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000
FXUS63 KLOT 051613
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1013 AM CST THU DEC 5 2013

.DISCUSSION...
345 AM CST

MAIN FORECAST FOCUS IS ON COLDER TEMPERATURES WHICH WILL AFFECT THE
MIDWEST THROUGH THE PERIOD...AND THE POTENTIAL FOR SOME LIGHT SNOW
MAINLY SOUTHEAST OF THE CWA TONIGHT AND FRIDAY...AND ACROSS MUCH OF
THE FORECAST AREA SUNDAY AND SUNDAY NIGHT. A PUSH OF EVEN COLDER
ARCTIC AIR APPEARS POSSIBLE EARLY NEXT WEEK IN THE WAKE OF THE
WEEKEND SYSTEM...LIKELY ENHANCED BY FRESH SNOW COVER.

STRONG LOW PRESSURE WAS OVER EASTERN LAKE SUPERIOR EARLY THIS
MORNING...WITH ITS TRAILING COLD FRONT WELL EAST OF THE FORECAST
AREA FROM LOWER MICHIGAN TO THE LOWER OHIO VALLEY AND ARKLATEX.
COLDER/DRIER ARCTIC AIR WAS STEADILY SPREADING INTO THE MIDWEST WITH
TEMPS IN THE TEENS/LOWER 20S AND DEW POINTS IN THE SINGLE DIGITS AND
TEENS. COLD ADVECTION WILL LIKELY OFFSET ANY DIURNAL WARMING TODAY
WITH TEMPS EXPECTED TO DIP A LITTLE MORE THIS MORNING THEN BECOME
STEADY. THOUGH APPROACHING AIR MASS IS DRIER...THERE IS CONSIDERABLE
LOW CLOUDINESS WITHIN COLD ADVECTION AND EVEN A NARROW BAND OF
FLURRIES OR LIGHT SNOW ACROSS FAR EASTERN IOWA AND NORTHWEST
ILLINOIS AT THIS HOUR. THIS APPEARS TO BE DRIVEN BY SHORT WAVE
ENERGY AND A TIGHTENING IN THE THERMAL GRADIENT AROUND NEAR 800
MB...WHICH THE RAP DEPICTS NICELY WITH AN AREA OF FRONTOGENESIS
CLOSELY TIED TO CURRENT RADAR RETURNS. THE RAP CONTINUES TO SPREAD
THIS FORCING INTO NORTH CENTRAL AND FAR NORTHERN IL EARLY THIS
MORNING BEFORE RAPIDLY WEAKENING IT PRIOR TO MIDDAY. LOW LEVEL CLOUD
COVER WILL LIKELY ERODE BY THIS AFTERNOON...THOUGH PRESENCE OF
STRONG SOUTHWESTERLY JET ALOFT AND ADDITIONAL MINOR AMPLITUDE
MID-LEVEL WAVES WILL LIKELY KEEP SOME DEGREE OF MID/HIGH CLOUDS
AROUND THROUGH THE DAY. FARTHER TO THE SOUTHEAST...SERIES OF FAST
MOVING MID-LEVEL DISTURBANCES WILL HELP TIGHTEN LOW LEVEL BAROCLINIC
ZONE ACROSS THE OHIO VALLEY...FRONT FRONTOGENETIC BANDING DEVELOPING
SNOW FROM SOUTHERN MISSOURI INTO OHIO. MODEL GUIDANCE CONTINUES TO
FOCUS AXIS OF ACCUMULATING SNOWFALL SOUTHEAST OF THE CWA LATER TODAY
INTO FRIDAY...WITH FARTHEST NORTH SOLUTIONS CLIPPING SOUTHEAST TIER
OF WFO LOT CWA COUNTIES WITH AT BEST A MINOR ACCUMULATION OF LESS
THAN A HALF INCH WITH DISTURBANCES THIS EVENING AND AGAIN FRIDAY
MIDDAY/AFTERNOON.

