Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 272014
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
314 PM CDT MON OCT 27 2014

.DISCUSSION...
252 PM CDT

RAIN/THUNDER CHANCES AND COOLER TEMPERATURES WILL BE THE INITIAL
FOCUS OF THE FORECAST THIS PERIOD...WITH OTHER HIGHLIGHTS A SECOND
NOTICEABLE COOL DOWN LATE IN THE WEEK WITH MODERATION EARLY NEXT
WEEK. AFTERNOON WATER VAPOR IMAGERY SHOWS UPPER RIDGING NOW OVER THE
GREAT LAKES WITH A CLOSED UPPER LOW PIVOTING EASTWARD ACROSS THE
NORTHERN PLAINS WITH ITS AXIS EXTENDING DOWN INTO TEXAS. LOW
AMPLITUDE RIDGING IS BUILDING OVER THE WEST COAST IN RESPONSE TO A
CLOSED LOW OFF THE BRITISH COLUMBIA COAST. AT THE SURFACE...HIGH
PRESSURE IS OVER THE EAST COAST WITH AN ELONGATED SURFACE LOW
STRETCHING FROM SOUTHERN MANITOBA TO CENTRAL IOWA BACK INTO THE
SOUTHERN PLAINS.

TONIGHT...THE CLOSED UPPER LOW OVER THE NORTHERN PLAINS WILL MOVE
EASTWARD THIS EVENING WITH A STRENGTHENING UPPER JET TRACKING
NORTHEASTWARD OVER THE GREAT LAKES AHEAD OF IT. THIS WILL ALLOW THE
ELONGATED SURFACE LOW TO THE WEST OF THE AREA TO CONSOLIDATE AND
DEEPEN OVER CENTRAL AND NORTHERN LAKE MICHIGAN INTO TUESDAY MORNING.
THIS WILL IN TURN BRING A COLD FRONT ACROSS THE AREA TONIGHT. WATER
VAPOR SHOWS SEVERAL WAVES EMBEDDED IN THE SOUTHWESTERLY MID/UPPER
FLOW AHEAD OF THE TROUGH THIS AFTERNOON. THE FIRST WAVE IS MOVING
INTO CENTRAL ILLINOIS PRODUCING SCATTERED SHOWERS AND WILL CONTINUE
TO TRACK NORTHEAST ACROSS THE SOUTHERN CWA LATE THIS AFTERNOON.
ANOTHER WAVE NOTED MOVING ACROSS EASTERN KANSAS INTO MISSOURI WILL
TRACK NORTHEAST ACROSS THE AREA EARLY TO MID EVENING WITH YET
ANOTHER WAVE LOOKING TO AFFECT THE AREA OVERNIGHT. DEEP LAYER
MOISTURE CONTINUES TO INCREASE WITH SOUNDINGS SHOWING ELEVATED
INSTABILITY ACROSS THE REGION ABOVE A STOUT LOW LEVEL CAP. EXPECT
THAT SOME SCATTERED SHOWER ACTIVITY WILL AFFECT THE FORECAST AREA IN
CONJUNCTION WITH THE FIRST AND SECOND WAVES...WITH SOME ISOLATED
THUNDER POSSIBLE. THIS WOULD BE TIMED LATE THIS AFTERNOON THROUGH
EARLY EVENING. FROM THAT POINT THE APPROACHING COLD FRONT SHOULD
BECOME MORE INVOLVED IN SHOWER AND THUNDER DEVELOPMENT WITH UPPER
FEATURES SUCH AS THE INCREASING JET MOVING INTO THE AREA. THE FRONT
SHOULD ARRIVE IN THE WESTERN CWA TOWARD MID EVENING WHICH SHOULD
INTRODUCE AN INCREASE IN SHOWER AND THUNDERSTORM COVERAGE INTO THE
EARLY OVERNIGHT. AM THINKING THAT NORTHWESTERN AREAS MAY SEE ONLY
SCATTERED ACTIVITY WITH COVERAGE INCREASING OVER THE CENTRAL CWA
FROM UP NEAR WAUKEGAN SOUTHWEST TOWARD THE MENDOTA AREA WITH
STEADILY INCREASING COVERAGE EAST OF THERE. HAVE REFINED POPS TO
SHOW LOWER POTENTIAL IN THE WEST AND NORTHWEST AND OVERALL DELAYED
THE HIGHEST POPS UNTIL AFTER MIDNIGHT OR SO...KEEPING EVENING VALUES
AT CHANCE OR LOWER. AM NOT EXPECTING ABUNDANT RAINFALL BUT ISOLATED
HEAVIER DOWNPOURS WILL BE POSSIBLE...ESPECIALLY OVERNIGHT SOUTH AND
EAST OF CHICAGO WHERE BEST ORGANIZATION IS EXPECTED. ACTIVITY WILL
BEGIN TO END FROM THE WEST OVERNIGHT AS DRIER AIR STARTS TO SPREAD
IN.

