Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 230250
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
850 PM CST MON DEC 22 2014

.UPDATE...
850 PM CST

NO CHANGES PLANNED TO FORECAST FOR TONIGHT AS UPPER LOW TO OUR
WEST CONTINUES TO DUMBBELL SLOWLY NORTHEAST FROM THE CORN BELT
TOWARD WISCONSIN. ASSOCIATED SFC LOW OVER SOUTHERN IOWA WILL MOVE
NORTHEAST IN CONCERT WITH THE UPPER LOW AND DRAG A WARM FRONT
NORTH ACROSS THE CWA OVERNIGHT INTO EARLY TUESDAY MORNING. THE
RESULT DRIZZLE AND FOG OVERNIGHT PUNCTUATED WITH PERIODS OF RAIN
ALONG WITH SLOWLY RISING TEMPS AS THE DIFFUSE WARM FRONT LIFTS
SLOWLY NORTH. TRENDS IN THE GRIDS ALREADY HAVE A GOOD HANDLE ON
THIS AND NO CHANGES ARE PLANNED.

OF MUCH GREATER CONCERN CONTINUES TO BE THE POTENTIAL FOR A WINTER
STORM AFFECTING PORTIONS OF THE REGION WEDNESDAY...CHRISTMAS EVE
DAY. THE 00Z WRF-NAM IS TRICKLING IN AND APPEARS TO HAVE FINALLY
CAVED IN AND JOINED THE CAMP OF WHAT THE REMAINDER OF THE GLOBAL
MODELS HAD SHOWN ON WITH THE 12Z RUNS. LACK OF RUN-TO-RUN
CONSISTENCY AND HIGH DEGREE OF VARIABILITY BETWEEN VARIOUS MODELS
HAS MADE THIS A CHALLENGING SYSTEM TO FORECAST FOR...HOWEVER THE
12Z GLOBAL MODELS CAME INTO MUCH BETTER AGREEMENT AND NOW THE 00Z
WRF-NAM SEEMS TO HAVE JOINED THAT CAMP WHICH COULD POSE A
SIGNIFICANT WINTER STORM THREAT TO A POTENTIALLY LARGE PORTION OF
OUR CWA WEDNESDAY.

THE EXPLOSIVE DEEPENING OF THE SYSTEM PROGGED IN THE MODELS WITH
500MB...ALONG WITH AMPLE MOISTURE...AND ELEVATED INSTABILITY ALL
LEAVE ME VERY CONCERNED THAT WE COULD BE LOOKING AT A VERY HEAVY
AND WET SNOW AFFECTING THE REGION WEDNESDAY. THE NEGATIVE MID-
LEVEL THETA-E LAPSE RATES AND LAPSE RATES GREATER THAN 6C/KM FROM
600-300MB POINT TOWARD THE POTENTIAL FOR UPRIGHT CONVECTION WITHIN
THE DEFORMATION BAND. THERMAL PROFILES IN THE MODELS ARE MARGINAL
FOR SNOW...ESPECIALLY DOWN LOW...HOWEVER GIVEN THE EXTREMELY
IMPRESSIVE DYNAMICS SHOWN IN THE LATEST GUIDANCE WOULD ANTICIPATE
STRONG ASCENT TO OVERWHELM IN MARGINAL WARMTH DOWN LOW AND LIKELY
ALLOW FOR HEAVY WET SNOW IN THE DEFO BAND.

NOT UNCOMMON FOR RAPIDLY DEEPENING SYSTEMS TO TRACK A BIT FARTHER
WEST THAN MODELS FORECAST...WHICH COULD PUSH THE HEAVY SNOW BAND
FARTHER WEST IN OUR CWA WITH MORE OF A MIX EAST. THE STRONG
FRONTOGENETIC FORCING AND WEAK INSTABILITY WOULD FAVOR A RATHER
NARROW BUT INTENSE BAND OF PRECIPITATION...MAKING THE PLACEMENT OF
ANY HEAVY SNOW BAND VERY DIFFICULT TO PINPOINT.

