502
FXUS63 KLOT 130800
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
300 AM CDT TUE AUG 13 2013
.DISCUSSION...
256 PM CDT
FORECAST CONCERNS THIS AFTERNOON CONTINUE TO CENTER ON SHRA/TSRA
TRENDS AND EVOLUTION THIS AFTERNOON AND EVENING AS WEAK COLD FRONT
SAGS SOUTH ACROSS THE CWA...THEN SHIFT TO COOLER THAN NORMAL TEMPS
ESPECIALLY TUESDAY AND WEDNESDAY AS CANADIAN HIGH PRESSURE BUILDS
SOUTHEAST ACROSS THE REGION.
THIS AFTERNOON/TONIGHT...
WEAK COLD FRONT WAS SAGGING SLOWLY SOUTHEAST ACROSS NORTHERN
ILLINOIS AT MID-AFTERNOON...WITH 19Z/2PM CDT SURFACE ANALYSIS
PLACING THE BOUNDARY ROUGHLY SOUTH OF WAUKEGAN...TO JUST EAST OF
STERLING...TO NEAR GALESBURG. SHRA/TSRA HAVE BEEN SLOW TO DEVELOP
ALONG AND AHEAD OF THE FRONT SINCE LATE MORNING...LARGELY IN
RESPONSE TO A FAIRLY DENSE SHIELD OF THICK CIRRUS OFF CONVECTION TO
OUR SOUTHWEST ACROSS MISSOURI. HOWEVER...ISOLATED TO SCATTERED
SHRA/TSRA HAVE BEGUN TO DEVELOP/INCREASE IN COVERAGE OVER THE PAST
1-2 HOURS AHEAD OF THE FRONT AS ANTICIPATED. EXPECTATION IS THAT
THIS ACTIVITY WILL CONTINUE TO DEVELOP THIS AFTERNOON AS FRONT
PUSHES INTO WEAKLY TO MODERATELY UNSTABLE ENVIRONMENT ACROSS THE
CENTRAL AND SOUTHERN PARTS OF THE CWA. AREAS AHEAD OF THE
FRONT...AND ALONG COMBINED FRONTAL/LAKE BREEZE BOUNDARY WORKING
SOUTH ALONG THE LAKE APPEAR TO BE FAVORED AREAS FOR CONTINUED GROWTH
GENERALLY ACROSS THE SOUTHEAST HALF OF THE CWA. SPC/RAP MESOANALYSIS
AT 19Z INDICATED EFFECTIVE BULK SHEAR REMAINS IN THE 25-30 KT RANGE
SIMILAR TO THE 0-6 KM LAYER VALUE...ALONG WITH SEVERAL HUNDRED J/KG
OF DOWNDRAFT CAPE PARTICULARLY ALONG THE SURFACE FRONT ITSELF. THUS
WHILE WIDESPREAD SEVERE STORMS NOT LIKELY...A FEW OF THE STRONGER
CELLS COULD PRODUCE ISOLATED 50-60 MPH WIND GUSTS LATER THIS
AFTERNOON. ALSO...PWATS AROUND 1.5 INCHES IN MOIST AXIS WITHIN/AHEAD
OF FRONTAL ZONE COULD FAVOR SOME LOCALLY HEAVY DOWNPOURS WITH FAIRLY
SLOW EASTERLY CELL MOVEMENT IF ANY SIGNIFICANT TRAINING OCCURS. AREA
OF STORMS SHOULD CONTINUE TO SLIDE SOUTHEAST WITH THE FRONT INTO
THIS EVENING...WITH PRECIP ENDING FROM THE NORTH. SCATTERED
THUNDERSTORMS WERE ALSO NOTED FARTHER NORTH ACROSS WISCONSIN THIS
AFTERNOON...BENEATH DIGGING SHORT WAVE AND WITH SECONDARY COLD
FRONTAL BOUNDARY IN THAT AREA. THIS SECONDARY FRONT WILL MOVE SOUTH
INTO OUR FORECAST AREA TONIGHT...THOUGH SHRA/TSRA COVERAGE ACROSS WI
SHOULD DECREASE AS THE FRONT ARRIVES WITH UNFAVORABLE DIURNAL
ARRIVAL AFTER SUNSET. THUS HAVE EVENING POPS ACROSS THE SOUTHEAST...
