AREA FORECAST DISCUSSION
NWS QUAD CITIES IA IL
AFDMLI 245 PM CDT TUE AUG 31 1999
...GLOBALLY...
GLOBAL MOSAIC OF WATER VAPOR SHOWS A STRONG JET RUNNING ACROSS THE
WESTERN HEMISPHERE AND INTO EASTERN ASIA. DARKNESS PLACES THE
STRENGTH AT 90-100 KNOTS WITH JET STREAKS OF 120+ KNOTS WHICH HAVE
BEEN CONFIRMED BY ROAB AND AIRCRAFT REPORTS. THE CURRENT 5 WAVE
PATTERN IS QUITE PROGRESSIVE AND IS STARTING A TRANSITION INTO A
MORE STABLE 3 WAVE PATTERN. THE KEY FEATURES BRINGING THIS CHANGE IS
MAINLY THE STRONG JET AND TO A LESSER DEGREE THE CURRENT SHORTWAVE
MOVING ACROSS GREENLAND.
WATER VAPOR SHOWS THE BERING SEA SHORTWAVE AND ITS ASSOCIATED 120+
KNOT JET STREAK IS JUST STARTING TO BREAK DOWN THE GULF OF ALASKA
RIDGE. AS THIS HAPPENS THE CURRENT WEST COAST TROF WILL CUT OFF AS
ALL JET ENERGY STAYS NORTH. THIS WILL ALLOW THE SHORTWAVE AND JET
STREAK OVER NORTHERN JAPAN/SEA OF OKHOTSK TO MOVE EAST AND START
CARVING OUT A NEW LONGWAVE TROF AROUND THE DATELINE. WHILE THIS IS
ALL GOING ON THE CURRENT GREENLAND LOW WOULD MOVE TO FAR NORTHERN
SIBERIA AND CARVING OUT A LONGWAVE TROF THERE WITH A DEEP POLAR
VORTEX. WAVELENGTH WOULD THEN PLACE A RIDGE OVER MOST OF NORTH
AMERICA WITH A LONGWAVE TROF SOUTH OF GREENLAND. DURING THIS
TRANSITION A 4 WAVE PATTERN WILL BRIEFLY BE SEEN WITH TROFS OFF BOTH
COASTS OF NORTH AMERICA WITH A RIDGE OVER THE CENTER OF THE
CONTINENT.
DENNIS IS THE FLY IN THE OINTMENT WITH IT CAUSING ALL SYSTEMS TO
SLOW DOWN ACROSS THE CONTINENT. WITH THE GLOBAL MODELS INDICATING
WEEK STEERING CURRENTS ALOFT DENNIS SHOULD PROVE TO BE A NUISANCE
INTO THE WEEKEND.
OVERALL NET RESULT HERE WILL BE TEMPERATURES AT OR A LITTLE ABOVE
NORMAL FOR THE SECOND HALF OF THE WEEK AND INTO LABOR DAY WEEKEND
WITH PRECIPITATION AT OR BELOW NORMAL. FOR THE FIRST FULL WEEK OF
SEPTEMBER TEMPERATURES AND PRECIPITATION SHOULD BE AROUND NORMAL.
...LOCALLY...
WATER VAPOR AND PROFILER DATA HAS A WEAK SHORTWAVE MOVING INTO IOWA
AT H5 FROM NEBRASKA WHICH HAS RESULTED IN THE DEVELOPMENT OF
CONVECTION THAT IS CROSSING THE MISSOURI. THIS SHORTWAVE CONTINUES
TO SLOWLY WEAKEN AS IT SLAMS INTO THE UPPER RIDGE RUNNING FROM THE
SOUTHERN PLAINS INTO THE EASTERN GREAT LAKES. LLJ FROM OKLAHOMA INTO
THE NORTHERN PLAINS HAS BEEN SLOWLY STRENGTHENING WITH IT UP TO 35-45
KNOTS. IMPRESSIVE SHORTWAVE ALONG THE BRITISH COLUMBIA/ALBERTA
BORDER OF CANADA WITH JET LEVEL HEIGHT FALLS OF 220 DEKAMETERS AT
SPOKANE. VISIBLE IMAGERY HAS DISSIPATING AC/CS ACROSS THE FORECAST
AREA WITH THE LIKELY CAUSE BEING A COMBINATION OF A MESO RIDGE NOTED
AT H3 IN THE 12Z UPPER AIR AND THE DRY AIR BETWEEN 900 AND 750 MB IN
THE DVN SOUNDING. 18Z SURFACE ANALYSIS HAS THE RIDGE AXIS RUNNING
FROM LAKE HURON TO THE KFTW. A WEAK LOW WAS NEAR KPHP WITH A WARM
FRONT RUNNING ESE TO KFSD. MOISTURE ANALYSIS HAS MID 50 DEW POINTS
ACROSS THE AREA WITH 60 DEW POINTS APPROACHING THE I35 CORRIDOR. THE
LLJ HAS ALLOWED DEW POINTS IN THE 70S TO POOL ACROSS THE DAKOTAS.
