🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1146 AM CST FRI MAR 1 2013

DISCUSSION...
919 AM CST

UPDATED THE GRIDS FOR THIS MORNING...MAINLY TO ACCOUNT FOR ONGOING
LAKE EFFECT SNOW SHOWERS.

LATEST RADAR IMAGERY DEPICTING SOME LIGHT RETURNS ACROSS ALL OF
NORTHERN ILLINOIS WITH AN ISOLATED AREA OF FAIRLY DECENT
RETURNS...PRIMARILY ACROSS SOUTHEAST COOK COUNTY AND NORTHWEST
LAKE COUNTY IN INDIANA. UNDER THE LIGHTER RETURNS LIGHT SNOW
CONTINUES WITH VIS STAYING CONFINED TO ONLY THE 3-6 MILE
RANGE...WITH VIS LIKELY AT OR UNDER 1 MILE UNDER THE STRONGER
RETURNS. THESE SNOW SHOWERS ARE IN RESPONSE TO PERSISTENT
NORTH/NORTHEAST FLOW OFF OF A MODERATELY UNSTABLE LAKE...AS WELL
AS WEAK LARGE SCALE LIFT. LATEST SURFACE ANALYSIS INDICATING A
SURFACE TROUGH SITUATED ACROSS THE SOUTHERN HALF OF THE
LAKE...EXTENDING BACK NORTHWEST FROM A WEAK SURFACE LOW OVER
IN/KY. THIS HAS RESULTED IN AN AREA OF BETTER CONVERGENCE THIS
MORNING...WHICH HAS THEN PROVIDED THIS ISOLATED AREA OF STRONGER
SNOW SHOWERS. LATEST SHORT TERM GUIDANCE IS INDICATING THIS TROUGH
TO SLOWLY EXIT/DAMPEN WITH ALL SURFACE WINDS BECOMING MORE
NORTHERLY...AND LOSS OF BETTER CONVERGENCE. SO DO EXPECT THIS
ISOLATED AREA OF STRONGER SNOW SHOWERS TO PERSIST OVER THE NEXT
COUPLE OF HOURS...ONCE AGAIN PRIMARILY AFFECTING ONLY SOUTHEAST
COOK AND NORTHWEST LAKE COUNTY IN INDIANA. UNDER THESE STRONGER
RETURNS IT WOULD NOT BE OUT OF THE QUESTION FOR SOME ADDITIONAL
ACCUMULATIONS...WITH 1 TO 3 INCHES NOT OF THE REALM OF
POSSIBILITY THROUGH MID DAY. LIGHT SNOW SHOWERS ARE THEN EXPECTED
TO CONTINUE INTO THE AFTERNOON...BUT WITH ONLY LIGHT ACCUMULATIONS
ANTICIPATED.

RODRIGUEZ

//PREV DISCUSSION...
330 AM CST

FORECAST CONCERNS ARE LAKE EFFECT SNOW SHOWER POTENTIAL THE NEXT FEW
DAYS...COLD TEMPERATURES OVER THE WEEKEND...AND A POSSIBLE SNOW
PRODUCING SYSTEM EARLY NEXT WEEK.

SHORT TERM...TODAY THROUGH SUNDAY NIGHT...

SLIGHTLY COLDER AIR MASS WILL SETTLE OVER THE REGION TODAY ON
CONTINUED NORTHERLY FLOW...WITH A LARGE AREA OF HIGH PRESSURE
BUILDING IN FROM THE NORTH. THIS SHOULD KEEP HIGHS A FEW DEGREES
COOLER THAN YESTERDAY. LAKE EFFECT SET UP IS QUITE MARGINAL
THERMODYNAMICALLY...BUT COMPENSATING SOMEWHAT WILL BE GOOD
CONVERGENCE AND FETCH WITH NORTH-NORTHEAST LOW LEVEL FLOW. THE
CONVERGENCE AND EVEN SOME ENHANCED LIFT IS BEST ALIGNED OVER
ILLINOIS AND PORTION OF NW INDIANA LAKEFRONT THROUGH MID/LATE
MORNING...SO BUMPED UP POPS SOME FROM PREVIOUS FORECAST. COULD SEE
SOME VERY LIGHT ACCUMULATIONS. OUTSIDE OF LAKE EFFECT...THERE MAY BE
SOME FLURRIES INLAND. ALTHOUGH STRATUS CAN CLEAR OUT QUICKER THIS
TIME OF YEAR DUE TO HIGHER SUN ANGLE...THINK THE NAM IS BEING WAY
TOO AGGRESSIVE WITH CLEARING TREND FROM NORTH. USED RAP GUIDANCE FOR
NEAR TERM SKY FORECAST. MOST OF CWA SHOULD SEE OVC THROUGH DAYLIGHT
HOURS.

