Areal Flood Advisory 21, NWS Milwaukee, WI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI
301 PM CDT FRI SEP 12 2008

.DISCUSSION...FORECAST FOCUS ON HEAVY RAIN TONIGHT AND
SATURDAY...FOLLOWED BY MORE HEAVY RAIN POSSIBILITIES SUNDAY WITH
THE REMNANTS OF IKE AND MUCH DRIER WEATHER FOR THE EXTENDED.

.SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT.

THERE ARE TWO MAIN CONCERNS FOR THIS FORECAST PACKAGE. ONE IS THE
HEAVY RAIN TONIGHT AND SATURDAY MORNING...AND THE OTHER IS THE
EVENTUAL PATH OF IKE. FIRST THING IS FIRST...THE HEAVY RAIN
TONIGHT. MANY THINGS ARE COMING TOGETHER FOR A HEAVY RAIN EVENT
LATER TONIGHT. ONE IS THE RICH MOISTURE SURGING NORTH INTO THE
UPPER MIDWEST FROM THE REMNANTS OF TROPICAL STORM LOWELL IN THE
EASTERN PACIFIC AND HURRICANE IKE IN THE NORTHWESTERN GULF.
PRECIPITABLE WATER VALUES INCREASE TO AROUND 2.3 INCHES LATER
TONIGHT. THESE ARE NEAR RECORD NUMBERS FOR THIS TIME OF YEAR. WARM
CLOUD DEPTHS ALSO RISE TO NEAR 13.5 KFT...WITH NEAR COMPLETE
SATURATION TO 25 KFT MAKING PRECIPITATION EFFICIENCY VERY HIGH.
ANOTHER FAVORABLE ELEMENT FOR HEAVY RAIN IS A 50 KNOT LLJ POINTED
RIGHT TOWARD SOUTHERN WI AND THE RIGHT ENTRANCE REGION OF THE
UPPER JET OVER THE AREA. ALSO...MBE VELOCITIES FALL TO 5 TO 10
KNOTS LATE...FAVORING SLOW MOVING TORRENTIAL RAIN BANDS. THE AREA
OF STRONG 925-850 MB MOISTURE CONVERGENCE DOES SWING THROUGH
WITHIN 6-9 HOURS...BUT DURING THAT TIME THE HEAVIEST RAIN SHOULD
BE OCCURRING.

6-HOUR FLASH FLOOD GUIDANCE VALUES RANGE FROM 2.5 TO 3.5 INCHES
ACROSS THE SOUTH HALF. THESE AREN/T EXTRAORDINARY VALUES TO
INDICATE FLASH FLOOD POTENTIAL...HOWEVER ONE WOULD THINK IT
WOULDN/T NECESSARILY TAKE THIS MUCH TO MAKE THE GROUND SATURATED
AGAIN AFTER GUSTAV/S RAIN LAST WEEK. A RUN THROUGH THE LOCAL
FLASH FLOOD CHECKLIST SHOWS ALL ELEMENTS COME OUT AS VERY
FAVORABLE EXCEPT FOR FLASH FLOOD GUIDANCE VALUES. QPF ON ALL THE
MODELS AVERAGE BETWEEN 2 TO 3 INCHES BY 21Z SATURDAY...WITH THE
NEW 12Z HI-RES ECMWF BRINGING 3 TO 3.5 INCHES. GIVEN THE
SIGNIFICANT MOISTURE ADVECTION AND DYNAMICS...I CERTAINLY HAVE THE
REQUIRED 50 PERCENT CONFIDENCE LEVEL NEEDED FOR A WATCH DESPITE
HIGH FFG NUMBERS. THUS...WILL ISSUE A FLOOD WATCH FOR THE
SOUTHERN HALF OF THE CWA FROM 05Z TONIGHT THROUGH 21Z SATURDAY. IT
MAY NEED TO BE EXTENDED NORTHWARD A TAD THIS EVENING SHOULD IT
APPEAR THE HEAVIEST SWATH OF RAIN SETS UP IN THAT DIRECTION.

