FXUS63 KPAH 011101
AFDPAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PADUCAH KY
501 AM CST TUE DEC 1 2009

.DISCUSSION...
THIS DISCUSSION IS SOMEWHAT DELAYED...GIVEN THE COMPLEXITY AND
DETAILS ASSOCIATED WITH A DEFINITELY SUB-ADVISORY MIXED
PRECIPITATION EVENT WEDNESDAY NIGHT. THE MOST SIGNIFICANT CHANGE
MADE TO THIS PACKAGE WAS TO REFINE THE POP/WEATHER/WEATHER TYPE
FROM WEDNESDAY THROUGH EARLY THURSDAY. ALTHOUGH SOME ADJUSTMENTS
WERE MADE FOR COLLABORATIVE CONTINUITY WITH SURROUNDING NWS
OFFICES...THE MAJORITY OF THE FORECAST INVOLVING MIXED
PRECIPITATION TYPES /RAIN AND SNOW/ FELL WITHIN THE PARAMETERS OF
THE TOP DOWN METHOD.

THE LAST TWO RUNS OF THE DETERMINISTIC NAM-WRF/ECMWF/GFS APPEAR TO
BE MORE CONSERVATIVE AND CONSISTENT IN BRINGING RAINFALL TO THE
THE WFO PAH FORECAST AREA /SRN BORDER COUNTIES OF SERN MO AND W
KY/ AFTER MIDNIGHT TONIGHT. ALTHOUGH SUFFICIENT HIGH LEVEL
MOISTURE WILL ALREADY BE IN PLACE /CIRRUS SHIELD ALREADY
OVERSPREADING THE LWR MS VALLEY AT 10Z TUESDAY/...DOWNWARD
SATURATION OF THE LAYER BELOW 700 MB WILL NOT TAKE PLACE UNTIL
SHORTLY AFTER DAYBREAK WEDNESDAY. 

HOWEVER...AFTER DAYBREAK WEDNESDAY...SIGNIFICANT BAROCLINICITY
WILL COME INTO PLAY AS THE CLOSED LOW /JUST NOW EJECTING OUT OF
NORTHERN MEXICO INTO EXTREME WESTERN TEXAS/ OPENS UP AND RACES TO
THE WEST KY/WEST TN BORDER BY 00Z THURSDAY/. THE MOST SIGNIFICANT
AND EFFECTIVE LIFT WILL COME FROM THE DEFORMATION ZONE...AS IT
SHIFTS FROM THE NORTHEAST TO NORTHWEST QUADRANT OF THE UPPER LOW
AS IT SHEARS OUT AS AN OPEN WAVE. THE ECMWF/GFS/NAM-WRF ARE
SIMILAR IN THE PLACEMENT OF THIS FEATURE...BUT DIFFER IN THE
INTENSITY AND SCOPE OF THE FEATURE INTO THE LOWER MS VALLEY LATE
WEDNESDAY AND WEDNESDAY NIGHT. 

OF MORE SIGNIFICANCE TO THE CHANGE IN PRECIPITATION TYPE WEDNESDAY
NIGHT...IS THE SLIGHTLY SLOWER TRANSLATION OF THE POSITIVELY
TILTED UPPER TROUGH TOWARD THE MIDDLE MS VALLEY WEDNESDAY NIGHT.
NEARLY ALL THE GUIDANCE SUGGESTS THAT THE APPROACHING TROUGH IN
THE NORTHERN PLAINS AND THE UPPER LOW/SHORTWAVE SHEARING OUT
ACROSS THE LOWER MS/OH VALLEYS WILL BE OUT OF PHASE. GIVEN THAT
THE LEADING EDGE OF THE COLDER AIR/BAROCLINIC ZONE WAS STILL OVER
NORTHERN MONTANA AT 00Z TUESDAY...THERE IS SOME CREDENCE TO THE
TIMING OF THE APPROACHING UPPER MIDWEST TROUGH IN THE LATEST SUITE
OF THE NUMERICAL GUIDANCE.

FOR THE WFO PAH FORECAST AREA...THIS WILL MEAN A MORE NORMAL
DIURNAL RANGE IN TEMPERATURES ON WEDNESDAY...WITH A SLOWER ONSET
OF COLDER AIR REACHING LOWER LEVELS AND AT THE SURFACE BEHIND THE
CLOSED SURFACE LOW. THE NUMERICAL MODELS STILL VARY IN THE
RELATIVE LOCATION OF THE SURFACE LOW MOVING UP FROM THE EAST TEXAS
COAST...BUT I AM MORE INCLINED TO ACCEPT THE FURTHER WEST
PLACEMENT OF THE ECMWF/NAM-WRF/SREF GUIDANCE INTO MIDDLE TN/KY BY
00Z THURSDAY. 

WITH THAT IN MIND...THE EASTWARD TRANSLATION/DEEPENING/DOWNWARD
TRANSPORT OF THE COLD LAYER ALOFT WILL BE RESTRICTED SOMEWHAT.
THEREFORE...THE IMPACT OF A RAIN/SNOW MIX WILL BE TEMPERED BY AN
RELATIVE LOSS OF MOISTURE AND LIFT WITH TIME LATE WEDNESDAY NIGHT.
WITH THIS IN MIND...LITTLE...IF ANY MEASURABLE ACCUMULATION OF
SNOW WILL BE POSSIBLE. SHOULD ANY SNOW ACCUMULATION OCCUR...IT WILL
BE WELL UNDER A HALF INCH AND MAINLY ON GRASSY SURFACES OVER SRN
IL/SWRN IN. AT THIS TIME...I HAVE LOW CONFIDENCE ON ANY MEASURABLE
ACCUMULATION AT ALL.

FOR TEMPERATURE...WIND...CLOUD COVER...DEWPOINTS AND
POP/WEATHER...UTILIZED THE NAM-WRF AT THE PRIMARY INPUT TO THE
NUMERICAL GUIDANCE...WITH SMALLER WEIGHTING TO THE GFS/SREF/ECMWF/RUC.

BEYOND THURSDAY...I CONTINUED THE CURRENT DRY FORECAST AND BELOW
NORMAL TEMPERATURES THROUGH SATURDAY. NO CHANGES WERE MADE TO THE
FORECAST BEYOND SATURDAY WITH THIS PACKAGE.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE QUAD STATE TODAY AND FOR MUCH 
OF TONIGHT. AS A STORM SYSTEM APPROACHES THE REGION FROM THE SOUTH 
LATE TONIGHT...LIGHT RAIN AND MVFR CEILINGS MAY SPREAD OVER MUCH OF 
WEST KENTUCKY...INCLUDING KPAH AND KOWB...BY 12Z WEDNESDAY.  WINDS 
WILL GENERALLY BE UNDER 10KTS TODAY AND TONIGHT. HOWEVER...AS THE 
STORM SYSTEM APPROACHES...SOUTH WINDS AT THE BEGINNING OF THE PERIOD 
WILL GRADUALLY BACK FROM SOUTH THROUGH SOUTHEAST AND EVENTUALLY TO 
NORTHEAST TONIGHT. THE CHANGE IN WIND DIRECTION WILL BEGIN THIS 
AFTERNOON IN THE SOUTH...AND THIS EVENING IN THE NORTH.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
MO...NONE.
IL...NONE.
IN...NONE.
&&

$$

AVIATION/FIRE WEATHER...DRS
SHORT/LONG TERM PUBLIC...SMITH