Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 140056 AAA
AFDLOT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
756 PM CDT SUN JUL 13 2014

.DISCUSSION...
755 PM CDT

ANY CHANGES MADE TO THE GOING FORECAST WERE SMALL AND BASED ON
OBSERVATIONAL TRENDS. CONTINUE TO SEE SCATTERED SHOWERS FOCUSED
ALONG THE CONVERGENCE OF THE COOL FRONT. THESE WEAK SHOWERS ARE
ABLE TO PRODUCE SOME MINI-GUST FRONTS DUE TO STEEPENED LOW-LEVEL
LAPSE RATES AS SEEN ON THE 00Z DVN SOUNDING...WITH SOME 25-30 MPH
GUSTS NEAR THESE BASED ON OBSERVATIONS INCLUDING HERE AT THE WFO
WHERE ONE SUCH SHOWER JUST MOVED OVER. WITH THE LOSS OF HEATING
AND AS A SHEARED MID-LEVEL WAVE OVER THE AREA DEPARTS...THESE
SHOWERS SHOULD CONTINUE TO SHOW A DISSIPATING TREND AS THE MOVE
SOUTHEAST.

HAVE MADE SOME ADJUSTMENTS TO HOURLY FORECAST TEMPERATURES BASED
ON COOLING/WET- BULBING CAUSED BY THE SPOTTY SHOWERS. NO
ADJUSTMENTS MADE TO LOWS THOUGH WHICH LOOK ON TRACK. LATE TONIGHT
THERE REMAINS A SMALL CHANCE FOR A LIGHT SHOWER OR TWO AS ANOTHER
SHEARED WAVE MOVES ALONG THE MID-LEVEL BAROCLINIC ZONE AHEAD OF
THE ANOMALOUSLY DEEP UPPER TROUGH SINKING SOUTH OUT OF CANADA.
CHANCE REMAINS LOW ENOUGH NOT TO INCLUDE IN OVERNIGHT FORECAST AT
THIS TIME.

MTF

&&

.PREV DISCUSSION...
330 PM CDT

MAIN FORECAST CONCERNS/CHALLENGES ARE WITH PRECIP TRENDS TONIGHT
THROUGH MONDAY...AS WELL AS COOLING TRENDS THROUGH MID WEEK.

THE DEPARTING FRONT THIS MORNING MADE WAY FOR A CLEARING TREND
TODAY WITH CLEAR TO PARTLY CLOUDY SKIES OBSERVED ACROSS THE AREA.
DESPITE SOME WEAKER CAA TODAY...TEMPERATURES WERE STILL ABLE TO
REBOUND INTO THE LOW TO MID 80S WITH A WARMER AND MOIST
ENVIRONMENT STILL IN PLACE THIS AFTERNOON. AM MONITORING RADAR
IMAGERY THIS AFTERNOON AS ISOLATED SHOWERS CONTINUE TO DEVELOP OUT
AHEAD OF APPROACHING SURFACE TROUGH SITUATED FROM IOWA NORTHEAST
INTO WISCONSIN. THIS ACTIVITY REMAINS ISOLATED WITH THUNDER
CURRENTLY NOT BEING OBSERVED. AS THIS TROUGH SLOWLY DIPS
SOUTH...OBSERVED SPEED MAX OVER IOWA WILL SHIFT OVERHEAD THROUGH
EARLY EVENING. THIS COULD AID IN FURTHER DEVELOPMENT OF THESE
SHOWERS INTO THE EARLY EVENING ACROSS NORTH CENTRAL AND NORTHEAST
ILLINOIS...BUT STILL AM ANTICIPATING THEM TO REMAIN MORE ISOLATED.
OVERALL FORCING WILL REMAIN WEAK AND WITH LOW LEVEL LAPSE RATES
EXPECTED TO MODERATE...DONT ANTICIPATE MUCH IN TERMS OF THUNDER.
IF ANY THUNDER DOES OCCUR IT WILL CONSIST OF A RUMBLE OF THUNDER
OR TWO AND BE CONFINED ACROSS THE WESTERN CWA...WHERE HIGHER
DEWPOINT AXIS RESIDES.

