000
FXUS63 KLOT 220912
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
412 AM CDT TUE OCT 22 2013
.DISCUSSION...
343 AM CDT
USUALLY IT WOULD BE ANOTHER FEW WEEKS OR EVEN LONGER IN OUR
FORECAST AREA BEFORE INTERROGATING SEVERAL OF THE FORECAST
INGREDIENTS THAT WERE THE FOCUS THIS MORNING. CHALLENGES WERE
LOOKING AT PRECIP TYPE AND INTENSITY TODAY...TO SEE IF SOME PLACES
COULD GET SOME BRIEF ACCUMULATION NAMELY NEAR THE I-80 /FIRST SUCH
ACCUMULATION IN OVER 15 YEARS AND LONGER FOR SOME/...AND THEN
LOOKING AT LAKE EFFECT PRECIP POTENTIAL AND TYPE FOR MAINLY PORTER
COUNTY INDIANA LATE TONIGHT AND WEDNESDAY MORNING.
SYNOPSIS...VIGOROUS SHORT WAVE ENTERING WESTERN IA EARLY THIS
MORNING ON A 120+ JET IS BEGINNING TO CURL MORE SE AROUND THE BASE
OF THE LONG WAVE TROUGH RESPONSIBLE FOR OUR COLD LATE OCTOBER
SNAP. A SURFACE REFLECTION IS DEPICTED IN OBSERVATIONS WITH A
BROAD LOW ACROSS WESTERN IA WHICH WILL TIGHTEN AND DEEPEN THROUGH
THIS MORNING. SOME LIGHT RAIN IS SEEN ACROSS PARTS OF CENTRAL AND
WESTERN IA IN THE WARM AIR ADVECTION FLANK OF THIS CLIPPER.
MODEL PREFERENCE...MODEL GUIDANCE IS IN GOOD GENERAL AGREEMENT
OF THIS WAVE TRANSITIONING EAST SOUTHEAST THIS AFTERNOON ACROSS
CENTRAL IL WITH SOME VARIABILITY IN LATITUDE OF ITS KEY
FEATURES...WHICH OF COURSE DICTATE ALL THE KEY ELEMENTS SUCH AS
PLACEMENT OF HEAVIER PRECIP AND SUBTLE THERMAL DIFFERENCES THAT
INFLUENCE WHERE TRANSITION TO RA/SN MIGHT OCCUR. HAD LEANED A
LITTLE MORE TO THE 00Z NAM WHICH HAS VERY CLOSE SUPPORT BY THE 00Z
EC AND THE 03Z SREF MEAN. IT SHOULD BE NOTED THIS 00Z NAM HAD
SHIFTED SLIGHTLY FURTHER SOUTH THAN ITS PRECEDING 18Z RUN...AND
NOW THE 06Z SOLUTION HAS EVEN SHIFTED A BIT FURTHER SOUTH. THE
TREND FROM THAT OF TIGHTENING THE PRECIP GRADIENT ON THE NORTH
SIDE HAS ALSO BEEN NOTED WHICH MAKES GOOD SENSE WITH HOW THIS
SYSTEM IS ORIENTED AND ITS GENERALLY COMPACT NATURE AND QUICK
MOVEMENT. THE MAIN POINT WITH THIS IS IT BASICALLY KEEPS PRECIP
OUT OF CHICAGO. HAVE NOT GONE THAT SHARP BUT HAVE TIGHTENED THAT
NORTHERN GRADIENT.
TODAY AND TONIGHT...THE UPPER WARM AND MOIST ASCENT AHEAD OF THE
CLIPPER SHOULD LEAD TO A BLOSSOMING OF RADAR ECHOES ACROSS THE
WESTERN FORECAST AREA BY LATER THIS MORNING. GOOD CONVERGENCE AND
THERMAL GRADIENT TIGHTENING IN THE MID-LEVELS...WITH SUPPORTIVE
LAPSE RATES...WILL LIKELY CREATE SOME TEMPORARY MORE INTENSE WEST-
TO-EAST CORRIDORS. MOST HIGH RESOLUTION GUIDANCE BOTH LOCALLY AND
NATIONALLY TEND TO INDICATE THIS...ALTHOUGH AGAIN WITH
DISCREPANCIES IN HOW FAR NORTH...SOME AS FAR NORTH AS AROUND
I-88...OTHERS TOWARD PEORIA TO BLOOMINGTON/NORMAL.
