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FXUS63 KLOT 271203
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
703 AM CDT TUE AUG 27 2013
.DISCUSSION...
324 AM CDT
FORECAST CHALLENGES THIS MORNING INCLUDE TODAY`S HEAT...THEN
TEMPS/POPS AS WE TEETER ON THE PERIPHERY OF UNSEASONABLY STRONG LATE
SUMMER UPPER HOT DOME INTO THE WEEKEND.
THE CARCASS OF SUNDAY NIGHT`S MCS PROVIDED A SHROUD OF FAIRLY DENSE
CIRRUS OVER THE AREA MONDAY AFTERNOON HOLDING TEMPS ABOUT 2-4F
COOLER THAN EXPECTED. ONCE AGAIN EARLY THIS MORNING MCS ACTIVITY IS
FESTERING ABOUT TO OUR NORTH ON THE PERIPHERY OF THE UPPER RIDGE.
CONVECTIVELY ENHANCED VORT DRIVING THIS MCS APPEARS TO BE PUSHING
INTO NORTHERN PORTIONS OF LAKE MICHIGAN ALREADY WITH BACK EDGE OF
CIRRUS SHIELD MOVING INTO NW WISCONSIN. HRRR SEEMS TO HAVE A GOOD
HANDLE ON ONGOING CONVECTIVE TRENDS AND IT...ALONG WITH THE MAJORITY
OF THE OTHER GUIDANCE...SUGGESTS THAT TODAY`S CIRRUS DEBRIS WILL
CLEAR OUT BY EARLY AFTERNOON AND ALLOW FOR STRONG HEATING TO TAKE
PLACE...ESPECIALLY NORTHWESTERN CWA. LOW LEVEL THERMAL RIDGE
MOVING INTO THE AREA WITH 925MB TEMPS PROGGED TO RISE TO 26-28C
NORTHWESTERN CWA WHICH WOULD EASILY SUPPORT MIDDLE IF NOT UPPER
90S. IN THE PAST HOUR CONVECTION HAS FLARED OVER NORTHEAST MINNESOTA
WHICH IS CONCERNING AS 850-300MB THICKNESS SUGGESTS THAT COULD SINK
SOUTHWARD AND SPREAD MORE CIRRUS ACROSS THE AREA TODAY.
GIVEN POTENTIAL FOR DEBRIS CLOUDINESS TO LINGER AGAIN TODAY SPLIT
THE DIFFERENCE FOR HIGHS BETWEEN WHAT PROGGED 925MB TEMPS WOULD
SUPPORT AND WHAT CLIMO INFLUENCED MOS NUMBERS FORECAST. ASSUMING
DEWPOINTS DON`T MIX OUT AND ASSUMING CIRRUS CLEARS OUT THEN WE WOULD
GET CLOSE TO THE 105F HEAT INDEX ADVISORY CRITERIA THIS AFTERNOON
NORTH/NORTHWEST AREAS. GIVEN A FORECAST OF CONDITIONS NOT MEETING
CRITERIA AND LOW CONFIDENCE IN THEM EVEN GETTING AS BAD AS I HAVE IN
THE GRIDS...WOULD NOT NORMALLY ISSUE AN ADVISORY...BUT WITH
HEADLINES OUT FOR AREAS NORTH/WEST OF OUR CWA AND POTENTIAL FOR NEAR
RECORD HIGHS DECIDED TO MAKE THE W/W/A MAP PRETTY AND HOIST AN
ADVISORY FOR NORTH CENTRAL ILLINOIS. IF TRENDS THIS MORNING SUGGEST
THAT CIRRUS WONT BE A BIG DEAL THEN DAY SHIFT COULD CONSIDER
EXTENDING THE ADVISORY FARTHER EAST INTO THE CHICAGO AREA.
ANOTHER SHORTWAVE OVER THE EASTERN DAKOTAS THIS MORNING IS PROGGED
TO RIDE THE RIM OF THE RIDGE EAST INTO WISCONSIN THIS AFTERNOON AND
WILL PROBABLY RE-IGNITE THUNDERSTORMS TO OUR NORTH THIS AFTERNOON
WITH THESE STORMS POTENTIALLY CLIPPING OUR NORTHERN AND NORTHEASTERN
CWA THIS EVENING AS THE MOVE SOUTHEASTWARD. MODERATELY STRONG MID
LEVEL FLOW AND FAIRLY STRONG INSTABILITY (ASSUMING TDS DONT MIX OUT)
COULD SUPPORT A LOW END SEVERE THREAT WITH ANY STORMS THAT DO
MEANDER INTO OUR CWA TONIGHT.
