000
FXUS63 KLBF 272043
AFDLBF
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
245 PM CST MON DEC 27 2010
.SYNOPSIS...
AT 19Z...HAND ANALYSIS OF THE SURFACE DATA SHOWED A WARM FRONT
FROM A LOW IN WESTERN MANITOBA...ACROSS THE DAKOTAS AND NEBRASKA...
THROUGH KANSAS. IN THE UPPER LEVELS...WATER VAPOR IMAGERY SHOWED A
SMALL VORTEX ALONG THE MONTANA-SASKATCHEWAN BORDER. A DEVELOPING
WAVE WAS INDICATED NEAR THE FOUR-CORNERS REGION. WIND PROFILES AT
500MB SHOW THE UPPER TROUGH HAD PASSED VALENTINE...THEDFORD...AND
LEXINGTON.
&&
.SHORT/MEDIUM RANGE...TONIGHT THROUGH WEDNESDAY NIGHT.
THE UPPER TROUGH WILL MOVE ON THROUGH BY EARLY EVENING...
BRINGING AND END TO THE PERSISTENT STRATUS THAT HAS AFFECTED CENTRAL
AND WESTERN NEBRASKA FOR THE LAST FEW DAYS. AS THE UPPER RIDGE MAKES
ITS WAY OVER THE PLAINS...THE WEATHER TONIGHT AND THROUGH TUESDAY
AND INTO THE DAY WEDNESDAY WILL BE DRY WITH COLD NIGHTS AND MILD
DAYTIME TEMPERATURES.
BY LATE WEDNESDAY...A BOUNDARY LAYER TRAJECTORY FROM THE TEXAS
GULF COAST BEGINS PUMPING SOME MOISTURE ONTO THE CENTRAL PLAINS.
WIND AND MOISTURE TRANSPORT INDICATE SOME ISENTROPIC UPGLIDE IN THE
285-290K LAYER ACROSS CENTRAL AND SOUTH CENTRAL NEBRASKA BY
WEDNESDAY EVENING. THIS WILL TRANSPORT MOISTURE INTO THE LAYER...
BUT SATURATION IS NOT EXPECTED UNTIL THE COLD FRONT BEGINS COMING
THROUGH WEDNESDAY EVENING. AREAS MOST LIKELY TO RECEIVE
PRECIPITATION WEDNESDAY NIGHT ARE ALONG AND EAST OF A
VALENTINE-THEDFORD-NORTH PLATTE LINE. ACCORDING TO FORECAST
SOUNDINGS...TEMPERATURES BENEATH THE SATURATED LAYER ARE EXPECTED TO
BE 0-3C...SO PRECIPITATION THAT DOES OCCUR WEDNESDAY EVENING AND
OVERNIGHT IS LIKELY TO BE RAIN WITH THE POTENTIAL FOR FREEZING ALONG
AND NORTH OF A BROKEN BOW-MULLEN LINE.
.EXTENDED...THURSDAY THROUGH MONDAY.
A DEEP UPPER-LVL TROUGH IS STILL SET TO IMPACT THE REGION LATE
THIS WEEK WITH A DISTINCT RISK OF A WINTER STORM. THERE IS MINIMAL
SPREAD AMONG MODEL SOLUTIONS IN THIS BASIC IDEA...BUT DETAILS ARE
STILL A WHOLE DIFFERENT STORY. THE MAIN PROBLEMS AT THIS JUNCTURE
ARE 1/ THE FACT THAT THE ENERGY MAY COME OUT AS MORE THAN ONE S/W
TROUGH AND 2/ SOME DISAGREEMENT OVER HOW FAR SOUTH THE MAIN 500-HPA
TROUGH WILL DIG BEFORE EJECTING NE. BOTH OF THESE HAVE CAN HAVE A
DRAMATIC EFFECT ON BOTH TIMING AND TRACK OF THE DYNAMICS IMPORTANT
TO THE SNOW FCST. WHERE 1/ IS CONCERNED...THE ECMWF FROM 00Z STILL
HOLDS THE ENERGY BACK AS ONE MAIN PIECE...ALLOWING IT TO GENERATE A
MORE POTENT STORM THAT TRACKS NW OF THE AREA WITH THE MAIN IMPACT
ACROSS THE DAKOTAS. THAT SOLUTION IS ON THE NW SIDE OF THE GUIDANCE
ENVELOPE...WHICH IS OFTEN NOT A BAD PATH TO FOLLOW WHEN A POTENT
TROUGH IS ACCOMPANIED BY INCREASING DOWNSTREAM RIDING. ON THE OTHER
HAND THE 00Z...06Z...AND 12Z GFS NOT ONLY HAVE A MORE SIGNIFICANT
LEAD S/W OVER THE NRN PLAINS THU...BUT ALSO DIG THE 500-HPA TROUGH
MORE BEFORE EJECTING IT INTO THE PLAINS. THAT WOULD ALLOW A GREATER
IMPACT OVER MORE OF WESTERN AND NC NEB AS EVEN IF A LAST-MINUTE NW
TRACK WAS FAVORED...THE WAVE WOULD BE MAKING ITS TURN FAR ENOUGH TO
THE EAST THAT THE 700-HPA LOW WOULD PASS EAST OF KLBF-KONL. THE 00Z
GEFS IS SOMETHING OF A MIDDLE GROUND AND REFLECTS CONTINUITY OF THE
GOING FCST THE BEST...SO IT WAS MAINLY NUDGED TO THAT SOLUTION.
