Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 271745
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1245 PM 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 18Z...

* SOUTHEAST WINDS JUST UNDER 10KTS THIS AFTERNOON.
* POSSIBLE LAKE BREEZE AND EASTERLY WINDS FRIDAY AFTERNOON. CMS

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

MAIN FORECAST CONCERN AND CHALLENGE WILL BE WIND DIRECTIONS THRU
THE PERIOD. WINDS ARE GENERALLY LIGHT SOUTHEASTERLY ACROSS THE
AREA EARLY THIS AFTERNOON WITH SPEEDS UNDER 10KTS THOUGH THERE IS
SOME VARIABILITY TO THE WIND DIRECTIONS. A LAKE BREEZE HAS FORMED
ACROSS FAR NORTHEAST IL. THIS LAKE BREEZE MAY TURN WINDS A BIT
MORE EASTERLY...IF IT MAKES IT TO ORD/MDW...BUT SPEEDS SHOULD
REMAIN UNDER 10KTS. SPEEDS WILL DIMINISH A BIT THIS EVENING WITH
SOUTHERLY WINDS EXPECTED FRIDAY MORNING. LOW PRESSURE OVER THE
PLAINS WILL MOVE TO NORTHEAST IA BY FRIDAY EVENING. THE GRADIENT
WILL TIGHTEN FRIDAY AFTERNOON WITH SOUTH/SOUTHEASTERLY WINDS
AROUND 10KTS FRIDAY AFTERNOON. ITS POSSIBLE A LAKE BREEZE WILL
REACH ORD/MDW MID/LATE FRIDAY AFTERNOON TURNING WINDS MORE TO THE
EAST/SOUTHEAST WITH THE POTENTIAL FOR 10KTS WITH THIS LAKE BREEZE.

ANY PRECIPITATION FROM THE APPROACHING LOW PRESSURE SHOULD BE
AFTER THE 30 HR ORD TAF AND PERHAPS EVEN AFTER 00Z AT RFD. SOME
LIGHT/SHALLOW GROUND FOG IS POSSIBLE OVERNIGHT INTO FRIDAY
MORNING. NOT EXPECTING ANYTHING TOO WIDESPREAD AND CONFIDENCE IS
LOW SO NO MENTION WITH THIS FORECAST.

CU/STRATOCU MAY BE SLOW TO SCATTER OUT ACROSS FAR NORTHEAST IL AND
NORTHWEST INDIANA THIS AFTERNOON BUT CIGS SHOULD REMAIN AT OR JUST
ABOVE 3KFT. CMS.

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

* MEDIUM FOR WINDS THRU THE PERIOD. CMS

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

FRIDAY NIGHT...SHRA AND CHC TSRA. MAINLY VFR. SOUTHEAST WINDS.

SATURDAY...SHRA AND SCHC TSRA. MVFR OR LOWER PSBL. SOUTH WINDS
BCMG NORTHWEST OVERNIGHT.

SUNDAY...VFR. NORTH WINDS BCMG SOUTH OVERNIGHT.

MONDAY AND TUESDAY...VFR. SOUTH WINDS.

WEDNESDAY...VFR. SOUTHEAST WINDS.

JEE

&&

.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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