723
FXUS63 KDVN 090920
AFDDVN
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
320 AM CST TUE FEB 9 2010
SYNOPSIS
VERY COMPLICATED WEATHER SCENARIO OUT THERE THIS MORNING. AT THE
SURFACE...A DEEP INVERTED TROUGH EXTENDS FROM A SURFACE LOW OVER
MISSISSIPPI NORTH ACROSS ILLINOIS...THEN CURVING BACK NORTHWEST
ACROSS THE QUAD CITIES AND INTO NORTHERN IOWA. A SECONDARY TROUGH
EXTENDS FROM CENTRAL IOWA SOUTHWEST TO SE NEBRASKA. IN THE
VICINITY OF THIS SECONDARY TROUGH IS A LARGE CLEAR SLOT...LEAVING
MUCH OF SOUTHWEST IOWA IN THE CLEAR. THIS AREA IS IN THE VICINITY
OF THE 500MB UPPER LOW AND A COLD POOL AT 850MB. THE 850MB LOW IS
SOMEWHAT TO THE SOUTHEAST OVER SOUTH CENTRAL IOWA...AT LEAST
ACCORDING TO THE 08Z RUC ANALYSIS. THIS VERY COMPLICATED SYSTEM
HAS PRODUCED WIDESPREAD SNOW OVER MUCH OF THE MIDWEST...CURRENTLY
OVER ALL OF ILLINOIS AND TO THE EAST...EASTERN AND NORTHERN
IOWA...AND SOUTHERN WISCONSIN AND MINNESOTA.
..LE..
SHORT TERM
TODAY AND TONIGHT.
THIS RATHER POTENT UPPER LOW WILL CONTINUE TO DRIVE SOUTHEAST
ACROSS WESTERN IOWA THIS MORNING AND INTO MISSOURI BEFORE SWINGING MORE
EAST TO CROSS INTO INDIANA AND OHIO TONIGHT. ISENTROPIC AND WARM
AIR ADVECTION FRONTOGENESIS FACTORS WHICH DOMINATE OUR SNOW PRODUCTION RIGHT
NOW WILL LOSE SOME OF THE ISENTROPIC LIFT AS WE GET MORE INTO THE
DEFORMATION ZONE PRECIPITATION WHICH IS FOLLOWING THE UPPER LEVEL
LOW. THE AREA OF MAXIMUM SNOWFALL AMOUNTS IS LIKELY TO CURVE
SOUTH FROM NORTHERN IOWA...AFFECT ABOUT THE NORTHERN HALF OF THE
FORECAST AREA...AND THEN ACROSS THE NORTHERN HALF OF ILLINOIS AND
INDIANA. WHAT IS INTERESTING ABOUT THIS EVENT IS THAT THOUGH WE
ARE GETTING THE BEST DYNAMICS NOW...VISIBILITIES IN THE SNOWFALL
IS GENERALLY BETWEEN 1 AND 2 MILES...CORRELATING WITH ONLY ABOUT A
QUARTER TO A HALF IN PER HOUR OF SNOW ACCUMULATION. THIS IS
EXPLAINED WELL BY THE PRECIPITABLE WATER VALUES DOWN AROUND 3
TENTHS AS OF 00Z. THUS...THOUGH THE DENDRITIC GROWTH ZONE IS A
RESPECTABLE 150MB DEEP...WE JUST ARE NOT GETTING ANY HEAVY
SNOWFALL RATES. SNOWFALL TOTALS AS OF AROUND MIDNIGHT HAVE ONLY
RANGED FROM 2-3 INCHES IN THE EAST...TO 4-5 INCHES IN THE
WEST...EVEN THOUGH WE HAVE BEEN SNOWING FOR MORE THAN 24 HOURS
NOW. THIS TREND WILL CONTINUE TODAY...WITH ONLY ANOTHER 2 TO 4
INCHES EXPECTED IN THE FORECAST AREA TODAY...AND AN INCH OR LESS
TONIGHT. THE DRY SLOT MENTIONED IN THE SYNOPSIS IS GOING TO BRUSH
OUR SOUTHWESTERN COUNTIES THIS MORNING...SHUTTING DOWN SNOW THERE
FOR A PERIOD OF TIME BEFORE IT STARTS UP AGAIN THIS AFTERNOON AND
EVENING. THERE SHOULD EVEN BE A BRIEF PERIOD OF SUN. THIS REALLY WILL
REDUCE OUR EVENT TOTALS IN THE SOUTHWESTERN COUNTIES...AND THERE
AM ONLY GOING FOR EVENT TOTALS IN THE 3 TO 6 RANGE...WHERE THE
REMAINDER OF THE REGION IS STILL LOOKING AT 5 TO 9 INCHES. THE
MAIN JUSTIFICATION FOR THE WINTER STORM WARNING HOWEVER WILL BE
THE WIND WHEN IT KICKS UP BEHIND THIS SYSTEM THIS AFTERNOON.
SUSTAINED WINDS WILL BE IN THE 15 TO 25 RANGE...WITH GUSTS TO OVER
30 MPH DURING THE AFTERNOON...AND CONTINUING INTO THE OVERNIGHT
HOURS. THIS HAS SOME PRETTY SIGNIFICANT IMPACTS ON ROAD CLEARING
EFFORTS...AND CAN CAUSE SOME SIGNIFICANT VISIBILITY ISSUES.
THUS...THE WARNING AND ADVISORY THAT WERE ISSUED YESTERDAY REMAIN
INTACT. SNOW SHOULD END FAIRLY RAPIDLY FROM WEST TO EAST THIS
EVENING AND OVERNIGHT...AND ALL SNOW SHOULD BE OUT OF THE AREA BY
WEDNESDAY MORNING. ..LE..
