Area Forecast Discussion
Issued by NWS Northern Indiana

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829
FXUS63 KIWX 181759 CCA
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
148 PM EDT MON MAY 18 2015

.SYNOPSIS...
ISSUED AT 135 PM EDT MON MAY 18 2015

A COLD FRONT DROPPING ACROSS THE AREA THIS AFTERNOON WILL BRING A
CHANCE FOR SHOWERS AND ISOLATED THUNDERSTORMS THROUGH EARLY EVENING.
COOLER AND DRIER AIR WILL SETTLE IN FOR THE WEEK. HIGHS THIS
AFTERNOON WILL REACH THE LOWER 80S. LOWS TONIGHT WILL DROP BACK
INTO UPPER 40S TO LOWER 50S. EXPECT MUCH COOLER TEMPERATURES TUESDAY
AND WEDNESDAY...WITH HIGHS IN THE MID 50S TO LOWER 60S.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 358 AM EDT MON MAY 18 2015

DRIER MID LEVEL AIR HAS ADVECTED INTO MOST OF THE FORECAST AREA THIS
MORNING. DEEPEST MOISTURE THROUGH THE MORNING HOURS IS EXPECTED TO
RESIDE ACROSS APPROXIMATELY THE SOUTHEAST THIRD OF THE AREA WHERE
SOME UPSWING IN 850-700 HPA MOISTURE TRANSPORT IS EXPECTED OVER THE
NEXT SEVERAL HOURS. 3-6 HOUR RAP PROGS DO INDICATE ABOUT 250-500
J/KG OF INSTABILITY LINGERING ACROSS THESE LOCATIONS EXPERIENCING
STRONGER MOISTURE TRANSPORT THIS MORNING...SO HAVE CONTINUED TO
CONCENTRATE LOW CHANCE POPS ACROSS THE SOUTHEAST THIS MORNING AS
ADDITIONAL WEAK SHORT WAVES TRACK NORTHEAST THROUGH THE OHIO VALLEY
IN ASSOCIATION WITH SOUTHERN STREAM.

ATTENTION FOR THE REMAINDER OF THE DAY WILL CENTER ON EVOLUTION OF
SFC COLD FRONTAL BOUNDARY AND POTENTIAL OF ANY ADDITIONAL SCATTERED
CONVECTION THIS AFTERNOON/EARLY EVENING. INSPECTION OF SHORT TERM
TRENDS SUGGESTS NAM MAY BE JUST A BIT SLOW WITH COLD FRONT ACROSS
THE MID MS RVR VALLEY...AND THE NAM CONTINUES TO BE SOMEWHAT OF A
SLOW OUTLIER THROUGH THE NEXT 24 HOURS WITH THE FRONTAL PROGRESSION.
EVEN WITH A PROGRESSION SOMEWHAT FASTER THAN THE NAM...PREFRONTAL
WARMING SHOULD SUPPORT HIGHS IN THE LOWER 80S FOR MOST LOCATIONS
THIS AFTERNOON. OUTSIDE OF FRONTAL CONVERGENCE...MID/UPPER LEVEL
FORCING IS EXPECTED TO BE QUITE WEAK...ALTHOUGH WATER VAPOR EARLY
THIS MORNING DOES DEPICT SOME INTEGRITY TO AN UPSTREAM WAVE LIFTING
NORTHEAST OUT OF OKLAHOMA/SOUTHEAST MISSOURI.  THIS FEATURE SHOULD
RAPIDLY EJECT NORTHEAST...BUT BECOME LESS POTENT AS IT BECOMES MORE
HIGHLY SHEARED ACROSS THE OHIO VALLEY/SOUTHERN GREAT LAKES REGION
LATER THIS AFTERNOON/THIS EVENING.

SOME OF THE NAM AND HIRES OUTPUT DOES INDICATE A NARROW ZONE OF
PREFRONTAL CONFLUENCE WITH SOME SUBSTANTIAL MOISTURE POOLING. HAVE
SOME SUSPICION THE SLOWER NAM OUTLIER MAY BE A BIT AGGRESSIVE IN
TERMS OF THE MAGNITUDE OF THIS MOISTURE POOLING AND HAVE GENERALLY
TRENDED AWAY FROM THE INSTABILITY MAGNITUDES ADVERTISED BY THE NAM
ALONG THIS PREFRONTAL CORRIDOR. HAVE MAINTAINED LOW TO MID CHANCE
TSRA FOR TODAY GIVEN AT LEAST SOME WEAK MID/UPPER FORCING LIFTING
INTO THE AREA WITH AFOREMENTIONED OK/MO VORT...ALTHOUGH OUTSIDE OF
ANY AREAS OF PREFRONTAL MOISTURE POOLING...LOW LEVELS MAY MIX OUT
ENOUGH TO TEMPER INSTABILITY QUITE A BIT BY LATER THIS AFTERNOON. IF
SCATTERED CONVECTION IS ABLE TO DEVELOP THERE MAY BE TWO PRIMARY
CORRIDORS FOR INITIATION...ONE ALONG ANY WEAK PREFRONTAL CONVERGENCE
ZONE...AND THEN ALONG THE ACTUAL FRONTAL BOUNDARY. SEVERE STORMS ARE
NOT EXPECTED IN TRENDING TOWARD NON-NAM GUIDANCE FOR THERMAL
PROFILES...AND ALSO GIVEN MORE OF THE SUBSTANTIAL DEEP LAYER SHEAR
WILL BE POST-FRONTAL.

