Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 230923
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
323 AM CST TUE DEC 23 2014

.SHORT TERM...

TODAY...

(FORTHCOMING)



TONIGHT THROUGH WEDNESDAY NIGHT...
323 AM CST

HIGH IMPACT WINTER STORM WITH HEAVY WET SNOW POSSIBLE CHRISTMAS EVE.

GENERALLY SPEAKING...WE ARE IN A MUCH BETTER PLACE WITH RESPECT TO
MODEL AGREEMENT THIS MORNING THAN WE WERE LAST NIGHT OR EVEN 12
HOURS AGO. HOWEVER...THERE REMAIN SOME DIFFERENCES WITH RESPECT TO
THE EXACT TRACK AND SPEED/TIMING OF THE SYSTEM. OTHER FORECAST
BUGABOO IS THE MARGINAL LOW LEVEL THERMAL PROFILES DURING THE HEART
OF THE EVENT.

WHILE THERE ARE STILL DIFFERENCES...ONE THING THE COLLECTION OF
MODEL GUIDANCE HAS NOW COME AROUND TO IS THE IDEA OF AN EXPLOSIVELY
DEEPENING CYCLONE DEVELOPING TRACKING NORTH FROM THE GULF COAST
TODAY TO LOWER MICHIGAN BY CHRISTMAS EVE NIGHT. EVENING UPPER AIR
ANALYSIS DEPICTED 170KT 250MB JET SAMPLED BY 00Z RAOB AT SALT LAKE
CITY...WITH THIS JET DIVING DOWN THE BACK SIDE OF THE LONG WAVE
TROUGH AND PROGGED TO SHARPEN/AMPLIFY SOUTHERN FLANK OF THE TROUGH
OVER THE SOUTHERN PLAINS TODAY. THE UPPER JET EMERGES FROM THE BASE
OF THE TROUGH WEDNESDAY GUIDANCE DEPICTS A RAPIDLY DEEPENING CYCLONE
WITH CIRCULATION PROGGED TO CLOSE OFF ALL THE WAY UP TO 300MB.

THE RAPID DEEPENING AND QUICK MOVEMENT OF THE MID/UPPER LEVEL LOW
WILL LIKELY RESULT IN EXTREMELY IMPRESSIVE LARGE SCALE FORCING WITH
AN IMPRESSIVE TROWAL/DEFORMATION ZONE LIKELY TO DEVELOP ON THE
WESTERN FLANK OF THE CYCLONE. VERY STRONG LOW/MID LEVEL
FRONTOGENESIS WILL OCCUR WITHIN THE TROWAL AND COINCIDENT WITH THE
RAPIDLY STRENGTHENING CYCLONE WILL PROMOTE STRONG VERTICAL
CIRCULATION. GUIDANCE ALSO SUGGESTS WEAKLY STABLE TO UNSTABLE
CONDITIONS ALOFT WITHIN THIS TROWAL WITH NEGATIVE EPV AND STEEP
MID-HIGH LEVEL LAPSE RATES...WHICH GIVEN THE STRONG FRONTOGENESIS
WOULD TEND TO FAVOR A NARROWER BUT INTENSE PRECIPITATION BAND.
GFS/WRF-NAM BOTH INDICATE THE PRESENCE OF SOME WEAK UPRIGHT
INSTABILITY WHICH GIVEN THE FORCING COULD ALLOW FOR UPRIGHT
CONVECTION WITHIN THE DEFO BAND AND POSSIBLY A COUPLE LIGHTNING
STRIKES.

WHILE GUIDANCE IS IN PRETTY GOOD AGREEMENT ON THE ABOVE...THERE ARE
A COUPLE OF KEY AREAS OF LINGERING UNCERTAINTY. FIRST...MODEL
BOUNDARY LAYER THERMAL PROFILES ARE FORECAST TO BE QUITE MARGINAL
FOR SNOW AND EVEN MORE MARGINAL FOR ACCUMULATING SNOW. DESPITE THE
MARGINAL THERMAL PROFILES...AM FAIRLY CONFIDENT THAT DYNAMIC COOLING
WITHIN THE DEFO BAND WILL LIKELY EASILY COOL THE BOUNDARY LAYER
SUFFICIENTLY TO SUPPORT ACCUMULATING SNOW...PARTICULARLY WHERE THE
HEAVIEST PRECIP OCCURS. THE 02/07/13 SYSTEM IS A PRIME EXAMPLE OF A
CASE OF VERY STRONG DYNAMIC COOLING THAT WAS VERY POORLY HANDLED BY
THE MODELS AND RESULTED IN NEARLY 10 INCHES OF SNOW WHERE MODELS
GUIDANCE FORECAST SFC TEMPS IN THE MID TO UPPER 30S.

