HAMweather.com Area Forecast Discussion for County Warning Area LOT

Thu, 16 Feb 2012 07:06:00 GMT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1109 PM CST WED FEB 15 2012

.DISCUSSION...
823 PM CST

THERE WERE SOME SLIGHT ADJUSTMENTS MADE TO THE GOING FORECAST BUT
THE GENERAL THEME OF A WET NIGHT WITH TEMPERATURES DROPPING AT A
SNAILS PACE STILL LOOKS GOOD.

AFTER SOME DRIER AIR ALOFT AND LIMITED FOCUS...RAIN HAS SPREAD
OVER A LARGE PORTION OF THE AREA THIS EVENING. THIS APPEARS TO BE
THANKS IN PART TO A LEAD SHORT WAVE PER WV/PROFILER ANALYSIS AND
ASSOCIATED BETTER 850-700MB CONVERGENCE THAT LINES UP WELL WITH
RADAR RETURNS. GIVEN THE RUC ANALYSIS OF THIS FIELD...TRENDS ON
RADAR...AND HIGH AVAILABLE MOISTURE /BASICALLY 200 PERCENT OF
NORMAL PWATS ON DVN AND ILX RAOBS/...THE RAIN AXIS SHOULD
CONTINUE FOR SEVERAL HOURS WHILE GRADUALLY EXPANDING/EVOLVING
SOUTHEAST AS FORCING FROM THE MAIN UPPER LOW APPROACHES. THIS WAVE
STILL HAS A CLOSED CIRCULATION ON WATER VAPOR AND IS TRIGGERING
CONVECTION ACROSS WESTERN MO. BUT THIS IS FORECAST TO DAMPEN IN
THE MORE CONFLUENT FLOW...LIMITING MORE INTENSE FORCING AND
DYNAMIC COOLING EFFECTS. SO WITH MORE LIMITED FORCING HAVE ADDED
DRIZZLE AND FOG MENTION IN THE FORECAST...ESPECIALLY OVERNIGHT. 
THE LOW T/TD SPREADS AND DECREASING FLOW WITH THE BAGGY SURFACE 
TROUGH MOVING ACROSS SHOULD FURTHER AID IN SOME AREAS OF LIGHT
FOG.

IT STILL IS POSSIBLE TO HAVE SOME WET NON-ACCUMULATING SNOW
TOWARDS THE NORTHWEST FORECAST AREA LATE TONIGHT AND MAYBE A BRIEF
MIX TOWARDS CHICAGO AS THE SYSTEM DEPARTS THU MORNING. WHILE THE
DVN RAOB DID COME IN AROUND 1-2C COOLER IN THE LOWER PROFILE THAN
THE 00Z RUC AND 18Z NAM...IT STILL DEPICTED NEARLY A 4500 FT THICK
LAYER NEAR OR ABOVE FREEZING. IN THE ABSENCE OF STRONG COLD AIR
ADVECTION...A DECREASING CHANCE FOR SNOW IN THE FORECAST
CONTINUES. AS FOR THUNDER...SOME WEAK POSITIVE INSTABILITY IS
PRESENT ON THE ILX SOUNDING WHEN LIFTING PARCELS FROM 850 MB...BUT
THE BETTER ELEVATED INSTABILITY GRADIENT WHICH LINES UP WITH
ONGOING CONVECTION IN MO LOOKS TO STAY TO OUR SOUTH. POCKETS OF
ISOLATED HEAVIER RAIN HAVE BEEN SEEN PER DUAL-POL ANALYSIS. GOING
QPF FORECAST LOOKS ALRIGHT ON MAGNITUDE...BUT DID SHRINK THE AREAL
COVERAGE OF THE FORECAST ONE QUARTER OF AN INCH AND GREATER.

MTF

//PREV DISCUSSION...

