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FXUS63 KLOT 271405
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
905 AM CDT SUN APR 27 2014
.DISCUSSION...
339 AM CDT
A PERIOD OF ACTIVE WEATHER WILL UNFOLD THE NEXT SEVERAL DAYS
ACROSS THE MIDDLE OF THE COUNTRY...WITH THE FORECAST AREA
APPEARING MORE LIKELY TO EXPERIENCE SOME OF THIS. A PROBABLE
EPISODE OF HEAVY RAINFALL SHOULD UNFOLD TONIGHT ALTHOUGH IMPACTS
ARE LIKELY TO BE MINIMAL WITH THIS. THE CHANCE FOR SEVERE STORMS
WILL BEGIN THIS EVENING AND PERSIST INTO TUESDAY...BUT THE MOST
FAVORED WINDOW AND OF GROWING CONCERN IS MONDAY AFTERNOON AND
EVENING.
SYNOPSIS...
THE MAIN INSTIGATOR TO THE UPCOMING WEATHER IS A CLOSED UPPER LOW AT
THE SOUTHERN END OF A NEGATIVELY TITLED TROUGH THAT IS MOVING
INTO WESTERN OK/KS THIS MORNING. TO QUANTIFY THE ANOMALOUS NATURE
OF THIS LOW...THIS CENTER OF CIRCULATION FROM THE MID-LEVELS TO
THE SURFACE IS AT 3-4 STANDARD DEVIATIONS BELOW NORMAL FOR THIS
TIME OF YEAR. LARGE SCALE DIVERGENCE AND DIFFLUENCE ALOFT AHEAD OF
THIS FEATURE IS ALLOWING FOR BROAD BUT STRONG LOW-LEVEL WARM AIR
ADVECTION ACROSS THE MIDWEST WITH NUMEROUS AREAS OF SHOWERS AND
STORMS EARLY THIS MORNING. THE UPPER LOW WILL BEGIN TO SLOW ITS
ADVANCE EASTWARD THIS AFTERNOON INTO TONIGHT...EVEN PROBABLY
BECOMING STATIONARY AT TIMES THROUGH TUESDAY. BUT AS IT GYRATES
WITH SEVERAL MID-LEVEL JET MAXIMUMS/IMPULSES ROTATING AROUND
IT...CONVECTION IS ANTICIPATED TO BE FREQUENT AHEAD OF IT AND
IN PARTICULAR NEAR AND AHEAD OF ITS SURFACE BOUNDARIES. THE
SURFACE LOW IS EXPECTED TO REMAIN JUST EAST OF THE UPPER LOW CENTER
AND DRIFT OVER OR NEAR THE FORECAST AREA TUESDAY NIGHT. THE SYSTEM
WILL THEN OCCLUDE AND GIVEN THE HIGH AMPLIFIED PATTERN IS LIKELY
NOT TO MOVE MUCH THE LATTER HALF OF THE WEEK...MEANING BELOW
NORMAL TEMPS AND CHANCES FOR SHOWERS.
MUCH OF TODAY...
THE WARM AIR ADVECTION ENVELOPE ALONG WITH INCREASING PWATS ARE
SPREADING OVER THE FORECAST AREA THIS MORNING FROM THE
WEST-SOUTHWEST. A SUBTLE IMPULSE CENTERED IN EASTERN IOWA AS OF 3 AM
HAS HELPED TO FOCUS SOME OF THIS AND AS IT CONTINUES TO MOVE
NORTHEAST THIS MORNING SHOULD BRING SOME SHOWER ACTIVITY AND
ISOLATED THUNDER WITH IT...MAINLY FOR THE WESTERN AND NORTHERN
FORECAST AREA. THERE LOOKS TO BE A LULL IN COVERAGE OR EVEN SHOWER
ACTIVITY ALTOGETHER DURING THE LATE MORNING THROUGH MID AND MAYBE
EVEN LATE AFTERNOON BEHIND THIS LEAD WAVE. STIFF EASTERLY WINDS
WILL ONCE AGAIN MEAN ONSHORE FLOW INTO NORTHEAST IL AND
TEMPERATURES HOLDING WELL BELOW LATE APRIL NORMALS. LOCAL
CLIMATOLOGY THIS TIME OF YEAR FOR SUCH ONSHORE FLOW WOULD PRESENT
A TEN DEGREE GRADIENT FROM OUTLYING AREAS OF THE CHICAGO METRO TO
DOWNTOWN AND CONTINUE TO ADVERTISE THAT IN THE FORECAST. ALREADY
AS OF 3 AM...TEMPERATURES ARE IN THE LOW TO MID 60S IN PARTS OF
DOWNSTATE IL. WHILE THIS IS NORTH OF THE WARM FRONT...60S ARE
EXPECTED TO REACH INTO THE FORECAST AREA WITH EVEN LOWER 70S
POSSIBLE TOWARD THE I-74 CORRIDOR.