AM RELUCTANT TO MAKE ANY WHOLE SALE CHANGES TO THE 4TH PERIOD OF
THE FORECAST ON AN EVEN SHIFT FOLLOWING A QUICK GLANCE AT THE
LATEST NAM...HOWEVER IF THE 00Z GLOBAL GUIDANCE COMES IN LOOKING
SIMILAR TO THEIR 12Z RUNS AND ANYTHING LIKE THE 00Z WRF-NAM THEN
THE MIDNIGHT SHIFT WILL PROBABLY NEED TO CONSIDER WINTER STORM
WATCH FOR AT LEAST PORTIONS OF THE AREA. WHILE IT LOOKS TO BE A
VERY HIGH WATER CONTENT SNOWFALL...THE CONVECTIVE ALLOWING MODELS
ARE SPITTING OUT QPF VALUES THAT COULD SUPPORT 6+ INCHES OF SNOW
EVEN WITH SNOW:LIQUID RATIOS AS LOW AS 7-8:1. WILL UPDATE THE SPS
AND HIT THINGS HARDER IN THAT PRODUCT FOR NOW.

IZZI

&&

.SHORT TERM...
1227 PM CST

TONIGHT THROUGH CHRISTMAS DAY...
MAIN CHALLENGES FOR TONIGHT WILL BE ON PLACEMENT OF PRECIP ALONG
WITH HOW MUCH RAIN WILL FALL ACROSS THE FORECAST AREA. THE
NORTHERN STREAM SFC LOW HAS BECOME WRAPPED UP ACROSS THE
DAKOTAS...WITH THE MID-LVL TROUGH AXIS STRETCHING SOUTH THROUGH
THE SOUTHERN PLAINS. LLVL FLOW ACROSS THE FORECAST AREA HAS BECOME
MORE EASTERLY...WITH A FEED OF SOME SLIGHTLY DRIER AIR AT THE SFC
INTO NORTHERN IL/NORTHWEST IN. THIS HAS BEEN ABLE TO DELAY THE
ONSET OF STEADY PRECIP UNTIL LATER THIS AFTN. SFC TEMPS HAVE BEEN
SLOW TO WARM...AND LATEST GUIDANCE INDICATES THAT MAX TEMPS MAY
NOT OCCUR UNTIL JUST AFT 00Z AS A STEADY CONVEYOR OF WARMER/MOIST
AIR ADVECTS NORTH.

LOCAL SOLUTIONS BRING THE BETTER SLUG OF PRECIP INTO THE FORECAST
AREA THIS EVENING...THEN SLOWLY LIFT THE STEADIER RAIN NORTH
OVERNIGHT. QPF TOTALS HAVE DECREASED FROM EARLIER SOLUTIONS...BUT
HAVE ONLY MADE MINOR ADJUSTMENTS TO THE GOING FORECAST. THE SFC LOW
OVER THE DAKOTAS WILL BE SLIDING SOUTHEAST TOWARDS SOUTHEAST IOWA
OVERNIGHT...WITH A DRY WEDGE BECOMING ENTRAINED OVER MISSOURI.
GUIDANCE HAS BEEN LEANING TOWARDS POSSIBLY AN EARLIER ARRIVAL OF THE
DRY SLOT INTO THE FORECAST AREA JUST BEFORE DAYBREAK TUE. GIVEN THE
CONTINUED UNCERTAINTIES ON TRACK/PLACEMENT OF THE LOW...HAVE HELD
ONTO SOME LGT RAIN AT DAYBREAK THEN LIFT THE PRECIP SHIELD NORTH OF
INTO WISC BY MID-MORNING TUE. TEMPS OVERNIGHT MON SHUD BE NEARLY
STEADY...THEN AFT MIDNIGHT WITH A 850MB THERMAL RIDGE OF 3-6 DEG C
LIFTING NORTH EXPECT TEMPS TO BE SLOWLY RISING INTO THE LOW 40S.

