Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 242208
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
508 PM CDT THU JUL 24 2014

.DISCUSSION...
323 PM CDT

THE MAIN WEATHER CONCERNS DURING THE PERIOD WILL FOCUS ON THE SHOWER
AND THUNDERSTORM CHANGES LATE FRIDAY THROUGH SATURDAY NIGHT...ALONG
WITH THE THREAT FOR SEVERE STORMS AND EXTREMELY HEAVY RAINFALL ACROSS
PORTIONS OF THE AREA SATURDAY NIGHT.

SURFACE HIGH PRESSURE IS CURRENTLY MOVING OVERHEAD THIS
AFTERNOON...AND IT IS RESULTING IN ANOTHER GREAT WEATHER DAY ACROSS
THE REGION. THIS HIGH WILL CONTINUE TO SHIFT EASTWARD ACROSS THE
OHIO VALLEY AND THE EASTERN SEABOARD DURING THE DAY FRIDAY. AS THIS
OCCURS...THE LOWER LEVEL FLOW WILL VEER SOUTHERLY...ALLOWING
MOISTURE TO BEGIN RETURNING TO THE AREA INTO SATURDAY.
MEANWHILE...IN THE MID AND UPPER LEVELS...A SIGNIFICANT DISTURBANCE
AND ASSOCIATED SPEED MAX...CURRENTLY NOTED IN WATER VAPOR IMAGERY
ACROSS THE PACIFIC NORTHWEST...IS EXPECTED TO SHIFT EASTWARD ALONG
THE INTERNATIONAL BOARDER INTO SATURDAY ATOP A STOUT RIDGE ACROSS
THE SOUTHWESTERN CONUS. THIS SHOULD SET UP AN ENHANCED BELT OF 85+
KT WESTERLIES IN THE UPPER LEVELS FROM THE NORTHERN HIGH
PLAINS...EASTWARD ACROSS THE UPPER MIDWEST AND INTO THE GREAT LAKES
REGION DURING MUCH OF THE WEEKEND. ULTIMATELY...THIS PATTERN WILL
SET UP ANOTHER SHORT PERIOD OF VERY WARM AND HUMID WEATHER...ALONG
WITH A COUPLE PERIODS FOR CONVECTION THROUGH MUCH OF THE WEEKEND.

AN IMPRESSIVE THERMAL RIDGE...ASSOCIATED WITH THE STOUT 994+ DAM
UPPER RIDGE OVER THE SOUTHWEST...IS CURRENTLY IN PLACE ACROSS THE
PLAINS AND INTER-MOUNTAIN REGION. IN FACT...MORNING RAOBS INDICATE
IN EXCESS OF 24 DEGREE CELSIUS TEMPERATURES AT 850 MB FROM EASTERN
MONTANA AND ACROSS THE PLAINS STATES. THIS THERMAL RIDGE WILL BE
ADVECTED EASTERLY TONIGHT THROUGH SATURDAY IN RESPONSE TO THE
DEVELOPMENT OF A 30+ KT SOUTHWESTERLY LOW LEVEL JET OUT AHEAD OF A
EAST-SOUTHEASTWARD MOVING LOWER LEVEL AREA OF LOW
PRESSURE...CURRENTLY OVER THE NORTHERN HIGH PLAINS. THIS WILL SET UP
ANOTHER WARMING TREND FRIDAY INTO SATURDAY...WITH SATURDAY LIKELY TO
BE A ANOTHER VERY WARM AND HUMID DAY ACROSS THE AREA.

ALSO...WITH THE INCREASING HEAT AND HUMIDITY...SO TO WILL OUR
CHANCES FOR THUNDERSTORMS. SHOWER AND THUNDERSTORM ACTIVITY IS
CURRENTLY ONGOING ACROSS THE EASTERN DAKOTAS...SOUTHWESTERN
MINNESOTA AND INTO NORTHWESTERN SECTIONS OF IOWA ALONG THE
NORTHEASTERN PERIPHERY OF THE SOUTHWESTERN UPPER RIDGE. THIS ACTIVITY
SHOULD CONTINUE TO DEVELOP ACROSS IOWA TONIGHT AS STRONG THETA E
ADVECTION SETS UP ACROSS THE CORN BELT ALONG THE NOSE OF A 40 KT
SOUTHWESTERLY JET. IT APPEARS THAT THE REMNANTS OF THIS ACTIVITY
WILL MOVE ACROSS NORTHERN ILLINOIS AND NORTHWESTERN INDIANA DURING
THE DAY FRIDAY AS THE BETTER MOISTURE SHIFTS EASTWARD. SHOWERS AND A
FEW ELEVATED THUNDERSTORMS WILL BE POSSIBLE AS A RESULT OF THIS ON
FRIDAY.

