684
FXUS64 KMRX 181139
AFDMRX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MORRISTOWN TN
639 AM EST WED DEC 18 2024
..NEW AVIATION
SHORT TERM
(TODAY AND TONIGHT)
ISSUED AT 319 AM EST WED DEC 18 2024
KEY MESSAGES:
1. LINE OF HEAVY RAIN SHOWERS AND EMBEDDED THUNDERSTORMS EXPECTED TO
CROSS WEST TO EAST TODAY. A VERY LOW RISK OF A SEVERE WIND GUST OR A
TORNADO.
DISCUSSION:
WIDESPREAD FOG AND LOW CLOUDS THIS MORNING ARE EXPECTED TO IMPROVE
AS SUNLIGHT ARRIVES THIS MORNING, THOUGH HEAVY CLOUD COVER WILL
PERSIST UP TO THE ARRIVAL OF THE MAIN WEATHER EVENT. A DIFFUSE
WARM FRONT FROM THE OVERNIGHT IS NORTH OF TENNESSEE TODAY,
ALLOWING FOR STRONGER SURFACE WINDS AND BETTER MIXING AT THE
SURFACE AND HELPING TO LIFT THE CLOUDS.
A MIDLATITUDE UPPER TROUGH IS CURRENTLY LOCATED OVER THE GREAT
PLAINS AND WILL PUSH ALL THE WAY THROUGH TENNESSEE BY THURSDAY
MORNING. AHEAD OF THE TROUGH AXIS, A COLD FRONT WILL BE BRINGING A
MORE SEASONABLE AIRMASS DOWN INTO THE MID-SOUTH. A LENGTHY LINE
OF HEAVY RAIN SHOWERS AND A FEW EMBEDDED THUNDERSTORMS ARE
EXPECTED TO BE PACING AHEAD OF THE FRONT, AND THAT WILL BE THE
MAIN FOCUS FOR TODAY. THE RISK LEVEL FROM SPC IS A MARGINAL RISK
AND THAT SEEMS TO BE A FAIR ASSESSMENT. THIS IS A HIGH SHEAR LOW
CAPE SETUP, WITH MEAN CAPE FROM HREF AROUND 300J SURFACE BASED.
EFFECTIVE SHEAR AROUND 40 KNOTS, AND ENOUGH TURNING IN THE 0-1 KM
LAYER THAT CAN'T BE IMMEDIATELY DISMISSED. GFS DEPICTS THE
MODERATE DIVERGENCE AND OTHER DYNAMICS WEAKENING AS THE LINE
APPROACHES OUR AREA, AND THOSE FACTORS, COMBINED WITH THE HEAVY
CLOUD COVER EXPECTED TODAY, HELP EXPLAIN WHY THIS IS A MARGINAL (1
OF 5) RISK EVENT.
LEADING EDGE OF STORMS HAS BEEN CONSISTENTLY DEPICTED BY
AVAILABLE CAM GUIDANCE AS FALLING APART AS IT MOVES FROM WEST TO
EAST ACROSS EAST TENNESSEE THROUGH THE AFTERNOON, AND ONCE THE
INITIAL EDGE CLEARS A LOCATION, THE SEVERE RISK IS OVER AND THE
REGULAR RAIN SHOWERS TAKE OVER THEREAFTER. TODAY IS LIKELY ONE OF
THOSE DAYS WHERE ODDS ARE NOTHING SEVERE HAPPENS, BUT IF SOMETHING
WERE TO HAPPEN, THE ENVIRONMENT AT LEAST HAS THE EXPLANATION
ALREADY MODELED. MORE LIKELY ANY CONVECTION WILL HELP PULL DOWN
STRONG WIND GUSTS TO THE SURFACE, CLOSER TO SPS STRENGTH. WITH THE
EXPECTED DETERIORATION OF THE LINE AS THE DAY PROGRESSES, THE BEST
RISK FOR A SPIN-UP TORNADO IS THEN THE PLATEAU, WHERE STORMS ARE
MOST LIKELY TO BE STRONG ENOUGH TO SUPPORT A HAZARD. EVERYTHING
SHOULD BE OUT OF HERE BY TONIGHT, AND THE LOW CLOUDS AND
ADDITIONAL PATCHY FOG WILL STICK AROUND OVERNIGHT.
LONG TERM
(THURSDAY THROUGH TUESDAY)
ISSUED AT 319 AM EST WED DEC 18 2024
KEY MESSAGES:
1. TURNING COLDER FOR THURSDAY THROUGH THE WEEKEND.
2. LIGHT SNOW ACCUMULATIONS POSSIBLE OVER THE MOUNTAINS OF E TN AND
SW VA FRIDAY INTO SATURDAY.
3. DRY SUNDAY THROUGH TUESDAY, WITH A GRADUAL WARMING TREND FOR
MONDAY AND TUESDAY.
