Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211956
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
256 PM CDT MON APR 21 2014

.DISCUSSION...
1051 AM CDT

SOME HIGHER TEMPORAL RESOLUTION HAS BEEN MADE TO THE RAIN
LIKELIHOOD ALONG WITH THE CHANCES FOR THUNDER FOR THE REST OF
TODAY AND TONIGHT...BUT ALL IN ALL MAIN THEME OF THE FORECAST
CONTINUES. THERE WERE TEMPERATURE ADJUSTMENTS TO BUMP FORECAST
HIGHS DOWN SEVERAL DEGREES ACROSS MANY COMMUNITIES...ESPECIALLY
NORTH.

THE LONG WAVE TROUGH WITH TWO MAIN SHORT WAVE CENTERS IS MOVING
EASTWARD INTO THE MISSISSIPPI VALLEY LATE THIS MORNING. WHILE THE
TWO MAIN CENTERS WILL SPLIT THE FORECAST AREA...SEVERAL MINOR
IMPULSES EMBEDDED WITHIN THE TROUGH AND JUST OVERALL BROAD ASCENT
AND DEEPER MOISTURE CONTINUE TO HAVE RAIN ENCROACHING ON THE
AREA. ONE LEAD VORT CENTER NEAR ST LOUIS IS MOVING NORTHEAST AND
THIS WILL LIKELY CONTINUE TO SPREAD SHOWERS ACROSS CENTRAL IL AND
INTO THE SOUTHERN FORECAST AREA EARLY THIS AFTERNOON. THIS HAS
OUTPACED THE INSTABILITY SO NOT EXPECTING MUCH THUNDER WITH THAT.
THIS STILL MAY BE ENOUGH TO DROP TEMPERATURES IN SOUTHERN
LOCATIONS A COUPLE DEGREES AS WELL...BUT WITH TEMPERATURES ALREADY
AT OR ABOVE 70 AT SEVERAL SITES IN THE SOUTHERN CWA HAVE NOT
ADJUSTED FORECAST HIGHS AT THIS TIME. FOR THE NORTHERN FORECAST
AREA...THICKENING CLOUDS AND NEARBY SHOWER ACTIVITY HAVE ALREADY
STUNTED TEMPERATURES WITH FREEPORT AND STERLING HAVING HELD STEADY
IN THE PAST HOUR. HAVE LOWERED TEMPERATURES BY SEVERAL DEGREES
OVER NORTH CENTRAL IL AND THERE LIKELY WILL BE SOME FLUCTUATIONS
AS RAIN MOVES IN.

THE MAIN PRECIP COVERAGE AXIS AS WELL AS THUNDER CHANCES WILL COME
WITH THE DEEPER CONVERGENCE/FORCING AHEAD OF THE TROUGH AS IT
SHIFTS WEST-TO-EAST OVER THE AREA FROM MID AFTERNOON THROUGH MID-
EVE. IN GENERAL THAT WAS GOING IN THE FORECAST AND HAVE JUST
INCREASED MAGNITUDE OF POPS AND REFINED TIMING. A SOMEWHAT DIFFUSE
RIBBON OF INSTABILITY ASSOCIATED WITH MID-UPPER 50S LOW-LEVEL DEW
POINTS IS FAVORED TO SUPPORT ISOLATED TO SCATTERED EMBEDDED
THUNDER WITHIN THIS AXIS. PRECIP MAY LINGER AFTER FROPA A
BIT...BUT SHOULD BE EAST OF ROCKFORD BY SUNSET AND EAST OF CHICAGO
BY 10 PM OR SO.

MTF/KREIN

//PREV DISCUSSION...
305 AM CDT

MAIN FORECAST CONCERN IS WITH RAIN/THUNDER CHANCES AND TRENDS TODAY
WITH COLD FRONT MOVING THROUGH THE FORECAST AREA THIS AFTERNOON AND
EARLY THIS EVENING. PATTERN REMAINS AMPLIFIED AND PROGRESSIVE
THROUGH THE WEEK...WITH ANOTHER ORGANIZED CHANCE OF PRECIP AND A
FRONTAL PASSAGE THURSDAY AND YET ANOTHER LATER NEXT WEEKEND.

