Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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000
FXUS64 KFWD 171809
AFDFWD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
1209 PM CST WED DEC 17 2014

.AVIATION...
A VERY TRICKY 18 UTC TAF CYCLE LATE THIS MORNING FOR THE METROPLEX
AND WACO TAF SITES. THE MAIN CONCERNS/IMPACTS WILL BE THE
THREAT FOR PRECIPITATION/THUNDER AS WELL AS THE POTENTIAL FOR IFR
CIGS/VISBY THIS EVENING THROUGH THE OVERNIGHT HOURS.

FOR THE METROPLEX TAF SITES...REGIONAL RADARS THIS MORNING
REVEALED SCATTERED SHOWERS ALONG AND NORTH OF THE INTERSTATE 20 CORRIDOR.
THE 12 UTC FWD RAOB AND RECENT AMDAR DATA REVEALED A VERY DRY
LAYER OF AIR PERSISTED AROUND THE 3-5 KFT AGL. TRENDS IN VAD WINDS
FROM THE KFWS WSR-88D INDICATED THAT VIRGA BELOW THE SUB-CLOUD
REGION WAS AIDING IN GRADUAL SATURATION OF THE AFORMENTIONED LAYER.
THIS TREND IS EXPECTED TO CONTINUE THROUGH THE AFTERNOON HOURS.
GUIDANCE FROM THE RAP/NAM HAS DONE A DECENT JOB IN HANDLING THIS
TOP-DOWN MOISTENING PROCESSES AND THUS THIS FORECAST LEANS
SOMEWHAT HEAVIER ON THE OUTPUT FROM THESE MODELS. HI-RES MODEL
GUIDANCE CONTINUES TO INDICATE THAT MORE WIDESPREAD PRECIPITATION
WILL BE POSSIBLE THIS AFTERNOON. THE INSTABILITY AXIS AS NOTED BY THE
RAP MESOANALYSIS SUGGESTS SOME POTENTIAL FOR THUNDER AND AS A
RESULT...HAVE INCLUDED VCTS AT THE METROPLEX TAF SITES FOR SEVERAL
HOURS THIS AFTERNOON. AS GREATER FORCING ARRIVES...MORE WIDESPREAD
PRECIPITATION SHOULD DEVELOP...AND A CONTINUED MOISTENING OF THE
LOW LEVELS SHOULD ENSUE. THIS MOISTENING WILL RESULT IN CIGS AND
VISBY DETERIORATING QUICKLY LATE THIS AFTERNOON AND INTO THE
EVENING HOURS. IFR CIGS AND VISBY ARE EXPECTED ACROSS THE
METROPLEX TAF SITES AROUND 0200 UTC. THE ONSET OF THE
DETERIORATING CIGS/VISBY WILL BE MOST LIKELY BE TIED TO AREAS OF
LIGHT RAIN/DRIZZLE DEVELOPING THIS EVENING AND THROUGH MIDNIGHT.
THERE IS THE POTENTIAL FOR LIFR CIGS/VISBY...BUT THERE REMAINS
SOME UNCERTAINTY IN THIS AND AS A RESULT...WILL DEFER TO LATER
SHIFTS. VISBY SHOULD IMPROVE AROUND MIDNIGHT WITH CONTINUED IFR CIGS
EXPECTED TO PERSIST THROUGH THURSDAY MORNING.

FOR THE WACO TAF SITE...MVFR CIGS JUST TO THE SOUTH OF THE TAF
SITE SHOULD SLOWLY LIFT NORTHWARD AS LOW LEVEL WARM/MOIST AIR
ADVECTION OVERSPREADS THE REGION. UNLIKE AREAS TO THE
NORTH...PRECIPITATION HAS BEEN SLIGHTLY SLOWER TO BREAKOUT
HERE THIS AM AND THUS A SLIGHTLY DEEPER LAYER OF DRIER AIR ALOFT
REMAINS. PRECIPITATION SHOULD GRADUALLY INCREASE AS THE LARGER
SCALE FORCING GRAZES CENTRAL TEXAS. AREAS OF LIGHT PRECIPITATION
SHOULD DEVELOP TOWARDS THE MID TO LATE AFTERNOON HOUR. AREAS OF
LIGHT RAIN AND DRIZZLE WILL SUPPORT IFR CIGS THROUGH TONIGHT AND
INTO THE EARLY PORTION OF THURSDAY. THEREAFTER...CIGS SHOULD
IMPROVE TO MVFR.

