Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 011158
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
658 AM CDT WED APR 1 2015

.SHORT TERM...
400 AM CDT

THROUGH THURSDAY...

APRIL WILL GET OFF TO A WARM AND BREEZY START TODAY...BUT THE
QUIET WEATHER WILL QUICKLY CHANGE TONIGHT. W/V IMAGERY THIS
MORNING SHOWS MIDLEVEL RIDING BUILDING INTO THE MIDWEST...WITH
STRONG UPPER DISTURBANCE RIDING OVER THE TOP OF THE RIDGE IN THE
NORTHERN HIGH PLAINS. THIS HAS SPURRED CYCLOGENESIS AND A ~995 MB
LOW OVER WESTERN NORTH DAKOTA. HEIGHT RISES WILL BE ACCOMPANIED BY
STRONG 850/925 MB WARM ADVECTION...WITH VALUES OF +10 TO +15
CELSIUS AT 925 MB SUPPORTIVE OF HIGHS IN THE LOWER TO EVEN MID
70S IN A FEW SPOTS...AIDED BY CLOSE TO FULL SUN. EXCEPTION WILL BE
FAR NORTHEAST IL SHORE BEING KEPT COOLER BY SOUTHEAST WINDS.
SLIGHT ONSHORE COMPONENT IS EXPECTED EARLIER IN THE DAY IN
DOWNTOWN CHICAGO...RESULTING IN A SLIGHTLY SLOWER WARMING TREND
THAN INLAND AREAS...BUT 70 IS STILL A GOOD BET IN THE LATE
AFTERNOON.

REGARDING THE WINDS...OUR AREA WILL BE SANDWICHED BETWEEN
DEPARTING HIGH PRESSURE RIDGE TO THE EAST AND FRONTAL TROUGH OF
STRONG LOW PRESSURE OVER THE NORTHERN PLAINS. AS FRONTAL TROUGH
ADVANCES...PRESSURE GRADIENT WILL TIGHTEN AND FAIRLY IMPRESSIVE
PRESSURE FALLS WILL RESULT IN INCREASINGLY GUSTY SOUTH
WINDS...ESPECIALLY IN NORTH CENTRAL ILLINOIS. THE STRONG SOUTHERLY
WINDS WILL CONTINUE TONIGHT...THOUGH WITH SOME UNCERTAINTY ON TOP
END MAGNITUDES. THE GUSTY WINDS AND CONTINUED WAA WILL RESULT IN
QUITE A MILD NIGHT TONIGHT...WITH LOWS IN THE LOWER TO MID
50S...WHICH IS ON PAR WITH NORMAL HIGHS FOR EARLY APRIL.

ATTENTION WILL TURN TO STRONG ~50 KT 850 MB LOW LEVEL JET LATE
TONIGHT AND UPPER DISTURBANCE PUNCHING INTO FRONTAL ZONE. GUIDANCE
INDICATES ENOUGH ELEVATED INSTABILITY SUPPORTIVE OF MAINTAINING
THUNDER IN GRIDS...BUT QUESTION IS TIMING OF POSSIBLY WEAKENING
AREA OF SHOWERS AND STORMS INTO NORTHWEST CWA. SOME CONVECTIVE
ALLOWING GUIDANCE KEEPS CONVECTIVE ACTIVITY OUTSIDE OF THE CWA
UNTIL AFTER 9Z AND OVERALL HAVE SLOWED TRENDS A BIT FROM PREVIOUS
FORECAST.

