FXUS63 KDVN 030928
AFDDVN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
328 AM CST THU DEC 3 2009

.SYNOPSIS...
WE HAVE HAD A FAIRLY MAJOR PHASING OCCUR BETWEEN THE SOUTHERN AND
NORTHERN STREAMS OCCUR IN THE LAST 24 HOURS. LAST NIGHT WE HAD
TWO SEPARATE SYSTEMS...ONE DIVING OUT OF CANADA AND ANOTHER IN
SOUTHERN TEXAS.  WE STILL HAVE TWO DISTINCT LOW CENTERS...BUT THEY
ARE BOTH PART OF A LARGER UPPER LEVEL TROUGH WHICH COVERS NEARLY ALL
OF THE CONUS.  THE SOUTHERN LOW HAS SWUNG NORTHEAST UP INTO KENTUCKY
AS OF 00Z...AND IS MOVING FAST ENOUGH THAT AS OF 08Z IT IS ALREADY
UP INTO OHIO AND SHOULD BE IN WESTERN PENNSYLVANIA/NEW YORK BY 12Z.
THIS SYSTEM HAS NOW MOVED FAR ENOUGH EAST THAT WE HAVE NO SENSIBLE
WEATHER FROM IT.  THE NORTHERN UPPER LOW IS WHAT WILL BE OF CONCERN
TO US IN THE NEXT 24 HOURS.  IT WAS LOCATED OVER SOUTHERN NORTH
DAKOTA AT 00Z...AND APPEARS TO HAVE ROTATED SOUTH INTO EASTERN SOUTH
DAKOTA AS OF 08Z...AND IS CONTINUING TO MOVE IN A SOUTHEAST
DIRECTION.  THESE POSITIONS ARE BASED OFF SATELLITE IMAGERY AND THE
RUC MODEL.  SNOW SHOWERS ARE ACCOMPANYING THIS UPPER LOW...CURRENTLY
AFFECTING PORTIONS OF SOUTH DAKOTA...SOUTHERN MINNESOTA...AND ARE
BEGINNING TO GET INTO NORTHWEST IOWA.  A TIME/DISTANCE ANALYSIS OFF
THE CURRENT MOVEMENT WOULD BRING THESE SNOW SHOWERS INTO THE
FORECAST AREA AROUND MID MORNING TODAY.    ..LE..