EARLY MORNING SURFACE MAP DEPICTS A TROUGH OF LOW PRESSURE EXTENDING
FROM NORTHWEST MINNESOTA SOUTHWEST THROUGH THE SIOUX FALLS AREA AND
INTO THE CENTRAL PLAINS. A COMPLEX UPPER TROUGH WAS OVER CENTRAL
NORTH AMERICA WITH AT LEAST THREE DISTINCT SHORT WAVE DISTURBANCES
FROM NORTH TO SOUTH PER 00Z RAOB AND CURRENT GOES VAPOR IMAGERY. THE
UPPER TROUGH IS PROGGED TO CONTINUE TO PROPAGATE EAST ACROSS THE
AREA TODAY...WITH SURFACE LOW PRESSURE LIFTING NORTH OF THE CWA AND
DEEPENING ACROSS THE NORTHERN GREAT LAKES IN RESPONSE TO THE
STRONGER NORTHERN STREAM SHORT WAVE DIGGING ALONG THE US/CANADIAN
BORDER. LOW LEVEL MOISTURE PLUME FROM TEXAS TO THE UPPER MISSISSIPPI
VALLEY HAS BEEN LIMITED SOMEWHAT BY PIECEMEAL NATURE OF UPPER
FORCING ALONG UPPER TROUGH AND BLOCKING OF CENTRAL/EASTERN GULF BY
EXPANSIVE SURFACE HIGH PRESSURE RIDGE WHICH EXTENDS FROM THE LOWER
MISSISSIPPI VALLEY TO OFF THE NEW ENGLAND COAST. HOWEVER...STARTING
TO SEE DEVELOPMENT OF ISOLATED SHRA/TSRA INTO FAR NORTHERN
IL/SOUTHERN WI EARLY THIS MORNING IN ADVANCE OF APPROACHING SHORT
WAVE AND ABOVE SUBTLE ELEVATED WARM FRONTAL ZONE WHICH STRETCHES
EAST/SOUTHEAST FROM SIOUX FALLS LOW. GUIDANCE RUNS IN GENERAL
AGREEMENT THAT PRECIP CHANCES WILL SLOWLY EXPAND ACROSS THE REGION
BY THIS AFTERNOON AS SURFACE COLD FRONT TRACKS EAST INTO THE AREA AS
SURFACE LOW DEEPENS TO OUR NORTH. COMBINATION OF LARGE SCALE ASCENT
ASSOCIATED WITH MID-LEVEL TROUGH AND LOW LEVEL FRONTAL CONVERGENCE
RESULTS IN STEEPENING LAPSE RATES BY AFTERNOON...WITH FORECAST
SOUNDINGS DEPICTING ABOUT 300-700 J/KG MLCAPE IN PRE-COLD FRONTAL
ENVIRONMENT. THIS IS EXPECTED TO BE SUFFICIENT TO PRODUCE INCREASING
COVERAGE OF SHRA/TSRA AS THE FRONT MOVES EAST ACROSS THE REGION THIS
AFTERNOON AND EARLY THIS EVENING...THOUGH DEEP SHEAR REMAINS
RELATIVELY WEAK WITH MAIN UPPER JET CORE WELL TO OUR NORTH. PRECIP
CHANCES THEN DIMINISH RAPIDLY FROM THE WEST/NORTHWEST THIS EVENING
AS FRONT PULLS AWAY TO THE EAST AND DRIER AIR PUSHES IN FROM THE
NORTHWEST. LOW LEVEL THERMAL RIDGE AHEAD OF COLD FRONT SHOULD EASILY
SUPPORT TEMPS IN THE 70S AGAIN TODAY...WITH SOUTHWEST BOUNDARY LAYER
FLOW 10-15 KT ENOUGH TO PREVENT ANY SIGNIFICANT LAKE BREEZE
DEVELOPMENT.

STRONG SHORT WAVE DIGS SOUTHEAST ACROSS THE GREAT LAKES REGION
TONIGHT...WITH COOL/DRY LOW LEVEL ADVECTION IN THE WAKE OF
ASSOCIATED SURFACE LOW AND COLD FRONT RESULTING IN COOLER TEMPS
TONIGHT AND TUESDAY...WITH NORTHERLY WINDS AHEAD OF SURFACE HIGH
PRESSURE WHICH BUILDS INTO THE AREA TUESDAY NIGHT AND WEDNESDAY.
WHILE IT WILL BE SUNNY...AFTER A COUPLE OF DAYS WELL INTO THE
70S...LOWER 60S APPEARS TO BE ABOUT THE WARMEST ACROSS OUR FAR
WESTERN/SOUTHERN COUNTIES ON TUESDAY AND WEDNESDAY WHILE NORTHEAST
WINDS OFF LAKE MICHIGAN LIMIT CHICAGO METRO AREA TO MID-UPPER
50S...WITH SOME 40S RIGHT ALONG THE LAKE. LOWS MAY DIP INTO THE LOW
30S IN SOME SPOTS TUESDAY NIGHT AS WINDS DIMINISH WITH THE APPROACH
OF THE RIDGE AXIS.

