Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 202318
AFDGRB

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREEN BAY WI
618 PM CDT WED MAY 20 2015

UPDATED AVIATION PORTION FOR 00Z TAF ISSUANCE

.SHORT TERM...TONIGHT AND THURSDAY
ISSUED AT 251 PM CDT WED MAY 20 2015

MAIN FCST CHALLENGE FOR TNGT TO BE ON TEMPS AS THE RACE WL BE ON
BETWEEN DECREASING CLOUDS AND SUNRISE FOR POSSIBLE FROST FORMATION
ACROSS THE NORTH. MAIN ISSUE FOR THU WL BE ON PCPN POTENTIAL AS A
CDFNT SAGS SOUTH THRU THE REGION.

THE 19Z MSAS SFC ANALYSIS INDICATED A RDG OF HI PRES STRETCHED
FROM THE NRN/CNTRL PLAINS EWD INTO SRN SECTIONS OF THE GREAT
LAKES. A WEAK AREA OF LO PRES WAS LOCATED NEAR THE MID-MS VALLEY
WITH A QUASI-STNRY BOUNDARY DRAPED EAST THRU THE TN VALLEY. VSBL
SATL IMAGERY SHOWED MID/HI CLOUDS OVERSPREADING MOST OF WI WITH
ANY LIGHT PCPN MOVING ALONG THE WI/IL BORDER.

CLOUDS ASSOCIATED WITH THE SFC LOW MOVING TOWARD THE OHIO RIVER/TN
VALLEYS AND A TIGHT BAROCLINIC ZONE OVER SRN IL/CNTRL INDIANA...WL
LINGER OVER MOST OF WI THIS EVENING BEFORE GRADUALLY PULLING AWAY
FROM NW TO SE OVRNGT. MODELS DO SHOW A SHORTWAVE TROF DROPPING SE
THRU WI DURING THE COURSE OF THE NGT...BUT A LACK OF LIFT AND
FORCING WOULD PRECLUDE MUCH IN THE WAY OF CLOUDS...LET ALONE ANY
PCPN. MIN TEMPS TO BE THE MAIN FCST ISSUE FOR TNGT AS 8H TEMPS ARE
WARMER AND THERE ARE SOME BOUNDARY LAYER WINDS (AROUND 20 KTS)
THRU THE NGT. HOWEVER...THE AIR MASS OVER WI IS STILL QUITE COOL
BY MAY STANDARDS AND SFC WINDS WL BE RATHER LIGHT...SO EXPECT
TEMPS TO STILL DROP TO SLIGHTLY BELOW NORMAL READINGS. LOOK FOR
MINS TO RANGE FROM THE MID TO UPR 30S IN THE NORTH...TO THE LWR
40S ACROSS E-CNTRL WI. HAVE MENTIONED SOME PATCHY FROST ACROSS THE
NORTH...BUT DO NOT BELIEVE FROST WL BE WIDESPREAD ENUF TO WARRANT
A HEADLINE AT THIS TIME.

A PREVAILING NW FLOW ALOFT WL ALLOW FOR A CDFNT TO DROP SOUTH THRU
NRN WI THU MORNING AND CNTRL/E-CNTRL WI THU AFTERNOON. THE
ATMOSPHERE STILL LOOKS TOO STABLE OVER NRN WI WHEN THE FRONTAL
PASSAGE OCCURS...THUS DO NOT ANTICIPATE ANY PCPN OVER THAT PART OF
THE STATE. HOWEVER...MODELS DO INDICATE BETTER INSTABILITY OVER
CNTRL/E-CNTRL WI IN THE AFTERNOON AS CAPES RISE TO AROUND 300
J/KG...LI`S DROP TO AROUND -1 AND MID-LEVEL LAPSE RATES STEEPEN TO
6.5 C/KM. THIS MARGINAL INSTABILITY...COUPLED WITH LIFT FROM THE
CDFNT...MAY BE ENUF TO PRODUCE LIGHT RAIN SHWRS/ISOLATED TSTMS
GENERALLY SOUTH OF A LINE FROM MERRILL TO WAUSAUKEE THU AFTERNOON.
MAX TEMPS TO RANGE FROM THE LWR 60S NEAR THE MI BORDER...TO THE
LWR 70S OVER PARTS OF CNTRL WI.