ON THU...CONFIDENCE IS HIGH THAT THE ARCTIC FRONT WILL SURGE TO
THE SOUTH WITH STRONG LOW/MID LVL CAA. THE 12Z NAM AND GFS HAVE THE
850-HPA TEMPS AS LOW AS -15 C BY 00Z FRI. WE THUS USED THESE MODELS
TO SHOW NON-DIURNAL /FALLING/ TEMPS DURING THE DAY. THERE/S A RISK
OF -RA OR -FZRA OVER AT LEAST THE EAST HALF OF THE AREA WITH FROPA
AS MODELS SUGGEST THE LOW-LVL COOLING WILL OUTPACE THAT ALOFT. WE
MAY END UP WITH ONLY MODEST VERTICAL MOTION BTWN WAVES AFTER 18Z...
BUT WE DID RETAIN CHANCE POPS FOR -SN IN THAT PERIOD.
THU NIGHT-FRI NIGHT...THE DETAILS OF THE MAIN +SN EVENT REMAIN
TOO UNCERTAIN TO FOCUS ON MANY SPECIFICS...BUT OUR CONFIDENCE IN
ACCUMULATING SNOW OVER PART OF WESTERN AND NC NEB DOES CONTINUE TO
INCREASE. IT/S WHICH PART THAT IS QUESTIONABLE. THE LAST FEW GEFS
CYCLES HAVE SLOWLY CONVERGED ON A 700-HPA LOW TRACK FROM KMCK-KFAR
THAT WOULD FAVOR NW NEB FOR +SN. WHAT IS A BIT CONCERNING IS THE
FACT THAT THE 00Z GEFS MEMBERS HAVE SHOWN A BIT MORE SPREAD IN THEIR
QPF SOLUTIONS AND SFC LOW POSITIONS. TWO OF THOSE LOWS STILL REMAIN
AS FAR WEST AS THE EASTERN DAKOTAS AS THE ECMWF CONTINUES TO SHOW.
GIVEN THIS...WE HAVE RETAINED HIGH CHANCE POPS CENTERED OVER NW NEB
IN THESE PERIODS...BUT HELD OFF FROM GOING WITH LIKELY WORDING YET.
WINDS STILL LOOK LIKE A GIVEN WITH THE CAA AND 850-HPA WINDS OF
30-45KTS BEHIND THE MAIN WAVE /MOST LIKELY DURING THE DAY FRI/.
THE 00Z ECMWF...GFS...AND GEFS ALL SHOW DECREASING AMPLITUDE TO
THE 500-HPA FLOW SAT-MON WITH SEASONABLY STRONG NW FLOW PREVAILING.
SLOW BUT STEADY WARMING RESULTS WITH 850-HPA TEMPS GOING FROM -10 C
AT 00Z SUN TO -4 C BY 00Z MON. WE HAVE TAKEN A CONSERVATIVE APPROACH
TO THE AMOUNT OF SFC WARMING SHOWN IN THE FCST THOUGH SINCE 1/ PART
OF THE AREA MAY HAVE A DECENT SNOW FIELD BY THEN AND 2/ WEAK SFC
FLOW IS MODELED WITH A SFC HIGH ANCHORED TO OUR WEST. THE TEMP FCST
WAS THUS TRENDED TO THE COLDER SIDE OF THE 00Z GEFS MEMBERS. RECENT
ECMWF AND GFS RUNS DO SHOW A WEAK S/W ON MON...BUT MOISTURE WILL BE
LACKING SO IT DOES NOT LOOK LIKE A PRECIP PRODUCER. TIMING/AMPLITUDE
UNCERTAINTIES IN THOSE SOLUTIONS LOWER CONFIDENCE IN ITS FCST ANYWAY.
&&
.AVIATION...
HIGH PRESSURE WILL CONTINUE TO DOMINATE THE AREA WITH VFR
CONDITIONS EXPECTED THE NEXT 24 HOURS. NORTHWEST WINDS WILL BE
BREEZY...10 TO 20 KTS...INTO THE EVENING...HOWEVER ONCE THE SUN SETS
AND THE BOUNDARY LAYER DECOUPLES EXPECT LIGHT AND VARIABLE.
NORTHWEST FLOW ALOFT WILL PROMOTE SOME MID/HIGH CLOUDS SLIDING
ACROSS THE REGION...MAINLY AOA SCT/BKN100.
&&
.LBF WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT/MEDIUM RANGE...SPRINGER
EXTENDED...SCHULTZ
AVIATION...MASEK