ANY LINGERING SHOWERS THIS EVENING WILL DIMINISH AS THEY DRIFT
SOUTHEAST THROUGH NORTHEAST ILLINOIS...AND WITH DRY CONDITIONS
EXPECTED TO PREVAIL FOR THE REMAINDER OF THE EVENING. CONFIDENCE
DOES LOWER WITH REGARD TO PRECIP TRENDS LATER TONIGHT INTO EARLY
MONDAY MORNING...WITH GUIDANCE VARYING WITH DEGREE OF FORCING WITH
APPROACHING MID/UPPER LEVEL FEATURES AND WITH THE APPROACHING COLD
FRONT. AS THESE FEATURES ARRIVE LATER TONIGHT...LARGE SCALE ASCENT
WILL OF COURSE BE INCREASING BUT IT DOES APPEAR IS IF THE BETTER
FORCING SHOULD REMAIN TO THE WEST OF THE CWA FOR MOST OF THE
NIGHT. HAVE GONE WITH A DRY FORECAST LATER TONIGHT...BUT WILL NEED
TO MONITOR TRENDS FOR POSSIBLE EARLY TIMING OF SHOWERS. IF THE
ARRIVAL DOES COME EARLIER THAN EXPECTED...NORTH CENTRAL ILLINOIS
WILL BE THE LOCATION TO POSSIBLY OBSERVE THESE SHOWERS. THIS
TIMING DIFFERENCES COULD PLAY INTO MONDAY POP FORECAST AS
WELL...ESPECIALLY WITH THE FRONT PUSHING SOUTH THROUGH THE CWA AND
WITH AN APPROACHING LARGE UPPER LEVEL LOW DIPPING SOUTH THROUGH
THE UPPER MIDWEST. BEST CHANCES FOR PRECIP ON MONDAY WILL LIKELY
BE BY EARLY AFTERNOON WITH DIURNAL SHOWER DEVELOPMENT EXPECTED
POST FROPA ACROSS NORTH CENTRAL ILLINOIS AND WITH BETTER
DEVELOPMENT LIKELY OUT AHEAD OF FRONT ACROSS EAST CENTRAL ILLINOIS
AND NORTHWEST INDIANA. NARROW RIBBON OF INSTABILITY ALONG WITH
SLIGHTLY HIGHER DEWPOINTS COULD AID FOR SOME THUNDERSTORM
DEVELOPMENT OUT AHEAD OF FRONT...BUT AM NOT OVERLY EXCITED FOR
THUNDER AS INSTABILITY WILL REMAIN WEAK.

PRECIP TO EXIT THROUGH MONDAY EVENING...WITH MUCH COLDER AIRMASS
FILTERING SOUTH WHILE LARGE UPPER LEVEL LOW MOVES OVERHEAD. AFTER
MID TO UPPER 70S TEMPS ON MONDAY...EXPECT COOLING TREND TO AROUND
70 DEGREES ON TUESDAY AND WEDNESDAY WHILE NIGHT TIME LOWS DROP
INTO THE 50S. POSSIBLE SCATTERED SHOWERS ON TUESDAY AS CYCLONIC
FLOW STILL IN PLACE BUT WITH DRY CONDITIONS EXPECTED THROUGH THE
REST OF THE WORK WEEK.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* CHANCES FOR A BRIEF SPRINKLE THIS EVENING...CHANCES FOR SHOWERS
  AND THUNDERSTORMS MONDAY AFTERNOON.

BMD

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

WEAK COLD FRONT WILL DROP SOUTH ACROSS THE TERMINALS TONIGHT WITH
LITTLE IF ANY IMPACTS EXPECTED. WINDS MAY BRIEFLY VEER TO THE WNW OR
NW BUT SHOULD THEN BACK MORE TO A WESTERLY DIRECTION OVERNIGHT AS
WINDS DECOUPLE AND SLACKEN OFF. A FEW LIGHT SHOWERS/SPRINKLES MAY
REACH THE TERMINALS THIS EVENING BUT THE MODERATE RAIN SHOWERS
FARTHER WEST WILL STAY WEST/SOUTHWEST TONIGHT. AN UPPER LEVEL
DISTURBANCE WILL DROP ACROSS THE REGION TOMORROW AFTERNOON AND
SHOULD KICK OFF SHOWERS AND THUNDERSTORMS SOMEWHERE IN THE REGION.
FOR NOW IT APPEARS THE BEST INSTABILITY MAY STAY ALONG AND NEAR A
LINE FROM VYS NORTHEAST TO MDW OR GYY. TO THE NORTH OF THIS
LINE...VERY MODEST INSTABILITY MAY SUPPORT A FEW LIGHT/SCATTERED
SHOWERS...BUT THE MORE ROBUST CONVECTION SHOULD STAY SOUTH OF THAT
LINE. OUTSIDE OF ANY LOW PRECIP CHANCES THIS TAF PERIOD...VFR
CONDITIONS WILL PREVAIL WITH SOME SORT OF WESTERLY COMPONENT TO THE
WINDS.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW CONFIDENCE IN PRECIP AT ORD/MDW. MDW HAS SLIGHTLY BETTER
  CHANCES THAN ORD MONDAY AFTERNOON.
* HIGH CONFIDENCE IN REMAINING FORECAST ELEMENTS.

