FXUS63 KLBF 060807
AFDLBF

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTH PLATTE NE
307 AM CDT SAT SEP 6 2008

.SYNOPSIS...

BROAD MID/UPR LVL TROUGH IS CENTERED OVER THE CENTRAL GREAT
PLAINS...WITH WEAK HIGH PRESSURE AT THE SFC LOCATED BENEATH THE
TROUGH AXIS. A 115 KT JET STREAK DIVING INTO THE WESTERN SIDE OF THE
TROUGH WILL REACH THE CENTRAL PLAINS THIS AFTERNOON...FORCING A
WEAK FRONTAL BOUNDARY TO MOVE EAST ACROSS THE REGION. AHEAD OF THE
APPROACHING UPR LVL JET...ISENTROPIC UPGLIDE ABOVE THE COOL/STABLE
PBL IS FORCING SHOWER DEVELOPMENT OVER WESTERN NEB. THIS ACTIVITY
WILL CONTINUE TO SPREAD EAST ACROSS MUCH OF THE FCST AREA THROUGH
SATURDAY MORNING.

&&

.DISCUSSION...

A PLUME OF MOISTURE IS FCST TO SURGE NORTH ON THE 305 K ISENTROPIC
SFC OVER SOUTHWEST NEB THROUGH SATURDAY MORNING. THIS APPEARS TO
OCCUR IN RESPONSE TO AN UPR LVL DISTURBANCE DROPPING SOUTHEAST OUT
OF ALBERTA AND INTO THE CENTRAL PLAINS THROUGH THIS AFTERNOON. THE
STRONGEST ISENTROPIC UPGLIDE WILL BE FOCUSED OVER THE SOUTHERN HALF
OF THE CWA...FAVORING SHOWERS AND PERHAPS A FEW RUMBLES OF THUNDER
/THOUGH ITS HARD TO FIND CAPE IN SHORT TERM MODEL GUIDANCE
SUPPORTING THE LIGHTNING CURRENTLY OBSERVED OVER THE PANHANDLE/.
A CLEARING TREND OVER THE WESTERN HALF OF THE CWA DURING THE
AFTERNOON WILL FAVOR BOUNDARY LAYER DESTABILIZATION. THIS WILL
OCCUR BENEATH COLD MID LVL AIR SPREADING SOUTHEAST INTO THE
CWA...WHICH MAY YIELD SUFFICIENT INSTABILITY FOR SHOWERS AND
ISOLATED THUNDERSTORMS...FOCUSED ALONG AN AREA OF LOW LVL CONVG
LOCATED WEST OF HIGHWAY 83. OTHERWISE...MORNING SHOWERS OVER
WESTERN/SOUTHWESTERN NEB ARE FCST TO SPREAD EAST INTO THE
REMAINDER OF THE CWA THROUGH THIS AFTERNOON. MODEL GUIDANCE THEN
SHIFTS THE UPR WAVE EAST OF THE REGION AFTER 00Z SUNDAY...WITH
STRONG/DRY ISENTROPIC DOWNGLIDE DEVELOPING OVER THE CWA. THIS
SHOULD BRING A FAIRLY QUICK END TO PRECIP FOR MOST LOCATIONS. A
WEAK COLD FRONT IS ALSO FCST TO MOVE ACROSS THE CWA SATURDAY
NIGHT...WITH HIGH PRESSURE THEN SAGGING SOUTH INTO THE REGION BY
12Z SUNDAY.

AN INVERTED TROUGH IS FCST TO DEVELOP OVER THE NEB PANHANDLE/EASTERN
COLORADO SUNDAY AFTERNOON...INDICATIVE OF INCREASING ASCENT AHEAD OF
A STRONG UPPER LEVEL DISTURBANCE DROPPING SOUTH TOWARD THE REGION
FROM THE NORTHERN HIGH PLAINS. BOTH THE NAM AND GFS DEVELOP A STRONG
MERIDIONAL TEMPERATURE GRADIENT IN THE 850-500 MB LAYER DURING
SUNDAY NIGHT. STRONG CONVERGENCE ACTING ON THIS HORIZONTAL
TEMPERATURE GRADIENT YIELDS INCREDIBLE LOW TO MID LVL
FRONTOGENESIS OVER THE NORTHERN HALF OF THE CWA. IN
ADDITION...INTENSE NORTHERLY MOISTURE TRANSPORT AND ISENTROPIC
UPGLIDE ARE PRESENT. THE COMBINATION OF INTENSE MESOSCALE ASCENT
AND RAPIDLY MOISTENING ATMOSPHERE WILL SUPPORT A WIDESPREAD
PRECIPITATION EVENT OVER THE CWA SUNDAY NIGHT INTO PART OF MONDAY
MORNING.

