FXUS63 KLOT 251040
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
440 AM CST FRI DEC 25 2009
.DISCUSSION...
415 AM CST
KLOT 88D BACK IN SERVICE WITH SPECIAL THANKS TO EL TECH STAFF
CALLED OUT TO THE OFFICE VERY EARLY THIS HOLIDAY MORNING.
MAIN CONCERNS THIS CHRISTMAS MORNING IS HEAVY LIQUID PRECIP AND
FLOOD POTENTIAL/ASSOCIATED HEADLINES TODAY. IN ADDITION...WINDY
CONDITIONS WORTHY OF SOME ATTENTION...ALONG TO TRANSITION TO COLDER
TEMPERATURES THIS AFTERNOON WITH PRECIPITATION CHANGING TO SNOW.
LINGERING LIGHT SNOW BENEATH UPPER LOW THIS WEEKEND MAY RESULT IN
LONG DURATION/LIGHT ACCUMULATION POTENTIALLY ADDING UP TO A FEW
INCHES BY END OF WEEKEND.
MASSIVE LONG WAVE TROUGH ACROSS MUCH OF CENTRAL NORTH AMERICA THIS
MORNING...WITH VIGOROUS MID LEVEL SHORT WAVE WRAPPING CYCLONICALLY
NORTHWARD FROM THE LOWER MISSISSIPPI VALLEY INTO SOUTHERN IL. 50-70
KNOT SOUTHERLY LOW LEVEL WINDS NOTED IN EVENING RAOBS ACROSS THE
GULF COAST STATES PRODUCING IMPRESSIVE MOISTURE TRANSPORT INTO THE
MIDWEST...WHICH HAS MAINTAINED PERSISTENT RAIN ACROSS THE FORECAST
AREA EARLY THIS MORNING. 00/06Z GUIDANCE IN GOOD AGREEMENT IN
TAKING MID LEVEL VORT NORTH/NORTHWEST INTO NORTHERN IA BY MIDDAY.
STRONGEST VERTICAL MOTION OCCURRING AHEAD OF THIS FEATURE...WITH
GRADUAL NORTH/NORTHEASTWARD SHIFT OF HEAVIER PRECIPITATION ACROSS
NORTHERN PARTS OF THE AREA AND INTO WISCONSIN EARLY THIS MORNING.
00Z WRF/GFS AND 06Z RUC ALL IN RELATIVELY GOOD AGREEMENT IN
PRODUCING ANOTHER 0.50-0.80" LIQUID QPF ACROSS NORTHEAST 1/2 OF CWA
IN 06-18Z PERIOD BEFORE FORCING WEAKENS BY LATE MORNING WITH PASSAGE
OF MID LEVEL VORT AND DRY SLOT INTRUSION OF RAPIDLY OCCLUDING
SYSTEM. KLOT 88D APPEARS TO BE OVERESTIMATING PRECIP AMOUNTS ACROSS
WESTERN CWA DUE TO BRIGHT BANDING OR MIXED PHASE PRECIP ALOFT...BUT
SURFACE REPORTS THROUGH 06Z INDICATE TOTALS SINCE WEDNESDAY ROUGHLY
1.75 INCHES ACROSS WESTERN IL COUNTIES...WITH PERHAPS 0.80 INCHES
ACROSS EAST CENTRAL/NORTHEAST IL. AMOUNTS TAPER TO A QUARTER INCH OR
LESS FARTHER TO THE EAST ACROSS NORTHWEST INDIANA. THIS RAINFALL ON
TOP OF SNOW MELT AND FROZEN SOILS INDICATES CONTINUED POTENTIAL FOR
FLOODING ACROSS THE AREA TODAY...THUS WHILE HEAVIER RAIN TAPERING
OFF THIS MORNING...FEEL MAINTAINING CURRENT FLOOD HEADLINES THROUGH
MIDDAY EXPIRATION WARRANTED.
COLDER AIR WRAPS INTO FORECAST AREA FROM THE SOUTHWEST THIS
AFTERNOON AS SURFACE LOW CONTINUES TO OCCLUDE OVER IOWA. FORECAST
SOUNDINGS AND LOW LEVEL THICKNESS FIELDS SUGGEST PRECIP SHOULD
REMAIN LIQUID THROUGH MIDDAY...WITH SURFACE TEMPS ABOVE FREEZING.
