FXUS63 KOAX 070901
AFDOAX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
400 AM CDT THU APR 7 2011
.DISCUSSION...
...MAIN FORECAST PROBLEM IS POPS...
08Z SURFACE ANALYSIS INDICATED DRY DEWPOINTS OVER THE FORECAST
AREA...MOSTLY IN THE UPPER 20S WITH A FEW LOWER 30S. SURFACE RIDGE
AXIS EXTENDED FROM SOUTH CENTRAL NEBRASKA THROUGH EAST CENTRAL
NEBRASKA AND INTO NORTH CENTRAL IOWA. WELL SOUTH OF THE FORECAST
AREA WAS THE FRONTAL ZONE THAT WAS JUST SOUTH OF KANSAS. RADARS
OVER THE HIGH PLAINS SHOWED A FEW LIGHT SHOWERS IN SOUTHWEST
NEBRASKA AND WESTERN KANSAS...BUT MUCH OF THIS MAY BE VIRGA GIVEN
THE DRY NEAR SURFACE LAYER. OTHERWISE THE PRIMARY FEATURE OF THE
UPPER LEVEL SYNOPTIC PATTERN OVER NORTH AMERICA CONTINUED TO BE THE
DIGGING TROF ALONG THE WEST COAST AND ASSOCIATED HEIGHT FALLS.
FOR THE PAST FEW DAYS THE FOCUS HAS BEEN ON RAIN CHANCES THURSDAY
THEN AGAIN THROUGHOUT THE WEEKEND. REGARDING THE POPS FOR TODAY AND
TONIGHT...THE PRIMARY FORCING MECHANISM HAS BEEN ISENTROPIC LIFT IN
THE 295-305K LAYER. THIS FORCING IS PRESENT WHERE THE RADAR RETURNS
ARE LOCATED...AND THE MODELS ARE IN AGREEMENT WITH SPREADING THIS
VERTICAL MOTION OVER THE FORECAST AREA TODAY AND INTO TONIGHT.
HOWEVER THE MODELS HAVE NOT REPRESENTED THE DRY LOW LEVEL AIR
WELL... AND HAVE THEREFORE BEEN OVER-FORECASTING THE RAIN. 00Z NAM
HAS TRENDED MUCH DRIER...HOWEVER THE GFS AND ECMWF CONTINUE TO
ADVERTISE WIDESPREAD LIGHT RAIN BY THIS AFTERNOON. GIVEN THE DRY
AIR IN PLACE...POPS HAVE BEEN DRASTICALLY LOWERED FOR THIS
MORNING... AND THE AFTERNOON POPS WERE PUSHED A COUPLE HOURS LATER
AND ALSO REDUCED SOME. THUNDER POTENTIAL TODAY LOOKS VERY LOW GIVEN
THE STABILITY OF THE AIRMASS.
FOCUS THEN TURNS TO THE PATTERN ON SATURDAY INTO SUNDAY AS THE
WESTERN US UPPER TROF APPROACHES THE PLAINS AND THE SURFACE WARM
FRONT MOVES BACK NORTH. THE SCENARIO THAT WAS EXPECTED IN PREVIOUS
FORECASTS CONTINUES TO LOOK ON TRACK WITH ONLY MINOR EXCEPTIONS.
MODELS ARE NOW IN BETTER AGREEMENT ON A SLIGHTLY SLOWER SOLUTION FOR
THE EASTWARD MOVEMENT OF THE UPPER TROF. REASONABLE AGREEMENT IS
ALSO FOUND WITH THE LOCATION OF THE FRONT OVER NORTHEAST NEBRASKA BY
00Z SUNDAY. THUS MOST OF THE FORECAST AREA WILL BE IN THE WARM
SECTOR SATURDAY. GFS AND NAM ARE 50 TO 100 MILES DIFFERENT ON THE
LONGITUDINAL POSITION OF THE DRY LINE WITH THE NAM SOLUTION EAST AS
FAR AS COLUMBUS-LINCOLN. BOTH MODELS INDICATE A TRIPLE POINT
SOMEWHERE NEAR ALBION-NORFOLK AREA 00Z SUNDAY. WARM SECTOR SURFACE
DEWPOINTS SHOULD BE IN THE 60S BY SATURDAY AFTERNOON. MODERATE
INSTABILITY AND STRONG DEEP LAYER SHEAR WILL SUPPORT SUPERCELLS ONCE
THE CAP BREAKS. STORM DEVELOPMENT MAY BE SLIGHTLY LATER THAN
PREVIOUSLY THOUGHT BASED ON THE SLOWER MOVEMENT OF THE UPPER TROF
AND A SLIGHTLY STRONGER CAP...BUT STORMS SHOULD EVENTUALLY DEVELOP
AND RAPIDLY EXPAND IN COVERAGE THROUGHOUT THE EVENING SATURDAY.
ON SUNDAY THE DEEP UPPER LOW AND ASSOCIATED COLD CORE IS PROGGED TO
MOVE NEAR THE KANSAS-NEBRASKA BORDER WITH GOOD AGREEMENT BETWEEN THE
GFS AND ECMWF. THIS PATTERN MAY SUPPORT LOW TOPPED SUPERCELLS
SUNDAY AFTERNOON. KEPT THE GOING POPS FOR SATURDAY NIGHT INTO
SUNDAY.
&&
.AVIATION...
TAF SITES KOFK/KOMA/KLNK
06Z TAFS
VFR CONDITIONS SHOULD PREVAIL OVER THE TAF SITES THROUGH AT LEAST
18Z AND POSSIBLY UNTIL AFTER 00Z. HIGH PRESSURE WILL CONTINUE TO
MOVE AWAY FROM THE AREA WITH EAST OR SOUTHEAST LOW LEVEL WINDS
THROUGH THE PERIOD. EXPECT THAT CLOUDINESS WILL THICKEN TODAY...WITH
CEILINGS GRADUALLY DECREASING. RAIN CHANCES INCREASE AFTER ABOUT 21Z
AT ALL THREE SITES. AT LEAST FOR NOW...THUNDERSTORM CHANCES APPEAR
LOW ENOUGH TO LEAVE OUT OF THE TAFS.
&&
.OAX WATCHES/WARNINGS/ADVISORIES...
NE...NONE.
IA...NONE.
&&
$$
DISCUSSION...NIETFELD
AVIATION...MILLER