Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 311141
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
641 AM EST SAT JAN 31 2015

.AVIATION...

WARM AIR ADVECTION WILL BRING VARYING CLOUD TO SOUTHEASTERN MICHIGAN
TODAY. MVFR CIGS WILL SWING THROUGH THE NORTHERN TAF SITES THIS
MORNING IN LINE WITH GREATEST MOISTURE AXIS EARLY...THEN FILL IN
ACROSS ALL OF SOUTHEASTERN MICHIGAN LATER THIS AFTERNOON. LATEST
INDICATIONS ARE A SHIELD OF LIGHT SNOW WILL LIFT NORTHWARD INTO
LOWER MICHIGAN AFTER MIDNIGHT...THE BEGINNING OF A LONG DURATION
WINTER STORM SYSTEM. WELL ORGANIZED DYNAMICS AT THE START SUGGESTS
IFR VSBYS IN SNOW AT THE START.

AT DTW...TIMING OF LIGHT SNOW AT DTW IS 8Z. SNOWFALL RATES OF A
QUARTER INCH TO A THIRD OF AN INCH PER HOUR IS EXPECTED AT ONSET.


//DTW THRESHOLD PROBABILITIES...

* HIGH CONFIDENCE IN CIGS BELOW 5000 FT TODAY AND TONIGHT.

* HIGH CONFIDENCE IN PRECIPITATION TYPE OF SNOW.

&&

.PREV DISCUSSION...ISSUED 359 AM EST SAT JAN 31 2015

SHORT TERM...TODAY AND TONIGHT

A PROGRESSIVE LONGWAVE PATTERN OVER NORTH AMERICA HAS A LOW
AMPLTITUDE SHORTWAVE RIDGE STRUCTURE PUSHING INTO THE CENTRAL GREAT
LAKES THIS MORNING. STEADY DIFFUSE WARM AIR ADVECTION IS ONGOING
LOCALLY ON THE LEAD WING OF ISENTROPIC ASCENT. NEAR SURFACE
EQUIVALENT POTENTIAL PROGS HAVE THE NEAR SURFACE WARM FRONT EAST OF
GEORGION BAY AND OHIO ALREADY BY 18Z. IT IS THIS SYSTEM RELATIVE
ISENTROPIC ASCENT THAT IS FEEDING LIGHT RADAR RETURNS ACROSS
PORTIONS OF WISCONSIN AND NORTHERN LOWER MICHIGAN. REGIONAL SURFACE
OBSERVATIONS REPORT SNOWFLAKES REACHING THE GROUND...BUT ARE LIMITED
TO THOSE LOCATIONS CLOSE TO LAKE MICHIGAN...WHERE THE LAKE
CONTRIBUTION IS HELPING LOW LEVEL SATURATION. MODELS SUPPORT HIGHER
SFC DEWPOINTS WASHING RAPIDLY EASTWARD BY MIDDAY. PERHAPS THIS MAY
ALLOW SOME FLURRIES TO REACH THE GROUND NEAR SAGINAW BAY...BUT FOR
NOW WILL LEAVE OUT OF THE FORECAST. MOISTURE PROGS ARE VERY NOISY
AND MAKES THE CLOUD FORECAST DIFFICULT. MID CLOUD IS FORECASTED TO
VACATE THE PREMISES THIS MORNING BEFORE LOWER CLOUD ARRIVES FROM THE
WEST. SUSPECT THERE COULD BE SOME OPPORTUNITY AT SOME SUNSHINE
THROUGH MIDDAY...BEFORE SKIES CLOUD UP FOR THE AFTERNOON.
REGARDLESS...WILL NOT BE MOVING OFF OF THE INHERITED SKY FORECAST
WHICH IS PESSIMISTIC/OVERCAST. TEMPERATURES TODAY WILL FEEL MUCH
WARMER THAN YESTERDAY...WITH READINGS SOME 8 TO 10 DEGREES WARMER.