THE SAME GENERAL IDEA OF DECENT CONVERGENCE BUT VERY MARGINAL
THERMODYNAMICS AND INVERSION HEIGHTS CONTINUES THROUGH SATURDAY WITH
THE LAKE EFFECT. WILL CONTINUE TO CARRY CHANCE POPS...THOUGH THEY
WILL SLIDE INTO NW INDIANA ON SATURDAY AS NORTHERLY FLOW BECOMES
JUST WEST OF DUE NORTH. SKIES SHOULD START TO CLEAR BY DAYBREAK OVER
THE NW CWA...SO TEMPERATURES SHOULD DROP PRETTY WELL THERE GIVEN
THE GOOD RADIATION COOLING...PROBABLY INTO THE MID TEENS.

OUTSIDE OF LAKE EFFECT CONCERNS...THE ONLY FOCUS OVER THE WEEKEND
WAS TEMPERATURES WITH HIGH PRESSURE BUILDING OVERHEAD. FOR HIGHS
ON SATURDAY...SUSPECT THAT THE MARCH SUN WILL GO TO WORK ON THE
SNOW COVER...ESPECIALLY FOR AREAS THAT DIDNT SEE MUCH THIS WEEK.
FOR MARCH 1-15...WITH 850 TEMPS AROUND -10C AS IS PROGGED ON
SATURDAY...MEDIAN HIGH AT ORD IS 34 DEGREES. WENT JUST UNDER THAT
IN NORTHERN CWA/ESPECIALLY IN THE NORTHEAST WHERE SNOW PACK IS
DEEPEST...YIELDING HIGHS AROUND 30. TO THE SOUTH OF I-80...SUSPECT
WE`LL SEE SOME MID TO EVEN UPPER 30S AS SUN EATS AWAY AT MEAGER
SNOW COVER.

THEN SUNDAY NIGHT...THERE IS A STRONG SIGNAL FOR GOOD RADIATIONAL
COOLING...ESPECIALLY ON THE ECMWF...WHICH MAY BE A BIT TOO
AGGRESSIVE WITH ITS ZERO TO SLIGHTLY BELOW ZERO NUMBERS FOR NORTH
CENTRAL IL. BASICALLY CONTINUED IDEA OF PREVIOUS FORECAST WITH
POTENTIAL FOR SINGLE DIGIT LOWS WEST OF FOX RIVER. ON SUNDAY...THE
COLD START MAY KEEP TEMPS IN CHECK SOMEWHAT...BUT MARCH SUN ANGLE
AND LONGER DAYLIGHT HOURS EXPECTED TO COMPENSATE...IN ADDITION TO
SLIGHTLY WARMER TEMPS ALOFT. MEDIAN ORD HIGHS FOR DAYS WITH 850
TEMPS AS PROGGED ON SUNDAY IN -6 TO -8 RANGE IS 38. DIDNT GO THIS
MILD DUE TO COLD START AND SNOW COVER OVER NORTHERN CWA...BUT
BUMPED UP TEMPS A FEW DEGREES FROM PREVIOUS. IF SNOW COVER CAN BE
ELIMINATED ON SATURDAY OVER SOUTH...WOULDNT BE SURPRISED IF SOME
LOCATIONS TAG 40 DEGREES...BUT FOR NOW WENT WITH UPPER 30S IN
CENTRAL IL. INCREASING CLOUD COVER SHOULD RESULT IN LOWS SEVERAL
DEGREES WARMER SUNDAY NIGHT THAN SATURDAY NIGHT.