MEDIUM RANGE FORECASTS REMAIN CONSISTENT IN BRINGING ANOTHER RATHER
AMPLIFIED UPPER TROUGH EAST OF THE ROCKIES ON WEDNESDAY...WITH
SURFACE LOW PRESSURE DEVELOPING ACROSS THE PLAINS. MOISTURE RETURN
LOOKS TO BE A LITTLE MORE ROBUST WITH THIS FEATURE...WITH MOISTENING
AND SATURATION IN ISENTROPIC WARM AIR ADVECTION POTENTIALLY YIELDING
SOME SHOWERS ACROSS PARTS OF NORTHWEST IL BY WEDNESDAY AFTERNOON...
AND ACROSS MUCH OF THE FORECAST AREA WEDNESDAY NIGHT. THE GREATEST
CHANCE FOR PRECIP IS THURSDAY...AS OCCLUDING COLD FRONT PLOWS EAST
ACROSS THE CWA. WHILE TEMPS WARM...POSITION OF TRIPLE POINT IN
OCCLUSION REMAINS ACROSS/SOUTH OF OUR AREA AND DOES NOT PORTENT
TEMPS AS WARM AS THIS PAST WEEKEND WITH GENERALLY 60S ANTICIPATED
AND PERHAPS ONLY 50S ALONG NORTH SHORE IN SOUTHEAST FLOW. MODELS
DEPICT SECOND DISTURBANCE ROTATING AROUND UPPER LOW TO OUR NORTH
THURSDAY NIGHT INTO FRIDAY...WRAPPING A SECONDARY COLD FRONT
SOUTHEAST ACROSS THE AREA AND MAINTAINING SOME POTENTIAL FOR SHOWERS
ACROSS ESPECIALLY NORTHERN/EASTERN PARTS OF THE FORECAST AREA. SOME
DIFFERENCES ARISE WITH ORIENTATION OF EASTERN NORTH AMERICAN TROUGH
WHICH RESULTS BY NEXT WEEKEND... THOUGH LOOKS TO BE ANOTHER COOLER
THAN NORMAL PERIOD WITH NORTH/NORTHEAST FLOW FOR US TO THE WEST OF
THE UPPER TROUGH.
WHILE RUN TO RUN CONSISTENCY EXISTS IN GFS AND ECMWF LONG RANGE
SOLUTIONS...ECMWF REMAINS FASTER WITH DEPARTING EAST COAST LONG WAVE
TROUGH AND BRINGS ANOTHER LOW/WARM FRONT INTO THE REGION (BUT TO OUR
SOUTH) LATE SATURDAY AND SUNDAY. HAVE GENERALLY FOLLOWED THE ECMWF
TREND WHICH BRINGS PRECIP CHANCE BACK IN NORTH OF THE WARM FRONT
SATURDAY NIGHT/SUNDAY.

RATZER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* SHRA WITH ISOLATED/SCATTERED TSRA DURING THE MID-AFTERNOON
  THROUGH MID-EVE.
* CHANCE FOR A BRIEF PERIOD OF MVFR CIGS EARLY THIS EVE.
* SOUTHWEST TO NORTHWEST WINDS THIS AFTERNOON BECOMING NORTHWEST
  BY EARLY-MID EVE
* WINDS BECOMING NORTHEAST TUESDAY AFTERNOON.

//DISCUSSION...UPDATED 18Z...