THURSDAY MORNING INTO THE EARLY AFTERNOON APPEARS TO BE PRIME
TIME FOR WIDESPREAD SHOWERS AND THUNDERSTORMS IMMEDIATELY AHEAD OF
THE FRONT. HAVE CONTINUED TO LEAN TOWARD A SLOWER FRONTAL
PASSAGE...KEEPING ESPECIALLY THE SOUTHEAST CWA IN THE WARM SECTOR
INTO THE EARLY-MID AFTERNOON. ONLY A FEW HUNDRED JOULES OF CAPE
PROGGED AND OF A TALL/NARROW PROFILE...BUT STRONG LOW LEVEL SHEAR
AND DECENT LOW LEVEL CAPE SUPPORTS THE MARGINAL RISK FOR SEVERE
WEATHER PER SPC OUTLOOK. MAIN THREAT WOULD BE GUSTY WINDS GIVEN
MARGINAL LAPSE RATES...THOUGH CANT RULE OUT AN ISOLATED TORNADO
THREAT IN OUR SOUTHEAST...ESPECIALLY IF A SECONDARY LOW CAN FORM
ALONG THE COLD FRONT. BIGGER CONCERN COULD BE A PERIOD OF HEAVIER
RAIN GIVEN ANOMALOUSLY HIGH 1.25 TO 1.5 INCH PWATS ADVECTED INTO
THE AREA. AFTER THE WARM START TO THE DAY...EXPECTING HIGHS IN THE
MID TO UPPER 60S...WITH A FEW SPOTS IN THE SOUTHEAST CWA POSSIBLY
TAGGING 70. THIS IS DESPITE THE CLOUDS AND PRECIP.

RC

&&

.LONG TERM...
352 AM CDT

THURSDAY NIGHT...

PASSAGE OF THE COLD FRONT AND DRYING OF LOW LEVELS THURSDAY NIGHT
WILL PUSH PRECIPITATION TO THE FAR SOUTHERN PORTION OF THE FORECAST
AREA...AND MAYBE EVEN INTO CENTRAL ILLINOIS AND OUT OF THE LOCAL
AREA ALTOGETHER.

FRIDAY AND FRIDAY NIGHT...

THIS BREAK FROM PRECIPITATION LIKELY WILL BE SHORT LIVED AS THE
FRONT STALLS TO THE SOUTH AND A CENTER OF LOW PRESSURE MOVES ALONG
THIS STATIONARY BOUNDARY INTO THE OHIO VALLEY ON FRIDAY.
PRECIPITATION WRAPS AROUND THE NORTH SIDE OF THE LOW...AND FOR A
WHILE IT APPEARED THAT ACCUMULATING SNOW WOULD BE A GOOD POSSIBILITY
FRIDAY AFTERNOON AS COLDER AIR FILTERED INTO THE AREA. THE LATEST
GUIDANCE SUGGESTS THAT TEMPERATURES WILL REMAIN JUST A BIT TOO WARM
TO SUPPORT SNOW UNTIL VERY LATE IN THE DAY OR INTO FRIDAY EVENING
WHEN POPS DROP CONSIDERABLY WITH THE EXIT OF THE LOW.  BOTH THE GFS
AND ECM SHOW AN INCH OR SO OF SNOW LATE FRIDAY EVEN WITH MARGINAL
SFC TEMPERATURES.  AREAS SOUTH OF THE LAKE MAY HAVE THE BEST CHANCE
SINCE WITH NORTH WINDS THEY WILL NOT GET QUITE AS WARM DURING THE
EARLY PART OF THE DAY AND WILL COOL FASTER TOWARD EVENING.

SATURDAY AND EASTER SUNDAY...

HIGH PRESSURE SLIDING TO OUR SOUTH AND INCREASING HEIGHTS ALOFT WILL
SUPPORT A QUIET AND SEASONABLY MILD SATURDAY.  AS THE HIGH MOVES
SOUTHEAST AWAY FROM THE AREA SATURDAY NIGHT...THE NEXT FRONTAL ZONE
DROPS SOUTH ACROSS WISCONSIN.  RETURN FLOW ON THE WEST SIDE OF THE
DEPARTED HIGH WILL SUPPORT MILDER TEMPERATURES SUNDAY BUT ONLY
MARGINALLY INCREASED MOISTURE...SO AM NOT INCLUDING MORE THAN SLIGHT
CHANCES OF LIGHT RAIN SATURDAY NIGHT AND SUNDAY DESPITE THE PRESENCE
OF THIS BOUNDARY.

MONDAY AND TUESDAY...