THEN DRY ACROSS THE AREA AFTER MIDNIGHT.
TUESDAY THROUGH WEDNESDAY NIGHT...
SECONDARY COLD FRONT WILL BE PUSHING OUT OF THE SOUTHERN COUNTIES OF
THE CWA TUESDAY MORNING...WITH VERY COOL AND BREEZY NORTH-NORTHEAST
LOW LEVEL FLOW DEVELOPING IN SOUTHEASTERN PERIPHERY OF EXPANSIVE
AREA OF SURFACE HIGH PRESSURE WHICH WILL BE BUILDING SOUTHEAST FROM
CENTRAL CANADA. 850MB TEMPS COOL TO AROUND +8C BY EVENING...WITH LOW
LEVEL MODEL THERMAL PROGS SUGGESTING HIGHS MAY STRUGGLE TO BREAK 70
DEGREES ALONG THE IMMEDIATE LAKE MICHIGAN SHORE WHILE MAKING ONLY
MID 70S WELL INLAND ACROSS THE FORECAST AREA. SOME COLD-AIR STRATOCU
IS POSSIBLE...THOUGH DRYING LOW LEVELS ESPECIALLY DURING THE
AFTERNOON SHOULD ALLOW FOR A GOOD DEAL OF SUNSHINE TO COUNTERACT THE
CANADIAN AIR. WINDS WILL DIMINISH TUESDAY NIGHT AS THE HIGH
CONTINUES TO SPREAD SOUTHEAST INTO THE REGION...WITH THE WEAK
PRESSURE GRADIENT IN THE VICINITY OF THE RIDGE AXIS THEN PROVIDING
LIGHT WINDS ALONG WITH MAINLY CLEAR SKIES FOR COOL OVERNIGHT
TEMPERATURES BOTH TUESDAY NIGHT AND WEDNESDAY NIGHT. AREAS ON BOTH
NIGHTS SHOULD EASILY REACH THE LOWER 50S AWAY FROM THE CHICAGO HEAT
ISLAND...WITH A FEW CHILLY UPPER 40S A GOOD BET IN SOME OF THE
TYPICALLY COLDER SPOTS IN THE OUTLYING SUBURBAN AND RURAL AREAS.
WEDNESDAY WILL AGAIN REMAIN A GOOD 7-12 DEGREES BELOW OUR MID-AUGUST
NORMALS WITH MID-70S FOR THE MOST PART UNDER THE COOL POOL ALOFT...
AND LAKE MICHIGAN SHORE AREAS AROUND 70 WITH A LAKE BREEZE
ENHANCEMENT OF THE DECREASING SYNOPTIC NORTHERLY FLOW. AT THIS TIME
IT APPEARS THAT RECORD LOW MAX TEMPS WHICH ARE GENERALLY IN THE MID
60S SHOULD NOT BE IN JEOPARDY TUE-WED...THOUGH RECORD LOWS FOR
WEDNESDAY AND THURSDAY MORNING ARE IN THE LOW/MID 40S AT ROCKFORD
AND ITS NOT IMPOSSIBLE THAT A FEW RURAL SPOTS MAY APPROACH THESE LOW
TEMPS WEDNESDAY OR THURSDAY MORNING.
THURSDAY THROUGH MONDAY...
