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000
FXUS63 KLOT 240316
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1016 PM CDT THU MAY 23 2013

.DISCUSSION...
840 PM CDT

FOR EVENING UPDATE...

GOING FORECAST HANDLING CURRENT TRENDS WELL...AND NO SIGNIFICANT
CHANGES MADE.

EVENING WATER VAPOR IMAGERY INDICATES STRONG SHORT WAVE DIGGING
SOUTHEAST OF THE LOCAL AREA INTO EASTERN CONUS LONG WAVE TROUGH.
STRONG SUBSIDENCE IN THE WAKE OF THIS FEATURE AND DRY ADVECTION IN
LOW LEVELS CONTINUES TO CLEAR CLOUDS OUT FROM THE NORTH. SURFACE
HIGH PRESSURE BUILDING SOUTH FROM THE FAR UPPER MIDWEST WILL SPREAD
ACROSS THE UPPER MISSISSIPPI VALLEY TONIGHT AND ACROSS THE WESTERN
LAKES FRIDAY. WEAKENING GRADIENT/DECREASING WINDS...COOL AND DRY
AIRMASS AND CLEAR SKIES WILL LIKELY ALLOW TEMPS TO COOL INTO THE
MID/UPPER 30S ACROSS PARTS OF NORTH CENTRAL IL AND THE FOX VALLEY
REGION OF NORTHEAST IL. THESE TEMPS CONTINUE TO LOOK REASONABLE
GIVEN LOW LEVEL PARCEL TRAJECTORY FORECASTS AND UPSTREAM
OBSERVATIONS. SOME PATCHY FROST POSSIBLE...MAINLY ACROSS NORTH
CENTRAL IL AND INTO THE FOX RIVER VALLEY WHERE THE GRADIENT IS
EXPECTED TO BE WEAKER CLOSER TO DEVELOPING SURFACE RIDGE AXIS TO OUR
WEST/NORTHWEST BY MORNING. SLIGHTLY MORE WIND FARTHER EAST/SOUTHEAST
AND SOME DOWNWIND TEMP MODERATION SOUTH OF LAKE MICHIGAN SUGGEST
FROST POTENTIAL RATHER LOW THERE.

HAVE ONLY MADE MINOR TWEAKS TO SKY COVER/HOURLY TEMP GRIDS...AND
UPDATED TEXT FORECASTS FOR CLEAR SKIES AND DECREASED WINDS FROM THIS
AFTERNOON.

RATZER

//PREV DISCUSSION...
300 PM CDT

THE MAIN FORECAST CONCERNS...WILL FIRST FOCUS ON TEMPERATURES AND
THE CHANCES FOR FROST TONIGHT...THEN ATTENTION WILL FOCUS ON SHOWER
AND THUNDERSTORM CHANCES THROUGH THE PERIOD.

AN UNSEASONABLY STRONG AREA OF HIGH PRESSURE AROUND 1030 MB IS
CURRENTLY BUILDING SOUTHWARD ACROSS ONTARIO AND INTO THE UPPER
MIDWEST. THIS SURFACE HIGH IS EXPECTED MAINTAIN ITS STRENGTH AS IT
SETS UP ACROSS THE MID AND UPPER MISSISSIPPI VALLEY OVERNIGHT
TONIGHT. THE MAGNITUDE OF THIS HIGH IS EXPECTED TO RANK AROUND 3
SIGMA ABOVE NORMAL FOR LATE MAY. REGIONAL SATELLITE IMAGERY SHOWS
SKIES QUICKLY CLEARING FROM NORTH TO SOUTH ACROSS CENTRAL AND
SOUTHERN WISCONSIN EARLY THIS AFTERNOON AS MUCH DRIER
AIR...ASSOCIATED WITH THE HIGH...MOVES SOUTHWARD. I EXPECT THIS
CLEARING TREND TO CONTINUE ACROSS MY AREA WITH SKIES BECOMING CLEAR
AREA-WIDE THIS EVENING. THE COMBINATION OF CLEAR SKIES AND LIGHT
WINDS WILL ALLOW FOR AN UNSEASONABLY COLD NIGHT ACROSS THE AREA
TONIGHT. CONSIDERING TEMPERATURES WILL START THE EVENING ONLY IN THE
UPPER 40S TO LOW 50S...IT WILL NOT TAKE MUCH TO ALLOW TEMPERATURES
TO FALL INTO THE MID TO UPPER 30S AWAY FROM THE LAKE. PATCHY FROST
IS STILL POSSIBLE...BUT IT APPEARS THE BEST POTENTIAL WILL BE ACROSS
MY WESTERN CWA ACROSS NORTH CENTRAL ILLINOIS...WHERE THE WINDS ARE
EXPECTED TO WEAKEN BELOW 5 KT LATER TONIGHT. FARTHER EAST ACROSS
NORTHEASTERN ILLINOIS...IT APPEARS A THE PRESSURE GRADIENT MAY
REMAIN JUST STRONG ENOUGH TO KEEP A 5 KT WIND GOING OUT OF THE
NORTHEAST...WHICH WOULD LIMIT THE OVERALL FROST THREAT...EXCEPT IN
WELL PROTECTED LOW AREAS. NO HEADLINES ARE PLANNED AS IT APPEARS TO
BE AN OVERALL MARGINAL EVENT WITH ONLY PATCHY FROST EXPECTED.

