Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
349 AM CDT SAT AUG 30 2014

.DISCUSSION...
344 AM CDT

THE MAIN FORECAST CONCERNS WILL CENTER AROUND CONVECTIVE CHANCES AND 
TIMING THROUGH THE PERIOD...ALONG WITH THE THREAT FOR STRONG TO 
SEVERE CONVECTION...ESPECIALLY MONDAY.

WATER VAPOR IMAGERY EARLY THIS MORNING INDICATES TWO MID/UPPER LEVEL 
DISTURBANCES OF INTEREST FOR TODAY AND TONIGHTS WEATHER. THE FIRST IS 
LOCATED OVER IOWA AND INTO EXTREME NORTHERN MISSOURI...AND THIS IS 
DRIVING AN AREA OF SHOWERS AND SOME EMBEDDED STORMS INTO WESTERN 
ILLINOIS AT THIS HOUR. THIS WAVE IS EXPECTED TO LIFT NORTHEASTWARD 
ACROSS WISCONSIN LATER THIS MORNING...AND AS IT DOES SO...A FEW 
SCATTERED SHOWERS AND A THUNDERSTORM OR TWO CAN BE EXPECTED ACROSS 
NORTHERN ILLINOIS. A SECOND DISTURBANCE IS CURRENTLY LOCATED ACROSS 
THE EASTERN DAKOTAS SOUTHWARD INTO EASTERN NEBRASKA. THIS WAVE IS 
EXPECTED TO SHIFT EASTWARD ACROSS WISCONSIN AND NORTHERN ILLINOIS 
DURING THE AFTERNOON TODAY. THESE TWO DISTURBANCES WILL HELP INDUCE 
A UPPER LEVEL 60+ KT SPEED MAX ACROSS THE LOWER GREAT LAKES 
REGION...ESSENTIALLY SETTING UP PORTIONS OF MY AREA TO LINE UP WITHIN 
THE FAVORABLE RIGHT ENTRANCE REGION OF THE UPPER LEVEL JET MAX THIS 
AFTERNOON AND INTO TONIGHT. THIS SHOULD HELP TO DRIVE AN AREA OF 
SHOWERS AND THUNDERSTORMS ACROSS PORTIONS OF THE AREA ALONG AND 
AHEAD OF AN APPROACHING SURFACE COLD FRONT.  

IT APPEARS THE BEST CHANCES FOR MORE ROBUST CONVECTIVE DEVELOPMENT 
WILL BEGIN BY EARLY TO MID AFTERNOON OVER OR JUST TO THE SOUTH AND 
EAST OF THE CHICAGO AREA. HIRES CONVECTIVE ALLOWING MODELS CONTINUE 
TO INDICATE THE FAVORABLE LOCATIONS FOR AFTERNOON CONVECTIVE 
ACTIVITY WILL PRIMARILY BE ALONG AND SOUTH OF A CHICAGO TO OTTAWA 
LINE. ALTHOUGH MORNING SHOWERS AND CLOUD COVER COULD HAMPER 
INSOLATION SOME ACROSS THE AREA...IT APPEARS THAT WITH DEW POINTS IN 
THE LOWER 70S...MLCAPES SHOULD END UP AROUND 1500 J/KG...ESPECIALLY  
ACROSS THE SOUTHEASTERN HALF OF MY AREA. THE INCREASING MID AND AN 
UPPER LEVEL WIND FIELD ALSO LOOKS TO INCREASE THE DEEP LAYER SHEAR 
UP TO 25 TO 30 KT...WHICH COULD SUPPORT A FEW STRONGER STORMS 
CAPABLE OF STRONG WIND GUSTS. THESE STORMS WILL ALSO LIKELY PRODUCE 
SOME TORRENTIAL DOWNPOURS GIVEN THE HIGH PWAT AIRMASS IN PLACE. 
STORM MOTIONS OF 30 TO 40 KT SHOULD LIMIT THE RESIDENCE TIME OF 
THESE STORMS OVER A GIVEN AREA...MAKING EXTREME RAINFALL AMOUNTS IN 
A GIVEN AREA UNLIKELY.  THESE STORMS LOOK TO LINGER INTO THE 
EVENING HOURS TONIGHT...ESPECIALLY ACROSS MY FAR SOUTHEASTERN 
COUNTIES IN NORTHWESTERN INDIANA AND EAST CENTRAL ILLINOIS. 

