Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200738
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
138 AM CST SAT DEC 20 2014

.SHORT TERM...
237 PM CST

THROUGH SATURDAY...

UNFORTUNATELY IT LOOKS LIKE WE WILL REMAIN LOCKED IN THE CLOUDS YET
AGAIN TONIGHT...DESPITE A CLEARING WEDGE THAT EXTENDS FROM
WESTCENTRAL IL TO SOUTHERN IN. WITH HIGH PRESSURE OVERHEAD THE
ABILITY TO MIX TO THE SFC DRIER AIR HAS BEEN NONEXISTENT. WITH THE
CONSIDERABLE SOLAR SHIELDING THIS AFTERNOON...SFC TEMPS HAVE BEEN
UNABLE TO WARM MUCH. TEMPS IN MOST LOCATIONS HAVE REMAINED ARND 30
TO A FEW LOW 30S ACROSS THE FAR SOUTHERN CWFA. OVERNIGHT THE SFC
RIDGE WILL DRIFT EAST...ALLOWING LGT/VRB WINDS TO SLOWLY TURN
SOUTHWESTERLY BY DAYBREAK SAT. OVERNIGHT LOWS WILL GENERALLY BE IN
THE LOW/MID 20S...BUT IF CLOUDS REMAIN THICK ENOUGH IT IS PLAUSIBLE
THAT TEMPS COULD HOLD IN THE MID/UPR 20S. GUIDANCE HAS INDICATED A
WEAK LOBE OF VORTICITY ALONG THE NORTHERN STREAM PUSHING EAST ACROSS
THE NORTHERN PLAINS OVERNIGHT...ARRIVING ACROSS THE GREAT LAKES
REGION AROUND SAT MORNING. WHILE IT APPEARS MOST OF THE NEAR SFC
WILL REMAIN DRY...THERE IS SOME WEAK FORCING IN THE MID-LVLS ALONG
WITH MARGINAL SATURATION THAT SOME FLURRIES COULD OCCUR DURING THE
MORNING HOURS. WITH YET ANOTHER DAY OF CONTINUED CLOUDINESS SAT...IT
APPEARS LESS FAVORABLE FOR ANY BUMP TO THE TEMPS. SO HAVE NUDGED
TEMPS DOWN A DEGREE TO THE MID 30S ACROSS THE CWFA.

SATURDAY NIGHT WILL GENERALLY REMAIN DRY...HOWEVER THERE IS ANOTHER
WEAK VORT TRYING TO LIFT NORTHEAST ACROSS IOWA LATE SAT NGT/EARLY
SUN. WITH A SLIGHT WARM LAYER TRYING TO LIFT ACROSS THE
REGION...HOWEVER HAVE HELD ONTO MAINLY LGT SNOW. THERE IS SOME
CONCERN THAT TEMPS ALOFT COULD BE WARM ENOUGH TO FULLY MELT ANY
DENDRITES. THE OTHER CONCERN WORTH MENTIONING IS THAT THE LIFT/OMEGA
IS LACKING LATE SAT NGT...SO DROPLET SIZE COULD BE SMALL AND INSTEAD
OF ANY DENDRITES THERE COULD END UP BEING PATCHY FREEZING
DRIZZLE...MAINLY AFTER MIDNIGHT.

BEACHLER

&&

.LONG TERM...
237 PM CST

SUNDAY THROUGH SATURDAY...

500MB TROUGH WILL BEGIN TO DIG SOUTH ACROSS THE CENTRAL CONUS SUN
INTO MON...WITH A SERIES OF MID-LVL VORTS PROGGED TO SLIDE SOUTHEAST
ACROSS THE CANADIAN ROCKIES INTO THE NORTHERN PLAINS. THE MAIN FOCUS
WILL BE ON A STRONGER SHORTWAVE DEVELOPING SUN NGT/EARLY MON ACROSS
THE DAKOTAS...EXPECTED TO SLIDE SOUTHEAST ACROSS NORTHERN IL.
ALTHOUGH CONFIDENCE IN THE TIMING/PLACEMENT OF THIS PATH IS LOW
GIVEN CONTINUED CORRECTIONS BASED ON THE LATEST GUIDANCE CYCLES.

