FXUS64 KLIX 030053
AFDLIX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NEW ORLEANS LA
653 PM CST WED DEC 2 2009
.UPDATE...
.SOUNDING DISCUSSION...
DRY AIR HAS MOVED INTO THE MID LEVELS BRINGING THE PRECIPITABLE
WATER VALUE DOWN TO 0.55 INCHES. A BROKEN STRATOCUMULUS DECK IS
PRESENT AS SHOWN ON THE SOUNDING NEAR 4500 FEET. A STRONG
SUBSIDENCE INVERSION IS PRESENT ABOVE 850 MB WHILE WINDS ARE
GENERALLY OUT OF THE WEST OR SOUTHWEST THROUGHOUT THE
ATMOSPHERE...STRONG MID AND UPPER LEVEL WINDS.
&&
.PREV DISCUSSION... /ISSUED 359 PM CST WED DEC 2 2009/
SHORT TERM...
A VERY UNUSUAL SNOW EVENT LOOKS TO DEVELOP OVER PORTIONS OF THE
FORECAST AREA WITHIN THE NEXT 72 HOURS. FOR THIS EVENT TO
DEVELOP...SEVERAL FACTORS MUST COME TOGETHER AT THE RIGHT TIME.
CURRENTLY...FORECAST GUIDANCE HAS A FAIRLY GOOD HANDLE ON THE
MID-UPPER LEVEL DYNAMICS OF THIS SYSTEM...AND ARE IN VERY GOOD
AGREEMENT ON THE TIMING AND STRENGTH OF THE UPPER LEVEL TROUGH. IN
FACT...IT APPEARS THAT SOME VERY VIGOROUS UPPER LEVEL LIFT WILL BE
TAKING PLACE ACROSS THE REGION LATE FRIDAY INTO EARLY SATURDAY.
THIS WILL PLAY A ROLE IN THE SNOWFALL EXPECTED OVER THE NORTHERN
THIRD OF THE CWA. THE DIFFERENCES ARISE IN THE LOW LEVEL
DEVELOPMENT OF THE LOW PRESSURE SYSTEM...AND THE TIMING OF THE DRY AIR
INTRUSION AT AROUND 850MB LATE FRIDAY NIGHT INTO EARLY SATURDAY
MORNING. ALL OF THE MODELS DEPICT A STRONG GULF LOW DEVELOPING
OVER THE WESTERN GULF OF MEXICO ON FRIDAY...AS A POSITIVELY TILTED
UPPER LEVEL TROUGH AXIS DIVES SOUTHWARD THROUGH THE SOUTHERN
PLAINS. THIS LOW WILL FORM IN A REGION OF ENHANCED BAROCLINICITY
CENTERED AROUND A STALLED FRONTAL BOUNDARY. THE LOW WILL CONTINUE
TO DEEPEN IN RESPONSE TO A JET COUPLET THAT IS EXPECTED TO DEVELOP
OVER THE WESTERN GULF FRIDAY AFTERNOON. BY FRIDAY EVENING...THE
LOW WILL BE OVER THE CENTRAL GULF OF MEXICO. STRONG MOISTURE
ADVECTION IN THE MID-LEVELS WILL TAKE PLACE FRIDAY EVENING ACROSS
THE REGION AS THE LOW MOVES THROUGH THE GULF. THIS MID-LEVEL
MOISTURE...COMBINED WITH SOME UPPER LEVEL MOISTURE MOVING IN WITH
THE UPPER LEVEL TROUGH AXIS WILL BE THE FUEL THAT FEEDS THE
PRECIPITATION EVENT FRIDAY NIGHT. AS STATED BEFORE...THE MODELS
ARE IN GOOD AGREEMENT ON THIS SITUATION.
AT THE SAME TIME THAT THE MOISTURE BEGINS TO FEED INTO THE
REGION...COLDER AIR WILL BE DRAWN SOUTHWARD IN RESPONSE TO THE
STRENGTHENING SURFACE LOW IN THE GULF. TEMPERATURES WILL BEGIN TO
FALL ACROSS THE CWA FRIDAY AFTERNOON AS THIS COLDER AIR MOVES IN.
THE MOST NOTICABLE TEMPERATURE CHANGES WILL BE FROM 800MB DOWN TO
THE SURFACE. MODEL SOUNDINGS SHOW A GOOD 5 DEGREE DROP IN
TEMPERATURES FROM 18Z FRIDAY TO 06Z SATURDAY. THIS DROP PLACES THE
LOWER ATMOSPHERE INTO A MORE FAVORABLE ZONE FOR SNOW DEVELOPMENT
FOR AREAS ACROSS SOUTHWEST MISSISSIPPI AND PARTS OF SOUTHEAST
LOUISIANA NORTH OF I-12. AS A RESULT...EXPECT SNOW TO DEVELOP
SOMETIME IN THE LATE EVENING HOURS FRIDAY AND LINGER THROUGH THE
NIGHT. ACCUMULATIONS ARE HARD TO PINPOINT AT THIS TIME...BUT SOME
ACCUMULATION WILL BE POSSIBLE.
