Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220818
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
318 AM CDT TUE JUL 22 2014

.DISCUSSION...
233 AM CDT

TODAY THROUGH TONIGHT...
THE FOCUS FOR TODAY WILL BE ON THE DEVELOPING CAP IN THE
ATMOSPHERE...AND WHEN IT MAY BEGIN TO WEAKEN AND ALLOW CONVECTION TO
DEVELOP LATER THIS AFTERNOON INTO THIS EVENING.

WARM/HUMID START TO THE DAY WITH TEMPS EARLY THIS MORNING STRUGGLING
TO DIP BELOW THE UPR 60S...WITH A HANDFUL OF POINTS HOLDING ARND 70.
DEW PTS HAVE HELD IN THE MID/UPR 60S...WITH THE 70 DEG DEW PTS
BEGINNING TO KNOCK ON THE DOOR STEPS OF THE WESTERN CWFA EARLY THIS
MORNING. PRESSURE GRADIENT CONTINUES TO BE HELD JUST WEST OF THE
FORECAST AREA...AS THE SFC RIDGE NOW CENTERED OVER THE MID-ATLANTIC
REGION WAS STILL PROVIDING SOME INFLUENCE EARLY THIS MORNING. THE
GRADIENT OVER THE CENTRAL PLAINS WILL SHIFT EAST AND ARRIVE OVERHEAD
LATER TODAY...WITH INCREASED MIXING AND A STEADY SOUTHERLY FLOW OF
WARM/MOIST AIR ADVECTING ACROSS NORTHERN IL/NORTHWEST IN.

IR/WATER VAPOR IMAGERY EARLY THIS MORNING DEPICTS THE CHANNEL OF
ATMOS MOISTURE STRETCHING FROM OKLAHOMA NORTH THROUGH THE UPPER
MIDWEST...AS A COMPLEX OF THUNDERSTORMS WAS SLIDING EAST TOWARDS
LAKE SUPERIOR/UPPER PENINSULA THIS MORNING. FOR OUR AREA TODAY
ABUNDANT SUNSHINE IS EXPECTED...WITH JUST SOME HIGH BASED CLOUDS.
STRONG THERMAL RIDGE ARRIVES WITH 850MB TEMPS NEARING 20-22 DEG C.
SFC TEMPS HAVE BEEN CONSISTENTLY PROGGED TO APPROACH 89-92 DEG
ACROSS THE CWFA...AND THIS COUPLED WITH THE ANOMALOUSLY HIGH DEW PTS
NEARING 73-76 DEG...THE POTENTIAL EXISTS THAT HEAT INDEX VALUES OF
ARND 100 DEG WILL DEVELOP THIS AFTN. FOR A FEW LOCATIONS WEST OF
I-39...HEAT INDEX VALUES COULD HOVER ARND 102 DEG THIS AFTN
BEFORE SUNSET. MIXING HEIGHTS SHUD INCREASE TO ARND
2500-3000FT...WHICH COULD BRING SOME SUBSIDENCE TO THE SFC. IF THIS
WERE TO MATERIALIZE THEN SFC TEMPS COULD RISE ANOTHER 1-2 DEG TO
PERHAPS 92-95 DEG.