OVERALL...THE UPPER LEVEL FLOW PATTERN WILL MAINTAIN AN UPPER RIDGE
OVER THE WESTERN CONUS/ROCKIES REGION...AND BROAD UPPER TROUGHING
ACROSS EASTERN CANADA AND THE EASTERN CONUS. THIS WILL ALLOW THE
SPRAWLING SURFACE HIGH OVER OUR REGION AT MID-WEEK TO CONTINUE TO
DRIFT SLOWLY SOUTHEAST AND WEAKEN ACROSS THE SOUTHERN GREAT LAKES
AND OHIO VALLEY REGION THROUGH FRIDAY. EXTENDED GUIDANCE THEN IS IN
FAIRLY GOOD AGREEMENT IN DEVELOPING ADDITIONAL UPPER TROUGHING AND
EVENTUALLY A WEAK CLOSED UPPER LOW OVER THE LOWER MISSOURI AND LOWER
OHIO VALLEYS NEXT WEEKEND. THIS KEEPS A WEST-EAST SURFACE HIGH
PRESSURE RIDGE ACROSS THE GREAT LAKES REGION TO OUR EAST...WITH
LITTLE QUALITY LOW-LEVEL MOISTURE RETURN FROM THE GULF EXCEPT FOR
FARTHER TO THE WEST ACROSS THE SOUTHERN/HIGH PLAINS. THIS GENERALLY
FOCUSES GREATER PROBABILITY FOR PRECIPITATION CHANCES TO REMAIN
WEST/SOUTH OF OUR IMMEDIATE FORECAST AREA THROUGH THE PERIOD...WITH
A VERY GRADUAL MODERATION IN TEMPS IN THE THURSDAY-MONDAY PERIOD
BACK TO ABOUT AVERAGE BY SUNDAY/MONDAY. SOME INCREASE IN LOW-CHANCE
POPS COULD BE ARGUED AS THE UPPER LOW/TROUGH DEVELOPS LATE IN THE
PERIOD...THOUGH APPEARS TO LOW/IFFY AT THIS DISTANCE HAND HAVE
MAINTAINED DRY FORECAST THROUGH THE EXTENDED.
RATZER
&&
.AVIATION...
//ORD AND MDW CONCERNS...UPDATED 06Z...
* SHORT PERIODS OF BKN MVFR CIGS POSSIBLE TIL ABOUT 10Z.
* OCCASIONAL WIND GUSTS NEAR 20 KT DEVELOPING TOWARD 07Z AS COLDER
AIR PUSHES IN WITH ANOTHER COOL FRONTAL PASSAGE. MORE FREQUENT
GUSTS DEVELOP MID MORNING.
* POTENTIAL FOR SOME MVFR LAKE EFFECT CLOUDS THIS AFTERNOON AND
EVENING.
MDB
//DISCUSSION...UPDATED 06Z...
A SECONDARY COLD FRONT IS QUICKLY PUSHING SOUTHWARD INTO NORTHEAST
ILLINOIS AND WILL CROSS THE TERMINALS OVER THE NEXT FEW HOURS.
ORD/MDW/GYY WILL LIKELY SEE THE MOST NOTICEABLE AFFECTS GIVEN
PROXIMITY TO THE LAKE AND EXPECT PERIODIC WIND GUSTS TO AROUND 20
KT TO DEVELOP. WINDS WILL ALSO LIKELY TURN A BIT MORE
NORTHEASTERLY. A BAND OF SCT-BKN MVFR CLOUDS IS ALSO MOVING
SOUTHWARD BEHIND THE FRONT AND WILL LIKELY HOLD TOGETHER AND CROSS
THE TERMINALS. CLOUD COVERAGE HAS BEEN INCREASING AND DECREASING
SO EXPECT MAINLY SCATTERED COVERAGE BUT BRIEF BROKEN COVERAGE IS
POSSIBLE. DPA AND RFD WILL LIKELY BE LESS AFFECTED BY THE WINDS
BUT THE SCT-BKN STRATUS WILL AFFECT THESE TERMINALS AS WELL. DRIER
AIR WILL BE MOVING IN BRINGING AN END TO ANY VSBY RESTRICTIONS.
NORTHEAST WIND GUSTS WILL BECOME MORE FREQUENT THROUGH TUESDAY
MORNING WITH THE HIGHEST GUSTS LIKELY AT GYY GIVEN CLOSE PROXIMITY
TO THE LAKE. CLOUD COVER WILL BE A CHALLENGE. THE COLD AIRMASS
WILL LIKELY SUPPORT SCT-BKN LAKE EFFECT CLOUD COVER DOWNWIND OF
THE LAKE INTO NORTHWEST INDIANA SO WILL CARRY SCT MVFR CLOUDS AT
GYY. THE COOL AIR ADJACENT TO ANY LAKE EFFECT CLOUDS WILL LIKELY
ACT TO STABILIZE THE AIRMASS OVER LAND ACROSS NORTHEAST ILLINOIS
SO EXPECT A VOID IN CLOUD COVER IN THE CHI METRO WHILE SCT-BKN
STRATOCU LIKELY DEVELOPS FURTHER INLAND. HIGH PRESSURE WILL BE
BUILDING IN SO WINDS WILL QUICKLY DROP OFF TUESDAY EVENING WITH
THE BULK OF THE STRATOCU DISSIPATING AS WELL. LAKE EFFECT CLOUD
POTENTIAL WILL BE THE WILDCARD AS IT MAY CONTINUE TO IMPACT GYY
AND POSSIBLY TRANSLATE WESTWARD INTO CHICAGO. CONFIDENCE IN LAKE
EFFECT CLOUDS TRENDS IS PRETTY LOW.
