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000
FXUS63 KDVN 182355
AFDDVN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
655 PM CDT SAT MAY 18 2013

.SYNOPSIS...
ISSUED AT 208 PM CDT SAT MAY 18 2013

AN ELONGATED AREA OF LOW PRESSURE EXTENDED FROM THE SOUTHERN
PLAINS INTO THE THE WESTERN DAKOTAS THIS AFTERNOON. AN ILL DEFINED
WARM FRONT ARCED SOUTHEAST FROM THE LOW OVER THE DAKOTAS THROUGH
THE MID MISSISSIPPI VALLEY. THE REGION WAS RAIN FREE THIS
AFTERNOON WITH THE CLOSED UPSTREAM PRECIP LOCATED OVER KANSAS AND
OKLAHOMA. THIS AREA OF SHOWERS AND ISOLATED THUNDERSTORMS WAS
OCCURRING AHEAD OF S/W MOVING NORTHEAST THROUGH WESTERN KS PER
RECENT WATER VAPOR IMAGERY.

&&

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 208 PM CDT SAT MAY 18 2013
THE OVERALL SYNOPTIC PATTERN REMAINS UNCHANGED THROUGH SUNDAY
MORNING WITH THE UPPER TROF REMAIN OVER THE ROCKIES AND THE PLAINS
SURFACE LOW MOVING VERY LITTLE. DURING THE DAY SUNDAY A S/W EJECTS
OUT OF THE BASE OF THE TROF AND BECOMES NEGATIVELY TILTED AS LIFTS
INTO MO BY LATE SUNDAY AFTERNOON.

FOR TONIGHT THE MAIN CONCERN WILL BE THUNDERSTORM CHANCES LATE
TONIGHT AS THE SW KANSAS S/W LIFTS NORTHEAST THROUGH WESTERN IOWA.
EXPECT STORMS TO DEVELOP LATE THIS AFTERNOON OR EARLY THIS EVENING
OVER THE CENTRAL AND SOUTHERN PLAINS AS THE WRN KS S/W INTERACTS
WITH THE DEVELOPING LLJ AND ELEVATED CAPES AROUND 2000J/KG. AS THE
S/W MOVES THROUGH WESTERN IA LATE THIS EVENING/EARLY SUNDAY
MORNING THE STORMS SHOULD MOVE NORTHEAST REACHING EAST CENTRAL
IOWA EARLY SUNDAY MORNING. MODELS SHOW CAPES WANING BY THIS TIME AS
WELL AS A WEAKENING LLJ SUGGESTING ANY STORMS WILL BE WEAKENING AS
THEY REACH EAST CENTRAL IOWA. SINCE THE STORMS ARE EXPECTED TO BE
DISSIPATING AS THE REACH EASTERN IOWA...HAVE KEPT LATE NIGHT POPS
LIMITED TO THE WESTERN THIRD AND IN THE LOW CHANCE OR SLIGHT
CHANCE CATEGORY. GIVEN THE MOIST AIR MASS ALREADY IN PLACE AND CLOUDS
BLOWING OFF OF CONVECTION APPROACHING FROM THE WEST WE SHOULD SEE
LOWS TONIGHT IN THE 60S.

FOR SUNDAY HAVE GONE WITH BROAD BRUSH CHANCE POPS DURING THE
AFTERNOON. THE LOW LEVELS WILL DESTABILIZE WITH LOW TO MID 60
DEWPOINTS IN PLACE AND TEMPERATURES IN THE 80S. HOWEVER WITH THE
MAIN UPPER TROF WELL TO THE SOUTHWEST AND NO COHERENT SURFACE
BOUNDARY IN PLACE...THE THREAT OF ORGANIZED STORMS WILL BE
LOW...MAKING IT DIFFICULT TO PIN POINT ANY ONE AREA FOR HIGHER
POPS. THE LACK OF AN OBVIOUS TRIGGER AND WEAK DEEP LAYER SHEAR WILL
ALSO MEAN THE THREAT FOR SEVERE STORMS DURING THE AFTERNOON WILL
BE LOW.

