Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 312201
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
501 PM CDT FRI JUL 31 2015

.SHORT TERM...
222 PM CDT

THROUGH SATURDAY...

A BOUNDARY IS FINALLY EXITING THE SOUTHEAST CWA WITH A FEW ISOLATE
SHOWERS FIRING ALONG IT OVER NORTHWEST INDIANA. HIGH PRESSURE IS
SPREADING OVER THE UPPER MIDWEST WHILE LOW PRESSURE REMAINS OVER
HUDSON BAY.  BETWEEN AMPLE MIXING AND A TIGHTENED PRESSURE GRADIENT
BETWEEN THE HIGH AND LOW...WEST TO NORTHWEST WINDS ARE GUSTING UP TO
30 MPH THIS AFTERNOON. GUSTS WILL CONTINUE UNTIL MIXING EASES THIS
EVENING.  EXPECTING DRY CONDITIONS THROUGH THE NIGHT WITH LOWS
AROUND 60 EXCEPT FOR MID TO UPPER 60S DOWNTOWN.

THE NAM12 FEATURES A WEAK VORT STREAMER MOVING THROUGH SOUTHERN WI
AND NORTHERN IL EARLY SATURDAY MORNING. THE NAM ALSO INDICATES
ISOLATED SHOWERS AND MAYBE A THUNDERSTORM WILL FORM IN ASSOCIATION
WITH THE VORTICITY STREAMER. HAVE VERY LOW CONFIDENCE IN ANY PRECIP
OCCURRING AND COVERAGE...BUT ALSO DID NOT HAVE ENOUGH CONFIDENCE TO
GO DRY EITHER. SO A FEW ISOLATED LIGHT RAIN SHOWERS WITH PERHAPS A
COUPLE OF STRIKES OF LIGHTNING ARE POSSIBLE ALONG THE WI/IL STATE
LINE EARLY SATURDAY MORNING...THEN EAST OF A ROCKFORD TO MIDWAY
AIRPORT LINE MID TO LATE SATURDAY MORNING.  ANY LINGERING SHOWERS
WILL THEN MOVE SOUTHEAST OVER NW INDIANA EARLY SATURDAY AFTERNOON.
MY CONFIDENCE DECREASES RAPIDLY AFTER THE SUN COMES UP AS WE WARM
AND DRY DUE TO MIXING. THEREFORE...KEEP AN EYE OUT FOR A STRAY
SHOWER OR STORM SATURDAY MORNING OVER FAR NORTHEAST IL AND NW
INDIANA...BUT THE REST OF THE DAY TIME HOURS SATURDAY LOOK DRY.

RAISED HIGH TEMPS OVER WEST CENTRAL AND CENTRAL IL BY A DEGREE OR
TWO AS THE WARMEST LOCATIONS WILL BE UNDER THE WARMEST UPPER LEVEL
AIR. THE WHOLE WARNING AREA WILL SEE HIGHS IN THE MID TO UPPER 80S
WITH HEAT INDICES ABOUT THE SAME AS THE HIGH TEMPS.  WEST WINDS ARE
ONCE AGAIN EXPECTED BUT WITH THE WEAKER GRADIENT...GUSTS IN THE
AFTERNOON WILL ONLY BE UP TO 20 MPH.

JEE

&&

.LONG TERM...
304 PM CDT

SATURDAY NIGHT THROUGH FRIDAY...

MAIN CONCERN FOR THE LATTER PORTION OF THE WEEKEND FROM SATURDAY
NIGHT THROUGH SUNDAY NIGHT WILL BE THE CHANCES FOR THUNDERSTORMS
THROUGH THIS PERIOD AS WELL AS TEMPERATURES FOR SUNDAY AFTERNOON.
THE MAIN ADJUSTMENTS TO THE GOING FORECAST WERE TO TREND A LITTLE
SLOWER WITH THE PROGRESSION OF THE NEXT COLD FRONT DROPPING THROUGH
THE AREA FROM NORTHWEST TO SOUTHEAST.

THE LARGER SCALE MODELS ARE TRENDING TO A MORE CONSISTENT SOLUTION
THAT THE SHORTER RANGE GUIDANCE WITH THE PROGRESSION OF THE FRONT
AND THE UPPER LEVEL PATTERN DRIVING THE ASSOCIATED SFC FEATURES.
THE GENERAL TREND THROUGH THE REMAINDER OF THE WEEKEND AND EVEN INTO
EARLY NEXT WEEK WILL BE TO CONTINUE TO AMPLIFY THE UPPER RIDGE OVER
THE WRN CONUS WHILE A DEEP UPPER LOW REMAINS PARKED INVOF HUDSON
BAY...WHILE BROAD UPPER TROUGHING CONTINUES OVER THE ERN CONUS.
THIS WILL KEEP THE UPPER MIDWEST THROUGH THE GREAT LAKES UNDER NWLY
FLOW ALOFT.

