356  
FXUS63 KIWX 221735  
AFDIWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE NORTHERN INDIANA  
1235 PM EST MON DEC 22 2014  
   
SYNOPSIS
 
 
ISSUED AT 1127 AM EST MON DEC 22 2014  
 
LOW PRESSURE MOVING THROUGH THE NORTH CENTRAL UNITED STATES WILL  
BE GIVING THE REGION SOME RAIN LATER TODAY THROUGH TONIGHT. COULD  
HAVE A BREAK IN THE RAIN TOMORROW BEFORE A LOW MOVES UP FROM THE  
SOUTH. THIS SECOND LOW IS EXPECTED TO BRING FIRST RAIN AND THEN  
SNOW ON WEDNESDAY...WITH SNOW CHANCES CONTINUING INTO WEDNESDAY  
NIGHT. HIGH TEMPERATURES TODAY WILL RANGE FROM AROUND 40 IN  
SOUTHERN MICHIGAN TO THE LOWER 50S IN SOUTHEASTERN INDIANA. LOWS  
TONIGHT WILL RANGE FROM THE UPPER 30S IN SOUTHERN MICHIGAN TO THE  
LOWER 40S NEAR THE OHIO RIVER.  
 

 
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 400 AM EST MON DEC 22 2014  
 
AN IMPRESSIVE 170 KT EAST PACIFIC UPPER JET NOSING INTO THE PAC  
NW/ROCKIES WILL CONTINUE TO HELP AMPLIFY A DOWNSTREAM CENTRAL CONUS  
LONGWAVE TROUGH TODAY INTO TONIGHT. 00Z MODELS IN GOOD AGREEMENT IN  
THIS JET ENERGY CARVING OUT A DEEP CLOSED CIRCULATION INVOF NRN  
MO/IA BY TONIGHT. WARM/MOIST CONVEYOR BELT IN DEEPENING SSW FLOW  
AND INCREASING DIFFLUENCE ALOFT EAST OF ASSOCIATED HEIGHT FALLS  
WILL WORK INTO THE AREA LATER TODAY-TONIGHT BRINGING GOOD CHANCES  
FOR RAIN ALONG AND IN ADVANCE OF AN OCCLUDED FRONT. INCREASING  
POPS FOR LIGHT RAIN (HIGHEST WEST OF I-69) THIS AFTERNOON/EVE WILL  
BE TIED TO THE INITIAL WARM ADVECTION/ISENTROPIC SURGE (3-4 G/KG  
PUSH WITHIN 30-40 KTS OF CROSS ISOBARIC FLOW ON 290-295K SFC'S).  
LOW WET BULB ZERO HEIGHTS A BIT CONCERNING FOR A BRIEF PERIOD OF  
LIGHT FREEZING RAIN AT ONSET OF PCPN NORTHERN HALF OF FA (MAINLY  
NORTHEAST ZONES...OR POSSIBLY SOME LIGHT FZDZ WESTERN ZONES LATER  
THIS MORNING. HOWEVER...PROBABILITY OF OCCURRENCE REMAINS TOO LOW  
TO INCLUDE IN THE FCST WITH BULK OF GUIDANCE QUICKLY WARMING TEMPS  
ABOVE FREEZING. DEEP UVM/MOISTURE PLUME WITH PWATS NEARING 0.80  
INCHES WITHIN THETA-E RIDGE OVERSPREADS THE LOCAL FROM SW TO NE  
OVERNIGHT. A 3-6 HOUR PERIOD OF MORE MODERATE RAIN SHOWER ACTIVITY  
WILL RESULT WITH CATEGORICAL POPS WARRANTED.  
 