DISCUSSION:
WE START THE PERIOD WITH A BROAD UPPER TROUGH OVER THE EASTERN CONUS
AND SURFACE HIGH PRESSURE EXTENDING INTO OUR AREA FROM THE NORTH.
THURSDAY WILL BE DRY, AND WHILE IT WILL BE COLDER THAN TODAY WE
WILL STILL SEE HIGH TEMPERATURES CLOSE TO SEASONAL NORMALS.
A SIGNIFICANT SHORT WAVE WILL DIVE SOUTHEAST THROUGH THE BROADER
UPPER TROUGH FRIDAY AND A STRONGER COLD FRONT WILL MOVE THROUGH OUR
AREA. MOISTURE LOOKS LIMITED, BUT NORTHERN AREAS WILL LIKELY SEE
SOME SHOWERS/SNOW SHOWERS AS THIS SYSTEM MOVES THROUGH. SNOW SHOWERS
AND FLURRIES WILL LINGER IN THE NORTHWEST FLOW AND COLD ADVECTION
FRIDAY NIGHT INTO SATURDAY MORNING ESPECIALLY NORTH AND MOUNTAINS.
SOME LIGHT SNOW ACCUMULATIONS WILL BE POSSIBLE, MAINLY OVER THE
HIGHER ELEVATIONS OF SW VA AND THE E TN MOUNTAINS WHERE OROGRAPHIC
FORCING WILL ENHANCE LIFT. WILL INCLUDE MENTION OF THIS IN THE HWO.
SURFACE HIGH PRESSURE TO OUR NORTH SATURDAY NIGHT WILL MOVE EAST AND
WILL EXTEND INTO OUR AREA THROUGH MONDAY. SUNDAY WILL CONTINUE TO BE
COLD WITH WELL BELOW NORMAL TEMPERATURES, THEN A GRADUAL WARMING
TREND WILL START MONDAY AND TEMPERATURES BY TUESDAY WILL BE BACK TO
NEAR SEASONAL NORMALS. THE NEXT SYSTEM WILL BE APPROACHING BY THE
END OF THE LONG TERM PERIOD, BUT RIGHT NOW IT APPEARS ANY
PRECIPITATION WILL HOLD OFF UNTIL AFTER THE DAY TUESDAY.
AVIATION
(12Z TAFS)
ISSUED AT 631 AM EST WED DEC 18 2024
FOG AND IFR CEILINGS SHOULD RAPIDLY IMPROVE BY OR AROUND 14Z AS
WARM FRONT FEATURE LIFTS NORTH OUT OF TN. OTHERWISE MVFR CIGS ARE
EXPECTED TO DOMINATE TODAY, WITH BRIEF REDUCTIONS TO IFR
CONDITIONS DURING PASSAGE OF RAIN A FEW EMBEDDED TS THIS
AFTERNOON IN A SOMEWHAT ORGANIZED LINE. GUSTY WINDS TO 20 KNOTS
AHEAD OF AND DURING LINE PASSAGE. PERSISTED MVFR AFTER 00Z BUT
WITH LOW CONFIDENCE.
PRELIMINARY POINT TEMPS/POPS
CHATTANOOGA AIRPORT, TN 70 39 54 36 / 90 40 0 0
KNOXVILLE MCGHEE TYSON AIRPORT, TN 68 38 51 36 / 90 60 0 0
OAK RIDGE, TN 65 36 51 33 / 100 40 0 0
TRI CITIES AIRPORT, TN 65 36 50 35 / 90 70 0 10
MRX WATCHES/WARNINGS/ADVISORIES
NC...DENSE FOG ADVISORY UNTIL 9 AM EST THIS MORNING FOR CHEROKEE-CLAY.
TN...DENSE FOG ADVISORY UNTIL 9 AM EST /8 AM CST/ THIS MORNING FOR
ANDERSON-BLEDSOE-BLOUNT SMOKY MOUNTAINS-BRADLEY-CAMPBELL-
CLAIBORNE-COCKE SMOKY MOUNTAINS-EAST POLK-GRAINGER-HAMBLEN-
HAMILTON-HANCOCK-HAWKINS-JEFFERSON-KNOX-LOUDON-MARION-
MCMINN-MEIGS-NORTH SEVIER-NORTHWEST BLOUNT-NORTHWEST CARTER-
NORTHWEST COCKE-NORTHWEST GREENE-NORTHWEST MONROE-RHEA-
ROANE-SEQUATCHIE-SEVIER SMOKY MOUNTAINS-SOUTHEAST GREENE-
SOUTHEAST MONROE-SULLIVAN-UNICOI-UNION-WASHINGTON TN-WEST
POLK.
VA...DENSE FOG ADVISORY UNTIL 9 AM EST THIS MORNING FOR LEE-RUSSELL-
SCOTT VA-WASHINGTON VA.
SHORT TERM...WELLINGTON
AVIATION...WELLINGTON
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