Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 201811
AFDOAX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
111 PM CDT SAT SEP 20 2014

.SHORT TERM...(TODAY THROUGH MONDAY)
ISSUED AT 311 AM CDT SAT SEP 20 2014

...PRIMARY FORECAST CONCERNS ARE POPS THIS MORNING...

AREA OF SCATTERED THUNDERSTORMS CONTINUES TO BE ACTIVE OVER
EASTERN NEBRASKA AND SOUTHWEST IOWA...CONFINED MAINLY NORTH OF
INTERSTATE 80 THUS FAR. OVERALL THIS AREA HAS SLOWLY SHIFTED
SOUTH DURING THE EARLY MORNING HOURS AND HAS MAINTAINED MOSTLY
WEAK UPDRAFTS. AIRMASS AHEAD OF THE CONVECTION...OVER SOUTHEAST
NEBRASKA AND SOUTHWEST IOWA...REMAINS SOMEWHAT UNSTABLE WITH
DEWPOINTS IN THE MID TO UPPER 60S. MOST UNSTABLE CAPE IS AROUND
1000 J/KG ALTHOUGH LIFTED PARCEL LEVELS ARE AROUND 2000 METERS.
LOW LEVEL JET IS AROUND 50 KNOTS ON KOAX VWP BUT HAS VEERED TO THE
WSW. ALL OF THIS POINTS TO CONTINUED SUPPORT OF CONVECTION FOR THE
NEXT FEW HOURS BUT REMAINING WELL BELOW SEVERE LEVELS...AND
MOSTLY ELEVATED.

HOW LONG IT WILL LINGER...AND SPECIFICALLY WHERE...SEEMS TO BE
TIED TO THE VERTICAL MOTION CAUSING IT. THE VERTICAL MOTION IS
MOSTLY BEING ATTRIBUTED TO MODERATELY STRONG FRONTOGENESIS IN THE
MID LEVELS THAT IS OCCURRING ABOVE THE SURFACE COLD FRONT THAT WAS
ANALYZED AT 07Z EXTENDING FROM NORTHWEST IOWA INTO CENTRAL
NEBRASKA. THIS IS FORECAST BY MOST SHORT TERM MODELS TO EXIT THE
FORECAST AREA BY EARLY AFTERNOON. HAVE TAPERED POPS FROM NORTH TO
SOUTH THROUGH THE MORNING AND HAVE ALL AREAS DRY BY 22Z. GENERALLY
EXPECT AREAS FROM INTERSTATE 80 NORTH TO BE DRY BY LATE MORNING...
BUT KEPT A SMALL CHANCE IN THE FAR SOUTHEAST CORNER OF NEBRASKA
AND SOUTHWEST IOWA DURING EARLY AFTERNOON BASED ON PROXIMITY TO
DECELERATING SURFACE COLD FRONT AND OUTFLOW BOUNDARY FROM ON-
GOING CONVECTION THAT IS EXPECTED TO SETTLE IN THAT AREA.

COOLER AND DRIER AIRMASS MOVES INTO THE FORECAST AREA TONIGHT WITH
SURFACE HIGH BUILDING INTO THE PLAINS SUNDAY. PLEASANT WEATHER
WITH LIGHT WINDS EXPECTED SUNDAY. MONDAY WILL BE NICE AS WELL...
ALTHOUGH THE SURFACE HIGH SHOULD MOVE TO THE MISSISSIPPI RIVER
VALLEY MONDAY AFTERNOON WITH A RETURN FLOW OF SOUTH WINDS SETTING
UP MONDAY.

.LONG TERM...(MONDAY NIGHT THROUGH FRIDAY)
ISSUED AT 311 AM CDT SAT SEP 20 2014

...MAIN CHALLENGE IS POPS MONDAY NIGHT INTO THURSDAY...

THE MODELS ARE IN GOOD AGREEMENT WITH AN UPPER RIDGE OVER THE
PLAINS MONDAY...AND A PRONOUNCED UPPER LEVEL LOW MOVING THROUGH
THE WESTERN US. REMARKABLE SIMILARITY BETWEEN MODEL SOLUTIONS AT
12Z TUESDAY WITH UPPER LOW CIRCULATION NEAR THE BLACK HILLS AND
STRONG SOUTHERLY LOW LEVEL FLOW OVER NEBRASKA BRINGING A RETURN OF
LOW LEVEL MOISTURE. MAJORITY OF DETERMINISTIC MODELS AND ENSEMBLE
MEMBERS SHOW RAIN OVER SOUTH DAKOTA...NEBRASKA...AND KANSAS AT 12Z
TUE...THEN THE FEATURES AND ASSOCIATED RAIN MOVES EAST OVER THE
FORECAST AREA BETWEEN 12Z TUESDAY AND 18Z WEDNESDAY. WE CONTINUED
THE CHANCE POPS DURING THAT PERIOD...WITH LIKELY POPS TUESDAY FOR
PARTS OF THE AREA.

MODEL SOLUTIONS DIVERGE DRASTICALLY FROM WEDNESDAY NIGHT INTO
FRIDAY...THEREFORE CONFIDENCE DROPS CONSIDERABLY. THE GFS SUGGESTS
A CONTINUATION OF THE WET PATTERN BY DROPPING THE UPPER LOW INTO
NEBRASKA...WHEREAS THE ECMWF ALLOWS THE LOW TO BE SHEARED OFF TO
THE EAST BY THE BROADER SCALE UPPER RIDGE FLOW...WHICH LOOKS MORE
REALISTIC. KEPT THE CHANCE POPS WEDNESDAY NIGHT BUT WENT DRY FOR
THURSDAY AND FRIDAY BASED ON THE ECMWF.

&&

.AVIATION...(18Z TAFS FOR KOFK...KLNK AND KOMA.)
ISSUED AT 104 PM CDT SAT SEP 20 2014

SURFACE COLDFRONT AT 18Z EXTENDED FROM JUST NORTHWEST OF KOMA TO
NORTHWEST OF KLNK. FRONT SHOULD BE THROUGH BOTH LOCATIONS BY 19Z.
WINDS NORTH OF THE FRONT WILL BE NORTHWEST AROUND 10 KNOTS. SOME
GUSTS UP TO 15 KNOTS IS EXPECTED THIS AFTERNOON AT KOFK. VFR
CONDITIONS ARE EXPECTED BEHIND THE FRONT AS HIGH PRESSURE CENTERED
OVER EASTERN MONTANA DROPS SOUTHEASTWARD.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...NONE.
IA...NONE.
&&

$$

SHORT TERM...NIETFELD
LONG TERM...NIETFELD
AVIATION...SMITH


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