.LONG TERM...THURSDAY NIGHT THROUGH WEDNESDAY
ISSUED AT 251 PM CDT WED MAY 20 2015

NO SIGNIFICANT CHANGES TO THE PREVIOUS FORECAST THIS PERIOD.

FOR THURSDAY EVENING...LATEST SUITE OF MODEL GUIDANCE WAS FAIRLY
CONSISTENT AND CONTINUED TO SUGGEST THERE WOULD STILL BE ENOUGH
FORCING WITH THE SURFACE FRONT AND LINGERING INSTABLITY TO KEEP
THE CHANCE OF SHOWERS IN THE FORECAST UNTIL ABOUT 02 OR 03Z
THUHRSDAY EVENING. SURFACE FRONT SHOULD BE WELL SOUTH OF THE
FORECAST AREA BY 06Z FRIDAY WITH FAIRLY DECENT LOW-LEVEL CAA
TAKING HOLD FOR THE REST OF THE NIGHT. SURFACE HIGH PRESSURE WILL
THEN SETTLE OVER THE FORECAST AREA FRIDAY INTO SATURDAY NIGHT.

WITH THE LATEST MODEL CYCLE...APPEARS THAT THE ECMWF IS BECOMING
THE OUTLIER AS FAR AS THE TIMING OF THE SOUTHERN STREAM SYSTEM FOR
LATER IN THE WEEKEND. NAM/GFS AND SREF WERE GENERALLY IN THE BALL
PARK...WHILE THE EC REMAINED CONSIDERABLY SLOWER. IGNORING THE EC
FOR NOW WOULD SUGGEST INTRODUCING LOW CHANCE POPS LATE SATURDAY
EVENING...THEN GRADUALLY RAMPING UP TO LIKELY BY SUNDAY
AFTERNOON. GIVEN THE UPSTREAM SPLIT FLOW REGIME WITH THE UPPER
CONFLUNCE ZONE SITUATED JUST NORTH OF THE WESTERN GREAT
LAKES...THE FORECAST WILL BE TENUOUS. IF THE NORTHERN STREAM BECOMES
A BIT MORE DOMINATE IN THE NEXT FEW FORECAST CYCLES...WE MAY SEE A
TREND MORE TOWARD THE EC. FOR NOW WILL KEEP THE GOING FORECAST
GENERALLY IN TACT.

UPPER FLOW APPEARS TO REMAIN RATHER ACTIVE AT LEAST THROUGH THE
EARLY PART OF NEXT WEEK WITH SEVERAL IMPULSES IN BOTH THE NORTHERN
AND SOUTHERN STREAMS. THUS CHANCE POPS REMIAN WARRANTED.
&&

.AVIATION...FOR 00Z TAF ISSUANCE
ISSUED AT 617 PM CDT WED MAY 20 2015

MAINLY VFR CONDITIONS TO PREVAIL TONIGHT INTO THURSDAY. ISOLATED
SHOWERS OR A THUNDERSTORM WILL BE POSSIBLE LATE THURSDAY MORNING
OVER THE NORTH AND THEN AFTERNOON OVER CENTRAL AND EAST CENTRAL
WISCONSIN. WINDS SHIFTING FROM SOUTHWEST TO NORTHWEST WILL MARK
THE PASSAGE AND LIKELY LOCATION OF SHOWERS. A FEW OF THE SHOWERS
MAY BRIEFLY CONTAIN MVFR CIGS.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....KALLAS
LONG TERM......ESB
AVIATION.......TDH


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