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 20Z...
* SCT'D SNOW SHOWERS THIS AFTERNOON
* WESTERLY WINDS GUSTING TO AROUND 20 KT UNTIL AROUND SUNSET
* OCNL MVFR CIGS THROUGH EARLY EVENING
IZZI
//DISCUSSION...UPDATED 20Z...
ONLY CHANGE TO GOING TAFS WAS TO INTRODUCE SOME GRAUPLE INTO THE
TEMPO. HAVE BEEN GETTING NUMEROUS REPORTS OF GRAUPLE IN ADDITION
TO SNOW WITH THESE SHOWER...INCLUDING A COUPLE REPORTS OF FAIRLY
HEAVY GRAUPLE. OTHERWISE NO CHANGES MADE TO 18Z TAFS.
IZZI
UPDATED 18Z...
COMBINATION OF COLD AIR ALOFT AND AN UPPER LEVEL DISTURBANCE
MOVING THROUGH THE REGION WILL LEAD TO FLURRIES WITH SOME OCNL
HEAVIER SNOW SHOWERS THIS AFTERNOON. HEAVIER BURSTS OF SNOW SHOULD
BE RELATIVELY BRIEF IN NATURE BUT COULD STILL RESULT IN VSBYS
DROPPING TO IFR. THERE IS A LOW END THREAT OF SOME GRAUPLE (GS)
WITH THE MORE INTENSE CONVECTIVE CELLS...BUT THAT THREAT IS TOO
LOW FOR INCLUDING IN TAFS.
CIGS WILL HAVE A TENDENCY THIS AFTERNOON TO TREND DOWN TO
MVFR...BUT GIVEN UPSTREAM OBS EXCEPT BASES TO BE RAGGED AND CIGS
SOMEWHAT VARIABLE THIS AFTERNOON. LOWER CIGS SHOULD CLEAR OUT THIS
EVENING THOUGH MID AND HIGH LEVEL CLOUDS WILL INCREASE AHEAD OF
NEXT SYSTEM PROGGED TO PASS NORTH OF THE AREA FRIDAY. MAIN EFFECT
FROM THIS SYSTEM WILL BE TO CAUSE WINDS TO BACK TO SOUTHERLY
FRIDAY MORNING AND INCREASE AGAIN FRIDAY WITH GUSTS OVER 20KT
ONCE AGAIN LIKELY DURING THE AFTERNOON.
IZZI
//ORD AND MDW CONFIDENCE...UPDATED 20Z...
* MEDIUM CONFIDENCE ON SHSN TODAY AND MVFR CIGS THRU EVENING
* HIGH CONFIDENCE ON ALL OTHER ASPECTS OF THE FORECAST
IZZI
//OUTLOOK FOR ORD/MDW FOR 00Z SATURDAY-12Z THURSDAY...UPDATED 12Z...
FRI NIGHT...VFR...WX NIL.
SAT...VFR...WX NIL.
SUN...VFR...CHANCE SHOWERS MVFR.
MON...VFR...CHANCE SHOWERS.
TUE...VFR...SLIGHT CHANCE SHOWERS.
WEDNESDAY...VFR...WX NIL.
TRS
&&
.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.
SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 9 PM THURSDAY.
GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...3 PM FRIDAY TO MIDNIGHT SATURDAY.
&&
$$
