Short Term Discussion

By: NWS
By: NWS

WELL THIS HAS BEEN A VERY COMPLEX AND DIFFICULT FORECAST TO EXAMINE...AND CERTAINLY ONE YOU DON`T SEE OFTEN IN NORTH FLORIDA. FOR THE PURPOSE OF THIS DISCUSSION...WILL PLACE THE MAIN EMPHASIS ON THE APPROACHING STORM SYSTEM. I WILL ONLY SAY THAT IT WILL BE COLD TONIGHT WITH TEMPS IN THE UPPER 20S AND HIGHS ON THURSDAY WILL BE STILL BELOW NORMAL...BUT MAINLY IN THE UPPER 40S TO LOW 50S. NOW...FOR THE WINTRY TYPE PRECIPITATION. THE MODELS CONTINUE TO BE IN AGREEMENT THAT AN AREA OF LOW PRESSURE WILL DEVELOP ACROSS THE WESTERN GULF OF MEXICO ON THURSDAY AFTERNOON NEAR THE TEXAS GULF COAST IN RESPONSE TO A MID LEVEL LOW. THIS LOW IS EXPECTED TO BE RELATIVELY WEAK...1006 TO 1008 MB RANGE...AND MOVE RAPIDLY EASTWARD. THE MID LEVEL FEATURE HAS BEEN PROGGED IN SEVERAL OF THE MODELS TO DAMPEN OUT WITH TIME AND DEVOLVE INTO A WEAK SHORTWAVE AS IT NEARS OUR REGION. THE ACTUAL STRENGTH OF THIS SHORTWAVE IS IMPORTANT TO THE FORECAST AS A STRONGER SHORTWAVE WILL TRANSLATE TO COLDER MID LEVEL TEMPS...BUT WILL ALSO TRANSLATE TO WARM THERMAL ADVECTION WITH A STRONGER SURFACE LOW...THUS NEGATING THE BENEFIT OF ANY DYNAMIC COOLING ALOFT. CLIMATOLOGY HAS SHOWN IN OUR REGION THAT THE BEST SNOW EVENTS SEEM TO TAKE PLACE WHEN A WEAK SURFACE WAVE (ON MANY OCCASIONS NOT EVEN A CLOSED LOW) MOVES ACROSS THE GULF OF MEXICO WITH A COLD AIRMASS ALREADY IN PLACE. THERE MAY WITH THESE SNOW MAKING SYSTEMS BE VERY LITTLE INFLUENCE FROM MID LEVEL SHORTWAVES...THUS LIMITING THE OVERALL IMPACT OF THERMAL ADVECTION IN THE 850-700 MB LAYER. WE NOW HAVE THE BENEFIT OF FOUR MODELS TO EXAMINE...THE GFS/NAM/EURO/CANADIAN. THE CANADIAN HAS ALWAYS BEEN THE COLDER SOLUTION WITH THIS SYSTEM...YET INDICATING A STRONGER SURFACE LOW. THIS WOULD SEEM TO BE COUNTERINTUITIVE BASED ON CLIMATOLOGY OF OUR REGION. THE EURO IS A GOOD COMPROMISE BETWEEN THE GFS AND CANADIAN...WHICH SHOWS SOME COOLER AIR TRYING TO REMAIN IN PLACE...BUT LESS THAN FAVORABLE PARTIAL THICKNESSES ON FRIDAY. UTILIZING MODEL SOUNDINGS FROM A TOP DOWN APPROACH SHOWS THAT THE THERMAL PROFILES WILL INITIALLY START OUT WITH WARM MELTING LAYERS NEAR THE SURFACE...UP TO ABOUT 900 MB ON THURSDAY NIGHT. THROUGH EVAPORATIVE COOLING (DEWPOINT DEPRESSIONS IN THE 925-850 MB LAYER APPROACH 20 DEGREES)...WE SHOULD BE ABLE TO SEE SOME HYDROMETEORS SURVIVE THE TRANSIT TO THE SURFACE AS SNOW. SLEET IS NOT POSSIBLE BECAUSE THERE IS NO RE-FREEZING LEVEL. THE PRECIP WILL EITHER BE A RAIN...SNOW...OR A RAIN SNOW MIX ON THURSDAY NIGHT/FRIDAY MORNING. BY FRIDAY MORNING...ENOUGH PRECIP IS LIKELY