LONG TERM...MONDAY THROUGH THURSDAY

MAIN FOCUS IS ON VERY ENERGETIC CLIPPER SYSTEM EARLY NEXT WEEK.
THERE HAVE BEEN FAIRLY SIGNIFICANT RUN TO RUN CHANGES WITH THE
TIMING AND TRACK OF THE STRONG UPPER LOW AND SURFACE REFLECTION.
WOULD EXPECT THIS TO CONTINUE UNTIL UPPER WAVE MAKES LANDFALL IN
PACIFIC NORTHWEST SATURDAY NIGHT ALLOWING FOR SAMPLING BY RAOB
NETWORK. AS IT STANDS RIGHT NOW...00Z GUIDANCE HAS SHIFTED BACK
NORTH...WHICH WOULD PUT MOST OF CWA IN LINE FOR A POTENTIALLY
ACCUMULATING SNOW. COULD SEE THE START OF SOME WAA SNOWS AHEAD OF
SYSTEM ON MONDAY...SO BLENDED POPS WERE MAINTAINED.
HOWEVER...BIGGEST CHANGE WAS TO GREATLY INCREASE POPS ABOVE
ALLBLEND MONDAY NIGHT AND TUESDAY IN LINE WITH A SLOWER
PROGRESSION OF THE UPPER LOW AND ATTENDANT SURFACE LOW FAVORED BY
THE 00Z ECMWF...CANADIAN...GEFS AND HPC. GFS APPEARS MUCH TOO FAST
IN CLEARING THE SYSTEM OFF TO THE EAST. FOR NOW...BUMPED UP POPS
TO MID RANGE CHANCE GIVEN UNCERTAINTY.

AS CURRENTLY MODELED...THE UPPER LOW WITH THIS SYSTEM LOOKS QUITE
STRONG...WITH A TIGHT BAROCLINIC ZONE ALONG/AHEAD OF ITS TRACK. THUS
THERE COULD BE A NICE SWATH OF SNOW PUT DOWN SOMEWHERE OVER/NEAR THE
REGION. ON THE NEGATIVE SIDE...TEMPERATURES AS IS USUALLY THE CASE
IN MARCH WOULD BE NEAR FREEZING...SO THE SNOW WOULD HAVE TO COME
DOWN HARD TO ACCUMULATE EFFICIENTLY. LOTS OF TIME TO MONITOR THIS
SYSTEM.

FOR THE REST OF THE EXTENDED AND BEYOND...AFTER A FAIRLY COOL DAY ON
WEDNESDAY BEHIND THE EARLY WEEK SYSTEM...THERE COULD BE BRIEF WARM UP
TO NORMAL TO ABOVE NORMAL AS THE PATTERN TRANSITIONS FROM AN EASTERN
TROUGH/WESTERN RIDGE (POSITIVE PNA) CONFIGURATION TO A WESTERN
TROUGH/EASTERN RIDGE (NEGATIVE PNA) PATTERN. WITH RIDGING FIRST
AMPLIFYING INTO CENTER OF CONUS...WE COULD SEE A FEW DAYS OF
TEMPERATURES AT LEAST BACK INTO THE 40S.

HOWEVER...THE LONG RANGE ENSEMBLES LOOKING BEYOND THIS 7 DAY
FORECAST PERIOD ARE HOLDING ONTO THE IDEA OF MAINTAINING HIGH
LATITUDE BLOCKING/NEGATIVE NAO AND NEGATIVE AO THROUGH MID MARCH.
IN ADDITION...RIDGING MAY AMPLIFY OVER OR NEAR ALASKA WHICH ALLOWS
FRESH COLD AIR TO INFILTRATE CANADA AND THE NORTHERN CONUS. BOTTOM
LINE IS ANY WARM UP COULD BE QUICKLY FOLLOWED BY A RETURN TO THE
COLDER THAN NORMAL PATTERN. NO MARCH 2012 PATTERN IS SEEN IN THE
EXTENDED FOR ANYONE HOPING FOR AN EARLY SUMMER AGAIN.

RC

&&

AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...


