Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250558
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1158 PM CST MON NOV 24 2014

.UPDATE...

830 PM CST

MINOR ADDITIONAL TWEAKS TO GOING FORECAST TONIGHT...MAINLY TO
REFINE POPS FOR ADDITIONAL SCATTERED SNOW SHOWERS/FLURRIES
OVERNIGHT AND TO LOWER TEMPS A BIT WITH PERSISTENT COLD ADVECTION.

REPORTS OF FREEZING DRIZZLE HAVE TAPERED OFF AT MID-EVENING AS
ANTICIPATED PER FORECAST SOUNDING TRENDS. A COMPARISON OF ILX/DVN
00Z RAOBS SHOWS THE MINOR DIFFERENCE IN LOW LEVEL SATURATION DEPTH
WITH THE TOP OF THE ILX MOIST LAYER JUST BELOW 700 HPA AND -14C
WHILE AT DVN IT IS AT 650 HPA AND -18C AND THUS MORE LIKELY TO
CONTAIN ICE AND PRODUCE SNOW. RAP SOUNDING AND IR SATELLITE
TRENDS INDICATE THAT FREEZING DRIZZLE SHOULD BE LESS OF AN ISSUE
FOR REMAINDER OF THE EVENING AS THE COLUMN CONTINUES TO COOL AND
MOISTURE DEEPENS AGAIN WITH APPROACH OF SHORT WAVE APPROACHING
FROM NORTHEAST IA. THIS WAVE IS EXPECTED TO CONTINUE TO PRODUCE
SCATTERED SNOW SHOWERS AS IT MOVES INTO THE CWA OVERNIGHT...THOUGH
HRRR/NAM BOTH INDICATE A DECREASE IN COVERAGE AFTER MIDNIGHT OVER
OUR NORTHERN CWA. THUS WHILE FLURRIES/SNOW SHOWERS WILL BE AROUND
THROUGH MUCH OF THE NIGHT...IT APPEARS THAT ANY LIGHT ADDITIONAL
MEASURABLE SNOW (JUST A COUPLE TENTHS PERHAPS) WILL BE LIMITED TO
NORTHERN PARTS OF NORTH CENTRAL IL. SNOW SHOWERS SHOULD TAPER OFF
TO JUST FLURRIES AFTER 07-09Z (2-4 AM CST) WITH PASSAGE OF
NORTHEAST IA VORT...BASED ON EXTRAPOLATION OF RADAR LOOP.

OTHERWISE...HAVE LOWERED MIN TEMPS JUST A BIT OVERNIGHT AS WE WERE
ALREADY WITHIN A COUPLE OF DEGREES OF FORECAST MINS OUT WEST.
BLENDED IN A BIT OF MAV/MET MOS GUIDANCE WHICH LOWERED MINS ABOUT
2 DEGREES IN MANY PLACES. WINDS CONTINUE TO GUST ABOVE 35 MPH IN A
FEW SPOTS...BUT WILL GRADUALLY DIMINISH OVERNIGHT AS GRADIENT
SLOWLY WEAKENS AND SURFACE PRESSURE RISE MAXIMA CONTINUES TO EXIT
TO OUR NORTHEAST.

RATZER

&&

.SHORT TERM...
302 PM CST

THROUGH WEDNESDAY...

TONIGHT...

PRIMARY SNOW BAND/DEFORMATION AXIS HAS SHIFTED NORTH BUT
WILL STILL SEE OCCASIONAL SNOW SHOWERS MOVING INTO NORTHERN ILLINOIS
INTO THE EVENING RUSH. SOME OF THESE COULD DROP VISIBILITY DOWN
UNDER 1 MILE AT TIMES...ESPECIALLY AS WINDS CONTINUE TO GUST CLOSE
TO 45 MPH AT TIMES. GOING TO SHIFT END TIME OF ADVISORY FOR
NORTH/NW CWA TO 00Z...WITH THE WIDESPREAD ACCUMULATING SNOW
ALREADY HAVING ENDED. COULD SEE SOME LINGERING LIGHT
SNOW/FLURRIES LATE EVE/OVERNIGHT AS ADDITIONAL UPPER LEVEL ENERGY
TRAVERSES AREA IN CYCLONIC FLOW ALOFT. STRATUS DECK WILL
COUNTERACT LOW LEVEL CAA TO KEEP TEMP FALL IN CHECK TONIGHT...BUT
STILL ANTICIPATING LOW- MID 20S BY DAYBREAK TUESDAY.

TUESDAY/TUES NIGHT...