15-BAIN/92-TR


&&

.UPDATE...
SHOWERS AND A FEW THUNDERSTORMS STARTING TO EXPAND IN COVERAGE
ACROSS NORTH CENTRAL TEXAS THIS MORNING...AHEAD OF A VIGOROUS
UPPER LEVEL SYSTEM LIFTING ACROSS SOUTHERN NEW MEXICO. ONGOING
FORECAST OF CATEGORICAL POPS STILL LOOKS GOOD FOR THIS
AFTERNOON...AS FORCING FOR ASCENT PROMOTES EASTWARD EXPANSION OF
RAIN SHIELD. RAIN/CLOUDS SHOULD INHIBIT TEMPERATURES ACROSS THE
NORTHERNMOST COUNTIES...SO LOWERED READINGS IN THESE AREAS A FEW
DEGREES IN THE LATEST UPDATE. REMAINDER OF FORECAST LOOKS GOOD AT
THIS POINT.

BRADSHAW

&&

.PREV DISCUSSION... /ISSUED 356 AM CST WED DEC 17 2014/

EARLY MORNING WATER VAPOR SATELLITE IMAGERY SHOWED A SHORTWAVE
TROUGH OVER NORTHERN BAJA CALIFORNIA AND FAR NORTHWESTERN
MEXICO...MOVING EAST TOWARDS TEXAS. TO THE SOUTHEAST OF THIS
TROUGH...WATER VAPOR SATELLITE IMAGERY SHOWED A GOOD PLUME OF
TROPICAL PACIFIC MOISTURE STREAMING NORTHEAST OVER NORTHERN MEXICO
AND MOST OF TEXAS. REGIONAL RADARS WERE ALREADY PICKING UP SOME
ELEVATED PRECIPITATION RETURNS OVER WEST CENTRAL TEXAS AT 0830Z
(230 AM CST). SURFACE OBSERVATIONS INDICATED THAT RAINFALL WAS NOT
YET REACHING THE GROUND UNDER THESE RETURNS. THE 00Z FWD RAOB WAS
VERY DRY...WITH AN OBSERVED PRECIPITABLE WATER OF A THIRD OF AN
INCH...SO IT WILL TAKE SOME TIME TO MOISTEN UP THE ATMOSPHERE
BEFORE ANY SUBSTANTIAL RAINFALL IS POSSIBLE ACROSS THE REGION
THIS MORNING.

TODAY...THE FACT THAT THE 00Z FWD RAOB WAS SO DRY...AND YET MODELS
CONSISTENTLY BRING A 70 TO 100 PERCENT CHANCE OF MEASURABLE
RAINFALL ACROSS THE CWA...IS A MAJOR SOURCE OF CONCERN WITH
REGARDS TO TODAY`S RAINFALL FORECAST. TO ASSESS THE REALISM OF
MODELS ADVERTISING THAT WE WILL MOISTEN UP FROM THE MID-LEVELS
DOWN THROUGH THE LOWER TROPOSPHERE QUICKLY TODAY WE HAVE TO KNOW
WHERE THIS MOIST AIR IS GOING TO COME FROM. IF WE KNOW THE SOURCE
REGION FOR THE MOIST MID-LEVEL AIR FORECAST TO BE IN PLACE
TODAY...WE CAN LOOK AT UPSTREAM RAOBS TO ASSESS HOW LIKELY
FORECAST MODELS ARE TO VERIFY THIS RAPID INCREASE IN MOISTURE
ACROSS NORTH AND CENTRAL TEXAS.

TO ASSESS THIS...WENT AHEAD AND RAN THE HYSPLIT MODEL OVER NORTH
TEXAS CALCULATING BACKWARDS TRAJECTORIES FROM THE MIDDLE
TROPOSPHERE OVER NORTH TEXAS AT 21Z TODAY (3 PM THIS AFTERNOON)
BACK TO 00Z YESTERDAY EVENING. WENT AHEAD AND DROPPED BACKWARDS
TRAJECTORIES INTO THE HYSPLIT MODEL FROM APPROXIMATELY THE 700 MB
LEVEL DOWN TO THE 850 MB LEVEL USING THE 17/00Z NAM AS THE BASE
MODEL FOR THE HYSPLIT RUNS. THIS ANALYSIS INDICATED THAT OUR MID-
TROPOSPHERE AIR THIS AFTERNOON WILL MOVE OVER THE REGION FROM THE
TEXAS/MEXICO BORDER...FROM DEL RIO SOUTHEAST TO LAREDO. THIS MEANS
THAT WE CAN LOOK AT THE 00Z DEL RIO RAOB TO GET SOME IDEA OF WHAT
THE MID-LEVEL AIR MASS OVER NORTH TEXAS MAY BE COMPOSED OF THIS
AFTERNOON. THE 00Z DEL RIO RAOB WAS ESSENTIALLY SATURATED FROM THE
850 MB LEVEL TO THE 700 MB LEVEL. MAYBE MORE IMPORTANTLY...THE
MOISTURE CONTENT IN THIS LAYER WAS AROUND 5 TIMES THE MOISTURE
CONTENT OBSERVED ON THE 00Z FWD RAOB. IN GENERAL...THIS IS GOOD
NEWS FOR THOSE THAT WANT RAIN AS IT LOOKS LIKE THE CONSENSUS MODEL
FORECAST FOR A RAPID MOISTENING OF THE MIDDLE TROPOSPHERE OVER THE
CWA TODAY IS FIRMLY BACKED UP BY OBSERVED DATA UPSTREAM.

