🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1227 PM CST THU FEB 28 2013

DISCUSSION...
349 AM CST

PESKY UPPER LOW WILL CONTINUE TO BE THE SOURCE OF OCCASIONAL
DRIZZLE/FLURRIES/LIGHT SNOW ACROSS THE REGION TODAY...THOUGH FORCING
IS EXPECTED TO WEAKEN AS THE CENTER OF THE CIRCULATION CONTINUES TO
DRIFT SLOWLY AWAY TO THE EAST AND SMALLER MID-LEVEL VORT MAXES
ROTATING ABOUT IT BECOME WEAKER.

EARLY MORNING GOES WATER VAPOR IMAGERY SHOWS THE UPPER LOW
TRANSITIONING FROM THE EASTERN GREAT LAKES TOWARD THE NORTHEASTERN
STATES. SEVERAL SMALLER SHORT WAVES WERE NOTED WRAPPING
SOUTHWESTWARD ACROSS THE WESTERN LAKES REGION...ONE ACROSS EASTERN
IA/MO AND WESTERN IL...ANOTHER MOVING SOUTHWEST ACROSS LAKE MICHIGAN
AND YET ANOTHER OVER CENTRAL/NORTHERN LOWER MICHIGAN. REGIONAL RADAR
MOSAIC DEPICTS PERSISTING PATCHES OF LIGHT SNOW/SNOW SHOWERS OF
VARIOUS INTENSITY ASSOCIATED WITH THESE FEATURES. GUIDANCE GENERALLY
WEAKENS THESE VORTS AS THEY ROTATE TO THE SOUTHWEST THROUGH THIS
MORNING AND GRADUALLY DECREASES THE CYCLONIC CURVATURE OF THE FLOW
WHICH SUGGESTS PRECIP MAY BECOME LESS WIDESPREAD...THOUGH WITH
PERSISTENT MOIST LOW LEVELS IT WILL BE EASY TO MAINTAIN AT LEAST
OCCASIONAL FLURRIES OR VERY LIGHT SNOW THROUGH THE DAY AND TONIGHT
BEFORE MORE SUBSTANTIAL DRYING OF LOW LEVELS OCCURS. SURFACE TEMPS
AT/ABOVE FREEZING AND RELATIVELY MILD THERMAL PROFILES THROUGH THE
MOIST LAYER...COMBINED WITH RELATIVELY WEAK FORCING...SHOULD LIMIT
SNOW/LIQUID RATIOS AND PREVENT ANY SIGNIFICANT ACCUMULATIONS BEYOND
A FEW TENTHS OF AN INCH. NORTH/NORTHEAST FETCH OFF THE LAKE WILL
KEEP CONVERGENCE ALONG THE IL/INDIANA SHORE AREAS MOST LIKELY TO SEE
BETTER CHANCES FOR THESE MINOR ACCUMS. AREAS WHERE DEEPER CLOUD
ELEMENTS ERODE AT TIMES MAY SEE A LITTLE DRIZZLE OR "SNIZZLE"...AS
CLOUD TOPS MAY OCCASIONALLY BE WARMER THAN ICE NUCLEATION TEMPS.
PREDOMINANT PRECIP TYPE SHOULD BE A WET SNOW WHERE/WHEN MORE
SIGNIFICANT PRECIPITATION OCCURS. LIGHT SNOW/SNOW SHOWERS SHOULD
TAPER TO FLURRIES TONIGHT/EARLY FRIDAY AS THE OLD UPPER LOW
CONTINUES TO DRIFT FARTHER AWAY...AND WEAK MID-LEVEL RIDGING
GRADUALLY DEVELOPS OVER THE UPPER MIDWEST.

THE EXCEPTION MAY BE FOR AREAS ALONG/NEAR THE LAKE MICHIGAN SHORE
INTO THE WEEKEND...AS LONG NORTH-NORTHEAST FETCH MAINTAINS MOIST
CONVERGENCE WITHIN MARGINALLY CONDUCIVE ENVIRONMENT FOR PRODUCING
WEAK LAKE EFFECT CLOUDS/FLURRIES OR LIGHT SNOW SHOWERS. WHILE FETCH
AND CONVERGENCE ARE FAVORABLE...LAKE-MODIFIED THERMODYNAMIC PROFILES
ARE NOT IMPRESSIVE THROUGH FRIDAY WITH 850 MB TEMPS WARMER THAN -10
C ONLY PRODUCING SFC-850 DELTA-T OF 12-14 C. COLDER AIR DOES
GRADUALLY SPREAD IN BEHIND THE SLOWLY DEPARTING LOW PRESSURE SYSTEM
LATE FRIDAY NIGHT THROUGH SATURDAY NIGHT. MODIFIED SOUNDINGS LOOK
THE BEST SATURDAY NIGHT INTO EARLY SUNDAY...AS DELTAS APPROACH 17 C
AND THE EQUILIBRIUM LEVELS INCREASE TO 7000 FT OR BETTER PER
WRF-NAM. THE WIND FIELD QUICKLY WEAKENS SUNDAY HOWEVER...AS SURFACE
HIGH PRESSURE RIDGE APPROACHES FROM THE WEST. WITH THESE
EXPECTATIONS HAVE GENERALLY MENTIONED CHANCE POPS FOR LIGHT SNOW
SHOWERS AND FLURRIES NEAR THE LAKE THROUGH SATURDAY...WITH CHANCE
POPS FOR SNOW SHOWERS SATURDAY NIGHT/SUNDAY MORNING MAINLY ACROSS
NORTHWEST INDIANA. GUIDANCE DOES VARY A BIT WITH THE LOW LEVEL WIND
DIRECTION AHEAD OF THE RIDGE...WHICH SUGGESTS ANY LAKE SNOW BAND
COULD BE ORIENTED DIFFERENTLY BUT WILL CONTINUE TO MONITOR THIS OVER
THE NEXT DAY OR TWO.