.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 208 PM CDT SAT MAY 18 2013

MAIN FOCUS IS ON THUNDERSTORM CHANCES AND POTENTIAL FOR SEVERE
STORMS SUNDAY NIGHT THROUGH MONDAY. SLOW MOVING UPPER LOW OVER THE
PLAINS WILL KEEP THE POTENTIAL FOR SHOWERS AND THUNDERSTORMS...ALONG
WITH ABOVE NORMAL TEMPERATURES...GOING UNTIL IT FINALLY EXITS MID TO
LATE WEEK. HIGH PRESSURE AT THE SURFACE SHOULD BRING DRY
CONDITION AND TEMPERATURES CLOSER TO NORMAL LATE WEEK WITH RAIN
CHANCES INCREASING OVER THE WEEKEND.

SUNDAY NIGHT...DEVELOPING UPPER LEVEL DIFLUENCE AND A MID LEVEL VORT
MAX AHEAD OF THE UPPER LEVEL JET DIVING INTO THE WESTERN U.S.
TROUGH...IS SHOWN BY MODEL CONSENSUS ROUNDING THE UPPER TROUGH AND
LIFTING NORTHEAST...OVERSPREADING ESPECIALLY THE WESTERN HALF OF THE
FORECAST AREA IN THE EVENING. EVEN COMPENSATING FOR MODEL DEWPOINTS
LIKELY TRENDING A FEW DEGREES TOO HIGH...SURFACE BASED CAPES WILL
LIKELY BE IN EXCESS OF 2000 J/KG IN THE EARLY EVENING...DEPENDENT ON
WHERE ANY AFTERNOON CONVECTION FIRES. AT THE SAME TIME...A MID LEVEL
50 KT PLUS SPEED MAX WILL PROVIDE AT LEAST MODERATE LEVELS OF DEEP
SHEAR. UNCERTAINTIES FOR CONVECTIVE DEVELOPMENT INCLUDE THE LACK OF
A STRONG SURFACE BOUNDARY AND THE STRENGTH OF THE CAPPING ELEVATED
MIXED LAYER. OVERALL...SIGNALS POINT TOWARD A COMPLEX DEVELOPING IN
THE FAR WEST OR JUST NORTHWEST OF THE FORECAST AREA IN THE
EVENING...SPREADING NE...THEN E-NE FOLLOWING CORFIDI VECTORS ACROSS
AT LEAST THE NORTHERN HALF OF THE FORECAST AREA OVERNIGHT. THERE MAY
ALSO BE ADDITIONAL DEVELOPMENT AWAY FROM ANY COMPLEX AS THE
NOCTURNAL JET...SHOWN AROUND 40 KT KICKS IN...FEEDING GULF MOISTURE
INTO THE REGION WHILE STEEPENING MID LEVEL THERMAL AND THETAE
LAPSE RATES. AS MENTIONED IN THE PREVIOUS DISCUSSION...THIS SETUP
SUPPORTS LARGE HAIL...DAMAGING WIND AND LOCALLY HEAVY RAINFALL. SPC
HAS A SLIGHT RISK OVER THE ENTIRE AREA IN THE CURRENT DAY 2 OUTLOOK
WHICH RUNS THROUGH 12Z MONDAY. HAVE INCREASED POPS TO CATEGORICAL IN
THE FAR NW WITH LIKELIES ELSEWHERE. HIGH DEWPOINTS WILL KEEP
TEMPERATURES IN THE 60S THROUGH THE NIGHT.

