Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011955
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
255 PM CDT MON SEP 1 2014

.DISCUSSION...
1115 AM CDT

FOR MORNING UPDATE...

THUNDERSTORM AND SEVERE WEATHER THREAT APPEARS TO HAVE DIMINISHED
SIGNIFICANTLY FOR MUCH OF THE FORECAST AREA TODAY...DUE TO CLOUD
COVER AND DRIER AIR WHICH HAS SPREAD EAST ACROSS NORTHERN IL WITH
OUTFLOW FROM OVERNIGHT MCS TO OUR WEST. SEE UPDATED SPC DAY 1
OUTLOOK WHICH REMOVES SLIGHT RISK GENERALLY ALONG/WEST OF I-57.
HAVE ALSO BOOSTED TEMPS ACROSS THE SOUTHEAST TO THE LOW-MID 80S
WHERE TEMPS QUICKLY WARMING.

LEAD SHORT WAVE...POSSIBLY ENHANCED BY MCS/MCV FROM NOCTURNAL MCS
OVER IOWA...HAS MOVED INTO NORTHERN IL THIS MORNING ACCOMPANIED BY A
PRE-FRONTAL TROUGH AND SHIFT TO WESTERLY WINDS GENERALLY WEST OF THE
FOX RIVER VALLEY AS OF 11 AM. EXTENSIVE CLOUD COVER..AND LOWER SFC
DEW POINTS BEHIND THE BOUNDARY HAVE STABILIZED ATMOSPHERE...WITH
HIGHER THETA-E AIR CHARACTERIZED BY LOWER 70S DEW POINT TEMPS NOW
MAINLY ALONG/EAST OF THE INTERSTATE 57 CORRIDOR. WHILE SOME EROSION
OF CLOUD COVER WAS NOTED ACROSS WESTERN IL...IT APPEARS THAT THE
MAIN INSTABILITY AXIS WILL BE ACROSS THE FAR EASTERN CWA AND AREAS
FARTHER EAST. HIGH-RES CONVECTION ALLOWING GUIDANCE INCLUDING THE
HRRR AND NATIONAL/LOCAL ARW RUNS INDICATE LITTLE IN THE WAY OF
ROBUST CONVECTION DEVELOPING ACROSS NORTHERN IL...WITH DIURNAL
DESTABILIZATION AND STRONGER THUNDERSTORM INITIATION FOCUSED EARLY
THIS AFTERNOON ALONG OUR EASTERN CWA BOUNDARY. THUS IT APPEARS OUR
SEVERE WEATHER THREAT HAS DECREASED SIGNIFICANTLY...AND SPC HAS
REMOVED MUCH OF THE CWA WEST OF I-57 FROM THEIR DAY 1 SLIGHT RISK.

THE SYNOPTIC COLD FRONT STILL REMAINS TO OUR WEST...FROM JUST EAST
OF THE TWIN CITIES TO SOUTHWEST IOWA AT 11 AM...AND THIS WILL MOVE
EAST THROUGH THE AFTERNOON AND EVENING. GUIDANCE DOES DEPICT A LINE
OF SHOWERS/ISOLATED THUNDERSTORMS DEVELOPING AHEAD OF THE FRONT LATE
THIS AFTERNOON AND EVENING...THOUGH SEVERE THREAT APPEARS VERY LOW
IN RATHER WEAK MLCAPE ENVIRONMENT. POTENTIAL FOR STRONGER STORM
DEVELOPMENT DOES REMAIN LATER TONIGHT ALONG THE FRONT AS MAIN SHORT
WAVE MOVES ACROSS THE AREA...THOUGH TRENDS FROM 12Z GUIDANCE
INDICATE ONLY A SMALL PORTION OF OUR SOUTHEAST CWA MAY BE
AFFECTED. HAVE TRIMMED BACK POPS QUITE A BIT ACROSS THE NORTHERN
3/4 OR SO OF THE CWA AND MAY BE ABLE TO TRIM EVEN FURTHER
DEPENDING ON TRENDS OVER THE NEXT FEW HOURS.

