Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 231929
AFDGRR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
329 PM EDT SAT APR 23 2016

LATEST UPDATE...
SYNOPSIS/SHORT TERM/LONG TERM/MARINE

.SYNOPSIS...
ISSUED AT 302 PM EDT SAT APR 23 2016

A SLOW MOVING LOW PRESSURE SYSTEM OVER SOUTH DAKOTA WILL TRACK
EAST OVER THE NEXT COUPLE OF DAYS. SOUTHERLY WINDS AHEAD OF THIS
SYSTEM WILL DRAW UP MILDER AND INCREASINGLY MOIST AIR FROM THE
SOUTH. THE CHANCES FOR SHOWERS AND THUNDERSTORMS WILL ALSO BE ON
THE RISE...ESPECIALLY MONDAY INTO MONDAY NIGHT. A MUCH COOLER
AIRMASS MOVES IN ON THE BACKSIDE OF THE DEPARTING STORM FOR
TUESDAY.

&&

.SHORT TERM...(THIS EVENING THROUGH MONDAY NIGHT)
ISSUED AT 302 PM EDT SAT APR 23 2016

A COUPLE OF CONCERNS WITH THE FORECAST IN THE SHORT TERM. OVERALL
THE CHANCE FOR SEVERE WEATHER IS NOT ZERO...AND AN ELEVATED
POTENTIAL FOR SOME AREAS TO SEE MORE THAN 2 INCHES OF RAIN EXISTS.

FIRST WILL BE THE TIMING AND STRENGTH OF THE STORMS AND THE AMOUNT
OF RAIN. CURRENTLY SEEING THUNDERSTORMS OVER NORTH DAKOTA...WHERE
THE MODELS ARE NOT FORECASTING ANY RAIN. SO RIGHT
AWAY...CONFIDENCE THAT THE MODELS HAVE THE CORRECT SOLUTION ARE
LOW. THE MODELS GENERALLY HAVE MOST OF THE RAIN THROUGH MONDAY AM
NORTH OF MY CWA. HOWEVER THE 1000/500 MB THICKNESS LINES ARE
NORTHWEST TO SOUTHEAST. COMMONLY WITH ELEVATED CONVECTION...THE
RAIN ENDS OF DIVING SOUTHEAST ALONG THE THICKNESS GRADIENT. SO I
KEPT THE RELATIVELY HIGH POPS GOING MAINLY FOR THE NORTHERN HALF
OF MY CWA STARTING SUNDAY AND GOING INTO MONDAY AM.

THEN MONDAY AFTERNOON INTO MONDAY NIGHT. THE WAVE OF LOW PRESSURE
TRACKS EASTWARD THROUGH SOUTHERN LOWER MI. SURFACE BASED
INSTABILITY IS SHOWN FOR THE REGION AS THE LOW TRACKS THROUGH. LOW
LEVEL CONVERGENCE STRENGTHENS AS THE UPPER LEVEL TAKES ON AN
ANTICYCLONIC CURVATURE OVER THE GREAT LAKES REGION. AS A LOW LEVEL
JET MOVES IN...AND PWAT VALUES CLIMB TO OVER 200 PCT ABOVE
NORMAL...HIGH CONFIDENCE THAT SHOWERS AND THUNDERSTORMS WILL
DEVELOP. DEEP LAYER SHEAR WILL INITIALLY BE ON THE LOW SIDE FOR
SEVERE WEATHER...BUT BY 00Z TUESDAY IT IS PLENTY FAVORABLE FOR
ORGANIZED CONVECTION. THIS IS WHEN THE INSTABILITY WILL BE
SHIFTING SOUTH...BUT A WINDOW FOR SEVERE WEATHER MAY EXIST IN THE
AFTERNOON.

ANOTHER CONCERN IS THE TEMPERATURES. FORECAST SOUNDINGS MONDAY
SHOW A SHALLOW COLD AIRMASS OVER THE NORTHEAST LOWER. THIS COLD
AIRMASS TRACKS SOUTHWESTWARD THROUGH THE CWA AS THE HEAVY RAIN
WILL BE FALLING MONDAY EVENING. WE MAY EVEN SEE PARTS OF THE AREA
WITH TEMPERATURES IN MID TO UPPER 30S WITH STEADY
RAIN...ESPECIALLY AROUND MT PLEASANT MONDAY EVENING.



.LONG TERM...(TUESDAY THROUGH SATURDAY)
ISSUED AT 302 PM EDT SAT APR 23 2016

TUESDAY WILL BE MAINLY DRY AS HIGH PRESSURE BUILDS SOUTH ACROSS THE
CWA. A WIDESPREAD FROST WEDNESDAY MORNING IS LOOKING INCREASINGLY
LIKELY AS UPPER CONFLUENT FLOW AND UPSTREAM RIDGING WILL PROVIDE A
GOOD SETUP FOR CLEARING.

