000
FXUS63 KLOT 191602
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1102 AM CDT SUN MAY 19 2013

.DISCUSSION...
501 AM CDT

FORECAST FOR TODAY INTO LATE IN THE WEEK PRIMARILY HINGES ON HOW
LONG IT TAKES FOR THE UPPER LOW THAT HAS BEEN FORMING OVER UT
YESTERDAY AND OVERNIGHT TO REACH AND THEN FINALLY MOVE EAST OF THE
LOCAL AREA.

MODEL RUNS OVER THE LAST COUPLE OF DAYS HAVE ALL BEEN GRADUALLY
SLOWING ITS EASTWARD PROGRESS AND ARE QUITE SIMILAR IN LOCATION OF
THE UPPER LOW AS IT MOVES VERY SLOWLY ENE ACROSS THE PLAINS. MODELS
ARE ALSO VERY SIMILAR IN THE GRADUAL WEAKENING OF THE UPPER LOW
DURING TUE NIGHT AND WED AS IT MOVES TO THE UPPER MIDWEST...AND WITH
THE REMAINING UPPER TROUGH STILL HANGING BACK ACROSS THE GREAT
LAKES...OH VALLEY AND MS VALLEY DURING THU.

WITH THE VERY SLOW PROGRESSION OF THIS SYSTEM ACROSS THE CENTRAL
U.S. THE POTENTIAL FOR SHOWERS AND THUNDERSTORMS WILL HANG OVER THE
LOCAL AREA FROM LATE TODAY TIL AT LEAST THU BEFORE THE UPPER TROUGH
MOVES FURTHER OVER THE GREAT LAKES AND MIDWEST. ECMWF 00Z RUN IS
MOST AGGRESSIVE WITH EASTWARD PROGRESS DURING THE LATTER PART OF THE
WEEK...AS WAS THE PREVIOUS 12Z RUN.

OTHER MODELS ARE SLOWER WITH ABSORBING THE GREAT LAKES UPPER
TROUGH INTO THE DEEP VORTEX PROGGED TO SETTLE SOUTHWARD OVER
EASTERN CANADA AND NORTHERN NEW ENGLAND WED THROUGH FRI BUT ALL
HAVE THE UPPER TROUGH AXIS MOVING E OF THE LOCAL AREA BY FRI WITH
A SURFACE RIDGE SPREADING SSE ACROSS THE REGION THU NIGHT THROUGH
SAT.

AHEAD OF THE SLOW MOVING SYSTEM VERY WARM AND MOIST AIR WILL
CONTINUE TO SPREAD FROM THE SOUTHERN AND CENTRAL PLAINS NNE
ACROSS THE MID MS VALLEY TODAY AND TONIGHT. FORECAST LOCAL
TEMPERATURE PROFILES SHOW A CAPPING INVERSION IN PLACE TODAY BUT
ERODED AWAY THIS EVENING. SURFACE DEW POINTS HAVE HELD STEADY
ACROSS THE AREA DURING THE OVERNIGHT AND WILL GRADUALLY RISE AS
INCREASING SE TO SSE LOW LEVEL FLOW ADVECTS MID 60S DEW POINTS
OVER THE AREA. WITH RISING LOW LEVEL MOISTURE...THE REMOVAL OF
THE CAP AND SLIGHT COOLING IN THE MID LEVELS BY THIS EVENING CAPE
VALUES CLIMB TO 1.0 TO 1.5K BY LATE AFTERNOON AND TO 1.5 TO 2.0K
DURING THE EARLY EVENING.

ALL MODELS SHOW INDICATIONS OF A SHORT WAVE TROUGH ROTATING ACROSS
NORTHERN MO AND IA TONIGHT. THIS WILL PROVIDE LARGE SCALE UVV TO
SUSTAIN CONVECTION EXPECTED TO BE ONGOING UPSTREAM THIS AFTERNOON
AND SPREADING OVER THE MID MS VALLEY DURING THE AFTERNOON. WITH
INSTABILITY AND UPPER FORCING CHANCES FOR THUNDERSTORMS WILL
SPREAD OVER MUCH OF FA LATE THIS AFTERNOON AND THIS EVENING...
ESPECIALLY THE NW HALF.

CONVECTIVELY INDUCED VORT MAXIMA ARE LIKELY TO MOVE NE AND E UP
AND OVER THE UPPER RIDGE...CURRENTLY OVER THE MS VALLEY...AS THE
RIDGE SHIFTS GRADUALLY E TO THE WESTERN GREAT LAKES AND THE LOWER
OH VALLEY BY 00Z. HOWEVER...WITH A CAP IN PLACE DURING THE
DAYTIME FORESEE ONLY SLIGHT CHANCES OF STORMS REACHING THE
WESTERN FA BEFORE LATER AFTERNOON.

