Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040443
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1143 PM CDT SUN MAY 3 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 03Z...

* SHRA DISSIPATING OVER SRN LAKE MICHIGAN ATTM.

* SE WINDS AROUND 10 KT BECOMING SOUTH OVERNIGHT.

* CHANCE OF SHRA AND TSRA POSSIBLE AGAIN LATE TONIGHT THROUGH
  LATE MONDAY MORNING AHEAD OF COLD FRONT.

* WIND SHIFT TO NORTHEAST LATE MONDAY MORNING/EARLY MONDAY
  AFTERNOON WITH COLD FRONTAL PASSAGE.

* CHANCE OF ADDITIONAL SHRA/TSRA MONDAY EVENING.

RATZER

//DISCUSSION...UPDATED 00Z...

RELATIVELY SMALL AREA OF SHRA/TSRA MOVING ACROSS NORTHEAST IL
EARLY THIS EVENING...DRIVEN BY SMALL MID-LEVEL CIRCULATION (MCV)
WHICH ORIGINATED FROM THUNDERSTORM CLUSTER OVER NEBRASKA LAST
NIGHT. THIS FEATURE MOVING SLOWLY BUT STEADILY ALONG...AND STORMS
WILL LIKELY DIMINISH IN INTENSITY/LIGHTNING AS THEY MOVE EAST AND
CLEAR CHICAGO TERMINALS FROM WEST-EAST BETWEEN 0130-0230Z THIS
EVENING. SURFACE WIND FIELD HAS BEEN DISTURBED BY MODEST OUTFLOW
AND WEAK MESOSCALE SFC HIGH PRESSURE BUBBLE...WITH WINDS SHIFTING
NORTHEAST AND THEN EXPECTED TO VEER CLOCKWISE THROUGH NORTHEAST
TO EAST AND EAST-SOUTHEAST OVER THE NEXT SEVERAL HOURS. SPEEDS
WILL LIKELY BE NEAR 10 KT.

ADDITIONAL THUNDERSTORMS WERE OCCURRING FROM THE NORTHERN LAKES
SOUTHWEST THROUGH MSP-OMA CORRIDOR ALONG AN APPROACHING COLD
FRONT. MOST MODEL GUIDANCE IN DECENT AGREEMENT IN EVENTUALLY
BRINGING THIS ACTIVITY INTO NORTHERN IL/NORTHWEST DURING THE PRE-
DAWN AND EARLY MANURING HOURS MONDAY...AHEAD OF THE COLD FRONT
WHICH SAGS ACROSS THE AREA DURING THE DAY. HOWEVER...BROAD AREA OF
SOUTHWEST FLOW ACROSS THE REGION TONIGHT AHEAD OF THIS FEATURE MAY
ALSO RESULT IN SCATTERED TSRA DEVELOPMENT...ALONG AND NORTH OF
PSUEDO-WARM FRONT/MOISTURE GRADIENT CURRENTLY FROM ABOUT RST-BDF.
CELLS CURRENTLY JUST EAST OF BDF ARE TRACKING ALONG THIS BOUNDARY.
SUSPECT THE HIGH-RES GUIDANCE...SUCH AS THE HRRR...ARE A BIT
OVERDONE WITH CONVECTION FILLING IN TO THE NORTHWEST...THOUGH WILL
HAVE TO MONITOR. BETTER CONFIDENCE IN FRONTAL ACTIVITY BEYOND
09-10Z.

ON MONDAY...SHRA/TSRA AHEAD OF COLD FRONT SHOULD BE MOST NUMEROUS
DURING THE MORNING...WITH FRONT SAGGING SOUTH OF THE TERMINALS IN
THE AFTERNOON. THE FRONT WILL BEGIN TO LIFT BACK NORTH AS A WARM
FRONT LATER MONDAY NIGHT INTO TUESDAY...WITH ANOTHER ROUND OF
SHRA/TSRA POSSIBLE BUT LARGELY BEYOND THE CURRENT 24-30 HOUR TAF
PERIOD.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH IN SE WINDS LATE THIS EVE.

* MEDIUM IN SHRA/TSRA DEVELOPING AGAIN BEYOND 09-10Z. LOW IN ANY
  SHRA/TSRA DEVELOPING BETWEEN 03-09Z.

* HIGH FOR WIND SHIFT MIDDAY MONDAY...MEDIUM FOR TIMING.

RATZER

//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.

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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