Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 232331
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
631 PM CDT THU APR 23 2015

.SHORT TERM...
359 PM CDT

FOR THE REMAINDER OF THIS AFTERNOON AND TONIGHT...

CONDITIONS ACROSS THE AREA WILL REMAIN CLEAR THROUGH THE
EVENING...WITH PERHAPS SOME INCREASING MID AND HIGH LEVEL
CLOUDINESS CLOUDINESS TOWARD DAYBREAK. WITH A VERY COOL...DRY
AIRMASS IN PLACE...AND WITH WINDS EXPECTED TO DROP OFF TO
LGT/VRBL... TEMPERATURES OVERNIGHT SHOULD CONTINUE TO BE WELL
BELOW NORMAL...DROPPING INTO THE LOWER 30S. A FROST ADVISORY WILL
BE ISSUED FOR THE ENTIRE CWA FOR THE PERIOD FROM MIDNIGHT THROUGH
13Z. SOME ISOLATED...LOW LYING... SHELTERED AREAS COULD SEE TEMPS
DROP BELOW 32F...BUT DO NOT ANTICIPATE BELOW FREEZING TEMPERATURES
TO BE WIDESPREAD ENOUGH TO NECESSITATE A FREEZE WARNING.

KREIN

&&

.LONG TERM...
359 PM CDT

FOR FRIDAY THROUGH THURSDAY...

THE TREND OF BELOW NORMAL TEMPERATURES WILL CONTINUE FROM FRIDAY AND
INTO EARLY NEXT WEAK.  THE LONG WAVE UPPER LEVEL PATTERN IS EXPECTED
TO BE FAIRLY PROGRESSIVE OVER THE NEXT FEW DAYS WITH COLD ADVECTION
UNDER NORTHWEST FLOW ALOFT KEEPS TEMPERATURES ON THE COOLER SIDE.
UPPER RIDING IS EXPECTED TO CROSS THE REGION OVERNIGHT FRIDAY
NIGHT...BUT THE HEIGHT RISES WILL OCCUR AT A DIURNALLY UNFAVORABLE
TIME TO HAVE ANY SIGNIFICANT IMPACT ON THE MAX TEMPS FOR FRIDAY OR
SATURDAY.  BY LATE FRIDAY NIGHT...A SOUTHERN STREAM SHORTWAVE
LIFTING OUT OF THE SOUTHERN ROCKIES WILL BEGIN TO PHASE WITH A NRN
STREAM SHORTWAVE DROPPING OUT OF THE NRN ROCKIES...LEADING TO A
CLOSED UPPER LOW OVER THE LOWER MISSOURI VALLEY BY SATURDAY MORNING.
THIS SYSTEM WILL BRING THE NEXT ROUND OF PCPN TO THE AREA.  THE
VARIOUS MODELS ARE IN GENERALLY GOOD AGREEMENT ON THE SETUP WITH THE
DEVELOPING UPPER LOW.  THE NAM IS TRENDING A LITTLE FASTER WITH THE
SFC REFLECTION...BUT THIS SLIGHT TIMING DIFFERENCE DOES NOT APPEAR
TO HAVE A SIGNIFICANT ON THE FORECAST FOR THE SENSIBLE WEATHER
FEATURES.  THE SCENARIO IS SHAPING UP TO POTENTIALLY BRING A
SIGNIFICANT AMOUNT OF WIDESPREAD RAINFALL TO THE REGION.  THE SFC
PATTERN WILL BE CONDUCIVE TO DRAWING GULF MOISTURE NORTHWARD FROM
THE GULF OF MEXICO AS THE SFC LOW TRACKS THROUGH NORTHERN MISSOURI
SATURDAY MORNING.  RAIN SHOULD BEGIN TO OVERSPREAD THE REGION FROM
THE SOUTHWEST LATE FRIDAY NIGHT INTO EARLY SATURDAY MORNING.  AN
AREA OF MODERATE TO...OCCASIONALLY HEAVY...RAIN SHOULD CONTINUE
THROUGH SATURDAY AS THE MAIN SFC LOW CROSSES THE REGION BY EARLY
SATURDAY EVENING.  QPF AMOUNTS WILL BE THE BIGGEST CONCERN...WITH A
FAIR SPREAD OF AMOUNTS ADVERTISED BY THE VARIOUS MODELS.  HAVE
GENERALLY TRENDED THE QPF CLOSE TO THE WPC 6 HOURLY QPF...BUT HAVE
INCREASED AMOUNT SOME IN DEFERENCE TO GUIDANCE SUGGESTING FORECAST
PWATS OF 1.0 TO ALMOST 1.5 INCHES AS THE SYSTEM CROSSES THE REGION.
CLIMATOLOGICALLY...THESE PWAT VALUES ARE IN EXCESS OF THE 90TH
PERCENTILE.  SUCH HIGH PWATS WOULD BE INDICATIVE OF THE POTENTIAL
FOR LOCALLY HEAVY RAINFALL.  HOWEVER...THE REGION SHOULD REMAIN
NORTH OF THE ASSOCIATED WARM FRONT...WITH THE GREATEST INSTABILITY
TO THE SOUTH OF THE CWA...OVER CENTRAL AND SOUTHERN ILLINOIS.
SO...WHILE THERE COULD BE SOME LOCALLY HEAVIER RAINFALL IN THE BAND
OF ISENTROPIC LIFT NORTH OF THE WARM FRONT...ANY SIGNIFICANT THUNDER
POTENTIAL SHOULD BE SOUTH OF THE CWA. THE RAIN SHOULD COME TO AN END
SATURDAY EVENING AS THE SYSTEM PASSES OFF TO THE EAST.  PRELIMINARY
STORM TOTAL PCPN AMOUNTS ARE 1 TO 1.5 INCHES OVER MUCH OF THE
CWA...WITH ONLY THE FAR NERN PORTIONS OF THE CWA SEEING A LITTLE
LESS.  HOWEVER...GIVEN THE NEAR RECORD LEVEL PWATS ANTICIPATED...THE
STORM TOTAL QPF COULD BE ON THE CONSERVATIVE SIDE...ESPECIALLY IF
THE SYSTEM TRACKS A LITTLE FARTHER TO THE NORTH AND A CONVECTIVE
ELEMENT CAN BE INTRODUCED.