SUNDAY STILL APPEARS TO BE THE PICK DAY OF THE WEEKEND...WITH A 
TEMPORARY BREAK FROM THE SHOWER AND THUNDERSTORM ACTIVITY...WHICH 
WILL RETURN BY LABOR DAY. THE WEATHER PATTERN WILL BECOME ACTIVE 
AGAIN EARLY NEXT WEEK AS A SIGNIFICANT UPPER LEVEL DISTURBANCE AND 
ASSOCIATED SPEED MAX...EMANATING FROM THE NORTHEASTERN 
PACIFIC...DIGS ACROSS THE NORTHERN CONUS. THIS LOOKS TO SUPPORT A 
SURFACE LOW...WHICH SHOULD SHIFT NORTHEASTWARD ACROSS THE UPPER 
MIDWEST AND INTO ONTARIO BY MONDAY. CONVECTIVE ACTIVITY...POSSIBLY 
STRONG TO SEVERE WILL BE FAVORED LATE SUNDAY FROM KANSAS 
NORTHEASTWARD ACROSS ACROSS IOWA AND MINNESOTA AHEAD OF AN 
APPROACHING COLD FRONT. THIS COLD FRONT IS EXPECTED TO MOVE INTO 
NORTHERN ILLINOIS DURING THE DAY MONDAY...AND IT APPEARS THE DIURNAL 
TIMING OF THIS FRONT...AND UPPER LEVEL SYSTEM...COULD BE FAVORABLE 
FOR STRONG TO SEVERE CONVECTION ACROSS THE AREA MONDAY AFTERNOON AND 
EVENING...ESPECIALLY CONSIDERING THE DEEP LAYER SHEAR IS EXPECTED TO 
BE MORE THAN ADEQUATE FOR ORGANIZED STRONG CONVECTION.

UNCERTAINTY INCREASES A BIT LATER MONDAY NIGHT INTO TUESDAY...AS THE 
VARIOUS MODEL GUIDANCE ARE IN DISAGREEMENT IN HOW FAST THE SURFACE 
COLD FRONT WILL PUSH ACROSS MY ENTIRE AREA. THE ECMWF IS SUGGESTING 
A SLOWER MOVEMENT OF THE FRONTAL BOUNDARY INTO TUESDAY AS IT 
SUGGESTS THAT ANOTHER MID LEVEL IMPULSE WILL SHIFT OVER THE AREA 
LEADING TO THE DEVELOPMENT OF A SECONDARY SURFACE LOW OVER THE AREA. 
IF THIS SOLUTION WERE TO PAN OUT...IT COULD SET THE STAGE FOR SOME 
VERY HEAVY RAINFALL AMOUNTS ACROSS PORTIONS OF THE AREA INTO TUESDAY 
MORNING.  

DURING THE MID TO LATER PORTION OF THE WEEK IT APPEARS THINGS WILL 
BE ON A WARMING TREND...ESPECIALLY BY THURSDAY AS MID LEVEL HEIGHTS 
BUILD SUBSTANTIALLY ACROSS THE AREA. THIS COULD LEAD TO A RUN AT 
UPPER 80 TO LOWER 90 TEMPERATURES FOR THURSDAY. I CONTINUED WITH 
SLIGHT CHANCE POPS DURING THE LATER PERIOD...MAINLY DUE TO 
UNCERTAINTIES ON WHETHER OR NOT ANY SMALL SCALE DISTURBANCES WILL TRY 
TO SPARK OFF CONVECTION OVER THE AREA. HOWEVER...WITH THE BUILDING 
HEIGHTS AND WARMING TEMPERATURES ALOFT...THE AREA COULD END UP 
CAPPED TO CONVECTIVE DEVELOPMENT ON WEDNESDAY AND THURSDAY.   

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* CHANCE OF SHRA/TSRA MIDDAY SATURDAY

* SWLY WINDS GUSTING TO 20KT SATURDAY AFTERNOON.

KREIN

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

SCT SHRA AND ISOLD TSRA CONTINUE AS THE AREA REMAINS ENTRENCHED IN
A WARM/MOIST AIRMASS. IN THE ABSENCE OF ANY SFC OR MID LEVEL
FOCUS...PCPN SHOULD REMAIN FAIRLY ISOLATED. HAVE BEEN TRACKING A
A FEW PATCHES OF PERSISTENT RAIN MOVING INTO NCNTRL/NERN IL WHICH
SHOULD HOLD TOGETHER LONG ENOUGH TO BRING SOME WETTING RAIN TO
THE CHICAGO AREA TERMINALS. HOWEVER...ANY PCPN SHOULD BE FLEETING.
FOR THE REMAINDER OF THE NIGHT AND INTO SATURDAY MORNING...WILL
CARRY A DRY FORECAST THOUGH THERE SHOULD BE SOME SCT SHRA OR ISOLD
TSRA WITH LITTLE CHANGE IN THE AIRMASS.