MOISTURE WILL BE ADVECTING NORTH OF THIS LOW MON...WITH PRECIP
ARRIVING MON MORNING LIKELY AS MAINLY RAIN OR PERHAPS A RAIN/SNOW
MIX ACROSS THE NORTHERN CWFA. THEN A QUICK PUSH OF MUCH WARMER AIR
WILL BRING TEMPS INTO THE UPR 30S/NEAR 40 MON AFTN AND SHOULD TURN
ANY PRECIP TO PLAIN RAIN. AS MON NGT PROGRESSES...THE LOW IS PROGGED
TO STEADILY DEEPEN AND LIFT NORTHEAST ACROSS NORTHERN IL EARLY TUE.
FROPA CURRENTLY PROGGED FOR TUE EVE...BRINGING COLDER AIR BACK INTO
THE FORECAST AREA AND TURNING RAIN OVER TO LGT SNOW.

MON NGT WITH THE WARM AIR STEADILY ADVECTING NORTH...TEMPS WILL
REMAIN IN THE MID/UPR 30S. TUE TEMPS COULD PUSH EVEN
WARMER...DEPENDING ON THE PLACEMENT/TIMING OF FROPA. CURRENT
INDICATIONS ARE THAT TEMPS WILL PUSH INTO THE LOW/MID 40S TUE AFTN.

CHRISTMAS EVE THROUGH CHRISTMAS DAY...
ENSEMBLES ALONG WITH OPERATIONAL SOLUTIONS HAVE BEEN TARGETING THE
SYSTEM BECOMING WRAPPED UP OVER THE GREAT LAKES REGION AND POSSIBLY
TRANSITIONING INTO ANOTHER LOW OVER SOUTHERN ONTARIO/SOUTHWEST
QUEBEC CHRISTMAS DAY. A TIGHT GRADIENT WILL REMAIN OVER THE REGION
BOTH CHRISTMAS EVE/CHRISTMAS DAY...WITH TEMPS RETURNING TO MORE
SEASONAL CONDITIONS IN THE LOW/MID 30S.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* MVFR CEILINGS LIKELY BECOMING VFR BEFORE SUNRISE. EXPECTED TO
  LOWER TO MVFR AGAIN THIS EVENING.

* LIGHT/VARIABLE WINDS BECOMING LIGHT SOUTH-SOUTHWEST BY MIDDAY.

RATZER

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

WEAK SURFACE HIGH PRESSURE RIDGE MOVING SLOWLY EAST ACROSS THE
AREA THIS MORNING. CALM OR LIGHT/VARIABLE WINDS AND MVFR CLOUDS
TRAPPED BENEATH SHALLOW INVERSION PERSISTING. SURFACE OBS INDICATE
MVFR CEILINGS ERODING FROM THE SOUTHWEST HOWEVER...WITH THE
POTENTIAL FOR CONDITIONS TO BECOME VFR AT TAF SITES DURING THE
PRE-DAWN HOURS. QUESTION IS HOW LONG VFR LASTS...AS EXTENSIVE AREA
OF MVFR/IFR CEILINGS WEST OF THE MISSISSIPPI RIVER WILL EVENTUALLY
SPREAD INTO THE AREA TONIGHT. KRFD BEING FARTHER WEST LOOKS TO BE
BACK INTO MVFR CONDITIONS PERHAPS AS EARLY AS MID-LATE MORNING.
CEILINGS WILL LIKELY LOWER INTO IFR RANGE LATE THIS EVENING INTO
EARLY SUNDAY...WITH THE POTENTIAL FOR SOME FLURRIES OR FREEZING
DRIZZLE DEVELOPING SUNDAY MORNING.