FOLLOWING THE PASSAGE OF THIS SYSTEM...COOL HIGH PRESSURE IS
EXPECTED TO BUILD OUT OF CANADA AND ACROSS THE UPPER MISSISSIPPI
VALLEY...HELPING KEEP TEMPERATURES BELOW NORMAL INTO MONDAY.  A
WARMING TREND IS EXPECTED TO BEGIN BY TUESDAY AS THE SFC HIGH SLIDES
OFF TO THE EAST AND UPPER RIDING BEGINS TO DEVELOP OVER THE UPPER
MISSISSIPPI VALLEY AND WESTERN GREAT LAKES REGIONS.  THE LONGER
RANGE GUIDANCE SHOWS A MORE COMPLEX PATTERN SETTING UP FOR NEXT WEEK
WITH A CUT OFF UPPER LOW DEVELOPING OVER THE SOUTH HIGH PLAINS AND
WEAK RIDGING REMAINING OVER THE UPPER MISSISSIPPI VALLEY AND WESTERN
GREAT LAKES REGIONS.  GIVEN THIS GENERAL TREND...TEMPERATURES SHOULD
BE ON THE INCREASE...WITH DIMINISHING PCPN POTENTIAL.  THE GFS/ECMWF
GENERALLY AGREE ON KEEPING THIS CUT OFF LOW WELL TO THE SOUTH...SO
IT SHOULD HAVE LITTLE INFLUENCE ON THE LOCAL WEATHER.  A SHORTWAVE
IS BEING ADVERTISED TO DROP OF THE NRN ROCKIES AND ACROSS THE NRN
PLAINS MONDAY NIGHT AND TUESDAY AND COULD BRING THE NEXT CHANCE OF
PCPN TO THE AREA...HOWEVER...THE LATEST MODEL RUNS WANT TO TRACK
THIS SHORTWAVE TO THE SOUTH AND WEST OF THE AREA...EVENTUALLY BEING
ABSORBED INTO THE UPPER LOW TO THE SOUTH.  SO...HAVE GONE WITH A DRY
FORECAST FOR SUNDAY THROUGH THE MIDDLE OF NEXT WEEK TO GO ALONG WITH
THE STEADY WARMING TREND.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* EASTERLY WINDS FRIDAY.
* CHANCE OF SHOWERS FRIDAY AFTERNOON/EVENING. CMS

