AREA FORECAST DISCUSSION
NWS CHICAGO IL
AFDCHI 900 PM CST SAT JAN 13 2001
SURFACE LOW PRESSURE OVER CENTRAL KANSAS THIS EVENING...WITH BAND OF
WARM ADVECTION PRECIPITATION ARCHING NORTHEAST INTO ILLINOIS AND
IOWA. PRECIPITATION TYPE RANGES FROM SNOW OVER NORTHWEST IOWA...TO A
MIX OF PELLETS AND FREEZING RAIN OVER EAST CENTRAL IOWA...WITH RAIN
PREDOMINANT INTO MISSOURI AND ILLINOIS. 01Z ACARS AIRCRAFT SOUNDING
OUT OF OHARE SHOWS POCKET OF VERY WARM AIR ALOFT...WITH A TEMP OF
NEARLY +6 C AT 7000 FT. DVN AND ILX 00Z SOUNDINGS INDICATE A LOT OF
DRY AIR IN THE LOW LEVELS HOWEVER...SO THAT WARM POCKET MAY COOL
RAPIDLY WITH THE ONSET OF PRECIPITATION. 1000-850 AND 850-700
THICKNESS PROGS INDICATE MAINLY RAIN...BUT WITH EVAPORATIVE COOLING
COULD HAVE SOME WET FLAKES AT TIMES...ESPECIALLY NORTHWEST. WITH
THAT IN MIND...HAVE UPDATED ZONES WITH RAIN OR RAIN MIXED WITH SNOW
DEVELOPING NORTHERN 2/3RD OF COUNTIES...WITH FAR SOUTHERN COUNTIES
LIKELY REMAINING RAIN.
SURFACE TEMPS HAVE BEEN HOLDING FAIRLY STEADY IN THE 32 TO 34 DEGREE
RANGE THROUGH THE EVENING...WITH THE EXCEPTION OF NORTHWEST INDIANA
WHERE DRIER AIR HAS LET TEMPS FALL JUST BELOW FREEZING. EXPECT THAT
AS SURFACE CIRCULATION MOVES INTO MISSOURI THAT SFC PRESSURE
GRADIENT WILL INCREASE...WITH WARM ADVECTION KEEPING TEMPS NEARLY
STEADY OR EVEN RISING A BIT DURING THE NIGHT. THEREFORE THINK THE
ONLY PART OF THE FORECAST AREA THAT MAY SEE SOME FREEZING RAIN WOULD
BE NORTHWEST INDIANA. RISING TEMPS AND RELATIVELY SHORT DURATION OF
HEAVIER PRECIP WILL PRECLUDE ANY SIGNIFICANT GLAZING PROBLEMS...AND
TEMPS HAVE BEEN ABOVE FREEZING LATELY TO HELP ROADS. INITIAL WARM
ADVECTION PRECIP SHOULD SHUT OFF IN EARLY THE MORNING WITH DRY
SLOT...THEN SCATTERED LIGHT SHOWERS DURING THE DAY SUNDAY. BETTER
CHANCE SUNDAY AFTERNOON AS UPPER LOW MOVES ACROSS THE AREA. ALL
PRECIP SHOULD BE LIGHT...AND WITH SURFACE TEMPS ABOVE FREEZING ON
SUNDAY SHOULD BE NO SIG PROBLEMS.
.CHI...NONE
$$
RATZER
AREA FORECAST DISCUSSION
NWS CHICAGO IL
AFDCHI 300 PM CST SAT JAN 13 2001
THE CYCLONE OVER SOUTHEAST KANSAS IS FORECASTED TO MOVE NORTHEAST TO
EXTREME NORTHWEST ILLINOIS AND SOUTHWEST WISCONSIN BORDER BY 03 UTC
MONDAY NIGHT. USED THE 850 MB WIND FLOW AND IMPLIED TEMPERATURE
ADVECTION FROM THE 12 UTC ANALYSIS AND MORE RECENT PROFILER DATA.