TEMPS WILL REMAIN A LITTLE SHY OF NORMAL TODAY THROUGH FRIDAY...WITH
LOW-MID 30S FOR HIGHS AND LOWS TONIGHT GENERALLY IN THE 20S...UPPER
20S PARTICULARLY EAST/SOUTHEAST WHERE CLOUD COVER IS MOST LIKELY TO
PERSIST. ADVECTION OF INCREASINGLY COLDER AIR FOR THE WEEKEND SHOULD
KEEP HIGHS IN THE 20S/AROUND 30...WITH LOWS DIPPING TO THE TEENS/LOW
20S BY SATURDAY MORNING. BETTER CHANCES FOR CLEARING SATURDAY NIGHT
ACROSS NORTH CENTRAL IL...COMBINED WITH SIGNIFICANT SNOW COVER AND
DECREASING WINDS...MAY VERY WELL SET THE STAGE FOR SOME SINGLE DIGIT
TEMPS THERE BY EARLY SUNDAY...BEFORE WE MODERATE AGAIN MON-TUES.

ON THE LARGER SCALE...A LARGE UPPER LEVEL RIDGE DEVELOPS ACROSS THE
WESTERN CONUS BY THE WEEKEND...INDUCING DOWNSTREAM AMPLIFICATION OF
THE LONG WAVE TROUGH OVER THE EASTERN US...JUST OFF TO OUR EAST.
ADDITIONAL SHORT WAVE ENERGY DIGS INTO THE WESTERN SIDE OF THE
TROUGH SUNDAY ACROSS THE MIDWEST...THOUGH BY THAT TIME OUR LOW/MID
LEVELS ARE DRY AND MAINLY ONLY SOME THICKENING HIGH CLOUD COVER
RESULTS. THE UPPER RIDGE PUSHES EAST ACROSS THE AREA INTO MONDAY AS
THE TROUGH MOVES OFF TO OUR EAST...WHILE A DEEP SHORT WAVE IS
INDICATED BY LONGER RANGE GUIDANCE TO MOVE ACROSS THE NORTHERN
ROCKIES AND INTO THE NORTHERN PLAINS BY MONDAY MORNING. THIS SYSTEM
THEN IS PROGGED TO DIG SOUTHEAST ACROSS THE MISSOURI VALLEY MONDAY
NIGHT AND ACROSS THE LOWER OHIO VALLEY TUESDAY. WARM ADVECTION
PRECIPITATION LOOKS TO DEVELOP NORTH/NORTHEAST OF THE TRACK OF THE
LOW AND ACROSS OUR FORECAST AREA IN THE FORM OF LIGHT SNOW MONDAY
INTO TUESDAY...BY WHICH TIME THE LONGER RANGE MODELS DEVELOP SOME
DIFFERENCES IN HANDLING UPPER TROUGH OVER THE REGION. IN EITHER
CASE...IT APPEARS A PERIOD OF SOME LIGHT ACCUMULATING SNOW IS
POSSIBLE MON/TUES WITH COLDER WINTRY TEMPS HANGING ON THROUGH
MID-WEEK. SOME HOPE FOR A LITTLE WARM-UP LOOMS BY THE END OF NEXT
WEEK.

RATZER

&&

AVIATION...

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

* MVFR CEILINGS PERSISTING INTO THE OVERNIGHT HOURS.

* THREAT OF LAKE EFFECT SNOW BAND MOVING FAR ENOUGH INLAND THIS
  EVENING TO REACH ORD FIELD.

* POTENTIAL FOR MVFR VISIBILITIES IN LAKE EFFECT SNOW...AND OF
  CEILINGS BEING REDUCED TO BELOW 015.

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

A SHORT WAVE ROTATING AROUND THE NW AND WESTERN END OF THE
ELONGATED UPPER LOW THAT WAS IN PLACE FROM THE MID MS VALLEY ENE
TO NEW ENGLAND LATE WED AND WED NIGHT MOVED ACROSS THE LOCAL AREA
FOLLOWED BY ANOTHER DISTURBANCE THIS MORNING. THESE ENHANCED SNOW
FALL ENOUGH TO PRODUCE SOME MINOR ACCUMULATIONS.