PROBLEMS THIS PACKAGE ARE ESSENTIALLY CLOUDS/TEMPERATURES AND WHEN
TO REINTRODUCE PRECIPITATION.
ROUND 2 OF CRAY 3 PROBLEMS HAVE CONCLUDED AT NCEP. NUF SAID. 12Z
MODELS APPEAR TO HAVE INITIALIZED OK WITH THE NGM SEEMING TO PLACE
THE SURFACE RIDGE BEST AT 18Z BUT THE ETA LOOKS BETTER ON THE
STRENGTH OF THE LLJ IN THE PLAINS. DENNIS IS THE KEY TO THE ENTIRE
FORECAST AND WITH IT SPINNING OFF THE COAST IT HAS APPLIED THE
ATMOSPHERIC BRAKES TO THE MOVEMENT OF FEATURES ACROSS THE CONTINENT.
ALL MODELS MOVE THE RIDGE VERY LITTLE TO THE EAST CAUSING IT TO FORM
A REX BLOCK WITH DENNIS ON THE EAST COAST. QG FORCING IS ESSENTIALLY
NIL ACROSS THE AREA WITH CONDENSATION PRESSURE DEFICITS STAYING AT
200-300 MB BEFORE EVENTUALLY DROPPING TO 100-200 MB. RH FIELDS
CONTINUE TO SHOW A SHARP GRADIENT OF MOISTURE WITH TIME SECTIONS
INDICATING MOST OF THE MOISTURE TO BE IN THE MID TO HIGH LEVELS. THE
CURRENT LLJ IN THE PLAINS GRADUALLY SHIFTS EAST BUT NEVER REALLY
GETS MUCH BEYOND THE I35 CORRIDOR. LAPSE RATES TO BE 6-7 C WHICH
SHOULD ALLOW SLIGHTLY STRONGER WINDS TO MIX DOWN FROM ALOFT.
IN A NUTSHELL DRY CONDITIONS TO CONTINUE WITH SOME CLOUDS. WITH THE
LLJ BEING A LITTLE CLOSER WEDNESDAY NIGHT A FEW MORE CLOUDS SEEMS
APPROPRIATE. TEMP WISE FWC NUMBERS LOOK TOO WARM (ESPECIALLY THE
MAXES) WITH THE COOLER FAN NUMBERS PREFERRED. PERSISTANCE AND
TRAJECTORY METHODS YIELD GRADUALLY INCREASING MAX/MIN TEMPERATURES
OVER THE NEXT FEW DAYS.
...EXTENDED...
GIVEN CRAY PROBLEMS DON/T KNOW HOW MUCH FAITH TO PUT INTO THE MRF.
HOWEVER MRF IS IN LINE WITH THE OTHER GLOBAL MODELS. COLD FRONT JUST
MOVING INTO THE NORTHERN PLAINS TODAY WILL STALL NORTH OF THE AREA
LATE THIS WEEK. GIVEN CURRENT DRY LOWER LEVELS AND THE LACK OF ANY
FORCING MECHANISM THAT WOULD CREATE PRECIP CAN/T SEE ANY REASON TO
KEEP THE RAIN CHANCE IN FOR SATURDAY. ADDITIONALLY THE TREND IN THE
MRF HAS BEEN TO LOWER POPS AS WELL.
COORDINATED WITH DSM...LSE...CHI...SPI.
.DVN...NONE.
NC