WITH HIGH CONFIDENCE IN SIGNIFICANT MOISTURE RETURN TO THE REGION
TODAY...WENT AHEAD AND BUMPED UP POPS INTO THE 70 TO 80 PERCENT
RANGE ACROSS THE CWA FOR THIS AFTERNOON AND EVENING. KEPT POPS IN
THE 20 TO 60 PERCENT RANGE FROM EAST TO WEST THIS MORNING ACROSS
THE CWA AS IT REALLY WILL TAKE SOME TIME FOR THIS MOISTURE RETURN
TO TAKE PLACE. FORECAST SOUNDINGS ARE SATURATED IN THE MIDDLE
ATMOSPHERE ALL ALONG THE INTERSTATE 35 CORRIDOR BY 21Z. ONCE THIS
OCCURS...ANY LIFT SHOULD RESULT IN MEASURABLE RAINFALL ACROSS THE
AREA. CONFIDENCE IN LIFT SPREADING OVER THE REGION TODAY IS HIGH
AS WELL AS THE SHORTWAVE TROUGH APPEARS TO BE WELL INITIALIZED IN
TERMS OF POSITION AND INTENSITY COMPARING MODEL DATA TO SATELLITE
IMAGERY. LARGE SCALE FORCING FOR ASCENT IS MAXIMIZED ALONG THE
INTERSTATE 35 CORRIDOR THIS AFTERNOON...AND THEN MOVES EAST OF THE
INTERSTATE 35 CORRIDOR AFTER 00Z (6 PM CST) THIS EVENING. FORECAST
SOUNDINGS CONTINUE TO ADVERTISE 1-200 J/KG OF ELEVATED CAPE WHEN
THE STRONGEST FORCING FOR ASCENT MOVES FROM WEST TO EAST ACROSS
THE CWA TODAY...SO LEFT ISOLATED THUNDER IN THE FORECAST THIS
AFTERNOON AND EVENING. RAINFALL AMOUNTS ARE GENERALLY EXPECTED TO
BE LIGHT...AND AVERAGE ABOUT ONE QUARTER OF AN INCH ACROSS NORTH
AND CENTRAL TEXAS.

TONIGHT AND THURSDAY...RAINFALL IS EXPECTED TO COME TO AN END FROM
WEST TO EAST ACROSS THE CWA THIS EVENING INTO THE OVERNIGHT HOURS.
WEAK FORCING FOR SUBSIDENCE BEHIND TODAY`S SHORTWAVE TROUGH SHOULD
KEEP DRY WEATHER IN PLACE FOR MUCH OF THE AREA ON
THURSDAY...HOWEVER ANOTHER UPPER LEVEL TROUGH WILL BE MOVING THIS
WAY DURING THE DAY ON THURSDAY. THIS TROUGH IS EXPECTED TO TAKE A
MORE SOUTHERLY TRACK THAN TODAY`S TROUGH...AND AS A RESULT...ITS
PRIMARY INFLUENCE ON THE REGION ON THURSDAY WILL BE TO INDUCE
CYCLOGENESIS OVER THE HILL COUNTRY THROUGHOUT THE DAY. THIS LOW-
LEVEL CYCLOGENESIS WILL RESULT IN SOME PERSISTENT LIFT IN THE
LOWER ATMOSPHERE OVER THE FAR SOUTHEASTERN CWA ON THURSDAY. WENT
AHEAD AND LEFT A SMALL CHANCE OF RAIN IN THE FORECAST FOR THESE
LOCATIONS AS A RESULT. OTHERWISE...EXPECT MUCH OF WEDNESDAY NIGHT
AND THURSDAY TO REMAIN DRY ACROSS MOST OF NORTH AND CENTRAL TEXAS.