* WINDS BECOMING NORTH-NORTHEAST EARLY THIS AFTERNOON.

* MVFR CIGS.

* LAKE EFFECT SNOW SHOWERS POSSIBLE AGAIN TONIGHT...LOWER MVFR VIS
  POSSIBLE.

KJB

//DISCUSSION...UPDATED 18Z...

THE LAKE EFFECT SNOW SHOWER ACTIVITY CONTINUES TO ABATE ACROSS THE
TERMINALS EARLY THIS AFTERNOON AS THE LOWER LEVEL CONVERGENCE WEAKENS
IN RESPONSE TO AN INVERTED SURFACE TROUGH MOVING ONSHORE ACROSS
NORTHEASTERN ILLINOIS. WINDS ARE BECOMING NORTH-NORTHEASTERLY IN
THE WAKE OF THIS SURFACE TROUGH...AND I EXPECT ALL OF THE EASTERN
TERMINALS TO BE 010-020 BY OR SHORTLY AFTER 18 UTC. ANY ADDITIONAL
SNOW SHOWER ACTIVITY THIS AFTERNOON LOOKS TO BE VERY LIGHT...WITH
MINIMAL IMPACTS TO THE VISIBILITY EXPECTED.

THE SURFACE FLOW WILL REMAIN OUT OF THE NORTH NORTHEAST INTO
SATURDAY AS A SURFACE RIDGE OF HIGH PRESSURE CONTINUES TO BUILD
EASTWARD ACROSS THE REGION. SOME ADDITIONAL LAKE EFFECT SNOW
SHOWER ACTIVITY IS POSSIBLY TONIGHT ACROSS THE EASTERN TERMINALS
AS THE LAKESHORE CONVERGENCE INCREASES IN RESPONSE TO A LAND BREEZE.
HOWEVER...THE OVERALL THERMODYNAMIC SETUP IS SLOWLY BECOMING
SLIGHTLY LESS FAVORABLE FOR LAKE EFFECT SNOW. THUS ONLY SOME MVFR
VIS IS CURRENTLY EXPECTED...WITH A SMALL POSSIBILITY OF A SHORT
PERIOD OF LOWER VIS NOT OUT OF THE QUESTION TONIGHT.

KJB


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM TO HIGH CONFIDENCE ON NORTH-NORTHEAST WINDS THROUGH THIS AFTERNOON.

* LOW TO MEDIUM CONFIDENCE ON ADDITIONAL LAKE EFFECT SNOW SHOWERS TONIGHT.

KJB

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

SATURDAY NIGHT...MVFR CIGS POSSIBLE EARLY.

SUNDAY...VFR.

MONDAY...CHANCE OF LIGHT SNOW IN THE AFTERNOON. MVFR POSSIBLE
LATE.

MONDAY NIGHT INTO TUESDAY...GOOD CHANCE OF SNOW. IFR PROBABLE.

WEDNESDAY...MVFR CIGS POSSIBLE.

THURSDAY...VFR EXPECTED.

IZZI

&&

MARINE...
222 AM CST

MODERATELY STRONG NORTH WINDS LIKELY TO CONTINUE THROUGH SATURDAY
WITH OCCASIONAL GALE FORCE GUSTS A POSSIBILITY...ESPECIALLY LATER
THIS AFTERNOON INTO THE EVENING...BUT GENERALLY THINK ANY GALES
WILL BE INFREQUENT ENOUGH TO KEEP OUT OF THE FORECAST. WINDS WILL
BEGIN TO DIMINISHING LATER SATURDAY INTO SUNDAY AS RIDGE AXIS
APPROACHES AND THEN MOVES ACROSS THE LAKE SUNDAY NIGHT. ANOTHER
SYSTEM WILL PASS TO THE SOUTHWEST OF THE LAKE EARLY/MIDDLE PART OF
NEXT WEEK WITH WINDS FRESHENING UP A BIT...BUT SYSTEM SHOULD BE
WEAK ENOUGH AND FAR ENOUGH AWAY TO NOT CAUSE ANY MAJOR
WINDS/WAVES.

IZZI

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 3 AM
     SUNDAY.

     SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 9 AM SUNDAY