845 PM CDT
LAST LITTLE SHOWER CONTINUES TO FADE OUT ACROSS NORTHERN LAKE
COUNTY INDIANA AT THIS HOUR WITH A FEW LINGERING SHOWERS ACROSS
SOUTHERN WISCONSIN AND OUT OVER THE LAKE TO THE NORTH AND EAST. CLOUD
COVER HAS DIMINISHED ACROSS A GOOD CHUNK OF THE NORTHERN AND
WESTERN CWA WITH HIGH CIRRUS STILL LINGERING ACROSS THE FAR
SOUTHERN CWA FROM PONTIAC TO RENSSELAER. ANOTHER AREA OF CIRRUS IS
RACING EASTWARD ACROSS CENTRAL AND WESTERN IOWA AND WILL BE
OVERHEAD BY LATE EVENING. THIS WILL LIKELY BRING A PERIOD OF
STEADIER TEMPS EARLY TONIGHT BUT EXPECT THE CLOUDS TO MOVE OFF TO
THE EAST BY EARLY MORNING AS IT THINS. THEREFORE SHOULD SEE A DROP
OFF IN TEMPS PRIOR TO DAYBREAK. HAVE TWEAKED HOURLY TEMP TRENDS TO
ACCOUNT FOR THIS AS WELL AS ERRATIC TEMPS CURRENTLY IN PLACE
THANKS TO THE EARLIER SHOWER ACTIVITY. WILL MAINTAIN THE FROST
ADVISORY BUT LOWER TEMPS IN THE TYPICALLY COOL ROCHELLE TO AURORA
CORRIDOR BY A FEW DEGREES.
MDB
//PREV DISCUSSION...
327 PM CDT
MAIN CONCERN FOR THE REMAINDER OF THE AFTERNOON AND INTO THE
EVENING WILL FOCUS OF THE SCATTERED SHOWERS THAT DEVELOPED THIS
AFTERNOON. AFTER SKIES CLEARED OUT EARLIER TODAY...SUNSHINE AND
COLD AIR ALOFT GENERATED VERY STEEP LAPSE RATES...QUICKLY
TRIGGERING SCATTERED SHOWERS OVER NRN IL. GIVEN THE STRONG
INSTABILITY AND COLD AIR ALOFT...OCCASIONAL GRAUPEL WILL BE
POSSIBLE WITH THE STRONGER SHOWERS. SHOWERS SHOULD DISSIPATE
AROUND SUNSET.
AFTER SUNSET...AS SKIES CLEAR OUT AND SFC HIGH PRESSURE BUILDS
ACROSS THE CWA...WINDS WILL QUICKLY BECOME LIGHT AND VARIABLE.
WITH A DRY AIR MASS IN PLACE...AND DEW POINTS IN THE LOW
30S...TEMPERATURES ACROSS MUCH OF THE AREA WILL ALSO DROP INTO
THE LOWER 30S. THUS...FROST IS LIKELY TONIGHT FOR MUCH OF THE
CWA. IMMEDIATE LAKEFRONT AND LOCATIONS AWAY FROM HEAVILY
URBANIZED AREAS WILL BE MOST PRONE TO FROST.
BEGINNING TOMORROW...THE REGION WILL BE IMPACTED BY A SERIES OF
NORTHERN STREAM SHORTWAVES WHICH WILL BRING PERIODS SHOWERS TO AT
LEAST PORTIONS OF THE CWA OVER THE NEXT SEVERAL DAYS.
THE FIRST IN THE SERIES OF THESE SYSTEMS WILL TRACK ACROSS
SOUTHERN CANADA...DRAGGING A WEAK COLD FRONT ACROSS THE AREA
FRIDAY AFTERNOON AND EVENING. WITH THE STRONGEST FORCING LIKELY
TO BE WELL TO THE NORTH...THE NORTHERN TIER COUNTIES WILL BE MOST
SUSCEPTIBLE TO ANY PCPN. BY FRIDAY NIGHT...THE FOCUS OF PCPN WILL
TRANSITION TO LAKE EFFECT RAIN SHOWERS FOR LAKE AND PORTER
COUNTIES IN NWRN INDIANA AND OVER LAKE MICHIGAN. AS THE COLDEST
AIR ASSOCIATED AN UPPER TROUGH MOVES ACROSS THE LAKE FRIDAY
NIGHT...THERE WILL ONCE AGAIN BE A SLIGHT CHANCE FOR SOME THUNDER
TO ACCOMPANY THE HEAVIER LAKE EFFECT SHOWERS. BY SATURDAY
MORNING...THE UPPER TROUGH WILL SHIFT OFF TO THE EAST AS A RIDGE
BUILDS ACROSS THE MISSISSIPPI VALLEY AND INTO THE UPPER MIDWEST.
EVEN COOLER AIR WILL OVERSPREAD THE AREA BY SATURDAY MORNING.
