Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 221738
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1238 PM CDT TUE JUL 22 2014

.DISCUSSION...
1011 AM CDT

.UPDATE...
JUST SOME MINOR TWEAKS THIS MORNING TO ACCOUNT FOR SOME ADDITIONAL
CLOUD COVER SLIDING THROUGH THIS MORNING OUT AHEAD OF THE COLD
FRONT. THIS MAY ACT TO KEEP TEMPS FROM GETTING INTO THE LOW 90S. OTHERWISE
HIGHS IN THE UPPER 80S TO CLOSE TO 90 SEEM REASONABLE...SO HEAT
INDEX VALUES SHOULD GENERALLY REMAIN AT 100 OR LESS.

WITH REGARDS TO CONVECTIVE TIMING THIS EVENING. MODELS ARE SHOWING
A BRIEF WINDOW OF THE CAP BREAKING FROM NW TO SE THIS EVENING.
BEST TIME FRAME WOULD SUGGEST STORMS BEGINNING IN SOUTHERN
WISCONSIN IN THE 22-23Z TIME FRAME AND SLIDING SOUTHEAST...MOVING
THROUGH THE METRO AREA BETWEEN 1Z-4Z...OR 8 TO 11 PM.  STILL SOME
MIXED SIGNALS ON SHORT TERM GUIDANCE AS TO HOW WIDESPREAD COVERAGE
WILL BE...ESPECIALLY FARTHER WEST/SOUTHWEST WHERE THE CAP IS
STRONGER AND HEIGHT FALLS ARE WEAKER. HAVE GONE WITH LIKELY STORMS
CLOSER TO THE LAKE. GIVEN STRONG INSTABILITY...MODERATE SHEAR...EXPECT
STORMS THAT DO DEVELOP TO HAVE THE POTENTIAL FOR STRONG
WINDS...LARGE HAIL...AND HEAVY RAIN.

KMD

//PREV 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 18Z...

* GUSTY SSW WINDS INTO THE LOW TO MID 20 KT RANGE THIS AFTERNOON.
* SCATTERED THUNDERSTORMS DEVELOPING IN THE AREA THIS EVENING
  AHEAD OF A COLD FRONT. BEST TIMING APPEARS TO BE IN THE 01 TO 04
  UTC TIME FRAME.
* WINDS SHIFTING TO NORTH THIS EVENING WITH COLD FRONTAL
  PASSAGE...THEN NORTHEAST.
* POTENTIAL FOR A BRIEF PERIOD OF LOW CIGS BEHIND FRONT.
* NORTHEASTERLY WINDS (020-030 DIRECTION) AROUND 14 KT WITH SOME
  GUSTS UP TO 20 KT ON WEDNESDAY.

KJB

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

A COLD FRONT IS EXPECTED TO MOVE ACROSS THE TERMINALS THIS EVENING.
THE ATMOSPHERE AHEAD OF THIS FRONT IS BECOMING VERY WARM...MOIST AND
HENCE VERY UNSTABLE. THEREFORE...THE THREAT FOR AT LEAST SOME
THUNDERSTORMS IS STILL ON TAP FOR THE AREA AROUND THE EARLY TO MID
EVENING HOURS TODAY. THE MAIN QUESTION THAT REMAINS...HOWEVER...IS
HOW WIDESPREAD THE THUNDERSTORM ACTIVITY WILL BE ACROSS THE TERMINAL
AIRSPACE THIS EVENING. IN SPITE OF THE PRESENCE OF A VERY UNSTABLE
ATMOSPHERE...WARMER TEMPERATURES ALOFT WILL ACT TO INHIBIT
DEVELOPMENT UNTIL VERY LATE THIS AFTERNOON/EARLY EVENING WHEN THE
SURFACE FRONT AND AN ACCOMPANYING MID LEVEL ATMOSPHERIC DISTURBANCE
APPROACH THE AREA. IT APPEARS THAT ALTHOUGH THE FORCING FOR ASCENT
FROM THIS APPROACHING DISTURBANCE WILL REMAIN VERY MODEST...IT SHOULD
BE ENOUGH TO ALLOW SOME THUNDERSTORMS TO DEVELOP ACROSS NORTHERN
ILLINOIS...SOUTHERN WI AREA AROUND THE 00 UTC TIMEFRAME...THEN SHIFT
EAST-SOUTHEASTWARD. HOWEVER...IT APPEARS THE AREAL COVERAGE COULD BE
FAIRLY LOW INITIALLY DUE TO INHIBITING FACTOR OF WARMER AIR ALOFT.
THEREFORE...THIS SHEDS SOME DOUBT AS TO WHETHER OR NOT THE TERMINALS
WILL BE DIRECTLY IMPACTED BY ANY OF THESE STORMS. IN SPITE OF THESE
UNCERTAINTIES...I WILL CONTINUE WITH THE MENTION OF THUNDERSTORMS
AFTER 00 UTC THROUGH AROUND 04 UTC THIS EVENING.

ONCE THE COLD FRONT SHIFTS ACROSS THE AREA BY LATE EVENING THE WINDS
WILL BECOME NORTH-NORTHEASTERLY OVERNIGHT AND INTO WEDNESDAY. IT
APPEARS THE WINDS WILL REMAIN GUSTY BEHIND THE FRONT...WITH
NORTHEASTERLY (020-030 DIRECTION) WINDS EXPECTED DURING THE DAY
WEDNESDAY...WITH GUSTS UP TO 20 KT.

KJB


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* MEDIUM-HIGH WITH WIND TRENDS/TIMING/MAGNITUDE.
* MEDIUM WITH THE GENERAL TIMING FOR TSRA THIS EVENING BUT LOW
  CONFIDENCE IN COVERAGE.
* LOW WITH MVFR CIGS BEHIND THE COLD FRONT.

KJB

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

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

BMD

&&

.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...ILLINOIS NEARSHORE WATERS UNTIL 10 PM TUESDAY.

&&

$$

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