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

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000
FXUS64 KFWD 211143 AAB
AFDFWD

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE FORT WORTH TX
643 AM CDT THU AUG 21 2014

.AVIATION...
/12Z TAFS/

MAINLY VFR WITH BREEZY SOUTH WINDS THROUGH FRIDAY MORNING. BRIEF
ROUNDS OF MVFR CIGS WILL POSE CHALLENGES EACH MORNING BETWEEN
SUNRISE AND MID MORNING...MAINLY AT WACO. STRONG 30-35 KT LLJ FOR
THIS TIME OF THE YEAR IS EXPECTED TO VEER MORE FRIDAY MORNING IN
RESPONSE TO NORTHERN ROCKIES UPPER TROUGH HAVING MOVED FURTHER
EAST. FOR THIS MORNING ONLY...MVFR CIGS 2.5 KFT WILL BE PATCHY
ENOUGH THAT A TEMPO GROUP AT EASTERN DFW AIRPORTS WILL LIKELY
SUFFICE. MIXING SHOULD OCCUR QUICKLY EACH DAY AND DO NOT EXPECT
MVFR CIGS TO LAST MORE THAN 2-3 HRS.

THE AFOREMENTIONED LLJ SHOULD MIX EASILY DOWN BY LATE MORNING WITH
SOUTH WINDS AROUND 15 KTS WITH OCCASIONAL GUSTS 20-25 KTS. PRESSURE
FALLS TO THE WEST WILL ALSO KEEP WINDS UP...EVEN INTO TONIGHT WITH
A TIGHT SURFACE PRESSURE GRADIENT PRESENT.

05/

&&


.PREV DISCUSSION... /ISSUED 542 AM CDT THU AUG 21 2014/
REMARKABLY STOUT LLJ FOR THIS TIME OF YEAR (35-40KTS) HAS
RESULTED IN WIDESPREAD (ALBEIT THIN) STRATUS ALONG THE BALCONES
ESCARPMENT. A FIBROUS DECK HAS INVADED SOUTHERN PORTIONS OF THE
CWA EARLY THIS MORNING...AND A FEW FRAGMENTS WILL LIKELY REACH THE
I-20 CORRIDOR AFTER DAYBREAK. AS THESE STRONG WINDS MIX WITH THE
SURFACE LAYER LATER THIS MORNING...WIND GUSTS WILL PICK UP AGAIN.
SUNNY SKIES WILL PUSH TEMPERATURES BACK UP INTO THE UPPER 90S TO
NEAR 100 DEGREES...BUT THE WIND WILL HELP TAKE AN EDGE OFF THE
HEAT. WITH THE BEST MOISTURE FLUX INTO LOUISIANA TODAY...EXPECT
THE WESTERN EXTENT OF THE SEA BREEZE ACTIVITY THIS AFTERNOON WILL
BE IN FAR SOUTHEAST TEXAS.

THE TROUGHING IN THE WESTERN STATES IS PROGGED TO BE A BIT WEAKER
WITH THE MAIN UPPER LOW TAKING A SHARP LEFT TURN INTO THE PRAIRIE
PROVINCES LATE IN THE WEEKEND. AS A RESULT...THE MID-LEVEL RIDGE
IN THE SOUTHEAST WILL BE ABLE TO BUILD FARTHER WEST THE PREVIOUSLY
PROGGED. THE RIDGE WILL BE NEATLY STACKED VERTICALLY ABOVE THE
BOUNDARY LAYER...FROM 10KFT ABOVE MSL TO AROUND 35KFT...WHICH WILL
ASSURE SUBSIDENCE WILL DOMINATE INTO NEXT WEEK. HOWEVER...ITS
INTENSITY IS STILL RATHER ORDINARY FOR AUGUST...AND ITS PLACEMENT
EAST OF TEXAS SHOULD PREVENT TEMPERATURES FROM SOARING INTO THE
TRIPLE DIGITS. IN ADDITION...BREEZY CONDITIONS WILL CONTINUE THE
REMAINDER OF THE WEEK...WHICH WILL PREVENT OPTIMAL HEATING OF THE
SURFACE LAYER. THIS WILL REDUCE HIGH TEMPERATURES 1-2 DEGREES FROM
WHAT WOULD OTHERWISE BE EXPECTED...MEANING THAT ACHIEVING TRIPLE-
DIGIT HIGHS WILL BE A LITTLE MORE DIFFICULT. A NIGHTLY LLJ WILL
MEAN REPEATED STRATUS INTRUSIONS...PRIMARILY INTO CENTRAL TEXAS.
THESE MORNING CLOUDS WILL BE SHORT-LIVED AND WILL DO LITTLE TO
PREVENT HOT AFTERNOONS. HEAT INDEX VALUES WILL EASILY TOP 100 BUT
SHOULD REMAIN BELOW 105.

