Area Forecast Discussion
Issued by NWS Jackson, KY

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000 FXUS63 KJKL 302114 AFDJKL AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE JACKSON KY 414 PM EST FRI JAN 30 2015 .SHORT TERM...
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(THIS EVENING THROUGH SATURDAY NIGHT) ISSUED AT 308 PM EST FRI JAN 30 2015 SURFACE HIGH PRESSURE PASSING OVER THE REGION TONIGHT INTO SATURDAY WILL PROVIDE MOSTLY CLEAR SKIES AND LIGHTER WINDS. THE ELEMENTS FOR A LATE WEEKEND WEATHER SYSTEM WILL BE COMING TOGETHER OVER THE WESTERN CONUS ON SATURDAY...WITH WARM/MOIST AIR SPREADING NORTHWARD ON ISENTROPIC LIFT OVER THE GREAT PLAINS DURING THE DAY. HIGH CLOUDS FROM THIS WILL BE CARRIED EASTWARD AND RIDE OVER AN UPPER LEVEL RIDGE TO BRING US INCREASING CLOUDS IN THE AFTERNOON. THE SURFACE LOW WILL BEGIN TO TAKE SHAPE ON SATURDAY NIGHT AND SHOULD BE OVER MISSOURI BY 12Z SUNDAY. MODELS HAVE COME INTO BETTER AGREEMENT CONCERNING THIS SYSTEM...AND NOW POINT TO A MORE NORTHERLY TRACK THAN WHAT LOOKED PROBABLE IN EARLIER MODEL RUNS. THIS WILL BE DEALT WITH IN MORE DETAIL IN THE LONG TERM DISCUSSION. THIS TREND IN THE MODELS RESULTS IN WARM AIR ADVECTION SETTING IN ON SATURDAY NIGHT...AND MOST OF THE PRECIP ON SATURDAY NIGHT BEING TO OUR N AND NW. WILL ONLY CARRY 20-40 PERCENT POPS IN THE JKL FORECAST AREA. HAVE USED PRECIP TYPE OF SNOW FOR MOST OF THE AREA...WITH RAIN/SNOW MIX IN OUR SW. TEMPS IN SOUNDINGS MAY SUPPORT MORE IN THE WAY OF RAIN. HOWEVER... THE LOW LEVELS ARE NOT SATURATED IN THE SOUNDINGS. IF PRECIP MAKES IT TO THE GROUND...WOULD EXPECT SOME EVAPORATIONAL COOLING TO ALLOW SNOW. .LONG TERM...(SUNDAY THROUGH FRIDAY) ISSUED AT 412 PM EST FRI JAN 30 2015 AS WE HEAD INTO THE EXTENDED PERIOD...AN UPPER LOW DISPLACED IN THE SOUTHWESTERN CONUS EJECTS A SHORTWAVE INTO THE SOUTHERN PLANS WHILE ANOTHER WAVE OVER THE NORTHERN STREAM OF THE POLAR FRONT JET MOVES SOUTHEAST ACROSS THE UPPER PLAINS. BY SUNDAY THESE TWO FEATURES MERGE OVER THE MIDWEST AND MOVE EAST THROUGH THE OH VALLEY. AT THE SURFACE...A STRONG LOW DEVELOPS AND TRACKS RIGHT ALONG/JUST NORTH OF THE OH RIVER. MODELS ARE FINALLY STARTING TO SHOW CONTINUITY IN THEIR RUNS...WHICH IS GIVING MORE CONFIDENCE TO THE FORECAST. IF THE GFS HOLDS TRUE BASED ON THE LAST 4 MODEL RUNS...THE CENTER OF THE LOW WILL TRACK ALONG AND JUST SOUTH OF THE OHIO RIVER. ONE THING TO NOTE HOWEVER IS THAT THE LAST 2 MODEL RUNS...12Z AND 6Z TODAY...SHOW AN INCREASE IN THE