LATEST MEDIUM RANGE GUIDANCE DELAYS PRECIPITATION WITH THE NEXT
SYSTEM TO WEDNESDAY NIGHT INTO THURSDAY. MENTIONED ISOLATED THUNDER
THURSDAY TO ACCOUNT FOR SOME ELEVATED CONVECTION...ALTHOUGH WOULD
REALLY LIKE TO SEE BETTER LOW LEVEL THETA-E ADVECTION ACROSS THE
FRONT COMPARED TO THE DEEP AND FAIRLY LIGHT ZONAL FRONT-PARALLEL
FLOW PATTERN THAT IS CURRENTLY ADVERTISED. REGARDLESS OF THUNDER
POTENTIAL...THIS SETUP HAS POTENTIAL TO PRODUCE SIGNIFICANT RAIN
SOMEWHERE AND WILL NEED TO BE WATCHED.

HIGHS FOR MUCH OF THE PERIOD WILL BE AROUND 60 WHICH IS NEAR OR JUST
SLIGHTLY BELOW NORMAL.


&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 139 PM EDT SAT APR 23 2016

HIGH PROBABILITY FOR VFR CONDITIONS AT ALL TERMINALS THROUGH THE
PERIOD. AS NOTED IN THE PREVIOUS DISCUSSION...A LAKE BREEZE HAS
SHIFTED WINDS TO THE SOUTHWEST AT THE MKG TERMINAL. BY THIS
EVENING MKG WILL EMULATE THE OTHER TERMINALS WITH LIGHT NORTHEAST
WINDS GRADUALLY VEERING TO THE SOUTHEAST OVERNIGHT.

RAIN WILL SPREAD SOUTH ACROSS THE AREA SUNDAY AFTERNOON AND
EVENING BEYOND THE VALID TIME OF THIS TAF PACKAGE.


&&

.MARINE...
ISSUED AT 302 PM EDT SAT APR 23 2016

STRENGTHENING SOUTHERLY WINDS SUNDAY NIGHT MAY ALLOW FOR
CONDITIONS TO APPROACH SMALL CRAFT ADVISORY. CONFIDENCE WAS TOO
LOW TO GO WITH THAT AT THIS TIME. OTHERWISE THE AREA WILL SEE A
RISK FOR THUNDERSTORMS INCREASING ESPECIALLY LATER SUNDAY NIGHT
INTO MONDAY NIGHT. WILL NEED TO MONITOR THE FOG RISK LATER SUNDAY
NIGHT INTO MONDAY AS SURFACE DEWPOINTS CLIMB THROUGH THE 50S.
WATER TEMPERATURES ARE STILL IN THE 40S.

&&

.HYDROLOGY...
ISSUED AT 1242 PM EDT SAT APR 23 2016

MINOR FLOODING IS POSSIBLE MONDAY AND MONDAY NIGHT. UNTIL THEN...
ONLY LIGHT RAIN IS EXPECTED ACROSS THE MUSKEGON RIVER BASIN ON
SUNDAY.

A RISK FOR HEAVY RAINFALL EXISTS MONDAY AND MONDAY NIGHT AS A QUASI-
STATIONARY BOUNDARY PUSHES SOUTH THROUGH LOWER MICHIGAN. GIVEN THE
PRESENCE OF MODEST INSTABILITY... THUNDERSTORMS ARE POSSIBLE. A WARM
CLOUD DEPTH OF ABOUT 10 KFT AND FAIRLY SATURATED SOUNDING SUGGEST
ANY STORMS WILL BE EFFICIENT RAIN PRODUCERS. VERTICAL UNIDIRECTIONAL
FLOW ORIENTED PARALLEL TO THE SURFACE BOUNDARY MAY RESULT IN
TRAINING OF MODERATE TO HEAVY SHOWERS OVER THE SAME AREAS FOR A FEW
HOURS MONDAY AFTERNOON THROUGH MONDAY NIGHT.

1 TO 2 INCHES OF RAIN IS POSSIBLE THROUGH MONDAY NIGHT. TUESDAY AND
WEDNESDAY LOOK FAIRLY DRY... WITH RAIN CHANCES RETURNING WEDNESDAY
NIGHT. MINOR AREAL FLOODING IS POSSIBLE MONDAY EVENING AND MONDAY
NIGHT... ESPECIALLY WITH HEAVIER RAINFALL THAT PERSISTS OVER
URBAN/POOR DRAINAGE AREAS. WE WILL LIKELY SEE SOME RIVERS RISE BACK
ABOVE BANKFULL NEXT WEEK.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LM...NONE.
&&

$$

SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...TJT
AVIATION...TJT
HYDROLOGY...EBW
MARINE...MJS



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