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January 12-14 Winter Storm Part 2


Hoosier

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AREA FORECAST DISCUSSION...UPDATED

NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL

845 PM CST WED JAN 11 2012

.DISCUSSION...

840 PM CST

AN INITIAL ANALYSIS HAS BEEN MADE OF 00Z UPPER AIR DATA AS WELL AS

THE 12.00 NAM AND 11.21 SREF. HAVE MADE MODIFICATIONS TO THE START

TIME OF THE SNOWFALL WHERE CONFIDENCE HAS INCREASED...BUT THE

WINTER WEATHER ADVISORY START TIMES STILL LOOK VALID.

RIGHT NOW THE AREA IS NICELY LOCATED UNDER A WEDGE OF SUPPRESSION

AND DRY AIR BETWEEN THE LOW OVER THE OHIO RIVER VALLEY AND THE

INCOMING DIGGING TROUGH AND ARCTIC FRONT. THIS SHOULD HOLD FOR A

WHILE AND LATEST GUIDANCE HAS CAUGHT ONTO THIS WITH SLOWER

PROGRESSION OF THE POST-FRONTAL FORCING AND SNOW ACROSS IA AND

INTO WESTERN MO. THE SURFACE COLD FRONT WILL CONTINUE

EAST...TURNING WINDS NORTHWEST AND BRINGING THE TEMPERATURES DOWN.

ALREADY THE CLEARING HAS ALLOWED FOR COOLING...AND THERE IS ALMOST

NO DOUBT THAT PRECIPITATION TYPE ONSET WILL BE SNOW GIVEN THE

COOLING COLUMN. THE NAM/RUC FORCING AND HRRR REFLECTIVITY HAVE

FORCING FOR PRECIPITATION STILL WEST OF INTERSTATE 39 BY 12Z...AND

STILL WEST OF CHICAGOLAND BY 15Z. THE SREF ENSEMBLE MEMBERS ALSO

HAVE A TIGHTER PACKING THROUGH 15Z FOR INCREASED CONFIDENCE...AND

THIS PACKING IS ON A SLOWER SOLUTION. HAVE ADDED HIGHER TEMPORAL

RESOLUTION POPS TO BETTER DEFINE THE ONSET TIME.

A NICE -37C 500MB COLD POCKET ASSOCIATED WITH THE WESTERN TROUGH

WILL DIG SOUTHEAST INTO THE LATE MORNING AND THE PHASING OF THE

SYSTEMS WILL RAPIDLY BEGIN. LARGE SCALE OMEGA INCREASES ACROSS THE

AREA WITH SATURATION WORKING ITS WAY ACROSS CHICAGOLAND AND

NORTHWEST IN. SO ONSET IN CHICAGOLAND STILL LOOKS TO BE PRIOR TO

NOON...AND LIKELY BETWEEN 9 AM AND NOON FOR MUCH OF THE AREA. THE

AFTERNOON INTO THE EARLY EVENING FOR MOST OF THE FORECAST AREA

CONTINUES TO BE THE PERIOD MOST FAVORED FOR HEAVIER SNOW. THIS IS

BECAUSE OF 1. STRONGER UPPER LEVEL SUPPORT 2. THE TIGHTEST MID-

LEVEL THERMAL GRADIENT SUPPORTING TEMPORARY BURSTS OF

MODERATE/HEAVY ACTIVITY WITHIN BANDS 3. SNOW-TO-WATER EFFICIENCIES

INCREASING WITH A COOLING COLUMN AND DEEPER DENDRITIC

GROWTH...ESPECIALLY AFTER 3 PM...AND 4. LAKE ENHANCED SUPPORT INTO

NORTHWEST INDIANA /WHICH ACTUALLY CONTINUES INTO THE NIGHTTIME

HOURS. THIS IS THE AREA THAT WILL BE PARTICULARLY WATCHED FOR

POSSIBLE UPGRADE TO A WARNING IN TIME.