MONDAY AND MONDAY NIGHT...SUBSEQUENT SHORTWAVES IN THE SOUTHWEST FLOW
AND LOW LEVEL BOUNDARIES LINGERING FROM OVERNIGHT CONVECTION WILL
TRIGGER ADDITIONAL THUNDERSTORMS. DEPENDING ON THE FORECAST
MODEL...SURFACE BASED CAPES MAY AGAIN REACH WELL ABOVE 2000 AND
POSSIBLY HIGHER THAN 4000 J/KG. THIS WILL BE GREATLY DEPENDENT HOW
THE OVERNIGHT CONVECTION DISSIPATES AND THE COVERAGE OF ASSOCIATED
CLOUD COVER. LATEST MODELS CONTINUE TO SUGGEST DEEP LAYER SHEAR WILL
BE STRONGER AS THE UPPER JET APPROACHES FROM THE SOUTHWEST. AT THE
SAME TIME...THE DEEPENING UPPER LOW OVER NEB WILL COOL TEMPERATURES
ALOFT...PROVIDING A WEAKER CAP THAN SUNDAY...AND EVEN STEEPER MID
LEVEL LAPSE RATES. THIS WILL BE MORE SUPPORTIVE OF SUPERCELL
THUNDERSTORMS ESPECIALLY IN THE AFTERNOON AND EARLY EVENING HOURS.
THESE SUPERCELLS WILL CONTINUE THE SEVERE WEATHER THREATS OF LARGE
HAIL AND DAMAGING WIND...AND INTRODUCE AN INCREASED RISK FOR
TORNADOES. SPC MAINTAINS A SLIGHT RISK ENCOMPASSING THE ENTIRE
FORECAST AREA FOR DAY 3...WHICH COVERS MONDAY THROUGH MONDAY NIGHT.
WILL KEEP POPS IN THE HIGH CHANCE RANGE IN THE DAYTIME DUE TO MORE
LIMITED COVERAGE...THEN INCREASE TO LIKELY POPS FOR OVERNIGHT WHEN
MORE WIDESPREAD COVERAGE OF SHOWERS AND THUNDERSTORMS IS EXPECTED.
TEMPERATURES REMAIN CLOSE TO PERSISTENCE IN A LARGELY UNMODIFIED
ENVIRONMENT WITH HIGHS IN THE 80S AND LOWS IN THE 60S.

THE UPPER LOW GRADUALLY WEAKENS AND SWEEPS EAST OF THE AREA BY
THURSDAY. THIS WILL KEEP CHANCES FOR SHOWERS AND AT LEAST ISOLATED
THUNDERSTORMS GOING THROUGH THURSDAY WITH TEMPERATURES TRENDING BACK
CLOSER TO LATE MAY NORMALS ALONG WITH LOWER HUMIDITY. HIGH PRESSURE
THAT FOLLOWS WILL PROVIDE DRY WEATHER THROUGH FOR AT LEAST FRIDAY.
SATURDAY...LOW CONFIDENCE SLIGHT CHANCES FOR RAIN ARE INTRODUCED
BASED ON THE MORE PROGRESSIVE ECMWF THAT BRINGS A WARM FRONT INTO
THE AREA. THE GFS MAINTAINS A DRY FORECAST WITH THE AREA UNDER THE
INFLUENCE OF HIGH PRESSURE RETREATING SLOWLY TOWARD THE GREAT
LAKES.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z SUNDAY EVENING)
ISSUED AT 639 PM CDT SAT MAY 18 2013

VFR CONDITIONS EXPECTED TO PREVAIL FOR MOST IF NOT ALL OF THE
TAF CYCLE. COULD SEE SOME PATCHY FOG WITH MVFR VSBYS LATE
TONIGHT...BUT NEGATIVE FACTORS INCLUDE INCREASING HIGH CLOUDS
FROM PLAINS CONVECTION AND INCREASING PRESSURE GRADIENT
RESULTING IN S/SE WINDS SUSTAINED AROUND 10 KTS WITH SOME
LOCAL GUSTS POSSIBLE. DUE TO THE UNCERTAINTY HAVE ONLY HINT OF
LIGHT FOG WITH VSBYS 6SM ATTIM. HAVE MAINTAINED MENTION OF VCSH
AT KCID AND KBRL OVERNIGHT UNTIL MID AM SUNDAY FOR POTENTIAL
OF WEAKENING CONVECTION ATTENDANT TO DIMINISHING LOW LEVEL JET.
SHOULD SEE SHOWERS AND STORMS REDEVELOP WITH HEATING BY MID TO
LATE SUNDAY AFTERNOON IN BROAD SOUTHERLY FLOW AHEAD OF AN UPPER
LEVEL DISTURBANCE EMERGING INTO THE PLAINS. MORE WIDESPREAD
SHOWERS AND STORMS SUGGESTED SUNDAY EVENING. WINDS SUNDAY WILL
BE GUSTY AT TIMES FROM THE S/SE AT 10-20 KTS.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...DLF
SHORT TERM...DLF
LONG TERM...SHEETS
AVIATION...05