FXUS63 KEAX 221152
AFDEAX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KANSAS CITY/PLEASANT HILL MO
551 AM CST MON DEC 21 2009
...UPDATED AVIATION DISCUSSION...
.DISCUSSION...
/455 AM CST MON DEC 21 2009/
ELONGATED PV ANOMALY (OR STRING OF PV ANOMALIES) DIVING SOUTH ALONG
THE PACIFIC NORTHWEST COAST TOWARDS A WEAKNESS IN THE HEIGHT FIELD
ACROSS THE DESERT SOUTHWEST WILL BECOME THE MAIN PLAYER IN STRONG
CNTRL CONUS CYCLOGENIC EVENT THIS WEEK. OF MORE IMMEDIATE CONCERN IS
A WEAK FRONTAL BOUNDARY AND DENSITY CURRENT SURGE THROUGH CNTRL
NEBRASKA WORKING TOWARDS THE CWA THIS MORNING. NAM-WRF SEEMED TO
CAPTURE THIS SHALLOW BOUNDARY MOST ACCURATELY...ESSENTIALLY WASHING
THE BOUNDARY INTO A QUASI-STATIONARY WARM FRONT THIS AFTERNOON...AS
LOW LEVEL MOISTURE CONVERGES FROM ALL DIRECTIONS. DIFFICULT
TEMPERATURE FORECAST FOR THE NEXT 24 HOURS...AS UPPER 40S AND LOWER
50S NOT THAT FAR AWAY ALONG THE I-40 CORRIDOR ATTEMPT TO WORK
NORTH...YET BOUNDARY LAYER MAINTAINS AN ELY FETCH...AND INCREASING
CLOUD COVER HELPS MUTE ANY ADDED DIURNAL EFFECTS.
CORRESPONDINGLY...WITH ADDED CLOUD COVER AND GRADIENT ELY WINDS
RESPONDING TO LEE SFC CYCLOGENESIS IN THE SRN HIGH PLAINS...FAIL TO
SEE HOW OVERNIGHT LOWS WOULD DROP MUCH AT ALL (AND EVEN POSSIBLY
RISE WITH A SURGE OF HIGHER THETA-E AIR OVERNIGHT).
MODELS HAVE DEFINITELY TRENDED TOWARDS THE PREVIOUSLY PREFERRED
ECMWF TYPE SOLUTION...BRINGING THE MAIN COLD CORE CIRCULATION CENTER
FROM NEW MEXICO THROUGH CNTRL KANSAS...AND TOWARDS FAR NWRN MISSOURI
BY THURSDAY EVENING (OPERATIONAL GFS STILL REMAINS ON THE ERN
PERIPHERY OF ALL GUIDANCE...WITH THE NAM-WRF TOWARDS A MORE WRN
EXTREME). IT SHOULD BE STATED THAT ON A GLOBAL SYNOPTIC SCALE WITH
CURRENT MODEL RESOLUTION AS IT IS...THIS TYPE OF SPREAD IS ACTUALLY
QUITE REASONABLE...THE PROBLEM BEING SO MUCH OF THE SENSIBLE WEATHER
IS DRIVEN ON THE SUB-SYNOPTIC SCALE...AND ONLY SMALL PERTURBATIONS
IN THE INITIALIZATION FIELDS CAN CAUSE A SHIFT OF ONLY A FEW GRID
POINTS...AND THUS COMPLETELY AFFECTING THE FORECAST PROCESS AND
IMPACT PERSPECTIVE (CONVERSELY THINK OF IT IN THE OPPOSITE
PERSPECTIVE...IF THE SPREAD OF A FORECAST RIDGE OF HIGH PRESSURE HAD
THIS TYPE OF UNCERTAINTY...THE IMPACT BASE WOULD BE QUITE
NEGLIGIBLE).
REGARDLESS...IT IS ALSO BECOMING APPARENT THAT SEVERAL SHORTWAVES
EMANATING FROM THE MAIN HEIGHT ANOMALY CENTER WILL PROPAGATE THROUGH
THE REGION PRIOR TO A REINFORCING WAVE FROM THE NRN STREAM PV
RESERVOIR CONSOLIDATING INTO A BAROTROPICALLY STACKED CIRCULATION
OVER NRN MISSOURI BY FRIDAY MORNING. THIS IS A TYPICAL PROCESS FOR
WAVES EJECTING ACROSS THE SRN ROCKIES...AND WILL BE MANIFESTED BY
SEVERAL INSTANCES OF ISENTROPICALLY INDUCED LIFT THROUGH THE AREA
OFFSET BY SINKING MOTION WITH THE PASSAGE OF EACH SHORTWAVE. GRADUAL
WEAK LIFT WILL BEGIN LATER THIS AFTERNOON...THOUGH WITHIN A
RELATIVELY SHALLOW LAYER...LIKELY ONLY RESULTING IN SOME DRIZZLE AND
SPRINKLES. NRN TIER OF COUNTIES MAY BE FLIRTING RIGHT AROUND THE
FREEZING MARK..AND AT THIS TIME THINKING ANY VERY LIGHT PRECIP WOULD
HAVE LITTLE IMPACT...ONLY PARTIALLY FREEZING ON WELL EXPOSED
SURFACES (IE. TREES AND CARS ETC). SIMILAR THINKING ALSO APPLIES FOR
TONIGHT WITH LITTLE CHANGE IN TEMPERATURE EVEN AFTER DARK...AND HAVE
OPTED TO NOT ISSUE ANY HEADLINES FOR THE TIME BEING.