TO HAVE OCCURRED TO DIMINISH ANY ADDITIONAL EVAPORATIVE COOLING EFFECTS. THUS...ANY ADDITIONAL COOLING (OR HEATING) WILL TAKE PLACE THROUGH DYNAMIC EFFECTS. ALL THE GLOBAL MODELS SEEM TO IMPLY INCREASING TEMPERATURES IN THE 850-700 MB LAYER IN AN AREA OF WARM ADVECTION AHEAD OF THE LOW FROM APALACHICOLA...TO TALLAHASSEE TO ALBANY. IN THE AREA ALONG AND EAST OF THIS LINE...HYDROMETEORS ARE EXPECTED TO ENCOUNTER ENOUGH OF A MELTING LAYER TO MAKE IT TO THE SURFACE MAINLY AS RAIN...THOUGH SOME SNOW MIXING IN CANNOT BE RULED OUT AT THIS TIME ACROSS OUR GEORGIA COUNTIES...ESPECIALLY ALONG AND NORTH OF US-82. FURTHER BACK TO THE WEST...THE PREDOMINANT PRECIP TYPE MAY BECOME SNOW BY LATE MORNING AND EARLY AFTERNOON AS THE EURO AND GFS SOUNDINGS ARE SHOWING THERMAL PROFILES THROUGH A DEEP LAYER AT OR JUST BELOW FREEZING. EVEN THOUGH THE MODELS ARE SHOWING A NEARLY ISOTHERMAL TO SLIGHTLY COOL PROFILE FROM THE CLOUD LAYER DOWN TO ABOUT 500 FT ABOVE THE SURFACE...CONDITIONS NEAR THE SURFACE WILL NOT BE CONDUCIVE ENOUGH TO SUPPORT ACCUMULATIONS ON ALL SURFACES. THUS...WE FEEL BEST AT THIS TIME INDICATING ACCUMULATIONS ON GRASSY AND ELEVATED SURFACES. SINCE THE LIKELIHOOD OF ACCUMULATIONS IS GREATEST WHERE THE COLDER AIR IS LOCATED...WE HAVE...AFTER COORDINATION WITH NEIGHBORING OFFICES...OPTED TO ISSUE A WINTER STORM WATCH FOR OUR SOUTHEAST ALABAMA COUNTIES AND A FEW COUNTIES IN SOUTHWEST GEORGIA. IT IS IMPORTANT TO NOTE THAT IN THIS AREA WE ARE INDICATING SNOWFALL AMOUNTS FROM A DUSTING TO ONE INCH POSSIBLE. FUTURE FORECASTS MAY REQUIRE THIS TOTAL TO BE ADJUSTED ONE WAY OR THE OTHER...THUS EVERYONE IS ADVISED THAT THIS IS SIMPLY A WATCH THAT IMPLIES ACCUMULATING SNOW IS POSSIBLE...BUT NOT LIKELY...AT THIS TIME. ACROSS THE CENTRAL AND EASTERN PART OF THE REGION...HAVE ISSUED AN SPS TO HIGHLIGHT THE POTENTIAL FOR A RAIN/SNOW MIX ON FRIDAY...BUT NOT EXPECTING ANY ACCUMULATIONS IN THIS AREA. AGAIN...LOOKING AT THE FORECAST SITUATION...THE OVERALL CONSENSUS OF THE MODELS IS FOR SNOW...OCCASIONALLY MIXING WITH RAIN ACROSS THE WINTER STORM WATCH AREA WITH ACCUMULATIONS UP TO 1 INCH POSSIBLE...MAINLY ON GRASSY AND ELEVATED SURFACES. FURTHER EAST...INTO THE ALBANY/VALDOSTA AREA...RAIN WILL BE MOST LIKELY...OCCASIONALLY MIXING WITH SNOW WITH NO ACCUMULATIONS. AREAS IN TALLAHASSEE...PANAMA CITY AND OTHER COASTAL LOCALES CAN EXPECT GENERALLY RAIN WITH AN OUTSIDE SHOT AT A FEW FLAKES...BUT NO ACCUMULATIONS. PRECIP WILL BE COMING TO AN END ON FRIDAY NIGHT WITH COOLER AIR LIKELY FOR THE WEEKEND.


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