Area Forecast Discussion
Issued by NWS

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FXUS61 KPBZ 212346
AFDPBZ

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PITTSBURGH PA
746 PM EDT THU MAY 21 2015

.SYNOPSIS...
COOL TEMPERATURES WILL REMAIN THROUGH LATE WEEK. A WEAK
DISTURBANCE WILL BRING SHOWERS MAINLY FROM PITTSBURGH SOUTH
THIS MORNING.

&&

.NEAR TERM /THROUGH FRIDAY/...
EVENING UPDATE...
A RELATIVELY STOUT VORT MAX AND ACCOMPANYING MID- LEVEL WAVE IS
DRIVING THROUGH NORTHERN OHIO CURRENTLY. THIS VORT MAX...EVIDENT
ON WATER VAPOR IMAGERY...IS RESPONSIBLE FOR RE- GENERATING RAIN
SHOWERS THIS EVENING...DESPITE THE RELATIVE LACK OF DEEP MOISTURE.
CURRENT MESOSCALE ANALYSIS PROGS SHOW THE PRECIPITABLE WATER
VALUES AT 0.75" OR LESS...WITH THE BULK OF THAT MOISTURE CONFINED
TO BELOW 700MB.

NEVERTHELESS...THE VIGOROUS NATURE OF THE WAVE IS
SQUEEZING OUT THE REMNANT MOISTURE...PRODUCING MORE THAN SPRINKLES
AND DRIZZLE AT THIS POINT. MODIFIED THE FORECAST TO INCLUDE
SHOWERS THROUGH LATE EVENING. THE WAVE WILL PUSH EAST OF THE
FORECAST AREA BY MIDNIGHT...SO THE FORECAST THEREAFTER REMAINS
LARGELY UNTOUCHED.
TAX

PREVIOUS DISCUSSION...
A WEAKER PIECE OF MID-LEVEL MOISTURE ALONG A 925-700 MB THERMAL
GRADIENT SEEMS TO WORK ITS WAY SOUTHWARD ACROSS THE AREA IN THE
NORTHWESTERLY FLOW THROUGH FRIDAY MORNING AND EARLY AFTERNOON.
MODEL SATURATION OF THE UNSTABLE LOWER FRONTOGENETICAL LAYER IS
NOT VERY CONVINCING...NOR IS LIFT PARTICULARLY DEEP WITH THIS
FEATURE. IT WOULD SEEM ISOLATED SHOWERS WILL BE THE BEST THAT
WILL LIKELY BE MUSTERED...SO POPS WERE MAINTAINED IN THIS
CATEGORY. LIKEWISE...WITH LESS THAN CONVINCING SATURATION ALONG
THE BOUNDARY...AND A GIGANTIC SLUG OF DRY AIR WITH LAYER MEAN RH
VALUES AROUND 15 PERCENT IN ITS WAKE...SKIES SHOULD RAPIDLY CLEAR
BY EVENING. FRIES

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
WITH SKIES CLEARING OUT QUICKLY FRIDAY EVENING AND HIGH PRESSURE
RESIDING OVER WESTERN PENNSYLVANIA BY SUNRISE
SATURDAY...CONDITIONS WILL BE PRIME FOR A NIGHT OF RADIATIONAL
COOLING. RAW MODEL GUIDANCE FROM THE 12Z GFS IS NEARLY 10 DEGREES
COLDER THAN THE NAM/SREF...ALTHOUGH MOS LOWS BETWEEN THE NAM AND
GFS ARE RELATIVELY SIMILAR. WITH THE IDEAL RADIATIONAL COOLING
CONDITIONS...HAVE NUDGED THE FORECAST IN THE DIRECTION OF THE RAW
GFS FORECAST. OVERALL...THIS GENERALLY BROUGHT FRIDAY NIGHT LOWS
DOWN 2-3 DEGREES...BUT DID NOT NEED TO EXPAND MENTION OF FROST TOO
MUCH FROM PREVIOUS FORECAST. CONSIDERING LOW TEMPERATURE FORECAST
ALONG THE INTERSTATE 80 CORRIDOR IS RIGHT AROUND
FREEZING...DECIDED TO ISSUE A FREEZE WATCH FOR SEVERAL COUNTIES.

HIGH PRESSURE WILL MOVE OFFSHORE SATURDAY NIGHT...AND LOW PRESSURE
DEVELOPING ACROSS THE CENTRAL PLAINS WILL SPREAD A WIDE SWATH OF
SHOWERS AND THUNDERSTORMS ACROSS THE CENTRAL UNITED STATES. HIGH
CLOUDS WILL MOVE INTO THE AREA SATURDAY AND SATURDAY NIGHT...WITH
LOWER CLOUDS MOVING IN FOR SUNDAY. HOWEVER...APPEARS THAT ANY
LIFTING MECHANISM WILL REMAIN TOO FAR WEST TO PRODUCE ANY SHOWERS
LOCALLY. SUNSHINE ON SATURDAY WILL ALLOW HIGHS TO RISE A FEW
DEGREES WARMER THAN FRIDAY...WITH SOUTHERLY WINDS ON SUNDAY
BRINGING ANOTHER INCREASE TO HIGHS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
UPPER RIDGING OVER THE SOUTHEAST CONUS IS EXPECTED TO DEVELOP BY
EARLY NEXT WEEK...AS SHORTWAVES ADVANCE ALONG THE NORTHERN PERIPHERY
OF THE RIDGE. MAINTAINED CHANCE POPS FOR THE HOLIDAY FORECAST AS THE
STRENGTH OF RIDGING AND PLACEMENT OF A SURFACE BOUNDARY ACROSS THE
AREA REMAIN IN QUESTION. SOUTHWEST FLOW IS PROGGED TO DEVELOP
THEREAFTER WITH OCCASIONAL WEAK EMBEDDED CROSSING SHORTWAVES. THIS
WILL CONTINUE PERIODIC CHANCES FOR SHOWERS/STORMS.

USED WPC BASED ENSEMBLE MEAN GUIDANCE WITH DOWNWARD ADJUSTMENTS TO
POPS TO HELP SMOOTH OUT INDIVIDUAL MODEL DIFFERENCES...ESPECIALLY
FOR MONDAY. TEMPERATURES SHOULD AVERAGE ABOVE SEASONAL LEVELS.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
HIGH PRESSURE AND DRIER AIR ARE QUICKLY MOVING IN FROM THE WEST.
VFR CIGS WILL BREAK UP TO SCT CLOUDS DURING THE EVENING
HOURS...AND VFR WITH LIGHT WINDS WILL CONTINUE THROUGH THE NIGHT.
A FRONT SINKS INTO THE AREA ON FRIDAY. MOISTURE IS LIMITED...AND
GUIDANCE INDICATES THAT SHRA SHOULD REMAIN WIDELY SCATTERED AND
CIGS SHOULD REMAIN VFR. WINDS WILL KICK UP OUT OF THE WEST AS THE
FRONT APPROACHES...AND NORTHWEST WINDS GUSTING TO 15-25 KNOTS ARE
EXPECTED BEHIND THE FRONT. SKIES WILL THEN CLEAR TOWARDS SUNSET.
CL

.OUTLOOK.../FRIDAY THROUGH TUESDAY/...
RESTRICTIONS ARE POSSIBLE WITH A LATE SUNDAY AND MONDAY WARM
FRONT.

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
MD...NONE.
OH...NONE.
PA...FREEZE WATCH FROM LATE FRIDAY NIGHT THROUGH SATURDAY MORNING
     FOR PAZ007>009-015-016.
WV...NONE.

&&

$$



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