THE DETERMINISTIC GFS THEN INDICATES THAT HIGH PRESSURE WILL BECOME
CENTERED OVER NEBRASKA ON MONDAY BEHIND THE PASSING UPR LVL WAVE.
THIS SOLUTION IS SUPPORTED FOR THE MOST PART BY SREF/ECMWF/GEFS AS
WELL. MET GUIDANCE FORECASTS HIGHS IN THE LOW TO MID 50S ON
MONDAY...COMPARED TO 60S BY THE MAV. AT THIS TIME WILL IGNORE THE
COOLER MET AND MATCH THE MET...WHICH STILL RESULTS IN 2-5 DEG
REDUCTION IN TEMPERATURES FOR MOST LOCATIONS. WITH CLEAR SKIES...AND
SFC HIGH PRESSURE RESIDING CLOSE TO THE CWA...MONDAY NIGHT COULD
POTENTIALLY BE COLD. HOWEVER...THE FCST AREA WILL RESIDE ON THE
WEST SIDE OF THE SFC HIGH...SO SOUTHERLY WINDS WILL LIKELY
INCREASE DURING THE NIGHT. IN ADDITION...SFC WETNESS WILL BE LARGE
DUE TO RECENT RAINFALL...WHICH MAY OFFSET STRONGER RADIATIONAL
COOLING. FOG WILL LIKELY BE FAVORED WITH THIS PATTERN...AND HAVE
ADDED IT TO THE FCST.

STRONG SOUTHERLY RETURN FLOW SETS UP ON TUESDAY AFTERNOON AND
NIGHT...RESULTING IN WARMER TEMPERATURES AND AN INCREASE IN LOW LVL
MOISTURE. ISENTROPIC UPGLIDE AND MOISTURE TRANSPORT BECOME MAXIMIZED
OVER THE NORTHEAST CORNER OF THE CWA TUESDAY NIGHT...AHEAD OF A
PLUME OF WARM H7 AIR. THIS MAY FAVOR SHOWERS...AND POSSIBLY
ELEVATED THUNDERSTORMS DUE TO THE STEEP MID LVL LAPSE RATES
ASSOCIATED WITH THIS AIRMASS. BY WEDNESDAY AFTERNOON...SFC CONVG
INCREASES WITH A DEVELOPING COLD FRONT...AND IS COLLOCATED WITH A
POTENTIALLY UNSTABLE ATMOSPHERE IN WESTERN NEB. IT IS TEMPTING TO
ADD A SLIGHT CHANCE OF SHOWERS/THUNDERSTORMS...BUT WILL HOLD OFF
AND LET THE DAY SHIFT REEVALUATE ADDITIONAL MODEL GUIDANCE.
PREVIOUS FCST HAS POPS GOING WEDNESDAY NIGHT AS THE COLD FRONT
SWEEPS EAST ACROSS THE CWA...AND THIS STILL APPEARS TO BE ON
TRACK.

&&

.AVIATION...

RAINFALL ASSOCIATED WITH A LARGE UPPER LEVEL DISTURBANCE...WILL
PERSIST AT THE KLBF AND KVTN TERMINALS THROUGH THE EARLY AFTERNOON
HOURS. MVFR CEILINGS ARE LIKELY IN THE VICINITY OF RAIN SHOWERS
THROUGH 21Z THIS AFTERNOON. CEILINGS WILL IMPROVE TO VFR HEIGHTS
AFTER 21Z AS THE UPPER LEVEL DISTURBANCE LIFTS EAST. ISOLATED
THUNDERSTORMS WILL BE POSSIBLE IN THE VICINITY OF THE HEAVIEST
SHOWERS...BUT WITH AREAL COVERAGE OF THUNDERSTORMS EXPECTED TO BE
MINIMAL...WILL FORGO MENTION OF TSRAS IN THE 12Z TAF ISSUANCE.

&&

.LBF WATCHES/WARNINGS/ADVISORIES...NONE.
&&

$$

GARNER/CLB


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