CALM OR LIGHT/VARIABLE WINDS LESS THAN 5 KT WILL BECOME SOUTH-
SOUTHWEST AT 5 KT BY EARLY AFTERNOON AND REMAIN LIGHT SOUTH
THROUGH TONIGHT.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM IN CEILING TRENDS AND VFR DURATION TODAY.

* HIGH IN WIND TRENDS.

RATZER

//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY...MVFR CIGS.  SOUTH WINDS.

MONDAY...MVFR CIGS. BRIEF PERIOD OF RAIN OR MIXED RAIN AND SNOW.

TUESDAY...MVFR CIGS. CHANCE OF RAIN.

WEDNESDAY...MVFR CIGS. BLUSTERY - GUSTY WEST WINDS. CHANCE OF
SNOW.

THURSDAY...VFR.

ED F

&&

.MARINE...
137 AM CST

THE MAIN MARINE FORECAST CONCERNS THIS MORNING FOCUS ON A COUPLE OF
PERIODS OF STRONG WINDS OVER THE NEXT SEVERAL DAYS...SOUTH AND
SOUTHEAST WINDS 25-30 KT DEVELOPING SUNDAY AND CONTINUING THROUGH
MONDAY EVENING...THEN THE POTENTIAL FOR WEST HIGH GALES LATE
WEDNESDAY THROUGH THURSDAY...CHRISTMAS EVE AND CHRISTMAS DAY.

IN THE NEAR TERM...WEATHER IS QUITE TRANQUIL ACROSS LAKE MICHIGAN
THIS MORNING AS DIFFUSE HIGH PRESSURE WAS DRIFTING SLOWLY EAST
ACROSS THE REGION. THE RIDGE AXIS WILL SHIFT EAST OF THE LAKE THIS
AFTERNOON ALLOWING CURRENT VARIABLE WINDS TO BECOME SOUTHERLY. SOUTH
WINDS WILL GRADUALLY INCREASE LATE TONIGHT AND SUNDAY AS AN AREA OF
LOW PRESSURE DEVELOPS SOUTHEAST IN THE LEE OF THE NORTHERN ROCKIES
WITH A CORRESPONDING TIGHTENING OF THE SURFACE PRESSURE GRADIENT
BETWEEN THE LOW AND THE HIGH TO EAST OF THE LAKES. MODEL FORECAST
SOUNDINGS SUPPORT WINDS 20-25 KT SUNDAY AFTERNOON INCREASING TO 30
KT SUNDAY NIGHT/MONDAY...BECOMING A BIT MORE SOUTHEAST LATE MONDAY
AS THE SURFACE LOW MOVES INTO THE UPPER MIDWEST.

THE SURFACE LOW IS EXPECTED TO FILL A BIT ACROSS THE REGION TUESDAY
AS IT OCCLUDES. MODEL FORECASTS CONTINUE TO DEPICT A COMPLEX
EVOLUTION BY MID-WEEK HOWEVER...WITH A SECOND AREA OF LOW PRESSURE
FARTHER SOUTH ALONG THE COLD FRONT DEEPENING AND WRAPPING NORTHWARD
INTO THE GREAT LAKES WEDNESDAY AND THURSDAY. SPREAD REMAINS AMONGST
MODEL FORECASTS OF THE EXACT TRACK AND DEPTH OF THIS LOW...BUT IT
DOES APPEAR THAT A DEEPENING SUB-980 MB LOW WILL TRACK NORTH ACROSS
THE GREAT LAKES REGION CHRISTMAS EVE INTO CHRISTMAS DAY. SUCH A LOW
PASSING EAST OF LAKE MICHIGAN SUPPORTS THE POTENTIAL FOR THE
DEVELOPMENT OF WESTERLY GALES ON THE LAKE...PERHAPS HIGH GALES ABOVE
40 KT...FROM WEDNESDAY AFTERNOON OR EVENING THROUGH THE DAY
THURSDAY.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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