July 1st Discussion from NWS for Big Bend / Apalachee Bay system evolution
Posted by AlligatorPointer on 7/1/2010, 5:09 pm
http://forecast.weather.gov/product.php?site=NWS&issuedby=TAE&product=AFD&format=CI&version=1&glossary=1

FXUS62 KTAE 011942
AFDTAE

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
342 PM EDT THU JUL 1 2010

.SHORT TERM (TONIGHT THROUGH SATURDAY NIGHT)...DEEP TROPICAL AIRMASS IS IN PLACE ACROSS THE TRI-STATE AREA THIS AFTERNOON. WARM RAIN PROCESSES ARE DEFINITELY IN PLAY AS THE DROP SIZE IS VERY SMALL AND THERE HAS BEEN VERY LITTLE LIGHTNING OBSERVED.
FORTUNATELY...THE STRONGEST CONVECTION HAS REMAINED
OFFSHORE...ALONG WITH ANY POTENTIAL FLOODING IMPACTS. ANY HEAVIER CELLS WHICH DO DEVELOP OVER LAND WILL NEED TO BE MONITORED CLOSELY THIS EVENING FOR FLOODING CONCERNS.

THE CONVECTION IS BEING TRIGGERED BY A COMBINATION OF A COLD FRONT ACROSS CENTRAL GEORGIA/ALABAMA AND A WEAK LOW TO MID-LEVEL VORT MAX CENTERED OVER THE BIG BEND AND APALACHEE BAY. 18Z OBS INDICATE A 1018MB SFC LOW CENTERED OVER APALACHEE BAY.

THE EVOLUTION OF THIS VORT MAX AND SFC REFLECTION PROVIDE THE FORECAST CHALLENGE FOR THE NEXT SEVERAL DAYS. THE 12Z OPERATIONAL GFS AND CANADIAN MODELS GRADUALLY ALLOW A SFC LOW TO ORGANIZE AND DEEPEN ALONG THE NORTHEASTERN GULF COAST. THE GFS LOW TRACKS SLOWLY WEST AND MEANDERS FOR SEVERAL DAYS ALONG THE PANHANDLE COAST. THE CANADIAN TRACKS THE LOW TOWARDS THE NATURE COAST AND
ACROSS THE FLORIDA PENINSULA. THE 12Z NAM AND 00Z ECMWF ARE MUCH LESS BULLISH...PREFERRING TO KEEP THE DISTURBANCE AS AN INVERTED TROUGH (NAM) OR A VERY WEAK SFC LOW (ECMWF) AND TRACKING IT SLOWLY WEST. A CLOSER LOOK AT THE PRECIP FIELDS IN THE GFS SHOWS VERY HIGH QPF OVER THE BIG BEND THIS AFTERNOON AND TONIGHT FROM STABLE PRECIPITATION PROCESSES WITH LOWER AMOUNTS FROM THE CONVECTIVE PARAMETERIZATION. THIS IS OFTEN A SIGN OF GRID-SCALE CONVECTIVE FEEDBACK WHICH WE BELIEVE IS HELPING THE GFS TO SPURIOUSLY SPIN-UP THE SFC LOW. A SIMILAR FEEDBACK MAY BE SPINNING THE LOW UP IN THE CANADIAN SOLUTION. IN CONTRAST...THE PARALLEL RUN OF THE GFS...WITH UPDATED CONVECTIVE SCHEMES...DOES NOT SHOW ANY SPIN-UP
OF A SFC LOW IN THE NORTHEASTERN GULF.

WITH THAT IN MIND...MUCH PREFER THE WEAKER SOLUTION OF THE
NAM/ECMWF FOR THIS FORECAST PACKAGE. HOWEVER...THE CURRENT VORT MAX WILL CONTINUE TO BE MONITORED...ESPECIALLY IF DEEP CONVECTION
OVER APALACHEE BAY PERSISTS AND DEEPENS OVERNIGHT.

BACK TO THE ECMWF SOLUTION...WITH THE WEAK SFC LOW TRACKING SLOWLY WEST OVER THE NEXT TWO DAYS...SHOULD SEE THE DRIER AIRMASS ASSOCIATED WITH THE STATIONARY FRONT FILTER INTO THE REGION...ESPECIALLY THE NORTHERN ZONES. BEGINNING FRIDAY...WILL TRIM POPS BACK THROUGH SATURDAY WITH THE HIGHEST VALUES ALONG THE COAST AND OFFSHORE AND LOWER VALUES WELL INLAND. LIKEWISE...WILL GO ABOVE THE MAV GUIDANCE FOR TEMPS FOR INLAND AREAS WHERE SOME
SUNSHINE IS EXPECTED TO BREAK THROUGH FOR FRIDAY AFTERNOON AND SATURDAY.

.LONG TERM (SUNDAY THROUGH THURSDAY)...ALTHOUGH THERE ARE SOME LARGE SCALE SIMILARITIES BETWEEN THE MODELS THROUGH THIS PERIOD...THE SMALLER SCALE DIFFERENCES NOTED OVER OUR AREA AND ACROSS THE REGION ONLY INCREASE THE FORECAST UNCERTAINTY. THE ECMWF/GEM/GFS ENSEMBLE CONTINUE TO ADVERTISE DRIER AIR SPREADING SOUTH OVER THE AREA IN THE WAKE OF THE PREVIOUSLY DISCUSSED FRONTAL BOUNDARY...WHILE THE GFS/CANADIAN KEEP THE SRN HALF OF THE
CWA IN MORE OF A WET PATTERN WITH THE FRONTAL BOUNDARY STALLING OVER THE REGION. SIMILAR TO THE PAST FEW NIGHTS...WE WILL CONTINUE TO LEAN MORE TOWARD CLIMATOLOGY WITH 20 TO 40 POPS THROUGH THE
AFTERNOON/EVENING PERIODS.

--

.MARINE...THE PRESSURE GRADIENT IS FORECAST TO REMAIN RELATIVELY WEAK THROUGH FRIDAY OVER THE COASTAL WATERS. HOWEVER...WILL CONTINUE TO SEE SOME SWELL APPROACHING FROM THE SOUTHWEST GULF WHICH WILL KEEP SEAS ELEVATED...ESPECIALLY OVER THE WESTERN WATERS. BY FRIDAY NIGHT...NO MATTER WHAT HAPPENS WITH THE WEAK SFC
LOW OVER APALACHEE BAY...THE GRADIENT WILL TIGHTEN AS HIGH
PRESSURE BUILDS DOWN THE ATLANTIC SEABOARD. THIS WILL PUSH EAST TO SOUTHEAST WINDS UP TO AROUND 15 KNOTS FOR SATURDAY AND SUNDAY. A MODERATE SOUTHEAST FLOW WILL THEN CONTINUE INTO NEXT WEEK WITH HIGH PRESSURE CENTERED OFF THE EAST COAST.
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North GOM - JAC, 7/1/2010, 8:00 am
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