A PIECE OF UPPER VORTICITY IS STILL PROGGED TO EJECT FROM THE
TUTT SOUTH OF BROWNSVILLE...AND THE SOUTHERLY FLOW BENEATH IT WILL
GUIDE IT NORTHWARD OVER THE LONE STAR STATE. THE POLAR JET WILL
EFFECTIVELY SHEAR IT...BUT THE TROUGHINESS IN THE UPPER REACHES
OF THE TROPOSPHERE WILL LINGER THROUGH THE WEEKEND. EVEN SO...THE
FEATURE WILL QUICKLY BECOME AN OPEN WAVE AND SHOULD BE UNABLE TO
DISRUPT THE DEEP SUBSIDENCE BENEATH IT.

AS THE UPPER LOW ENTERS CANADA...THE FLOW ALOFT INTO THE CENTRAL
PLAINS WILL DIMINISH CONSIDERABLY. A CORRESPONDING WEAKENING OF
THE PERSISTENT LEE TROUGH WILL MEAN LIGHTER SURFACE WINDS ACROSS
NORTH AND CENTRAL TEXAS. THIS WILL BRING AN END TO THE MORNING
STRATUS AND WILL MAKE THE HEAT FEEL A LITTLE MORE OPPRESSIVE...
BUT IT MAY ALSO ALLOW THE SEA BREEZE TO REAWAKEN.

THE GFS AND ECMWF SOLUTIONS DIVERGE NEXT WEEK...WITH THE ECMWF
DEVELOPING A CUT-OFF LOW AND THE GFS FAR MORE PROGRESSIVE. THE
CANADIAN IS A COMPROMISE BETWEEN THE TWO...BUT LEANS TOWARD THE
GFS IN THAT IT KEEPS THE FLOW UNIMPEDED. DESPITE CONSIDERABLE
SPREAD AMONG THE GFS ENSEMBLE MEMBERS...THE CONSENSUS FAVORS A
FRONTAL PASSAGE LATE NEXT WEEK. HAVE INTRODUCED NORTH WINDS AND
SLIGHT PRECIP CHANCES ON THURSDAY NEXT WEEK...BUT THESE WILL NOT
APPEAR IN THE PUBLIC FORECAST UNTIL THE AFTERNOON PACKAGE TODAY.

25

&&

.FIRE WEATHER...
HOT AND RAIN-FREE CONDITIONS WILL PERSIST THROUGH THE WEEKEND. A
BROAD UPPER HIGH WILL BE CENTERED ACROSS THE WESTERN GULF STATES
AND WILL RESULT IN HIGHS BETWEEN THE MID 90S AND 102 DEGREES
ACROSS THE AREA. HOTTEST HIGH TEMPERATURES WILL BE ACROSS THE
FAR WESTERN COUNTIES WHERE DEEPER MIXING WILL RESULT IN LOWER
AFTERNOON HUMIDITY VALUES EACH DAY BETWEEN 20 AND 30 PERCENT.
HOWEVER...20 FOOT WINDS WILL REMAIN SUSTAINED BETWEEN 10 TO 15 MPH
WITH OCCASIONAL GUSTS EXCEEDING 20 MPH. A WEAK SURFACE LOW NEAR
THE WESTERN HALF OF NORTH TEXAS SHOULD KEEP WINDS FROM BECOMING
TOO STRONG. THIS AREA WILL NEED TO BE WATCHED CLOSELY IN THE
COMING DAYS FOR A HIGH FIRE DANGER...PARTICULARLY THOSE AREAS
NORTHWEST OF A GAINESVILLE...DECATUR...COMANCHE LINE. FUEL MOISTURE
LEVELS AT BOTH POSSUM KINGDOM RESERVOIR AND MILLER/S CREEK RAWS
OBSERVATIONS WERE ALREADY BELOW 10...INDICATING SOME DRYING OF
FINE FUELS ACROSS THE AREA.

OTHERWISE...AREAS FURTHER TO THE EAST MAY SEE OCCASIONAL HIGHER
GUSTS BUT WILL ALSO SEE RICHER GULF MOISTURE AND AFTERNOON
HUMIDITY VALUES REMAINING ABOVE CRITICAL LEVELS. THE HIGHER
HUMIDITY ACROSS THE EASTERN AREAS SHOULD HOLD HIGH TEMPERATURES
INTO MID TO UPPER 90S. THE FIRE DANGER SHOULD REMAIN LOW INTO EARLY
NEXT WEEK...AS WIND SPEEDS REMAIN BELOW 15 MPH.

