Area Forecast Discussion
Issued by NWS Amarillo, TX

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FXUS64 KAMA 190348 AAB
AFDAMA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE AMARILLO TX
1048 PM CDT MON MAY 18 2015

.AVIATION...
NO REAL CHANGES TO THE FORECAST OF 6 HOURS AGO...OTHER THAN A COUPLE
OF TIMING ADJUSTMENTS.  MVFR CEILINGS WILL LOWER INTO IFR RANGE
BETWEEN 07Z AND 10Z TUESDAY.  OCCASIONAL THUNDERSTORMS EXPECTED AT
ALL TERMINALS AFTER 07Z TUESDAY WITH OCCASIONAL LIFR CONDITIONS
LIKELY.  IFR CEILINGS EXPECTED TO PERSIST THROUGH THE DAY AND EVENING
ON TUESDAY.  COLD FRONT EXPECTED TO BRING SURFACE WIND SHIFT TO
NORTHERLY AROUND 06Z WEDNESDAY AT NORTHERN TERMINALS...WITH NO
IMMEDIATE IMPROVEMENT EXPECTED.

COCKRELL

&&

.PREV DISCUSSION... /ISSUED 638 PM CDT MON MAY 18 2015/

AVIATION...
MODERATE TO STRONG EASTERLY UPSLOPE FLOW WILL CONTINUE TO DRAW LOW-
LEVEL MOISTURE INTO THE FORECAST AREA.  BY 05Z TUESDAY...EXPECT MVFR
CEILINGS TO DEVELOP.  RAPID DETERIORATION OF FLIGHT CATEGORIES
EXPECTED TO FOLLOW WITH WITH ALL TERMINALS LOWERING TO IFR CEILINGS
BY 09Z TUESDAY.  THUNDERSTORMS EXPECTED AT ALL TERMINALS AFTER 07Z
TUESDAY WITH OCCASIONAL LIFR CONDITIONS LIKELY.  THESE POOR FLYING
CONDITIONS ARE EXPECTED TO PERSIST THROUGH THE DAY ON TUESDAY.

COCKRELL

PREV DISCUSSION... /ISSUED 357 PM CDT MON MAY 18 2015/

DISCUSSION...

AN ACTIVE WEATHER PATTERN WILL CONTINUE THROUGH THE END OF THE
FORECAST PERIOD WITH SEVERAL CHANCES FOR RAIN. CHALLENGES FOR THIS
FORECAST INCLUDE HEAVY RAIN/FLOODING POTENTIAL...FRONTAL
POSITION...TEMPERATURES...AND POSSIBILE WINDOW FOR SEVERE
THUNDERSTORM DEVELOPEMENT. THESE UNCERTAINTIES ARE LARGELY A RESULT
OF SUBTLE DIFFERENCES WITH FLOW PATTERN ALOFT AND REFLECTION AT THE
SURFACE. IN THE NEAR TERM...CONFIDENCE IS HIGH ON YET ANOTHER
WIDESPREAD PRECIPITATION EVENT TONIGHT THROUGH AT LEAST EARLY
TOMORROW. THIS EVENT SHOULD BE FAIRLY LOW IMPACT IN THE ABSCENCE OF
SIGNIFICANT INSTABILITY ALTHOUGH SATURATED SOILS PARTICULARLY IN THE
EAST RAISES SOME CONCERN FOR MINOR FLOODING. EVEN AN INCH OR TWO IN
A 36 HOUR PERIOD MAY BE ENOUGH FOR SOME FLOOD CONCERNS.

PRESSURE ADVECTION AND SATURATION ON ISENTROPIC SURFACES MAXIMIZE FOR
A PERIOD BEGINNING EARLY TONIGHT IN THE WEST AND SPREADING EAST
THROUGH THE NIGHT. THE BULK OF THIS SHOULD MOVE EAST OF THE AREA BY
LATE MORNING OR EARLY AFTERNOON. PWAT VALUES OF AROUND THE 99TH
PERCENTILE SUGGEST POTENTIAL FOR HEAVY RAINFALL BUT PROGRESSIVE
NATURE SHOULD LIMIT A MORE SIGNIFICANT FLOOD CONCERN. THE PLAN IS TO
HOLD OFF ON A FLOOD WATCH AND HANDLE THIS EVENT WITH FLOOD
ADVISORIES WHERE NECESSARY AS THE EVENT UNFOLDS THROUGH THE NIGHT
AND INTO TOMORROW MORNING.

