000
FXUS63 KLOT 012007
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
207 PM CST SAT MAR 1 2014
.DISCUSSION...
1158 AM CST
DID A QUICK UPDATE TO THE GRIDS FOR THIS AFTERNOON TO MOVE UP THE
TIMING OF THE ONSET OF THE SNOW THIS AFTERNOON ACROSS NORTHERN
PORTIONS OF THE CWA. STILL LOOKS LIKE STRONG MID LEVEL FRONTOGENESIS
WILL SUPPORT AN AREA OF FAIRLY IMPRESSIVE SNOWFALL ACROSS THE AREA
LATE THIS AFTERNOON INTO TONIGHT...SPREADING GRADUALLY SOUTH ACROSS
THE CWA. WILL LIKELY BE NUDGING STORM TOTAL SNOWFALL AMOUNTS
DOWNWARD SOME...PROBABLY MORE INTO THE 3-5" RANGE ACROSS THE
CWA...WITH MANY AREAS POSSIBLY WINDING UP ON THE LOWER END OF THAT
RANGE. EVEN SO...A COUPLE/FEW HOURS OF STRONG ASCENT COUPLED WITH
WEAKER STABILITY ALOFT COULD ALLOW FOR SOME BRIEFLY IMPRESSIVE
SNOWFALL RATES. ALSO APPEARS AS THOUGH SNOWFALL WILL BE WRAPPING UP
EARLIER THAN PREVIOUSLY THOUGHT...WITH THE MAJORITY DONE NORTH OF
I-80 BY MIDNIGHT.
IZZI
//PREV DISCUSSION...
400 AM CST
MAIN CONCERN FOR THE FORECAST PERIOD WILL BE THE POTENTIAL FOR HEAVY
SNOW THROUGH THE WEEKEND.
FOR THE REMAINDER OF THE OVERNIGHT HOURS...A WEAK SHORTWAVE INVOF
THE LEFT EXIT REGION OF A STRONG UPPER LEVEL JET IS PROVIDING THE
DYNAMICS TO GENERATE A QUICK ROUND OF SNOW EARLY THIS MORNING. WITH
ONLY WEAK SFC TROUGHING AND VERY MODEST LOW LEVEL FORCING...THE SNOW
IS ALMOST ENTIRELY RELYING ON UPPER LEVEL DYNAMICS TO SUPPORT THE
SNOW GENERATION. LATEST RADAR TRENDS INDICATE THAT THE BAND OF SNOW
EXTENDS FROM WAUKEGAN TO PERU AND IS MOVING VERY QUICKLY TO THE
EAST. THE SNOW WILL END SHORTLY IN THE ROCKFORD AREA AND WILL
QUICKLY PUSH PAST THE CHICAGO METRO AREA SHORTLY AFTER
DAYBREAK...AND EVEN ENDING OVER THE FAR EASTERN PORTIONS OF THE CWA
BY LATE THIS MORNING. SINCE THE SYSTEM IS MOVING SO QUICKLY...ONLY
AROUND AN INCH OF ACCUMULATION IS EXPECTED.
THE MAIN SNOW CONCERN IS SETTING UP FOR LATE THIS AFTERNOON...AND
PERSISTING ACROSS PORTIONS OF THE REGION UNTIL LATE SUNDAY NIGHT.
GIVEN THE LONG DURATION OF THE EVENT...HAVE OPTED TO ISSUE A WINTER
WEATHER ADVISORY FOR THE ENTIRE CWA. THE ADVISORY WILL BE IN 2
SEGMENTS...BEGINNING EARLIER FOR LOCATIONS NORTH OF THE ILLINOIS AND
KANKAKEE RIVERS...AND THEN SPREADING SOUTH OVERNIGHT TONIGHT.