 
   
LONG TERM
 
(TUESDAY THROUGH SUNDAY)  
ISSUED AT 400 AM EST MON DEC 22 2014  
 
NOT REALLY A SURPRISE THAT MODELS HAVE COME BACK TO A VERSION OF  
THEIR PREVIOUS SOLUTIONS WITH REGARDS TO THE UPCOMING WINTER STORM  
THIS WEEK. WHILE UNCERTAINTY CONTINUES IT IS LIKELY THAT WITH THE  
INITIAL SAMPLING OF THE JET ENERGY OVER THE PACIFIC NW BY RAOB  
NETWORK MAY FINALLY BE HELPING MODELS HONE IN ON EXPECTED LARGE  
SCALE EVOLUTION. RATHER GOOD LARGE SCALE MODEL AGREEMENT NOW WITH  
CURRENT RUN AND FOR MOST PART PREVIOUS RUN. HOWEVER...THIS IS BY FAR  
A LOCK WITH NUMEROUS QUESTIONS REMAINING REGARDING CRITICAL THERMAL  
DETAILS AND IMPORTANT EXACT TRACK OF ACTUAL LOW PRESSURE CENTERS.  
 
LONG TERM PERIOD BEGINS TUESDAY WITH QUESTIONS DEVELOPING WITH  
RESPECT TO HOW MUCH MEASURABLE PCPN WILL BE AROUND BETWEEN SYSTEMS.  
LATEST GFS SHOWING A RATHER PRONOUNCED DRY SLOT INVADING THE AREA  
EARLY BEHIND DEPARTING FIRST WAVE OF MOISTURE AS INITIAL LOW  
OCCLUDES AND BEGINS TO FILL. 00Z ECMWF HINTING AT THIS AS WELL BUT  
DOES NOT APPEAR TO BE AS PRONOUNCED. ABUNDANT LOW LEVEL RESIDUAL  
MOISTURE WILL BE LEFT ACROSS THE AREA AND NUMEROUS SUBTLE SHORT  
WAVES AND VORTICITY CENTERS BEING SHOWN ALONG WITH PRONOUNCED DEEP  
SOUTHERLY FLOW. EXPECT THIS TO LOCK LOW CLOUDS IN WITH EITHER  
DRIZZLE OR LIGHT SHOWERS POSSIBLE THROUGH THE DAY. THUS WILL LEAVE  
RATHER HIGH POPS IN FORECAST FOR NOW CLOSER TO ECE GUIDANCE BUT  
ACTUAL PCPN AMOUNTS WILL LIKELY BE ON THE LIGHTER SIDE TUE INTO TUE  
NIGHT.  
 
THE STRONG PACIFIC JET ENERGY WILL DIVE SOUTH AND CARVE OUT A LARGE  
SCALE TROUGH OVER THE CENTRAL CONUS BY TUESDAY. THIS WILL LEAD TO  
CYCLOGENESIS NEAR THE GULF COAST OR EVEN OVER THE GOMEX TUESDAY.  
THIS LOW WILL DEEPEN AS IT MOVES ALMOST DUE NORTH INTO THE OHIO  
VALLEY AND UPPER GREAT LAKES ON WEDNESDAY. THIS WILL SPREAD A GOOD  
DEAL OF RAIN BACK ACROSS THE AREA WITH AMOUNTS UPWARDS OF AN INCH  
POSSIBLE WEDNESDAY. THE CRITICAL EXACT TRACKS OF THE MID AND LOW  
LEVEL LOW PRESSURE CENTERS REMAIN IN QUESTION AS 850-500MB LOWS  
CLOSE OFF. WHILE SPREAD IN 00Z MODELS HAS NARROWED  
CONSIDERABLY...MINUS THE WEAK NAM12 OUTLIER...THE SPREAD IS STILL  
LARGE ENOUGH TO KEEP UNCERTAINTY HIGH WITH RESPECT TO TIMING AND  
LOCATION OF CHANGEOVER FROM RAIN TO SNOW AND RESULTANT AMOUNTS.  
 