317 PM...PRIMARY FORECAST FOCUS REMAINS APPROACHING SYSTEM WITH
MAINLY RAIN/SHOWERS TONIGHT.

AFTER A BRIEF PERIOD OF SOME SUN THIS MORNING...LOW CLOUDS SPREAD 
NORTH AND DEVELOPED ACROSS THE AREA ALONG WITH SOME SHOWERS MIDDAY 
WHICH HAVE SINCE MOVED OVER THE LAKE. FAIRLY STRONG UPPER WAVE TO
MOVE ACROSS THE AREA TONIGHT...THOUGH IT SLOWLY WEAKENS WITH TIME.
SCATTERED SHOWERS CURRENTLY DEVELOPING FROM KANSAS INTO SOUTHERN
IOWA AND THIS ACTIVITY WILL CONTINUE NORTHEAST ACROSS THE AREA
THIS EVENING INTO EARLY THURSDAY MORNING. LATEST GUIDANCE
SUGGESTS A TREND OF THE HEAVIEST RAIN...OR THE POTENTIAL FOR THE
HEAVIEST RAIN...FALLING ACROSS THE SOUTHERN CWA AND POINTS SOUTH
AND EAST. SO HAVE BACKED OFF ON QPF AMOUNTS SOME ESPECIALLY NORTH
WITH STRONGER CONVECTION FURTHER SOUTH TAKING SOME OF THE
AVAILABLE MOISTURE/QPF. THOUGH SINCE THERE COULD STILL BE SOME
CONVECTIVE ELEMENTS...LOCALIZED RAINFALL TOTALS OF A HALF TO THREE
QUARTERS OF AN INCH ARE STILL POSSIBLE. THERE IS ALSO SOME SMALL
INSTABILITY ALOFT THAT MAY CAUSE A RUMBLE OR TWO OF THUNDER...
MAINLY ACROSS THE FAR SOUTHERN CWA AND WILL CONTINUE WITH NO
MENTION OF THUNDER.

AS FOR ANY SNOW POTENTIAL...NOT MUCH COLD AIR TO WORK WITH THIS
SYSTEM AND DEW POINTS ALREADY CREEPING UP INTO THE MID 30S ACROSS
NORTHERN IL. ANY SNOW WOULD LIKELY RESULT FROM PRECIP COOLING THE
COLUMN ON THE BACK SIDE OF THE PRECIP LATE TONIGHT INTO EARLY
THURSDAY MORNING...RATHER THAN SHARP COLD AIR ADVECTION. AND BY
THE TIME THERE IS ENOUGH COLD AIR FOR PRECIP TO CHANGEOVER TO
SNOW...IT APPEARS PRECIP WILL BE ENDING. THUS HAVE MAINTAINED A
CHANCE OF SNOW ACROSS THE FAR NORTHWEST CWA OVERNIGHT AND THEN A
CHANCE OF RAIN OR SNOW SHOWERS MIXED THURSDAY MORNING...BUT EVEN
IF PRECIP WERE TO GO OVER TO SNOW...GROUND WOULD LIKELY BE WARM
ENOUGH AND WET ENOUGH THAT LITTLE OR NO ACCUMULATION WOULD RESULT.
AND AS NOTED...HUNG ONTO SLIGHT/CHANCE POPS THURSDAY MORNING BUT
ITS ALSO POSSIBLE THAT PRECIP MAY END BY SUNRISE MOST AREAS. IN
ADDITION...IF HEAVIER RAIN/SHOWERS DO END UP FURTHER SOUTH...
PRECIP MIGHT END UP BEING MORE DRIZZLE.

TEMPERATURES CURRENTLY IN THE LOWER 40S WILL SLOWLY FALL INTO THE
MID/UPPER 30S BUT FOR THE MOST PART WILL BE STEADY...PROBABLY NOT
DROPPING SIGNIFICANTLY UNTIL WIDESPREAD PRECIP BEGINS FALLING.
LOWS ACROSS THE NORTHWEST MAY DIP DOWN TO FREEZING BY MORNING BUT
MOST OF THE CWA LIKELY TO STAY JUST ABOVE FREEZING THROUGH
MORNING. CLOUDS BEGIN CLEARING BY LATE THURSDAY MORNING AND WITH
SOME SUN IN THE AFTERNOON...SHOULD SEE TEMPS WARM INTO THE LOWER
40S THURSDAY.