INTENSITY OF THE LOW TO FROM 1012MB TO 1006MB. EVEN THE LATEST 0Z AND 12Z ECMWF IS IN STRONG AGREEMENT WITH THE GFS. AND THOUGH IT IS TOWARDS THE END OF ITS RUN...THE NAM80 IS FINALLY STARTING TO SHOW AGREEMENT WITH BOTH THE GFS AND ECMWF AS WELL...SHOWING 1005MB LOW TRACKING ALONG THE OHIO RIVER DURING THE SAME TIME PERIOD. THE 12Z GEM STILL SEEMS TO BE TOO FAR SOUTH WITH THE SYSTEM...SO DID NOT USE THIS MODEL IN THE FORECAST FOR THIS EVENT. ALL THINGS CONSIDERED...IF THE GFS/ECMWF/NAM TRACKS DO PAN OUT...BY 12Z SUNDAY THE PRECIP WILL LIKELY BE SPREADING ACROSS THE CWA FROM THE NW...QUICKLY ENCOMPASSING MUCH OF THE CWA WITHIN A FEW HOURS. WHILE THE MOIST LAYER WILL BE WELL BELOW FREEZING...AND SURFACE TEMPS WILL BE VERY NEAR THE FREEZING MARK...A WARM/DRY NOSE JUST ABOVE THE SURFACE WILL KEEP TEMPS IN THE LOW LEVELS JUST ABOVE THE SURFACE WELL ABOVE FREEZING. THE SHALLOWNESS OF THIS DRY LAYER WILL LIKELY RESULT IN A QUICK WETBULBING DOWN TO THE SURFACE...WITH MOISTURE OVERCOMING AND HITTING THE SURFACE FROM NORTH TO SOUTH. GIVEN THE COLD MOIST LAYER...DESPITE THE WARMING NEAR THE SURFACE...EXPECT MOST OF THE PRECIP TO STILL FALL AS SNOW...POSSIBLY MIXING WITH RAIN IN THE FAR SOUTH WHERE THE WARMEST LOW TEMPS WILL OCCUR. SOUTHERLY WINDS AT THE SURFACE WILL THEN ALLOW TEMPS TO RISE THROUGHOUT THE DAY TO ABOVE FREEZING IN THE FIRST 6-7KT FEET. THIS SHOULD ALLOW ENOUGH TIME FOR ICE CRYSTALS TO MELT AS THEY FALL AND PRECIPITATION TO CHANGE TO RAIN. TEMPERATURES WILL INCREASE INTO THE LOW TO MID 40S ACROSS THE CWA BY THE AFTERNOON...WITH WARMEST TEMPS EXPECTED IN THE SOUTHERN CWA. THE COLD FRONT ITSELF SHOULD PASS THROUGH THE REGION SOMETIME AFTER 6Z MONDAY. AT THIS POINT...DRY AIR WILL BEGIN WORKING IN THE UPPER LEVELS...AND MUCH COLDER AIR WILL BEGIN FILTERING IN AS WINDS SHIFT TO A MORE NWRLY DIRECTION IN THE LOW LEVELS. DURING THIS TRANSITION...WHILE QPF IS EXPECTED TO LESSON...PRECIP WILL LIKELY QUICKLY TRANSITION BACK FROM RAIN TO FROZEN PRECIP LATE IN THE NIGHT...NEAR 12Z MONDAY. NOTE...THIS IS A MUCH LATER TRANSITION TIME THAN WHAT WAS PREVIOUSLY FORECAST WITH A SLIGHTLY MORE NORTHWARD TRACK. SHALLOW MOISTURE AND LIGHT SNOW IS EXPECTED TO PERSIST THROUGHOUT THE DAY MONDAY UNDER STRONG NW FLOW. THIS WILL LEAD TO UPSLOPE FLOW AND BETTER CHANCES FOR ACCUMULATING SNOW AMOUNTS ACROSS THE HIGHER TERRAIN. THE 6Z TUESDAY SOUNDING LOOKS TO FINALLY PULL IN ENOUGH LOW LEVEL DRY AIR TO CUT OFF ANY LINGER PRECIP...THOUGH SOME LOW CLOUDS MAY CONTINUE TO LINGER