PREFERENCE STILL LIES WITH THE WESTERN MOST SOLUTIONS AS WE HAVE  
STATED PREVIOUSLY TENDS TO VERIFY BEST WITH STRONG DYNAMICAL AND  
NEGATIVELY TILTED WINTER STORMS. THE PARALLEL GFS AND GEFS MEANS ARE  
SIMILAR IN THIS IDEA...TAKING SFC LOW FROM NEAR SDF TO KFWA WED.  
HOWEVER...THE DEGREE OF NEGATIVE TILT THIS SYSTEM TAKES AND THE  
TIMING OF THE PHASING STILL VARY AMONGST THE MODELS AND WILL  
ULTIMATELY DECIDE THE TRACK. REGARDLESS OF THE VARIOUS  
TRACKS...CURRENT INDICATIONS STILL SUPPORT A LATER CHANGEOVER TO  
SNOW WEDNESDAY AFTERNOON. WITH A VERY WET AND RELATIVELY WARM GROUND  
FROM RAIN IT WILL BE DIFFICULT FOR SNOW TO ACCUMULATE INITIALLY. THE  
CAVEAT HERE IS THE DEGREE OF DYNAMICAL FORCING AND COOLING NEAR THE  
TRACK OF THE 700MB LOW AND STRENGTH OF DEFORMATION ZONE ON THE WEST  
SIDE OF THE LOW TRACK. EXPECT WITH THIS RAPIDLY INTENSIFYING SYSTEM  
THAT WE WILL SEE AN IMPRESSIVE TROWAL TAKE SHAPE. THERMAL PROFILES  
TAKEN VERBATIM ON CURRENT RUNS WOULD SUPPORT MAINLY RAIN WITH A WET  
SNOW MIX WED AFTERNOON CHANGING OVER TO ALL SNOW IN THE EVENING  
BEFORE ENDING OVERNIGHT.  
 
EXPECTED ACCUMULATIONS REMAIN A LONG SHOT AND STILL HIGHLY  
UNCERTAIN. HOWEVER...WE ARE NEARING THE TIME PERIOD FOR INCLUSION  
WITHIN GRIDS SO BEST ESTIMATES AT THIS TIME SUPPORT A SWATH OF 1 TO  
3 INCHES ON BACK SIDE. AGAIN...VERY EARLY AND LOW CONFIDENCE  
PREDICTION. THOSE WITH TRAVEL PLANS OR CONCERNS SHOULD CONTINUE TO  
MONITOR UPDATED FORECASTS THROUGH THE EARLY PART OF THE WEEK LEADING  
UP TO THE STORM.  
 
SEVERAL CLIPPER SYSTEMS TO FOLLOW FOR LATE WEEK INTO NEXT WEEKEND.  
MODEL CHAOS WITH INITIAL BIG SYSTEM LEADS TO LITTLE PREDICTABILITY AND  
CONFIDENCE IN LATER PERIODS WITH THESE FAST MOVING SYSTEMS. IT DOES  
APPEAR A RETURN TO MORE WINTER LIKE TEMPERATURES IS ON THE HORIZON  
FOR THE WEEK 2 PERIOD WITH ARCTIC AIR POISED TO MOVE SOUTH.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)  
ISSUED AT 1212 PM EST MON DEC 22 2014  
 
WARM FRONT OVER THE OHIO VALLEY WILL LIFT SLOWLY N-NE CAUSING  
DETERIORATING AVIATION CONDITIONS AS RAIN OVERSPREADS THE AREA  
THIS AFTN/EVE... CONTG TONIGHT... THEN TRANSITIONING TO DZ TUE  
MORNING AS THE FRONT MOVES NORTH OF THE AREA. CURRENT VFR  
CONDITIONS ACROSS THE AREA SHOULD LOWER TO IFR THIS EVE WITH LIFR  
PSBL LATE TONIGHT AND TUE MORNING.  
 

 
   
IWX WATCHES/WARNINGS/ADVISORIES
 
 
IN...NONE.  
MI...NONE.  
OH...NONE.  
LM...NONE.  

 
 

 
 
SYNOPSIS...LOTHAMER  
SHORT TERM...STEINWEDEL  
LONG TERM...LASHLEY  
AVIATION...JT  
 
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