BY TOMORROW MORNING...THE SHORT TERM MODEL GUIDANCE IS PUSHING A
WEAK COLD FRONT ACROSS THE AREA IN ASSOCIATION WITH A STRONGER
UPPER LEVEL TROUGH. THE UPPER TROUGH AND SFC FRONT SHOULD PROVIDE
BETTER FOCUS FOR MORE ORGANIZED THUNDERSTORMS ACTIVITY.
HOWEVER...THE TIMING OF THE FRONT WILL PLAY A SIGNIFICANT ROLE IN
THE TS COVERAGE AS IT SHOULD MOVE NRN IL BY LATE MORNING...AT A
TIME BEFORE MAX DIURNAL WARMING. THE HIGH RESOLUTION MODEL
GUIDANCE SUGGESTS THAT TS COVERAGE SHOULD INCREASE AS THE FRONT
MOVES THROUGH ECNTRL IL AND NWRN IN DURG THE EARLY AFTERNOON
HOURS. SO...WILL MAINTAIN THE PROB30 GROUP FOR THE TIME THAT THE
FRONT SHOULD BE INVOF THE TERMINALS...THOUGH THERE IS A BETTER
CHANCE FOR MORE ORGANIZED...WIDESPREAD TS INVOF THE COLD FRONT AS
IT PUSHES SOUTH AND EAST OF THE TERMINALS SATURDAY AFTERNOON.

WINDS SHOULD GENERALLY BE SOUTHERLY AT LESS THAN 10KT THROUGH THE
REMAINDER OF THE NIGHT AND INTO SATURDAY MORNING. AS THE TROUGH
ASSOCIATED WITH THE COLD FRONT APPROACHES THE REGION...WINDS
SHOULD BECOME MORE SWLY WITH GUSTS TO ARND 20KT FOR THE LATE
MORNING INTO EARLY EVENING. WINDS SHOULD DROP OFF AND BECOME
LGT/VRBL OVERNIGHT SATURDAY NIGHT AS HIGH PRESSURE SPREADS ACROSS
THE AREA.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* LOW-MEDIUM CONFIDENCE IN PRECIP TRENDS.

* MEDIUM-HIGH CONFIDENCE IN WINDS.

KREIN

//OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z...

SUNDAY...VFR.

MONDAY...PERIOD OF TSRA LIKELY. 

TUESDAY AND WEDNESDAY...PREDOMINANTLY VFR.

THURSDAY...CHANCE TSRA. 

IZZI

&&

.MARINE...
300 AM CDT

A PERIOD OF ACTIVE WEATHER IS SETTING UP FOR LAKE MICHIGAN FOR
THE NEXT SEVERAL DAYS. A SERIES OF WEAK LOW PRESSURE SYSTEMS WILL
TRACK ACROSS THE UPPER GREAT LAKES REGION INTO CANADA DRAGGING
WEAK COLD FRONTS ACROSS THE LAKE. MODERATE SOUTH TO SOUTHWESTERLY
WINDS WILL PERSIST ACROSS THE LAKE THROUGH MUCH OF SATURDAY UNTIL
THE FIRST OF THE COLD FRONTS SWEEPS ACROSS THE LAKE LATE THIS
AFTERNOON AND THIS EVENING...TURNING WINDS TO NORTHWESTERLY
TONIGHT. A WEAK RIDGE OF HIGH PRESSURE WILL BRIEFLY BUILD IN
BEHIND THE FRONT...ALLOWING WINDS TO DROP OFF AND BECOME MORE
VARIABLE IN DIRECTION EARLY SUNDAY. BY SUNDAY AFTERNOON...ANOTHER
DEVELOPING LOW OVER THE NORTHERN PLAINS WILL LEAD TO STRENGTHENING
SOUTHERLY WINDS FOR MONDAY AND MONDAY EVENING...UP TO 25 KT...BUT
THE NEXT FRONT WILL CROSS THE LAKE MONDAY NIGHT. THIS FRONTAL
PASSAGE WILL NOT BE AS ABRUPT AS THE FIRST WITH WINDS ONLY VEERING
SOUTHWESTERLY. WINDS SHOULD INCREASE AGAIN TO UP TO 25KT
IMMEDIATELY FOLLOWING THE FRONTAL PASSAGE MONDAY NIGHT...BUT SHOULD
THEN WEAKEN SLIGHTLY INTO MIDWEEK WHILE REMAINING GENERALLY
SOUTHWESTERLY. 

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO



National Weather Service Glossary of Abbreviations