845 PM CST
FOR EVENING UPDATE...
ONLY MINOR ADJUSTMENTS TO GOING FORECAST...MAINLY WITH RESPECT TO
SKY COVER GRIDS OVERNIGHT.
AFTERNOON CONVECTIVE SNOW/GRAUPEL SHOWERS HAVE DISSIPATED WITH UPPER
TROUGH AND COLD POOL MOVING EAST OF THE FORECAST AREA AND LOSS OF
DIURNAL ENHANCEMENT OF LOW LEVEL LAPSE RATES. SUBSIDENCE AND HEIGHT
RISES ALREADY DEVELOPING ACROSS THE AREA IN THE WAKE OF THE
DEPARTING UPPER TROUGH...WITH DEVELOPING SUBSIDENCE INVERSION
ALREADY EVIDENT IN KDVN 00Z RAOB. GOES IR SATELLITE IMAGERY ALSO
DEPICTS THE RESULTING EROSION IN LOW LEVEL CLOUD COVER ACROSS A
LARGE PORTION OF NORTHEAST AND EAST CENTRAL IL. A FAIRLY EXTENSIVE
AREA OF LOW CLOUDS DOES LINGER ACROSS WESTERN/CENTRAL WISCONSIN AND
THE UPPER MISSISSIPPI VALLEY...THOUGH EROSION IS NOTED ON THE
EASTERN PERIPHERY ACROSS NORTH CENTRAL IL. THIS MAKES CLOUD FORECAST
A LITTLE TRICKY FOR THE LATE EVENING/OVERNIGHT HOURS ESPECIALLY OVER
NORTH CENTRAL ILLINOIS WHERE OVERCAST SKIES WILL COVER A PORTION OF
THE AREA WHILE OTHERWISE ERODING.
TO COMPLICATE OVERALL SKY COVER IS AREA OF HIGH/MID CLOUDS SPREADING
OUT OF THE EASTERN DAKOTAS AND WESTERN MINNESOTA...WHERE WARM
ADVECTION IS ALREADY STRENGTHENING IN PROGRESSIVE FLOW REGIME.
GUIDANCE WOULD SUGGEST AN INCREASE IN SOME OF THIS HIGH CLOUD INTO
NORTHERN IL TOWARD MORNING. THUS HAVE TRENDED GRIDS A LITTLE
CLOUDIER NORTHWEST...WITH VARIABLE CLOUDINESS WORDING IN THE ZFP...
WHILE GENERALLY CARRYING A PARTLY CLOUDY FORECAST OVERNIGHT ACROSS
NORTHEAST IL/NORTHWEST INDIANA. THIS WILL ALSO IMPACT OVERNIGHT LOW
TEMP FORECAST...AND BASED ON THIS AND SFC DEW POINT TRENDS SINCE
THIS AFTERNOON HAVE BUMPED UP A DEGREE OR TWO IN SOME OF THE COLDER
SPOTS ACROSS NORTH CENTRAL IL.
OTHERWISE...FORECAST APPEARS IN GOOD SHAPE AND HAVE MADE NO OTHER
SIGNIFICANT CHANGES THIS EVENING.
RATZER
//PREV DISCUSSION...
312 PM CST
THE MAIN ISSUE DURING THE FORECAST PERIOD WILL BE THE POTENTIAL FOR
ANOTHER SIGNIFICANT PRECIPITATION EVENT EARLY NEXT WEEK.
HOWEVER...THE LATEST MODEL GUIDANCE AND ENSEMBLES STILL SUGGEST A
LARGE DEGREE OF UNCERTAINTY WITH THE EVOLUTION OF THE UPPER LOW OFF
THE WEST COAST OF AMERICA. DUE TO THIS HIGH UNCERTAINTY...I SIDED
CLOSER TO A PERSISTENCE FORECAST FOR EARLY NEXT WEEK.
THIS AFTERNOON...COLD TEMPS ALOFT ASSOCIATED WITH A DIGGING SHORT
WAVE TROUGH...HAS PRODUCED SOME STEEP LOW AND MID LEVEL LAPSE RATES
ACROSS THE REGION. THIS HAS SET UP A DECENT AREA OF SCATTERED SNOW
SHOWERS ACROSS NORTHEASTERN ILLINOIS AND SOUTHERN LAKE MICHIGAN.
