FXUS63 KLOT 261207
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
607 AM CST SAT DEC 26 2009
.DISCUSSION...
346 AM CST
ATTENTION REMAINS FIXED ON LARGE CUTOFF UPPER LOW OVER THE MIDWEST
AND ON PRECIPITATION TRENDS AND SNOWFALL AMOUNTS ACROSS FORECAST
AREA INTO SUNDAY AS SYSTEM VERY SLOWLY DRIFTS EAST.
00Z UPPER AIR ANALYSIS DEPICTS APPROXIMATE CENTER OF LARGE SCALE LOW
ALONG IA/MO BORDER...WITH CURRENT GOES VAPOR LOOP SHOWING SEVERAL
SMALLER SCALE SHORT WAVES/VORT CENTERS ROTATING COUNTER-CLOCKWISE
ABOUT. THESE SMALLER EMBEDDED DISTURBANCES TO GOVERN PERIODS OF
PRECIP ACROSS THE AREA THROUGH SUNDAY...BEFORE UPPER TROUGH AXIS
FINALLY SHIFTS EAST OF THE FA. ONE SUCH WAVE LIFTED ACROSS NORTHERN
IL/NORTHWEST IN LAST EVENING...AND HAS LIFTED INTO SOUTHERN WI ALONG
WITH ASSOCIATED MID LEVEL COLD POOL. NEXT IS NOTED DIGGING ACROSS
SOUTHWEST IA/NORTHWEST MO...AND GUIDANCE INDICATES THIS FEATURE WILL
ROTATE ACROSS IL THROUGH THIS AFTERNOON. LIGHT SNOW HAS ALREADY
PICKED UP IN ADVANCE OF THIS FEATURE FROM WEST CENTRAL INTO
NORTHEAST IL...THOUGH THIS MAY BE MORE ASSOCIATED WITH IMPRESSIVE
MID LEVEL WARM ADVECTION IN WAKE OF INITIAL COLD POOL WITH LAST
EVENINGS WAVE...AND CONVERGENCE NOTED IN LOWER/MID LEVEL PROFILER
WINDS ACROSS MO/WESTERN IL. 03Z SREF APPEARS TO CAPTURE TREND WELL
WITH INCREASING 3 HOUR POPS SHOWING GOOD AGREEMENT WITH RADAR
DEPICTED SNOW AREA. SREF WOULD SUGGEST THIS AREA OF LIGHT SNOW TO
CONTINUE TO DEVELOP NORTHEAST ACROSS NORTHERN IL TODAY...BEFORE VORT
LIFTS NORTHEAST OF AREA THIS EVENING AND PRECIP DIMINISHES FROM THE
WEST. GOOD AGREEMENT IN QPF OUTPUT FROM 06Z WRF/00Z GFS/03Z SREF
WITH 0.10-0.15 INCHES TODAY WHICH BASED ON SNOW/LIQUID RATIOS OF
APPROXIMATELY 18 TO 1 FROM WRF BUFFER OUTPUT WOULD YIELD 2-3 INCHES
OF SNOW ACCUMULATION THROUGH 00Z...AND PERHAPS ANOTHER 1 INCH THIS
EVENING. APPEARS THAT NORTHWEST 2/3 OR SO OF CWA WOULD BE IN
POSITION FOR BEST ACCUMULATION...WITH AMOUNTS TAILING OFF TO AN INCH
OR LESS FAR SOUTHEAST. FLUFFY NATURE OF SNOW...COMBINED WITH SOUTH
WINDS GUSTING AROUND 20 MPH MAY CAUSE SOME DRIFTING ON ESPECIALLY
RURAL ROADS...THOUGH FORECAST AMOUNTS A BIT ON THE LOW SIDE OF
ADVISORY CRITERIA. GIVEN POTENTIAL FOR SOME TRAVEL IMPACT WILL
HIGHLIGHT IN HWO THIS MORNING...THOUGH WILL MONITOR AND DEFER TO DAY
SHIFT FOR NOW FOR ANY POTENTIAL HEADLINE ISSUANCE.
