Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 211731
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1231 PM EST FRI NOV 21 2014


.AVIATION...

//DISCUSSION...

HIGH PRESSURE CENTERED OVER THE OHIO VALLEY WILL KEEP MAINTAIN VFR
CONDITIONS TODAY...BUT THERE WILL BE A SOME CU THAT DEVELOPS BETWEEN
2500 AND 3500 FEET. THE HIGH WILL SHIFT TOWARDS THE EAST COAST
TONIGHT...ALLOWING CLOUDS TO SPREAD INTO THE AREA. A WARM
FRONT...INITIALLY ELEVATED...WILL THEN LIFT UP INTO LOWER MICHIGAN
LATE TONIGHT AND SATURDAY. THIS WILL ALLOW A WARMER LAYER OF AIR TO
SETTLE IN OVER SUB-FREEZING AIR AT THE SURFACE...POTENTIALLY LEADING
TO FREEZING RAIN AND SLEET AS MOISTURE AND LIFT ARRIVE WITH THE
FRONT. A PERIOD OF FREEZING RAIN OR SLEET WILL BE POSSIBLE BEFORE 14-
16Z BEFORE STRONGER WARM AIR ADVECTION COMMENCES AND PRECIPITATION
CHANGES TO ALL RAIN. THE FRONT WILL ALSO ALLOW CEILINGS TO FALL TO
MVFR...WITH POSSIBLY A DROP TO IFR MID-MORNING SATURDAY.

IN ADDITION...THE LOW-LEVEL JET WILL NOSE INTO THE AREA LATE TONIGHT
AND REMAIN OVERHEAD TOMORROW RESULTING IN A STRONG CORE OF 40-50 KT
WINDS ABOVE 1500 FEET. LOW-LEVEL WIND SHEAR WAS THEREFORE ADDED TO
THE TAFS AFTER 08Z.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILINGS FALLING BELOW 5000 FEET AFTER 09Z
  SATURDAY.

* MEDIUM CONFIDENCE WITH AT LEAST A LITTLE FREEZING RAIN AT ONSET OF
  PRECIPITATION BETWEEN 08Z-13Z SATURDAY. HIGH CONFIDENCE
  PRECIPITATION WILL THEN CHANGE TO RAIN DURING THE MID-MORNING.

&&

.PREV DISCUSSION...ISSUED 343 AM EST FRI NOV 21 2014

SHORT TERM...TODAY AND TONIGHT

AMPLIFIED UPPER LEVEL RIDGE AXIS WILL SLIDE OVER THE CENTRAL GREAT
LAKES THIS AFTERNOON/EVENING. INVERSION HEIGHTS WILL BE CRASHING
DOWN THIS MORNING...SUB 4000 FEET...EFFECTIVELY ENDING ANY THREAT
OF ADDITIONAL LAKE EFFECT FLURRIES/SNOW SHOWERS BY AROUND
NOON...WITH WARM ADVECTION PATTERN KICKING IN THIS AFTERNOON...AS
850 MB TEMPS RISE INTO THE NEGATIVE LOW SINGLE NUMBERS TOWARD 00Z
SATURDAY. STILL...WITH THE COLD START...LATE NOVEMBER INSOLATION
AND SNOW COVER ACROSS MUCH OF SOUTHEAST MICHIGAN...MAX TEMPS WILL
PROBABLY FALL SHORT OF 30 DEGREES (BASED ON THE 925 MB TEMPS
FORECAST OF -9 C). CLEAR SKIES AND WINDS ATTEMPTING TO DECOUPLE
EARLY THIS EVENING SHOULD ALLOW MOST LOCATIONS TO DIP INTO UPPER
TEENS TO AROUND 20 DEGREES BEFORE CLOUDS AND INCREASING WARM
ADVECTION PATTERN TAKES OVER. STRUNG OUT UPPER LEVEL PV
ADVECTION/GOOD ISENTROPIC ASCENT (285-290-295 K) VERY LATE TONIGHT
(9-12 Z) WILL SUPPORT A CHANCE OF SOME TYPE OF FROZEN
PRECIPITATION...WITH BORDERLINE 3 C WARM LAYER (925-850 MB)
INDICATED. ANY FREEZING RAIN/SLEET/SNOW WILL ULTIMATELY BE
DEPENDENT ON PRECIPITATION RATE/WET BULBING EFFECTS...AS 850-700
MB WET BULB TEMP REMAINS AT OR BELOW FREEZING THROUGH 12Z SATURDAY
(PER 00Z NAM).

LONG TERM...

SATURDAY MORNING...EXTENSIVE PATTERN OF WARM AIR ADVECTION WORKING
INTO THE EXITING ARCTIC AIRMASS WILL BE WELL UNDER WAY BY EARLY
SATURDAY.  SLIVER OF STRONGER 285-290K ISENTROPIC ASCENT LODGED
ALONG THE SOUTHERN PERIPHERY OF A WEAK PV FILAMENT WILL TRANSLATE
THROUGH SE MICHIGAN DURING THE LATTER HALF OF THE MORNING PERIOD.
THIS WILL MAINTAIN A CHANCE FOR LIGHT PRECIPITATION PRODUCTION
DURING THIS TIME.  STEADY WARMING THROUGH THE COLUMN AT THE SAME
TIME WILL LEAVE A FIRM WARM NOSE/MELTING LAYER AT 900-800
MB...RECENT SOUNDINGS ON THE NAM AND GFS SHOWING A MEAN TEMPERATURE
OF 3-4C WITHIN THIS LAYER.  SOME LAG TO THE RESPONSE IN THE NEAR
SURFACE LAYER WILL PRESENT A NARROW 3-4 HOUR WINDOW /12-16Z/ FOR
LIGHT FREEZING RAIN AS THIS LAYER WORKS TO CLIMB ABOVE FREEZING.  A
RAPIDLY WARMING THERMAL PROFLE AND THE LIMITED DURATION/INTENSITY
AND CORRESPONDING LOW QPF /A FEW HUNDREDTHS/ WILL CONTAIN THE
POTENTIAL FOR ICE ACCRETION... PERHAPS GENERATING A FEW SLICK SPOTS
ON ELEVATED SURFACES.