LAKE ENHANCED COLD FRONT WILL MOVE INTO THE CWA EARLY WEDNESDAY
MORNING PROGRESSING SOUTHWARD INTO OUR SOUTHERN CWA BY AFTERNOON.
BETTER FORCING ALOFT SHOULD REMAIN TO OUR EAST BUT AM CONCERNED THAT
MOISTURE/INSTABILITY POOLING AND LOW LEVEL CONVERGENCE ALONG THE
FRONT COULD BE ENOUGH TO KICK OFF A THUNDERSTORM WEDNESDAY
AFTERNOON. SLIGHTLY COOLER AIR MASS WILL FILTER IN BEHIND THE
FRONT...WITH COOLING MOST NOTICEABLE NEAR LAKE MICHIGAN. AHEAD OF THE
FRONT WILL BE ANOTHER STEAMY DAY AND WITH BETTER DEWPOINT POOLING
COULD ACTUALLY END UP BEING HOTTER/MORE HUMID IN OUR SOUTHERN CWA
TOMORROW THAN TODAY...BUT THAT WILL HINGE ON THE AMOUNT OF
CLOUDINESS SOMETHING WHICH WILL BE MORE EASILY DIAGNOSED TONIGHT.
LOW LEVEL BAROCLINIC ZONE HANGS UP IN THE NEIGHBORHOOD THROUGH THE
END OF THE WEEK WITH WEAK NORTHWEST FLOW IMPULSES LIKELY TO RIPPLE
AROUND THE PERIPHERY OF THE HEAT DOME MEANS AT LEAST SOME POTENTIAL
FOR THUNDERSTORMS TONIGHT UNTIL STRONG COLD FRONT MOVES THROUGH THIS
WEEKEND. HAVE KEPT FORECAST DRY FOR NOW WEDNESDAY NIGHT THROUGH
THURSDAY BUT NW FLOW`S LIKE THIS ARE NOTORIOUS FOR PRODUCING
"SURPRISES" SO STAY TUNED!
HEADING INTO THE SECOND HALF OF THE LABOR DAY WEEKEND THE DIFFERENCE
BETWEEN THE 27/00Z RUN OF THE ECMWF WHEN COMPARED TO THE 25/12Z RUN
IS ABOUT AS DRAMATIC I`VE EVER SEEN IN THE WARM SEASON. FOR DAYS
MEDIUM RANGE MODELS HAD BEEN ADVERTISING RECORD BREAKING HEAT AND
NOW AFTER STARTING TO TREND SHARPLY COLDER WITH THE 26/12Z RUN THE
LATEST ECMWF NOW FORECASTS NEAR RECORD COLD TEMPERATURES BY MONDAY
WITH H8 TEMPS NEARING 0C! THE REMAINDER OF THE MEDIUM RANGE MODELS
HAVE BEEN TRENDING TOWARD THIS EARLIER FROPA/STRONG COLD PUSH
SOLUTION BUT STILL LAG WELL BEHIND THE UNSEASONABLY COLD ECMWF. IF
ECMWF WERE TO VERIFY WITH THIS INTENSE COOL SNAP THEN OUR FORECAST
WILL LIKELY TAKE A COUPLE MORE CYCLES TO CATCH UP WITH SUCH AN
ABRUPT CHANGE BUT HAVE TRENDED TEMPS DOWN SIGNIFICANTLY FROM THE
INITIALIZED ALLBLEND GUIDANCE...THOUGH IF LATEST EC VERIFIES HIGHS
MONDAY WOULD LIKELY BE IN THE 60S! NEEDLESS TO SAY FORECAST
CONFIDENCE IN THIS TIME PERIOD WILL REMAIN LOW UNTIL WE BEGIN TO SEE
MODELS CONVERGE ON A SOLUTION AND REMAIN MORE STABLE FROM RUN TO RUN.
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