RATZER

//PREV DISCUSSION...
340 AM CDT

ENTERING SEPTEMBER AND METEOROLOGICAL AUTUMN WE STILL FIND THE
FORECAST CHALLENGES TO BE MORE SUMMER-LIKE...AT LEAST EARLY ON IN
THE FORECAST. THAT IS STORM AND HEAVY RAIN POTENTIAL TODAY AND
TONIGHT...THEN STORM CHANCES WEDNESDAY NIGHT AND FRIDAY WHICH
SANDWICH A POSSIBLE 90 PLUS DEGREE DAY ON THURSDAY. LESS BUT
STILL SOME TIME WAS SPENT ON THE FINAL CHALLENGE OF HOW MUCH OF A
COOL DOWN OCCURS THE FIRST FULL WEEKEND OF THIS NEW METEOROLOGICAL
SEASON.

SYNOPSIS...
A LONG WAVE TROUGH EXTENDING SOUTHWARD THROUGH CANADA INTO THE NORTH
CENTRAL U.S. HAS A VERY PROGRESSIVE SHORT WAVE FEATURE MOVING
THROUGH ITS SOUTHERN BASE. THIS IS SPEEDING EAST-NORTHEAST ACROSS
IA EARLY THIS MORNING. A SURFACE COLD FRONT MOVING EAST LAGS THIS
SHORT WAVE...EXTENDING FROM NORTHERN MN THROUGH THE TX PANHANDLE
EARLY THIS MORNING...WITH MULTIPLE SURFACE WAVES IN THE 1003MB
NEIGHBORHOOD ALONG IT. NORTHWEST SURFACE FLOW BEHIND THIS FRONT IS
NOT PARTICULARLY STRONG SO THIS FRONT WILL CONTINUE TO BE A SLOW-
MOVER TODAY...NOT EXPECTED TO ENTER THE FORECAST AREA UNTIL THIS
EVE. BEING WITHIN A WARM SECTOR UNDER QUICKENING UPPER/MID-LEVEL
FLOW PRESENTS THE STORM CHANCES THESE 24 HOURS.

THIS MORNING...
A LOOSELY DEFINED MCS MOVING EASTWARD INTO NORTHWEST IL EARLY THIS
MORNING WANED CONSIDERABLY WITH WARMING CLOUD TOPS AND OUTFLOW
EXTENDING WELL AHEAD OF IT. SEEING AN UPTICK ACROSS WEST CENTRAL IL
LIKELY ASSOCIATED WITH LIFT OF THE AFOREMENTIONED PROGRESSIVE SHORT
WAVE. HIGH RESOLUTION MODELS GREATLY VARY ON WHAT WILL HAPPEN EVEN
JUST A FEW HOURS OUT...BUT WOULD EXPECT SYNOPTICALLY TO SEE SOME
CONVECTION EVOLVE INTO THE FORECAST AREA THIS MORNING...POSSIBLY
ONLY THE WESTERN/SOUTHWESTERN AREA BUT STILL CANT RULE OUT SCATTERED
ACTIVITY MOVING EASTWARD THROUGH THE ENTIRE AREA INCLUDING
CHICAGO DURING MID/LATE MORNING. DESPITE A FAIRLY STRONG LOW-LEVEL
JET INTO ANY ACTIVITY...THE CONTINUED BROAD AREA OF STORMS ACROSS
EASTERN KS INTO WEST/SOUTHWEST MO THROUGH EARLY TO MID MORNING
SHOULD LESSEN MOISTURE TRANSPORT AND THUS ABILITY TO REALIZE STEEP
LAPSE RATES ALOFT FOR MORE ROBUST ACTIVITY...AND MAYBE EVEN
THUNDER IN GENERAL NORTH OF INTERSTATE 80.

THIS AFTERNOON AND TONIGHT...
DESPITE THIS BEING IN THE FIRST 12 HOURS OF THE FORECAST...DUE TO
INFLUENCE OF MORNING STORMS AND LARGE MODEL DIFFERENCES...CONFIDENCE
ON SPECIFICS IN THESE PERIODS REMAIN LOW. THERE SHOULD BE SOME
FORM OF OUTFLOW BOUNDARY OR BOUNDARIES FROM MORNING ACTIVITY/CLOUD
COVER THAT LINGER INTO THE FORECAST AREA INTO THE AFTERNOON...MOST
FAVORED SOUTH OF INTERSTATE 80. AS WARMING AND DESTABILIZATION
OCCURS WITH DEW POINTS AROUND 70F...COULD SEE SCATTERED ACTIVITY
DEVELOP NEAR ANY SUCH BOUNDARIES DURING THE MID TO LATE AFTERNOON.
THE SECOND UPPER WAVE ACROSS WESTERN WY THIS MORNING WILL BEGIN TO
APPROACH THE REGION AS WELL BY LATE IN THE DAY...WITH FORCING FROM
THIS POSSIBLY AIDING IN SCATTERED STORM DEVELOPMENT ACROSS MUCH OF
THE REGION. EVEN EARLY THIS MORNING THERE ARE A HANDFUL OF STORMS
ACROSS THE UPPER MIDWEST NOT LINKED TO ANY SPECIFIC SURFACE
FEATURE. INCREASING DEEP LAYER SHEAR OF 35 TO 50 KT WILL SUPPORT
ISOLATED TO SCATTERED SEVERE DISCRETE STORMS IN THE REGION.

