Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 291939
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
339 PM EDT TUE JUL 29 2014

.SHORT TERM... TODAY AND TONIGHT

DIURNAL CLOUDS AND CONVECTION HAVE BEEN BLOSSOMING IN THE WAKE OF
A BAND OF MID CLOUDS. THESE MID CLOUDS /ASSOCIATED WITH THE LEAD
EDGE OF A PLUME OF HIGHER LOW-MID LEVEL MOISTURE/ WILL BE EXITING TO
THE EAST OF THE AREA BY FORECAST ISSUANCE. THE COLD AIR ALOFT IS A
RESULT OF AN ANOMALOUSLY DEEP UPPER TROUGH EXTENDING ACROSS THE ERN
US. WITH 500MB TEMPS AROUND -17C OVER SE MI...MID LEVEL LAPSE RATES
WILL REMAIN STEEP WELL INTO THE NIGHT. THE 19Z SURFACE ANALYSIS
SHOWS A TROUGH AXIS EXTENDING FROM THE THUMB DOWN TO SOUTH-CENTRAL
LOWER MI. MOISTURE POOLING ALONG THIS TROUGH HAS SENT SFC DEWPOINTS
INTO THE MID TO UPPER 50S...RESULTING IN 0-1KM ML CAPE ON THE ORDER
OF 500 J/KG. THE COMBINATION OF THIS WEAK INSTABILITY AND COLD AIR
ALOFT WILL CONTINUE TO SUPPORT BRIEF SHOWERS/THUNDERSTORMS INTO THIS
EVENING THAT WILL BE CAPABLE OF PRODUCING SMALL HAIL. THE SURFACE
TROUGH WILL MOVE INTO THE ERN PORTIONS OF THE AREA /PORT HURON-
DETROIT-MONROE/ BY 21Z. THUS SCATTERED-NUMEROUS CONVECTION IS
EXPECTED TO CONTINUE DEVELOPING TOWARD THE EAST OVER THE NEXT COUPLE
OF HOURS.

THE LOSS OF DAYTIME HEATING DURING THE EVENING WILL LEAD TO A
DIMINISHING TREND IN COVERAGE AND INTENSITY OF RESIDUAL CONVECTION.
IN LIGHT OF WHAT WAS OBSERVED UPSTREAM YESTERDAY AND CONSIDERING THE
STEEP MID LEVEL LAPSE RATES...THIS PROCESS SHOULD BE GRADUAL. THE
REGIONAL WATER VAPOR IMAGE SHOWS A MID LEVEL SHORT WAVE IMPULSE OVER
WRN LAKE SUPERIOR...EMBEDDED WITHIN THE NORTHWEST FLOW. THIS WAVE
WILL TRACK ACROSS SW LOWER MI INTO NRN OHIO TONIGHT. SOME ADDED
ASCENT ASSOCIATED WITH THIS FEATURE COUPLED WITH A RESIDUAL LOW
LEVEL MOISTURE AXIS OVER FAR SRN MI WILL WARRANT A CHANCE OF
LINGERING SHOWERS/ISOLATED THUNDERSTORMS INTO THE OVERNIGHT. THERE
WILL BE SOME OPPORTUNITY FOR CLEARING TONIGHT...ESPECIALLY ACROSS
THE SAGINAW VALLEY AND THUMB. WITH SURFACE DEWPOINTS HOLDING WELL IN
THE 50S UP THROUGH THE NRN GREAT LAKES...THERE WILL BE A CHANCE OF
SOME FOG LATE TONIGHT PREDICATED UPON THE DEGREE OF RADIATIONAL
COOLING.

&&

.LONG TERM...

LARGE UPPER LEVEL LOW CENTERED OVER THE SOUTHERN END OF JAMES BAY
WILL DRIFT SOUTHWEST...DROPPING INTO NORTHEASTERN ONTARIO BY
THURSDAY MORNING.

MULTIPLE SPOKES OF ENERGY/SHORTWAVE TROUGHS WILL BE ROTATING AROUND
THE MAIN CIRCULATION...WITH ONE MOVING THROUGH NORTHERN LOWER
MICHIGAN LATE TOMORROW/TOMORROW EVENING...AS 500 MB TEMPS FALL INTO
THE NEGATIVE UPPER TEENS TO AROUND -20 C. THERE LOOKS TO BE A SHARP
MOISTURE GRADIENT STRADDLING SOUTHERN LOWER MICHIGAN TOMORROW...AS
SEEN IN 850-700 MB THETA-E FIELDS...AND THERE IS A QUESTION MARK ON
HOW MUCH MOISTURE AND SUBSEQUENT INSTABILITY WILL BE AROUND TO
SUPPORT SHOWERS AND ISOLATED THUNDERSTORMS...AS NAM IS THE MOST
AGRESSIVE...BUT LATEST EURO HAS ALSO TRENDED MORE UNSTABLE. WITH THE
FAVORABLE LATE DAY TIMING AND MAGNITUDE OF THE MID LEVEL COLD
POOL...CERTAINLY THINK SCATTERED POPS (ESPECIALLY TOWARD OHIO
BORDER) ARE WARRANTED WITH ISOLATED THUNDERSTORMS.