OTHER THAN POSSIBLY A FEW FLURRIES IN NORTHEAST
IL/NW IN IN THE AM AS SOME LIFT REMAINS OVER THE AREA...LAKE EFFECT
FOCUS WILL LIKELY BE TO OUR EAST WITH WESTERLY FLOW. QUESTION IS
DOES STRATUS ERODE DURING AFTERNOON INTO EVENING. AS IS
TYPICAL...WRF/NAM GUIDANCE IS MOST BULLISH ON LOW LEVEL
MOISTURE...WHILE REMAINDER OF GUIDANCE DOES ENABLE DRY AIR TO MOVE
IN LATE DAY INTO TUESDAY EVENING. CONFIDENCE IS NOT THE
HIGHEST...BUT HAVE TRENDED A BIT MORE OPTIMISTIC WITH CLOUD COVER.
WARM ADVECTION WILL RAMP UP ALOFT IN ADVANCE OF CLIPPER SYSTEM
TOMORROW NIGHT...BUT WITH A TRANSIENT SURFACE RIDGE OVER THE
AREA...CLEAR/PCLOUDY SKIES COULD ENABLE A MORE SIGNIFICANT
TEMPERATURE FALL FROM HIGHS IN THE UPPER 20S-LOW 30S. THIS IS
PARTICULARLY THE CASE FOR AREAS THAT PUT DOWN SNOW COVER TODAY. HAVE
ADJUSTED LOW TEMPS DOWN A FEW DEGREES...TO UPPER TEENS IN NORTHWEST
TO LOW-MID 20S ELSEWHERE.

WEDNESDAY...

DECIDED TREND ON GUIDANCE AND ENSEMBLES TODAY HAS BEEN TO TRACK
CLIPPER MAINLY SOUTH OF THE CWA...AND HAVE ADJUSTED POPS DOWNWARD
CHANCE SOUTHWEST CWA AND SLIGHT CHANCE ELSEWHERE. UPPER LEVEL ENERGY
ASSOCIATED WITH THIS SYSTEM WAS STILL OFF OF THE BRITISH COLUMBIA
COAST AT RAOB TIME. GIVEN TENDENCY FOR LARGE SHIFTS IN THE SHORT
TERM GUIDANCE WITH THE TRACK OF SUBTLE FEATURES SUCH AS
CLIPPERS...HAVE NOT GONE WITH A COMPLETELY DRY FORECAST YET.
HOWEVER...IF TRENDS CONTINUE...ONLY THE FAR SOUTHWEST CWA WOULD SEE
ANY SENSIBLE WX IMPACTS. TEMPS LOOK TO RISE INTO THE LOW-MID
30S...WITH A FEW DEGREES ABOVE THIS POSSIBLE IF CLIPPER TRACKS
COMPLETELY SOUTH OF THE CWA. THIS WILL BE IN ADVANCE OF NEXT SURGE
OF MODIFIED ARCTIC AIR FOR THANKSGIVING DAY.

RC

&&

.LONG TERM...

WEDNESDAY NIGHT THROUGH TUESDAY...

A SECONDARY TROUGH AXIS WILL CROSS THE REGION WEDNESDAY NIGHT...BUT
OUTSIDE OF BULLISH CANADIAN MODEL...LITTLE SUPPORT FOR ANYTHING MORE
THAN SLIGHT POPS. BIG STORY IS GLANCING BLOW OF ARCTIC AIR SENDING
H85 TEMPS INTO THE LOW-MID MINUS TEENS CELSIUS ON THANKSGIVING
DAY...ONLY SUPPORTING HIGHS FROM THE MID 20S TO LOWER 30S...COLDEST
WHERE THERE WILL STILL BE SOME LEFTOVER SNOW COVER. LAKE EFFECT
FOCUS LOOKS TO BE EAST OF OUR NW IN COUNTIES...BUT WILL STILL NEED
TO MONITOR DUE TO FAVORABLE PARAMETERS.

UPPER LEVEL PATTERN CONTINUES TO LOOK TO QUICKLY SHIFT TO MORE ZONAL
ON LATEST GUIDANCE...SUPPORTING A RAPID MODERATING TREND. PRIOR TO
THAT...GUIDANCE DOES BREAK OUT SOME LIGHT WAA PRECIP ON FRIDAY AS AN
UPPER LEVEL WAVE MOVES OVER THE REGION. BETTER FORCING LOOKS TO BE
INTO SOUTHERN WI AT THIS TIME...BUT STILL A WAYS OUT. HAVE INDICATED
SLIGHT POPS FOR LIGHT SNOW...WITH A MIX OR TRANSITION TO RAIN BY
AFTERNOON FOR PARTS OF AREA AS BOUNDARY LAYER WARMS ABOVE FREEZING.
LOW PRESSURE OVER NORTHERN PLAINS LOOKS TO ADVECT IN A MUCH WARMER
AIR MASS ON SOUTHWEST FLOW ON SATURDAY. H85 TEMPS WARMING TO
POSSIBLY UPWARDS OF +10C COULD SPELL READINGS WELL INTO THE 50S (!)
FOR AT LEAST PORTIONS OF AREA IF TRENDS HOLD. DESPITE FLOW REMAINING
ZONAL...SIGNS THAT COLD AIR OVER CANADA AND FAR NORTHERN TIER COULD
TRY TO OOZE SOUTH LATER INTO THE WEEKEND INTO EARLY NEXT WEEK. THIS
WOULD SET UP A BAROCLINIC ZONE AND POSSIBLY LOW PRECIP CHANCES
RETURNING.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* PERSISTENT SCATTERED LIGHT SNOW SHOWERS...BUT REMAINING VFR.