THE VARIOUS MODELS SHOW A SERIES OF IMPULSES DROOPING OUT OF CANADA
IN THE FAST NWLY FLOW ALOFT...AND AS USUAL...IT IS THE TYPICAL
TIMING AND INTENSITY DIFFERENCES AMONG THE MODELS IN HOW THEY HANDLE
THESE FAST MOVING SHORTWAVES WILL HAVE A MAJOR IMPACT ON THE
SENSIBLE WEATHER FEATURES.  GIVEN THE RELATIVE CONSISTENCY BETWEEN
THE GFS/ECMWF...HAVE LARGELY BASED THE FORECAST POPS/WX/QPF ON A 50-
50 BLEND OF THESE 2 MODELS.  THE NET EFFECT HERE IS THAT SATURDAY
NIGHT SHOULD BE DRY...WITH INCREASING CHANCES FOR SHOWERS AND
THUNDERSTORMS ACROSS THE REGION ON SUNDAY. THE FORECAST GRIDS FOR
SUNDAY ARE A BIT OF A BROADBRUSH FOR CHANCE SHRA/TSRA ACROSS THE
ENTIRE CWA THROUGH THE DAY AND INTO THE EARLY EVENING...LARGELY IN
DEFERENCE TO THE RETURN OF HIGH DEWPOINT AIR TO THE REGION IN THEN
SWLY SFC FLOW AHEAD OF THE COLD FRONT. ALSO...A NRN STREAM SHORTWAVE
DROPPING THE THE FAST NWLY FLOW ALOFT WILL ALSO HELP ORGANIZE SOME
OF THE TS.  THE GFS/ECMWF ARE BOTH VERY CONSISTENT ON TRACKING THE
SHORTWAVE THROUGH THE AREA BTWN 12-18Z SUNDAY WITH THE BEST FORCING
OVER THE SWRN PORTIONS OF THE CWA...THOUGH SOME TSRA/SHRA CANNOT BE
RULED OUT ELSEWHERE.  WITH TEMPERATURES EXPECTED TO REACH THE MIDDLE
TO UPPER 80S ON SUNDAY AND DEWPOINTS IN THE LOWER 70S AWAY FROM THE
URBANIZED AREAS OF THE CHICAGO METRO...THE AFTERNOON HOURS WILL ALSO
BE PRIMED FOR AT LEAST SCT CONVECTIVE ACTIVITY.  CURRENTLY FEEL THAT
THE TIMING OF THE FROPA...DURG THE LATE EVENING HOURS...WILL BE THE
MAIN LIMITING FACTOR IN THE TS CHANCES FOR SUNDAY EVENING AND INTO
THE OVERNIGHT HOURS.  THERE IS SOME HINT OF WEAK SHORTWAVE ENERGY
RIPPLING THROUGH THE NWLY FLOW ALOFT...BUT WITH THE TIMING OF THE
FROPA EXPECTED TO BE AFTER THE TIMING OF MAX HEATING...WILL CONTINUE
TO BROADBRUSH CHANCE POPS THROUGH THE EVENING.  WHILE THERE IS THE
POSSIBILITY OF MORE ORGANIZED CONVECTION ALONG THE COLD
FRONT...TIMING AT THIS POINT IS A LITTLE TOO UNCERTAIN TO TRY AND
PIN DOWN ANY SPECIFIC AREA AND TIME WITH HIGHER POPS.

FOLLOWING THE PASSAGE OF THE COLD FRONT LATE SUNDAY NIGHT OR EARLY
MONDAY MORNING...FORECAST CONFIDENCE WILL WANE THROUGH THE REMAINDER
OF THE FORECAST PERIOD AS THE UPPER LEVEL PATTERN DEAMPLIFIES INTO A
MUCH MORE LOW AMPLITUDE PATTERN ALOFT...WITH THE FAST WESTERLIES
ACROSS THE NRN CONUS.  THE MODELS INDICATE A SERIES OF FAST MOVING
SHORTWAVES MOVING THROUGH THE BELT OF STRONGER WESTERLY FLOW
ALOFT...KEEPING PERIODIC CHANCES FOR PCPN ACROSS THE AREA THROUGH
THE PERIOD.  HIGH PRESSURE BUILDING ACROSS THE NRN PLAINS AND INTO
THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES WILL ALSO HELP TO
SUPPRESS THE GREATER CHANCES FOR PCPN TO THE SOUTH OF THE
CWA...WHILE SETTING UP GENERALLY ELY FLOW AT THE SFC...RANGING FROM
NELY-SELY THROUGH MUCH OF THE WEEK.  THIS PATTERN WILL BE CONDUCIVE
TO A COOLER TEMPERATURE TRENDS...WITH HIGHS RANGING FROM 5-10
DEGREES BELOW NORMAL...IN THE MIDDLE TO UPPER 70S.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 22Z...