LATE THIS AFTERNOON INTO TONIGHT...
THE MAIN SURFACE WARM FRONT LOCATED FROM WESTERN MO TO NORTHERN MS
VERY EARLY THIS MORNING WILL MARCH NORTHWARD THIS AFTERNOON.
CONVECTIVE DEVELOPMENT IS EXPECTED ALONG THE ENTIRE LENGTH OF THIS
FRONT FROM THE PLAINS TO THE LOWER OHIO VALLEY THIS AFTERNOON...AS
WELL AS NORTH OF IT BY LATE IN THE DAY AS RICHER MOISTURE IS
TRANSPORTED NORTHWARD. SOME STORMS...POSSIBLY STRONG/SEVERE...MAY BE
APPROACHING BOTH THE WESTERN AND SOUTHERN FORECAST AREAS BY LATE IN
THE DAY.
AREAS OF CONVECTION ARE EXPECTED TO PROPAGATE/DEVELOP NORTHEASTWARD
ACROSS THE FORECAST AREA DURING THE EVENING WITH THE GREATEST
COVERAGE OVERNIGHT. ELEVATED INSTABILITY WILL CONTINUE TO GROW
ALTHOUGH PROFILES INDICATE MAINLY TALL/NARROW INSTABILITY IN THE
VERTICAL...AND SOMEWHAT MARGINAL VALUES...WITH LITTLE IN THE -10C
TO -30C RANGE. SO WHILE NOT EXPECTING MUCH FOR SEVERE DEVELOPMENT
IN THE FORECAST AREA...WILL NEED TO KEEP A CLOSE EYE ON REMNANT
ORGANIZED SEVERE CONVECTION FROM THE SOUTHWEST/WEST AND IF IT CAN
MAKE IT INTO THE FORECAST AREA THIS EVE. PWATS WILL APPROACH 1.50
INCHES BY OVERNIGHT...WHICH IS THE 95TH-99TH PERCENTILE FOR
APRIL/MAY. THERE IS CERTAINLY STOUT REPLENISHING MOISTURE
TRANSPORT ON A 50KT PLUS LOW-LEVEL JET ALONG WITH DEEP WARM CLOUD
DEPTHS. PROPAGATION OF HEAVY-RAIN PRODUCING STORM CLUSTERS LOOKS
TO BE ENOUGH TO PREVENT RAINFALL AMOUNTS GETTING TOO OUT OF
HAND...AND SOMEWHAT DRY ANTECEDENT CONDITIONS WILL HELP AS WELL TO
LIKELY KEEP THE FLASH FLOOD THREAT LOW THROUGH TONIGHT...AS LEAST
IN MOST AREAS. HAVE ONE TO ONE AND A HALF INCHES FORECAST BUT AS
ALWAYS WITH CONVECTIVE HIGH PWAT SCENARIOS SOME MAXIMUMS HIGHER
THAN THE AVERAGE ARE INEVITABLE.
MONDAY...
MORNING RAIN AND EMBEDDED CONVECTION IS LIKELY WHICH IS EXPECTED TO
SHIFT NORTHWARD. THE SURFACE LOW ON A COMPOSITE OF THE NAM/GFS/EC
IS 988MB NOT FAR WEST OF OMAHA BY MID-AFTERNOON. THE SURFACE WARM
FRONT ON ALL THESE GUIDANCE MEMBERS IS EXPECTED TO BE MOVING
NORTHWARD INTO THE SOUTHERN FORECAST AREA DURING THE DAY MONDAY
AND LIKELY DIABATICALLY ENHANCED WITH THE CLOUDS/RAIN NORTH OF
THIS FEATURE.
