Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040525
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1225 AM CDT MON MAY 4 2015

.UPDATE...
1130 PM CDT

MONITORING ISOLATED SHOWER/THUNDERSTORM DEVELOPMENT ACROSS NORTH
CENTRAL ILLINOIS LATE THIS EVENING...AS WELL AS AN APPROACHING
LINE OF THUNDERSTORMS OVER EASTERN IOWA AND SOUTHWEST WISCONSIN.

BROAD WAA WITH LLJ ORIENTED INTO NORTHERN ILLINOIS IS ALLOWING FOR
ISOLATED DEVELOPMENT AT THIS TIME AND WITH WEAK INSTABILITY IN
PLACE...STRONGEST CELLS ARE PRODUCING PEA SIZE HAIL. THIS WILL
LIKELY CONTINUE OVER THE NEXT COUPLE OF HOURS OUT AHEAD OF
ADDITIONAL DEVELOPMENT TO THE NORTHWEST OF THE CWA. HAVE THIS
LINE OF SHOWERS/THUNDERSTORMS TO THE CWA AROUND 1 AM CDT...BUT
WITH SOME UNCERTAINTY STILL IN PLACE AS TO THE OVERALL EVOLUTION
OF THIS PRECIP AFTER ITS ARRIVAL. AS ITS ALREADY WEAKENING AND
SLOWING SLIGHTLY WITH OUTFLOW PUSHING AHEAD OF IT AT THIS
TIME...LIKELY SCENARIO IS FOR THIS LINE TO MAKE IT TO NORTHERN
ILLINOIS WITH ONLY A MARGINAL/ISOLATED STRONG TO SEVERE THREAT.
STILL THINK THE MORE LIKELY SCENARIO TONIGHT IN TERMS OF HAZARDS
WILL BE SMALL HAIL WITH HEAVY RAINFALL PRIMARILY OVER NORTHERN
ILLINOIS.

RODRIGUEZ

&&

.SHORT TERM...
303 PM CDT

THROUGH MONDAY...

COMPACT BUT FAIRLY VIGOROUS MCV MOVING INTO NORTHWEST ILLINOIS
MID AFTERNOON WILL CONTINUE TO PUSH ACROSS FAR NORTHERN ILLINOIS
THROUGH EARLY THIS EVENING. CONVECTION HAS RE-FLARED ENTERING INTO
THE DIURNALLY FAVORABLE PERIOD AND WITH WEAK DESTABILIZATION
TAKING PLACE IN ADVANCE OF THE MCV WOULD ANTICIPATE SHOWERS AND
THUNDERSTORMS TO CONTINUE TO MARCH STEADILY EASTWARD ACROSS MAINLY
NORTHERN HALF OF THE CWA...LIKELY MOVING OUT ONTO THE LAKE EARLY
THIS EVENING.

SHOULD SEE A LULL IN THE CONVECTION FOR A TIME IN THE WAKE OF THE
MCV BEFORE CHANCES OF SHOWERS AND THUNDERSTORMS BEGIN TO RAMP BACK
UP AGAIN LATER TONIGHT WITH THE APPROACH OF THE COLD FRONT FROM
THE NORTHWEST. GUIDANCE CONTINUES TO HIT QPF PRETTY HARD LATE
TONIGHT INTO FIRST HALF OF THE DAY MONDAY AND AM MAINTAINING
LIKELY POPS...BUT GIVEN THAT WE WILL BE ENTERING A TYPICAL
CONVECTIVE DOWNTIME AM QUITE LEARY ABOUT ABOUT SHOWERS AND
THUNDERSTORMS BEING AS WIDESPREAD AND PRECIPITATION HEAVY AS MODEL
WOULD SUGGEST.

SYNOPTIC FRONT...POTENTIALLY MODIFIED BY CONVECTION TONIGHT INTO
MONDAY MORNING...WILL BE IN THE PROCESS OF MAKING ITS WAY
SOUTHWARD MONDAY AND LIKELY GET AN ADDED BOOST FROM THE LAKE.
MODELS DIFFER ON TIMING OF THE FRONTAL PASSAGE MONDAY AND THIS
TIMING UNCERTAINTY ALONG WITH THE POTENTIAL FOR CONVECTION AND/OR
CONVECTIVE DEBRIS ADD A LOT OF UNCERTAINTY TO THE TEMPERATURE
FORECAST FOR MONDAY. WHEN THE LAKE ENHANCED FRONT MOVES THROUGH
LOOK FOR TEMPS ALONG THE SHORE TO CRASH INTO THE 50S TO PERHAPS
40S RIGHT ALONG THE SHORE. CANNOT RULE OUT SOME MARINE FOG AS WELL
GIVEN THE COLD MARINE LAYER ENCOUNTERING 55F+ DEWPOINTS.