SATURDAY AFTERNOON AND EVENING...PERSISTENT BUT WEAK ASCENT WITHIN A
STRONGLY SHEARED WEST-SOUTHWEST FLOW/ADVECTIVE PROCESS WILL SUSTAIN
A SATURATED PROFILE IN THE LOWEST 5 TO 6K FT DURING THE LATTER HALF
OF THE DAY AND INTO THE EVENING PERIOD.  SE MICHIGAN SETTLES ALONG
THE NORTHERN PERIPHERY OF AN ANTICYLONIC UPPER JET CORE DURING THIS
TIME...SOME DEGREE OF WEAKER FRONTAL FORCING OR CONVERGENCE HOLDING
IN PLACE.  SUFFICIENT DEPTH INTO THE TOP OF THE INVERSION LAYER TO
SUGGEST SOME POCKETS OF LIGHT RAIN OR DRIZZLE WILL EMERGE AT TIMES
IN THIS ENVIRONMENT... WORTHY OF A LOW POP MENTION.  LATE AFTERNOON
TEMPERATURES IN THE VICINITY OF 40 DEGREES.

REST OF THE WEEKEND...DYNAMIC SOUTHERN STREAM WAVE WILL EJECT
NORTHEAST OUT OF TEXAS SATURDAY NIGHT...REACHING THE GREAT LAKES BY
LATE SUNDAY.  THIS PROCESS WILL PROVIDE A RENEWED INCREASE IN MOIST
ISENTROPIC ASCENT DOWNSTREAM ANCHORED ON A SECONDARY WARM FRONTAL
BOUNDARY.   THIS FORCING WILL WORK ACROSS SE MICHIGAN EARLY
SUNDAY...YIELDING AN UPTICK IN RAIN CHANCES THROUGH THIS TIME.   THE
EXTENSIVE CLOUD COVER AND A SLIGHT EASTERLY COMPONENT TO THE LOW
LEVEL WIND FIELD WILL CAP HIGHS SUNDAY  IN THE MIDDLE AND UPPER 40S.
A DEEPER MOISTURE PLUME WILL THEN ACCOMPANY THE ARRIVAL OF THE MAIN
MID LEVEL ASCENT/VORTICITY AXIS DURING THE SUNDAY EVENING/NIGHT
PERIOD.  HIGH CONFIDENCE FOR A WIDESPREAD COVERAGE OF SHOWERS GIVEN
THE DEGREE OF FORCING AND MOISTURE QUALITY DURING THIS TIME...WORTHY
OF AN UPGRADE TO CATEGORICAL POPS.

EARLY NEXT WEEK...INITIAL PERIOD OF WEAK COLD AIR ADVECTION ENSUES
ON MONDAY AS THE ASSOCIATED DEEP SURFACE LOW EXITS NORTHWARD THROUGH
NORTHERN LAKE HURON.  A PERIOD OF STRONGER CONVERGENCE ALONG THE
PASSING COLD FRONT MAY PROVIDE AN ADDITIONAL ROUND OF SHOWERS DURING
THE DAYLIGHT PERIOD.  TRAILING NORTHERN STREAM TROUGH WILL THEN
PROVIDE A STRONGER DOWNWARD SPIRAL IN TEMPERATURES BY TUESDAY... 850
MB TEMPERATURE PROJECTED TO FALL BELOW -10C ONCE AGAIN.  THIS MOIST
CYCLONIC FLOW AND COLD AIR ADVECTION WILL PROVIDE THE NEXT
OPPORTUNITY FOR SNOW SHOWERS WITHIN MONDAY NIGHT AND TUESDAY
TIMEFRAME.

MARINE...

WINDS WILL EASE A BIT TODAY WHILE SHIFTING TO SOUTHWESTERLY IN
RESPONSE PASSING HIGH PRESSURE.  SOUTHWEST WINDS WILL STRENGTHEN
AGAIN HEADING INTO THE EVENING...A PERIOD OF PEAK WINDS IN EXCESS OF
20 KNOTS EMERGING THROUGHOUT LAKE HURON BY TONIGHT.  THE POTENTIAL
DOES EXIST FOR GUSTS TO APPROACH MARGINAL GALES ACROSS CENTRAL
PORTIONS OF THE LAKE TONIGHT THROUGH EARLY SATURDAY.  A GALE WATCH
IS NOW IN EFFECT FOR THIS CORRIDOR.  A DIMINISHING GRADIENT WILL
PROVIDE A GRADUAL DECREASE OF SOUTHERLY WIND SPEEDS SATURDAY NIGHT
AND SUNDAY.  SOUTHEAST FLOW WILL STRENGTHEN IN RESPONSE SUNDAY
NIGHT TO STRONG LOW PRESSURE TRACKING BY TO THE WEST.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...GALE WATCH FROM 9 PM EST THIS EVENING THROUGH SATURDAY MORNING FOR
     LHZ362-363.

     SMALL CRAFT ADVISORY FROM 5 PM THIS AFTERNOON TO 4 PM EST SATURDAY
     FOR LHZ421.

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

$$

AVIATION.....HLO
SHORT TERM...SF
LONG TERM....MR
MARINE.......MR


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