THE MAIN DIFFERENCES ARISE IN THE EXACT DURATION OF THE SNOWFALL.
THE GFS AND ECMWF BOTH DEPICT STRONG DRY AIR ADVECTION TAKING HOLD
BY 12Z...WITH THE STRONGEST DRYING IN THE MID-LEVELS.
HOWEVER...THE NAM CONTINUES TO HOLD MID-LEVEL MOISTURE IN THE
REGION FOR A FEW MORE HOURS...BEFORE SHOWING STRONG DRYING AROUND
18Z. AT THIS TIME...HAVE LEANED MORE TOWARD THE GFS AND
ECMWF...WITH DRYING TAKING PLACE EARLY SATURDAY MORNING.
LONG TERM...
HEADING INTO THE EXTENDED PERIOD...THE ENTIRE PATTERN WILL SHIFT
FROM A HIGHLY AMPLIFIED PATTERN TO ONE THAT IS MORE ZONAL AND
PROGRESSIVE. AFTER A BRIEF PERIOD OF SUBSIDENCE AND RIDGING IN THE
WAKE OF THE NEXT GULF LOW AND UPPER LEVEL TROUGH AXIS SATURDAY
NIGHT INTO EARLY SUNDAY...IT APPEARS THAT A FAST MOVING SHORT WAVE
TROUGH WILL BEGIN TO IMPACT THE REGION. IN ADVANCE OF THIS FAST
MOVING SYSTEM...MID-LEVEL FLOW WILL TURN SOUTHERLY AND BEGIN TO
TRANSPORT IN MOISTURE. THIS INCREASED MOISTURE WILL FIRST BE
NOTICTED OVER WESTERN ZONES AND WILL OVERSPREAD THE AREA SUNDAY
NIGHT. STRONG LIFT ALONG THE TROUGH AXIS WILL ALLOW FOR SOME
SHOWER DEVELOPMENT SUNDAY NIGHT AND MONDAY. IMMEDIATELY FOLLOWING
THIS SYSTEM...ANOTHER TROUGH WILL MOVE THROUGH MONDAY NIGHT. A
DEEPER TROUGH AXIS SHOULD THEN BEGIN TO INFLUENCE THE AREA ON
TUESDAY. STRONG PVA AND WARM AIR ADVECTION WILL ALLOW FOR A FAIRLY
UNSTABLE ENVIRONMENT TUESDAY INTO TUESDAY NIGHT. AT THIS TIME HAVE
KEPT SHOWERS IN THE FORECAST...BUT WOULD NOT BE SURPRISED TO SEE A
FEW THUNDERSTORMS DEVELOP.
AVIATION...
WATER VAPOR SATELLITE IMAGERY SHOWED DRY AIR ENTRAINMENT INTO THE
SYSTEM AND PRODUCE A LARGE BREAK IN CEILINGS FOR MSY...GPT AND MCB.
WRAP AROUND MOISTURE WILL BRING MVRF CEILINGS BACK INTO FOLD FOR ALL
TAF SITES OVERNIGHT. DRY AIR AT THE LOWER LEVELS MAINTAIN STABLE
CEILINGS OVER THE ARE...BUT NO VISIBILITY RESTRICTIONS ARE EXPECTED
THROUGH 18Z THURSDAY. 18
MARINE...
GALE OR NEAR GALE CONDITIONS HAVE TAPERED AND ONLY SMALL CRAFT
CONDITION EXIST. WITH THE LOW SHIFTING NORTH AND HIGH MOVING
IN...THE PRESSURE GRADIENT WILL RELAX TONIGHT AND SMALL CRAFT
CONDITION WILL DIMINISH BY MIDNIGHT. WINDS MAY PICK UP AGAIN LATE IN
THE WEEK AS ANOTHER AREA OF LOW PRESSURE POSSIBLY DEVELOPS AND
TRACKS NORTHEAST ACROSS THE CENTRAL GULF. 18
&&
.PRELIMINARY POINT TEMPS/POPS...
MCB 39 57 36 50 / 0 0 10 30
BTR 40 59 38 52 / 0 0 10 40
MSY 45 59 44 54 / 0 0 10 30
GPT 41 60 39 54 / 0 0 10 20
&&
.LIX WATCHES/WARNINGS/ADVISORIES...
LA...NONE.
GM...SMALL CRAFT ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR GMZ530.
SMALL CRAFT ADVISORY UNTIL 4 AM CST THURSDAY FOR GMZ550-555-570-
575.
MS...NONE.
GM... SMALL CRAFT ADVISORY UNTIL 4 AM CST THURSDAY FOR GMZ555-570-575.
&&
$$