THE AMPLIFYING LONGWAVE RIDGE AXIS IS THEN EXPECTED TO PUSH INTO THE
CENTRAL GREAT LAKES TONIGHT. AS A RESULT...INCREASING CONFLUENCE
ALOFT WILL OCCUR AS THE ENTRANCE REGION TO MASSIVE JET BRANCH WILL
POSITION OVER PORTIONS OF WISCONSIN...UP...LAKE MICHIGAN. THIS WILL
ALLOW A COUPLE OF THINGS TO HAPPEN: 1. DRAW MOISTURE AND SYNOPTIC
SCALE ASCENT NORTHWARD INTO LOWER MICHIGAN 2. ALLOW STRONG SURFACE
RIDGING TO EXPAND ACROSS NORTHERN WI/NORTHERN LOWER...SENDING A COLD
FRONT SOUTHWARD THIS AFTERNOON...REINFORCING MAIN BAROCLINIC ZONE
ALONG OHIO BORDER BY 12Z SUNDAY. STRONG MODEL CONSENSUS HAS REALLY
ZEROED IN ON A LIGHT SNOW SHIELD LIFTING NORTHWARD INTO SOUTHERN
LOWER MICHIGAN AFTER 7 OR 8Z TONIGHT. A TIGHT CYCLONIC VORTICITY
GRADIENT IS ALSO FORECATED...INDICATIVE OF STRONG THETA E/MOISTURE
GRADIENT. HIGH CONFIDENCE NOW EXISTS IN MEASURABLE PRECIPATION AND
RAISED POPS TO CATEGORICAL FOR MANY AREAS. SNOWFALL AMOUNTS WILL BE
THE HEAVIEST NEAR THE OHIO BORDER WHERE 1 TO 2 INCHES WILL BE
POSSIBLE. AN INCH IS EXPECTED FOR THE I94 CORRIDOR...WITH LESS THAN
AN INCH FROM THE NORTHERN DETROIT SUBURBS THROUGH FLINT.

LONG TERM... SUNDAY THROUGH THURSDAY

MOST SIGNIFICANT SNOWFALL OF THE SEASON REMAINS ON TARGET TO IMPACT
A GOOD PORTION OF SOUTHEAST MICHIGAN SUNDAY AND SUNDAY NIGHT.  00Z
MODEL SUITE ARRIVING A TOUCH NORTHWARD WITH THE OVERALL DOWNSTREAM
PROJECTION OF THIS STORM SYSTEM...AN ADJUSTMENT THAT TRANSLATES INTO
A UNIFORM UPWARD TREND IN THE SNOWFALL ACCUMULATION POTENTIAL
THROUGHOUT THE LOCAL AREA.  THIS UPGRADE NOW BRINGS EXPECTED
ACCUMULATION WITHIN THE 5 TO 10 INCH FROM THE OHIO BORDER NORTH INTO
THE M-59 CORRIDOR /HIGHEST TOTALS SOUTH/...3 TO 6 INCHES I-69 TIER
NORTH INTO SAGINAW...AND 1 TO 2 INCHES NORTHERN THUMB AND
MIDLAND/BAY.  THIS EVOLUTION INTRODUCES THE POSSIBILITY FOR PORTIONS
OF THE M-59 CORRIDOR TO APPROACH WARNING LEVEL SNOWFALL...CONFIDENCE
LEVEL JUST HIGH ENOUGH TO GO AHEAD WITH A WATCH INCLUSION WITH THIS
FORECAST CYCLE.  NO ADDITIONAL HEADLINE ADJUSTMENTS PLANNED...A LATE
SATURDAY NIGHT ONSET STILL AFFORDING US ONE ADDITIONAL MODEL CYCLE
TO CONTRACT ON THE DETAILS FURTHER BEFORE PROVIDING APPROPRIATE
HEADLINE DECISIONS.