THIS LOOKS TO BE A POTENTIAL RIPE FOCUS FOR SEVERE
CONVECTION AS THE NEXT MID-LEVEL SPEED MAX ROTATES OVER EASTERN
IA/MO/CTRL AND NRN IL DURING MONDAY AFTERNOON INTO EVENING. MODEL
WIND AND INSTABILITY PROFILES ARE CERTAINLY SUPPORTIVE OF
SUPERCELLS GIVEN THE WARM FRONT PLACEMENT. THE POTENTIAL WOULD
EXIST FOR TORNADOES AND LARGE HAIL WITH THESE STORMS IF THEY CAN
DEVELOP. THERE OBVIOUSLY IS QUITE A BIT OF CONVECTION EXPECTED
WITH THIS SYSTEM THROUGH THE NEXT 24-30 HOURS AND THAT WILL
INFLUENCE MODEL SOLUTIONS GREATLY GIVEN THE MASSIVE AMOUNTS OF
LATENT HEAT RELEASE THE MODELS ARE DEPICTING AND PROPAGATING THE
OVER THESE UPCOMING PERIODS. SO WHILE HAVE NOT GOT TOO PRECISE WITH
THE WARM FRONT DEPICTION IN THE FORECAST...CONTINUE TO TIGHTEN THE
TEMPERATURE GRADIENT WITH UPPER 40S/LOWER 50S FORECAST ALONG PARTS
OF THE NORTHEAST IL LAKE SHORE...TO NEAR 80 IN THE SOUTHERN
FORECAST AREA...THOUGH LIKELY AM NOT TIGHT ENOUGH WITH THIS
AND IT WILL FURTHER BE REFINED AS CONFIDENCE ON EXACT PLACEMENT
GROWS. ALSO HAVE ADDED TEMPORAL RESOLUTION TO POPS TO INDICATE
MORNING AND LATE AFTERNOON/EVE BEING THE BEST TIMES. CIPS BEST
ANALOG MATCHES FOR THIS SOMEWHAT ATYPICAL SYNOPTIC SETUP THIS TIME
OF YEAR DO INDICATE SEVERE WEATHER POTENTIAL WITH A HANDFUL OF
TORNADOES ACROSS NW/NC/CTRL IL. WILL HIGHLIGHT FURTHER IN THE
HWO.
TUESDAY AND BEYOND...
WITH THE SURFACE LOW STILL TO THE WEST ON TUESDAY...IT WOULD BE
CONCEIVABLE THAT ENOUGH INSTABILITY WILL REMAIN PRESENT FOR SOME
POTENTIAL OF ISOLATED STRONGER STORMS ON TUESDAY. UNCERTAINTY THOUGH
ON CONVECTIVE SPECIFICS OF COURSE IS QUITE HIGH BY THAT FAR OUT.
WIND DIRECTION WILL INFLUENCE TEMPERATURES HIGHLY BUT WITH THE WARM
FRONT NORTH OF THE AREA...EXPECTING OFFSHORE FLOW WITH DOWNTOWN
CHICAGO LIKELY TO BE ITS WARMEST THIS WORKWEEK ON TUESDAY. WEDNESDAY
AND BEYOND HAVE MANY PERIODS WITH CHANCE/SLIGHT CHANCE OF SHOWERS AS
THE UPPER LOW ROTATES TO THE NORTH. IT CERTAINLY IS NOT A WASHOUT
PERIOD BUT JUST TOUGH TO REFINE SPECIFIC TIMES. IT LIKELY WILL BE
THE AFTERNOON AND EARLY EVE AS THE HIGHEST CHANCES...BUT THAT WILL
BE FURTHER REFINED IN TIME. TEMPERATURES ARE VERY FAVORED WITH THIS
SETUP TO BE BELOW NORMAL.
www.nws.noaa.gov/view/prodsByState.php?state=IL&prodtype=discussion