WHILE IT REMAINS DIFFICULT TO PINPOINT TIMING...IT WOULD SEEM THE
GREATEST COVERAGE IN REDEVELOPING STORMS THIS EVENING WOULD BE IN
THE SOUTHERN FORECAST AREA WHERE AFOREMENTIONED BOUNDARY MAY OCCUR
AND MODEST SOUTHWEST 850-925MB FLOW AND MOISTURE TRANSPORT
REMAINS. AS THE AREA BECOMES UNDER A RIGHT ENTRANCE REGION TO THE
UPPER JET...DO EXPECT INCREASING ASCENT TONIGHT IN THE REGION FOR
MORE ROBUST CONVECTION AND POTENTIAL SEVERE AND HEAVY RAIN
THREATS. RIGHT NOW A GUIDANCE OVERLAP WOULD INDICATE THE FAR
SOUTHEAST FORECAST AREA IS MOST FAVORED FOR THIS. PRECIPITABLE
WATERS AROUND TWO INCHES AND STORM MOTIONS NEARLY PARALLEL TO THE
UPPER AND MID LEVEL FLOW WILL PRESENT HEAVY RAIN THREATS IN
THE GENERAL CENTRAL TO SOUTHERN IL INTO IN REGIONS...BUT AGAIN AT
THIS TIME CONFIDENCE VARIES ON PLACEMENT. BECAUSE OF THAT WILL
CONTINUE WITH THE HYDROLOGIC OUTLOOK /ESF/ FOR THE AREAS HIT HARD
ON SATURDAY NIGHT WITH HEAVY RAINFALL.

TUESDAY AND WEDNESDAY...
THE FRONT SHOULD CLEAR THE FORECAST AREA SOMETIME TUESDAY MORNING.
WHILE DRY AIR ADVECTION WILL OCCUR...LIMITED CONVERGENCE REMAINS
IN THE SOUTHEAST FORECAST AREA FOR POSSIBLY A STORM OR TWO TUESDAY
BUT IT LOOKS TO BE A LOW CHANCE. TEMPERATURES ONLY COOL SLIGHTLY
AS HIGH PRESSURE QUICKLY MOVES ATOP THE AREA TUESDAY NIGHT AND
THEN EASTWARD.

WEDNESDAY NIGHT THROUGH FRIDAY...
WARM AIR ADVECTION AND A RAPID PRECIPITABLE WATER INCREASE IS
FORECAST WEDNESDAY NIGHT ON ALL GUIDANCE. FEEL THAT MODELS MAY BE
UNDER-DOING THE STORM CHANCES GIVEN FORECAST PROFILES OF STEEPENING
LAPSE RATES WITHIN A RAPIDLY MOISTENING COLUMN...AT LEAST IN THE
WESTERN FORECAST AREA. SO HAVE INCREASED POPS IN THAT AREA. AS FOR
THURSDAY...HAVE GONE DRY IN MANY AREAS COLLABORATING AND
UNDERCUTTING A MODEL BLEND OF POPS. THE SYNOPTIC PATTERN FAVORS A
HOT DAY AND BARRING ANY STRATUS ISSUES WITH WEDNESDAY NIGHTS WARM
AIR ADVECTION AND POSSIBLE PRECIP...SHOULD SEE THE 90 DEGREE MARK
REACHED OR EXCEEDED ACROSS A GOOD PART OF THE FORECAST AREA BASED
ON STRONG SOUTHWEST FLOW AND FORECAST 850MB AND 925MB TEMPERATURES
COMPARED TO CLIMATOLOGY. FOR NOW HAVE KEPT THE NORTH A LITTLE
BELOW THAT WITH THE POSSIBILITY OF MORNING CLOUDS. THE SYSTEM
COLD FRONT IS PRESENTLY FORECAST BY THE EC AND THE GFS TO COME
THROUGH THE AREA SOMETIME ON FRIDAY WHICH WOULD BRING STORM
CHANCES WITH IT...AS WELL AS FLOW TURNING OFF THE LAKE BEHIND ITS
PASSAGE.