THE MAIN FEATURE OF CONCERN IS A STRONG CLOSED UPPER LOW OFF THE
CENTRAL CALIFORNIA COAST. THE GUIDANCE HAS CONVERGED INTO
RELATIVELY GOOD AGREEMENT IN THE LARGER SCALE DETAILS WITH THE
SYSTEM EVOLVING INTO AN OPEN WAVE AS THE SYSTEM MOVES ON SHORE LATE
THIS AFTERNOON. THIS WILL SET UP A STREAM OF SWLY FLOW ALOFT AS
WEAK UPPER RIDGING BUILDS OVER THE CENTRAL CONUS AND THE UPPER
TROUGH DIGS OVER THE WEST. A BROAD UPPER LOW CENTERED OVER HUDSON
BAY...WITH TROUGHING THROUGH THE UPPER GREAT LAKES WILL SQUEEZE THE
UPPER LEVEL FLOW...SETTING UP VERY FAST FLOW ALOFT RIPPLES OF SHORT
WAVE ENERGY EJECTING OUT OF THE SOUTHERN ROCKIES WITH A DIRECT
TRAJECTORY ACROSS THE MIDDLE MISSISSIPPI VALLEY TO THE GREAT LAKES.
AT THE SURFACE...POLAR HIGH PRESSURE WILL BUILD OUT OF CANADA AND
SPREAD ACROSS THE NORTHERN PLAINS AND INTO THE MIDDLE AND UPPER
MISSISSIPPI VALLEY TODAY. AFTER A RELATIVELY MILD FRIDAY...THIS
AREA OF HIGH PRESSURE WILL BRING AN ARCTIC AIRMASS BACK TO THE REGION
AT THE SFC. THE SNOW WILL AGAIN LARGELY BE DRIVEN BY UPPER LEVEL
DYNAMICS...WITH THE BAND OF STRONGEST FORCING SAGGING SOUTHWARD
THROUGH THE CWA. CURRENTLY...ANTICIPATE THAT THE SNOW WILL BEGIN
OVER THE NWRN PORTIONS OF THE CWA...INCLUDING THE ROCKFORD METRO
AREA BY AROUND 3PM THIS AFTERNOON AND GRADUALLY SPREAD SOUTH AND
EAST...WITH HEAVIER SNOW REACHING THE CHICAGO METRO AREA DURG THE
EARLY EVENING HOURS. AS THE UPPER LEVEL TROUGHING EXTENDING FROM
HUDSON BAY BACK TOWARD SOUTH CENTRAL CANADA BEGINS TO DIG SOUTHWARD
ACROSS THE UPPER GREAT LAKES LATE TONIGHT AND EARLY SUNDAY...THE
AREA OF SNOW WILL BE QUICKLY FORCED SOUTH...WITH SNOW ENDING OVER
THE NORTHERN TIER COUNTIES BY EARLY SUNDAY MORNING. HOWEVER...THE
MAIN SHORTWAVE TROUGH ASSOCIATED WITH THE PACIFIC SYSTEM WILL EJECT
OUT OF THE SOUTHERN ROCKIES BY SUNDAY MORNING AND TRACK ACROSS THE
PLAINS TO THE MIDDLE MISSISSIPPI VALLEY BY SUNDAY NIGHT...AND THEN
PUSH QUICKLY EAST OF THE AREA BY EARLY MONDAY MORNING. WHILE THE
CANADIAN TROUGH WILL FORCE AN END TO THE SYNOPTIC SNOW OVER THE NRN
PORTIONS OF THE BY EARLY SUNDAY MORNING...THE PACIFIC SHORTWAVE
ENERGY WILL KEEP THE SNOW POTENTIAL GOING OVER THE SRN PORTIONS OF
THE CWA THROUGH LATE SUNDAY NIGHT. ALL TOTAL...EXPECT AROUND 3 TO 5
INCHES ACCUMULATION OF RELATIVELY DRIER SNOW OVER THE NORTHERN
PORTIONS OF THE CWA...AND 5 TO 7 INCHES OVER THE SOUTHERN PORTIONS
OF THE CWA...THOUGH THE SNOW OVER THE SOUTHERN LOCATIONS MAY LIKELY
CONTAIN SLIGHTLY HIGHER LIQUID WATER CONTENT DUE TO THE LONGER
DURATION OF THE PACIFIC SYSTEM IMPACTING THE REGION.