GRAUPEL HAS ALSO BEEN REPORTED WITHIN THE MORE ROBUST CONVECTIVE
ELEMENTS. THESE SHOWERS WILL QUICKLY DROP VISIBILITIES UNDER A MILE
AND MAY RESULT IN SOME MINOR ACCUMULATIONS ON ELEVATED AND GRASSY
SURFACES. ALTHOUGH TEMPERATURES ARE IN THE UPPER 30S TO NEAR 40 IN
MOST AREAS...SURFACE WET BULB TEMPERATURES WILL ALLOW TEMPERATURES
TO DROP LOCALLY INTO THE LOWER TO MID 30S AS THE SNOW SHOWERS
AFFECT A REGION. I EXPECT THESE SNOW SHOWERS TO QUICKLY ABATE AND
MOVE OVER AND DOWNSTREAM OF LAKE MICHIGAN BY EARLY THIS EVENING.
A DECENT WARMING TREND WILL BE THE NEXT STORY FOR THE WEEKEND AS
SURFACE HIGH PRESSURE SETS UP ACROSS THE SOUTHEASTERN CONUS. THIS
WILL ALLOW FOR SOUTHWEST WINDS TO INCREASE AND TRANSPORT MUCH WARMER
TEMPERATURES ACROSS NORTHERN ILLINOIS. I EXPECT THAT MID TO UPPER
50S ARE LIKELY BY SATURDAY...WITH SOME LOW 60S DEFINITELY
POSSIBLE.
AS MENTIONED ABOVE...UNCERTAINTY BEGINS TO INCREASE BY EARLY NEXT
WEEK. THE 12 UTC NAM AND GFS INDICATE THAT A COLD FRONT...ASSOCIATED
WITH LOW PRESSURE ACROSS THE UPPER MISSISSIPPI VALLEY AND SOUTHERN
CANADA...WILL PUSH ACROSS NORTHERN ILLINOIS SUNDAY AFTERNOON.
HOWEVER...THE 12 UTC ECMWF AND GEM INDICATE A MUCH SLOWER
PROGRESSION OF THIS FRONTAL ZONE...KEEPING IT WEST OF MY AREA ON
SUNDAY. THE TIMING OF THIS FRONT WILL HAVE LARGE CONSEQUENCES ON THE
AMOUNT OF RAINFALL THE AREA WILL EXPERIENCE ON MONDAY. IT APPEARS
THE MAIN DIFFERENCES WITH THIS TIMING AMONGST THE MODELS IS DUE IN
LARGE PART TO THE ECMWF AND GEM FORECASTING THE AMPLIFICATION OF A
MID LEVEL RIDGE ACROSS THE SOUTHEASTERN CONUS ON SUNDAY. THIS SAME
BEHAVIOR IS NOT APPARENT IN THE GFS. THE SLOWER AND MORE AMPLIFIED
ECMWF AND GEM WOULD ALLOW THE MAIN NORTHERN STREAM SHORT WAVE
DIGGING ACROSS THE PACIFIC NORTHWEST TO PROGRESS SLOWER AND DIG
DEEPER ACROSS THE WESTERN CONUS. THIS WOULD PRESENT A SCENARIO
THAT WOULD FAVOR A FASTER EJECTION AND MOVEMENT OF THE UPPER LOW
OFF THE WEST COAST OF THE CONUS. ULTIMATELY THIS PATTERN WOULD SET
UP THE POTENTIAL FOR ANOTHER SIGNIFICANT RAINFALL EVENT FOR
PORTIONS OF NORTHERN ILLINOIS.
IF...HOWEVER...A GFS SOLUTION VERIFIES THE FRONTAL ZONE WILL BE WELL
TO OUR SOUTH BY MONDAY...ESSENTIALLY KEEPING A MAJORITY OF THE REGION
DRY. ALTHOUGH THE ECMWF APPEARS TO BE AN OUTLIER RELATIVE TO THE GFS
ENSEMBLES...THERE ARE A COUPLE OF ENSEMBLE SOLUTIONS THAT SUPPORT A
SCENARIO SIMILAR TO THE 12 UTC ECMWF. THEREFORE...THE ENVELOP OF
SOLUTIONS STILL APPEARS TO BE LARGE. WITH THIS IN MIND...NO MAJOR
CHANCES WERE MADE TO THE EXTENDED PERIOD.