ALSO USED THE 700 AND 300 MB AND SURFACE ANALYSIS AT 12 UTC AND
PROFILER 2100 UTC MAPS AND SURFACE MAPS. WILL FORECAST RAIN AND
FREEZING RAIN FOR THE FAR NORTHWEST COUNTIES IN NORTHWEST ILLINOIS
AND A CHANCE OF FREEZING RAIN FOR AROUND CHICAGO TONIGHT. ACARS
SOUNDING SHOWS A STRONG INVERSION AROUND 7,000 FT OVER ORD AT 1613
UTC. THAT INVERSION WILL HANG AROUND AND WITH THE SNOW COVER AND
NIGHTFALL THERE MAY BE SOME FREEZING RAIN. USED A COMBINATION OF THE
AVN AND ETA MODELS WITH THE ABOVE ANALYSIS FOR PRECIPITATION TYPE
AND TEMPERATURES. THE 1000 TO 850 THICKNESS AND 850 TO 700 THICKNESS
SHOW LIQUID PRECIP THROUGH SUNDAY AND INTO SUNDAY NIGHT. EVEN THOUGH
THE 850 TO 700 LAYER COOLS MUCH FASTER THAN THE LOWER LAYER. WILL
NOT INTRODUCE SNOW IN THE FORECAST UNTIL LATE SUNDAY NIGHT AND
MONDAY MORNING. THAT IS WHEN THERE IS COLD AIR BEHIND THE CYCLONE
WILL MOVE INTO THE FORECAST AREA. USED THE MRF AND 3 TO 7
TEMPERATURE FORECAST FOR THE EXTENDED FORECAST. SINCE THE FREEZING
RAIN EPISODES ARE EXPECTED TO BE BRIEF WILL NOT ISSUE AN ADVISORY
AT THIS TIME.
COORDINATION...MKX...DVN...ILX
$$
WHW
AREA FORECAST DISCUSSION
NWS NORTHERN INDIANA
AFDFWA 1053 PM EST SAT JAN 13 2001
FOG BEGINNING TO MIX IN INDIANA COUNTIES. VISIBILITIES IMPROVING
SLOWLY BUT MOST AREAS REPORTING HALF MILE OR BETTER. EXCEPTION IN
NW OHIO AND HILLSDALE MICHIGAN WHERE FOG IS HOLDING ON. WL CONT
ADVISORY THOSE AREAS AND DROP OVER INDIANA.
NEXT CONCERN IS PCPN IN ILLINOIS MOVING N. 00Z SOUNDINGS AND ACARS
SHOWING LARGE AREA OF DRY AIR IN LOWER LEVELS ABOVE BL AND H8 TEMPS
OF +6C OVR AREA. ETA AND MESOETA DROP H8 TEMPS WITH ONSET OF PCPN
AND OMEGA BUT STILL KEEP ABOVE ZERO. EVAP COOLING APPEARS TO BE
TAKING PLACE AT SFC WITH LEADING EDGE OF COLDEST CLOUD TOPS. LAF AND
HUF HAVE DROP 2 TO 3 DEGREES LAST COUPLE HOURS. SW FA STILL AROUND 30
TO 33 SO ANY DROP THESE AREAS WILL POSE THE RISK FOR SOME FREEZING OR
FROZEN PCPN AT ONSET BEFORE WAA AND RISING TEMPS TAKE OVER. TIMING OF
PCPN INTO THIS AREA ALSO LOOKS TO BE AFTER 200 AM SO WL UP POPS TO
LIKELY CATEGORY. NEW ETA/NGM SUPPORT FASTER TIMING AND INCREASED
POPS. REST OF FCST REMAINS UNCHANGED BUT NEW MODELS TAKING UPPER LOW
LITTLE FURTHER NW AND N LEAVING MUCH OF FA IN DRY SLOT WITH ONLY
LIGHT QPF AFTER INTITIAL WAA SURGE.
.IWX...DENSE FOG ADVISORY CONTINUES NW OHIO AND HILLSDALE MICHIGAN.
LASHLEY