BROAD LOW PRESSURE ACROSS WI/U.P. OF MICHIGAN WILL CONSOLIDATE
GOING INTO THIS EVE AND STEER A COLD FRONT ACROSS THE AREA EARLY
THIS EVE. AHEAD OF THIS...FORCING AND MOISTURE WILL BE ENOUGH TO
CONTINUE AREAS OF SHOWERS. THERE LIKELY WILL BE AN UPTICK IN
COVERAGE DURING THE MID-LATE AFTERNOON ACROSS NORTHERN/CENTRAL
IL. HOW MUCH THUNDER IS EMBEDDED IN THAT REMAINS SOMEWHAT
UNCERTAIN...BUT IT LIKELY TO BE ON THE ORDER OF 30-50 PERCENT.
HAVE TRENDED TO A VCTS FOR THAT REASON. IN SOME OF THE MORE MODEST
SHOWERS NEAR THE FRONT THERE HAVE BEEN OBSERVATIONS OF BRIEF IFR
VISIBILITY TO THE WEST AND FEEL THAT COVERAGE OF AT LEAST SHOWERS
AHEAD OF THE FRONT WILL BE HIGH ENOUGH HERE TO MAINTAIN THAT 3SM
MENTION IN THE TAFS. MVFR CIGS ARE LESS COMMON SO HAVE NOT
INCLUDED IN TAFS. WITH FROPA THIS EVE WILL COME DRYING ALONG WITH
NORTHWEST GUSTS WHICH ARE FAVORED TO LAST INTO THE DAY TUESDAY.
GIVEN THE NNW TRAJECTORY OF SYNOPTIC SURFACE WINDS AND THE
DIMINISHING SPEEDS BY MIDDAY TUE...CLIMATOLOGY WILL LIKELY TAKE
OVER FOR A NORTHEAST WIND TO DEVELOP OFF THE LAKE AT ORD AND MDW.

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH IN SHRA OCCURRING AND WINDS REMAINING BETWEEN 190 AND 320
  THIS AFTERNOON AND EARLY EVE INCLUDING UNDER AND NEAR SHRA.
* LOW IN TSRA OCCURRING. MEDIUM THAT IF THEY DO OCCUR IT WOULD BE
  BETWEEN 22Z-01Z.
* LOW IN WHETHER MVFR CIGS CAN DEVELOP.
* HIGH IN WIND SHIFT TO NORTHEAST TUESDAY AFTERNOON BUT LOW IN
  TIMING.

//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z...

TUESDAY NIGHT...VFR.

WEDNESDAY...CHANCE OF SHOWERS...OTHERWISE VFR.

THURSDAY...A PERIOD OF SHRA/TSRA LIKELY.

FRIDAY...SLIGHT CHANCE OF SHOWERS...OTHERWISE VFR.

SATURDAY...CHANCE OF SHOWERS LATE...OTHERWISE VFR.

SUNDAY...CHANCE OF SHOWERS...OTHERWISE VFR.

MTF/RODRIGUEZ

&&

.MARINE...
253 PM CDT

A BROAD LOW PRESSURE SYSTEM WILL MOVE ACROSS THE GREAT LAKES THIS
EVENING AND TONIGHT DRAGGING A COLD FRONT DOWN LAKE MICHIGAN FROM
MID TO LATE THIS EVENING. NNW WINDS WILL PICK UP BEHIND THE FRONT
WITH A WINDOW OF 30 KT GUSTS POSSIBLE. A SMALL CRAFT ADVISORY
REMAINS IN EFFECT FROM MID EVENING THROUGH TOMORROW MORNING THOUGH
WILL BE PARTICULARLY MARGINAL FOR THE ILLINOIS SHORES. EXTENDED
THE ADVISORY INTO THE AFTERNOON FOR INDIANA THOUGH WHICH WILL HAVE
HIGHER WAVES FOR A FEW MORE HOURS INTO THE AFTERNOON. HIGH
PRESSURE WILL ALLOW WINDS TO DIMINISH SOME TUESDAY ALLOWING LAKE
BREEZES TO DEVELOP...ESPECIALLY ON THE WESTERN SIDE OF THE LAKE
WHERE THE GRADIENT IS MORE RELAXED. THE HIGH WILL MOVE OVERHEAD
WEDNESDAY WITH WINDS BECOMING LIGHT AND VARIABLE FOR A
PERIOD...AND AGAIN ALLOWING LAKE BREEZES TO DEVELOP IN THE
AFTERNOON. SOUTHEASTERLY FLOW BEGINS TO RAMP UP WEDNESDAY NIGHT
INTO THURSDAY AS LOW PRESSURE TAKES SHAPE OVER THE CENTRAL PLAINS
AND LIFTS INTO THE UPPER MIDWEST. SOUTHEAST SUSTAINED WINDS TO
AROUND 25 KT ARE EXPECTED...THOUGH GUSTS SHOULD STAY IN CHECK AS A
STABLE MARINE LAYER DEVELOPS. BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...10 PM MONDAY TO 3
     PM TUESDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...10 PM MONDAY TO 10
     AM TUESDAY.

&&

$$

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