Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 270730
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
230 AM CDT THU AUG 27 2015

.SHORT TERM...
225 AM CDT

THROUGH FRIDAY...

CANADIAN HIGH PRESSURE IS CENTERED FROM DIRECTLY OVER THE FORECAST
AREA NORTHWARD THROUGH LAKE SUPERIOR THIS MORNING. A FEW UPPER 40S
BEING OBSERVED IN OUTLYING LOCATIONS UNDER A CLEAR SKY OUTSIDE OF
A FEW LINGERING STRATUS PATCHES IN NORTHWEST INDIANA. EXPECT THAT
SOME DIURNAL CUMULUS WILL FORM TODAY AWAY FROM THE LAKE
SHADOW...AND LIKELY A LITTLE MORE PREVALENT EAST OF THE I-55
CORRIDOR INTO NORTHWEST INDIANA. ALL IN ALL THOUGH MORE SUNSHINE
THAN THE PAST COUPLE DAYS. LIGHT ONSHORE FLOW WILL DEVELOP AND
POSSIBLY IN THE FORM OF A TRADITIONAL LAKE BREEZE ACROSS NORTHEAST
ILLINOIS AND FAR NORTHWEST INDIANA THIS AFTERNOON. THIS WILL
PROBABLY DROP TEMPERATURES INTO THE 60S IN AREAS NEAR THE SHORE.
AFTERNOON HIGHS ELSEWHERE LOOK TO BE IN THE MID 70S AS THE AIR
MASS BEGINS TO MODIFY. MORE CLOUDS ON FRIDAY WILL LIKELY LEVEL OUT
THE MODIFYING...WITH HIGHS MID TO UPPER 70S FORECAST FROM NORTH TO
SOUTH.

MTF

&&

.LONG TERM...
225 AM CDT

FRIDAY NIGHT THROUGH WEDNESDAY...

A BROAD AREA OF INCREASED SUBTROPICAL MOISTURE IS SEEN ON MORNING
WATER VAPOR IMAGERY OVER THE INTERMOUNTAIN WEST AND WESTERN
PLAINS. WITHIN THIS ARE A FEW EMBEDDED SHORT WAVES...WITH TWO KEY
ONES ACCORDING TO NUMERICAL MODEL GUIDANCE BEING OVER NORTHERN
WYOMING AND SOUTHWEST COLORADO AS OF 230 AM. MODEL SOLUTIONS ARE
SPLIT ON WHICH OF THESE...IF EITHER...BECOME MORE DOMINANT ON
FRIDAY AS THEY TRACK EAST. A DEEPENING OF A LOW AMPLITUDE
SHORTWAVE TROUGH FROM THESE WAVES IS EXPECTED...WHICH SHOULD HELP
RAIN COVERAGE AND FOCUS WITHIN THE UPPER MIDWEST BY FRIDAY
EVENING. THE QUESTION IS AT WHAT LATITUDE WILL THIS BE AND THE
FURTHER SOUTH THE GREATER LIKELIHOOD FOR SHOWER POTENTIAL AND
DURATION IN THE CWA.

AT THIS TIME HAVE LEANED TOWARD A SLIGHTLY NORTH OF A MEAN
SOLUTION...CLOSER TO THE NAM AND EC. SUCH AN EVOLUTION WOULD STILL
APPEAR CLOSE ENOUGH FOR LIKELY RAIN SHOWERS WITHIN QG FORCING
FIELDS...ESPECIALLY NORTH OF INTERSTATE 80...FOR A PERIOD SOMETIME
FRIDAY NIGHT INTO SATURDAY MORNING. A 27.03 SREF MEAN FOR
ROCKFORD PROVIDES AROUND 0.40 OF AN INCH OF RAIN WITH AMOUNTS
DROPPING SOUTH AND EAST...ALTHOUGH QUITE A BIT OF SPREAD. HAVE
LEANED CLOSE TO A COLLABORATED WPC QPF WITH NO SIGNS OF ANY STRONG
MESOSCALE FORCING.

GUIDANCE ALSO DISAGREES ON HOW QUICKLY THE SURFACE TROUGH ASSOCIATED
WITH THIS SYSTEM MOVES EASTWARD. SOME MODELS SUCH AS THE EC HAVE IT
EAST OF THE AREA SATURDAY NIGHT...WHILE OTHER GLOBAL GUIDANCE LIKE
THE GFS AND GEM WEAKEN THE TROUGH JUST WEST OR OVER THE AREA GOING
INTO SUNDAY. BASICALLY CONTINUE TO GRADUALLY SHIFT RAIN CHANCES
EASTWARD THROUGH SATURDAY AFTERNOON AND NIGHT. MARGINAL INSTABILITY
MAY BE ENOUGH FOR ISOLATED TO SCATTERED THUNDER IN THE REGION
LATE FRIDAY NIGHT INTO SATURDAY...BUT SEVERE WEATHER IS UNLIKELY
BASED ON THE INSTABILITY AND MOISTURE SPECTRUM FORECAST.