STEADY DEEPENING OF THE SHORTWAVE NOW SEEN ON WATER VAPOR LIFTING
SOUTHEAST ACROSS BRITISH COLUMBIA WILL BE WELL UNDERWAY SUNDAY
MORNING ACROSS THE CENTRAL PLAINS...THIS PROCESS MAGNIFIED AS THE
TRAILING PV CATCHES THE ENERGY SHEARING OUT OF THE CLOSED LOW
DRIFTING INTO NORTHERN MEXICO.  WATER VAPOR PROVIDES AN IMPRESSIVE
REPRESENTATION OF THE AVALABLE MOISTURE CONTENT CURRENTLY TIED TO
THIS LOW...WITH A SUBSTANTIAL MID LEVEL THETA-E/WARM AIR ADVECTION
COMPONENT EMERGING THROUGHOUT TODAY.  THIS RESULTS AN AGGRESSIVE
NORTHEASTWARD EXPANSION OF THIS PLUME INTO SOUTHEAST MICHIGAN LATE
TONIGHT INTO EARLY SUNDAY.  INITIAL PERIOD OF ISENTROPIC ASCENT WILL
AUGMENT ANY EXISTING ASCENT SUPPLIED BY THE LINGERING 850-600 MB
FRONTAL CIRCULATION.  TRANSITION IN FORCING BY AFTERNOON/EVENING
TOWARD A STRENGTHENING REGION OF MID LEVEL DEFORMATION AS THE 850 MB
LOW CENTER TRANSLATES THROUGH THE NORTHERN OHIO VALLEY...THIS
PROCESS ALLOWING FOR SOME STRENGTHENING OF THE FRONTAL STRUCTURE.
THIS EVOLUTION EXPLAINS THE MORE ROBUST OUTPUT GENERATED BY THE NAM
RELATIVE TO THE OTHER MODEL SOLUTIONS...A SIZABLE INCREASE IN
FRONTOGENESIS NOTED AT A LEVEL /700-500 MB/ FAVORABLY POSITIONED
THERMALLY FOR GREATER DENDRITE PRODUCTION.  RELIANCE ON THE
ACCURANCY OF THIS HIGH END SOLUTION CARRIES STRONG DEPENDENCE ON
REALIZING THIS DEGREE OF FORCING AT THAT LEVEL...A LOWER CONFIDENCE
SOLUTION IN TERMS OF MAGNITUDE AT THIS TIME.  FOR THIS REASON...
FORECAST DETAILS LEAN IN FAVOR OF A ECMWF/GFS COMPROMISE...WHICH
STILL PROVIDE A HEALTHY RESPONSE UNDER A PROLONGED PERIOD OF MID
LEVEL DEFORMATION FORCING.

MODEL CROSS SECTION ANALYSIS CONTINUE TO DISPLAY A DEEP LAYER OF 2
G/KG OF SPECIFIC HUMIDITY...FAR SOUTHERN SECTIONS SEEING UPWARDS OF
3 G/KG DURING THE LATE AFTERNOON/EARLY EVENING PEAK TIME.  LOWER
STATIC STABILITY THROUGH THE COLUMN...A DOWNWARD TREND NOTED
RELATIVE TO RECENT MODEL CYCLES...BUT NO REAL SIGN OF A GREATER
LEVEL OF CONDITIONAL INSTABILITY WITH THIS EVENT.  A COOLING MID
LEVEL THERMODYNAMIC PROFILE UNDER STRENGTHENING NORTHEAST FLOW WILL
FAVOR A LOWERING TREND TO THE EFFECTIVE SNOWFALL RATIOS AS THE EVENT
PROGRESSES...PARTICULARLY ACROSS CENTRAL SECTIONS WHERE THE BULK OF
THE MOIST LAYER COOLS BELOW -10C BY AFTERNOON.  LATEST ASSESSMENT
YIELDS AN AVERAGE OF 13-15:1 THROUGHOUT THE EVENT.  WHILE THE
NORTHEAST GRADIENT WILL INTRODUCE THE POSSIBILITY OF SOME FORM OF
LAKE ENHANCEMENT...INVERSION HEIGHTS REMAINS SUPPRESSED GIVEN THE
DEGREE OF DRY AIR STILL PRESENT ACROSS NORTHERN LAKE HURON...WHILE
ESTABLISHING ANY DEGREE OF CONVERGENCE WILL BE DIFFICULT UNDER A
STEADILIY BACKING MEAN FLOW WITH TIME.  OVERALL LOOKING AT A LONG
DURATION EVENT OF LIGHT TO MODERATE SNOWFALL FROM LATE TONIGHT
THROUGH SUNDAY NIGHT...WITH SOME PERIODIC BURSTS OF HEAVIER SNOWFALL
CENTERED PRIMARILY FROM LATE AFTERNOON INTO THE EVENING PERIOD.