TUESDAY THROUGH THURSDAY...THE SURFACE LOW LIFTS TO JAMES BAY BY
TUESDAY EVENING WITH THE MAIN UPPER TROUGH AXIS NOT SWINGING BY
UNTIL EARLY WEDNESDAY. A STEADY FLOW OF COLD ADVECTION WILL BE
UNDERWAY TUESDAY BEHIND THE FRONT SO HIGHS MAY OCCUR EARLY IN THE
DAY BUT INCREASINGLY SUNNY SKIES MAY OFFSET THE COLD ADVECTION AND
ALLOW FOR STEADIER TEMPS. THERE WILL BE A MODEST INCREASE IN DEEP
LAYER MOISTURE AS THE TROUGH AXIS ROTATES BY WEDNESDAY MORNING SO AN
INCREASE OF CLOUDS MAY RESULT BUT AT THIS POINT ANY SHOWER POTENTIAL
WOULD BE VERY LOW AND WILL NOT HAVE ANY MENTION. LOW LEVEL FLOW WILL
TURN NORTHWESTERLY BUT IT APPEARS ANY LAKE EFFECT SHOWERS WOULD
REMAIN NORTH AND EAST OF THE AREA. THE LARGE SCALE TROUGH WILL
BROADEN OVER MUCH OF EASTERN NORTH AMERICA GOING INTO THURSDAY.
THERE HAS BEEN A CONSISTENT SIGNAL FOR A SHORTWAVE TO TRACK
SOUTHEAST ACROSS THE AREA THURSDAY BUT MOISTURE SHOULD BE MEAGER AND
FORCING NOT TERRIBLY COHERENT. WILL NEED TO WATCH THIS PERIOD FOR
SHOWER POTENTIAL BUT WILL MAINTAIN A DRY FORECAST FOR NOW. HIGHS
WILL BE IN THE 50S WEDNESDAY AND THURSDAY WITH WEDNESDAY BEING THE
COOLEST.

FRIDAY THROUGH THE WEEKEND...MEDIUM RANGE GUIDANCE CONTINUES TO
POINT TO A SHARPENING OF THE LONGWAVE TROUGH FRIDAY AS A STRONG
UPPER WAVE DIGS SOUTHWARD FROM CENTRAL CANADA. THIS IS EXPECTED TO
BRING H85 TEMPS OF NEAR -8C BY FRIDAY NIGHT WITH BREEZY NORTH LOW
LEVEL FLOW. THE TROUGH ITSELF COULD PRODUCE SOME SHOWERS THURSDAY
NIGHT BUT AGAIN THE AIRMASS LOOKS FAIRLY DRY AND THE FORCING QUICK
TO MOVE THROUGH. LAKE EFFECT PRECIP BECOMES A CONCERN WITH THE DUE
NORTH WIND AND VERY COLD AIRMASS BUT HIGH PRESSURE QUICKLY MOVING IN
MAY KEEP FAVORABLE INVERSION HEIGHTS SHORT LIVED. HIGHS FRIDAY WILL
ONLY BE IN THE 40S AND CURRENT FORECAST VALUES COULD BE A LITTLE
OPTIMISTIC...THOUGH SUNSHINE MAY HELP OFFSET THE STRONG COLD
ADVECTION SLIGHTLY. BEYOND THAT SHARP UPPER RIDGING IS EXPECTED TO
BUILD ACROSS THE PLAINS AND MISSISSIPPI RIVER VALLEY AS A DEEP
TROUGH SPREADS INTO THE WESTERN U.S. IT IS STILL NOT CLEAR HOW
QUICKLY THIS WILL SHIFT EASTWARD SO THE SPEED WITH WHICH THE
ACCOMPANYING WARMING TREND OCCURS IS STILL IN QUESTION BUT
MODERATION SHOULD BEGIN BY SUNDAY. HAVE LOW CONFIDENCE BY MONDAY
HAVING SEEN SEVERAL VARIATIONS WITH HOW THE UPPER FLOW PATTERN
EVOLVES OVER THE PAST FEW DAYS...WITH LATEST GUIDANCE SHOWING A
NORTHERN STREAM TROUGH PUSHING THE UPPER RIDGE WITH AN ASSOCIATED
SURFACE LOW OVER SOUTHERN CANADA BRINGING A FRONT ACROSS THE AREA
EARLY NEXT WEEK.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* SOUTHWEST WINDS GUSTING TO 20-25KT THROUGH LATE AFTERNOON.

* SHRA/RA LIKELY LATER THIS EVENING INTO THE OVERNIGHT WITH A
  CHANCE FOR TSRA.