AFTER THIS WAVE PASSES THROUGH SATURDAY AFTERNOON...A LULL IN RAIN
WILL OCCUR BETWEEN THAT TIME AND SUNDAY MORNING. THEN THE CENTER
OF IKE SHOULD MAKE HIS WAY INTO THE LOWER GREAT LAKES SUNDAY
AFTERNOON. VERY HEAVY RAIN ON THE NORTHWEST SIDE OF THE STORM
COULD BRING MAJOR FLOODING ISSUES TO SOUTHERN WISCONSIN IF IT
TRACKS FAR ENOUGH TO THE WEST. THE 12Z GFS CONTINUED TO SHOW A
WESTWARD JOG...AS DID THE CANADIAN. THE GFDL AND HWRF ARE THE
FURTHEST WEST WITH A TRACK ACROSS NORTHERN INDIANA. THESE TWO
MODELS ARE OFTEN PREFERRED TO HANDLE TROPICAL SYSTEMS. THE ECMWF
INSISTS ON AN EASTERLY TRACK. THE TREND FOR CUT OFF SYSTEMS SUCH
AS THIS IS TO ALWAYS TRACK FURTHER WEST THAN MODELS ORIGINALLY
PROJECT...THEREFORE I INCREASED POPS TO CATEGORICAL OVER THE FAR
SOUTHEAST AND TAPERED IT OFF TO THE WEST. THE MOST CONCERNING
THING LIES WITH THE ACCUMULATED RAINFALL FORECASTS FROM THE
HURRICANE MODELS WHICH INDICATE 5+ INCHES COULD COME WITH IKE. IKE
IS A MASSIVE STORM WITH LOADS OF MOISTURE. STAY TUNED...

.LONG TERM...MONDAY THROUGH NEXT FRIDAY.

HIGH PRESSURE WILL BUILD INTO THE MIDWEST AND SLOWLY PUSH EAST
FOR NEXT WEEK. MAINLY CLEAR CONDITIONS AND A WARMING TREND IS
EXPECTED DURING THIS TIME...ALLOWING THE AREA TO DRY OUT FOR
SEVERAL DAYS.

&&

.AVIATION...EXPECT MAINLY MVFR CEILINGS THROUGH EARLY EVENING AT TAF
SITES...WITH SOME VFR AT TIMES AT KMKE AND KMSN. MAIN AREA OF IFR
CEILINGS AND MVFR/IFR VISIBILITIES WITH RAIN SHIELD ENTER TAF SITES
AROUND 04Z TO 05Z SATURDAY. PREVAILING LIFR CONDITIONS EXPECTED WITH
BEST UPWARD VERTICAL MOTION FIELDS FROM 09Z TO 15Z SATURDAY...AS LOW
LEVEL JET NOSE AND FOCUSED WARM AIR ADVECTION MOVE ACROSS THE AREA.
VLIFR CEILINGS AND VISIBILITIES ARE POSSIBLE AT TIMES FROM MID
EVENING THROUGH MIDDAY SATURDAY WITH ANY CONVECTIVE HEAVY RAIN
BANDS.

&&

.MARINE...AS THE LOW PASSES OVER THE AREA ON SATURDAY...SOUTHWEST
WINDS ACROSS THE NEARSHORE WATERS MAY GUST TO 22 KNOTS AT
TIMES...WITH THOSE WINDS JUST OFF OF THE SURFACE. HOWEVER...DO NOT
EXPECT THE GUSTS TO BE FREQUENT ENOUGH FOR A SMALL CRAFT ADVISORY
ISSUANCE.

A BETTER CHANCE FOR SMALL CRAFT ADVISORY WINDS COMES ON SUNDAY
AFTERNOON AND NIGHT...AS THE CORE OF THE REMNANTS OF IKE PASS TO THE
SOUTHEAST. SUSTAINED NORTHEAST TO NORTH WINDS TO 25 KNOTS ARE
POSSIBLE DURING THIS TIME.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...FLOOD WATCH FROM MIDNIGHT CDT TONIGHT THROUGH SATURDAY AFTERNOON
     FOR WIZ062>072.

LM...NONE.
&&

SHORT/LONG TERM...BORGHOFF
AVIATION/MARINE...WOOD

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