FXUS63 KOAX 081037
AFDOAX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
437 AM CST TUE MAR 8 2011
.DISCUSSION...
...MAIN FORECAST PROBLEM IS SNOW AMOUNTS AND PRECIP TYPE OF RAIN
VS SNOW...
WATER VAPOR IMAGERY WAS SHOWING THE DEVELOPING EXTRA TROPICAL
CYCLONE MOVE THROUGH THE SOUTHWEST AND JUST STARTING TO EMERGE
INTO THE SOUTHERN HIGH PLAINS EARLY THIS MORNING. WARM CONVEYOR
BELT WAS BECOMING ESTABLISHED WITH RAIN SPREADING FROM THE TEXAS
PANHANDLE NORTH INTO KANSAS. THE UPPER LOW HAD A -24C COLD CORE AT
500 MB OVER ALBUQUERQUE AT 00Z...WITH NUMEROUS LIGHTNING STRIKES
NOTED AROUND THE 4-CORNERS REGION EARLIER MONDAY EVENING. WATER
VAPOR ALSO SHOWED A DECENT SUB-TROPICAL MOISTURE PLUME BEING
WRAPPED INTO THE WARM CONVEYOR BELT. OVER NEBRASKA...ANOTHER AREA
OF WEAK ISENTROPIC LIFT WAS NOTED ON THE 290K SURFACE...WITH A
BROAD AREA OF LIGHT SNOW FALLING MAINLY NORTH OF INTERSTATE 80.
THIS LIGHT SNOW WAS MOVING NORTH AND SHOULD MOSTLY BE OUT OF THE
FORECAST AREA BY SUNRISE...LITTLE OR NO ACCUMULATION HAS BEEN
REPORTED WITH THIS SNOW.
FORECASTER REASONING AND MODEL GUIDANCE ARE IN GOOD AGREEMENT ON
THE GENERAL PRECIPITATION AMOUNTS ASSOCIATED WITH SYSTEM MOVING
THROUGH THE PLAINS TODAY AND TONIGHT. ONE SIGNIFICANT TIMING
ADJUSTMENT WAS TO START THE MAIN PRECIPITATION A BIT LATER THAN
EARLIER FORECAST DUE TO A SLOWER EVOLUTION OF THE WARM CONVEYOR
BELT PRECIPITATION. 00Z MODELS REPRESENTED THIS WELL AND THEREFORE THE
TIMING OF THE PRECIPITATION WAS SIMILAR AMONG THE SOLUTIONS.
HOWEVER THE MOST CHALLENGING ASPECT OF THIS FORECAST IS
DETERMINING THE PROPER PRECIPITATION PHASE...NAMELY RAIN VS. SNOW.
THAT IN TURN WILL DETERMINE THE FORECAST SNOWFALL AMOUNTS. THE
PRECIPITATION TYPE WILL MAINLY BE A FUNCTION OF THE NEAR GROUND
TEMPERATURES...THE DEEPER THERMAL PROFILE...AND ANY SUBTLE EFFECTS
FROM COOLING DUE TO MELTING OF THE SNOWFLAKES CAUSING LATENT HEAT
ABSORPTION. THE MODEL GUIDANCE APPEARED TO INITIALIZE THE SURFACE
TEMPERATURES TOO COOL...ALTHOUGH THE MODEL SOUNDINGS APPEARED TO
MATCH AN AIRCRAFT SOUNDING ASCENDING OUT OF OMAHA REASONABLY WELL
ABOVE THE SURFACE. MOST LOCATIONS IN NORTHEAST NEBRASKA HAVE A
COLD ENOUGH THERMAL PROFILE AND A COLD ENOUGH NEAR GROUND
TEMPERATURE FOR ALL SNOW...WHICH IS WHAT HAS BEEN REPORTED EARLY
THIS MORNING. HOWEVER AREAS FROM INTERSTATE 80 SOUTH HAVE SEEN A
MIX OF RAIN AND SNOW MAINLY DUE TO THE WARMER NEAR GROUND
TEMPERATURES. WHILE THE UPPER LOW REMAINS TO OUR WEST...WARM AIR
ADVECTION IN THE LOWER TO MID LEVELS WILL WARM THE THERMAL
PROFILE TODAY...WHICH WAS BEST REPRESENTED BY THE GFS. THIS WARMING
WILL LIKELY RESULT IN MORE RAIN THAN SNOW DURING THE DAYLIGHT
HOURS OVER MUCH OF THE FORECAST AREA EXCEPT THE FAR NORTHWEST. THE
CONCERN IS FOR THE ABOVE-MENTIONED POSSIBILITY OF SOME COOLING DUE
TO MELTING OF SNOWFLAKES WHICH COULD OCCUR SEVERAL HOURS INTO THE
RAIN EVENT AND IF THE THERMAL PROFILE IS CLOSE TO ISO-THERMAL.
WILL KEEP A MENTION OF A MIX OF RAIN AND SNOW MOST AREAS TO
ACCOUNT FOR THIS POSSIBILITY AND MONITOR AS THE EVENT UNFOLDS.