COLUMN COOLS RAPIDLY DURING THE AFTERNOON...WITH PRECIP EXPECTED TO
CHANGE TO LIGHT SNOW/SNOW SHOWERS FROM WEST TO EAST. HAVE BASED
HOURLY TEMP FORECAST ON WRF RAW SFC TEMPS...WITH NON-DIURNAL MAXIMA
OCCURRING THIS MORNING...THEN FALLING RATHER SHARPLY BY MID/LATE
AFTERNOON.
VERTICALLY STACKED STORM SYSTEM PROGGED TO TAKE UP RESIDENCE ACROSS
THE MIDWEST FOR THE WEEKEND...AS UPPER LOW PHASES WITH SECOND HEIGHT
MINIMA DIGGING THROUGH THE PLAINS. PHASED CIRCULATION THEN WOBBLES
SLOWLY EAST ACROSS THE IL/WI BORDER REGION THROUGH SUNDAY...
PRODUCING OCCASIONAL LIGHT PRECIP WITH A FEW HUNDREDTHS OF AN INCH
OF QPF EACH PERIOD. GUIDANCE POPS VARY A BIT THROUGH THE WEEKEND
THOUGH HAVE SIDED WITH HIGHER NUMBERS AS SUSPECT WE WILL SEE A
PERSISTENT LIGHT QPF EVENT WHICH THE STATISTICAL GUIDANCE IS
PROBABLY NOT HANDLING VERY WELL. WITH WRF INDICATING SNOW/LIQUID
RATIOS OF 15:1 OR BETTER...THIS SUGGESTS POTENTIALLY A HALF INCH TO
AN INCH OF SNOW ACCUMULATION EACH PERIOD...WITH A FEW INCHES
POSSIBLE IN TOTAL BY THE TIME THE LOW EVENTUALLY WANDERS OFF TOWARD
THE EAST SUNDAY NIGHT. CONFIDENCE NOT EXCEPTIONALLY HIGH WITH THESE
AMOUNTS...BUT AT LEAST IT WILL BE RELATIVELY LIGHT ACCUMULATION OVER
A LONG DURATION.
AS FOR WINDS IN SHORT TERM...HAVE SEEN EASTERLY WINDS APPROACHING
WIND ADVISORY CRITERIA LAST EVENING...THOUGH APPEARS THAT LOW HAS
REACHED ITS MINIMUM PRESSURE...WITH PRESSURE FALLS RELAXING ACROSS
IA/MN/WI/IL/IN LAST FEW HOURS. WHILE IT WILL BE WINDY TODAY...EXPECT
WINDS SHOULD REMAIN BELOW ADVISORY CRITERIA AT 20-30 GUST 40 THIS
MORNING WITH A DIMINISHING TREND THROUGH THIS AFTERNOON/EVENING
THOUGH STILL BLUSTERY.
NO SIGNIFICANT CHANGES BEYOND SUNDAY.
RATZER
&&
.AVIATION...
0600 UTC TAFS...FORECAST CONCERNS FOR THE PERIOD CONTINUE TO BE
WITH CEILINGS AND PRECIP AS WELL AS STRONG EASTERLY WINDS.
SOME OF THE IFR CEILINGS WHICH WERE GRADUALLY MAKING THEIR WAY
INTO THE CWA EARLIER THIS EVENING HAVE SINCE LIFTED WITH VFR
CEILINGS IN PLACE ACROSS MUCH OF THE REGION. THIS HAS SHIFTED
EARLIER THOUGHTS OF A QUICKER PROGRESSION TOWARDS IFR CEILINGS
TONIGHT INTO FRIDAY MORNING. HAVE BACKED OFF ON TIMING OF LOWER
CEILINGS...AND EVEN WITH THE POSSIBILITY OF IFR CEILINGS...MORE
TOWARDS THE 9Z TIME FRAME. ALTHOUGH WILL CONTINUE TO MONITOR
TRENDS TONIGHT FOR THE POTENTIAL OF LOWER CEILINGS. SAME THINKING
AS EARLIER FORECAST IN TERMS OF PRECIP TONIGHT THROUGH TOMORROW
WITH RAIN CONTINUING INTO MID DAY FRIDAY WITH A TRANSITION BACK
OVER TO ALL LIGHT SNOW IN THE AFTERNOON ON FRIDAY. CEILINGS
THROUGHOUT THIS TIME FRAME WILL HOLD AT VFR CONDITIONS...THEN
CONTINUING THROUGH THE END OF THE FORECAST PERIOD.