A RIDGE OF HIGH PRESSURE SLIDES BY THURSDAY NIGHT INTO FRIDAY WITH
SOUTHERLY FLOW RETURNING FRIDAY AFTERNOON. DEPENDING ON THE EXTENT
OF INCREASING CLOUDS FRIDAY AFTERNOON...TEMPS WARMING BACK INTO
THE MID/UPPER 40S SEEMS REASONABLE FROM THIS DISTANCE. MODELS THEN
BRING ACROSS ANOTHER WEAK SURFACE LOW FRIDAY NIGHT. APPEARS THE
BULK OF THE PRECIP WITH THIS SYSTEM WILL REMAIN ON THE NORTH SIDE
OF THIS LOW AND NORTH OF THE CWA...BUT A FEW RAIN OR SNOW SHOWERS
APPEAR POSSIBLE ALONG THE TRAILING COLD FRONT LATE FRIDAY NIGHT
INTO SATURDAY MORNING AND INTRODUCED LOW CHANCE POPS DURING THIS
TIME PERIOD. THIS COLD FRONT USHERS IN SOME COLDER AIR...THOUGH
THE MODELS DIFFER ON THE EXTENT OF JUST HOW COLD THIS AIR WILL
BE. DOESN/T APPEAR TOO COLD BUT HIGHS THIS WEEKEND LIKELY ONLY IN
THE 30S.

NEXT WEEK LOOKING BOTH UNSETTLED AND UNCERTAIN AS A POSSIBLE COLD
FRONT MOVES THROUGH THE AREA TUESDAY WITH A STRONGER SYSTEM
LIFTING NORTHEAST ACROSS THE OHIO VALLEY/SOUTHERN LAKES REGION
TOWARD MIDWEEK. CMS

.AVIATION...

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

 * IFR/LIFR CEILINGS AND VISIBILITIES TONIGHT.

 * ENDING TIME OF -RA AND POTENTIAL FOR -DZ OVERNIGHT.

 * TIMING OF RETURN TO VFR THU.

 * GUSTY NW WIND DEVELOPING THU. 

TRS

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

SYSTEM MOVING OVER THE MIDWEST IS RATHER DISORGANIZED BASED ON
CLOUD TOP TEMPS SEEN IR SATELLITE AND ON RADAR LOOPS. EVEN THOUGH
A WELL DEFINED UPPER LOW IS STILL SEEN OVER SW MO THERE APPEARS
TO BE MULTIPLE SMALLER SCALE IMPULSES EMBEDDED WITHIN THE OVERALL
LARGE SCALE FLOW.

MID LEVEL TROUGH IS CURRENTLY MOVING OVER SOUTHERN WI AND
NORTHERN IL. TROUGH AXIS AND TRAILING EDGE OF RAIN CURRENTLY OVER
THE AREA EXPECTED TO BE E OF CHI VICINITY BY 07Z BASED ON LATEST
MODEL PROGS AND EXTRAPOLATION OF RECENT RADAR IMAGES.
HOWEVER...ANOTHER AREA OF RAIN CURRENTLY OVER CENTRAL AND NE MO
INTO W CENTRAL IL WITH SOME DEVELOPMENT A BIT FURTHER DOWNSTREAM
TOWARDS N CENTRAL IL AHEAD OF THE UPPER LEVEL TROUGH. APPEARS THAT
MAIN PORTION OF THIS AREA WILL PASS JUST S OF THE ORD AND MDW
THOUGH A COUPLE SHOWERS STILL POSSIBLE TIL AXIS OF UPPER TROUGH
MOVES E AROUND 10Z. 

BETWEEN RAINFALL DURING THE EVENING AND CONTINUED WEAK LOW LEVEL
MOISTURE ADVECTION ANTICIPATE IFR CONDITIONS TO PREVAIL EVEN AFTER
THE WEAK SURFACE TROUGH PASSES THROUGH AS HIGHER DEW POINTS LAG A
BIT BEHIND THE TROUGH AND THERE IS ONLY A WEAK PUSH OF THE DRIER
AIR INTO THE AREA OVERNIGHT. MID AND UPPER TROUGHS PASS TO THE E
OVERNIGHT BUT LOWER LEVELS REMAIN SATURATED AND MAY SEE SOME
DRIZZLE DEVELOP DURING THE OVERNIGHT HOURS AS WEAK COLD AIR
ADVECTION IN THE LOWER LEVELS BEGINS. 

BY DAYBREAK A DRYING NW LOW LEVEL FLOW IS TO HAVE BECOME MORE 
ESTABLISHED IN THE WAKE OF THE SURFACE TROUGH. MODEL GUIDANCE
SHOWS QUICK CLEARING OUT OF THE REMAINING LOW LEVEL MOISTURE BY
MIDDAY THU SO EXPECT A QUICK JUMP FROM IFR CEILINGS TO PRETTY MUCH
SKC THEREAFTER THOUGH MAY A FEW STRATOCU AT LEAS INTO MID
AFTERNOON.