* POTENTIAL FOR A STRONGER SNOW SHOWER EARLY THIS FORECAST
  PERIOD...WITH POSSIBLE IFR VIS.

* MVFR CEILINGS THROUGH LATE MORNING.

RODRIGUEZ

//DISCUSSION...UPDATED 06Z...

LATEST RADAR IMAGERY SHOWING SNOW SHOWERS PERSISTING AROUND THE
TERMINALS THIS HOUR...BUT WITH NO REAL VIS RESTRICTION. THESE SNOW
SHOWERS AND VIS TRENDS WILL CONTINUE EARLY THIS MORNING BUT WITH
THE POSSIBILITY FOR A BRIER HEAVIER SNOW SHOWER EARLY...POSSIBLY
PROVIDING IFR VIS. HAVE MAINTAINED TEMPO GROUP WITH 2SM VIS BUT
DID PUSH IT BACK AN HOUR BASED ON LATEST TRENDS. ANY SNOW WILL
LIKELY COME TO AN END LATER THIS MORNING WHILE MVFR CEILINGS
SLOWLY LIFT TO VFR BY LATE MORNING...WITH VFR CEILINGS THEN
LINGERING INTO THE AFTERNOON. AS HIGH PRESSURE BUILDS ACROSS THE
REGION THIS AFTERNOON SHOULD SEE THESE CLOUDS SCATTER...WITH ONLY
HIGH LEVEL CLOUD THEN ANTICIPATED THROUGH THE REST OF THE FORECAST
PERIOD. THE STRONGER WESTERLY WINDS ARE CURRENTLY
DIMINISHING...WITH THIS TREND CONTINUING THROUGH THE REST OF THE
MORNING BUT WHILE STAYING WESTERLY.

RODRIGUEZ


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH CONFIDENCE WITH SCATTERED LIGHT SNOW SHOWERS.

* MEDIUM CONFIDENCE WITH ANY STRONGER SNOW SHOWERS EARLY.

* HIGH CONFIDENCE WITH MVFR CEILINGS...LOW MEDIUM CONFIDENCE WITH
  DURATION.

RODRIGUEZ

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

WEDNESDAY...SCHC SNOW. OTHERWISE VFR. SOUTHEAST TO EAST WINDS.

THURSDAY...A FEW FLURRIES POSSIBLE...OTHERWISE VFR. BREEZY NORTHWEST WINDS.

FRIDAY...SCHC SNOW. SOUTH WINDS.

SATURDAY...VFR. WEST-SOUTHWEST WINDS.

SUNDAY...VFR. NORTH WINDS.

MDB

&&

.MARINE...
235 PM CST

LOW PRESSURE THAT MOVED OVER THE STRAITS THIS MORNING AND DRAGGED A
COLD FRONT ACROSS THE LAKE WILL HAVE LIFTED INTO THE JAMES BAY AREA
LATE THIS EVENING.  STRONG WEST GALES ACROSS THE SOUTH HALF OF THE
LAKE WILL PERSIST INTO THE OVERNIGHT HOURS...WITH GUSTY BUT WEAKER
NORTHWEST WINDS ACROSS THE NORTHERN HALF OF THE LAKE.  SNOW MOVES
OUT TONIGHT EXCEPT FOR LINGERING LAKE EFFECT ALONG THE MICHIGAN
SHORE.

WINDS REMAIN WESTERLY BUT RELAX DURING THE DAY TUESDAY AND THEN
RELAX FURTHER BY WEDNESDAY IN A WEAK AND DISORGANIZED PRESSURE
GRADIENT.  BY WEDNESDAY NIGHT INTO THURSDAY THE PATTERN BECOMES
BETTER ORGANIZED AGAIN AND WINDS PICK UP FROM THE WEST TO NORTH.
WEATHER REMAINS MOSTLY QUIET THIS WEEK AFTER TONIGHT ASIDE FROM A
BRIEF SHOT OF SNOW AGAIN LATE WEDNESDAY AND THE MICHIGAN SIDE LAKE
EFFECT.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878 UNTIL 3 AM TUESDAY.

&&

$$

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