THURSDAY LOOKS FAIRLY SIMILAR WITH ANOTHER PIECE OF UPPER LEVEL
ENERGY ROTATING THROUGH....WITH TIMING SENSITIVITY. 30 KNOTS OF 0-6
KM WIND SHEAR AND INVERTED V SOUNDING MAKES WIND A BIT OF A CONCERN
IF ANY THUNDERSTORM IS IN FACT ABLE TO MATERIALIZE...AS WELL AS HAIL
WITH MAX CAP DENSITY CENTERED AROUND FREEZING LEVEL. ONCE
AGAIN...NAM IS INDICATING THE MOST INSTABILITY WITH UP TO 1500 J/KG
OF CAPE.

LOW LEVEL AIRMASS WILL BE SLOWLY MODIFYING AS 850 MB TEMPS GO FROM
UPPER SINGLE NUMBERS WEDNESDAY (SUPPORTING HIGH IN THE LOWER 70S) TO
LOWER TEENS ON THURSDAY (MID/UPPER 70S).

THE LONGWAVE PATTERN WILL CONTINUE TO SLOWLY DEAMPLIFY OVER THE
REMAINDER OF THE FORECAST PERIOD...BUT IT STILL LOOKS TO FEATURE A
GENERAL RIDGE WEST/TROUGH EAST PATTERN...PRECLUDING ANY BIG
WARM-UPS. UPPER TROUGH AXIS/COLD POOL WILL STILL BE OVERHEAD FRIDAY
AND SATURDAY SO DIURNALLY ENHANCED SHOWERS WILL BE A POSSIBILITY AT
LEAST THROUGH SATURDAY. TEMPERATURES ARE EXPECTED TO BE NEAR TO
SLIGHTLY BELOW NORMAL INTO EARLY NEXT WEEK.

&&

.MARINE...

A COOL BUT LIGHT NORTHWEST WIND WILL PERSIST OVER LAKE HURON AND
LOWER MICHIGAN THROUGH TOMORROW. THE WIND WILL REMAIN LIGHT THROUGH
THE END OF THE WEEK AND TEND TO SHIFT TOWARD THE SOUTHWEST BY FRIDAY
WITH INCREASING INFLUENCE OF WEAK HIGH PRESSURE BY THE WEEKEND.
OTHERWISE THERE WILL BE A LOW CHANCE OF BRIEF SHOWERS AND ISOLATED
THUNDERSTORMS...PRIMARILY NEAR THE SHORELINES OVER LAKE ST CLAIR AND
WESTERN LAKE ERIE.

&&

.AVIATION...ISSUED 111 PM EDT TUE JUL 29 2014

//DISCUSSION...

ENHANCED LOW LEVEL MOISTURE IN THE VICINITY OF A SURFACE TROUGH
SLOWLY PUSHING ACROSS SE MI AND VERY COLD AIR ALOFT WILL LEAD TO
DIURNAL ENHANCEMENT IN CLOUDS AND SHOWERS THIS AFTERNOON. THE DEGREE
OF LOW LEVEL MOISTURE DOES SUGGEST SOME MVFR BASED CLOUDS...
ESPECIALLY WITHIN THE SHOWERS. THERE IS LIKELY TO BE WIDE VARIATIONS
IN CLOUD BASES ACROSS THE AREA WITH AN OVERALL TREND TOWARD
HIGHER CIGS LATER IN THE DAY AS DIURNAL HEATING LIFTS THE
CONVECTIVE CLOUD BASES. THE LOSS OF DAYTIME HEATING THIS EVENING
SHOULD RESULT IN A DIMINISHING TREND TO THE CONVECTION. THE COLD
AIR ALOFT WILL CERTAINLY MAINTAIN A RISK OF LATE AFTERNOON
THUNDERSTORMS. THE INSTABILITY IS NOT EXTREMELY HIGH...SO
CONFIDENCE OF THUNDERSTORMS IMPACTING THE TERMINALS IS TOO LOW TO
MENTION IN THE TAFS.

FOR DTW...THE SFC TROUGH AXIS AND BETTER MOISTURE AND THUS THE
CONVECTIVE POTENTIAL WILL NOT ARRIVE INTO METRO DETROIT UNTIL LATE
THIS AFTERNOON /AROUND 21Z/. THE LATER ARRIVAL OF THE HIGHER LOW
LEVEL MOISTURE INTO METRO WILL SUPPORT SLIGHTLY HIGHER CLOUD BASES
THAN FARTHER NORTH BUT STILL INTO THE 3500 TO 5000 FT CATEGORY.

//DTW THRESHOLD THREATS...

* MEDIUM CONFIDENCE IN CEILING AOB 5000 FEET IN SHOWERS DURING THE
  AFTERNOON.

* LOW CONFIDENCE IN AN ISOLATED THUNDERSTORM DURING THE AFTERNOON.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

SHORT TERM...SC
LONG TERM....SF/DT
MARINE.......SF
AVIATION.....SC


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