THIS HEAVIER
PRECIP IS IMPORTANT BECAUSE MARGINAL THERMAL PROFILES WOULD NEED
THE DIABATIC COOLING TO THE WET BULB /OR COOLER/ TO ENABLE SNOW TO
MIX IN TO THE SURFACE. HAVE GENERALLY FAVORED THE
LEE/LASALLE/LIVINGSTON COUNTIES THE MOST THIS AFTERNOON...AS THAT
CORRIDOR OF WET BULB COOLING AT THE SURFACE /TEMPS DROPPING INTO
THE MID 30S/ SHOULD BE SHIFTING SOUTHEAST THROUGH THOSE COUNTIES.
IF SNOW CAN MATERIALIZE...TEMPORARY RATES COULD CERTAINLY BE
ENOUGH TO BRING DOWN VISIBILITY QUICKLY...AND ITS POSSIBLE THOUGH
NOT REALLY FAVORED TO SEE SOME LIGHT SLUSHY ACCUMULATION. LATE IN
THE DAY AND THIS EVE THE PROFILES BECOME MORE SUPPORTIVE TO SEE A
MIX ACROSS THE WHOLE AREA...BUT THE OVERALL STRONGER FORCING IS
WEAKENING AS THE CLIPPER TRANSITIONS AWAY. SO REALLY NOT EXPECTING
ANY ACCUMULATION ELSEWHERE IF ANY OTHER PLACES CAN ACTUALLY CHANGE
OVER. IN MUCH OF THE CHICAGO METRO /OUTSIDE OF THE FAR SOUTHWEST
AND SOUTH/ THE PROFILES JUST APPEAR TOO WARM TO SUPPORT A LONG
CHANGEOVER IF ANY GIVEN MORNING MODERATION IN TEMPS.
LAKE EFFECT...IN THE WAKE OF THE CLIPPER AND ITS SURFACE LOW
REFLECTION MOVING INTO INDIANA...WINDS WILL TURN MORE NORTHERLY
AND DOWN THE FETCH OF THE LAKE. IT APPEARS THEIR IS A 6 TO 12 HOUR
WINDOW OF GOOD N-S CONVERGENCE INTO MAINLY NW IN. MODERATE TO
EXTREME LOW-LEVEL INSTABILITY WILL BE IN PLACE OVER THE LAKE
GIVEN 850MB TEMPERATURES OF -4C TO -6C. NAM PROFILES OVER THE LAKE
GIVEN MODIFIED PARCELS NEAR THE SURFACE LAKE TEMPERATURES SUPPORT
CLOUD DEPTHS OF 15K FT. ITS NOTED THIS MORNING THAT THE AIR
TEMPERATURE AT THE SOUTH BUOY IS ONLY 42 DESPITE A WATER
TEMPERATURE OF 60. SO THE INSTABILITY MAY NOT BE AS EXTREME AS SOME
MODIFIED GUIDANCE SHOWS...BUT STILL AMPLE FOR DECENT SHOWERS.
PRECIP TYPE IS A REAL CHALLENGE AND AGAIN BASED ON INTENSITY. IT
WOULD SEEM ALONG THE IMMEDIATE LAKE SHORE WITH ONSHORE FLOW IT
WOULD BE TOUGH TO GET EVEN A CHANGEOVER. INLAND THERE IS CERTAINLY
THAT CHANCE AND IF THE CONVERGENCE IS STRONG...THEN THAT
INSTABILITY WILL TRAIN INLAND POTENTIALLY CREATING A PROBLEM FOR
QUICK ACCUMULATION IF COLD ENOUGH WITH HEAVY SUSTAINED RATES.
HAVE ADDED UNDER AN INCH OF ACCUMULATION IN EASTERN PORTER
COUNTY...BUT CERTAINLY THE CHANCE IS THERE FOR MORE. FOR NOW
THOUGH...THERMAL PROFILES ARE AWFULLY MARGINAL AND WITH THE CAVEAT
OF ONSHORE FLOW POTENTIALLY LEADING TO RAIN SHOWERS FOR MUCH OF
THAT TIME.
BEYOND...MORE TRANQUIL CONDITIONS PREVAIL HOWEVER STILL UNDER
NORTHWEST FLOW ALOFT...SO BELOW NORMAL TEMPERATURES CONTINUE. THE
BACK SIDE OF THE TROUGH SHIFTS EAST AS DOES ITS JET...SO THE PATH
FOR SHORT WAVES WILL BE MORE TO THE NORTH AND EAST BY
THURSDAY...MEANING LESS OF A CHANCE FOR PRECIP. LAKE EFFECT
ORIENTATION LOOKS TO BE EAST OF OUR INDIANA COUNTIES ON MOST OF
THESE DAYS.
MTF
www.nws.noaa.gov/view/prodsByState.php?state=IL&prodtype=discussion