TUE EXPECT THE SFC LOW TO BE SLOWLY WEAKENING ACROSS SOUTHWEST
WISC...WITH THE MID-LVL TROUGH AXIS EJECTING THE SECONDARY SFC LOW
ALONG THE WESTERN GULF. THIS SECONDARY LOW IS THE MAIN CONCERN FOR
WED AFTN-WED NGT ACROSS OUR REGION. THE DRY WEDGE SHUD KEEP PRECIP
WEST/NORTH OF THE FORECAST AREA THRU TUE MORNING...THEN WRAP AROUND
MOISTURE WILL BRING LGT RAIN BACK INTO THE AREA TUE AFTN/EVE. TEMPS
SHUD WARM INTO THE MID/UPR 40S...POSSIBLY TOUCHING 50 DEGREES IN THE
SOUTHEAST CWFA. THE SOUTHERN STREAM LOW WILL BE RAPIDLY DEEPENING
AND LIFTING NORTH ACROSS THE TENN VALLEY WED MORNING...THEN IT
BECOMES A CHALLENGE ON THE PLACEMENT OF THE LOW BEYOND WED MORNING.
GUIDANCE CONTINUES TO SHOW CORRECTIONS...WITH A FEW SOLUTIONS
BRINGING THE LOW NORTH INTO NORTHEAST IN AND A FEW FURTHER EAST.
THIS WILL BE A KEY PLAYER ON THE PLACEMENT OF THE POTENTIAL
DEFORMATION ZONE WHICH COULD BRING ACCUM SNOW TO THE FORECAST AREA
WED AFTN/EVE. TEMPS WILL BE FALLING DURING THE DAY WED...AND WITH
THE INCREASED FORCING...THIS COULD BR STRONG ENOUGH TO OVERCOME THE
WARMER AIR AT THE SFC AND ALLOW RAIN TO QUICKLY CHANGE TO SNOW AND
START ACCUMULATING. GEFS ENSEMBLES ARE BEGINNING TO DEMONSTRATE A
WEAK EASTERLY LLVL JET AT 850MB LIFTING NORTHEAST OVER THE FORECAST
AREA WED AFTN/EARLY EVE...WHICH WOULD SUPPORT THE IDEA OF A
DEFORMATION SNOW BAND FOR THE CENTRAL/EASTERN CWFA. CONSIDERABLE
UNCERTAINTY REMAINS WITH THIS SYSTEM...BUT EITHER WAY IT WILL IMPACT
A PORTION OF THE GREAT LAKES REGION ON ONE OF THE BUSIEST TRAVEL
DAYS OF THE YEAR.

FOR CHRISTMAS DAY...THE DOWNSTREAM RIDGE OVER THE CANADIAN MARITIME
WILL BEGIN TO WEAKEN AND ALLOW FOR THE 500MB TROUGH TO PUSH EAST OF
THE GREAT LAKES...LIFTING THE DEEPER MOISTURE/PRECIP NORTHEAST WITH
IT. MOST GUIDANCE COMES INTO SLIGHTLY BETTER AGREEMENT FOR HIGHS
ARND THE MID/UPR 30S...WITH POSSIBLY THINNING CLOUD COVER.

BEACHLER

&&

.LONG TERM...
252 PM CST

FRIDAY THROUGH MONDAY...

MAIN FORECAST CONCERNS THIS PERIOD SURROUND A PAIR OF STORM SYSTEMS.
THE FIRST TO PASS TO THE NORTH OF THE AREA FRIDAY-FRIDAY NIGHT AND
THE SECOND TO PASS TO THE SOUTH AND EAST (SHOULD CURRENT TRENDS
CONTINUE) ON SUNDAY.  FIRST SURFACE LOW DEVELOPS OVER THE CENTRAL
PLAINS THURS NIGHT IN RESPONSE TO THE DIGGING UPPER TROF OVER THE
WESTERN U.S.  MODELS SHOWING NRN IL GENERALLY DRY FRIDAY DURING THE
DAYTIME HOURS FRIDAY WITH MAX TEMPS RUNNING AROUND 10 DEG ABOVE
AVERAGE. A COLD FRONT THEN SWEEPS ACROSS THE AREA FRIDAY NIGHT
BRINGING A CHANCE OF RAIN OR SNOW.

MEANWHILE A SECONDARY UPPER LEVEL SHORTWAVE ROTATES INTO THE BASE
OF WESTERN U.S. TROUGH THEN ON SATURDAY AMPLIFYING THE SOUTHERN
STREAM. MODELS NOT COMPLETELY RESOLVING THIS SYSTEM YET AND HAVE
LEANED TOWARD MODEL CONSENSUS FOR A FORECAST SCENARIO WHERE A
SHARPER SHORTWAVE HELPS TO ORGANIZE SURFACE LOW PRESSURE NEAR THE
TX/LA GULF COAST SATURDAY. THIS LOW THEN MOVES NORTHEAST ACROSS
THE TN VALLEY AND INTO THE CENTRAL APPLICATIONS SUNDAY. WILL
CONFINE POPS WITH THIS SYSTEM TO JUST THE SOUTHEASTERN COUNTIES OF
THE FORECAST AREA CURRENTLY WITH TEMPS EXPECTED NEAR SEASONAL FOR
THE TIME OF YEAR.