A FAIRLY STRONG SURFACE RIDGE IS TO BE BUILDING SE ACROSS THE
PLAINS LATE TONIGHT AND DURING THU RESULTING A MODERATELY STRONG
WNW-NW LOW LEVEL FLOW OVER THE LOCAL AREA BY THE TIME THE CLOUDS
CLEAR OUT. WITH SURFACE HEATING MIXED LAYER PROGGED TO DEEPEN TO
1-1.5KM WHICH WOULD ALLOW MOMENTUM TRANSFER OF 20-25KT WINDS AT
THIS LEVEL MIXING DOWN TO THE SURFACE RESULTING IN SOME GUSTINESS
FROM MIDDAY UNTIL SUNSET THU. 

TRS

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

 * HIGH CONFIDENCE IN CONTINUED IFR REST OF NIGHT.

 * HIGH CONFIDENCE IN CONTINUED LIGHT WINDS REST OF NIGHT.

 * HIGH CONFIDENCE IN INCREASING NW WINDS DURING THU MORNING.

 * MODERATE CONFIDENCE IN ENDING TIME OF -RA.

 * MODERATE CONFIDENCE IN OCCURRENCE OF -DZ OVERNIGHT INTO PRE-
   DAWN THU.

 * MODERATE CONFIDENCE IN RAPID IMPROVEMENT FROM IFR DURING LATER
   THU MORNING.

TRS

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

FRIDAY...VFR. 

SATURDAY...POSSIBLE MVFR CIGS.

SUNDAY...VFR.

MONDAY...VFR.

TUESDAY...CHANCE RA AND/OR SN WITH MVFR. 

TRS

.MARINE...
203 PM CST

MAIN MARINE FORECAST CONCERNS ARE THE POTENTIAL FOR A COUPLE OF 
PERIODS OF 20-25 KT WINDS OVER THE NEXT FEW DAYS...THURSDAY INTO 
THURSDAY NIGHT AND AGAIN SATURDAY INTO SATURDAY NIGHT.

LOW PRESSURE WAS OVER THE SOUTHERN PLAINS OF OKLAHOMA THIS 
AFTERNOON...WITH A WEAKER TROUGH EXTENDING THROUGH THE UPPER MIDWEST 
TO ANOTHER LOW OVER ONTARIO. THE PLAINS LOW WILL WEAKEN AS IT MOVES 
EAST ACROSS ILLINOIS AND INDIANA INTO SOUTHWESTERN LOWER MICHIGAN BY 
THURSDAY MORNING...THEN WILL COMBINE WITH THE ONTARIO LOW AS THEY 
MOVE EAST OF THE LAKES...EVENTUALLY DEEPENING OVER SOUTHWEST QUEBEC 
BY THURSDAY EVENING. ONCE THE LOW AND TROUGH AXIS MOVE ACROSS LAKE 
MICHIGAN TONIGHT...WINDS WILL SHIFT TO THE WEST AND INCREASE INTO 
THE 15 TO 25 KT RANGE THURSDAY AS THE LOW DEEPENS OFF TO THE EAST. 
WINDS WILL GRADUALLY DIMINISH THURSDAY NIGHT...AS HIGH PRESSURE 
SLIDES INTO THE LOWER OHIO VALLEY AND WEAKER RIDGING EXTENDS NORTH 
INTO THE WESTERN LAKES FRIDAY MORNING. ANOTHER WEAK LOW WILL FOLLOW 
ACROSS LAKE MICHIGAN LATE FRIDAY NIGHT AND SATURDAY...WITH NORTHERLY 
WINDS PICKING UP AGAIN INTO THE 15 TO 25 KT RANGE SATURDAY AND 
SATURDAY NIGHT AS THE GRADIENT TIGHTENS FOR A TIME BETWEEN THE 
DEPARTING LOW TO THE EAST AND HIGH PRESSURE SPREADING ACROSS THE 
UPPER MIDWEST. THIS HIGH WILL SLOWLY SPREAD EAST ACROSS THE LOWER 
OHIO VALLEY AND WESTERN LAKES BY SUNDAY NIGHT...ALLOWING WINDS TO 
DIMINISH AHEAD OF THE RIDGE ON SUNDAY.

RATZER

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