AS THE SYNOPTIC SNOW TAPERS OFF OVER THE NORTHERN PORTIONS OF THE
CWA EARLY SUNDAY MORNING...THE SFC HIGH PRESSURE WILL SPREAD EAST
ACROSS THE UPPER MISSISSIPPI VALLEY AND THE GREAT LAKES REGION. THE
LARGER SCALE MODELS...IN PARTICULAR...THE GFS...HAVE BEEN
ADVERTISING THE POTENTIAL FOR A LAKE EFFECT BAND OF SNOW TO DEVELOP
OVER LAKE MICHIGAN BY SUNDAY MORNING. THE SHORTER RANGE HIGH RES
GUIDANCE IS NOW SHOWING A SIMILAR SETUP WITH SFC CONVERGENCE SETTING
UP OVER WESTERN LAKE MICHIGAN. THIS COULD BRING PERIODS OF LAKE
EFFECT SNOW SHOWERS TO FAR NERN IL...AND EVENTUALLY FAR NWRN INDIANA
AS THE LAKE PLUME MIGRATES EWD INTO EARLY MONDAY MORNING. GIVEN THE
EXTENSIVE ICE COVER ON LAKE MICHIGAN AND RELATIVELY COLD SFC WATER
TEMPERATURES...INVERSION HEIGHTS WILL BE RELATIVELY SHALLOW AND
EQUILIBRIUM LEVELS ONLY AROUND 5-6KFT AND CAPE VALUES OF 300-400
J/KG. THE MODEST CONVECTIVE DEPTH AND INSTABILITY WOULD SUGGEST
THAT SIGNIFICANT LAKE EFFECT SNOW IS NOT LIKELY...BUT PERIODS OF
LAKE EFFECT SNOW SHOWERS PRODUCING ADDITIONAL LIGHT ACCUMULATIONS
OVER THE CHICAGO METRO AREA ARE CERTAINLY POSSIBLE SUNDAY AND SUNDAY
NIGHT.
AS THE COLD HIGH PRESSURE SETTLES OVER THE REGION EARLY NEXT
WEEK...THE REGION CAN EXPECT A FEW MORE DAYS WITH LOW TEMPERATURES
RANGING FROM THE SINGLE DIGITS ABOVE ZERO TO THE SINGLE DIGITS BELOW
ZERO. BRISK NORTH WINDS SHOULD DRIVE WIND CHILL READING TO AROUND 5
TO 15 BELOW ZERO FOR SUNDAY MORNING. AS THE CENTER OF THE HIGH
SETTLES OVER IOWA BY MONDAY MORNING...COLD ADVECTION WILL DRIVE
TEMPERATURES BELOW ZERO FOR PORTIONS OF THE AREA GENERALLY WEST OF
THE I-57 CORRIDOR AND THE FOX VALLEY...WHERE TEMPERATURES COULD DIP
TO AROUND 5 BELOW ZERO...WITH WIND CHILLS TO 20 BELOW ZERO. THE
CENTER OF THE HIGH WILL MOVE OVER THE AREA FOR TUESDAY MORNING...SO
CLEAR SKIES...LIGHT WINDS AND FRESH SNOW COVER WILL ALLOW FOR MORE
WIDESPREAD LOWS BELOW ZERO DUE TO RADIATIVE COOLING.
FOR THE EXTENDED FORECAST PERIOD...FOR TUESDAY THROUGH FRIDAY...THE
LONGER RANGE GUIDANCE CONTINUES TO ADVERTISE A GRADUAL WARMING TREND
BY THE END OF THE PERIOD. WEDNESDAY SHOULD REMAIN RELATIVELY COOL
WITH HIGHS STILL ONLY IN THE MIDDLE 20S...BUT AS THE ARCTIC HIGH MOST
WELL TO THE EAST BY THURSDAY...SETTING UP A WARM ADVECTIVE
PATTERN...TEMPERATURES SHOULD REBOUND INTO THE LOWER 30S BY THURSDAY
AND COULD EVEN PEG 40F BY FRIDAY. A SERIES OF MID LEVEL SHORTWAVES
COULD BRING PERIODIC CHANCES FOR SOME LIGHT SNOW ON WEDNESDAY AND
THURSDAY...BUT NOTHING SIGNIFICANT IS LIKELY.
KREIN
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