LITTLE CHANGE IN THINKING WITH RESPECT TO TEMPERATURES TODAY THROUGH
SATURDAY...WITH PERSISTENT INTRUSION OF ARCTIC AIR INTO FRIDAY AND
SATURDAY AS SURFACE HIGH PRESSURE EVENTUALLY BUILDS INTO THE REGION.
GUIDANCE REMAINS IN DECENT AGREEMENT WITH THE GENERAL TREND OF
20S/LOW 30S TODAY...THEN HIGHS IN THE UPPER TEENS TO LOW 20S FRI/SAT
WITH LOWS RANGING FROM THE SINGLE DIGITS NORTHWEST TO THE TEENS
SOUTHEAST.

ENERGY NOTED IN WATER VAPOR IMAGERY CURRENTLY DIGGING SOUTH INTO
BRITISH COLUMBIA IS PROGGED TO AMPLIFY OVER THE WEST COAST OVER THE
NEXT COUPLE OF DAYS...THEN MOVE EAST ACROSS THE ROCKIES AND PLAINS
SUNDAY AND ACROSS THE GREAT LAKES REGION SUNDAY NIGHT. WHILE SOME
MINOR DETAIL DIFFERENCES REMAIN IN THE LONGER RANGE MODEL
GUIDANCE...THERE IS DECENT CONSENSUS IN A BROAD SURFACE LOW
DEVELOPING ACROSS THE REGION AND LIFTING NORTHEAST ACROSS THE GREAT
LAKES SUNDAY NIGHT AND EARLY MONDAY. DRY EASTERLY FLOW IN MODIFIED
ARCTIC AIR WILL LIKELY BE A BIT SLOW TO SATURATE AS WARM/MOIST
ADVECTION DEVELOPS IN ADVANCE OF THIS SYSTEM SUNDAY...THOUGH LIGHT
SNOW IS EXPECTED TO DEVELOP FROM THE WEST/SOUTHWEST THROUGH THE
AFTERNOON. SNOW IS THEN EXPECTED SUNDAY NIGHT INTO EARLY MONDAY
MORNING...WITH CURRENT MODEL QPF SUGGESTING PERHAPS 2-3 INCHES
ACROSS ESPECIALLY THE CENTRAL AND NORTHWEST PARTS OF THE FORECAST
AREA BEFORE TAPERING TO FLURRIES MONDAY MORNING. IF THIS SNOW COVER
DOES MATERIALIZE...ANOTHER BRIEF SHOT OF ARCTIC AIR WOULD LIKELY
PRODUCE COLDER TEMPS EARLY/MID NEXT WEEK. COLDEST PERHAPS WOULD BE
TUESDAY MORNING...WHEN WEAK RIDGE AXIS WOULD SUPPORT BEST
RADIATIONAL COOLING.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* GUSTY WEST WINDS SLOWLY DIMINISHING REST OF TODAY AND LOOSING
  GUSTS AT SUNDOWN.

TRS

//DISCUSSION...UPDATED 16Z...

ORD AND MDW HAVE BOTH LOST THEIR MVFR CIGS IN THE PAST HOUR.
VISIBLE SATELLITE LOOP SHOWS THAT IT WILL REMAIN VFR AT ORD WHILE
MDW WILL LIKELY BE SCT V BKN TIL 17Z. STILL SOME MVFR CIGS
REMAINING OVER SE AND E CENTRAL IA...FAR NW IL AND FAR S CENTRAL
WI BUT MODELS DRY UP THIS LOW LEVEL MOISTURE BEFORE IT MUCH
FURTHER E.

GENERAL TREND IN WIND SPEEDS HAS BEEN A VERY GRADUAL DECREASE AND
EXPECT THIS TO CONTINUE THIS AFTERNOON AS THE CENTER OF LOW PRES
IN EASTERN ONTARIO CONTINUES MOVING SLOWLY NE TO JAMES BAY BY
00Z. DIRECTION GENERALLY WESTERLY WITH SOME SUBTLE BACKING AND
VEERING NOTED BETWEEN 250 AND 280 DEG. WINDS WILL VEER MORE TO
WNW-NW LATE AFTERNOON-EARLY EVENING AS THE LARGE RIDGE OF HIGH
PRES CURVING SE FROM WESTERN CANADA TO ACROSS THE MID MS VALLEY
GRADUALLY MOVES A BIT FURTHER EASTWARD BEHIND THE DEPARTING
ONTARIO LOW.