COULD SEE SOME DIURNAL REDEVELOPMENT OR RE-INTENSIFICATION OF
CONVECTION MONDAY AFTERNOON ALONG THE FRONT...BUT GUIDANCE SUGGESTS
BETTER LARGE SCALE ASCENT WILL BE MOVING AWAY FROM THE STALLING OUT
FRONTAL BOUNDARY DURING THE AFTERNOON. AHEAD OF THE FRONT
TEMPERATURES SHOULD ONCE AGAIN SOAR INTO THE 70S...POTENTIALLY LOWER
80S WHERE RAIN AND CLOUD COVER HOLD OFF AND ALLOW FOR BETTER
HEATING.

IZZI

&&

.LONG TERM...
303 PM CDT

MONDAY NIGHT THROUGH SUNDAY...

QUASI-STATIONARY FRONT LIKELY TO TAKE UP RESIDENCE OVER THE AREA
MONDAY NIGHT AND POTENTIALLY THROUGH WEDNESDAY NIGHT DEPENDING ON
YOUR GUIDANCE OF CHOICE. CONVECTION WILL LIKELY ULTIMATELY PLAY A
ROLE IN FRONTAL TIMING...BUT CERTAINLY THE SLOWER NORTHWARD ADVANCE
ADVERTISED BY THE WRF-NAM WOULD SEEMS REASONABLE BASED ON THE
EXPECTED PERIODIC MCS ACTIVITY. ONE SUCH MCS LOOKS TO POTENTIALLY
AFFECT THE AREA MONDAY NIGHT INTO EARLY TUESDAY MORNING...BEFORE
RAIN CHANCES WITH THE FRONT DWINDLE A BIT AS UPPER HEIGHTS RISE
PLACING THE MORE FAVORED VORT TRACK WELL NORTH AND REMOVED FROM THE
FRONTAL BOUNDARY.

THERE IS THE POTENTIAL FOR FOG NORTH OF THE BOUNDARY MONDAY NIGHT
REALLY THROUGH WEDNESDAY...PARTICULARLY AT NIGHT INLAND AND JUST
ABOUT ANYTIME NEAR THE LAKE.

FRONT IS PROGGED TO BEGIN HEADING BACK NORTHWARD WEDNESDAY AND PLACE
THE MAJORITY OF OUR CWA IN THE WARM AND HUMID AIR MASS. THE ONE
POTENTIAL EXCEPTION WEDNESDAY COULD BE THE ILLINOIS LAKEFRONT WHERE
SOUTHEAST WIND CUTTING OFF THE LAKE COULD PREVENT THE FULL EFFECTS
OF THE WARMTH FROM REACHING THE BEACHES. BY WEDNESDAY NIGHT AND
THURSDAY A STRONGER SOUTHERLY FLOW SHOULD SEND THE FRONT WELL NORTH
INTO WISCONSIN WITH UNSEASONABLY WARM AND HUMID AIR MASS OVER THE
ENTIRE CWA THURSDAY THROUGH FRIDAY.

FRONT DOES LOOK TO SAG SOUTHWARD INTO THE VICINITY OF OUR CWA OVER
THE WEEKEND BRINGING THE THREAT OF OCCASIONAL SHOWERS/STORMS AND
PROBABLY SETTING UP A LARGE TEMPERATURE GRADIENT ONCE AGAIN WITH
WINDS OFF THE LAKE NORTH OF THE BOUNDARY POTENTIALLY KEEPING
LAKESIDE AREAS RATHER CHILLING WHILE LOCATIONS SOUTH OF THE FRONT
REMAIN WARM/HUMID.

IZZI

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* TSRA LIKELY IN THE PREDAWN TO DAYBREAK TIME FOR AT LEAST A
  COUPLE HOUR WINDOW. SCATTERED TSRA COULD BE SEEN BOTH BEFORE AND
  AFTER THIS.

* WINDS TURNING NORTHWEST WITH TSRA AND THEN LIKELY A PERIOD OF
  VARIABLE BEFORE SHIFTING NORTH-NORTHEAST MID TO LATE MORNING.

* MVFR CIGS LIKELY NEAR AND AFTER THE WIND SHIFT FOR A PERIOD
  DURING MIDDAY AND POSSIBLY THE ENTIRE AFTERNOON. CHANCE FOR IFR
  CIGS.

* CHANCE OF ADDITIONAL SHRA/TSRA THIS EVENING AND MORE SO
  OVERNIGHT.


//DISCUSSION...UPDATED 06Z...

A COMPLEX OF STORMS WILL CONTINUE TO SHIFT EAST-SOUTHEAST AND VERY
LIKELY ACROSS TAF SITES PRIOR TO OR NEAR DAYBREAK. THIS SHOULD
CONTINUE ON A GRADUAL WEAKENING TREND...BUT ENOUGH INSTABILITY TO
WARRANT TSRA PREVAILING FOR A COUPLE HOUR WINDOW AT NORTHERN IL
TAF SITES. BEHIND THE INITIAL PUSH OF STORMS...SOME REDEVELOPMENT
COULD OCCUR AS SEEN IN CENTRAL IA AS OF 05Z...BUT EVEN IF THAT
WERE TO OCCUR IT SHOULD BE MORE LIGHT AND MAYBE JUST SHRA.