* WEST-NORTHWEST WINDS GUSTING TO AROUND 25KT THROUGH 00Z.

* ISOLATED SHOWERS/THUNDERSTORMS POSSIBLE SATURDAY MORNING.

RODRIGUEZ/MTF

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

SURFACE TROUGH MOVING THROUGH THE AREA EARLY THIS AFTERNOON WITH
VFR CONDITIONS IN PLACE...AND WITH WEST/NORTHWEST WINDS INCREASING
AT THIS TIME. VFR CONDITIONS WILL REMAIN TODAY INTO TONIGHT WHILE
ANY SHOWER/THUNDERSTORM DEVELOPMENT THIS AFTERNOON REMAINS WELL
SOUTH OF THE TERMINALS. THESE STRONGER WESTERLY WINDS WILL LIKELY
PERSIST THROUGH EARLY EVENING...WITH EXPECTED GUSTS AROUND 25KT.
THERE DOES APPEAR TO BE SMALL WINDOW SATURDAY MORNING FOR ISOLATED
PRECIP TO MOVE ACROSS THE TERMINALS. THEY WILL LIKELY BE ONLY
SHOWERS...BUT WITH THUNDER DEFINITELY NOT OUT OF THE QUESTION.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH IN WIND SPEED/DIRECTION.

* LOW IN ANY SHOWERS/THUNDERSTORMS SATURDAY MORNING.

RODRIGUEZ/MTF

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

SATURDAY NIGHT...MAINLY VFR. CHANCE OF SHOWERS/THUNDERSTORMS LATE.

SUNDAY...CHANCE OF TSRA. SOUTHWEST WINDS.
MONDAY...CHANCE OF TSRA. NORTHWEST WINDS.
TUESDAY...DRY/VFR. NORTHEAST WINDS.
WEDNESDAY...SLIGHT CHANCE OF TSRA. EASTERLY WINDS.
THURSDAY...SLIGHT CHANCE OF RAIN.

BEACHLER

&&

.MARINE...
315 PM CDT

FOLLOWING THE PASSAGE OF A COLD FRONT...WINDS ACROSS THE WESTERN
GREAT LAKES HAVE INCREASED TO ARND 20 KT WITH OCNL GUSTS TO ARND
25 KT IN A MODEST NWLY GRADIENT BETWEEN DEEP LOW PRESSURE OVER
HUDSON BAY AND A RIDGE OF HIGH PRESSURE EXTENDING FROM THE NRN
PLAINS THROUGH THE LOWER MISSISSIPPI VALLEY. A SMALL CRAFT
ADVISORY WILL REMAIN IN EFFECT FOR THE IL NEARSHORE WATERS INTO
EARLY EVENING WHERE THE WINDS HAVE BEEN STRONGEST AND MOST GUSTY.
AS THE HIGH BUILDS EAST TONIGHT ACROSS THE MIDDLE MISSISSIPPI
VALLEY AND INTO THE OHIO VALLEY...THE PRESSURE GRADIENT WILL RELAX
AND WINDS WILL DIMINISH AND BECOME MORE WESTERLY THROUGH THE
EVENING AND OVERNIGHT TONIGHT...ENDING THE SMALL CRAFT ADVISORY
CONDITIONS. AS THE HIGH CONTINUES TO SETTLE INTO THE SERN CONUS
SUNDAY AND A COLD AND ANOTHER COLD FRONT DROPS PUSHES INTO THE NRN
PLAINS. BRISK SWLY WINDS WILL SET UP ACROSS THE LAKE SATURDAY
NIGHT AND SUNDAY...THOUGH THE GRADIENT IS NOT EXPECTED TO BE VERY
STRONG AND WINDS SHOULD ONLY BE ARND 10-20KT. THE NEXT COLD FRONT
SHOULD SWEEP ACROSS THE LAKE SUNDAY EVENING...TURNING WINDS BACK
TO NWLY AGAIN. HIGH PRESSURE BUILDING ACROSS THE NRN PLAINS AND
ACROSS THE UPPER GREAT LAKES EARLY NEXT WEEK SHOULD THEN LEAD TO A
PERIOD OF GENERALLY EASTERLY FLOW OVER LAKE MICHIGAN FOR MUCH OF
NEXT WEEK.

KREIN

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
     FRIDAY.

&&

$$

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