ADDITIONAL ELEVATED SHOWERS AND STORMS ARE EXPECTED DEVELOP ACROSS
THE AREA FRIDAY NIGHT AS THE LOW LEVEL JET NOSES UP ACROSS THE AREA.
MID LEVEL LAPSE RATES ARE EXPECTED TO STEEPEN ACROSS THE AREA INTO
FRIDAY NIGHT AS THE WARMER AIR GETS ADVECTED OVER THE AREA FROM THE
HIGH PLAINS. AS A RESULT OF THIS...IT APPEARS THERE COULD BE A SMALL
THREAT FOR HAIL WITH SOME OF THESE STORMS. HOWEVER...IT ALSO APPEARS
THAT THERE COULD BE A HEAVY RAIN THREAT WITH THIS ACTIVITY AS WELL.
THE SOUTHWESTERLY JET IS EXPECTED TO PUMP IN DEEPER
MOISTURE...RESULTING IN UPWARDS OF 1.75 PWATS ACROSS THE AREA FRIDAY
NIGHT. HOWEVER...ALTHOUGH THESE STORMS COULD PRODUCE TORRENTIAL
DOWNPOURS FOR A PERIOD...IT APPEARS THAT A QUICK STORM MOTION WILL
KEEP THE THREAT FOR SIGNIFICANT FLASH FLOODING LOW.

DURING THE DAY SUNDAY IT APPEARS THAT AN EML WILL SET UP ACROSS THE
AREA. THEREFORE...IN SPITE OF THE FACT THAT VERY HIGH THETA E AIR
WILL BECOME JUXTAPOSED BENEATH A CORRIDOR OF STEEP MID LEVEL LAPSE
RATES...RESULTING IN A MODERATELY TO STRONGLY UNSTABLE
ENVIRONMENT...IT APPEARS THE AREA MAY REMAIN CAPPED FOR MUCH OF THE
DAY LIMITING THE THREAT OF CONVECTIVE ACTIVITY. CONDITIONS ACROSS THE
AREA WILL LIKELY BECOME UNCOMFORTABLE AGAIN AS TEMPERATURES TOP OUT
IN THE MID TO UPPER 80S...WITH SOME AREAS AROUND 90 CERTAINLY
POSSIBLE. THESE TEMPERATURES COMBINED WITH DEW POINTS IN THE LOW TO
MID 70S WILL RESULT IN HEAT INDICIES FROM 95 TO NEAR 100.

CONVECTION LOOKS TO DEVELOP BY SATURDAY EVENING ACROSS PORTIONS OF
THE AREA AS BETTER MOISTURE CONVERGENCE SETS UP ALONG THE LOW LEVEL
JET AND MODEST HEIGHT FALLS ALOFT HELP WEAKEN THE CAP. CURRENTLY
THERE APPEARS TO BE SOME UNCERTAINTIES ON THE EXACT AREAS THAT WILL
BE UNDER THE THREAT FOR THESE STORMS SATURDAY NIGHT...WITH THE
NUMERICAL GUIDANCE SLOWING A BIT DISAGREEMENT. BUT WITH THE A
SURFACE BOUNDARY EXPECTED TO BE IN PLACE ACROSS CENTRAL ILLINOIS BY
SATURDAY NIGHT...IT APPEARS THE MAIN THREAT FOR THIS ACTIVITY WILL
BE MAINLY ACROSS MY SOUTHERN CWA. THEREFORE...I HAVE FOCUSED THE
HIGHEST POPS SOUTH OF INTERSTATE 80. THIS THUNDERSTORM ACTIVITY
COULD ALSO BE A SEVERE THREAT WITH A POSSIBLE FORWARD PROPAGATING MCS
GIVEN THE MAGNITUDE OF THE DEEP LAYER SHEAR AND THE INSTABILITY.
VERY HEAVY RAINFALL WILL AGAIN BE A THREAT WITH THESE
STORMS.

IT APPEARS CONDITIONS WILL REMAIN IN THE 80S AGAIN ON SUNDAY...WITH
A COLD FRONT EXPECTED TO MOVE ACROSS THE REGION DURING THE AFTERNOON
AND EARLY EVENING. ADDITIONAL SHOWERS AND STORMS COULD DEVELOP ACROSS
PORTIONS OF THE AREA IN THE AFTERNOON AS THE MAIN MID LEVEL IMPULSE
DROPS ACROSS SOUTHERN WISCONSIN.

IN THE EXTENDED PERIOD MONDAY THROUGH THURSDAY A UPPER LEVEL TROUGH
IS EXPECTED TO DIG ACROSS THE EASTERN HALF OF THE COUNTRY WITH THE
RIDGING...AND HENCE THE HEAT...RETROGRESSING BACK ACROSS THE WESTERN
CONUS. COOL AND PLEASANT CONDITIONS ARE EXPECTED THROUGH NEXT WEEK.
TEMPERATURES ARE EXPECTED TO ONLY BE IN THE 70S AGAIN...WITH ONLY A
SLOW WARMING TREND EXPECTED BY LATE WEEK.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* SLY-SWLY WINDS TO ARND 10KT TOMORROW.

