Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 230525
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1225 AM CDT WED JUL 23 2014

.MESOSCALE DISCUSSION...
807 PM CDT

THE HIGHEST THETA-E SURFACE AIR MASS OF THE YEAR HAS ADVECTED INTO
THE AREA IN ADVANCE OF A SOUTHWARD MOVING COLD FRONT ADVANCING
TOWARD THE CWA FROM SOUTHERN WISCONSIN. AN AXIS OF MID TO EVEN SOME
UPPER 70 DEGREE DEWPOINTS IS POOLING ACROSS NORTHERN ILLINOIS JUST
AHEAD OF THE FRONT. THE VERY HIGH THETA-E JUXTAPOSED BENEATH AN
IMPRESSIVE EML WITH VERY STEEP LAPSE RATES IS RESULTING IN A CLASSIC
LOADED GUN SITUATION WITH VERY STRONG INSTABILITY THAT IS CURRENTLY
LARGELY BEING HELD IN CHECK BY THE STRONG CAP AT THE BASE OF THE EML
BETWEEN 900-800 MB ON THE DVN EVENING SOUNDING. FRONTAL
CONVERGENCE ALONE HAS NOT BEEN TERRIBLY EFFECTIVE AT BUSTING
THROUGH THE CAP THUS FAR...THOUGH A FEW CELLS HAVE MADE STOUT BUT
RELATIVELY SHORT LIVED EFFORTS.

AS THE FRONT MOVES THROUGH THE CWA THIS EVENING WOULD NOT ANTICIPATE
MUCH BOUNDARY LAYER STABILIZATION TO TAKE PLACE AHEAD OF THE FRONT
DUE TO THE VERY HIGH DEWPOINTS IN PLACE LEADING TO RELATIVELY
LIMITED DIURNAL COOLING. WATER VAPOR IMAGERY THIS AFTERNOON AND
EVENING HAS SHOWN VERY WELL DEFINED SHORTWAVE TRACKING
EAST-SOUTHEAST FROM SOUTH DAKOTA NOW INTO NORTHERN IOWA AND SOUTHERN
MINNESOTA. AS THIS FEATURE CONTINUES EAST-SOUTHEASTWARD AND BEGINS
TO CATCH UP TO THE SURFACE FRONT...THE INCREASING LARGE SCALE ASCENT
ASSOCIATED WITH THE SHORTWAVE COULD RESULT IN A WEAKENING OF THE CAP
AND AN INCREASE IN STORM COVERAGE THIS EVENING ALONG THE FRONT AS IT
MOVES INTO OUR CWA. CONFIDENCE IN THIS SCENARIO IS NOT OVERLY
HIGH...SO NOT GOING TO MAKE ANY CHANGES TO POPS AT THIS POINT...BUT
THAT IS THE REASON THAT DESPITE THE PALTRY STORM COVERAGE THUS FAR
THAT I`VE BEEN RELUCTANT TO LOWER POPS. IF CAP DOES WEAKEN AND
SUSTAINED STORMS CAN FORM...THE VERY STRONG INSTABILITY AND MODEST
EFFECTIVE SHEAR WOULD BE ENOUGH TO SUPPORT AT LEAST A PULSE SEVERE
THREAT.

IZZI

&&

.DISCUSSION...
233 PM CDT

FORECAST FOCUS PRIMARILY ON SHORT TERM HEAT AND HUMIDITY AND
CONVECTIVE CHANCES THIS EVENING. AFTER THUNDERSTORMS TONIGHT...WE
WILL HAVE A COUPLE DAYS OF DRY AND COOLER CONDITIONS...FOLLOWED BY A
RETURN TO A POTENTIALLY MORE ACTIVE PATTERN FOR THE LATTER HALF OF
THE WEEK AND THROUGH THE WEEKEND.

THE LARGE UPPER RIDGE IS BEING FLATTENED BY AN UPPER WAVE
PROGRESSING THROUGH THE NORTHERN LAKES. DEWPOINTS ARE POOLING OUT
AHEAD OF THE COLD FRONT APPROACHING FROM THE NORTH AND WEST...SO
READINGS ARE GENERALLY WELL INTO THE 80S TO AROUND 90 AND DEWPOINTS
IN MANY AREAS INTO THE 70S WHERE IT STARTS TO FEEL QUITE
UNCOMFORTABLE...EVEN ALL THE WAY TO THE LAKEFRONT. LINGERING CONVECTION
FROM EARLIER TODAY IS SINKING SOUTH AND WEAKENING ALONG THE IA/MO
BORDER. RECENT THUNDERSTORM DEVELOPMENT HAS BEEN ONGOING IN
CENTRAL AND EASTERN IOWA OUT AHEAD OF THE COLD FRONT. THE STORMS
ARE TRYING TO GROW AGAINST THE CAP BUT HAVE OVERALL BEEN
RELATIVELY UNIMPRESSIVE ON SATELLITE TO THIS POINT. LATEST SPC/RAP
MESOANALYSIS SHOWS AN AREA OF VERY HIGH MIXED LAYER INSTABILITY IN
EASTERN IOWA AND EXTENDING INTO FAR NW ILLINOIS WHERE DEWPOINTS
ARE IN THE LOW TO MID 70S. UNFORTUNATELY IN THIS SITUATION THE BEST
MOISTURE CONVERGENCE IS NOT JUXTAPOSED WITH THE BETTER HEIGHT
FALLS AND STRONGER WINDS ALOFT ASSOCIATED WITH THE DISTURBANCE
GLIDING BY TO THE NORTH.

