Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 291403
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
903 AM CDT FRI AUG 29 2014

.DISCUSSION...
326 AM CDT

THE MAIN FORECAST CONCERNS WILL CENTER AROUND CONVECTIVE CHANCES AND
TIMING...AS WELL AS TEMPERATURES TODAY...AND THROUGH THE HOLIDAY WEEKEND.

CURRENTLY...THE REMNANT MCV FROM YESTERDAY EVENINGS SHOWERS AND
STORMS CONTINUES TO SHIFT ACROSS MY SOUTHERN COUNTIES EARLY THIS
MORNING. HOWEVER...ONLY A FEW LIGHT SHOWERS/SPRINKLES REMAIN ACROSS
THIS AREA...WITH DRIER AND FAIRLY STABLE CONDITIONS IN PLACE TO THE EAST.

ANOTHER AREA OF CONVECTION IS ONGOING TO OUR WEST ACROSS IOWA AND
INTO SOUTHERN MN. THIS ACTIVITY IS OCCURRING WITHIN A REGION OF GOOD
LOWER LEVEL WARM AIR ADVECTION/ISENTROPIC ASCENT ON THE DOWNSTREAM
PERIPHERY OF A REMNANT MID/UPPER LEVEL TROUGH OVER THE PLAINS STATES.
DOWN STREAM UPPER RIDGING OVER THE GREAT LAKES REGION IS LEADING TO A
STEERING FLOW TO THE NORTHEAST. THEREFORE...MOST IF NOT ALL OF THIS
CONVECTIVE ACTIVITY WILL SHIFT INTO SOUTHERN WISCONSIN AND PERHAPS
FAR NORTHWESTERN ILLINOIS THIS MORNING. IN SPITE OF THIS...I HAVE
LEFT SLIGHT CHANCE POPS IN THIS MORNING AS IT IS POSSIBLE THAT SOME
ADDITIONAL CELLS COULD DEVELOP IN ASSOCIATION WITH AN EASTWARD MOVING
OUTFLOW BOUNDARY...AS EVIDENT OFF OF THE DMX RADAR MOVING TOWARDS
EASTERN IOWA. THIS ACTIVITY...IF IT OCCURS IN ANY OF NORTHERN
ILLINOIS EARLY THIS MORNING...WOULD LIKELY BE ISOLATED TO WIDELY
SCATTERED...WITH THE MAIN FOCUS REMAINING NORTH.

A SURFACE WARM FRONT IS EXPECTED TO LIFT NORTHWARD OVER THE AREA
INTO THIS AFTERNOON...WITH A WARMER AIRMASS EXPECTED IN ITS WAKE. IN
SPITE OF THIS...HOWEVER...HIGH TEMPERATURES ACROSS THE AREA WILL BE
TRICKY AS IT APPEARS THAT SOME SCATTERED THUNDERSTORMS COULD DEVELOP
ACROSS PORTIONS OF THE AREA THIS AFTERNOON...AND HAMPER THE MAGNITUDE
OF THE WARMTH ACROSS SOME AREAS. MODEST HEIGHT FALLS ALOFT ARE FORECAST
ACROSS NORTHERN ILLINOIS IN ASSOCIATION WITH THE AFOREMENTIONED UPPER
TROUGH SHIFTING EASTWARD OVER THE MID MISSOURI VALLEY THIS
AFTERNOON. IN SPITE OF THE FACT THAT THE BEST LARGE SCALE FORCING
FOR ASCENT WITH THIS DISTURBANCE SHOULD REMAIN WEST OF THE AREA THIS
AFTERNOON...THE EXPECTED INCREASE IN LOW-LEVEL THETA E IS EXPECTED
TO PRODUCE AN UNCAPPED ATMOSPHERE FEATURING MLCAPES OF AROUND 1500
J/KG BY THIS AFTERNOON. A MAJORITY OF THE GLOBAL MODELS...AS WELL AS
THE HIRES CONVECTIVE ALLOWING MODELS FIRE UP ACTIVITY WITHIN THIS
REGIME THIS AFTERNOON. IT APPEARS WE WILL HAVE TO WATCH MORNING
CLOUD COVER...HOWEVER...AS THIS COULD INHIBIT THE DEGREE OF
WARMING A BIT THROUGH THE MORNING...AND POSSIBLY LEAD TO WEAKER
INSTABILITY THAN WHAT THE MODELS ARE CURRENTLY ADVERTISING.