MODELS USUALLY HAVE TROUBLE WITH IDENTIFYING AND MAINTAINING
MINOR DISTURBANCES ROTATING AROUND AN UPPER LOW...ESPECIALLY WHEN
NUMEROUS STRONG THUNDERSTORM COMPLEXES FORM. GIVEN LARGE
UNCERTAINTY IN WHEN ANY DISTURBANCES WILL BE MOVING OVER THE
LOCAL AREA WILL HAVE TO USE A BROAD BRUSH WITH POP VALUES MON
THROUGH THU AS IDENTIFYING TIMES OF ANY PEAKS AND LULLS IN
CONVECTIVE OCCURRENCE MINIMAL AT BEST IN THIS PATTERN.

CIRRUS FROM PLAINS THUNDERSTORMS HAS BEEN SPREAD E OVERNIGHT BUT
DISSIPATING AS IT REACHES THE UPPER RIDGE AXIS JUST W OF THE FA
AT PREDAWN. EXPECT A GRADUAL INCREASE IN HIGH CLOUDINESS AS THE
DAY PROGRESSES WITH THE RIDGE AXIS MOVING E OF THE FA BY EARLY
EVENING. WITH SUFFICIENT SUN BEFORE THICKER CLOUDS SPREAD E TEMPS
LIKELY TO HIT MID 80S AND POSSIBLY UPPER 80S...EXCEPT FOR ALONG
THE LAKE WHERE THE SE LOW LEVEL WINDS WILL KEEP LAKE COOLED AIR AT
THE IL SHORE.

WITH 850HPA TEMPS PROGGED TO REMAIN AROUND VALUES SEEN TODAY FOR
MON...GIVEN SOME SUN TEMPS ARE LIKELY BE SIMILAR TO TODAY OR EVEN
A DEGREE OR TWO WARMER AS TEMPS TO START OUT IN THE UPPER 60S AT
SUNRISE MON. INCREASING CONVECTION AND SHIFT EASTWARD OF THERMAL
AXIS BY TUE WILL RESULT IN TEMPS IN THE UPPER 70S AND LOWER 80S...
AND CONTINUED COOLING OFF MID AND LATE WEEK. BY FRI WITH THE LARGE
SURFACE RIDGE BUILDING SSE OVER THE AREA AND MUCH COOLER AIR
SPREADING DOWN FROM CENTRAL CANADA HIGHS ARE EXPECTED TO BE IN THE
MID AND UPPER 60S INLAND...WITH AREAS NEAR THE LAKE IN THE 50S AND
LOWER 60S WITH A MODERATELY STRONG ALMOST FULL-FETCH FLOW DOWN THE
LAKE AND ACROSS NW IND AND NE IL.

TRS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* LIGHT EAST TO SOUTHEAST WIND INCREASING TO 10-13KT WITH
  OCCASIONAL GUSTS TO 20KT EARLY THIS AFTERNOON. BECOMING MORE
  SOUTHERLY EARLY THIS EVENING.

* DRY WEATHER SHUD HOLD MUCH OF THE DAY...PSBL TSRA LATE THIS
  EVENING. MAY BRING PATCHY MVFR CIGS WITH PRECIP.

* WINDS DIMINISH OVERNIGHT TO ARND 8-10KT...THEN BREEZY CONDS
  RETURN DURING THE DAY MON. GUSTS COULD APPROACH 20-25KT.

PAW

//DISCUSSION...UPDATED 12Z...

ONGOING CONVECTION THIS MORNING ACROSS EASTERN PORTIONS OF
SOUTHEAST MINNESOTA STRETCHING SOUTH THROUGH EASTERN IOWA. THIS
AREA OF SHOWERS/THUNDERSTORMS WAS STEADILY DISSIPATING AND NOT
EXPECTED TO REACH NORTHERN IL/NORTHWEST IN AIRFIELDS. WITH WINDS
RELATIVELY LIGHT EARLY THIS MORNING AND ADDITIONAL MOISTURE
LIFTING OVERHEAD...VSBYS HAVE BEEN REDUCED SLIGHTLY TO AROUND
5-6SM AT MANY AIRFIELDS. THIS SHUD QUICKLY IMPROVE TO GREATER THAN
6SM BY 14Z. WINDS WILL BE VRB AT 4-6KT...THEN WITH SFC
HEATING...WINDS WILL QUICKLY TURN SE AND INCREASE IN SPEED TO ARND
10-12KT. BETTER MIXING WILL ALLOW OCCASIONAL GUSTS BY MIDDAY TO
20KT FROM A 160-180 DEG RANGE.