THIS SURFACE HIGH WILL DRIFT ACROSS NORTHEASTERN ILLINOIS DURING THE
DAY ON FRIDAY. THE ONSHORE FLOW SHOULD PRODUCE ANOTHER COOL DAY
ACROSS THE AREA...ESPECIALLY FOR AREAS NEAR THE LAKESHORE.

MID AND UPPER LEVEL HEIGHTS WILL BE ON THE RISE ACROSS THE WESTERN
GREAT LAKES ON FRIDAY AND FRIDAY NIGHT AS THE MID LEVEL RIDGE ACROSS
THE PLAINS BUILDS EASTWARD IN THE WAKE OF THE TROUGH CURRENTLY
MOVING ACROSS THE GREAT LAKES. CONCURRENT WITH THIS...A 35+ KT LOW
LEVEL JET WILL DEVELOP ACROSS THE THE PLAINS AND THEN VEER
SOUTHWESTWARD ACROSS THE MID MISSOURI VALLEY. THIS WILL LIKELY
RESULT IN DECENT MOISTURE RETURN ACROSS THE THE MID SECTION OF THE
NATION. IT APPEARS THAT SOME COMPLEXES OF THUNDERSTORMS WILL DEVELOP
FRIDAY NIGHT ALONG THE NOSE OF THIS LOW LEVEL JET AND IN THE
VICINITY OF A BAROCLINIC ZONE...WHICH SHOULD BE SOMEWHERE  ACROSS
EASTERN NEBRASKA AND INTO IOWA. THE STEERING LAYER FLOW LOOKS TO
TAKE ANY ACTIVITY TO THE EAST-SOUTHEAST...WHICH WOULD FAVOR MOST OF
THE REMNANT ACTIVITY TO REMAIN ACROSS WESTERN SECTIONS OF THE AREA
INTO SATURDAY MORNING. BECAUSE OF THIS...I HAVE CHOSEN TO PLACE THE
HIGHER POPS WEST...WITH ONLY SLIGHTS FARTHER EAST ACROSS
NORTHEASTERN ILLINOIS AND NORTHWESTERN ILLINOIS.

ADDITIONAL WAVES OF CONVECTIVE ACTIVITY IS EXPECTED THROUGH MUCH OF
THE WEEKEND. HOWEVER...AGAIN THE MAIN ACTIVE ZONE LOOKS TO BE MAINLY
WEST OF THE AREA FROM IOWA EAST-SOUTHEASTWARD ACROSS WESTERN
ILLINOIS. THEREFORE...THERE ARE ONLY SMALL CHANCES OF ANY OF THIS
ACTIVITY GETTING AS FAR EAST AS CHICAGO. IT APPEARS THE GFS IS BEING
AFFECTED BY CONVECTIVE ACTIVITY AND BECOMING TO AGGRESSIVE WITH
CONVECTIVE GENERATED SHORT WAVES. THEREFORE...I STRAYED AWAY FROM
ITS FORECASTS OF THESE COMPLEXES AFFECTING MOST OF NORTHERN ILLINOIS.
I HAVE AGAIN FAVORED SMALL POPS EAST INTO EARLY NEXT WEEK.