TONIGHT THE UPPER LOW MOVES JUST SOUTH OF THE FORECAST AREA AND
THE DEEP THERMAL PROFILE SHOULD COOL DUE TO ADIABATIC COOLING WHILE
THE VERTICAL MOTION IS MAXIMIZED AND COLD AIR ADVECTION WILL TAKE
PLACE FROM THE GROUND UP...THUS ALL AREAS SHOULD SEE A SWITCH TO
ALL SNOW AFTER DARK. UPPER LOW MOVES FAIRLY QUICKLY EAST WEDNESDAY
MORNING AND PULLS THE MATURING TROWAL WITH IT...THUS ENDING THE
SNOWFALL FROM WEST TO EAST WEDNESDAY MORNING. THE UPPER LOW IS
PROGGED TO DEEPEN AS IT MOVES TO OUR SOUTH AND WILL NEED TO
MONITOR SNOWFALL RATES WHILE INTENSE VERTICAL MOTION IS PRESENT.
SNOWFALL ACCUMULATION WILL BE HAMPERED BY ANY RAIN MIXING WITH THE
SNOW...AND A RELATIVELY SHORT DURATION OF LIFT. WE DO NOT EXPECT
MANY LOCATIONS TO SEE 12 HOURS OF ACCUMULATING SNOW. MIXING RATIO
VALUES WOULD SUPPORT NEARLY 6 INCHES OF SNOW IN SOME
AREAS... HOWEVER GIVEN THE ABOVE MENTIONED NEGATING FACTORS WE
HAVE KEPT SNOWFALL AMOUNTS UNDER 6 INCHES AND KEPT A WINTER
WEATHER ADVISORY IN EFFECT. ADVISORY WAS ADJUSTED FOR A LATER
START AND WE DID ADD THE FEW COUNTIES THAT WERE NOT PREVIOUSLY IN
IT. FORECAST SOUNDINGS DO NOT SUPPORT CONVECTIVE INSTABILITY
HOWEVER SOME CONDITIONAL SYMMETRIC INSTABILITY WILL LIKELY BE
FOUND JUST ABOVE THE STRONGEST FRONTOGENETICAL FORCING AROUND 06Z
IN OUR SOUTHEAST COUNTIES.
WIND FORECAST SEEMS TO BE ON TRACK WITH GUSTS AROUND 25 MPH BEHIND
THE FRONT. THIS MAY CAUSE SOME DRIFTING PROBLEMS BUT WIDESPREAD LOW
VISIBILITIES IN BLOWING SNOW ARE NOT EXPECTED.
TEMPERATURES ON THURSDAY SHOULD REBOUND AS WARM AIR ADVECTION IS
ESTABLISHED. SNOWCOVER MAY KEEP HIGHS DOWN A FEW DEGREES BUT
SUNSHINE AND WARMER AIRMASS WILL HELP.
NIETFELD
&&
.AVIATION...
TAF SITES KOFK/KOMA/KLNK
06Z TAFS
AN UPPER LEVEL SYSTEM THAT WILL MOVE FROM THE SOUTHERN ROCKIES
TOWARD THE MS VALLEY THROUGH TONIGHT WILL CAUSE SURFACE LOW PRESSURE
TO DEEPEN OVER NORTHERN MO BY 09/06Z. AHEAD OF THIS SYSTEM IFR CIGS
AND AREAS OF SNOW OR RAIN/SNOW MIXED PREVAILED...ALTHOUGH THE
PRECIPITATION SHOULD DECREASE THROUGH THE MORNING HOURS. EVEN
THOUGH IFR CIGS WERE FORECAST AT TAF SITES...SOME LOW LEVEL DRYING
WAS FORECAST TO SPREAD NORTH THROUGH KOMA/KLNK AND SOME IMPROVEMENT
IN CIGS WERE NOTED AS OF 08/08Z FROM SERN NEBRASKA INTO FAR
SOUTHWEST IOWA. IF THIS TREND PERSISTS THROUGH THE MORNING
HOURS...CIGS COULD IMPROVE AT KOMA/KLNK. HOWEVER...WOULD EXPECT
CIGS TO LOWER AGAIN AS PRECIPITATION INCREASES THIS AFTERNOON.
PRECIPITATION TYPE SHOULD BE GENERALLY SNOW AT KOFK AND SHOULD BE
MOSTLY SNOW AT KOMA AND KLNK AFTER 09/00Z OR 09/01Z ALTHOUGH SOME
MIX POSSIBLE WITH ANY PCPN THAT OCCURS FROM LATE MORNING INTO MID
AFTERNOON. WITH LIFT INCREASING THIS AFTERNOON VSBYS IN SNOW COULD
DROP TO 1/2 MILE OR LESS...ALTHOUGH TIMING OF THE LOWEST VSBYS COULD
BE A BIT LATER THAN 08/06Z TAFS INDICATED.
CHERMOK
&&
.OAX WATCHES/WARNINGS/ADVISORIES...
NE...WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 9 AM CST
WEDNESDAY FOR NEZ011-012-015>018-032>034-044-045-051>053-
066>068-089>093.
WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO 6 AM CST
WEDNESDAY FOR NEZ030-031-042-043-050-065-078-088.
IA...WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 9 AM CST
WEDNESDAY FOR IAZ043-055-079-090.
WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON CST
WEDNESDAY FOR IAZ056-069-080-091.
&&
$$