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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPBZ 290549
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
1249 AM EST THU JAN 29 2015

.SYNOPSIS...
CROSSING LOW PRESSURE WILL BRING A WINTRY MIX OF PRECIPITATION TO
THE UPPER OHIO REGION TODAY AND SNOW EARLY ON FRIDAY.

&&

.NEAR TERM /THROUGH TODAY/...
OVERNIGHT IN GOOD SHAPE. JUST NEED TO KEEP UP WITH THE TEMPS.

MODELS ARE COMING INTO GOOD AGREEMENT WITH TIMING OF PRECIPITATION
AHEAD OF FRONT AND THE STRONG WAA AS WELL. THE KEY IS WHAT WILL
THE SURFACE TEMPS DO. MODELS ARE INDICATING A SLOW RESPONSE TO THE
WAA AT THE SURFACE BUT I HAVE MY DOUBTS THAT IT WILL BE FAST
ENOUGH TO PREVENT SOME ICING AT THE BEGINNING OF THE EVENT. THERE
ARE A LOT OF FACTORS AT PLAY HERE WHICH MAKES THIS A VERY
DIFFICULT FORECAST. FIRST ALOFT. ALL THE MODELS ARE SHOWING A
STRONG SURGE OF WARMER AIR ENVELOPING MY ENTIRE CWA TODAY. THIS
SHOWS UP QUIET WELL IN THE THERMAL PATTERN AT 850MB AND IN THE LOW
AND MID-LEVEL THICKNESS PATTERNS. I HAVE DECIDED TO CHANGE VERY
LITTLE FROM PREVIOUS THINKING ON PRECIP TYPES. THE THING TO WATCH
WILL BE HOW THE ATMOSPHERE RESPONDS ONCE THE PRECIP BEGINS AND THE
WET BULBING EFFECTS COME INTO PLAY. MODEL SOUNDINGS SHOW A PROFILE
VERY CLOSE TO THE 0 DEGREE LINE WHICH WILL KEEP THE DIFFERENCE
BETWEEN LIQUID OR FROZEN VERY CLOSE. AT THE SURFACE. MODELS DO
SHOW SURFACE TEMPS RISING QUICKLY AS THE PRECIP APPROACHES.
HOWEVER...MY CONCERN IS THAT WITH SUCH COLD DEWPOINTS AT THE
SURFACE...WET BULBING WILL ALLOW FOR A PERIOD OF FREEZING
RAIN...AT LEAST IN OHIO AT THE ONSET OF THE EVENT. OTHER FACTORS
THAT NEED TO BE CONSIDERED ARE THE WIDESPREAD SNOW PACK AND
LOCALIZED EFFECTS...MAINLY SHELTERED AREAS WHERE TEMPS WILL TAKE
LONGER TO RECOVER.