SHORT TERM GUIDANCE STILL REMAINS MIXED TO THIS POINT AS TO HOW
WIDESPREAD THE COVERAGE WILL BE...BUT ANY THAT DO FORM STILL HOLD
THE POTENTIAL FOR STRONG WINDS...LARGE HAIL DUE TO THE MODERATE MID
LEVEL LAPSE RATES...AND LOCALLY HEAVY RAINFALL AS PWAT VALUES HOVER
ABOVE 2". WHAT WE CAN SAY WITH DECENT CONFIDENCE IS THE TIMING OF ANY
STORMS. SOME HI-RES GUIDANCE SUGGEST A BRIEF WINDOW OF THE CAPPING
TO BE ERODED IN NORTH CENTRAL INTO NE ILLINOIS IN THE 6-7 PM TIME
FRAME...MOVING THROUGH METROPOLITAN AREA BETWEEN 8-11PM...AND THEN
SHIFTING SOUTH AND EAST INTO NW INDIANA. CONFIDENCE IS LOW ON A FULL
LINE DEVELOPING AS SOME MODELS INDICATE AS THE CAPPING HOLDS IN OUR
SOUTHWESTERN AREAS WHERE WEAKER LOW LEVEL CONVERGENCE AND UPPER
FORCING WILL HAVE A HARDER TIME WITH THE CAP. STILL BELIEVE THERE
WILL BE SCATTERED ACTIVITY DURING THIS PERIOD. WOULD LIKE TO BE MORE
DEFINITIVE WITH THE POPS AT THIS TIME FRAME BUT WILL GENERALLY
CONTINUE THE 40-50 POP/SCATTERED COVERAGE FOR MOST AREAS AS THE CAP
LOOKS TO KEEPS THINGS IN CHECK AT LEAST AT THIS POINT. CLOSER TO
THE LAKE WE MAY GET SLIGHTLY BETTER HEIGHT FALLS LATER THIS
EVENING ALONG WITH THE FRONT.

THE FRONT WILL BE RATHER PROGRESSIVE...THUS THE WINDOW FOR STORMS
FOR ANY ONE AREA APPEARS TO BE PRETTY SHORT.  BY DAYBREAK THE FRONT
SHOULD LARGELY HAVE PASSED THROUGH THE ENTIRE COUNTY WARNING AREA.
THERE MAY BE SOME LOWER CLOUDS BEHIND THE FRONT ESPECIALLY CLOSER TO
THE LAKE IN THE ONSHORE FLOW PATTERN. COLD ADVECTION CONTINUES
THROUGH THE DAY THUS EXPECT SOME WARMING WITH SOLAR INSOLATION BEING
STRONG...BUT WE WILL LIKELY HAVE A QUICKER DROP OFF IN THE AFTERNOON
AND EVENING. LOOKS LIKE A DRY AND SEASONABLY COOL DAY WITH NW FLOW
ALOFT IN PLACE WHILE THE UPPER RIDGE IN THE CENTER OF THE COUNTRY
AMPLIFIES INTO SASKATCHEWAN AND MANITOBA.  NOT MUCH OF A CHANGE IN
THE PATTERN FOR THURSDAY AS HIGH PRESSURE MOVES OVERHEAD AND SHIFTS
EAST...MAINTAINING COOLER THAN NORMAL CONDITIONS UNDER ONSHORE FLOW.