MDB
//ORD AND MDW CONFIDENCE...UPDATED 06Z...
* HIGH CONFIDENCE IN SCT MVFR CLOUDS ARRIVING TOWARD 07Z...MEDIUM
CONFIDENCE THAT BKN MVFR PERIODS WOULD BE BRIEF.
* HIGH CONFIDENCE IN WINDS GUSTING TO NEAR 20 KT TOWARD 07Z...THEN
FREQUENCY MAY DECREASE OVERNIGHT...MORE FREQUENT GUSTS DURING
THE DAY.
* LOW CONFIDENCE IN LAKE EFFECT CLOUD POTENTIAL.
MDB
//OUTLOOK FOR ORD/MDW FOR 12Z WEDNESDAY-12Z MONDAY...UPDATED 00Z...
WEDNESDAY THROUGH SUNDAY...VFR.
IZZI
&&
.MARINE...
300 AM CDT
A SERIES OF COLD FRONTS HAS NOW PUSHED SOUTH OF LAKE MICHIGAN WITH A
FAIRLY TIGHT PRESSURE GRADIENT POSITIONED ACROSS THE LAKE WITH LOW
PRESSURE TO THE EAST OVER EASTERN ONTARIO AND A STRONG HIGH PRESSURE
SYSTEM TO THE WEST FROM MANITOBA ACROSS THE DAKOTAS. NORTH TO
NORTHEAST WINDS HAVE INCREASED ACROSS THE LAKE AND WILL REMAIN
STRONG THROUGH LATE AFTERNOON AT WHICH POINT SPEEDS WILL BEGIN TO
GRADUALLY EASE AS THE PRESSURE GRADIENT SLACKENS. WESTERN AND
NORTHERN SECTIONS OF THE LAKE WILL SEE WINDS DIMINISHING FIRST AS
THE GRADIENT WILL BE WEAKENING FROM NORTHWEST TO SOUTHEAST. WAVES
WILL CONTINUE TO BUILD THIS MORNING WITH THE LONG FETCH DIRECTING
THEM AT THE SOUTHERN SHORELINES. HAVE BUMPED WAVES UP TO 5 TO 8 FT
WITH OCCASIONAL 12 FT FOR THE SOUTHERN PARTS OF THE LAKE WITH WELL
EXPOSED BUOYS NEAR THE END OF THE DEVELOPING FETCH ALREADY SHOWING
WAVES IN THE 4 TO 6 FT RANGE...WITH A FEW PEAKS CLOSER TO 7 FT. WILL
CONTINUE WITH THE SMALL CRAFT ADVISORY AS-IS. THE HIGH WILL MOVE
SOUTHWARD AND SETTLE INTO ILLINOIS INTO WEDNESDAY AND THEN GRADUALLY
SPREAD EASTWARD ACROSS THE OHIO VALLEY THROUGH THE END OF THE WEEK.
NORTHERLY WINDS WILL LIKELY TURN ONSHORE WEDNESDAY WITH THE WEAK
PRESSURE GRADIENT IN PLACE BEFORE SHIFTING TO SOUTHERLY ACROSS MOST
OF THE LAKE WEDNESDAY NIGHT AS THE HIGH SHIFTS. SOUTHWEST TO WEST
WINDS ARE EXPECTED THROUGH THE REMAINDER OF THE WEEK BUT ONSHORE
FLOW WILL LIKELY SET UP ACROSS SOUTHERN PARTS OF THE LAKE WHERE THE
PRESSURE GRADIENT WILL REMAIN WEAK INTO THE WEEKEND.
MDB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...BEACH HAZARDS STATEMENT...ILZ006-ILZ014 UNTIL 4 AM WEDNESDAY.
IN...BEACH HAZARDS STATEMENT...INZ001-INZ002 UNTIL 7 AM WEDNESDAY.
LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 AM WEDNESDAY.
SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 7 AM WEDNESDAY.
&&
$$
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