Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210240
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
940 PM CDT MON APR 20 2015

.SHORT TERM...
248 PM CDT

THROUGH THURSDAY...

ANONYMOUSLY LOW MID-LEVEL HEIGHTS...ASSOCIATED WITH A LARGE AND VERY
SLOW MOVING MID/UPPER LEVEL LOW ACROSS THE NORTH CENTRAL
CONUS...WILL RESULT IN A PROLONGED PERIOD OF BELOW TO MUCH BELOW
NORMAL TEMPERATURES ACROSS MUCH OF THE GREAT LAKES REGION. AS
SUCH...THE MAIN FORECAST CONCERNS DURING THE SHORT TERM PERIOD WILL
THEREFORE LARGELY REVOLVE AROUND TEMPERATURES AND ALSO WINDS OVER
THE NEXT 24 TO 30 HOURS. IT APPEARS WE WILL HAVE A DECENT POTENTIAL
FOR SOME AREAS OF NORTHERN ILLINOIS TO APPROACH THE FREEZING MARK
FOR LOW TEMPERATURES BY MID-WEEK...SO FROST AND FREEZE ISSUES COULD
ARISE.

SATELLITE IMAGERY THIS AFTERNOON LOOKS MORE LIKE ITS LATE OCTOBER
RATHER THAN LATE APRIL...WITH COLD SEASON CLOSED CELLULAR CUMULUS
EVIDENT OVER THE REGION. TEMPERATURES ACROSS THE REGION ALSO
RESEMBLE FALL-LIKE CONDITIONS WITH TEMPERATURES UNDER THE CLOUD
COVER STRUGGLING TO MAKE IT NEAR 50 DEGREES. THEN OF COURSE THE
GUSTY WESTERLY WINDS UP AROUND 30 KT...CERTAINLY MAKES IT FEEL EVEN
COLDER THEN THE CURRENT TEMPERATURES SUGGEST.

ALTHOUGH WIND GUSTS WILL EASE THIS EVENING...WIND SPEED WILL
INCREASE AGAIN ON TUESDAY. A LARGE LOWER AND UPPER LOW WILL REMAIN
NEARLY STATIONARY ACROSS THE UPPER MIDWEST AND SOUTHERN ONTARIO INTO
TUESDAY...AND THIS WILL CONTINUE TO RESULT IN STOUT PRESSURE
GRADIENT ACROSS THE AREA...SUPPORTING ANOTHER WINDY DAY. IT APPEARS
THAT WIND GUSTS TUESDAY AFTERNOON COULD BE SEVERAL KNOTS STRONGER
THAN THEY WERE TODAY...AS DEEP ATMOSPHERIC MIXING OVER 8,000 FEET
AGL IS EXPECTED.  WITH WINDS OF 50 TO 60 KT AT THIS HEIGHTS...A
PERCENTAGE OF THIS MAGNITUDE WILL LIKELY BE MIXED TOWARDS THE
SURFACE. THIS LOOKS TO RESULT IN 35 TO POSSIBLY 40 KT WIND GUSTS
OVER THE AREA. BECAUSE OF THIS...A WIND ADVISORY MAY BE NEEDED FOR
TUESDAY AFTERNOON. HOWEVER...AFTER COLLABORATING WITH SURROUNDING
OFFICES...WE HAVE CHOSEN TO LET THE MIDNIGHT SHIFT GET ANOTHER LOOK
AT THINGS BEFORE LOCKING INTO A WIND ADVISORY HEADLINE. EITHER WAY
IT WILL BE A WINDY DAY TUESDAY.

IN ADDITION TO THE WINDS...A FEW SCATTERED SHOWERS ARE POSSIBLE
LATER TUESDAY AFTERNOON...MAINLY NORTHERN AREAS. SEVERAL SMALL SCALE
DISTURBANCES ARE EXPECTED TO ROTATE EAST-SOUTHEASTWARD THROUGH THE
LARGER SCALE MID/UPPER LEVEL LOW OVER THE UPPER MIDWEST. THE
COMBINATION OF THIS AND STEEP LOWER LEVEL LAPSE RATES DUE TO THE
UNSEASONABLY COLD AIR ALOFT...COULD SUPPORT A FEW AFTERNOON SHOWERS.
IT THESE SHOWERS DO INDEED DEVELOP FAR ENOUGH SOUTH INTO NORTHERN
ILLINOIS THE LOW FREEZING LEVELS AND STEEP LAPSE RATES THROUGH THE -
10 TO -20 C LEVELS COULD RESULT IN SOME GRAUPEL REACHING THE
SURFACE...AND OR AN ISOLATED LIGHTNING STRIKE. THOUGH CONFIDENCE IN
THIS IS LOW.