MEANWHILE THE SOUTHERN STREAM SHORTWAVE TROUGH WILL DEAMPLIFY/WEAKEN
WITH RESIDUAL 50-60 KNOT 500 MB SPEED MAX OVERSPREADING THE AREA BY
AFTERNOON. THIS IS WHEN SIGNIFICANT UNCERTAINTIES EMERGE. MODELS
HAVE STRUGGLED WITH MESOSCALE DETAILS AS OF LATE WHEN OVERNIGHT
CONVECTION IS EXTENSIVE THE PREVIOUS NIGHT AND TOMORROW WILL
PROBABLY BE NO EXCEPTION. MEAN FLOW ORIENTATION AND MAGNITUDE WOULD
GENERALLY FAVOR AN EASTWARD MIXING DRYLINE ALTHOUGH WITH DOWNWARD
TRENDS IN MOMENTUM BY PEAK MIXING AND FAIRLY DEEP LOW LEVEL MOISTURE
IT MAY BE DIFFICULT FOR THE DRYLINE TO SHARPEN SIGNIFICANTLY AND MIX
TOO FAR EAST. THERE IS SOME QUESTION ON THE LEVEL OF
DESTABILIZIATION THAT CAN OCCUR DURING THE AFTERNOON. CURRENT
THINKING IS THAT IN THE WAKE OF WIDESPREAD PRECIPITATION SOME
DIABATIC HEATING AND POCKETS OF AT LEAST MODEST INSTABILITY SHOULD
FORM BY MID AFTERNOON. SCATTERED CONVECTION SHOULD GRADUALLY
INCREASE IN INTENSITY AND POSE A MARGINAL SEVERE WEATHER THREAT
DURING THE AFTERNOON GIVEN SUFFICIENT DEEP LAYER SHEAR AND BACKED
LOW LEVEL WINDS. MOST MODELS SHOW A LOW LEVEL THETA-E AXIS EXTENDING
NORTHWESTWARD ACROSS ROUGHLY THE SOUTHWESTERN HALF OF THE TEXAS
PANHANDLE AND THIS MAY BE COINCIDENT WITH THE MOST INTENSE
CONVECTION DURING THE AFTERNOON. ALL OF THIS WILL DEPEND ON
DIFFICULT TO RESOLVE MESOSCALE FEATURES/BOUNDARIES THAT WILL
STRONGLY BE TIED TO OVERNIGHT/EARLY MORNING CONVECTION. A MORE
CONFIDENT FORECAST WILL REQUIRE AN ASSESSMENT OF OBSERVATIONAL TRENDS
LATE TONIGHT AND EARLY TOMORROW MORNING.

SOME OF THE GUIDANCE IS SLIGHTLY MORE PERTURBED WITH MEAN
SOUTHWESTERLY FLOW INTO WEDNESDAY AND THEREBY MORE AGGRESSIVE WITH A
SOUTHWARD INTRUSION OF COOL/DRY AIR MASS INTO AT LEAST THE NORTHERN
PORTION OF THE AREA. GFS IS THE OUTLIER AND LESS AGRESSIVE WITH
SOUTHWARD MOVEMENT OF THE COLD FRONT. HAVE SIDED WITH THE HIGHER
RESOLUTION NAM AND WRF AND THE SREF AND ECMWF FOR FRONTAL
TIMING...WINDS AND TEMPERATURES. WE HAVE CONSIERABLY COOLER
TEMPERATURES THAN MOST STATISTICAL GUIDANCE FOR WEDNESDAY. FORECAST
SOUNDINGS SHOW DRY SOUTHWESTERLY FLOW ALOFT ABOVE MOIST
NORTHEASTERLY/EASTERLY LOW LEVELS. GIVEN STEADY HEIGHT RISES AND
SIGNS OF CAPPING INVERSION AT THE TOP OF THE MOIST LOW LEVELS IN
FORECAST SOUNDINGS BELIEVE PROBABILITY OF PRECIPITATION IS QUITE LOW.
DEPENDING ON HOW THIS EVOLVES THERE IS SOME POTENTIAL FOR MOIST
UPSLOPE AND COOLER AIR MASS TO RESULT IN PERSISTIENT LOW CLOUDS AND
FOG THROUGH MUCH OF THE DAY ON WEDESDAY.

BY THURSDAY EVENING A LEAD SHORTWAVE APPROACHES AND HEIGHT FALLS
OCCUR SO RETAINED LOW PRECIPITATION PROBABITLIES. THE MAGNITUDE OF
ASCENT AND PRESENCE OF ANOMALOUS MOISTURE JUSTIFIES RELATIVELY HIGH
PRECIPITATION PROBABILITIES FOR THURSDAY NIGHT THROUGH FRIDAY
MORNING. AS THE MAIN UPPER LOW EJECTS NORTHEASTWARD FRIDAY NIGHT
INTO SATURDAY THERE WILL LIKELY BE ANOTHER PERIOD OF PRECIPITATION.
CONTINUED MOIST SOUTHEASTERLY LOW LEVEL TRAJECTORIES AND PWAT VALUES
OVER ONE INCH (POSSIBLY AGAIN REACHING OR EXCEEDING THE 75TH
PERCENTILE) RAISES CONCERN FOR FLOODING. THIS IS ESPECIALLY TRUE
GIVEN THE POTENTIAL FOR TWO SUCCESSIVE WAVES OF PRECIPITATION
(POTENTIALLY HEAVY) IN A RELATIVELY SHORT PERIOD OF TIME...ON
THURSDAY NIGHT AND AGAIN FRIDAY NIGHT/SATURDAY. TIMING/PLACEMENT OF
EACH ROUND OF PRECIPITATION IN THE EXTENDED REMAINS UNCERTAIN GIVEN
TYPICAL INCREASED SPREAD IN THE ENSEMBLES AND UNRESOLVED SMALLER
SCALE DETAILS.

BRB

&&

.AMA WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
OK...NONE.

&&

$$

03/16





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