NOW THE FOCUS TURNS TOWARDS THE CONVECTIVE POTENTIAL. IF STRICTLY
FOCUSED ON THE SFC CAPE/MID-LVL CAPE...GUIDANCE WOULD POINT TOWARDS
A VERY UNSTABLE ATMOS. HOWEVER THE MONKEY WRENCH IS THAT A STRONG
CAP CONTINUES TO BE PROGGED TO DEVELOP LATE THIS MORNING IN THE
MID-LVLS WITH CIN ARND 200J/KG LINGERING INTO THE EVENING HOURS. LCL
ARW8KM SOUNDINGS SHOWS THIS CAP HOLDING INTO THE EARLY EVENING
HOURS...WHICH WOULD LESSEN THE POTENTIAL FOR CONVECTION. THEN ARND
3Z WED THE CAP SHUD BE STEADILY WEAKENING AS GUIDANCE HAS BEEN
CONSISTENT IN THE THERMAL RIDGE DIMINISHING JUST AHEAD OF THE
FRONTAL BOUNDARY ARRIVAL. AFT 21Z 0-6KM SHEAR INCREASES TO NEARLY
30KTS OVERHEAD...SO THERE WILL EXIST A HAIL/WIND THREAT IF ANY
STORMS CAN BECOME STRONG/SEVERE. THE SPC HAS UPGRADED THE ENTIRE
NORTHERN IL/NORTHWEST IN FORECAST AREA INTO A SLIGHT RISK OF SEVERE
WEATHER TODAY. GIVEN THE PWAT VALUES OF ARND 2"...WHATEVER
CONVECTION IS ABLE TO DEVELOP BRIEF HEAVY DOWNPOURS COULD LEAD TO
FLOODING CONCERNS.

CONFIDENCE IN TEMPS...HIGH.
CONFIDENCE IN STORMS/TIMING/INTENSITY...MEDIUM.

WEDNESDAY THROUGH FRIDAY...
FRONTAL BOUNDARY ARRIVES EARLY WED MORNING ALONG WITH THE MID-LVL
TROUGH...AND A BUILDING BROAD SFC RIDGE. THIS WILL FRESHEN UP THE
AIR OVER THE REGION WITH MUCH COOLER/DRIER AIR EXPECTED ALONG A
NORTHERLY FLOW...WITH HIGHS GENERALLY IN THE 70S WED-FRI.

DRY CONDS LOOK SOLID THRU THURSDAY NIGHT...THEN FRI THE SFC RIDGE
WILL BEGIN TO DRIFT EAST WITH FLOW TURNING SOUTHERLY/SOUTHWESTERLY
ALONG WITH THE MID-LVL FLOW TRANSITIONING INTO A MORE FLAT/ACTIVE
FLOW. GUIDANCE CONTINUES TO POINT TOWARDS A WEAK WAVE DEVELOPING IN
THE CENTRAL ROCKIES EARLY FRI AND ARRIVING FRI AFTN/EVE OVER THE
FORECAST AREA. WHICH COULD BRING THE NEXT CHC FOR PRECIP/CONVECTION
THRU FRI NGT.

CONFIDENCE...HIGH.

SATURDAY THROUGH EARLY NEXT WEEK...
ENSEMBLES HAVE MAINTAINED A HIGH CONFIDENCE WITH MINIMAL SPREAD IN
THE LATER PERIODS WITH RESPECT TO THE MID-LVL FLOW TRYING TO
FLATTEN...HOWEVER THIS MAY ONLY BE FOR A BRIEF PERIOD THRU THE
WEEKEND. HIGH LATITUDE BLOCK OVER THE NORTH ATLANTIC WEAKENS
FRI/SAT...WITH ENSEMBLES DEMONSTRATING THIS FEATURE REDEVELOPING
LATE IN THE WEEKEND INTO EARLY NEXT WEEK. THIS WILL LIKELY ALLOW THE
HIGH AMPLITUDE RIDGE TO RETURN TO THE WESTERN CONUS AND STRETCH
NORTH THRU THE CANADIAN ROCKIES. BETWEEN THESE TWO RIDGES WILL
LIKELY EXIST A TROUGH OVER THE GREAT LAKES REGION IN THE EARLY NEXT
WEEK TIMEFRAME.

THE UPCOMING WEEKEND APPEARS TO BE RELATIVELY ACTIVE WITH THE
SEMI-ZONAL MID-LVL FLOW...SO HAVE LOW CHC POPS THRU THE WEEKEND INTO
MON. TEMPS SHUD WARM INTO THE LOW 80S SAT/SUN...ALTHOUGH
CLOUDS/PRECIP COVERAGE COULD HOLD TEMPS DOWN A LITTLE. THEN MOVING
INTO EARLY NEXT WEEK TEMPS SHUD COOL BACK INTO THE LOW/MID 70S.

CONFIDENCE...HIGH.

BEACHLER

&&

.AVIATION...