THE UPPER DISTURBANCE THAT IS MOVING BY TODAY WILL MERGE WITH A
LARGE UPPER LOW IN THE HUDSON BAY AREA THAT GETS BLOCKED BY A RIDGE
IT THE ATLANTIC BASIN. MEANWHILE AN UPPER LOW OFF THE PACIFIC COAST
WILL MOVE THROUGH THE CANADIAN PRAIRIES. WARM ADVECTION AHEAD OF A
SURFACE LOW EJECTING OUT OF THE NORTHERN ROCKIES ON FRIDAY LOOKS TO
TRIGGER A ROUND OF SHOWERS AND STORMS. A BIT MORE UNCERTAINTY TO THE
EVOLUTION ON THE WEEKEND AS A SURFACE WAVE ASSOCIATED WITH THE
PACIFIC LOW IS HANDLED QUITE DIFFERENTLY...THUS SOME TIMING CONCERNS
AS TO WHEN THE ASSOCIATED COLD FRONT SLIDES THROUGH...WHICH WILL
PROVIDE ANOTHER OPPORTUNITY FOR SHOWERS AND STORMS.

BEYOND THAT FORECAST CONFIDENCE INCREASES SOMEWHAT WITH
MODELS/ENSEMBLES DEPICTING A HIGHLY AMPLIFIED FLOW PATTERN WITH A
LARGE RIDGE IN THE WEST AND AN ANOMALOUS LOW IN THE EAST KEEPING THE
REGION UNDER NW FLOW FOR COOLER THAN NORMAL CONDITIONS WITH PERIODS
OF CLOUDS AND SHOWERS.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* POTENTIAL FOR MVFR CIGS OVERNIGHT.
* GUSTY NNE/NE WINDS TONIGHT THROUGH WEDNESDAY EVENING.

BMD

//DISCUSSION...UPDATED 06Z...

A COLD FRONT IS DROPPING SOUTH ACROSS THE REGION TONIGHT WITH A
BROKEN LINE OF THUNDERSTORMS ALREADY MOVING SOUTH OF THE TERMINALS.
BEHIND THE FRONT WINDS HAVE TURNED NORTHEAST WITH GUSTS INTO THE LOW
TO MID 20 KT RANGE. SOME STRATUS IS ALSO EVIDENT ON SPLIT CHANNEL IR
IMAGERY HUGGING THE LAKE MICHIGAN SHORES WITH CEILINGS BETWEEN
ROUGHLY 025 AND 035. FARTHER UPSTREAM VFR CONDITIONS PREVAIL WITH
JUST SOME PATCHY MID AND HIGH CLOUDS. WITH COLD ADVECTION INTO THE
AREA...MODERATE WINDS TO AROUND 30 KT ARE EVIDENT IN SOUNDINGS AS
LOW AS AROUND 010 AND MAY BE ABLE TO MIX DOWN TO THE SURFACE AT
TIMES OVERNIGHT RESULTING IN CONTINUED GUSTINESS THROUGH THE
OVERNIGHT HOURS. IN FACT...WINDS MAY NOT BEGIN TO TAPER UNTIL CLOSER
TO SUNSET WEDNESDAY EVENING.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH IN WIND DIRECTION...MEDIUM IN WIND GUSTS SPEEDS.
* LOW CONFIDENCE IN PREVAILING MVFR CIGS OVERNIGHT.
* HIGH CONFIDENCE IN VFR CONDITIONS THROUGH THE DAY WEDNESDAY.

BMD

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

THURSDAY...VFR. NORTHEAST WINDS.
FRIDAY...CHANCE OF TSRA DURING THE NIGHT.
SATURDAY-MONDAY...PERIODIC CHANCES OF TSRA.

MTF

&&

.MARINE...
300 PM CDT

LOW PRESSURE OVER SOUTHERN QUEBEC WILL CONTINUE TO DEEPEN AS IT
MOVES ACROSS QUEBEC TONIGHT WHILE HIGH PRESSURE MOVES OFF THE MID
ATLANTIC COAST. A STRONG COLD FRONT ASSOCIATED WITH THE LOW WILL
PUSH SOUTH DOWN THE LENGTH OF THE LAKE TONIGHT. MODERATELY STRONG
SOUTH TO SOUTHWEST WINDS WILL VEER TO NORTHERLY FOLLOWING THE
FRONTAL PASSAGE WITH WINDS INCREASING TO 30 KT WITH OCCASIONAL 35
KT GALE FORCE GUSTS POSSIBLE. A SMALL CRAFT ADVISORY ASSOCIATED
WITH THE STRONG NORTH WINDS WILL GO INTO EFFECT LATE TONIGHT AND
REMAIN IN EFFECT THROUGH THURSDAY MORNING WHEN WINDS AND WAVES
WILL TAPER OFF AS HIGH PRESSURE BEGINS TO BUILD ACROSS THE
REGION.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...BEACH HAZARDS STATEMENT...ILZ006-ILZ014 UNTIL 10 AM THURSDAY.

IN...BEACH HAZARDS STATEMENT...INZ001-INZ002 UNTIL 10 AM THURSDAY.

LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 AM THURSDAY.

&&

$$

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