TRS

PREVIOUS... UPDATED 12Z...

DESPITE POCKETS OF VFR THE TREND THIS MORNING IS MORE LIKELY TO BE
GRADUALLY IMPROVING MVFR. SLOWED DOWN THE ARRIVAL OF HIGHER CIGS
COMPARED TO EARLIER TAFS BASED ON UPSTREAM OBS AND LATEST GUIDANCE
PRODUCTS. GUSTY WEST WINDS WILL VERY GRADUALLY VEER NORTHWESTERLY
AND BY EARLY THIS EVENING WE SHOULD LOSE THE GUSTS. ALSO INCLUDED
PERIODS OF LIGHT SNOW THIS MORNING WHICH ACTUALLY IS EXPECTED TO
BE MORE LIKE FLURRIES AND HOPEFULLY NOT ANY SORT OF PROBLEM.

LENNING

//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* MEDIUM CONFIDENCE IN TIMING OF LOSS OF MVFR CEILINGS THIS
  MORNING.

* HIGH CONFIDENCE IN WIND TRENDS AND OTHER ELEMENTS.

TRS

//OUTLOOK FOR ORD/MDW FOR 00Z SATURDAY-12Z THURSDAY...UPDATED 12Z...

FRIDAY NIGHT AND SATURDAY...VFR.

SUNDAY...CHANCE OF SNOW...ESPECIALLY LATE. MVFR OR LOWER POSSIBLE.

MONDAY...CHANCE OF SNOW WITH MVFR OR LOWER EARLY.

TUESDAY AND WEDNESDAY...MAINLY VFR.

LENNING

&&

.MARINE...
321 AM CST

LOW PRESSURE CROSSING LAKE SUPERIOR INTO ONTARIO THIS MORNING
DRAGGED A COLD FRONT ACROSS THE LAKE LAST EVENING. WINDS TURNED
SOUTHWEST BEHIND THE FRONT BUT ARE VEERING TO WEST THIS MORNING
AND WILL CONTINUE VEERING TO NORTHWEST TONIGHT. IN THE TIGHTER
PRESSURE GRADIENT ACROSS THE NORTHERN HALF OF THE LAKE CLOSER TO
THE LOW...GALES TO 40 KTS WITH POSSIBLY HIGHER GUSTS ARE EXPECTED
TO PERSIST INTO THIS EVENING. THE SOUTHERN HALF OF THE LAKE ALSO
WILL BE WINDY...BUT GALE FORCE GUSTS ARE EXPECTED TO BE
CONSIDERABLY LESS FREQUENT ACROSS THE SOUTH HALF. LATEST 3 AM CST
SHIP OBS IN THE FAR SOUTHERN PART OF THE LAKE HAD WIND SPEEDS
ONLY IN THE TEENS...WHILE SHIP OBS IN THE NORTHERN LAKE HAD WINDS
IN THE MID 30 KNOT RANGE.

DEBATED WHETHER TO TRIM THE END OF THE SMALL CRAFT ADVISORY FOR
THE INDIANA NEARSHORE WATERS AS THE PRESSURE GRADIENT FALLS OFF
QUICKLY THIS EVENING. DECIDED WITH SUCH COLD AIR AND A NORTHWEST
FETCH...WAVES SHOULD BE EASY TO MAINTAIN...AND GUSTS INTO THE MID
20 KNOT RANGE MAY WELL PERSIST INTO THE OVERNIGHT HOURS.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ868 UNTIL 9 PM THURSDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 3 PM
     THURSDAY.

     SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 4 AM FRIDAY.

&&

$$

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