BEYOND...
NO CHANGES MADE TO THIS WEEKEND WHERE HIGHS LOOK TO BE AROUND 70
WITH DRY CONDITIONS BEHIND THE AFOREMENTIONED FRONT. HIGH PRESSURE
AROUND 1023MB IS FORECAST BY A BLEND OF GUIDANCE. GIVEN THE 925MB
TEMPERATURES FORECAST IF THAT HIGH IS CENTERED OVER THE FORECAST
AREA SOMETIME SUNDAY OR MONDAY NIGHTS /7TH OR 8TH/...LOWS IN
OUTLYING AREAS WOULD LIKELY BE IN THE 40S.

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* SHRA CHANCES THIS EVENING.

* GUSTY SOUTHWEST WINDS.

* POSSIBLE MVFR VSBYS OVERNIGHT TOWARD MORNING.

LENNING

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

CONVECTION HAS BLOSSOMED ALONG A LINE OVER LOWER MI BUT MANAGED TO
AVOID MOST OF THE LOCAL AREA THIS MORNING. AN ADDITIONAL CLUSTER
OF STORMS IS STARTING TO FORM OVER NORTHEAST IOWA ALONG A COLD
FRONT THAT IS MOVING THIS DIRECTION. TIMING OF THE IOWA ACTIVITY
INTO THE LOCAL AREA IS UNCERTAIN...BUT FEEL THAT 00Z TO 03Z
IS A GOOD FIRST ESTIMATE AT ORD AND MDW AS PER 18Z TAF.

CEILINGS HAVE SCATTERED EXCEPT FOR SOME LINGERING MVFR...BUT AM
NOT CONFIDENT THAT SCT-CLR SKIES WILL BE WIDESPREAD THIS
AFTERNOON GIVEN THE AMOUNT OF MOISTURE IN THE AIR. AREAS THAT HAD
CLEARED EARLIER ARE ALREADY STARTING TO FILL BACK IN WITH DIURNAL
HEATING...BUT FOR THE MOST PART ARE STAYING VFR.

OVERNIGHT TOWARD MORNING THERE IS A CHANCE OF LOWER VSBYS WITH
LIGHT WINDS AND MOIST AIR IN PLACE...BUT GUIDANCE SHOWS BETTER
CHANCES TOWARD RFD THAN IN THE MDW/ORD TERMINALS...LIKELY DUE TO
BETTER COOLING THAN IN THE CITY. HAVE NOT PUT THIS IN THE TAFS YET
BUT WILL MONITOR FOR LATER INCLUSION.

LENNING

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM TO HIGH CONFIDENCE FOR ALL ELEMENTS.

LENNING

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

TUESDAY NIGHT THROUGH THURSDAY...PREDOMINANTLY VFR.

FRIDAY...CHANCE OF SHRA/TSRA.

SATURDAY AND SUNDAY...PREDOMINANTLY VFR.

KJB

&&

.MARINE...

142 PM...LOW PRESSURE OVER NORTHERN ONTARIO WILL LIFT NORTHEAST
ACROSS NORTHERN QUEBEC TUESDAY. A TRAILING COLD FRONT FROM THIS
LOW WILL MOVE ACROSS THE LAKE TONIGHT. SOUTHWEST WINDS 15-25KTS
WILL DIMINISH THIS EVENING THEN SHIFT NORTHWESTERLY OVERNIGHT.
WEAK RIDGING WILL MOVE ACROSS THE SOUTHERN LAKES TUESDAY AND
TUESDAY NIGHT AS WINDS SHIFT BACK SOUTHERLY ACROSS THE LAKE. A
LARGE AREA OF LOW PRESSURE WILL DEVELOP AND DEEPEN OVER THE
PLAINS WEDNESDAY AND THEN LIFT NORTHEAST INTO ONTARIO THURSDAY
NIGHT AND FRIDAY. THE GRADIENT WILL TIGHTEN AHEAD OF THIS SYSTEM
WEDNESDAY NIGHT AND THURSDAY. A TRAILING COLD FRONT FROM THIS
LOW WILL THEN MOVE ACROSS THE LAKE FRIDAY. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 10 PM MONDAY.

&&

$$

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