FAR DEEPER AND MORE ROBUST LIFT WILL COMMENCE LATE TONIGHT AND
THROUGH WEDNESDAY MORNING...AS THE FIRST SHORTWAVE AND SURGE OF
HIGHER THETA-E AIR PUSHES NORTH FROM THE SRN PLAINS. MODEL SOUNDINGS
ALL INDICATE A DECENT MEASURE OF MUCAPE ASSOCIATED WITH A NOSE OF
SATURATED +4C TO +5C AIR AROUND H8 COINCIDENT WITH MODESTLY STEEP
MIDLEVEL LAPSE RATES. ANY CONVECTIVE ELEMENT WOULD CERTAINLY LOCALLY
ENHANCE RAINFALL AMOUNTS EARLY WEDNESDAY MORNING. IT IS VERY
CONCEIVABLE (AND ALMOST EXPECTED) THAT MUCH OF THE AREA WILL
EXPERIENCE A BREAK IN PRECIPITATION WEDNESDAY AFTERNOON...AWAITING
THE ARRIVAL OF THE NEXT PIECE OF ENERGY WEDNESDAY EVENING.
ANTICIPATE THE ENTIRE CWA WILL BE ABOVE THE FREEZING MARK FROM LATE
WEDNESDAY MORNING THROUGH EARLY THURSDAY AFTERNOON...WITH THE MOST
SIGNIFICANT ICING ACROSS THE I-80 CORRIDOR IN IOWA.
VERY INTENSE DEEPENING AND STRONG MATURING CYCLOGENESIS WILL TAKE
PLACE THROUGHOUT THE DAY ON THURSDAY...WITH THE OCCLUSION PROCESS
OCCURRING THURSDAY NIGHT. FIRST CONCERN REGARDING THIS SYSTEM IS THE
FLOODING POTENTIAL...AS PWATS AOA 2 STANDARD DEVIATIONS ABOVE
AVERAGE ADVECTING NORTH THROUGH THE WARM SECTOR IN CONCERT WITH DEEP
LIFT ASSOCIATED WITH A COLLOCATED LLJ AXIS...LEFT FRONT QUADRANT OF
A IMPINGING JET AXIS...AND NOTABLE VORTICITY ADVECTION/Q-VECTOR
CONVERGENCE. HINTS OF ELEVATED INSTABILITY REMAIN ON SOUNDINGS EVEN
INTO THURSDAY MORNING...SUGGESTING CONVECTIVELY ENHANCED
PRECIPITATION MAY BE POSSIBLE. HOWEVER...AXIS OF HEAVIEST RAINFALL
MAY ACTUALLY ALIGN FURTHER TO THE SOUTH AND EAST (CLOSER TO THE I-44
CORRIDOR)...AS IS TYPICAL WITH MORE PROGRESSIVE SYSTEMS. WILL HOLD
OFF ON ANY FLOOD WATCH FOR NOW...INSTEAD HIGHLIGHTING THE POTENTIAL
IN AN ESF AND HWO PRODUCT.