A FEW CUMULUS CLOUDS WILL DEVELOP LATER THIS MORNING...WITH BASES
GENERALLY ARND 4-5KFT AGL. COVERAGE SHUD REMAIN IN A SCT
CATEGORY...WITH SLIGHTLY BETTER COVERAGE LATE THIS AFTN. ENOUGH
RIDGING REMAINS IN PLACE TO KEEP PRECIP/THUNDERSTORMS WEST OF THE
AREA THRU SUNSET. GUIDANCE HAS SLOWED THE ARRIVAL OF CONVECTION
UNTIL AFT 06Z...AND THERE ARE SOME INDICATIONS THAT THIS COULD GET
PUSHED BACK UNTIL CLOSER TO DAYBREAK MON. COVERAGE OF
THUNDERSTORMS DOES NOT LOOK TO HIGH AT THIS POINT.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM/HIGH CONFIDENCE IN WIND TREND.
* MEDIUM/HIGH CONFIDENCE AIRFIELDS WILL REMAIN DRY THRU THIS
  EVENING.
* MEDIUM CONFIDENCE THUNDERSTORMS WILL HOLD OFF UNTIL AFT 06Z MON.

PAW

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

MONDAY NIGHT...PERIODS OF TSRA.
TUESDAY...PERIODS OF TSRA.
WEDNESDAY...LIKELY TSRA DURING THE DAY. CHC OF SHRA IN THE EVENING.
THURSDAY...CHC OF SHRA/TSRA EARLY.
FRIDAY THROUGH
SATURDAY...VFR.

BEACHLER

&&

.MARINE...
257 AM CDT

AN AREA OF HIGH PRESSURE CONTINUES TO DRIFT EAST...WITH MUCH LESS
OF AN INFLUENCE OVER LAKE MICHIGAN TODAY. MEANWHILE A WEAK FRONTAL
BOUNDARY WAS BEGINNING TO APPROACH THE SOUTHERN TIP OF THE LAKE. A
DEVELOPING AREA OF LOW PRESSURE CENTERED OVER THE CENTRAL PLAINS
WILL STRUGGLE TO LIFT NORTH TODAY...AS A SECOND AREA OF HIGH
PRESSURE BECOMES CENTERED ACROSS CENTRAL CANADA. ANOTHER FRONTAL
BOUNDARY WILL BEGIN TO DROP SOUTH ACROSS THE NORTHERN PORTION OF
THE LAKE THIS AFTERNOON/EVENING. AS THE CENTRAL PLAINS LOW
PRESSURE STRENGTHENS...THIS WILL AID IN LIFTING A WARM FRONT
ACROSS THE LAKE DURING THE DAY MONDAY. AS A RESULT MUCH WARMER AIR
WILL PUSH OVER THE COLDER LAKE SFC...CREATING A RATHER MODEST
STABLE ENVIRONMENT. THIS WILL POSE A CHALLENCE TO FORECASTING WAVE
HEIGHTS AS WINDS MAY BE BETWEEN 10 TO 20 KT ABOVE THE SFC...BUT
RIGHT AT THE SFC OF THE LAKE IT MAY NOT BE ABLE TO BUILD WAVES.
THIS MAY CONTINUE WAVES AT 2 FT OR LESS FOR A MUCH LONGER PERIOD
OF TIME...HOWEVER HAVE ADJUSTED THE OPEN WATERS FORECAST TO
REFLECT A SLIGHT INCREASE IN WAVES. AS CONFIDENCE IN THE STRENGTH
OF THE STABLE ENVIRONMENT IS LOW AT THIS TIME.

ANOTHER ITEM OF CONCERN IS THE EXPECTED DEVELOPMENT OF PATCHY
DENSE FOG. IN ADDITION TO THE MUCH WARMER AIR LIFTING NORTH ACROSS
THE LAKE...THIS AIRMASS WILL ALSO HAVE AN ABUNDANCE OF MOISTURE.
DEW POINTS WILL BE BETWEEN 5 AND 10 DEG WARMER THAN THE LAKE SFC
TEMPS...WHICH WILL MOST LIKELY RESULT IN PATCHY DENSE FOG. THIS
COULD LINGER THRU THE FIRST HALF OF THE WEEK.

THE LOW PRESSURE WILL BEGIN TO PUSH EAST TOWARDS THE MID
MISSISSIPPI VALLEY...REACHING THE OHIO VALLEY LATE IN THE WEEK.
THIS SHOULD ALLOW WARM FRONT TO SAG TO THE SOUTH DURING THE WEEK. DURING
THIS TIME...THE WIND DIRECTIONS WILL PRIMARILY BECOME EAST
NORTHEASTWARD 15 TO 25 KT NORTH OF THE FRONT. LATER IN THE
WEEK...AS THE SURFACE LOW PASSES EAST OF LAKE MICHIGAN...THE WINDS
LOOK TO BE RATHER STOUT UP TO 25 TO POSSIBLY AS HIGH AS 30 KT OUT
OF THE NORTH FOR A PERIOD AS A GOOD AREA OF HIGH PRESSURE BUILDS
EASTWARD ACROSS ONTARIO AND THE UPPER GREAT LAKES REGION.

BEACHLER

&&

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

$$

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