Forecast-Discussion
000
FXUS63 KLOT 111841
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1241 PM CST THU MAR 11 2010
.DISCUSSION...
340 AM CST
ACTIVE WEATHER PATTERN TO CONTINUE TODAY WITH MAIN FORECAST FOCUS ON
CONVECTIVE POTENTIAL LATER THIS AFTERNOON...HYDROLOGIC ISSUES WITH
ADDITIONAL RAIN AND RUNOFF FROM SNOWMELT AFFECTING AREA RIVERS...AND
THEN THE POTENTIAL FOR ADDITIONAL RAINFALL FRIDAY INTO SATURDAY
NIGHT.
SFC ANALYSIS AT 08Z THIS MORNING INDICATING WEAK SFC WARM FRONT
POSITIONED APPROXIMATELY FROM WEST CENTRAL ILLINOIS EXTENDING
NORTHEAST INTO THE FAR SOUTHERN SUBURBS OF CHICAGO. NORTH OF THIS
FRONT DID HAVE SOME LOWER VSBYS REPORTED AT UGN LAST EVENING WITH
SOUTHEAST WINDS OFF THE LAKE AND COLDER SFC TEMPS IN THE UPPER 30S
TO LOWER 40S. VSBYS HAVE BEEN ON AN IMPROVING TREND OVER PAST FEW
HOURS...AND IN THE 05Z-07Z TIMEFRAME EVEN SAW THESE VIS
IMPROVEMENTS OVER PORTIONS OF SOUTHEAST WISCONSIN...BUT LATEST
OBSERVATIONS INDICATE KENW BACK DOWN TO 1/2SM. GIVEN THESE LOWER
TRENDS ONCE AGAIN IN SOUTHEAST WISCONSIN...HAVE SOME HESITATION OF
CANCELLING THE DENSE FOG ADVISORY FOR MCHENRY/LAKE COUNTIES WHICH
IS SET TO EXPIRE AT 15Z. WILL MONITOR FOR TRENDS OVER NEXT
HOUR...BUT CURRENT THINKING IS THAT IT IS PROBABLY WISER TO ALLOW
THIS ADVISORY TO RUN THROUGH 15Z.
REGIONAL RADAR MOSAIC EARLY THIS MORNING INDICATING LARGE AREA OF
RAIN EXTENDING FROM SOUTHEAST IOWA INTO THE MID MS RIVER VALLEY.
WELL DEFINED UPPER LEVEL SHORT WAVE TROUGH ACROSS NORTHERN MISSOURI
INTO WEST CENTRAL ILLINOIS WILL ROTATE CYCLONICALLY BACK TO THE
NORTHWEST AROUND PARENT UPPER LOW ACROSS NORTHWEST MISSOURI WHICH
WILL ALLOW RAIN AND SOME EMBEDDED THUNDER TO LIFT NORTHWARD
THROUGH APPROXIMATELY SOUTHWESTERN HALF OF CWA EARLY THIS
MORNING...AND THEN ACROSS REMAINDER OF NORTHEAST
ILLINOIS/NORTHWEST INDIANA LATE THIS MORNING INTO THIS AFTERNOON.
BETTER POTENTIAL OF THUNDER THIS MORNING WILL LIKELY REMAIN ACROSS
WESTERN HALF OF AREA OUT TOWARD INTERSTATE 39 CORRIDOR WHERE WEAK
INSTABILITY AXIS WILL BE RESIDING. SYSTEM WILL BECOME VERTICALLY
STACKED TODAY...WITH SFC OCCLUDED FRONT PERHAPS WITH A SLIGHTLY
SLOWER PROGRESSION IN LATEST NAM/GFS GUIDANCE...ONLY REACHING INTO
FAR SOUTHWEST PORTIONS OF FORECAST AREA BY LATE THIS AFTERNOON.