EARLY NEXT WEEK THE GFS AND ECMWF SHOW SIMILAR AGREEMENT IN
DEVELOPING A LARGE LOW PRESSURE SYSTEM ACROSS THE PLAINS AND LIFTING
IT INTO THE WESTERN GREAT LAKES.  SHOWER AND THUNDERSTORM CHANCES
INCREASE MONDAY AND ESPECIALLY INTO TUESDAY AS THIS SYSTEM
APPROACHES BUT AT THIS RANGE WILL JUST MENTION A CHANCE OF TSRA FOR
BOTH DAYS.

LENNING

&&

.FIRE WEATHER...545 AM CDT

SOUTHERLY FLOW TODAY AND TONIGHT WILL GRADUALLY BRING INCREASED
MOISTURE TO THE AREA ALONG WITH MILD TEMPERATURES AND EVENTUALLY
THUNDERSTORMS ON THURSDAY. HOWEVER...PEAK WIND GUSTS AROUND 25
MPH THIS AFTERNOON WILL APPROACH MINIMUM RELATIVE HUMIDITY VALUES
OF AROUND 30 PERCENT...ESPECIALLY IN WINDIER AND DRIER AREAS WEST
OF I-39. WHEN WINDS ARE HIGHER THAN THE RELATIVE HUMIDITY...THERE
IS AN INCREASED RISK THAT FIRES WILL GET OUT OF CONTROL. AT THIS
POINT CONDITIONS DO NOT APPEAR TO BE WINDY AND DRY ENOUGH TO
PROMPT RED FLAG WARNINGS TODAY...BUT THOSE CONDUCTING PRESCRIBED
BURNS SHOULD USE EXTRA CAUTION.

LENNING

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 12Z...

* GUSTY SOUTH WINDS DEVELOPING THIS AFTERNOON.

* SMALL CHANCE FOR TSRA LATE TONIGHT/EARLY THURSDAY...WITH A
  BETTER CHANCE THURSDAY MORNING.

* CHANCE FOR LLWS TONIGHT IF WINDS ARE LESS GUSTY.

RC

//DISCUSSION...UPDATED 12Z...

VFR CONDITIONS ARE IN STORE THROUGH THE DAY TODAY...BUT SOUTH
WINDS WILL BECOME GUSTY AS PRESSURE GRADIENT TIGHTENS BETWEEN
DEPARTING HIGH PRESSURE TO THE EAST AND FRONTAL TROUGH OVER THE
PLAINS. FORECAST SOUNDINGS CONTINUE TO SUPPORT GUSTS IN THE 20-25
KT RANGE FOR THE EASTERN TERMINALS BY THE EARLY TO MID AFTERNOON
AND 25 TO 30 KT ACROSS NORTH CENTRAL IL...INCLUDING FOR RFD. THE
STRONG SOUTHERLY WINDS WILL CONTINUE TONIGHT...THOUGH GUST
MAGNITUDE AND FREQUENCY IS A BIT LESS CERTAIN. IF GUSTS BECOME
LESS FREQUENT THAN INDICATED IN CURRENT TAF...SOUTHWEST WINDS OF
50+ KT AT JUST ABOVE 1500 FT AGL WILL PRESENT THE CHANCE FOR LOW
LEVEL WIND SHEAR. LATER TONIGHT...A WEAKENING BAND OF SHRA/TSRA
IS LIKELY TO BE MAKING MAKE ITS WAY EAST AS FRONT APPROACHES...SO
MAINTAINED A PROB30 IN THE TAFS. RFD MAY HAVE BEST CHANCE TO
OBSERVE SHRA/TSRA ACTIVITY OVERNIGHT...BUT CONFIDENCE IS STILL LOW
THERE.

THERE IS A BETTER CHANCE FOR MORE WIDESPREAD SHOWERS AND STORMS ON
THURSDAY AS THE FRONT SLOWLY MOVES ACROSS THE AREA. INSTABILITY ON
MODEL SOUNDINGS IS NOT THAT IMPRESSIVE BUT LOW LEVEL JET WILL
REMAIN STRONG AND LIKELY SUPPORTIVE OF AT LEAST EMBEDDED TSRA
WITHIN SHRA ACTIVITY. MOIST AIRMASS COULD ALSO ENABLE HEAVIER
DOWNPOURS AND VSBY REDUCTIONS. FOR NOW...HAVE GONE WITH A
BROADBRUSH APPROCH IN THE ORD 30HR TAF...WITH THUNDER DETAILS TO
BE ADDED IN SUBSEQUENT TAF UPDATES.