THROUGH THE NIGHT. HIGHEST QPF AMOUNTS WILL BE ACROSS THE NORTHERN CWA AND POINTS JUST TO OUR NORTH...CORRESPONDING WITH THE LOW TRACK AS WELL AS THE CENTER OF BEST QVECTOR CONVERGENCE. HOWEVER...UPSLOPE FLOW COULD LEAD TO HIGHER AMOUNTS IN THE HIGHER TERRAIN IN THE FAR SE COUNTIES AS WELL. ALL TOGETHER...GENERALLY A HALF INCH OR LESS OF SNOWFALL IS EXPECTED SUNDAY MORNING AND SUNDAY NIGHT INTO MONDAY. HOWEVER...THE HIGHEST TERRAIN ABOVE 2500 FEET COULD SEE BETWEEN 1 AND 2 INCHES DUE TO THE UPSLOPE FLOW AND TEMPERATURES FALLING MORE QUICKLY AT THESE HIGHER ELEVATIONS SUNDAY NIGHT. BY MONDAY NIGHT...THE SHORTWAVE WILL HAVE FINALLY TRANSITIONED EAST OF THE REGION...PUTTING MUCH OF THE OHIO RIVER VALLEY AREA IN A MORE ZONAL FLOW PATTERN THROUGH MID WEEK. COLDER AIR WILL CONTINUE TO FILTER INTO THE REGION...WITH OVERNIGHT TEMPS MONDAY NIGHT/TUESDAY MORNING MODELS SINKING DOWN INTO THE LOW TEENS IN THE NORTH...AND INTO THE UPPER TEENS TO AROUND 20 IN THE SOUTH. MODELS LOSE SIGNIFICANT AGREEMENT BY THIS POINT...SO WILL HAVE TO RELY ON THE MODEL BLEND TO COME UP WITH A FORECAST SOLUTION. THE GFS SHOWS A DISTURBANCE CLIPPING OUR FAR NORTHERN COUNTIES WEDNESDAY...WHILE THE OTHER MODELS KEEP THIS SYSTEM WELL TO OUR NORTH. THIS IS A DOWNGRADE FROM PREVIOUS MODEL RUNS...AND AS A RESULT...POPS HAVE LOWERED COMPARED TO THE PREVIOUS FORECAST. ANOTHER SYSTEM COULD SWEEP FROM THE NW TO THE SE SOMETIME LATE WEDNESDAY NIGHT AND INTO THE DAY THURSDAY. THIS WILL RESULT IN PRECIP...AS WELL AS ANOTHER BURST OF COLDER AIR FILTERING INTO THE REGION BEHIND THE SYSTEM. AFTER COLLABORATING WITH THE SURROUNDING OFFICES...LOWERED TEMPS CONSIDERABLY FROM WHAT THE MODEL BLEND GAVE ON THURSDAY BASED ON THE AMOUNT OF COLD AIR FILTERING INTO THE REGION. WENT AHEAD AND INCLUDED SOME LIGHT SNOWFALL AMOUNTS WEDNESDAY NIGHT AND THURSDAY...BUT THIS IS SUBJECT TO CHANGE TO HIGHER OR LESSER AMOUNTS AS MODELS CONTINUE TO COME INTO BETTER AGREEMENT.
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&& .AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON) ISSUED AT 101 PM EST FRI JAN 30 2015 MVFR CEILINGS MAY LINGER EARLY IN THE PERIOD NEAR THE VA BORDER. OTHERWISE...VFR IS EXPECTED THROUGH THE PERIOD. NW WINDS GUSTING BETWEEN 15 AND 20 KTS WILL DIE DOWN BY EVENING...WITH LIGHT WINDS EXPECTED FOR THE REMAINDER OF THE PERIOD. && .JKL WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...HAL LONG TERM...JMW AVIATION...HAL

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