IT APPEARS THAT A WARM FRONT WILL LIFT NORTHEAST TOWARDS THE AREA ON
TUESDAY..AND THIS COULD PROVIDE SOME SLIGHTLY BETTER CHANCES OF
CONVECTIVE ACTIVITY ACROSS NORTHERN ILLINOIS AND NORTHWESTERN
INDIANA. ONCE THIS WARM FRONT LIFTS NORTH OF THE AREA...IT APPEARS
CONDITIONS WILL HEAT UP ACROSS THE AREA FOR THE MIDDLE TO END OF NEXT
WEEK. I HAVE CHOSEN TO GO WILL A DRY FORECAST IN THE EXTENDED...AS
IT APPEARS A STRONG MID LEVEL RIDGE WILL SET UP ACROSS THE CENTRAL
PORTION OF THE COUNTRY ESSENTIALLY BUCKLING THE MAIN BELT OF
WESTERLIES NORTH OF THE AREA. I HAVE RAISED TEMPERATURES SEVERAL
DEGREES FOR WEDNESDAY AND PUT EVEN WARMER CONDITIONS IN THE FORECAST
FOR THURSDAY OF NEXT WEEK. WE COULD BE PUSHING CLOSE TO 90 DEGREES
ACROSS THE AREA...AS THE CURRENT MODEL FORECASTS AREA SUGGESTING 18
TO 20 DEGREE CELSIUS 850 MB TEMPERATURES AND 20 TO 23 DEGREE 925
TEMPERATURES. LOCAL RESEARCH SUGGESTS THAT TEMPERATURES OF THIS
MAGNITUDE WOULD SUPPORT TEMPERATURES IN THE MID TO UPPER 80S THIS
TIME OF YEAR. I HAVE THEREFORE WENT ABOVE GUIDANCE FOR THIS
PERIOD.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* NONE. CMS

//DISCUSSION...UPDATED 00Z...

ONLY CONCERN THIS PERIOD ARE WINDS THIS EVENING WHICH WILL REMAIN
NORTH/NORTHEAST. SPEEDS/GUSTS WILL SLOWLY DIMINISH THROUGH THE
EVENING...THOUGH HOW FAST GUSTS COMPLETELY DIMINISH AT ORD/MDW/GYY
IS A BIT UNCERTAIN. FURTHER WEST AT RFD/DPA...GUSTS SHOULD
DIMINISH FAIRLY QUICKLY WITH SUNSET. SPEEDS WILL CONTINUE TO
DIMINISH TO 10KTS OR LESS OVERNIGHT AS A RIDGE OF HIGH PRESSURE
BUILDS SOUTH ACROSS THE REGION. THIS RIDGE WILL GRADUALLY SHIFT
EAST FRIDAY NIGHT INTO SATURDAY...MAINTAINING EASTERLY WINDS FOR
MUCH OF THE PERIOD...MORE NORTHERLY OFF THE LAKE AT GYY. WITH THE
EXCEPTION OF SOME POSSIBLE THIN CIRRUS FRIDAY...EXPECT SKC INTO
FRIDAY EVENING...WITH MID CLOUDS POSSIBLE FRIDAY NIGHT INTO
SATURDAY MORNING. CMS

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* MEDIUM CONFIDENCE REGARDING WIND SPEEDS/DIRECTIONS. CMS

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...

* SATURDAY...DRY. VFR.
* SUNDAY THROUGH TUESDAY...VFR. SLIGHT CHANCE OF TSRA.
* WEDNESDAY...GUSTY SOUTHWEST WINDS. CHANCE OF TSRA.

&&

.MARINE...
203 PM CDT

QUICKLY DEPARTING LOW PRESSURE FROM THE UPPER OHIO VALLEY HELPED
TO ENHANCE WINDS TO GALE FORCE ACROSS THE SOUTHERN HALF OF THE
LAKE ON THURSDAY. THESE WILL BE SLOW TO DIMINISH ACROSS THE FAR
SOUTH THIS EVE AND WAVES ACCORDINGLY WILL TAKE EVEN LONGER TO
SUBSIDE GIVEN THE FETCH. BECAUSE OF THAT...SMALL CRAFT ADVISORY
CONDITIONS IN NEAR SHORE AREAS OF IL AND IN WILL FOLLOW THE GALES
THROUGH AT LEAST FRIDAY MORNING. HIGH PRESSURE WILL ENVELOP THE
LAKE FRI NIGHT INTO THE HOLIDAY WEEKEND. LAKE BREEZES WILL BE SEEN
WITH SUCH A PATTERN. THE HIGH WILL WEAKEN EARLY NEXT WEEK WITH
GRADUAL RETURN FLOW SETTING IN.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 7 PM FRIDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 4 PM FRIDAY.

&&

$$

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