A WIND SHIFT TO THE WEST-NORTHWEST IS LIKELY WITH THE CONVECTION
INCLUDING A FEW GUSTS OVER 20 KT ESPECIALLY AT RFD WHERE THE GUSTS
MAY LAST CLOSE TO TWO HOURS. THE FORECAST FOR WIND DIRECTION THEN
GETS VERY CHALLENGING AS A PERIOD OF VARIABLE WIND IS LOOKING
LIKELY AT AND JUST AFTER DAYBREAK...BUT FOR EXACTLY HOW LONG AND
WHAT SPEED IS DIFFICULT TO SAY AT THIS POINT. A SURFACE COOL FRONT
WILL THEN SWING SOUTHWARD TURNING WINDS TO THE NORTH-NORTHEAST AT
ALL TAF SITES BY MIDDAY. MVFR CIGS ARE LIKELY TO ACCOMPANY THIS
MID SHIFT WITH THE CHANCE FOR IFR....ESPECIALLY FOR TAF SITES NOT
FAR FROM THE LAKE AS LAKE-COOLED AIR COLLIDES WITH DEW POINTS IN
THE MID-UPPER 50S.

LATER TONIGHT INTO EARLY TUESDAY MORNING WILL LIKELY SEE MORE
SHOWERS AND STORMS AS AN UPPER LEVEL DISTURBANCE ADVANCES ALONG
AND TO THE NORTH OF THE FRONTAL BOUNDARY THAT WILL BE LAYING TO
THE SOUTH OF THE TAF SITES. AT THIS POINT THINK BETTER CHANCES ARE
AFTER 06Z TUESDAY.


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* MEDIUM-HIGH IN TSRA OCCURRING AND TIMING.

* LOW ON SPECIFIC WIND DIRECTION DURING THE MORNING RUSH TIME AS A
  PERIOD OF VARIABLE WITHIN THAT TIME IS LIKELY.

* LOW-MEDIUM ON TIMING OF WIND SHIFT TO NORTH-NORTHEAST.

* MEDIUM-HIGH IN MVFR CIGS BUT LOW ON TIMING AND DURATION.

* MEDIUM IN SHRA/TSRA MONDAY OVERNIGHT.


//OUTLOOK FOR ORD/MDW FOR 12Z TUESDAY-12Z SUNDAY...UPDATED 00Z...

TUESDAY...SHRA WITH PERIODS OF TSRA LIKELY. MVFR LIKELY AT TIMES. EAST-
NORTHEAST WINDS.

WEDNESDAY...CHANCE OF TSRA...THOUGH PRIMARILY VFR. SOUTHEAST WINDS.

THURSDAY...VFR. SOUTH WINDS.

FRIDAY AND SATURDAY...CHANCE OF TSRA. SOUTH WINDS...BECOMING NORTH LATE
SATURDAY.

MTF/RATZER

&&

.MARINE...
253 PM CDT

BRISK SOUTH-SOUTHWEST WINDS PREVAILING ACROSS LAKE MICHIGAN WILL
EASE THIS EVENING.  A VERY STABLE VERTICAL PROFILE RESULTING FROM
THE WARM MOIST AIRMASS ADVECTING NORTH OVER COLD LAKE WATER ALSO IS
INHIBITING BETTER TRANSFER OF HIGHER MOMENTUM AIR ALOFT TO THE LAKE
SURFACE.

WITH WINDS EASING OFF THIS EVENING WILL ALLOW SMALL CRAFT ADVISORY
TO EXPIRE AS SCHEDULED.  RELAXING PRESSURE GRADIENT OVERNIGHT IN
ADVANCE OF APPROACHING COLD FRONT WILL CAUSE WINDS TO SLACKEN
FURTHER OVERNIGHT.  THE COLD FRONT ITSELF DROPS SOUTHEAST ACROSS
ABOUT THE NORTH HALF OF THE LAKE BY 0700-0800 CDT MONDAY... AND THEN
REACHES THE FAR SOUTH END OF THE LAKE BY MID AFTERNOON MONDAY.

WINDS WILL BECOME NORTHWEST AND THEN QUICKLY NORTH BEHIND THE FRONT
MONDAY... AS THE FRONT THEN STALLS JUST SOUTH OF LAKE MICHIGAN
MONDAY NIGHT.  WINDS ACROSS THE LAKE WILL BECOME NORTHEAST TO EAST
LATER MONDAY NIGHT AND TUESDAY... BEFORE THE FRONT LIFTS BACK NORTH
AS A WARM FRONT TUESDAY NIGHT AND WEDNESDAY.  GIVEN THE PRESENCE OF
A WARM MOIST AIRMASS WOULD EXPECT FOG TO DEVELOP OVER THE LAKE
TUESDAY INTO WEDNESDAY. AS THE FRONT THEN LIFTS BACK NORTH... THIS
WILL SET THE STAGE FOR SOUTHERLY WINDS ON THE LAKE FOR MUCH OF LATE
WEEK.

ED F

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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