WINDS HAVE CONTINUED TO INCREASE TONIGHT WITH THE APPROACH OF THE
AREA OF LOW PRESSURE TO THE SOUTH...AS TIGHT PRESSURE GRADIENT IN
PLACE THROUGHOUT NORTHEAST ILLINOIS ALONG WITH SOME HIGHER WINDS
JUST ABOVE THE SURFACE BEING TAPPED INTO. WINDS WILL NOT INCREASE
MUCH MORE THAN WHAT IS BEING OBSERVED TONIGHT...AS AREA OF LOW
PRESSURE WILL NOT DEEPEN MUCH WITH ITS TRACK NORTH. WIND GUSTS WILL
COME DOWN THROUGHOUT THE DAY ON FRIDAY...BUT MAY SEE AN INCREASE
ONCE AGAIN INTO THE EARLY EVENING HOURS AS COLDER AIR WRAPS AROUND
THIS SYSTEM.
RODRIGUEZ
&&
.MARINE...
439 AM CST
GALES INTO EARLY THIS EVENING AND THEN POTENTIAL FOR MORE GALES
MONDAY WILL CONTINUE TO BE THE MAIN MARINE ISSUES IN THIS FORECAST
PACKAGE.
SURFACE LOW ACROSS NORTHEAST MISSOURI APPEARS TO HAVE REACHED ITS
MAX INTENSITY AT 29.1 INCHES AS SFC LOW APPEARS TO BECOMING
CENTERED IN BETWEEN SFC PRESSURE RISE/FALL COUPLET EARLY THIS
MORNING. THIS LOW IS EXPECTED TO TAKE A SLIGHT NORTHWEST JOG THIS
MORNING INTO CENTRAL IOWA THIS AFTERNOON. AS THIS
OCCURS...DIRECTIONS WILL BECOME SOUTHEAST THIS AFTERNOON. HAVE NOT
MADE ANY CHANGES TO GOING HEADLINES WITH GALE WARNING ACROSS THE
SOUTH HALF THROUGH 00Z AND THROUGH 03Z FOR THE NORTH HALF OF THE
LAKE. 00Z MAY BE A BIT LONG FOR THE SOUTH HALF GIVEN LOW WILL
BEGIN FILLING THIS MORNING...BUT CANNOT RULE OUT AT LEAST
POTENTIAL FOR SOME LINGERING GALE GUSTS THIS AFTERNOON ACROSS THE
SOUTH HALF. OCCLUDED FRONT WILL LIFT NORTH ACROSS LAKE MICHIGAN
THIS EVENING WHICH WILL SHIFT WINDS TO THE SOUTH WITH CONTINUED
GUSTS TO 25 KT OVERNIGHT. WINDS TO CONTINUE TO LESSEN IN SPEED
SATURDAY AS FILLING LOW APPROACHES LAKE MICHIGAN. A SECONDARY
TROUGH OF LOW PRESSURE MARKING ARCTIC AIR MASS WILL TRACK ACROSS
NORTHERN GREAT LAKES SUNDAY NIGHT...AND DO SEE A POTENTIAL OF
GUSTS APPROACHING GALES MONDAY FROM THE NORTHWEST.
MARSILI
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FLOOD WATCH...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
ILZ032-ILZ033-ILZ039 UNTIL NOON FRIDAY.
IN...FLOOD WATCH...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL NOON
FRIDAY.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
LMZ874-LMZ876-LMZ878 UNTIL 6 PM FRIDAY.
GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868 UNTIL 9 PM FRIDAY.
&&
$$