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

* SSW WINDS BECOMING GUSTY FROM LATE TUESDAY MORNING THROUGH THE
  EVENING...THEN WINDS SHIFTING TO THE NORTHWEST THEN EVENTUALLY
  NORTHEAST TUESDAY EVENING/NIGHT.
* SCATTERED THUNDERSTORMS IN THE AREA TUESDAY EVENING.

BMD

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

OVERNIGHT...EXPECT RELATIVELY UNCHANGING WEATHER WITH VFR CONDITIONS
AND LIGHT SOUTH WINDS. CANNOT COMPLETELY RULE OUT SOME PATCHY FOG
AGAIN TONIGHT MAINLY AT GYY WHERE THE GRADIENT IS WEAKEST...BUT THE
SOUTH WINDS SHOULD STAY UP ENOUGH TO INHIBIT FOG BECOMING TOO BIG A
PROBLEM. INCREASING GRADIENT AND MIXING DURING THE DAY TUESDAY WILL
RESULT IN GUSTY CONDITIONS WITH LOW TO MID 20 KT GUSTS POSSIBLE AT
TIMES...ESPECIALLY DURING THE AFTERNOON HOURS. STRONG INSTABILITY IS
PROGGED TO DEVELOP AHEAD OF AN APPROACHING COLD FRONT BUT A STRONG
CAP WILL ALSO BE IN PLACE MOST OF THE DAY KEEPING A SOLID LID ON
DEEP CONVECTION. CIN ERODES SOME DURING THE EVENING AND MAY ALLOW
THUNDERSTORMS TO KICK OFF DURING THE EVENING...HOWEVER GIVEN THE
OVERALL STRENGTH OF THE CAP IT IS UNCERTAIN HOW MUCH COVERAGE THERE
WILL BE...AND IT MAY END UP BEING ON THE LOWER END AS IT COMES
ACROSS THE TERMINALS WITH BETTER COVERAGE DEVELOPING LATER TO THE
SOUTH OF TERMINALS.

BMD

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

* MEDIUM-HIGH CONFIDENCE ON WIND TRENDS/TIMING/MAGNITUDE.
* HIGH CONFIDENCE IN VFR CONDITIONS THROUGH MUCH OF THE
  PERIOD...OUTSIDE OF TSRA CHANCES TOMORROW EVENING.
* LOW-MEDIUM CONFIDENCE ON SCATTERED TSRA TOMORROW EVENING AT OR
  NEAR THE TERMINALS.

BMD

//OUTLOOK FOR ORD/MDW FOR 12Z WEDNESDAY-12Z MONDAY...UPDATED 00Z...

WEDNESDAY...MORNING LOW CLOUDS. GUSTY NORTHEAST WINDS.
THURSDAY...NORTHEAST WINDS.
FRIDAY...CHC OF TSRA DURING THE NIGHT.
SATURDAY-SUNDAY...CHC OF TSRA.

MTF

&&

.MARINE...
317 AM CDT

LOW PRESSURE IS MOVING INTO THE WESTERN GREAT LAKES EARLY THIS
MORNING AND WILL DEEPEN SLIGHTLY AS IT REACHES QUEBEC BY TONIGHT
WITH A COLD FRONT PUSHING SOUTH ACROSS LAKE MICHIGAN THIS EVENING.
SOUTH TO SOUTHWEST WINDS WILL CONTINUE TO RAMP UP THROUGH THE DAY
TODAY...ESPECIALLY OVER THE NORTH HALF OF LAKE MICHIGAN WHEN WINDS
TO 30 KT ARE EXPECTED AND A FEW GALE FORCE GUSTS REMAIN POSSIBLE.
BEHIND THE FRONT...WINDS TURN NORTH TO NORTHWEST TO AROUND 30 KT.
WINDS WILL BEGIN TO TAPER THE LATTER HALF OF THE WEEK AS HIGH
PRESSURE BEGINS TO BUILD ACROSS THE REGION.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...10 AM TUESDAY TO
     10 PM TUESDAY.

&&

$$

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