TEMPERATURES ARE EXPECTED TO DROP INTO THE UPPER 20S TO LOWER 30S
ACROSS ALL OF THE CWA...WITH THE EXCEPTION OF THE IMMEDIATE LAKE
FRONT AND DOWNTOWN CHICAGO. LOCATIONS WEST OF THE FOX VALLEY AND
NORTH OF I-80 MAY SEE TEMPERATURES DROP INTO THE UPPER 30S...SO A
FREEZE WARNING MAY BE REQUIRED FOR THE NORTHWESTERN PORTIONS OF
THE CWA. THE REMAINDER OF SATURDAY AND INTO SATURDAY NIGHT SHOULD
BE QUIET. THE NEXT UPSTREAM SYSTEM SHOULD MOVE ACROSS THE UPPER
MISSISSIPPI VALLEY BY SUNDAY MORNING. THE TRACK OF THIS SYSTEM
SHOULD BE MORE SOUTHERLY THAN THE PREVIOUS...WITH THE MAIN SFC LOW
TRACKING ALONG THE WISCONSIN BORDER. THUS SHOWERS SHOULD BE MORE
WIDESPREAD WITH GRAUPEL LIKELY AGAIN SUNDAY AND SUNDAY NIGHT.
MONDAY SHOULD BE QUIET...BUT ANOTHER SYSTEM IS BEING ADVERTISED
BY THE LONGER RANGE MODELS FOR THE MIDDLE OF NEXT WEEK. WHILE THE
MODELS ARE IN RELATIVELY GOOD AGREEMENT ON TRACK...TIMING AND
STRENGTH OF THE FIRST TWO SYSTEMS...THE GFS AND ECMWF ARE ABOUT
12-18 HOURS OUT OF PHASE AND THE ECMWF HAS MUCH HIGHER AMPLITUDE
TO THE UPPER TROUGH. IN THE ABSENCE OF A GOOD MODEL
CONSENSUS...WILL ONLY CARRY CHANCE POPS. WHILE THERE CERTAINLY
SHOULD BE MORE PCPN FOR MIDWEEK...THE TIMING IS HIGHLY
QUESTIONABLE.
KREIN
&&
.AVIATION...
//ORD AND MDW CONCERNS...UPDATED 08Z...
* POTENTIAL FOR IFR VSBYS IN PATCHY FOG EARLY AM.
* CHANCE OF CONVECTIVE SHOWERS VERY LATE AFTN/EVENING.
RATZER
//DISCUSSION...UPDATED 08Z...
FOG REMAINS QUITE PATCHY ACROSS THE AREA AT 08Z. VSBYS WHICH HAD
DROPPED AS LOW AS 3/4SM AT PWK EARLIER HAVE IMPROVED TO 7SM
BENEATH HIGH CLOUDS. STILL APPEARS THAT CIRRUS DECK WILL MOVE OFF
TO THE EAST OF THE AREA TOWARD SUNRISE ALLOWING STRONGER RADIATIVE
COOLING AND DECREASING TEMPERATURE-DEW POINT SPREADS...THOUGH AM
EVEN LESS CONFIDENT THAN EARLIER ABOUT FOG DEVELOPMENT AND IMPACTS
ON ORD/MDW/GYY TERMINALS. THEREFORE HAVE GONE A LITTLE MORE
OPTIMISTIC WITH 08Z AMENDMENTS.
RATZER
PREVIOUS DISCUSSION FROM 06Z...
MAIN CONCERN FOR TERMINALS THIS MORNING IS WITH THE POTENTIAL FOR
PATCHY DENSE FOG TO DEVELOP...MAINLY AT ORD/MDW/GYY WHERE THURSDAYS
AFTERNOON AND EVENING WET THE GROUND AND VERY WEAK SURFACE PRESSURE
GRADIENT IS NOW RESULTING IN CALM WINDS. BROKEN CIRRUS DECK
CURRENTLY OVERHEAD IN ASSOCIATION WITH UPPER JET STREAK IS
EXPECTED TO SHIFT EAST THE TERMINALS AROUND 10Z...WHICH WILL ALLOW
FOR STRONGER RADIATIONAL COOLING THROUGH SUNRISE AND DEVELOPMENT
OF PATCHY DENSE FOG. EVEN WITH HIGH CLOUDS OVERHEAD...VSBYS
ALREADY VARYING FROM 3/4SM TO 1 1/2SM AT NEARBY KPWK. CURRENT
THINKING IS THAT FOG WILL REMAIN SHALLOW/PATCHY UNTIL HIGH CLOUDS
CLEAR HOWEVER. SHOWERS DID NOT DIRECTLY IMPACT RFD AND DPA...WHICH
SUGGESTS PATCHY IS LESS OF A CONCERN AT THOSE LOCATIONS.