SNOW SHOULD DIMINISH FROM THE WEST EARLY THIS EVENING AS VORT
CONTINUES TO LIFT NORTHEAST OF FORECAST AREA...THOUGH OCCASIONAL
LIGHT SNOW WILL CONTINUE OVERNIGHT AS FRIDAY EVENINGS VORTEX WRAPS
BACK ACROSS THE AREA LATE TONIGHT/EARLY SUNDAY AS IT BECOMES MAIN
UPPER LOW CIRCULATION. SREF POPS...AS WELL AS THOSE FROM MAV/MET
GUIDANCE...DECREASE AFTER THE EARLY EVENING HOURS WITH MODEL QPF
ALSO DECREASING ACROSS FA OVERNIGHT. WITH THIS IN MIND HAVE TRENDED
FROM CATEGORICAL POPS ACROSS FA TODAY...TO SCATTERED SNOW SHOWERS
BEYOND THIS EVENING AND SUNDAY. MINOR ADDITIONAL ACCUMULATIONS
GENERALLY AN INCH OR LESS EXPECTED LATE TONIGHT/SUNDAY. UPPER TROUGH
THEN FINALLY OPENS UP AND MOVES OFF TO THE EAST SUNDAY NIGHT/MONDAY.
MID LEVEL DRYING BEHIND TROUGH SUGGEST NO MORE THAN SOME LINGERING
FLURRIES MAINLY SUNDAY NIGHT.
GUIDANCE COMING INTO BETTER AGREEMENT IN DIGGING ANOTHER SHORT WAVE
SOUTH OUT OF MANITOBA/WESTERN ONTARIO MONDAY...WITH REINFORCING PUSH
OF COLDER AIR ARRIVING LATE MONDAY. BASED ON THIS HAVE LOWERED MIN
TEMPS INTO THE SINGLE DIGITS AND LOWER TEENS MONDAY NIGHT/EARLY
TUESDAY ESPECIALLY ACROSS NORTH CENTRAL IL. AT THIS TIME...MODEL
TIME SECTIONS INDICATE SIGNIFICANT AMOUNT OF DRY AIR ABOVE BOUNDARY
LAYER THUS HAVE REFRAINED FROM ADDING ANY PRECIP WITH COLD FRONT.
NO CHANGES TO EXTENDED PORTION OF FORECAST BEYOND DAY 4 AT THIS
TIME.
RATZER
&&
.AVIATION...
606 AM CST
1200 UTC TAFS...SNOW AND IMPACT ON CIGS/VSBYS WILL BE THE MAIN
FORECAST CHALLENGE OVER THE NEXT 24 HOURS.
SEVERAL SHORT WAVES CONTINUE TO ROTATE THROUGH BROAD UPPER LEVEL
CYCLONIC CIRCULATION. THE STRONGEST OF THESE SHORT WAVES HAS NOW
MOVED NORTH INTO SOUTHERN WISCONSIN...ALTHOUGH NEXT VORT MAX
ACROSS CENTRAL IOWA WILL MOVE ACROSS THE REGION THIS EVENING. MOST
NOTABLE FORCING MECHANISM FOR SNOW TODAY WILL BE STRONG MID LEVEL
WARM ADVECTION ASSOCIATED WITH RETREATING MID LEVEL COLD POCKET
THAT ACCOMPANIED THE MORE VIGOROUS UPPER LEVEL WAVE THAT TRACKED
INTO SOUTHERN WISCONSIN. AREA OF LIGHT SNOW THAT DEVELOPED JUST
AFTER MIDNIGHT ACROSS WEST CENTRAL ILLINOIS SHOULD GRADUALLY
TRANSLATE NORTHEASTWARD THIS MORNING AND IMPACT TERMINALS. FEEL
THAT IFR VSBYS AND LOWER END MVFR CIGS WILL BE THE THEME TODAY.