THE ONLY CHANGE WITH THE THURSDAY AFTERNOON PORTION OF THE

FORECAST WAS TO SLIGHTLY INCREASE SNOWFALL AMOUNTS SOUTH OF

INTERSTATE 80. AMOUNTS ON THURSDAY AFTERNOON ACROSS THE AREA COULD

BE PRETTY HOMOGENEOUS...WITH TEMPORARY BURSTS/BANDS SUPPORTING

LOCALIZED HIGHER AREAS. THE SLOWER ONSET TIME MAY MEAN LESS

SNOWFALL IN THE MORNING ACROSS THE SOUTH THOUGH...SO OVERALL

FORECAST AMOUNTS CAME OUT FAIRLY CLOSE...WITH NO CHANGES FOR

CHICAGOLAND ON TOTALS.

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DVN just issued another update. Decided to hold off upgrading to a warning for now and wait and see if/where the best mesoscale forcing sets up.

ONCE AGAIN...A ROUGHLY 60 MILE WIDE SWATH OF OVER 6 INCHES

POSSIBLE...ALONG WITH CONSIDERABLE BLOWING/DRIFTING...STRETCHING

FROM MERCER AND HENRY COUNTY JUST SOUTH OF THE QUAD CITIES...TO

DELAWARE DUBUQUE COUNTIES IN THE NORTH. THIS REMAINS THE MOST

LIKELY AREA THAT MAY REACH OR EXCEED WARNING CRITERIA BASED ON THE

LATEST DATA.

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Update from ILX. Nice discussion.

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE LINCOLN IL

845 PM CST WED JAN 11 2012

.DISCUSSION...

ISSUED 845 PM CST WED JAN 11 2012

QUICK GLANCE AT THE 00Z NAM...AS WELL AS THE HRRR AND RUC MODELS...

INDICATES NO BIG CHANGES NEEDED TO HEADLINES OR SNOW SCENARIO

FORECAST. COLD FRONT NOW APPROACHING THE IL RIVER WITH GUSTY

NORTHWEST WINDS AND LOW CLOUDS. RADARS INDICATE THE SNOW IS LAGGING

BACK OVER CENTRAL IA...SO NO POPS THIS EVENING STILL LOOK ON TRACK.

THE NAM AND HRRR MODELS INDICATE THE LEADING EDGE OF THE SNOW

REACHING THE MIDDLE OF THE FORECAST AREA BY 12Z. WATER VAPOR LOOPS

SHOWING THE DEEPENING/AMPLIFYING TROUGH OVER THE PLAINS...WITH

INCREASING INFLECTION OVER NEB BEING THE BEGINNINGS OF THE LOW

CLOSING OFF. THE EXPANDING DARK AREA ON SATELLITE ALONG THE TROF

AXIS AND DOWN INTO THE INFLECTION OVER SOUTHERN NEB AND NORTHWESTERN

KS SUGGESTS THAT STRATOSPHERIC POTENTIAL VORTICITY IS POKING

DOWNWARD ON THE WESTERN FLANK OF THE TROUGH...WHICH WILL ACT TO

ACCELERATE THE DEEPENING PROCESS.

MOST OF THE SYNOPTIC MODEL OUTPUT AGREES ON TRACKING THE CENTER OF

THE DEEPENING UPPER LOW ACROSS THE SOUTHERN THIRD OF IL FROM LATE

MORNING TO LATE AFTERNOON THURSDAY. DURING THAT TIME...PV AND

DIFLUENCE WILL BE ROTATING THROUGH THE EASTERN QUADRANT AND INTO THE

NORTHEASTERN QUADRANT OF THE LOW...WHICH WILL EXTEND EAST/WEST

THROUGH CENTRAL IL. IN ADDITION...ISENTROPIC ANALYSES INDICATE A

DECENT UPGLIDE ON THE 300K-290K SURFACES IN THE SAME GENERAL

AREA...WITH SPECIFIC HUMIDITIES OF 1.5-2.5 G/KG. SO SNOW AMOUNTS OF

3-5 INCHES OVER THE NORTHERN HALF OF THE AREA STILL LOOK GOOD...WITH

POSSIBLE MESOSCALE ENHANCEMENT OF SNOW BANDS.

WILL FRESHEN UP THE WINTER WEATHER ADVISORY PRODUCT SHORTLY.

04

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