NEXT MID LEVEL DISTURBANCE AND ASSOCIATED SURFACE TROUGH CURRENTLY
OVER THE EASTERN DAKOTAS WILL MOVE INTO THE FORECAST AREA BY
TONIGHT. APPROACH OF TROUGH WILL ALLOW WINDS TO BACK TO THE
SOUTHWEST ONCE NOCTURNAL INVERSION MIXES OUT BY MID-MORNING.
MODEL FORECAST SOUNDINGS INDICATE INSTABILITY DEVELOPING AGAIN
DURING THE LATE AFTERNOON AND EVENING HOURS DUE TO COLD AIR ALOFT
WITH APPROACHING DISTURBANCE. HAVE INDICATED PROB30 FOR SHRA AT
THIS TIME AS LOW LEVEL AIR IS RELATIVELY DRY...THOUGH STEEP LOW
LEVEL LAPSE RATES ARE PROGGED TO EXTEND TO NEARLY 9000 FT.
RATZER
//ORD AND MDW CONFIDENCE...UPDATED 08Z...
* LOW CONFIDENCE IN PATCHY FOG AND IFR VSBYS THIS AM.
* MEDIUM CONFIDENCE IN TIMING/COVERAGE OF SHOWERS LATE TODAY AND
THIS EVENING.
RATZER
//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...
SAT...VFR. WX NIL.
SUN...PREVAILING VFR...CHC SHRA WITH LCL MVFR
MON...VFR. WX NIL.
TUE...VFR. WX NIL.
WED...VFR...SLGT CHC -SHRA.
TRS
&&
.MARINE...
155 AM CDT
SEVERAL UPPER LEVEL DISTURBANCES/CLIPPER SYSTEMS SHOULD AFFECT THE
WEATHER OVER LAKE MICHIGAN FOR THE NEXT FEW DAYS. EACH DISTURBANCE
LOOKS TO PROVIDE AN INCREASE IN THE WINDS AND GOOD CHANCES FOR LAKE
EFFECT/LAKE ENHANCED SHOWERS AND EVEN SOME THUNDERSTORMS.
THE FIRST CLIPPER SYSTEM WILL RACE SOUTHEASTWARD ACROSS THE LAKE
LATER TODAY AND TONIGHT. THE ASSOCIATED SURFACE TROUGH OF LOW
PRESSURE SHOULD INDUCE AN INCREASE IN THE SOUTH TO SOUTHWESTERLY
WINDS LATER THIS AFTERNOON AND DURING THE EVENING. THIS SHOULD
PROVIDE A WINDOW OF 15 TO 25 KT WINDS INTO THIS EVENING BEFORE THE
WINDS ABATE. THE OTHER CONCERN WITH THIS SYSTEM WILL BE THE THREAT OF
SOME THUNDERSTORMS AND WATERSPOUTS TONIGHT. VERY COLD TEMPERATURES
ALOFT ASSOCIATED WITH THE UPPER LEVEL DISTURBANCE SHOULD PROVIDE A
FAVORABLE THERMODYNAMIC SETUP FOR THUNDERSTORMS AND WATERSPOUTS. THE
WATERSPOUT THREAT WILL LIKELY BE THE HIGHEST DURING THE LATER
EVENING AND OVERNIGHT HOURS AS THE SURFACE TROUGH PROVIDES A GOOD
SURFACE CONVERGENCE ZONE. THIS THREAT WILL DIMINISH ON SATURDAY
MORNING.
THERE WILL BE A BRIEF QUIET PERIOD DURING THE DAY ON SATURDAY AS A
SURFACE RIDGE OF HIGH PRESSURE BUILDS IN ACROSS THE LAKE.
HOWEVER...ANOTHER CLIPPER SYSTEM LOOKS TO AFFECT LAKE MICHIGAN ON
SUNDAY AND SUNDAY NIGHT. THIS SYSTEM APPEARS TO BE THE STRONGEST OF
THE TWO...AND THEREFORE THE SOUTHERLY WINDS THAT IT INDUCES ON
SUNDAY COULD APPROACH LOW END GALE FORCE. WILL STAY CONSERVATIVE AND
CAP THE WINDS AT 32 KT AT THIS TIME. AT THE LEAST...THIS LOOKS TO
PRODUCE A GOOD CHANCE OF SMALL CRAFT CONDITIONS FOR THE NEAR SHORE
WATERS ON SUNDAY.
NEXT WEEK IT APPEARS THAT THE WEATHER PATTERN WILL REMAIN ACTIVE
ACROSS THE CENTRAL CONUS. HOWEVER...AT THIS TIME THE STRENGTH OF THE
STORM SYSTEMS REMAIN IN QUESTION.
KJB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...FROST ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
ILZ032-ILZ033-ILZ039 UNTIL 7 AM FRIDAY.
IN...FROST ADVISORY...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 7 AM
FRIDAY.
LM...NONE.
&&
$$
