Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 250317
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1017 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 03Z...

* SOUTH SOUTHEAST WINDS TONIGHT...THEN SOUTH SOUTHWEST EARLY
  FRIDAY.

* LOW CHANCE FOR SHRA FRIDAY MORNING.

* POSSIBILITY FOR TSRA FRIDAY AFTERNOON...WITH CHANCES
  INCREASING FOR SHRA/TSRA ONCE AGAIN FRIDAY NIGHT.

RODRIGUEZ

//DISCUSSION...UPDATED 00Z...

ACTIVITY ACROSS THE TERMINALS POSSIBLY INCREASING ON FRIDAY AS
CURRENT HIGH PRESSURE OVERHEAD SHIFTS EAST OF THE AREA...AND A
TROUGH OF LOW PRESSURE APPROACHES FROM THE WEST. IN THE NEAR TERM
AND INTO TONIGHT...ANY ACTIVITY WILL REMAIN WELL WEST OF THE
TERMINALS WITH CURRENT PRECIP OVER IOWA DROPPING TO THE SOUTH
WHILE DISSIPATING...AND WITH VFR CONDITIONS EXPECTED TONIGHT.
INCREASING MID/UPPER LEVEL SUPPORT FOR PRECIP WILL BEGIN TO
INCREASE LATE TONIGHT INTO EARLY FRIDAY MORNING AS A MID LEVEL
DISTURBANCE APPROACHES BUT WITH THE BULK OF ANY PRECIP STILL TO
REMAIN TO THE WEST. GUIDANCE IS HINTING AT THE POSSIBILITY FOR A
STRAY SHOWER OR TWO TO DEVELOP OUT AHEAD OF THE MAIN PRECIP EARLY
FRIDAY MORNING...BUT WITH THESE CHANCES REMAINING LOW AND WITHOUT
ANY THUNDER EXPECTED DID LEAVE OUT OF THE TAFS AT THIS TIME.
BETTER FORCING WITH PRECIP AND VFR CEILINGS SHOULD PUSH EAST BY
MID DAY INTO THE AFTERNOON TIME FRAME...BUT WITH SOME GUIDANCE
INDICATING THIS PRECIP COULD REMAIN WEST OF THE TERMINALS.
ALTHOUGH WITH THE LIKELIHOOD FOR ANY SHRA/TSRA WEST OF THE
TERMINALS TO DRIFT EAST TOWARDS THE TERMINALS INCREASING DURING
THIS TIME...HAVE MAINTAINED CURRENT THUNDER MENTION BUT DID
SHORTEN UP THE DURATION. CHANCES FOR SHRA/TSRA WILL DIMINISH
TOWARDS EARLY EVENING...BUT INCREASE ONCE AGAIN BY LATE EVENING AS
BEST FORCING TO MOVE OVERHEAD.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH CONFIDENCE WITH WIND SPEED AND DIRECTION.

* LOW CONFIDENCE WITH SHRA CHANCES FRIDAY MORNING.

* LOW MEDIUM CONFIDENCE WITH TSRA ACROSS THE TERMINALS FRIDAY
  AFTERNOON AND FRIDAY NIGHT...LOW MEDIUM CONFIDENCE WITH
  TIMING/DURATION.

RODRIGUEZ

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

SATURDAY...CHANCE OF TSRA.

SUNDAY...CHANCE OF TSRA.

MONDAY...VFR.

TUESDAY...VFR.

WEDNESDAY...SLIGHT CHANCE OF TSRA.

RODRIGUEZ

&&

.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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