THURSDAY NIGHT AND FRIDAY...THIS SECOND UPPER LEVEL TROUGH IS
EXPECTED TO BE STRONGER THAN TODAY`S UPPER TROUGH. THIS SHOULD
RESULT IN SIGNIFICANTLY HIGHER RAINFALL TOTALS WHEREVER THE BEST
FORCING FOR ASCENT SETS UP. UNFORTUNATELY...BECAUSE THIS SECOND
SYSTEM IS EXPECTED TO MAINTAIN A MORE SOUTHERLY TRACK THAN TODAY`S
SYSTEM...THIS AXIS OF HEAVIER RAINFALL APPEARS LIKELY TO REMAIN
SOUTH OF MOST OF THE CWA. THERE APPEARS TO BE TWO FEATURES TO KEEP
A CLOSE EYE ON DURING THIS EVENT...THE 850 AND 700 MB FRONTS.

MODELS SEEM TO CONSISTENTLY INDICATE THAT THE AXIS OF HEAVIEST
RAINFALL WILL REMAIN CLOSELY TIED TO THE DYNAMICS AND POSITION OF
THE 850 MB FRONT. NEARLY ALL GUIDANCE PAINTS AN AXIS OF 1-2 INCHES
OF RAIN ALONG AND JUST NORTH OF THIS FEATURE. THE CONSENSUS OF
GUIDANCE INDICATES THAT THIS FRONT WILL BE LOCATED NEAR A LINE
FROM PALESTINE TO KILLEEN FOR NEARLY 12 HOURS FROM LATE THURSDAY
NIGHT THROUGH FRIDAY MORNING. AS A RESULT...HAVE REASONABLE
CONFIDENCE THAT ANY RAINFALL TOTALS EXCEEDING ONE INCH DURING THIS
EVENT WILL REMAIN CONFINED TO LOCATIONS ALONG AND SOUTHEAST OF A
LINE FROM LAMPASAS TO WACO TO ATHENS...WHICH IS JUST NORTH OF THE
FORECAST POSITION OF THE 850 MB FRONT. TO THE NORTH OF THIS LINE
...LOW-LEVEL DYNAMICS CONSISTENTLY FAVOR COLD AIR ADVECTION AND
SUBSIDENCE...AND EXPECT RAINFALL TOTALS TO DROP OFF DRAMATICALLY
NORTHWEST OF THIS LINE AS A RESULT.

THE 700 MB FRONT APPEARS TO REPRESENT THE NORTHERN EDGE OF WHERE
MEASURABLE RAINFALL IS MOST LIKELY TO FALL. FOR NATIONAL WEATHER
SERVICE FORECAST PURPOSES...A 100 PERCENT CHANCE OF RAIN IS
VERIFIED REGARDLESS OF WHETHER ONE HUNDREDTH OF AN INCH OR 4
INCHES OF RAIN FALLS...SO THIS LINE IS IMPORTANT FOR FORECAST
PURPOSES. HOWEVER...RAINFALL THAT OCCURS IN BETWEEN THE 700 MB AND
850 MB FRONTS IS LIKELY TO AVERAGE BETWEEN ONE TENTH TO ONE
QUARTER OF AN INCH...SIGNIFICANTLY LESS THAN WHAT IS EXPECTED
SOUTHEAST OF THE 850 MB FRONT. WILL NOT GET INTO ALL OF THE
DYNAMICAL REASONS THAT THIS IS THE CASE...HOWEVER THE PRIMARY
REASON FOR THE DRAMATIC DIP IN RAINFALL AMOUNTS IS BECAUSE THE
QUANTITY OF MOISTURE DECREASES RAPIDLY WITH ALTITUDE IN OUR
ATMOSPHERE. EVEN THOUGH THE MID-LEVEL LIFT MAY BE NEARLY AS
STRONG AS THE LOW-LEVEL LIFT...THERE IS SIMPLY LESS WATER VAPOR TO
LIFT...AND THIS RESULTS IN A DRAMATIC DECLINE IN RAINFALL AMOUNTS.