KJB
&&
.AVIATION...
//ORD AND MDW CONCERNS...UPDATED 06Z...
* GUSTY SOUTHERLY WINDS FRIDAY. CMS
//DISCUSSION...UPDATED 06Z...
MAIN FORECAST CONCERN THIS PERIOD WILL BE THE WINDS. WESTERLY
WINDS UNDER 10 KTS WILL CONTINUE TO DIMINISH TO 5 KTS OR SO
OVERNIGHT AND TURN SOUTHWESTERLY. WINDS CONTINUE TO TURN SOUTHERLY
FRIDAY MORNING. IN ADDITION TO THE GRADIENT TIGHTENING SOME...
LOW LEVELS WILL BEGIN TO MIX BY LATE MORNING/EARLY AFTERNOON WITH
GUSTS IN THE MID 20 KT RANGE EXPECTED...WHICH SHOULD DIMINISH BY
EARLY FRIDAY EVENING.
BKN/OVC 4-6KFT DECK OVER NORTHWEST IL CONTINUES TO SLOWLY ERODE AS
MID/HIGH CLOUDS MOVE SOUTHEAST ACROSS THE NORTHERN PLAINS. THE
LOWER CLOUDS SHOULD CONTINUE TO DISSIPATE AS THEY SLOWLY MOVE EAST.
MID/HIGH LEVEL CLOUDS WILL INCREASE OVERNIGHT...THEN SCATTER OUT
FRIDAY AFTERNOON...VFR CIGS THROUGH THE PERIOD. CMS
//ORD AND MDW CONFIDENCE...UPDATED 06Z...
* MEDIUM CONFIDENCE REGARDING WIND SPEEDS/GUSTS FRIDAY. CMS
//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...
SATURDAY...VFR.
SUNDAY...WINDY. SOUTHWEST WINDS GUSTING 30+ KTS. CHANCE OF SHRA.
MONDAY...CHANCE OF SHRA. MVFR POSSIBLE.
TUESDAY...VFR. SLIGHT CHANCE OF SHRA.
WEDNESDAY...VFR. CMS
&&
.MARINE...
223 PM CST
UPCOMING PERIODS OF POTENTIAL GALES WILL BE THE PRIMARY CONCERN IN
THE OPEN WATER FORECASTS. WHILE THE STRONG LOW PRESSURE THAT MOVED
OVER THE AREA ON WEDNESDAY IS FAR NORTH OF THE AREA OVER ST JAMES
BAY...IT CONTINUES TO HAVE INFLUENCE WITH WEST NORTHWEST WINDS OVER
THE LAKE INTO THIS EVENING. THESE WILL SLOWLY DIMINISH AS A
QUICK-PACED WEATHER PATTERN RESUMES OVER THE REGION. AFTER A RIDGE
PASSES OVERNIGHT INTO FRIDAY MORNING...A CLIPPER SYSTEM WILL MOVE
RAPIDLY EAST TO THE IMMEDIATE NORTH OF THE LAKES. THE INFLUENCE OF
THIS WILL CAUSE SOUTHWEST WINDS TO COME UP QUICKLY FRIDAY
AFTERNOON...WITH POTENTIAL GALE FORCE GUSTS OVER THE NORTHERN LAKE
DUE TO THE RAPID PRESSURE FALLS AND MIXING POTENTIAL OVER THE LAKE.
ANOTHER POTENTIAL ROUND OF GALES IS SET FOR LATE SATURDAY NIGHT
THROUGH SUNDAY ACROSS MUCH OF THE LAKE DUE TO A DEEPER LOW PRESSURE
PASSING AGAIN JUST NORTH OF THE LAKES. BECAUSE OF THE LONGER
DURATION INFLUENCE...CONFIDENCE IN THIS EVENT IS ACTUALLY SLIGHTLY
HIGHER. ALSO...THE LIKELIHOOD OF NEEDING SMALL CRAFT ADVISORIES
FOR THE IL AND IN NEARSHORE ZONES FOR AT LEAST A PORTION OF THAT
TIME IS PROBABLE.
MTF
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 3 AM FRIDAY.
GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...3 PM FRIDAY TO MIDNIGHT SATURDAY.
&&
$$