THE BIGGER UNCERTAINTY AND WHAT IS KEEPING ME FROM JUMPING OUT WITH
A WINTER STORM WATCH AT THIS TIME IS THE TRACK DIFFERENCES IN THE
MODELS. WHILE THEY ARE NOT HUGE AND NOT SURPRISING GIVEN THE RAPIDLY
DEEPENING NATURE OF THE SYSTEM...THEY ARE SUBSTANTIAL ENOUGH TO MAKE
IT TOUGH TO PINPOINT THE PORTION OF OUR CWA MOST AT RISK. GIVEN
COMBINATION OF STRONG FRONTOGENESIS...INSTABILITY...AND MARGINAL
THERMAL PROFILES IT SEEMS LIKELY THE AREAL EXTENT OF HEAVY SNOW
WOULD NOT BE LARGE WITH POTENTIALLY SHARP CUT OFF ON BOTH THE
WESTERN AND EASTERN FLANKS OF THE SNOW BAND...SIMILAR TO WHAT WAS
SEEN IN THE 12/01/2006 SYSTEM WHICH IS ONE OF THE STRONGEST ANALOGS
FOR THIS SYSTEM AND PRODUCED LOCALLY OVER A FOOT OF SNOW.

THE CURRENT SNOWFALL FORECAST MAP WAS CREATED BLENDING VARYING
MODEL TRACK SOLUTIONS TOGETHER WHICH YIELD SNOWFALL AMOUNTS THAT
WILL LIKELY BE TOO LOW WHERE THE HEAVIEST SNOW FALLS AND TOO HIGH
OUTSIDE OF THAT BAND...BUT LIKELY THE BEST COMPROMISE THAT CAN BE
DONE FROM THIS DISTANCE GIVEN WHAT LOOKS TO BE A MESOSCALE HEAVY
SNOW THAT WILL SET UP. WHERE THE HEAVY SNOW BAND SETS UP...THINK IT
IS PRETTY LIKELY THAT VSBYS OF 1/4SM OR LESS WITH HEAVY WET SNOW ARE
LIKELY WITH SNOWFALL RATES >/= 2 INCHES PER HOUR POSSIBLE. THE PEAK
INTENSITY OF THE SNOW LOOKS TO COINCIDE WITH THE DAYTIME HOURS WHEN
MANY PEOPLE WILL LIKELY BE TRAVELING...SO ASSUMING THE BAND SETS UP
AS EXPECTED...IT COULD MAKE TRAVEL EXTREMELY DIFFICULT TOMORROW
AFTERNOON. SOME MODELS ARE AS FAR WEST AS NW IL INTO RFD (PARALLEL
GFS AND WRF-NMM) RANGING TO AS FAR EAST AS NORTHWEST INDIANA (00Z
ECMWF). WOULD TEND TO FAVOR THE MORE WESTERN SOLUTIONS AS RAPIDLY
DEEPENING CYCLONES TYPICALLY TEND TO END UP FARTHER WEST THAN
PROGGED WHICH COULD FAVOR CHICAGO AREA OR POINTS JUST WEST FOR THE
HEAVY SNOW THREAT. BUT WITH A STILL RELATIVELY LARGE SPREAD IN TRACK
SOLUTIONS WOULD BE DIFFICULT TO PIN POINT A SPECIFIC AREA FOR A
WINTER STORM WATCH WHEN IN REALITY THE HEAVY (6"+) SNOW AMOUNTS
COULD BE CONFINED TO <25MI WIDE SWATH. AFTER COORDINATION
INTERNALLY...WITH NEIGHBORING WFOS...AND WPC HAVE OPTED TO JUST HIT
THE THREAT HARD IN UPDATED SPS AND HOPEFULLY MORNING MODEL RUNS WILL
NARROW DOWN THE THREAT AREA ENOUGH TO ALLOW HEADLINES TO BE ISSUED
BY MIDDAY.