IF THE CONVECTION DOES DEVELOP AS  EXPECTED...MARGINAL DEEP LAYER
SHEAR OF 25 TO 30 KT COULD ALLOW FOR A FEW STRONGER STORMS CAPABLE
OF SOME ISOLATED HIGH WIND GUSTS. SOME TORRENTIAL DOWNPOURS WOULD
ALSO BE A THREAT FROM THESE STORMS GIVEN THE JUICY AIRMASS FEATURING
PWATS UP AROUND 1.75 INCHES. HOWEVER...EXTREME RAINFALL AMOUNTS
APPEAR UNLIKELY GIVEN STORM MOTIONS SHOULD BE TO THE NORTHEAST
AROUND 30 KT. SO THE STORMS SHOULD NOT BE OVER A GIVEN AREA TOO
LONG. MUCH OF THIS ACTIVITY APPEARS TO BE HIGHLY DIURNAL IN
NATURE...SO A RAPID DOWNWARD TREND IS EXPECTED AFTER SUNSET THIS
EVENING.

ANOTHER ROUND OF THUNDERSTORMS APPEARS LIKELY ON
SATURDAY...ESPECIALLY DURING THE MORNING AS THE UPPER LEVEL
TROUGH...AND AN ASSOCIATED SURFACE LOW AND COLD FRONT SHIFTS
EASTWARD ACROSS ACROSS SOUTHERN WI AND NORTHERN IL. THIS ACTIVITY
COULD LINGER INTO SATURDAY EVENING ACROSS MY FAR SOUTHEASTERN
COUNTIES. HOWEVER...IT APPEARS THAT FOLLOWING THIS FRONTAL PASSAGE
WE WILL BE GREETED WITH A NICE DRY SUNDAY ACROSS THE AREA.

UNFORTUNATELY...LABOR DAY APPEARS THAT IT COULD BE ANOTHER ACTIVE
WEATHER DAY ACROSS THE REGION. ANOTHER POTENT UPPER LEVEL
DISTURBANCE AND SURFACE FRONT IS FORECAST TO SHIFT OVER THE AREA BY
MONDAY. WE WILL ALSO HAVE TO WATCH FOR THE THREAT OF SEVERE
THUNDERSTORMS ON MONDAY AND MONDAY NIGHT AS A 45 TO 50 KT MID LEVEL
JET IS PROGGED TO DEVELOP ACROSS THE AREA. THIS COULD RESULT IN DEEP
LAYER SHEAR STRONG ENOUGH TO PRODUCE MORE ORGANIZED STRONG TO SEVERE
CONVECTION. VERY HEAVY RAINFALL WOULD AGAIN BE A CONCERN WITH THIS
ACTIVITY...ESPECIALLY IF THE FRONTAL ZONE STALLS OUT OVER THE AREA
MONDAY NIGHT. AT THIS TIME...HOWEVER...UNCERTAINTY IS HIGH ON WHERE
THE SURFACE FRONT MAY STALL AND HENCE WHAT AREAS COULD BE FAVORED FOR
EXTENDED PERIODS OF VERY HEAVY RAINFALL.

THE ACTIVE WEATHER PATTERN COULD SHIFT SOUTH OF THE AREA FOR A DAY
OR TWO INTO MID TO LATE WEEK BEFORE MORE PERIODS OF CONVECTION ARE
POSSIBLE. OVERALL...IT APPEARS TEMPERATURES WILL REMAIN ON THE WARM
SIDE OF AVERAGE...WITH THE POTENTIAL FOR SOME VERY WARM AND HUMID DAYS
LATER NEXT WEEK.

KJB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 14Z...

* ISOLATED-SCATTERED TSRA THIS AFTERNOON/EARLY EVENING.

* SOUTH-SOUTHEAST WIND AROUND 10 KT BECOMING SOUTH WITH GUSTS NEAR
  20 KT DEVELOPING BY MIDDAY.

* GREATER CHANCE FOR MORE ORGANIZED TSRA TOMORROW.

RATZER

//DISCUSSION...UPDATED 12Z...

FORECAST CONCERNS CONTINUE TO FOCUS ON SHRA/TSRA POTENTIAL
THROUGH THE PERIOD AS WELL AS GUSTY SOUTH WINDS DEVELOPING LATE
FRIDAY MORNING AND PERSISTING INTO THIS EVENING.

A WARM FRONT WILL SLOWLY LIFT NWD THROUGH THE DAY AND PUSH NORTH
OF THE TERMINALS BY LATE MORNING. WHILE THE SFC WARM FRONT IS
STILL TO THE SOUTH OF THE TERMINALS...SCT SHRA HAVE DEVELOPED IN A
ZONE OF WARM ADVECTION ALONG 25-30KT SWLY WINDS ALOFT. THE BULK OF
THE ACTIVITY IS CURRENTLY LOCATED OVER WRN IL...INVOF KRFD. SFC
WINDS ARE STILL SLIGHTLY EAST OF SOUTH...BUT AS THE WARM FRONT
PUSHES NORTH OF THE TERMINALS...WINDS WILL VEER TO SOUTHERLY WITH
GUSTS TO ARND 20KT OR SO.