GUSTY NORTHEAST WINDS WILL PROVIDE A BLOWING SNOW COMPONENT...
PARTICULARLY SUNDAY NIGHT /30 MPH RANGE/ WITH AN UPTICK IN THE
GRADIENT EXPECTED NORTH OF THE PASSING LOW.   THIS WILL ALSO
CONTRIBUTE TO A GREATER WIND CHILL COMPONENT HEADING INTO SUNDAY
NIGHT...LOW LEVEL ARCTIC AIR OOZING IN THROUGH THE PERIOD ALLOWING
LOW TEMPERATURES TO BOTTOM OUT IN THE LOWER AND MIDDLE SINGLE
DIGITS.  LATEST WIND CHILL CALCULATIONS POINT TOWARD READINGS
FALLING INTO THE -10 TO -20 DEGREE RANGE BY MONDAY MORNING.

MID LEVEL SUBSIDENCE TAKES HOLD MONDAY UNDER DEVELOPING DRIER
NORTHWEST FLOW.  ARCTIC AIR FULLY ENTRENCHED LOCALLY DURING THE DAY
WILL SEE LIMITED RECOVERY DESPITE THE PROSPECTS FOR SOME SUN.
LATEST 850/925 MB PROJECTIONS LEAVE HIGHS STRUGGLING INTO THE LOWER
AND MIDDLE TEENS BY AFTERNOON.  WEAK CLIPPER CRUISING THROUGH A
QUASI-ZONAL WEST-NORTHWEST UPPER LEVEL FLOW IS FORECAST TO REMAIN
SOUTH OF THE REGION MONDAY NIGHT.  A TRAILING AND STRONGER SYSTEM
WILL PROVIDE THE NEXT OPPORTUNITY FOR ACCUMULATING SNOWFALL BY LATE
TUESDAY INTO TUESDAY NIGHT.

MARINE...

MODEST SOUTHWESTERLY WINDS TODAY IN ADVANCE OF AN ARCTIC FRONT.
THIS FRONT SHIFTS THROUGH THIS EVENING.  WINDS WILL SHIFT TO
NORTHWEST AND THEN NORTHEAST TONIGHT AS COLDER AIR FUNNELS ACROSS
THE LAKE.   EXTENDED PERIOD OF FIRM NORTHEAST WINDS SUNDAY AND
SUNDAY NIGHT AS STRENGTHENING LOW PRESSURE PASSES THROUGH THE OHIO
VALLEY.  POTENTIAL FOR GUSTS INTO THE 30 KNOT RANGE...WITH ANY GALES
REMAINING LIMITED IN OCCURRANCE.  WINDS EASE INTO MONDAY AS HIGH
PRESSURE BUILDS INTO THE REGION.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...WINTER STORM WATCH FROM LATE TONIGHT THROUGH LATE SUNDAY NIGHT FOR
     MIZ063-068>070-075-076-082-083.

LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MICHIGAN WATERS OF LAKE ERIE...NONE.
&&

$$

AVIATION.....CB
SHORT TERM...CB
LONG TERM....MR
MARINE.......MR

YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).



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