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

LAKE BREEZE HAS MOVED WEST OF ORD EARLY THIS EVENING AND WILL
CONTINUE MOVING WEST UNTIL IT DISSIPATES THIS EVENING. EAST WINDS
WILL EVENTUALLY BECOME LIGHT AND VARIABLE THIS EVENING AND IF
THERE IS A PREVAILING WIND DIRECTION OVERNIGHT...IT WILL MOST
LIKELY BE LIGHT WESTERLY...BUT 5KTS OR LESS. SPEEDS WILL LIKELY
BECOME CALM IN MOST LOCATIONS AS HIGH PRESSURE MOVES ACROSS THE
AREA. AS THIS HIGH MOVES EAST FRIDAY...WINDS WILL TURN BACK EAST/
SOUTHEAST FRIDAY MORNING...THEN MORE EASTERLY IN THE AFTERNOON
WITH SPEEDS 10-15KTS POSSIBLE FRIDAY AFTERNOON INTO FRIDAY EVENING
WITH SPEEDS/GUSTS SLOWLY INCREASING FRIDAY NIGHT.

WARM AIR ADVECTION ALOFT WILL ALLOW MID CLOUDS TO DEVELOP BY
FRIDAY AFTERNOON AND SOME SHOWERS ARE POSSIBLE DURING THE
AFTERNOON/EVENING BUT CONFIDENCE REGARDING COVERAGE IS LOW AND
HAVE OPTED TO REMAIN DRY WITH THIS FORECAST. HOWEVER...IF SHOWERS
DO DEVELOP THEY WILL LIKELY PERSIST FOR AT LEAST A FEW HOURS. CMS

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH FOR WINDS THRU THE PERIOD.
* LOW FOR SHOWERS FRIDAY AFTERNOON/EVENING. CMS

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

* SATURDAY...RAIN. IFR/LIFR. STRONG/GUSTY EAST/NORTHEAST WINDS.
* SUNDAY THROUGH WEDNESDAY...DRY/VFR. NORTH/NORTHEAST WINDS.

&&

.MARINE...
147 PM CDT

ONSHORE FLOW IS EXPECTED TO DEVELOP ALONG THE ENTIRE ILLINOIS
NEARSHORE BY LATER THIS AFTERNOON AS A LAKE BREEZE INCHES INLAND.
WINDS SHOULD THEN BECOME VARIABLE AS HIGH PRESSURE PREVAILS INTO
FRIDAY. AN ONSHORE COMPONENT WILL DEVELOP AGAIN BY LATE FRIDAY
MORNING. THE PERIOD OF INTEREST IN THE MARINE FORECAST IS SATURDAY
AS FAIRLY DEEP LOW PRESSURE ADVANCES EASTWARD TO THE SOUTH OF THE
LAKE. THIS WILL RESULT IN STRENGTHENING EAST TO EAST-NORTHEAST
WINDS ACROSS THE SOUTHERN PART OF THE LAKE. THERE IS SOME
UNCERTAINTY ON HOW MUCH OF A PRESSURE GRADIENT WILL RESULT...BUT
EVEN GIVEN THAT SMALL CRAFT ADVISORY CRITERIA WINDS AND PROBABLY
WAVES ARE HIGHLY LIKELY. SOME GALE FORCE GUSTS ARE POSSIBLE IF
THIS LOW PRESSURE ENDS UP BEING FURTHER NORTH AND THUS CLOSER TO
THE LAKE. NORTHERLY WINDS ARE THEN EXPECTED TO PREVAIL WITH 30 KT
GUSTS LATER SUNDAY.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FROST ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
     ILZ023-ILZ032-ILZ033-ILZ039...1 AM FRIDAY TO 8 AM FRIDAY.

IN...FROST ADVISORY...INZ001-INZ002-INZ010-INZ011-INZ019...1 AM
     FRIDAY TO 8 AM FRIDAY.

LM...NONE.
&&

$$

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