WITH THE WEAKENING OF THE SURFACE LOW...LOOKS LIKE LINGERING CLOUDS
FROM THIS SYSTEM COULD BE A PROBLEM...ESPECIALLY IF A SLOWER
SOLUTION PANS OUT. CONCEIVABLY CLOUDS COULD LAST INTO THE DAY SUNDAY
AND LIMIT WARMTH. FOR NOW...HAVE KEPT SATURDAY HIGHS SEVERAL DEGREES
BELOW A SMART GUIDANCE BLEND.

INTO NEXT WEEK THE PATTERN CONTINUES TO LOOK SLOW-MOVING WITH THE
PRIMARY JET STREAM WELL NORTH INTO CANADA AND UPPER/MID-LEVEL
HEIGHTS ABOVE NORMAL ON THE SUITE OF LONG RANGE GUIDANCE. THE
PATTERN IS ONE THAT LOOKS MAINLY SUNNY FROM MONDAY ONWARD. THE EC
HAS QUICKER WARMING IN THE LOW-LEVELS THAN THE GFS...WITH 850MB
TEMPERATURES AT 19-20C BY MONDAY NIGHT INTO TUESDAY. HAVE GONE
WITH THE GUIDANCE BLEND AND A LITTLE WARMER BY WEDNESDAY. FLOW
STRENGTH LOOKS LIGHT ENOUGH THAT LAKE BREEZES COULD DEVELOP AND
WILL HAVE TO REFINE THOSE DETAILS IN TIME.

MTF

&&

.AVIATION...

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

* VRB WINDS BCMG SOUTHEAST LESS THAN 10 KT IN THE EARLY AFTN.

JEE

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

HIGH PRESSURE IS OVER THE REGION WITH CLEARING SKIES AND LIGHT AND
VARIABLE WINDS. A FEW LOCATIONS ARE REPORTING 3SM FOG BUT THINKING
WE ARE DRY ENOUGH WHERE FOG WILL NOT FORM AT THE TERMINALS.
HOWEVER IF FOG DOES FORM...DPA AND RFD WOULD HAVE THE BEST
CHANCES. THE HIGH DRIFTS EAST TODAY ALLOWING SOUTHEAST WINDS TO
BECOME SOUTHEAST LESS THAN 10 KT IN THE EARLY AFTN. WINDS WILL
LIKELY BE MORE EAST TO PSBLY NORTHEAST AT GYY. A LOW END VFR SCT
CLOUDS ARE EXPECTED AS WELL. WINDS REMAIN LIGHT AND BECOME SOUTH-
SOUTHEAST TONIGHT WITH INCREASING HIGH LEVEL CLOUDS FROM WEST TO
EAST.

JEE

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

* HIGH CONFIDENCE IN WINDS BECOMING SOUTHEAST AND REMAINING BELOW
  10 KT...MEDIUM IN TIMING.

JEE

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

FRIDAY...PRIMARILY VFR. CHANCE SHRA FRIDAY EVENING/NIGHT AND
POSSIBLE VSBY IMPACTS. SOUTHEAST TO EAST WINDS.

SATURDAY...CHANCE SHRA/TSRA AND ASSOCIATED VSBY
IMPACTS...OTHERWISE PRIMARILY VFR. SOUTH WINDS.

SUNDAY...SMALL CHANCE OF A SHRA/ISOLATED TSRA...OTHERWISE VFR.
NORTH TO NORTHEAST WINDS.

MONDAY AND TUESDAY...VFR.

RC

&&

.MARINE...
225 AM CDT

HIGH PRESSURE IS OVER THE LAKE THIS MORNING AND IT WILL SLOWLY DRIFT
EAST THROUGH FRIDAY AFTERNOON.  VARIABLE WIND DIRECTIONS ACROSS THE
LAKE WILL BECOME MAINLY SOUTH BY THIS EVENING AS THE HIGH SHIFTS
EAST.  THE NEXT LOW IS OVER THE CENTRAL PLAINS...AND WHILE GUIDANCE
STILL DIFFERS ON THE EXACT PATH OF THIS LOW...SIDING WITH THE
MAJORITY OF GUIDANCE MEMBERS THAT TAKE THE LOW OVER THE CENTRAL
PORTION OF THE LAKE SATURDAY.  SOUTHEAST WINDS INCREASE TO 20 KT
OVER THE SOUTHERN TWO THIRDS OF THE LAKE FRIDAY NIGHT AND SPREAD TO
THE WHOLE LAKE BY SATURDAY MORNING. AS THE LOW MOVES OVER THE
LAKE...WIND DIRECTION WILL VARY FROM EAST OVER THE NORTHERN END TO
WEST OVER THE SOUTHERN END.  THE LOW DISSIPATES OVER THE EASTERN
GREAT LAKES SUNDAY WITH A BROAD HIGH PRESSURE SYSTEM REMAINING OVER
THE LAKE THROUGH NEXT WEEK. LIGHT WINDS BECOME SOUTH MONDAY THROUGH
AT LEAST MID WEEK.

JEE

&&

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

$$

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