU...ACCOUNTING NICELY FOR THE MODEL OUTPUT 
SHOWING 2 DISTINCT PERIODS OF PRECIP. SO LOOK FOR RAIN TO BECOME 
MORE SPOTTY BY DAY BREAK WITH MORE DEVELOPING DURING THE MORNING AND 
ESPECIALLY WITH THE COLD FRONT.

DYNAMICS STILL LOOK PROMISING FOR A MARGINAL SEVERE EVENT. UPPER 
WINDS WILL BE QUITE STRONG AND THE POTENTIAL WILL BE THERE FOR THE 
STRONG WINDS TO BE MIXED DOWN TO THE SURFACE IN ANY CONVECTION THAT 
DEVELOPS. ALL MODELS SHOW SIGNIFICANT RIDGING IN COLD AIR JUST 
BEHIND THE FRONT INDICATIVE OF STRONG ANAFRONT SCENARIO. NEW ETA 
SHOWS DEEP FRONTOGENETIC FORCING ACCOMPANYING THE FRONT AS IT MOVES 
THRU THE REGION. THIS FORCING IS OVERLAID BY A DEEP LAYER OF 
NEGATIVE EPV AIR INDICATING THE POTENTIAL WILL BE THERE FOR BANDED 
PRECIP TO FORM. MID LEVEL LAPSE RATES INCREASE TO IN EXCESS OF 6C/KM 
AS VERY COLD AIR ACCOMPANIES STRONG HGHT FALLS ALOFT...MORE THAN 
260M/12HR AT 300MB...AS UPPER TROF AND PV CENTER SWING THRU. PROGGED 
K INDICES EXCEED 30 AND T-TOTALS EXCEED 50 BY AFTERNOON OVER MOST OF 
THE REGION. THE PRESENCE OF UPRIGHT INSTABILITY AS WELL AS CSI 
SUGGESTS THE BANDED PRECIP THAT MAY FORM COULD BE OF THE NARROW COLD 
FRONTAL RAINBAND (NCFR) TYPE. THIS HAS BEEN A RELIABLE SEVERE WX 
MAKER ON SEVERAL OCCASIONS IN THE PAST UNDER SIMILAR LOOKING 
SYNOPTIC CONDITIONS. WHAT IS MISSING FROM THIS SETUP...ACCORDING TO 
PROGS...IS PRESENCE OF A STRONG DOWNSTREAM JET ENTRANCE REGION. 
OTHER INGREDIENTS LOOK TO BE THERE HOWEVER. ITS JUST A QUESTION OF 
IF A WELL DEFINED NCFR CAN FORM AND ACT AS A CATALYST FOR SEVERE WX. 
WEAK COOL AIR DAMMING SCENARIO IS HINTED AT IN THE LOW LEVEL 
THETA/THETA-E PLOTS...SO IF STRONG WINDS MAKE IT TO SFC..THEY WILL 
NEED SOME CONVECTION TO DO THE WORK. WITH THUNDERSTORMS BREAKING OUT 
UPSTREAM WITH TEMPS IN THE 40S...HAVE TO BELIEVE POTENTIAL IS THERE.

LONGER RANGE...WILL TWEAK WED TOWARD NICER DAY AS PER NEW NOGAPS/AVN 
AND 12Z ECMWF. SIG STORMINESS ONCE AGAIN SEEN BY END OF WEEK...BUT 
SO FAR LOOKS WET AND NOT WHITE.

.CTP...FLASH FLOOD WATCH MONDAY SOUTHERN HALF OF CENTRAL PA. WILL 
ISSUE PHLSPSCTP FOR CONVECTIVE POTENTIAL.

LA CORTE







NNNN