IZZI

&&

.LONG TERM...
321 AM CST

THURSDAY THROUGH MONDAY...

(FORTHCOMING)


&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* PRIMARILY IFR CIGS OVERNIGHT...PROBABLY LIFTING TO MVFR OR
  POSSIBLY VFR DURING THE DAY.
* PERIODS OF LIGHT RAIN/DRIZZLE ENDING WITHIN THE NEXT 1 TO 2
  HOURS.

KJB

//DISCUSSION...UPDATED 06Z...

A SURFACE LOW  ACROSS IOWA WILL DRIVE A FRONTAL BOUNDARY NORTHWARD
ACROSS NORTHERN ILLINOIS BY DAYBREAK. ONCE THIS FRONT PASSES...THE
RAIN WILL COME TO AN END. IN FACT...CURRENT REGIONAL RADAR IMAGERY
ALREADY SHOWS THE LAST BATCH OF RAINFALL LIFTING NORTHWARD ACROSS
CENTRAL ILLINOIS. CURRENT TIMING PUTS THE BACK EDGE OF THIS RAIN
THROUGH MOST OF THE TERMINALS BY 10 UTC. CIGS LOOK LIKE THEY WILL
REMAIN IN THE 500 TO 700 FOOT RANGE FOR MUCH OF THE NIGHT.
HOWEVER...CIGS DO LOOK TO IMPROVE DURING THE DAY TODAY...POSSIBLY
TO VFR. OTHERWISE...EXPECT WINDS TO SHIFT SOUTH-SOUTHWESTERLY BY
DAY BREAK AS THE AFOREMENTIONED FRONTAL BOUNDARY LIFTS OVER THE
AREA.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* MEDIUM CONFIDENCE IN CIGS.

KJB

//OUTLOOK FOR ORD/MDW FOR 12Z WEDNESDAY-12Z MONDAY...UPDATED 00Z...

WEDNESDAY...RAIN LIKELY...POSSIBLY CHANGING TO HEAVY WET SNOW. IFR
OR LOWER PROBABLE.

THURSDAY...CHANCE OF MVFR CIGS...MAINLY EARLY.

FRIDAY...CHANCE OF MVFR CIGS. STRONG SOUTH WINDS POSSIBLE.

SATURDAY AND SUNDAY...CHANCE OF MVFR CIGS.

IZZI

&&

.MARINE...
217 AM CST

AN INITIAL AREA OF LOW PRESSURE ACROSS SOUTHERN IOWA THIS
MORNING...WILL SHIFT A WARM FRONTAL BOUNDARY FROM NORTH TO SOUTH UP
THE LAKE DURING THE DAY. THEREFORE...SOME STIFF EASTERLIES WILL
SHIFT SOUTHERLY BY LATE IN THE DAY. FOLLOWING THIS...IT STILL
APPEARS LIKELY THAT A SECOND RATHER SIGNIFICANT STORM SYSTEM WILL
RAPIDLY DEVELOP NEAR THE GULF COAST AND RACE NORTHWARD...WHILE
DEEPENING CONSIDERABLE...ACROSS THE OHIO VALLEY AND OVER THE CENTRAL
GREAT LAKES REGION BY CHRISTMAS EVE. DUE TO THE STRENGTH OF THIS
LOW...IT APPEARS THAT A PERIOD OF GALE FORCE WINDS WILL BE A DECENT
POSSIBLY...ESPECIALLY OVER THE OPEN WATERS CHRISTMAS EVE.
THEREFORE...I HAVE ISSUED A GALE WATCH FOR THE OPEN WATERS OF LAKE
MICHIGAN. DUE TO THE FAST MOVEMENT OF THE STORM...WINDS LOOK TO DROP
BELOW GALE FORCE BY CHRISTMAS DAY. STRONG WINDS AND HIGH WAVES ARE
ALSO EXPECTED OVER THE NEAR SHORE WATERS. HOWEVER...I HAVE OPTED
TO LEAVE THE GALE WATCH OUT OF THE NEAR SHORE WATERS AT THIS
TIME...THOUGH SOME GALES CANT BE RULED OUT.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 9 AM TUESDAY.

&&

$$

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