&&

.SHORT TERM...TODAY AND TONIGHT...
THE MAIN KEY FOR TODAYS FORECAST ARE THE MID LEVEL LAPSE RATES THAT
ACCOMPANY THE UPPER LEVEL LOW AND ITS COLD POOL ALOFT.  THE UPPER
LEVEL LOW HAS A NICE VORT MAX WITH IT THAT WILL MOVE ACROSS IOWA
TODAY AS THE UPPER LEVEL LOW SWINGS ACROSS THE STATE.  THIS...ALONG
WITH THE MID LEVEL LAPSE RATES OF 7 TO 8 DEG C/KM IN THE 850-700MB
LAYER SHOULD GENERATE A SIGNIFICANT BAND OF SNOW SHOWERS...
ESPECIALLY ALONG AND NORTH OF THE UPPER LOW TRACK AND IN THE STEEPEST
LAPSE RATES.  MODELS ARE IS FAIRLY GOOD AGREEMENT ON THIS...AND HAVE
FAIRLY HIGH CONFIDENCE ON THIS...AND HAVE RAISED POPS WELL OVER
GUIDANCE ACROSS THE NORTHERN CWA...AND HAVE LOWERED THEM SOUTH.
CURRENTLY AM LOOKING AT ABOUT A HALF INCH OF ACCUMULATION...THOUGH UP
TO AN INCH IS POSSIBLE IN NARROW BANDS UNDER THE STRONGER SHOW
SHOWERS. WITH THIS KIND OF SYSTEM WE ALSO TEND TO GET VERY DEEP
MIXING...AND WE MAY BE AS HIGH AS 850MB BY EARLY AFTERNOON.  THUS...
WE HAVE MAX TEMPERATURES IN THE 30-35 RANGE NEARLY EVERYWHERE.  ONE
TRICKY PART IS THAT THIS MORNING MOST LOCATIONS ARE HOLDING NEARLY
STEADY AROUND 30...HAVING DROPPED INTO THE LOW-MID 20S WHERE A NARROW
CLEAR SLOT HAS DEVELOPED OVER A PORTION OF EASTERN IOWA.  THE
REMAINDER OF THE SITES MAY DROP FARTHER YET...BUT THAT WOULD MEAN A
WARMER START TO THE DAY...AND WARMER MAX TEMPS...ESPECIALLY
NORTHWEST.  HOWEVER... THE STRONG COLD AIR ADVECTION AND AT LEAST
BROKEN CLOUDS WILL REALLY HOLD DOWN TEMPS TODAY.  TONIGHT THE GOOD
LAPSE RATES LINGER INTO THE EARLY EVENING...AND SO DESPITE THE UPPER
LOW AND VORT MAX MOVING OFF TO THE EAST...THINK WE WILL HAVE LIGHT
SNOW LINGERING INTO THE EVENING HOURS BEFORE TAPERING OFF TO FLURRIES
AS THE LAPSE RATES FINALLY MOVE EAST.  PART OF THIS DECISION IS THAT
SOUTH DAKOTA IS GETTING SNOW SHOWERS EVEN WHERE THE VORT MAX APPEARS
TO HAVE PASSED.  WITH CLOUDS LINGERING OVER THE AREA HAVE KEPT
TEMPERATURES AROUND 20...THOUGH TEENS WOULD BE LIKELY OF SOME CLEAR
SPOTS COULD DEVELOP.     ..LE..

.LONG TERM...FRIDAY THROUGH NEXT WEDNESDAY...
BRUNT OF LOW TO MID LEVEL COLD POOL WILL MOVE ACRS THE REGION ON FRI
WITH H85 MB TEMPS OF -14C TO -16C AND IMPRESSIVE H5 MB COLD POOL OF
-36 TO -38 C. ONLY ANOTHER ROUND OF DEEP MIXING WILL HELP SFC TEMPS
BREACH THE 30 DEGREE MARK...WITH MUCH OF THE NORTH HALF OF THE DVN
CWA PROBABLY STAYING IN THE 20S. SOME SLIGHT ISENTROPIC LIFT MAY
WRING OUT A FEW FLURRIES OUT OF ONGOING LLVL CLOUD DECK ON FRI...BUT
INCREASING SUBSIDENCE THROUGH THE COLUMN ESPECIALLY LATER IN THE DAY
MAY EVEN LIMIT THIS MINIMAL ACTIVITY. FCST SOUNDINGS SUGGEST THE
STRATUS WILL GET TRAPPED FOR AWHILE UNDER H75 MB INVERSION FRI
EVENING...BUT FEEL TOP-DOWN DRYING IN SUBSIDENCE REGIME WILL
EVENTUALLY CLEAR THINGS OUT AS THE NIGHT PROGRESSES AND SET THE STAGE
FOR A COLD SAT MORNING. LOWS IN THE MID TO EVEN LOWER TEENS POSSIBLE
IF THIS OCCURS ALONG WITH A LONG ENOUGH WINDOW OF SFC WIND DECOUPLING
BEFORE RETURN FLOW COMMENCES. FEEL STRATUS MIX OUT WILL OCCUR EVEN
SOONER THAN THE MODELS INDICATE FRI AFTERNOON ESPECIALLY IN THE
SOUTHWESTERN FCST AREA.