SECOND CONCERN IS THE POTENTIAL FOR WINTER PRECIPITATION AS THE COLD
CORE ALOFT PROGRESSES OVER THE AREA...AND DEEP COLUMN COOLING OCCURS
WITHIN A SATURATED ENVIRONMENT. OF NOTE FROM A CLIMATOLOGICAL
STANDPOINT...OVERALL HEIGHT ANOMALIES ARE IN THE 2-3 NORMALIZED
STANDARD DEVIATION RANGE (SIGNIFICANT BUT NOT OVERWHELMING)...WITH
OVERALL TEMPERATURES AND THICKNESSES HARDLY ANOMALOUS. THE MOST
SIGNIFICANT PORTION OF THIS SYSTEM IS THE DEEP SLP AND LOW LEVEL
WIND ANOMALIES...WITH PRESSURE ANOMALIES PEAKING IN THE 3-4 SD BELOW
AVERAGE RANGE...AND H8-H9 WIND ANOMALIES GREATER THAN 4 SD AWAY FROM
AVERAGE (AND ESPECIALLY FOR THE ELY WINDS NORTH OF THE WARM FRONT IN
IOWA FEEDING THE INTENSE TROWAL FORMATION TOWARD THE INVERTED TROUGH
STRUCTURE THROUGH CNTRL/ERN NEBRASKA). ASIDE FROM THE ICE/SLEET
PROBLEM ACROSS IOWA...THE KEY TO THE INTENSE SNOWFALL APPEARS TO BE
THE LOCATION AND STRUCTURE OF THIS INVERTED TROUGH AND DIRECT
CIRCULATION WITH THE INFLUX OF WARMER MOIST AIR ON THE ANOMALOUS
ELEVATED WARM CONVEYOR BELT. BEST COMBINATION OF ALL PARAMETERS
LOOKS TO BE FROM CNTRL KANSAS THROUGH CNTRL/ERN NEBRASKA/SWRN
MINNESOTA...WITH THE DISTINCT POSSIBILITY OF LOCALLY UP TO 20 INCHES
OF ACCUMULATION THERE. ANALYSIS OF CIPS ANALOG CASES REVEALS SEVERAL
GOOD CASES WITH BOTH SIMILAR UPPER LEVEL FEATURES AND SFC
TEMPERATURES (FEB 28 1987...FEB 24 2007...AND MAR 27 1983)...ALL OF
WHICH ARGUE FOR THIS CNTRL PLAINS TO UPPER MIDWEST CORRIDOR.
THINKING THE FAR NW CORNER OF MISSOURI STANDS THE BEST CHANCE OF
EXPERIENCING MORE SIGNIFICANT SNOWFALL.
OUTSIDE OF THIS AREA...MOST OTHER LOCATIONS MAY ACTUALLY SEE THE
MOST SNOW ON FRIDAY...AS A DRY SLOT MAY CUT OFF ICE NUCLEATION
THURSDAY NIGHT...BEFORE THE PV ANOMALY BECOMES RECHARGED AND
RECIRCULATED ABOUT THE PARENT LOW OVER NRN MISSOURI. MODELS ARE
ACTUALLY IN VERY GOOD AGREEMENT WITH THIS SCENARIO...DEPICTING A
GENERAL 1-2 INCH SNOWFALL FOR JUST ABOUT ALL LOCATION NORTH OF THE
MISSOURI RIVER FRIDAY. INCREASED POPS DURING THIS TIME PERIOD...BUT
ALSO KEPT TEMPERATURES FROM FALLING TOO QUICKLY GIVEN CLOUD COVER
AND THE STILL INFLUENTIAL BAROTROPIC NATURE TO THE MIDLEVEL
CIRCULATION. ALL TOLD...MAY EVENTUALLY NEED SOME SORT OF HEADLINES
FOR THE NRN HALF OF THE CWA (AND IN PARTICULAR THE NWRN CWA)
THURSDAY NIGHT THROUGH FRIDAY.
21
&&
.AVIATION...
FOR THE 12Z TAFS...TWO DIFFERENT IFR STRATUS FIELDS ARE HEADING
TOWARD THE TAF SITES...ONE FROM SOUTHERN IOWA...AND THE OTHER OUT OF
NEBRASKA. THE IOWA STRATUS IS BEING DRIVEN SOUTHWEST WITH BACKING
LOW LEVEL WINDS RESPONDING TO A WEAK SURFACE WAVE HEADING OUT OF
SOUTHEAST NEBRASKA. STRATUS IN NEBRASKA IS MOVING SOUTHEAST BEHIND
THE BOUNDARY ASSOCIATED WITH THIS WEAK SURFACE WAVE. TIMING FROM
SATELLITE WOULD HAVE THIS STRATUS ARRIVING MIDDAY TO EARLY AFTERNOON
TIME FRAME...WHICH IS A LITTLE SLOWER THAN WHAT GUIDANCE IS SHOWING.
WILL HAVE TO WATCH FOR ANY INCREASED FORWARD SPEED IN THESE CLOUD
DECKS...BUT FOR NOW WILL GO WITH SATELLITE TIMING. REGARDING
PRECIPITATION...NOT EXPECTING MUCH OTHER THAN DRIZZLE THIS EVENING
AS LOW LEVEL ISENTROPIC LIFT SETS UP...BUT WILL LEAVE THIS LIGHT
PRECIPITATION OUT AT THIS POINT AND FOCUS ON THE POTENTIAL OF RAIN
DEVELOPING TOWARD MIDNIGHT AS BETTER UPPER SUPPORT ARRIVES WITH THE
LEAD SHORT WAVE HEADING INTO THE REGION.
PC
&&
.EAX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
KS...NONE.
&&
$$