* WESTERLY WIND SHIFT LATE TONIGHT FOLLOWING A COLD FRONTAL
  PASSAGE. WESTERLY WINDS GUSTY UP AROUND 27 KT DURING THE DAY
  TUESDAY.

KJB

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

SURFACE LOW PRESSURE ACROSS IOWA EARLY THIS AFTERNOON WILL SHIFT
NORTHEASTWARD ACROSS THE UPPER GREAT LAKES REGION BY TUESDAY
MORNING...WHILE DEEPENING. THIS SYSTEM WILL DRIVE A COLD FRONT
ACROSS THE TERMINALS LATER TONIGHT...LIKELY AFTER 09-10 UTC.
SOUTH-SOUTHWESTERLY WINDS WILL CONTINUE ACROSS THE AREA PRIOR TO THE
ARRIVAL OF THIS FRONTAL ZONE...WITH WIND SPEEDS LIKELY TO REMAIN
GUSTY UP INTO THE MID 20 KT RANGE DURING MUCH OF THE AFTERNOON.
STRONG GUSTY WESTERLY WINDS WILL SET UP IN THE WAKE OF THE FRONTAL
ZONE INTO TUESDAY.

SCATTERED SHOWERS ARE ALSO EXPECTED TO DEVELOP ACROSS PORTIONS OF
THE AREA DURING THE EVENING AS MOISTURE CONTINUES TO BE DRIVEN
NORTHWARD. HOWEVER...IT APPEARS THE BETTER CHANCES OF SHOWERS AND
PERHAPS A FEW THUNDERSTORMS WILL BE OVERNIGHT TONIGHT ASSOCIATED
WITH THE APPROACHING COLD FRONTAL BOUNDARY. IT APPEARS THE DURATION
OF ANY THUNDERSTORMS WILL BE SHORT (AN HOUR OR TWO). CONDITIONS
SHOULD IMPROVE BY DAYBREAK TUESDAY...WITH DRIER AIR FILTERING IN
ACROSS THE AREA...BUT GUSTY WESTERLY WINDS WILL CONTINUE FOR MUCH OF
THE DAY TUESDAY.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH CONFIDENCE IN WIND TRENDS.

* MEDIUM CONFIDENCE IN TIMING OF PCPN ONSET THIS EVENING.

* LOW CONFIDENCE IN TSRA DIRECTLY IMPACTING THE TERMINALS.

* MEDIUM CONFIDENCE IN CIG TRENDS.

KJB

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

TUESDAY NIGHT...VFR/WX NIL.  WEST WINDS.

WEDNESDAY...VFR/WX NIL. WEST WINDS THROUGH THE DAY...BACKING TO
SLY OVERNIGHT.

THURSDAY...VFR/WX NIL. SLY WINDS THROUGH THE DAY...VEERING NWLY
OVERNIGHT.

FRIDAY...VFR/WX NIL. NORTH TO NORTHEAST WINDS.

SATURDAY...VFR/WX NIL. NORTH TO NORTHEAST WINDS.

SUNDAY...VFR/WX NIL. SOUTHEAST WINDS.

KREIN

&&

.MARINE...
230 PM CDT

A WARM FRONT LIFTING ACROSS THE LAKE TODAY HAS PRODUCED WIDELY
VARIABLE CONDITIONS FROM ONE LOCATION TO ANOTHER...AS WELL AS
WIDESPREAD THUNDERSTORMS ACROSS THE NORTHERN END. TONIGHT THE
FRONT WILL EXIT THE NORTHERN END OF THE LAKE AND BY MORNING A
CENTER OF LOW PRESSURE WILL FOLLOW IN ITS PATH. BEHIND THE
LOW...MORE UNIFORM CONDITIONS DEVELOP TOMORROW MORNING WITH
GENERALLY WEST TO SOUTHWEST WINDS FOR THE ENTIRE LAKE. THE
TIGHTEST PRESSURE GRADIENT WILL BE LOCATED OVER THE NORTHERN HALF
WHICH IS CLOSEST TO THE DEPARTING LOW...SO A PERIOD WITH OCCASIONAL
GALE FORCE GUSTS IS POSSIBLE TOMORROW AFTERNOON INTO THE EVENING
FOR THE NORTH HALF.

WINDS REMAIN GENERALLY WESTERLY AND CONDITIONS GRADUALLY RELAX
THROUGH THE REMAINDER OF THE WEEK UNDER THE INFLUENCE OF AN
APPROACHING RIDGE UNTIL ANOTHER LOW CROSSES THE LAKE THURSDAY
NIGHT. COLD NORTHERLY FLOW THEN DEVELOPS FOR FRIDAY AND MOST OF
SATURDAY.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 4 AM TUESDAY.

&&

$$

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