THE FLOW OVER THE LOCAL AREA HAS NOW TRANSITIONED FROM CYCLONIC
TO NEUTRAL DURING THE MID AND LATE MORNING HOURS AS THE MAIN
CENTER OF CIRCULATION OF THE UPPER LOW MOVED ENE TO OVER THE
ADIRONDACKS OF UPSTATE NY AS OF MIDDAY WITH A NARROW UPPER RIDGE
TO PROGRESS TO OVER THE UPPER MS VALLEY AND THE MID AND LOWER MO
VALLEY AS AN UPPER TROUGH MOVES E ACROSS THE NORTHERN AND CENTRAL
PLAINS. OUTSIDE OF A FEW INCONSEQUENTIAL FLURRIES OR VERY LIGHT
SNOW SHOWERS THE ONLY PRECIPITATION CONCERN WILL BE THE LAKE
EFFECT PLUME WHOSE WESTERN EDGE CURRENTLY JUST BRUSHES SE COOK
COUNTY WITH THE MORE SIGNIFICANT SNOW SHOWERS OCCURRING IN FAR NW
IN. MARGINAL DELTA-T FOR LAKE EFFECT SNOWS WITH CLOUD TOPS 5-6K FT
BUT FULL LENGTH FETCH HELPING TO MAXIMUM LOW LEVEL CONVERGENCE AND
MOISTURE INGESTION.

12Z WRFEMS HAD A GOOD HANDLE ON THE PLUME AS OF LATE MORNING SO
BASED MAGNITUDE OF SNOWFALL THREAT AND TIMING THEREOF FOR 18Z
TAF ON THE OUTPUT OF THIS MODEL. USING BOUNDARY LAYER DIVERGENCE
AND LOW LEVEL OMEGA PROGS THIS RUN KEEPS THE BAND OFF OF THE IL
SHORE WHILE PUSHING THE S END OF THE PLUME A BIT FURTHER E IN NW
IN DURING THE AFTERNOON. BY EARLY EVENING IT SHOWS RENEWED
DEVELOPMENT ALONG AND JUST OFF SHORE OF IL AND THEN GRADUALLY
BRINGS THE W EDGE OF THE SNOW BAND WESTWARD TO ORD 04-05Z. DURING
THE OVERNIGHT HOURS THE BAND IS PROGGED TO WEAKEN AS IT MOVES
FURTHER INLAND AND W OF ORD.

TRS


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

* MODERATE CONFIDENCE IN CEILING TRENDS.

* MODERATE CONFIDENCE IN TIMING OF GREATEST THREAT OF LAKE EFFECT
  SNOW SHOWERS.

* HIGH CONFIDENCE IN WIND TRENDS.

TRS

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

FRIDAY NIGHT THROUGH SUNDAY...VFR.
MONDAY THROUGH TUESDAY...CHANCE OF SNOW. MVFR.
WEDNESDAY..VFR.

BMD

&&

MARINE...
300 AM CST

AREA OF LOW PRESSURE LIES OVER THE EASTERN GREAT LAKES THIS
MORNING WHILE A STRONG AREA OF HIGH PRESSURE IS BUILDING
SOUTHEAST OVER HUDSON BAY WITH RIDGING EXTENDING SOUTH INTO THE
NORTHERN PLAINS. THIS LEAVES LAKE MICHIGAN UNDER A MODERATE
PRESSURE GRADIENT WITH NORTH WINDS OF 30 TO 35 KT POSSIBLE THROUGH
THE DAY. THE GRADIENT IS SLIGHTLY TIGHTER OVER THE NORTHERN
PORTIONS OF THE LAKE...WHICH LENDS TO SLIGHTLY MORE CONFIDENCE IN
THE GALE WARNING VERIFYING ACROSS THE NORTH. ACROSS THE
SOUTH...GUIDANCE SUGGESTS SHALLOW MIXING MAY TAP INTO A FEW
GALES...WITH BEST CHANCES THIS AFTERNOON AS AN AREA OF MODERATE
HEIGHT RISES PUSHES ACROSS THE LAKE. OVERALL THOUGH...THIS MAY END
UP BEING A SLIGHTLY BELOW GALE EVENT. THE AIR MASS IS NOT
PARTICULARLY COLD LEADING TO ONLY SHALLOW MIXING...WITH THE
COLDEST AIR NOT ARRIVING UNTIL FRIDAY AS THE GRADIENT BEGINS TO
SLACKEN. FOR NOW...WHILE DO NOT HAVE MUCH CONFIDENCE IN GALES
ACROSS THE LAKE...THE DIFFERENCE MAY ONLY BE A FEW KNOTS AND FEEL
THE BEST COARSE OF ACTION WILL BE TO LEAVE HEADLINE IN PLACE. HIGH
PRESSURE WILL SLOWLY BUILD ACROSS THE LAKE OVER THE WEEKEND
ALLOWING A DOWNWARD TREND IN WIND SPEEDS. AN AREA OF LOW PRESSURE
WILL MOVE ACROSS THE MIDWEST EARLY NEXT WEEK...BUT FOR NOW THIS
DOES NOT LOOK LIKE A HIGH END EVENT FOR THE LAKE. BMD

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 9 PM THURSDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 AM SATURDAY.

&&

$$