ED F

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* IFR/LIFR CIGS THROUGH TUESDAY MORNING...PROBABLY LIFTING TO MVFR
  OR POSSIBLY MVFR TUESDAY AFTERNOON
* PERIODS OF RAIN/DRIZZLE TONIGHT THROUGH EARLY TUESDAY MORNING

IZZI

//DISCUSSION...UPDATED 00Z...

DRIZZLY WITH WAVES OF RAIN TONIGHT INTO EARLY TUESDAY MORNING AS A
WARM FRONT LIFTS NORTH ACROSS THE REGION. IFR CIGS ARE LIKELY TO
DROP TO LIFR TONIGHT AND LINGER INTO TUESDAY MORNING. IN THE WAKE
OF WARM FRONTAL PASSAGE TUESDAY MORNING CIGS WILL LIKELY BEGIN TO
LIFT...MOSTLY LIKELY INTO MVFR RANGE THOUGH SOME GUIDANCE SUGGEST
CIGS UP TO VFR TUESDAY AFTERNOON AND NIGHT. WINDS WILL BECOME
SOUTHERLY BEHIND THE WARM FRONT TUESDAY WITH SOME OCNL GUSTS OF
15-20KT POSSIBLE IN THE AFTERNOON.

IZZI

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM-HIGH CONFIDENCE IN CIGS THROUGH TONIGHT...DECREASING
  CONFIDENCE IN CIGS FORECAST TUESDAY
* MEDIUM-HIGH CONFIDENCE IN PRECIP

IZZI

//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...
305 PM CST

LOW PRESSURE OVER THE CENTRAL MISSOURI VALLEY WILL LIFT NORTHEAST
TONIGHT WHILE WEAKENING. THIS WILL RESULT IN BRISK EAST-SOUTHEAST
WINDS OVERNIGHT...WITH GUSTS UP TO 25 TO 30 KT. THE STRONGEST
EASTERLY WINDS WILL SETTLE INTO THE NORTHERN PORTION OF THE LAKE
TUESDAY MORNING...WITH WINDS TURNING SOUTHERLY ON THE SOUTH HALF.
THE LOW PRESSURE WILL DISSIPATE OVER THE NORTHERN PORTION OF THE
LAKE TUESDAY NIGHT...BRINGING QUICKLY DIMINISHING SOUTHERLY WINDS
AND A PERIOD OF LIGHT/VRB OVER THE NORTH.

ATTENTION WILL THEN TURN TO ANOTHER QUICKLY DEVELOPING LOW PRESSURE
OVER THE LOWER OHIO VALLEY ON WEDNESDAY MORNING THAT WILL LIFT NORTH
TO THE CENTRAL GREAT LAKES AND STRENGTHEN RAPIDLY INTO WEDNESDAY
NIGHT...THEN FURTHER INTENSIFY OVER WESTERN/NW QUEBEC ON THURSDAY.
MODELS HAVE COME INTO BETTER AGREEMENT ON THIS OVERALL SCENARIO AND
AM CONCERNED FOR A PERIOD OF GALES ON THE NORTHERN PORTION OF THE
LAKE WEDNESDAY NIGHT CLOSEST TO THE BOMBING LOW. HAVE NOT ISSUED A
WATCH YET DUE TO LOWER CONFIDENCE BUT IF CURRENT TRENDS HOLD WILL
LIKELY NEED A HEADLINE. NORTH-NORTHWEST GALES ARE POSSIBLE ON THE
SOUTH HALF AS WELL DEPENDING ON THE TRACK/STRENGTH OF THE LOW...BUT
CURRENT GUIDANCE SUGGESTS MORE OF A 30 KT PREVAILING/OCNL GALE GUST
SCENARIO. A FAST UPPER PATTERN SUGGESTS A FAIRLY ACTIVE PERIOD
REMAINING OVER THE LAKE INTO THE WEEKEND.

IN THE NEARSHORE...HAD EXTENDED THE SMALL CRAFT FOR THE IL ZONES
THROUGH MID MORNING TUESDAY FOR LINGERING WAVES FROM BRISK
EAST-SOUTHEAST FLOW. WILL LIKELY NEED ANOTHER SMALL CRAFT BY
WEDNESDAY AFTERNOON INTO THURSDAY FOR STRONG WINDS AND BUILDING
WAVES.
RC

&&

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

&&

$$

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