* CHANCE TSRA TOMORROW AFTERNOON.

KREIN/RODRIGUEZ

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

THE CENTER OF A LARGE AREA OF HIGH PRESSURE HAS MOVED OVER NRN IL
AND THE HIGH EXTENDS FROM LAKE MICHIGAN THROUGH THE MIDDLE
MISSISSIPPI VALLEY. WITH THE EXCEPTION OF SOME DIURNAL CU
DEVELOPMENT...SKIES REMAIN MOSTLY CLEAR WITH WX NIL. THE MAIN
CONCERN FOR THE REMAINDER OF THE AFTERNOON WILL BE WIND
DIRECTION/SPEED. WITH THE CENTER OF THE HIGH TO THE NORTHWEST OF
THE TERMINALS...A WEAK NELY GRADIENT HAS SET UP AND SOME LAKE
ENHANCEMENT/EFFECT IS ALLOWING WINDS TO REACH 10-12KT WITH AN
OCCASIONAL HIGHER GUST. AS THE CENTER OF THE HIGH MOVES ACROSS THE
TERMINALS TONIGHT...WINDS WILL BECOME LGT/VRBL...BEFORE BECMG SLY-
SWLY TOMORROW AS THE CENTER OF THE HIGH SETTLES TO THE SOUTHEAST
OF THE TERMINALS. THE GRADIENT WILL REMAIN WEAK...WITH SPEEDS ARND
10KT LIKELY AGAIN TOMORROW. THE OTHER CONCERN FOR TOMORROW WILL BE
THE CHANCE FOR THUNDERSTORMS FROM THE LATE MORNING THROUGH THE
AFTERNOON HOURS. THE MODELS CONTINUE TO SUGGEST THAT A CLUSTER OF
THUNDERSTORMS MAY DEVELOP FORM OVER THE UPPER MIDWEST LATE TONIGHT
OR EARLY TOMORROW MORNING AND ON THE NOSE OF THE LOW LEVEL
JET AND THEN MOVE SOUTHEAST ALONG THE INSTABILITY GRADIENT. THE
INSTABILITY DOES NOT QUITE REACH AS FAR EAST AS THE CHICAGO METRO
TERMINALS WITH RFD THE MORE LIKELY SITE IN THE PATH OF THE TS.
HOWEVER...WILL GO WITH A PROB30 FOR THE END OF THE TAF PERIOD AT
THIS TIME IN DEFERENCE TO THE CHANCES THAT THE TRACK OF THE TS
CLUSTER COULD BE A LITTLE FARTHER EAST THAN CURRENTLY ANTICIPATED.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* LOW CONFIDENCE IN CHANCES FOR TS TO DIRECTLY IMPACT THE
  TERMINALS TOMORROW.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

KREIN/RODRIGUEZ

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

FRIDAY NIGHT THROUGH SUNDAY...PERIODIC CHANCES FOR TSRA.
MONDAY THROUGH WEDNESDAY...VFR.

BMD

&&

.MARINE...
235 PM CDT

A BROAD AREA OF HIGH PRESSURE IS CENTERED ACROSS THE UPPER MIDWEST
AND WESTERN GREAT LAKES REGION THIS AFTERNOON. THE PRESSURE GRADIENT
OVER THE SOUTHERN TIP OF THE LAKE IS WEAKENING AS THE HIGH MOVES
OVERHEAD BUT ENOUGH OF A GRADIENT TO KEEP WINDS AND WAVES A BIT
ELEVATED THROUGH SUNSET.  A LAKE BREEZE HAS DEVELOPED WITH
ONSHORE FLOW EXPECTED THROUGH THE EVENING. AS THE HIGH DRIFTS TO
SOUTHEAST TONIGHT...A SERIES OF LOW PRESSURE SYSTEMS WILL BEGIN TO
EMERGE FROM THE NORTHERN ROCKIES AND CANADIAN PRAIRIES THROUGH THE
NEXT FEW DAYS. SOUTH TO SOUTHWEST FLOW WILL DEVELOP AHEAD OF THESE
SYSTEMS THAT WILL EASE THE NORTH SWELL CURRENTLY OVER THE LAKE

THE FIRST LOW MOVING OUT OF THE NORTHERN ROCKIES IS PROGGED TO
WEAKEN AND DISSIPATE AS IT MOVES THROUGH ON SATURDAY. THE NORTHERN
LOW IS SLIGHTLY DEEPER AND WILL MOVE ACROSS THE GREAT LAKES SUNDAY
DRAGGING A COLD FRONT ACROSS LAKE MICHIGAN. BEHIND THE FRONT STRONG
NORTH/NORTHWEST WINDS APPROACHING 30 KT ARE EXPECTED ONCE AGAIN
SUNDAY NIGHT INTO MONDAY. THIS WILL BRING ANOTHER PERIOD OF LARGER
WAVES...ESPECIALLY OVER THE SOUTHERN HALF OF THE LAKE.

KMD

&&

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

$$

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