INSPECTION OF WATER VAPOR IMAGERY EARLY THIS MORNING DOES SUGGEST
ANOTHER UPSTREAM IMPULSE THAT WILL NEED WATCHING TRACKING THROUGH
THE SOUTHERN PLAINS THAT SHOULD PUSH INTO CENTRAL ILLINOIS LATE
THIS AFTERNOON/EARLY THIS EVENING. IN TERMS OF SEVERE THREAT...IT
STILL APPEARS THIS BOUNDARY WILL SERVE AS THE MAIN FOCUS FOR ANY
POTENTIAL THREAT. WITH SKINNY CAPE PROFILE...THERE WILL BE A HIGH
SENSITIVITY TO THERMODYNAMICS IN THE LOWEST LEVELS...AND OF COURSE
A POTENTIAL OF MORE SFC BASED INSTABILITY IF SOME SUN CAN BE
REALIZED THIS AFTERNOON WITH DRIER MID LEVEL PUNCH WORKING INTO
WESTERN PORTIONS OF THE AREA. SLIGHTLY BACKED LOW LEVEL FLOW
WILL LIKELY EXIST JUST TO THE EAST OF SFC OCCLUDED FRONT THIS
AFTERNOON. HOWEVER...MORE PRONOUNCED SHEAR PROFILE WILL LIKELY
EXIST THIS MORNING INTO EARLY THIS AFTERNOON...AND DOES APPEAR TO
SLACKEN OFF A BUT DURING MAX INSTABILITY WHICH IS ONE FACTOR
LIMITING SEVERE THREAT. STILL CANNOT COMPLETELY RULE OUT BRIEF
TORNADO THREAT GIVEN ABOVE FACTORS...BUT THREAT DOES APPEAR TO BE
LOWER END AT THIS TIME AND IN HWO WILL PROBABLY JUST LIMIT MENTION
TO SMALL HAIL THREAT ACROSS THE AREA. ALSO POSSIBLY SOME ISOLATED
GUSTINESS WITH CONVECTION THIS AFTERNOON IF SURFACE BASED
INSTABILITY CAN BE REALIZED ACROSS SOUTHERN PORTIONS OF CWA. DID
KEEP IDEA OF THUNDER MENTION FOR EASTERN HALF OF AREA GOING INTO
THIS EVENING WITH NORTHWEST TO SOUTHEAST INSTABILITY AXIS SHIFTING
EASTWARD. IN TERMS OF TEMPERATURES TODAY...MADE ONLY VERY MINIMAL
TWEAKS TO INHERITED TEMPS...WITH MID TO UPPER 50S NORTH TO LOWER
TO MID 60S SOUTH. A COUPLE OF CAVEATS...THE FIRST BEING THE
CONTINUED SLIGHTLY COOLER LAKE SIDE TEMPERATURES ACROSS NORTHERN
CHICAGO SUBURB NORTHSHORE AREAS...AND IF SOME SUN IS REALIZED
ACROSS FAR SOUTHWEST...COULD SEE FAIRLY NARROW AXIS OF EVEN WARMER
SFC TEMPS JUST AHEAD OF THE OCCLUDED BOUNDARY.
IN TERMS OF HYDROLOGY...SITUATION TODAY AND TONIGHT IS COMPLICATED
BY THE LIKELY NONUNIFORM PRECIPITATION DISTRIBUTION DUE TO
CONVECTIVE NATURE OF PRECIP ESPECIALLY LATER TODAY. STILL APPEARS
GREATEST THREAT AREA FOR RIVER FLOODING NEXT FEW DAYS WILL BE AREAS
WHICH HAVE EXPERIENCED GREATER RUNOFF DUE TO RECENT
SNOWMELT...ACROSS THE UPPER ROCK...FOX...AND DES PLAINES
WATERSHEDS. AFTER INITIAL RAIN BAND MOVES THROUGH THIS MORNING WITH
THE STRONGEST SYNOPTIC FORCING ACROSS THESE WATERSHEDS...THE
CONFIDENCE IN HEAVIER CONVECTIVE PRECIPITATION THIS AFTERNOON IS
MUCH LOWER ACROSS THESE AREAS. THEREFORE...HAVE HELD OFF ON ANY
FLOOD HEADLINES AT THIS TIME...AND WILL CONTINUE TO COVER THE
THREAT IN THE RIVER FLOOD WARNINGS THAT ARE CURRENTLY IN EFFECT.
FOR LATER THIS EVENING...DRY SLOT EXPECTED TO OVERSPREAD THE
AREA...WITH MAIN PRECIP FOCUS SHIFTING NORTH AND EAST OF THE AREA
WITH MOISTURE BECOMING MORE SHALLOW ONCE AGAIN. WEAK SFC RIDGING
WILL BUILD INTO THE AREA BEHIND OCCLUDED FRONT TONIGHT ALLOWING FOR
WEAK LOW LEVEL WIND PROFILE. PLENTY OF LOW LEVEL MOISTURE LIKELY
LINGERING AND FOG WILL ONCE AGAIN BE POSSIBLE. DID ADD THIS INTO
THE FORECAST FOR OVERNIGHT HOURS BUT HELD OFF ON DENSE FOG MENTION
AT THIS TIME.
