FXUS63 KEAX 102046
AFDEAX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KANSAS CITY/PLEASANT HILL MO
245 PM CST THU DEC 10 2009
.DISCUSSION...
SHORT TERM (TONIGHT THROUGH SUNDAY)...
PRIMARY FORECAST CHALLENGE FOR THE NEXT 48 HOURS WILL BE
TEMPERATURES...WHICH WILL BE HIGHLY CONTINGENT UPON ALIGNMENT AND
EROSION OF THE DEEPER SNOW PACK ACROSS THE CWA. TIGHT GRADIENT CAN
ALREADY BE SEEN TODAY...WHERE HIGHS HAVE PUSHED NEAR FREEZING OVER
THE SRN CWA...WHILE LANGUISHING IN THE TEENS OVER THE DEEPER SNOW.
NUMERICAL GUIDANCE APPEARS SLIGHTLY LESS BIASED THAN YESTERDAY...AND
HAVE ONLY PUSHED TEMPS AT THE LOWEST PACKAGE OVER THE SNOW COVER FOR
THE NEXT 24 HOURS...AND KEPT AROUND A GUIDANCE AVERAGE ELSEWHERE.
BIGGEST PROBLEM IS ANTICIPATING THE EFFECTIVE EDGE OF THE SNOW
INFLUENCE...AS 1) SLY WINDS WILL BE ADVECTING OFF WARMER BARE
GROUND...EFFECTIVELY DAMPENING OR NEGATING THE SNOW INFLUENCE SOME
DISTANCE AWAY FROM THE TRUE EDGE OF THE SNOW COVER...AND 2) DEPTH OF
SNOW WILL BE QUITE VARIABLE ALONG THE SRN FRINGES OF THE PACK
WHEREAS TRACE OR PATCHY COVER WILL HAVE MINIMAL INFLUENCE VERSUS A
SOLID 1 INCH ACCUMULATION. UNDOUBTEDLY...ADJUSTMENTS WILL BE
NECESSARY TO PINPOINT THE STRONGEST TEMPERATURE GRADIENT BEYOND
FRIDAY AFTERNOON.
WEEKEND FORECAST BECOMES COMPLICATED BY A SURGE OF ELEVATED (H8-H9
LAYER) HIGHER THETA-E AIR FROM THE WRN GULF COAST (IN ASSOCIATION
WITH A FEW FAST MOVING PROGRESSIVE SHORTWAVES) OVER THE REMNANTS OF
THE SNOW COVER. FIRST CONCERN IS THE POSSIBILITY OF DENSE ADVECTION
FOG...WITH AT LEAST MID/UPPER 20 DEWPOINT SFC AIRMASS ADVECTING
NORTH. TOO FAR IN THE FUTURE AND ENOUGH UNCERTAINTY TO REALLY
INCLUDE A STRONGER MENTION YET...BUT CERTAINLY SOMETHING WORTH
WATCHING. BIGGER CONCERN IS FOR DRIZZLE/SPRINKLE CHANCES LATER
SATURDAY AFTERNOON THROUGH SATURDAY NIGHT (THEN AGAIN ON SUNDAY
NIGHT). CERTAINLY NOT A BIG QPF PRODUCER...BUT GIVEN THE NOCTURNAL
TIMING AND POSSIBLE DETRIMENTAL COOLING INFLUENCE OF THE REMAINING
SNOW COVER...SFC TEMPERATURES MAY STAND BELOW FREEZING CREATING A
VERY LIGHT ICING EVENT. SEE LITTLE IN MODEL FORECASTS IN TIME AND
SPACE SUGGESTING SIGNIFICANT SATURATION OR UVV WITHIN THE ICE
CRYSTAL GROWTH LAYER...SO FEEL SNOWFALL POTENTIAL IS LOW (MAYBE SOME
SNOW GRAINS WOULD ACCOMPANY DRIZZLE/SPRINKLE MIX). AGAIN...SFC TEMPS
AND SNOW COVER EXTENT AND INFLUENCE ARE OF UTMOST IMPORTANCE TO THIS
FORECAST. KEPT TEMPERATURES CONSERVATIVELY NEAR A GUIDANCE AVERAGE
OVER THE WEEKEND...THOUGH MANY MODEL PARAMETERS ARGUE FOR EVEN
WARMER READINGS...AND MAY NEED TO BOOST TEMPS A COUPLE CATEGORIES IF
FUTURE TRENDS CONTINUE IN THIS DIRECTION (EFFECTIVELY ALLEVIATING
MUCH OF THE MINOR ICING CONCERN).
21
DAYS 5-7 (MONDAY THROUGH WEDNESDAY)
FOR THE START OF THE WORK WEEK...UPPER AIR FLOW WILL BECOME
MORE MERIDIONAL AS AN UPPER LEVEL TROUGH STARTS TO DIG INTO THE
NORTHERN PLAINS. MODELS ARE PICKING UP ON A WEAK AREA OF SFC LOW
PRESSURE TRACKING FROM THE PACIFIC NORTHWEST INTO THE GREAT PLAINS
BY MONDAY. MODELS SUGGEST THIS LOW PRESSURE REGION WILL GET SUCKED
INTO THE INFLUENCE OF THE UPPER LEVEL SYSTEM ON MONDAY...WHICH COULD
LEAD TO PRECIP OVER THE REGION. HOWEVER...DISCREPANCIES STILL EXIST
BETWEEN THE GFS/ECMWF MODELS REGARDING THE POSITIONING OF THIS AREA
OF SFC LOW PRESSURE OVER THE NORTHERN PLAINS. ECMWF SHOWS THE LOW
PRESSURE BEING SITUATED JUST EAST OF THE CWA MONDAY...TYING INTO THE
MOISTURE SUPPLY OF BOTH THE UPPER LEVEL SYSTEM OVER CANADA AND THE
GULF OF MEXICO...AND THUS SUGGESTING THE CHANCE OF PRECIP OVER MUCH
OF THE CWA. GFS ON THE OTHER HAND...POSITIONS THE SFC LOW FURTHER
NORTH COMPARED TO THE ECMWF. IT ALSO SHOWS LESS MOISTURE INFLUENCE
FROM THE GULF OF MEXICO. AS A RESULT...GFS KEEPING A MAJORITY OF THE
CWA DRY FOR MUCH OF MONDAY. DUE TO THESE DIFFERENCES...CONTINUE TO
KEEP POPS AT SLIGHT CHANCE UNTIL A BETTER CONSENSUS IS SEEN AMONGST
THE MODELS.
THE UPPER LEVEL TROUGH WILL BEGIN TO SHIFT TOWARD THE NORTHEAST BY
TUESDAY...AND WILL BE REPLACED BY AN UPPER LEVEL RIDGE OVER MUCH OF
THE WESTERN HALF OF THE U.S. SFC HIGH PRESSURE OVER THE REGION WILL
KEEP TEMPS UNSEASONABLY COOL AND CONDITIONS DRY THROUGH MID WEEK.
ACH
&&
.AVIATION...
WIDESPREAD VFR CONDITIONS CHARACTERIZED BY CLEAR SKIES...AND LIGHT
SWLY SFC WINDS GRADUALLY BACKING TO SOUTHERLY BY FRIDAY MORNING.
21
&&
.EAX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
KS...NONE.
&&
$$