RC

//ORD AND MDW CONFIDENCE...UPDATED 12Z...

* MEDIUM-HIGH IN WIND TRENDS.

* LOW IN TSRA POTENTIAL TONIGHT...HIGH THAT THERE WILL BE TSRA
  NEARBY DURING THURSDAY MORNING BUT LOW IN DIRECT IMPACTS TO
  TERMINALS.

* HIGH IN REMAINING ELEMENTS.

RC

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

THURSDAY NIGHT...VFR LIKELY.

FRIDAY...CHANCE LIGHT RAIN AND MVFR. GUSTY NORTH TO NORTHEAST
WINDS.

SATURDAY...VFR. CHANCE -RA AT NIGHT.

SUNDAY...CHANCE -RA. SOUTH WINDS SHIFTING TO EAST.

MONDAY...CHANCE -SHRA AND IFR/MVFR. SLIGHT CHANCE TSRA AT NIGHT.

TUESDAY...CHANCE TSRA AND MVFR.

RC

&&

.MARINE...
458 AM CDT

THE LAKE WILL BE IN BETWEEN A FRONTAL TROUGH OVER THE PLAINS AND
HIGH PRESSURE DEPARTING TO THE EAST TODAY. A TIGHTENING PRESSURE
GRADIENT WILL RESULT IN STRENGTHENING SOUTHERLY WINDS...ESPECIALLY
THIS EVENING THROUGH TONIGHT. HAVE ISSUED A SMALL CRAFT ADVISORY
FOR THE NEARSHORE STARTING AT 00Z THIS EVENING...THOUGH GIVEN
MAGNITUDE OF PRESSURE FALLS SPREADING OVER THE LAKE..IT IS
POSSIBLE THAT IT MAY NEED TO BE STARTED EARLY. EXPECTING SOUTHERLY
SPEEDS TO 30 KT AHEAD OF THE FRONT TONIGHT...WITH A VERY MILD AIR
MASS LIMITING MIXING. CANNOT RULE OUT A FEW GALE FORCE GUSTS
DESPITE THE STABLE CONDITIONS...ESPECIALLY ON THE CENTRAL AND
NORTHERN PORTION OF THE LAKE. THE MILD AND MOIST AIRMASS COULD
ALSO RESULT IN FOG DEVELOPING...THOUGH CONFIDENCE IS LOW IN HOW
WIDESPREAD IT WILL BECOME...SO HAVE GONE WITH PATCHY WORDING IN
THE GLF LATE TONIGHT THROUGH MID DAY THURSDAY.

FRONT TRAILING FROM LOW PRESSURE MOVING TO NEAR JAMES BAY BY
THURSDAY EVENING WILL CROSS THE LAKE DURING THURSDAY
AFTERNOON...ENDING THE SMALL CRAFT WINDS IN THE NEARSHORE...LATEST
FOR THE INDIANA SHORE. WEST WINDS RELAX THURSDAY NIGHT THEN TURN
NORTH FOR FRIDAY AS A LOW PRESSURE AREA RIDES UP THE OHIO VALLEY.
30 KT WINDS ARE POSSIBLE FOR THE SOUTH HALF DEPENDING ON THE TRACK
OF THE LOW...ALONG WITH SMALL CRAFT WINDS AND WAVES FOR THE
NEARSHORE. WINDS WILL BACK WEST AND THEN SOUTHWEST ON SATURDAY.
UNCERTAINTY INCREASES ON SUNDAY...AS SOME GUIDANCE INDICATES A
FAST MOVING LOW CROSSING THE LAKE...WHILE OTHER GUIDANCE HAS HIGH
PRESSURE OVER THE REGION.

RC

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...7 PM WEDNESDAY TO
     1 PM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...7 PM WEDNESDAY TO
     10 AM THURSDAY.

&&

$$

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