BMD

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

TUESDAY...SLIGHT CHANCE SHRA. PRIMARILY VFR.
WEDNESDAY THROUGH SATURDAY...VFR.

BMD

&&

.MARINE...
227 PM CDT

A MORE EARLY FALL-LIKE PATTERN WILL BUILD INTO THE REGION EARLY
THIS WEEK AS STRONG LOW PRESSURE OVER JAMES BAY THIS AFTERNOON
DEEPENS AND MOVES NORTHEASTWARD. THIS WILL BRING SEVERAL FRONTAL
BOUNDARIES SOUTHWARD DOWN THE LAKE WITH THE FIRST MOVING THROUGH
THIS EVENING...WITH ANOTHER PASSING LATER TOMORROW. WESTERLY WINDS
WILL TURN MORE SOUTHERLY TONIGHT THEN WESTERLY MONDAY. STRONG
HIGH PRESSURE WILL BE MOVING SOUTHWARD INTO THE NORTHERN PLAINS
BEHIND THE NEXT PASSING COOL FRONT. THIS WILL BRING A FAIRLY
STRONG PRESSURE GRADIENT ACROSS THE LAKE MONDAY EVENING AND MONDAY
NIGHT IN CONJUNCTION WITH A PUSH OF MUCH COOLER AIR. THIS WILL
LIKELY SET UP THE FIRST UNSTABLE CONDITIONS WE HAVE SEEN OVER THE
LAKE IN QUITE A WHILE SUPPORTING THE MIXING OF STRONGER NORTHWEST
WIND GUSTS TO THE SURFACE. THE STRONGEST GRADIENT WILL BE ACROSS
THE SOUTHERN PART OF THE LAKE AND IT DOES APPEAR THAT 20 TO 25 KT
GUSTS WILL BE LIKELY INTO MONDAY NIGHT. THE PRESSURE GRADIENT
WEAKENS TUESDAY ALLOWING WINDS TO DIMINISH BEFORE THE HIGH BUILDS
OVERHEAD WEDNESDAY. THE INDIANA NEARSHORE WATERS MAY REACH SMALL
CRAFT CRITERIA FOR WINDS AND WAVES MONDAY EVENING AND MONDAY
NIGHT. BY WEDNESDAY...ONSHORE FLOW WILL LIKELY DEVELOP WITH LIGHT
SPEEDS WHICH MAY PERSIST INTO FRIDAY AT WHICH POINT THE BULK OF
THE HIGH WILL BE TO THE EAST. A STEADIER SOUTHERLY FLOW SHOULD TRY
TO SET UP NEXT WEEKEND BUT THE MAIN PRESSURE GRADIENT WILL STILL
BE TO THE WEST SO DAYTIME ONSHORE FLOW MAY CONTINUE SOUTH WHERE
THE PRESSURE GRADIENT LOOKS TO BE WEAKER.

ONE OTHER ITEM FOR MONDAY NIGHT IS THE POTENTIAL FOR WATERSPOUTS.
LATEST GUIDANCE SHOWS 850 MB TEMPS COOLING TO AROUND +4 OR +5 C
INTO THE OVERNIGHT HOURS LEADING TO A LAKE-850 DELTA T OF AROUND
+12 C. SOME OF THE LATEST FORECAST SOUNDINGS SHOW A PERIOD OF DEEP
INSTABILITY WITH EQUILIBRIUM LEVELS IN THE 20-25 KFT RANGE USING
TODAYS WATER TEMPS AT THE MID LAKE BUOYS. THESE CONDITIONS WOULD
BE ON THE LOWER END OF WATER SPOUT POTENTIAL SO WILL CONTINUE TO
MONITOR FOR NOW.

MDB

&&

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

$$

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