MAIN AREA OF PRECIPITATION WILL EXIT THIS AFTERNOON AS THE SURFACE
FRONT SWINGS INTO CENTRAL PA. THE ONSET OF CAA WILL BE RIGHT
BEHIND THE EXITING FRONT.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
THERMAL PROFILES QUICKLY BECOME MORE UNSTABLE TONIGHT. COLD
ADVECTION BEGINS IN EARNEST ALOFT AS TWO SHORT WAVE TROUGHS ARE
SET TO CROSS THE AREA THROUGH FRIDAY MORNING. THIS WILL OCCUR
SIMULTANEOUSLY TO THE DRIER AIR COMING IN...HOWEVER THE DENDRITIC
GROWTH LAYER LOOKS TO CRASH TOWARD THE SURFACE TONIGHT IN THE
RIDGES...WELL BEFORE THE MOISTURE PEELS OFF. CONSENSUS SNOW TO
LIQUID RATIOS FROM THE NAM AND GFS VIA BUFKIT SUGGEST AROUND 25 TO
1 BEGIN OVERNIGHT. GIVEN INSTABILITY AND UNIDIRECTIONAL
UPSLOPE...THIS SHOULD RESULT IN VERY EFFICIENT SNOW GROWTH. MODEL
QPF IS GENERALLY ONLY A COUPLE OF TENTHS...AND GIVEN ICE COVER
UPSTREAM ON LAKE ERIE...A TON OF QPF IS VERY UNLIKELY.
HOWEVER...EVEN LIGHT QPF IN THIS CASE IS LIKELY TO ADD UP TO
SEVERAL INCHES OF SNOW. GIVEN THAT AHEAD OF THE SYSTEM ON THURSDAY
THERE COULD BE AN INCH OR SO IN SOME OF THE SHELTERED HIGHER
ELEVATIONS...AND ANOTHER 2 TO 5 SEEM LIKELY IN THE UPSLOPE THROUGH
FRIDAY WITH ADDITIONAL SYNOPTIC HELP VIA TWO ADDITIONAL WAVES
CROSSING THE AREA...3 TO 6 INCH TOTALS SEEM REASONABLE IN THE
RIDGES BY FRIDAY MORNING. WINTER WEATHER ADVISORIES WERE EXPANDED
TO ACCOMMODATE THIS.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
HIGH LEVELS OF UNCERTAINTY IN THE LONG TERM PARTICULARLY WITH THE
SYSTEM THAT COULD AFFECT THE AREA DURING THE SUNDAY/MONDAY TIME
FRAME. FOR NOW...CONSISTENCY WILL BE THE NAME OF THE GAME UNTIL
THE MODELS PROVIDE A BETTER PICTURE. COLD PATTERN WILL CONTINUE
THROUGH THE REMAINDER OF THE LONG TERM.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
-- Changed Discussion --
VFR OVERNIGHT THROUGH SUNRISE WITH THICKENING MID/HI CLOUDS.
PRECIPITATION RETURNS AFTER 14Z WITH LOW PRESSURE TRACKING TO THE
NORTH. EXPECT A DROP TO MVFR SOON AFTER PRECIP ONSET...WITH IFR
CIGS TO FOLLOW AT MOST TERMINALS DURING THE AFTERNOON AND EVENING.
PRECIP TYPES ARE THE MAIN DIFFICULTY. STILL THINK BEST CHANCES OF
FZRA WILL LIE IN OHIO AND AT FKL/DUJ DURING THE FIRST FEW HOURS OF
PRECIPITATION. CANNOT RULE OUT AT LEAST A BRIEF PERIOD OF FZRA
ANYWHERE SAVE PERHAPS MGW. CHANGOVER TO SNOW WILL OCCUR AFTER 00Z
AS A COLD FRONT SWEEPS THROUGH. WINDS WILL STRENGTHEN OUT OF THE
SOUTH AROUND MIDDAY AND REMAIN GUSTY AS THEY VEER TOWARDS SW/W
WITH TIME. WILL CONTINUE TO MONITOR FOR LLWS POSSIBILITIES.  CL

.OUTLOOK.../THURSDAY NIGHT THROUGH MONDAY/...
RSTRNS ARE LIKELY THRU ERLY FRI AS A CDFNT EXITS. WDSRPD RSTRNS
ARE LIKELY AGAIN WITH ADVNG LOW PRES SUN AND MON.
-- End Changed Discussion --

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...WINTER WEATHER ADVISORY FROM 11 AM THIS MORNING TO 10 AM EST
     FRIDAY FOR MDZ001.
OH...FREEZING RAIN ADVISORY FROM 8 AM THIS MORNING TO NOON EST
     TODAY FOR OHZ048-057.
PA...WINTER WEATHER ADVISORY FROM 11 AM THIS MORNING TO 10 AM EST
     FRIDAY FOR PAZ007>009-015-016-023-074-076.
WV...WINTER WEATHER ADVISORY FROM 7 PM THIS EVENING TO 10 AM EST
     FRIDAY FOR WVZ023-041.

&&

$$