FXUS61 KPBZ 250327
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
1027 PM EST THU DEC 24 2009

.SYNOPSIS...
A STRONG LOW PRESSURE SYSTEM MOVING NORTHWARD THROUGH THE MIDWEST
WILL PUSH A WARM FRONT WITH FREEZING RAIN AND RAIN THROUGH THE
UPPER OHIO VALLEY CHRISTMAS DAY FOLLOWED BY A COLD FRONT SATURDAY
WITH SCATTERED SNOW SHOWERS INTO EARLY NEXT WEEK. 

&&

.NEAR TERM /THROUGH FRIDAY/...
SLIGHTLY ADJUSTED TEMPERATURES AND ONSET TIMING OF FREEZING RAIN
AND CHANGEOVER TO RAIN WITH INCOMING WARM FRONT BASED ON BLEND OF
RECENT SREF MODEL OUTPUT...NAM MODEL PROFILES AND GFS LAMP. AS A
RESULT ALSO ADJUSTED AREAL EXTENT OF WINTER WEATHER ADVISORY TO
INCLUDE SEVERAL MORE COUNTIES NEAR THE OHIO RIVER. 

DUE TO INITIAL GROUND COLDNESS AND SURFACE LAYER DRYNESS...SURFACE
TEMPERATURES MAY HAVE TO GO ABOVE 35 BEFORE FREEZING RAIN CHANGES
COMPLETELY TO RAIN CHRISTMAS AFTERNOON.  

SECOND CONCERN IS SPEED OF WIND GUSTS IN DOWN SLOPE AREAS
IMMEDIATELY WEST OF THE APPALACHIAN MOUNTAIN RIDGES. NAM MODEL
PROFILES SUGGEST SPEEDS OF 30 TO 40 MPH CHRISTMAS DAY AND THIS IS
WHAT HAS BEEN FORECASTED. MENTIONED THIS AS AN ADDED HAZARD TO THE
ICING SITUATION NEAR THE APPALACHIAN MOUNTAINS. 

QPF AMOUNTS ARE PROGGED AS 0.40 TO 0.80 INCHES AND THERE CAN BE A DRY
SLOT SWEEPING ACROSS THE AREA DURING THE AFTERNOON. OHRFC EXPECTS
LIQUID TO BE ABSORBED INTO THE SNOW PACK...THUS NO FLOOD WATCH AND
A RATHER CONSERVATIVE RIVER FORECAST.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
JUDGING FROM RECENT SREF MODEL OUTPUT...EXPECT THE COLD FRONT TO
MOVE THROUGH FRIDAY NIGHT. THE RESULTING COOL DOWN WILL CHANGE
PRECIPITATION TO ALL SNOW BY SATURDAY NIGHT. HOWEVER POST FRONTAL
PRECIPITATION WILL BE SCATTERED SO WENT WITH CHANCE OF SNOW
SHOWERS SATURDAY NIGHT AND SUNDAY. 

FORECASTED TEMPERATURES USING A BLEND OF RECENT SREF MODEL OUTPUT
AND GFS MOS. 

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
UPPER LEVEL LOW OVER THE LOWER GREAT LAKES WILL MOVE INTO NEW 
ENGLAND BY TUESDAY. PERSISTENT LOW LEVEL NORTHWEST FLOW AND COLD AIR 
INSTABILITY WILL CONTINUE TO GENERATE SCATTERED SNOW SHOWERS THROUGH 
EARLY TUESDAY. 

SURFACE HIGH PRESSURE WILL BUILD INTO THE REGION LATE TUESDAY AND 
WEDNESDAY BRINGING DRY WEATHER. A LOW PRESSURE SYSTEM OVER THE 
SOUTHEASTERN STATES WILL BRING MOISTURE AND A CHANCE OF SNOW SHOWERS 
TO THE NORTHERN WEST VIRGINIA/WESTERN MARYLAND MOUNTAINS THURSDAY. 

CLOUD COVER WILL HOLD DAYTIME TEMPERATURES DOWN TO FEW DEGREES BELOW 
NORMAL THROUGH THE EXTENDED PERIOD.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
A POTENT LOW PRESSURE SYSTEM WILL LIFT NORTHWARD ACROSS THE
MISSOURI VALLEY OVERNIGHT...BRINGING LOW-LEVEL WIND SHEAR TO THE
TERMINALS BY THE EARLY MORNING AS INDICATED BY RECENT NAM MODEL
PROFILES. LOW-LEVEL MOISTURE RELATED TO THE SYSTEM WILL BRING
PRECIPITATION TO THE AREA AROUND 12Z FRIDAY.

SURFACE TEMPS NEAR FREEZING AT THE ONSET OF PRECIPITATION AND WARM
AIR ALOFT MAY INDUCE A PERIOD OF FREEZING RAIN ACROSS THE
TERMINALS WITH THE ONSET OF PRECIPITATION FRIDAY MORNING. TEMPO
GROUPS ARE FORECAST FOR FREEZING RAIN STARTING AT 11Z AT KZZV...
13Z-14Z AT KPIT/KAGC/KBVI/KHLG AND 15Z-16Z AT KFKL/KDUJ/KLBE.
PRECIPITATION WILL TURN OVER TO RAIN BY LATE MORNING EVERYWHERE
EXCEPT KDUJ/KFKL...WHERE -FZRA COULD PREVAIL FOR TWO TO THREE
HOURS BEFORE CHANGING OVER TO RAIN AROUND 20Z.

RAIN WILL CONTINUE TO PREVAIL MOSTLY ACROSS THE TERMINALS ON
FRIDAY AND FRIDAY NIGHT...WITH CEILINGS BECOMING MVFR WITH
MARGINAL IFR CONDITIONS POSSIBLE. SOUTHEASTERLY WINDS OF AROUND 15
KTS GUSTING TO 20-25 KTS FRIDAY EVENING WITH LOW-LEVEL WIND SHEAR
CONTINUING. BY SATURDAY MORNING...RAIN WILL TURN TO SNOW SHOWERS
WITH WIND SHEAR ALOFT DECREASING AND SURFACE WINDS BECOMING
LIGHTER...SHIFTING TO SOUTHWESTERLY.

MVFR/IFR RESTRICTIONS WITH SCATTERED SNOW SHOWERS ARE EXPECTED TO
PERSIST SATURDAY INTO EARLY NEXT WEEK AS THE UPPER LEVEL LOW
SLOWLY MOVES ACROSS THE GREAT LAKES INTO NEW ENGLAND.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...WINTER WEATHER ADVISORY FROM 6 AM TO 4 PM EST FRIDAY FOR 
     MDZ001.
OH...WINTER WEATHER ADVISORY FROM 6 AM TO 10 AM EST FRIDAY FOR 
     OHZ040-041-050.
PA...WINTER WEATHER ADVISORY FROM 6 AM TO 4 PM EST FRIDAY FOR 
     PAZ008-009-015-016-023.
     WINTER WEATHER ADVISORY FROM 6 AM TO 10 AM EST FRIDAY FOR 
     PAZ029-031.
     WINTER WEATHER ADVISORY FROM 6 AM TO NOON EST FRIDAY FOR 
     PAZ007-013-014-020>022-030-032.
WV...WINTER WEATHER ADVISORY FROM 6 AM TO 4 PM EST FRIDAY FOR 
     WVZ041.
     WINTER WEATHER ADVISORY FROM 6 AM TO 10 AM EST FRIDAY FOR 
     WVZ001>003.
     WINTER WEATHER ADVISORY FROM 6 AM TO NOON EST FRIDAY FOR 
     WVZ023.

&&

$$