THERE APPEARS TO BE A SLIGHTLY BETTER CHANCE FOR SOME RAIN SHOWERS
TUESDAY NIGHT...MAINLY ACROSS MY SOUTHERN AREAS AS YET ANOTHER MID
LEVEL DISTURBANCE SHIFTS OVER THE AREA. THIS COULD INTERACT WITH THE
LOWER LEVEL BAROCLINIC ZONE OVER CENTRAL PORTIONS OF THE STATE TO
RESULT IN A BAND OF ENHANCED FGEN...AND HENCE A FOCUS FOR FORCED
ASCENT OVER SOUTHERN SECTIONS OF THE AREA TO SUPPORT SOME RAIN
SHOWERS. AT THIS TIME I WILL ADD IN SOME LOW END POPS ACROSS MY
SOUTHERN CWA TO COVER THIS THREAT.

FOR WEDNESDAY AND THURSDAY WE LOOK TO CONTINUE UNDER THIS ABNORMALLY
CHILLY WEATHER PATTERN. HOWEVER...THE GOOD NEWS IS WE WILL LIKELY
SEE MORE SUN AND LIGHTER WINDS...ESPECIALLY BY THURSDAY.
THEREFORE...IT SHOULD NOT FEEL QUITE AS COLD AS TODAY. AT NIGHT
THOUGH WE COULD BE LOOKING AT NEAR FREEZING CONDITIONS BY THURSDAY
MORNING...THOUGH WIND SPEEDS MAY NOT DROP OFF QUITE ENOUGH TO
RESULT IN WIDESPREAD FROST UNTIL FRIDAY MORNING. THIS WILL NEED TO
BE WATCHED.

KJB

&&

.LONG TERM...
248 PM CDT

THURSDAY NIGHT THROUGH MONDAY...

UNSEASONABLY COOL TEMPERATURES AND PERIODIC SHOWERS PERSIST THROUGH
THE WEEKEND AS THE UPPER LOW AND ITS SURFACE COUNTERPART CONTINUE TO
SPIN OVER QUEBEC THROUGH THE END OF THIS WEEK AND INTO AT LEAST THE
EARLY PART OF NEXT WEEK.  WITH SPLIT FLOW ACROSS THE WESTERN
STATES...OUR BEST CHANCE OF PRECIP THROUGH THIS PERIOD APPEARS TO BE
FOCUSED IN THE SOUTHERN PORTION OF THE FORECAST AREA AS SHORTWAVES
GET EJECTED FROM THE SOUTHERN STREAM TROUGH.  ENSEMBLES HAVE A
DIFFERENT HANDLING OF THIS ENERGY THAN DO THE DETERMINISTIC
SOLUTIONS...AND A DETERMINISTIC SOLUTION LIKE THE ECMWF HAS ITS
SHORTWAVES QUITE A BIT STRONGER AND SLOWER AND FARTHER NORTH THAN
THE OTHER MODELS...BUT ALL AGREE THAT THE STRONGEST WAVE REACHES THE
AREA SOMETIME ON SATURDAY.  THIS WOULD BE THE SAME SYSTEM OUTLOOKED
BY SPC TO BRING A THREAT OF SEVERE WEATHER TO THE WESTERN GULF ON
FRIDAY...AND THEN PROGGED BY THE MODELS TO BOMB OUT ALONG THE
ATLANTIC COAST LATE IN THE WEEKEND AS IT GETS PULLED INTO THE TROUGH
OF THE QUEBEC LOW.

CONFIDENCE IS NOT ENOUGH TO GO WITH MORE THAN CHANCE POPS ON
SATURDAY...AND NO OTHER PERIOD HAS ENOUGH CONSISTENCY IN THE
GUIDANCE TO INCLUDE MENTIONABLE POPS AT ALL...THOUGH THIS DOES NOT
NECESSARILY MEAN THE ENTIRE EXTENDED PERIOD WILL REMAIN DRY.  BUT
EVEN IN THE MOST NORTHERN SOLUTION AS SHOWN BY THE ECMWF...GULF
WARMTH AND MOISTURE REMAIN CUTOFF TO THE SOUTH...SO PRECIP LOCALLY
WOULD LIKELY BE IN THE FORM OF SHOWERS RATHER THAN THUNDERSTORMS.