LATEST RUN 00Z MEDIUM RANGE SOLUTIONS SUGGEST THAT AS THE LARGE
MULTI-CENTERED UPPER LOW COMPLEX ENDS IT/S FUJIWARA DANCE AND
SHEARS APART SOME...IT WILL COMBINE WITH DIGGING GULF OF AK WAVE
ENERGY DUMPING DOWN THE WEST COAST TO INDUCE LOW AMPLITUDE UPPER
RIDGING ACRS MID TO UPPER MS RVR VALLEY REGION. THIS WOULD BRING A
FAIR WX AND THERMALLY MODIFYING PATTERN BACK CLOSER TO NORMAL FOR
THE FIRST HALF OF THE WEEKEND. BUT THE ENSEMBLES ALSO SUGGEST
THAT THE FLATTENED OUT AND DOMINANT SOUTHERN STREAM TO BE
PROGRESSIVE AND WILL STILL LOOK TO SHUTTLE A LEAD DISTURBANCE OUT OF
DEVELOPING SOUTHWESTERN CONUS TROFFINESS AND ACRS THE MIDWEST BY
LATE IN THE WEEKEND. DEPENDING ON THE MODEL THERE ARE VARIANCES WITH
THIS FEATURE TRYING TO PHASE/MOSTLY UNSUCCESSFULLY/ WITH REMNANTS
OF NORTHERN STREAM TROFFINESS...BUT MOST SHUTTLE A DEVELOPING SFC
WAVE/INVERTED TROF COMPLEX ACRS OR JUST SOUTH OF THE LOCAL AREA
FROM LATE SUNDAY INTO MONDAY. CURRENT INDICATIONS SUGGEST THAT THE
MAIN PRECIP WINDOW FROM THIS SYSTEM WOULD BE SUNDAY NIGHT INTO MONDAY
MORNING WITH THERMAL PROFILES SUGGESTING ALL SNOW. EARLY INDICATIONS
OF THE AVAILABLE MOISTURE AND FORCING PROGS SUGGEST 1-2 INCHES OR
POSSIBLY 1-3 INCHES OF SNOWFALL TO OCCUR BY MONDAY MORNING. WILL UP
POPS ACRS THE ENTIRE CWA TO MENTIONABLE CHC FOR SUNDAY NIGHT.

LONGER RANGE INDICATIONS THAT THE MAIN PIECE OF WAVE ENERGY TO EJECT
OUT IN SOUTHERN STREAM WITH ANOTHER AND MORE SIGNIFICANT SNOW EVENT
POSSIBLE FOR TUESDAY INTO WEDNESDAY BUT THIS HIGHLY DEPENDENT OF
COURSE ON HOW FAR NORTH OR SOUTH THE MAIN WAVE PROPAGATES AND PHASES
WHICH IS STILL VERY UNCERTAIN AT THIS POINT. NO CHANGES TO GRIDS
PAST MONDAY NIGHT FOR NOW.        ..12..

&&

.AVIATION...
CURRENT VFR CONDITIONS ARE EXPECTED TO BEGIN DETERIORATING BY MID
MORNING AS THE STEEPER LAPSE RATES MOVE IN AND A SIGNIFICANT
STRATOCUMULUS FIELD DEVELOPS...REPLACING THE CURRENT VFR STRATUS
DECK.  EXPECT IT WILL BE DOWN AROUND 25HFT AT FIRST...LIFTING TO
ABOVE 3KFT BY MID DAY AS THE DEEPER MIXING DRIES THE DEWPOINTS A
LITTLE.  WIDESPREAD SNOW SHOWERS WILL REDUCE VISIBILITIES DOWN TO
1-2SM BRIEFLY IN STRONGEST SHOWERS...AND THESE SHOULD ONLY AFFECT
KDBQ. KCID AND KMLI MAY SEE 2-3SM INSTEAD...WITH BETTER THAN 3SM AT
KBRL.  SNOW SHOWERS SHOULD DIMINISH BETWEEN 00Z AND 03Z...FLURRIES
WILL LINGER OVERNIGHT.  LOW CLOUDS WILL REMAIN...LIKELY BACK INTO
THE MVFR CATEGORY.     ..LE..

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.

&&

$$

LE/12