SOME ISOLATED CONVECTION MAY LINGER INTO THE EVENING ACROSS
SOUTHEAST HALF OF THE AREA...WITH MAIN STORY THROUGH THE REMAINDER
OF THE NIGHT CENTERING ON THE APPROACH OF A MUCH COOLER AND LESS
HUMID AIR MASS. SOME COLD ADVECTION STRATOCU MAY WORK INTO FAR
NORTHERN LOCATIONS OVERNIGHT...BUT OTHERWISE MOST LOCATIONS WILL
EXPERIENCE CLEARING SKIES IN THE EVENING.

&&

.LONG TERM...(TUESDAY THROUGH SUNDAY)
ISSUED AT 358 AM EDT MON MAY 18 2015

TRANQUIL WEATHER ANTICIPATED FOR MUCH OF THIS LONG TERM PERIOD.
SPLIT FLOW OVER WESTERN NOAM WITH A HIGHLY AMPLIFIED AND ENDURING
NORTHERN STREAM PATTERN ACROSS CANADA WILL LEAVE THE GREAT LAKES IN
A REGION OF STRONG CONFLUENCE AND GENERAL SUBSIDENCE FOR MOST OF
THIS WEEK. WILL ALSO BE MUCH COOLER BEHIND TODAY`S COLD FRONT WITH
PERSISTENT N/NW FLOW MAINTAINING AN EFFICIENT FEED OF CANADIAN AIR
INTO OUR REGION. EXPECT TEMPS ROUGHLY 10-15 DEGREES BELOW NORMAL
DURING THE MIDWEEK PERIOD. TUESDAY NIGHT WILL BE THE COLDEST PERIOD
WITH NUMEROUS SOURCES OF RAW AND STATISTICAL GUIDANCE SUGGESTING MID
30S IN OUR NORTHERN COUNTIES...WITH LOWER 40S IN OUR SOUTH. SOME
CONCERN FOR PATCHY FROST...ESPECIALLY IN OUR MICHIGAN COUNTIES AWAY
FROM THE LAKE. HOWEVER...HIGH BASED CLOUDS WILL BE INCREASING BY
WEDNESDAY MORNING AND MODELS SHOW ENOUGH OF A SURFACE PRESSURE
GRADIENT TO KEEP WINDS IN THE 5-10 MPH RANGE. GIVEN THESE CONCERNS
AND MARGINAL TEMPS WILL HOLD OFF ON MENTIONING ANY FROST IN THE
FORECAST AT THIS POINT AND CONTINUE TO MONITOR.

ONE WEAK SOUTHERN STREAM WAVE DOES EJECT OUT OF THE FOUR CORNERS AND
ATTEMPT TO PHASE WITH SUBTLE NORTHERN STREAM VORT MAX AS IT CROSSES
OUR AREA LATE WEDNESDAY. DEGREE OF DRY AND STABLE LOWER LEVELS IS
CONCERNING BUT FAVORABLE 300MB JET PLACEMENT AND SOME MODEST
CVA/HEIGHT FALLS COULD GENERATE A FEW LIGHT RAIN SHOWERS. NO MAJOR
QUALMS WITH INHERITED LOW CHANCE POPS BUT DID ADJUST THE TIMING UP A
LITTLE BIT BASED ON LATEST NAM AND ECMWF SOLUTIONS. NEXT CHANCE OF
RAIN THEREAFTER WILL NOT BE UNTIL SUNDAY WHEN SOUTHWEST FLOW FINALLY
RETURNS AHEAD OF NEXT WESTERN CONUS TROUGH EJECTING NORTHEAST. THIS
PATTERN SHIFT WILL ALSO SIGNAL THE RETURN OF WARMER AND MORE HUMID
CONDITIONS FOR AT LEAST THE LATTER HALF OF THE HOLIDAY WEEKEND.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 135 PM EDT MON MAY 18 2015

ABUNDANT CLOUD COVER AND LACK OF SIGNIFICANT INSTABILITY /
BOUNDARY LAYER MOISTURE IS PREVENTING CONVECTIVE DEVELOPMENT OVER
THE AREA. ISOLATED SHOWERS HAVE FORMED SOUTH AND EAST OF THE
FORECAST AREA AND EXPECT THIS TREND TO CONTINUE THROUGH THE EARLY
EVENING. AS OF 17Z THE SURFACE COLD FRONT IS LOCATED OVER THE
SOUTHERN TIP OF LAKE MICHIGAN...CROSSING THE IL/IN BORDER. IN THE
CLOUD FREE AREAS THE CU FIELD HAS EXPANDED IN THE PAST FEW HOURS
AND EXPECT THAT TREND TO CONTINUE THROUGH THE EVENING.

FRONTAL PASSAGE WILL BRING IN DRIER AIR AND EXPECT TO SEE AN
ABRUPT WIND SHIFT WITH THE PASSAGE. ADDITIONALLY ALL MODELS
INDICATE STRONG BOUNDARLY LAYER PRESSURE GRADIENT SO GUSTING WINDS
WILL PERSIST THROUGH MUCH OF THE FORECAST PERIOD. LLWS IS NOT
FORECAST...HOWEVER WITH THE ORIENTATION OF THE TROF/SFC
FRONT...EXPECT TO HAVE A BRIEF AND NARROW CORRIDOR OF TURBULENCE
DURING THE PASSAGE.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...SMALL CRAFT ADVISORY FROM 11 PM THIS EVENING TO 5 PM EDT TUESDAY
     FOR LMZ043-046.

&&

$$

SYNOPSIS...LEWIS
SHORT TERM...MARSILI
LONG TERM...AGD
AVIATION...LEWIS


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