AT ANY RATE...EVEN THROUGH RAINFALL AMOUNTS ARE LIKELY TO BE
LIGHT...THE 700 MB FRONT LOOKS LIKELY TO BE LOCATED NEAR THE
INTERSTATE 30 CORRIDOR DURING THIS EVENT...SO HAVE RELATIVELY HIGH
POPS (AROUND 60 PERCENT) IN THE FORECAST THIS FAR NORTH DESPITE
THE FACT THAT RAINFALL WILL BE MUCH LIGHTER IN NATURE THAN FOR
AREAS TO THE SOUTHEAST.

RAINFALL IS EXPECTED TO COME TO AN END FROM WEST TO EAST ACROSS
THE ENTIRE CWA FRIDAY AFTERNOON AND EVENING. SOME MODELS ARE
HANGING ON TO SOME WRAP AROUND MOISTURE ALONG AND NORTH OF
INTERSTATE 30 FRIDAY EVENING BEHIND THE UPPER TROUGH...SO LEFT
A LOW CHANCE OF RAIN IN THE FORECAST TO ACCOUNT FOR THIS. MODELS
GENERALLY INDICATE THAT THIS STORM SYSTEM WILL NOT GET WRAPPED UP
QUICKLY ENOUGH TO RESULT IN A SIGNIFICANT OCCLUSION OVER THE
SOUTH PLAINS IN ITS WAKE...BUT AN OCCLUSION IS A CLASSIC FEATURE
OF STRONG MID-LATITUDE CYCLONES...SO WILL KEEP AN EYE ON WRAP
AROUND MOISTURE IN LATER FORECASTS. IF MORE SIGNIFICANT MOISTURE
DOES GET WRAPPED UP BEHIND THIS SYSTEM...IT SHOULD ONLY RESULT IN
LIGHT RAIN AS ALL GUIDANCE IS WARM ENOUGH TO PREVENT LIGHT SNOW
FROM REACHING THE SURFACE AT THIS TIME.

THIS WEEKEND...MODELS CONTINUE TO INDICATE THAT FRIDAY`S UPPER
TROUGH WILL BE STRONG ENOUGH TO ADVECT ALL SIGNIFICANT MOISTURE
OFF TO THE EAST OR SOUTHEAST BY SATURDAY...SO MAINTAINED A DRY
FORECAST AT THIS TIME. MODELS CONTINUE TO INDICATE A RELATIVELY
STRONG UPPER TROUGH MOVING OVER THE SOUTH PLAINS IN NORTHWEST FLOW
ALOFT ON SUNDAY...BUT WITH LITTLE MOISTURE TO WORK WITH THIS MAY
ONLY RESULT IN A BRIEF INCREASE IN CLOUD COVER.

EARLY NEXT WEEK...MODELS ARE COMING INTO BETTER AGREEMENT WITH
REGARDS TO THE PASSAGE OF A STRONG COLD FRONT SOUTH ACROSS THE
SOUTHERN PLAINS MONDAY NIGHT INTO TUESDAY MORNING. WHILE THE FRONT
IS STRONG IN MODEL GUIDANCE...THE BULK OF THE ARCTIC AIR IN ITS
WAKE REMAINS WELL EAST OF THE REGION...SO THE COOL DOWN ASSOCIATED
WITH THE FRONT IS EXPECTED TO BE BRIEF AT THIS TIME. MODELS DO NOT
AGREE ON AVAILABLE MOISTURE FOR PRECIPITATION ASSOCIATED WITH THIS
FRONT. BECAUSE IT LOOKS LIKE NORTHWEST FLOW ALOFT WILL PERSIST
BEFORE THIS FRONT ARRIVES...SIDED WITH DRIER SOLUTIONS AS THIS
PATTERN REPRESENTS ONE IN WHICH IT IS DIFFICULT TO GET SIGNIFICANT
MOISTURE RETURN TO NORTH TEXAS.

CAVANAUGH



&&


.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX  46  43  61  47  51 /  80  60  10  40  50
WACO, TX              49  45  62  49  54 /  80  60  10  70  70
PARIS, TX             42  40  51  43  48 /  60  70  20  30  50
DENTON, TX            44  41  59  44  49 /  80  60  10  30  40
MCKINNEY, TX          43  41  57  44  50 /  80  60  10  40  40
DALLAS, TX            47  44  60  47  51 /  80  60  10  40  50
TERRELL, TX           48  43  58  47  50 /  60  70  10  50  50
CORSICANA, TX         50  46  63  49  52 /  50  60  20  70  70
TEMPLE, TX            49  46  64  50  55 /  60  50  20  80  80
MINERAL WELLS, TX     45  41  60  44  51 /  80  40  10  40  40

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

 /






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