INSTABILITY WILL INCREASE VERY QUICKLY AS THE WARM FRONT PASSES
TO THE NORTH AND WARM SECTOR AIR OVERSPREADS THE REGION BRINGING
INCREASING TEMPERATURES AND DEWPOINTS. WEAK MID LEVEL IMPULSES
SHOULD TRACK ACROSS THE AREA THIS AFTERNOON AND EVENING IN UPPER
LEVEL SWLY FLOW AS AN UPPER RIDGE PUSHES TO THE EAST AND A LARGER
SCALE UPPER TROUGH MOVES ACROSS THE CENTRAL PLAINS. THESE IMPULSES
WILL BE THE TRIGGERING MECHANISM FOR ADDITIONAL SHRA/TSRA...BUT IN
THE ABSENCE OF ANY SFC FORCING...ACTIVITY SHOULD GENERALLY BE
SCATTERED AND DIFFICULT TO TIME AT THE TERMINALS. HAVE PUSHED BACK
THE TIMING OF THE PCPN MENTION AT THE CHICAGO AREA TERMINALS WHEN
INSTABILITY SHOULD BE THE STRONGEST...BUT THIS IS A LOW CONFIDENCE
CHANGE AS IN SUCH A WARM/MOIST AIRMASS SCT SHRA/ISOLD TSRA COULD
DEVELOP AT ANY TIME.

A BETTER CHANCE FOR MORE ORGANIZED TS WILL COME LATE IN THE
PERIOD...POSSIBLY NOT UNTIL TOMORROW MORNING AS THE MAIN LONGWAVE
UPPER TROUGH AND A WEAK SFC COLD FRONT CROSS THE REGION.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 14Z...

* MEDIUM-LOW CONFIDENCE IN THUNDER CHANCES/COVERAGE AND SPECIFIC TIMING
  THROUGH THIS EVENING.

* MEDIUM CONFIDENCE IN MORE ORGANIZED TS TOMORROW MORNING.

* HIGH CONFIDENCE IN WIND TRENDS THROUGH THE PERIOD.

KREIN/RATZER

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

* SATURDAY NIGHT THROUGH SUNDAY NIGHT...VFR. WX NIL.

* MONDAY...CHANCE OF TSRA.

* TUESDAY...VFR. WX NIL.

* WEDNESDAY THROUGH THURSDAY NIGHT...SLIGHT CHANCE OF TSRA

KREIN

&&

.MARINE...
315 AM CDT

HIGH PRESSURE OVER THE EASTERN GREAT LAKES WILL MOVE EAST OFF THE
NEW ENGLAND COAST FRIDAY EVENING...WHILE A TROUGH OF LOW PRESSURE
DEVELOPS ACROSS THE CENTRAL PLAINS AND UPPER MIDWEST. THIS WILL
SET UP A PERIOD OF SOUTHERLY WINDS AND AS THE TROUGH MOVES
EAST...THE GRADIENT OVER THE LAKE WILL STRENGTHEN AND WINDS SHOULD
INCREASE TO THE 20-25 KT RANGE. A DEEPENING LOW CENTER WITHIN THE
TROUGH WILL MOVE TO WISCONSIN SATURDAY AND ACROSS NORTHERN LAKE
MICHIGAN SATURDAY EVENING. AS THE LOW TRACKS TO THE NORTHEAST...AN
ASSOCIATED COLD FRONT WILL MOVE DOWN THE LAKE SATURDAY AFTERNOON
AND EVENING...QUICKLY SHIFTING WINDS TO NWLY FOLLOWING THE FROPA.
THE LATEST MODEL GUIDANCE SUGGESTS THAT THE FRONT WILL WASH OUT
LATE SATURDAY NIGHT BEFORE REACHING THE SOUTH END OF THE LAKE AND
WINDS THERE SHOULD NEVER TRULY SHIFT NWLY...BUT RATHER BECOMING
LIGHT AND VARIABLE LATE SATURDAY NIGHT THROUGH SUNDAY MORNING. A
DEEPER LOW IS EXPECTED TO DEVELOP OVER THE DAKOTAS AND LIFT
THROUGH WESTERN ONTARIO SUNDAY NIGHT AND MONDAY BRINGING A RETURN
TO BRISK SOUTH TO SOUTHWEST WINDS TO THE LAKE FOR MONDAY AND
TUESDAY.

KREIN

&&

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

$$

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