LENNING

&&

.FIRE WEATHER...
443 PM CDT

...ERRATIC FIRE BEHAVIOR TUESDAY...

STRONG WINDS ARE LIKELY DURING THE DAY TUESDAY WITH WINDS EXPECTED
TO SLIGHTLY STRONGER THAN THEY WERE TODAY. GUSTS TOMORROW AFTERNOON
ARE EXPECTED TO BE IN THE 40 TO 45 MPH RANGE. DEEP MIXING INTO A DRY
AIRMASS WILL RESULT IN EFFECTIVE MIXING OF DEW POINTS DURING THE
DAY. CURRENT PROJECTIONS ARE FOR AFTERNOON MINIMUM RH VALUES TO FALL
INTO THE 25 TO 35 PERCENT RANGE HOWEVER SEVERAL MODELS HINT THAT WE
MAY TRIM AN ADDITIONAL FEW DEGREES OFF OF DEW POINTS RESULTING IN RH
VALUES CLOSER TO 20 PERCENT.

WINDS WILL LIKELY EXCEED RED FLAG WARNING CRITERIA AND MIN RH VALUES
WILL BE IN THE BALLPARK OF RED FLAG WARNING CRITERIA...HOWEVER GIVEN
YESTERDAYS HALF TO THREE QUARTERS OF AN INCH OF RAIN...10 HOUR FUEL
MOISTURE MAY NOT REACH RED FLAG CRITERIA. FASTER DRYING FUELS
HOWEVER WILL LIKELY DRY OUT QUICKLY GIVEN THE CONDITIONS TOMORROW
RESULTING IN AN ELEVATED FIRE DANGER.

BMD

&&

.AVIATION...

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

* STRONG WEST WINDS...POTENTIALLY GUSTING TO OR JUST OVER 40KT
  AT TIMES LATE TUES MORNING THROUGH THE AFTERNOON

IZZI

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

WINDS WILL CONTINUE TO EASE THIS EVENING WITH GUSTINESS LIKELY
BECOMING FAIRLY INFREQUENT AFTER SUNSET. WINDS LOOK TO INCREASE
RAPIDLY AGAIN AFTER SUNRISE TUESDAY WITH SUSTAINED WINDS RAMPING
UP INTO THE 20-30KT RANGE WITH GUSTS OF 35-40KT+ LOOKING LIKELY
FROM LATE MORNING THROUGH THE AFTERNOON BEFORE SUBSIDING QUICKLY
TUESDAY EVENING. OCNL VFR CIGS EXPECTED THROUGH THE
PERIOD...ESPECIALLY TUES AFTERNOON DURING HEATING OF THE DAY.
COULD BE AN ISOLD -SHRA TONIGHT OR TUESDAY...BUT BRUNT OF
ACTIVITY LOOKS TO REMAIN SOUTH OF THE TERMINALS AND ANYTHING THAT
WERE TO OCCUR WOULD BE BRIEF AND LIKELY LITTLE/NO IMPACT.

IZZI

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS

IZZI

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

WEDNESDAY THROUGH SUNDAY...PRIMARILY VFR.

IZZI

&&

.MARINE...
304 PM CDT

WESTERLY OFFSHORE WINDS CONTINUE TO GUST UP AROUND 30 KT THIS
AFTERNOON OVER THE NEAR SHORE WATERS...AND THESE WINDS WILL BE SLOW
TO DIMINISH THIS EVENING. ANOTHER WINDY DAY IS ON TAP FOR
TUESDAY...AND MAGNITUDES LOOK TO BE STRONGER THAN THEY WERE TODAY.
THEREFORE...IT APPEARS THAT GALES OF 35 TO 40 KT WILL BE A DECENT
BET ALONG THE ILLINOIS AND INDIANA NEAR SHORE WATERS DUE TO THE OFF
SHORE COMPONENT. HOWEVER...OVER THE OPEN WATERS IT APPEARS THAT THE
NEAR SURFACE MARINE STABLE LAYER WILL NOT ALLOW THESE STRONGER WINDS
TO REACH THE WATER SURFACE. THEREFORE...THE GALE FORCE WINDS MAY END
UP ONLY EXTENDING OFF SHORE FOR A FEW MILES...WITH WEAKER WINDS OVER
THE OPEN WATERS. DUE TO THIS EXPECTATION...A GALE WATCH IS BEING
ISSUED ONLY FOR THE NEAR SHORE WATERS FOR TUESDAY.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 9 AM TUESDAY.

     GALE WATCH...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...9 AM
     TUESDAY TO 10 PM TUESDAY.

&&

$$

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