PERIOD OF GREATEST ACCUMULATION IS EXPECTED TO OCCUR IN THE 18Z-
22Z TIME FRAME WHEN THE MID LEVEL WARM ADVECTION WILL BE MAXIMIZED
ACROSS THE AREA. THIS FORCING WILL LIFT NORTH OF THE AREA LATER
THIS EVENING BUT POTENTIAL FOR MORE SCATTERED LIGHT SNOW SHOWERS
OR FLURRIES WILL CONTINUE THROUGH THE OVERNIGHT HOURS AS THE SHORT
WAVE ACROSS WISCONSIN BEGINS TO ROTATE BACK SOUTHWARD ACROSS THE
AREA. HAVE INDICATED IMPROVEMENT BACK TO MVFR VSBYS BY EARLY THIS
EVENING BUT FUTURE FORECASTS MAY NEED TO CONSIDER EXTENDING IFR
MENTION INTO EARLY THIS EVENING. AT THIS TIME...EXPECTING SNOW
ACCUMULATIONS TO RANGE FROM 1 TO 3 INCHES TODAY WITH POSSIBLY
ANOTHER INCH THIS EVENING. CIGS EXPECTED TO REMAIN GENERALLY MVFR
ALTHOUGH PERIODS OF IFR CIGS POSSIBLE IN SHORTER DURATION MODERATE
PERIODS OF SNOW. SOUTH WINDS GUSTING TO AROUND 15 KNOTS TODAY WILL
GRADUALLY BECOME SOUTHWEST THIS EVENING AS THE LOW PRESSURE ACROSS
IOWA MOVES INTO SOUTHERN WISCONSIN...AND THEN EVENTUALLY BECOMING
WEST TOWARD SUNDAY MORNING.
MARSILI
&&
.MARINE...
327 AM CST
EVOLUTION OF WEAKENING SFC LOW ACROSS IOWA OVER NEXT 36
HOURS AND ARCTIC FRONT APPROACHING THE REGION EARLY MONDAY BRINGING
STRONGER WINDS WILL BE THE MAIN FORECAST ISSUES THIS MORNING.
SURFACE LOW ACROSS SOUTHERN IOWA CONTINUES TO FILL THIS MORNING AND
WILL BE APPROACHING CENTRAL PORTIONS OF LAKE MICHIGAN LATE THIS
EVENING. SOUTH WINDS WILL CONTINUE AND OBS INDICATING GUSTS TO
AROUND 25 KNOTS CONTINUE ACROSS SOUTHERN LAKE MICHIGAN THIS MORNING.
AS LOW CONTINUES TO FILL AND SFC PRESSURE GRADIENT SLACKENS OFF
LATER THIS MORNING SHOULD SEE GUSTS OF THIS MAGNITUDE BEGIN TO
SUBSIDE BACK INTO THE 15 TO 20 KT RANGE ACROSS SOUTHERN PART OF THE
LAKE. LOW VERY SLOW TO DEPART CENTRAL/NORTHERN LAKE MICHIGAN
SUNDAY...AND FOCUS FOR SUNDAY NIGHT WILL BE APPROACH OF ANOTHER
TROUGH ROTATING THROUGH BACKSIDE OF THIS LOW BRINGING SHOT OF STRONG
COLD AIR ADVECTION. SOME CONCERN FOR GALES DURING THIS PERIOD GIVEN
DEEP MIXING AND PRESENCE OF 40 KNOT 850 HPA FLOW. AT THIS TIME
BELIEVE NORTHWEST GALES WOULD REPRESENT JUST THE HIGHER END GUSTS
AND DO NOT HAVE ENOUGH CONFIDENCE TO GO WITH A GALE WATCH AT THIS
TIME FOR MONDAY...BUT WILL CONTINUE TO MENTION IN GLF MAINLY ACROSS
NORTHERN HALF OF LAKE WHERE MIXING HEIGHTS WILL BE DEEPER. WINDS
THEN BEGIN TO SUBSIDE MONDAY NIGHT AND TUESDAY AS STRONG HIGH
PRESSURE BEGINS TO BUILD IN FROM THE WEST.
MARSILI
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&
$$
