Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250729
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
229 AM CDT FRI JUL 25 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 06Z...

* CHANCES FOR MAINLY LIGHT SHRA MID-LATE AFTERNOON.

* CHANCES FOR TSRA AFTER 00Z.

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

HIGH PRESSURE ACROSS THE OHIO RIVER VALLEY WILL CONTINUE TO DEPART
TO THE EAST WHILE A TROUGH OF LOW PRESSURE INCHES EASTWARD FROM
THE DAKOTAS TODAY. SOUTHERLY FLOW WILL INCREASE TODAY AND WITH
THAT WILL COME MORE MOISTURE. REMNANT SHOWERS AND POSSIBLY A FEW
STORMS ARE LIKELY TO MAKE IT INTO NORTHWEST IL LATE THIS MORNING
INTO EARLY AFTERNOON. THE FOCUS FOR THESE SHOULD WANE INTO THE
AFTERNOON...HOWEVER OVERALL INSTABILITY ALOFT WILL BE ON THE
INCREASE...SO IT IS DIFFICULT TO SAY HOW SHOWERS WILL BEHAVE
FURTHER EASTWARD TOWARD CHICAGO. THUNDER POTENTIAL LOOKS TO BE
GREATER DURING THE EVENING INTO EARLY OVERNIGHT AS ANOTHER UPPER
DISTURBANCE MOVES OVER THE AREA. HOWEVER...WITH THE ATMOSPHERE
BEING MOIST AND UNSTABLE CANNOT RULE OUT A TSRA AT ANYTIME
TONIGHT.

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

* LOW IN PRECIPITATION COVERAGE AND TIMING THIS AFTERNOON AND
  TONIGHT. ALTHOUGH HIGH THAT ANY PRECIP PRIOR TO 00Z WOULD BE
  MAINLY LIGHT AND VFR.

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

SATURDAY-SUNDAY...CHANCE OF TSRA.
MONDAY-TUESDAY...VFR.
WEDNESDAY...SLIGHT CHANCE OF TSRA.

MTF

&&

.MARINE...
228 AM CDT

SOUTHERLY FLOW WILL INCREASE OVER THE LAKE TODAY AS HIGH PRESSURE
SCOOTS EASTWARD FROM THE OHIO RIVER VALLEY. THE SURFACE TROUGH
LEADING TO THE INCREASE IN WINDS WILL ARRIVE EARLY
SATURDAY...HOWEVER WEAKEN TO THE POINT WHERE IT WILL NOT SHIFT THE
WINDS MUCH IF ANY FROM THE SOUTH-SOUTHWEST. THE LOW PRESSURE
DRIVING THIS TROUGH WILL EVOLVE OVER THE LAKE ON SUNDAY TURNING
WINDS NORTHERLY AND DRIVING A SHARP INCREASE IN SPEEDS SUNDAY
NIGHT AND PERSISTING INTO MONDAY. FOR JULY STANDARDS THESE WIND
SPEEDS ONCE AGAIN WILL BE QUITE STRONG WITH EVEN POSSIBLY SOME
GALE FORCE NORTHERLY GUSTS LATE SUNDAY NIGHT INTO MONDAY MORNING.
WITH ONCE AGAIN A LONG FETCH WAVES SHOULD REACH HEIGHTS OF OVER 6
FT OVER MUCH OF THE LAKE WITH EVEN POSSIBLY UP TO 10 FT AGAIN
ACROSS THE FAR SOUTH...PRIMARILY NEAR THE INDIANA SHORE.

MTF

&&

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

$$

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