AN UNSETTLED PATTERN TO CONTINUE THROUGH THE WEEKEND...AS YET
ANOTHER SHORT WAVE ACROSS FOUR CORNERS REGION WILL GET INGESTED INTO
UPPER LOW AND WRAP BACK INTO THE AREA. TOUGH TO EXACTLY PINPOINT
EXACT TRACK OF THIS DISTURBANCE...BUT MUCH OF FRIDAY COULD
ACTUALLY BE DRY BEFORE THIS FORCING WRAPS BACK INTO THE AREA LATE
IN THE DAY OR EARLY EVENING. RAIN BECOMES MORE LIKELY AGAIN LATER
FRIDAY INTO AT LEAST SATURDAY NIGHT AS UPPER LOW WOBBLES ACROSS
THE AREA WITH ADDITIONAL VORTS WRAPPING BACK INTO NORTHERN
ILLINOIS. SOME DECENT MOISTURE ADVECTION WILL ACCOMPANY THESE
WAVES AND COULD LEAD TO SOME HEALTHY TOTAL RAINFALL AMOUNTS FOR
FRI-SAT...POSSIBLY IN EXCESS OF HALF AN INCH...SO WILL HAVE TO
MONITOR THIS PERIOD FOR ADDITIONAL RIVER FLOODING THREATS. UPPER
LOW TO FINALLY MOVE OUT OF THE AREA SUNDAY WITH UPPER RIDGING
BUILDING ACROSS UPPER MIDWEST INTO NORTHERN GREAT LAKES PROVIDING
SOME DRY WEATHER FOR LATE WEEKEND/EARLY NEXT WEEK. THE GOOD NEWS
IS THAT THIS RIDGING BUILDING IN FOR MIDDLE OF NEXT WEEK SHOULD
PREVENT ANY SIGNIFICANT COLD AIR INTRUSIONS AND KEEP TEMPERATURES
AT OR ABOVE NORMAL.
MARSILI
&&
.AVIATION...
1800 UTC TAFS...THE IFR CEILINGS WERE WITH THE SHOWERS AND
ISOLATED THUNDERSTORMS. THOSE CLOUDS HAVE MOVED NORTH. WE EXPECT
MVFR CLOUDS AND POSSIBLY VFR THIS AFTERNOON. THERE MAY BE SOME
AFTERNOON HEATING AS THE SUN BREAKS THROUGH THE CLOUDS. THIS WILL
CREATE ENOUGH INSTABILITY FOR THUNDERSTORMS THIS LATE AFTERNOON
AND EVENING. WE WILL ADD THEM TO THE FORECAST AT THIS TIME AND
THEN UPDATE AT 21 UTC. MORE LOW CLOUDS MAY MOVE INTO NORTHERN
ILLINOIS LATER TONIGHT. WE WILL FORECAST MVFR CLOUDS AT 1200 TO
1500 BROKEN. THE SURFACE LOW WILL REMAIN IN IOWA THROUGH FRIDAY
MORNING. THIS WILL PRODUCE A CIRCULATION WHERE BANDS OF CLOUDS
WILL MOVE THROUGH NORTHERN ILLINOIS AND NORTHWEST INDIANA TONIGHT.
&&
.MARINE...
255 AM CST
NOT MUCH CHANGE IN MARINE CONCERNS FOR THIS FORECAST PERIOD. LOW
PRESSURE LIFTING FROM MISSOURI IN TO IOWA TODAY AND DEEPENING
SLIGHTLY WILL TIGHTEN GRADIENT AND INCREASE EAST/NORTHEAST WINDS
ACROSS LAKE TODAY...BEFORE LOW WEAKENS/FILLS AND ALLOWS GRADIENT
TO RELAX ACROSS THE SOUTHERN PARTS OF THE LAKE TONIGHT. WEAKENING
LOW THEN GETS ABSORBED BY NEW AREA OF LOW PRESSURE DEVELOPING OVER
OHIO VALLEY ON FRIDAY...WITH NEW CENTER FORMING ACROSS EAST
CENTRAL IL/IN BY FRIDAY EVENING. THIS WILL SET UP RATHER PROLONGED
PERIOD OF EAST/NORTHEAST FLOW WITH STRONGEST GRADIENT GENERALLY
ACROSS NORTHERN 2/3 OF LAKE INTO THE WEEKEND. LOW WILL THEN PULL
OFF TO THE EAST ALLOWING WINDS TO DIMINISH GRADUALLY
SUNDAY/MONDAY. WHILE SOME GUSTS COULD APPROACH 30 KTS ON THE NORTH
END OF THE LAKE...OVERALL GUIDANCE CONTINUES TO SUPPORT MORE OF A
15-25 OR 20-25 KT SUSTAINED WIND FORECAST. NOT MUCH COLD AIR
COMING IN BEHIND DEPARTING LOW...WHICH ALSO FAVORS KEEPING 25 KTS
AT THIS TIME.
OTHER CONCERN HAS BEEN DENSE FOG ESPECIALLY OVER CENTRAL LAKE
MICHIGAN IN MOISTURE GRADIENT NORTH OF WARM FRONT. THIS LIKELY TO
REMAIN A PROBLEM THROUGH THIS MORNING...AND AGAIN TONIGHT...BEFORE
DRIER NORTHEAST FLOW REALLY TAKES HOLD ON FRIDAY.
RATZER
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&
$$
NWS LOT Office Area Forecast Discussion