05

&&

.CLIMATE...
LATE AUGUST FRONTAL PASSAGES ARE REMARKABLY COMMON. ALTHOUGH NORMAL
HIGH TEMPERATURES (BASED ON DFW DATA 1981-2010) GRADUALLY DECREASE
AFTER AUGUST 17 AT A STEADY RATE...THE AVERAGE TEMPERATURES TELL A
DIFFERENT STORY.

DAILY NORMALS ARE CREATED USING A HARMONIC FIT...A METHOD EMPLOYED
TO DAMPEN OUTLIERS. THE RESULT ASSURES A SMOOTH TRANSITION FROM ONE
DAY TO THE NEXT...BUT THIS SMOOTHING REMOVES INTRICACIES IN THE DATA
THAT CAN REVEAL MORE ABRUPT SEASONAL TRANSITIONS. FOR EXAMPLE...MUCH
OF THE DECREASE IN HIGH TEMPERATURES DURING LATE AUGUST OCCURS IN A
2-DAY SPAN BETWEEN AUGUST 26 AND 28.

THE DATA BELOW ARE BASED ON THE DALLAS/FORT WORTH CLIMATE RECORD
1981-2010.

               AVERAGE       NORMAL
             HIGH / LOW    HIGH / LOW

AUGUST 26    97.4 / 75.4     95 / 74
AUGUST 28    93.5 / 73.4     94 / 74

THE AVERAGE HIGH TEMPERATURE (1981-2010) ACTUALLY INCREASES BETWEEN
AUGUST 17 AND 26. THE SUDDEN 3.9-DEGREE DROP IN JUST 2 DAYS (AUGUST
26-28) IS THUS VERY SIGNIFICANT.

THESE ARE NOT SIMPLY ARTIFACTS OF A 30-YEAR DATA SET. USING THE
ENTIRE CLIMATE RECORD (WHICH BEGAN IN SEPTEMBER 1898) YIELDS A
DECREASE IN THE AVERAGE HIGH TEMPERATURE BETWEEN AUGUST 17 AND 26
(1.2 DEGREES) THAT IS STILL LESS THAN THE 2-DAY SPAN AUGUST 26-28
(1.4 DEGREES). AND YES...WHEN INCORPORATING THE ENTIRE CLIMATE
RECORD OF MORE THAN A CENTURY OF DATA...THE TIME FRAME OF THE
SIGNIFICANT DROP REMAINS AUGUST 26-28.

WHAT DOES THIS MEAN?

IT MEANS THAT HISTORICAL FRONTAL PASSAGES CLUSTER AROUND THESE
DATES...SHOWING A TENDENCY FOR THE FIRST SIGNIFICANT LATE SUMMER
FRONT TO OCCUR AT THIS TIME. IT LOOKS LIKE 2014 MAY FOLLOW SUIT. THE
GFS PROJECTS A FRONTAL PASSAGE NEXT WEEK ON AUGUST 28.

DOES THIS MEAN FALL HAS ARRIVED?

NOT QUITE YET...AS HIGH TEMPERATURES WILL LIKELY BE IN THE 90S EVEN
IF THE COLD FRONT ARRIVES. NORMAL (AND AVERAGE) HIGH TEMPERATURES
ARE STILL IN THE 90S IN EARLY SEPTEMBER.

ANOTHER SIGNIFICANT DROP IN AVERAGE TEMPERATURES OCCURS AROUND THE
AUTUMNAL EQUINOX...WHICH IS ONLY A MONTH AWAY. (THE BELOW ARE BASED
ON DFW DATA 1981-2010.)

                  AVERAGE       NORMAL
                HIGH / LOW    HIGH / LOW

SEPTEMBER 20    88.1 / 65.8     87 / 66
SEPTEMBER 24    83.1 / 62.6     85 / 64

PERHAPS THERE IS SOMETHING TO THE FIRST DAY OF FALL.

&&

.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX  99  78  99  78 100 /   0   0   0   0   0
WACO, TX             100  76 100  76 100 /   0   0   0   0   0
PARIS, TX             97  74  97  74  98 /   0   0   0   0   0
DENTON, TX            99  77  99  75 100 /   0   0   0   0   0
MCKINNEY, TX          97  75  98  74  99 /   0   0   0   0   0
DALLAS, TX            99  78  99  79  99 /   0   0   0   0   0
TERRELL, TX           97  75  98  75  98 /   0   0   0   0   0
CORSICANA, TX         98  75  98  75  98 /   0   0   0   0   0
TEMPLE, TX            99  76  98  74  98 /   0   0   0   0   0
MINERAL WELLS, TX     99  73  99  73 101 /   0   0   0   0   0

&&

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

&&

$$

05/25




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