LONG TERM...WEDNESDAY THROUGH MONDAY
TEMPERATURES ARE THE INITIAL CHALLENGE AS CANADIAN HIGH PRESSURE
BRINGS CLEARING SKIES OVER THE FRESH SNOW COVER WED AND WED NIGHT.
UPPER LEVEL NW FLOW THEN BECOMES INCREASINGLY MORE ACTIVE LATE WEEK
INTO WEEKEND WITH POTENTIAL FOR ONE OR MORE ROUNDS OF FLURRIES OR
LIGHT SNOW.
WED...N-NW WINDS REMAIN BRISK IN THE MORNING DUE TO INITIAL TIGHT
PRESSURE GRADIENT...THEN DIMINISH CONSIDERABLY BY AFTERNOON AS HIGH
BUILDS IN FROM THE NRN PLAINS. MID LEVEL SUBSIDENCE AND COLD
ADVECTION SUPPORT WINDS AT LEAST 15 TO 25 MPH IN THE MORNING...WHICH
COULD CONTINUE SOME DRIFTING SNOW EARLY. SUBSIDENCE AND INCOMING
VERY DRY MID AND UPPER LEVELS SHOULD CLEAR SKIES OVER ALL BUT THE
EAST BY MIDDAY. COLD ADVECTION NW FLOW THROUGH AFTERNOON SHOULD
LIMIT ANY WARMING AND KEPT HIGHS IN THE TEENS NORTH TO LOWER 20S
SOUTH.
WED NIGHT...SETUP APPEARS NEARLY IDEAL FOR STRONG RADIATIONAL
COOLING DUE TO THE RELATIVELY FRESH...DEEP SNOWCOVER AND RIDGE AXIS
OVERHEAD AT 12Z. HOWEVER...BOTH GFS AND ECMWF INDICATE A WEAK SHORTWAVE
THROUGH THE AREA AT THIS TIME WITH ACCOMPANYING HIGH LEVEL RH THAT
MAY RESULT IN SOME HIGH CLOUDS LATE NIGHT INTO THU. LOCAL MIN
TEMPERATURE TECHNIQUE FOR SIMILAR SCENARIOS WITH MOSTLY CLEAR
SKIES...SHARP LOW LEVEL INVERSION...AND RIDGE AXIS OVERHEAD USING
INPUT FROM H8 TEMPERATURES...WHICH MODIFY BY THIS TIME TO SEVERAL
DEGREES WARMER THAN THIS MORNING...WOULD INDICATE WIDESPREAD SINGLE
DIGITS. THIS IS CLOSE TO WHAT MOS GUIDANCE IS FORECASTING. BUT WITH
WIDESPREAD SINGLE DIGIT SUB ZERO MINS ALREADY IN FORECAST...AM
HESITANT TO GO BEYOND A MINOR UPWARD NUDGE AND LET LATER SHIFTS
RE-EVALUATE.
THU THROUGH SAT...THERE IS CONTINUED POOR CONSISTENCY BETWEEN MODELS
AND FROM RUN TO RUN WITH TIMING OF A WEAK SHORTWAVE THU NIGHT INTO
FRI...AND ALSO WITH YET ANOTHER CLOSED H5 LOW DROPPING SOUTH OUT OF
CANADA INTO THE AREA LATE SAT OR EARLY SUN. FOR NOW...WILL CONTINUE
SLIGHT CHANCE POPS FOR SNOW FRI AND FRI NIGHT WITH TEMPERATURES
MODERATING BACK TO CLOSER TO NORMAL. WEEKEND SYSTEM CERTAINLY
LOOKING MORE SIGNIFICANT IN TERMS OF POTENTIAL FOR MEASURABLE SNOW
AND WILL NEED TO INTRODUCE POPS WITHIN THE NEXT COUPLE FORECAST
CYCLES ONCE SOME CONSENSUS CAN BE REACHED.
..SHEETS..
AVIATION
AFOREMENTIONED CLEAR SLOT SHOULD AFFECT THE KBRL TERMINAL FOR
SEVERAL HOURS THIS MORNING WITH VFR CONDITIONS BEFORE MAINLY MVFR
CLOSE BACK IN FOR THE AFTERNOON AND EVENING HOURS. FOR SEVERAL
HOURS THIS AFTERNOON AND EVENING THERE SHOULD BE A PERIOD OF IFR
CONDITIONS AS THE STRONGEST SNOW BAND MOVES OVERHEAD. FOR THE
REMAINING TAF SITES...PREVAILING MVFR CONDITIONS WITH TEMPORARY
IFR CONDITIONS WILL CONTINUE THROUGH TODAY AND INTO TONIGHT...
BEFORE GRADUALLY IMPROVING TO VFR AS THE SNOW ENDS TONIGHT.
..LE..
DVN WATCHES/WARNINGS/ADVISORIES
IA...WINTER STORM WARNING UNTIL 6 AM CST WEDNESDAY FOR CLINTON-
JACKSON-SCOTT.
WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR BENTON-
BUCHANAN-CEDAR-DELAWARE-DES MOINES-DUBUQUE-HENRY IA-IOWA-
JOHNSON-JONES-KEOKUK-LINN-LOUISA-MUSCATINE-WASHINGTON.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR JEFFERSON-
LEE-VAN BUREN.
IL...WINTER STORM WARNING UNTIL 6 AM CST WEDNESDAY FOR BUREAU-CARROLL-
HENDERSON-HENRY IL-JO DAVIESS-MCDONOUGH-MERCER-PUTNAM-ROCK
ISLAND-STEPHENSON-WARREN-WHITESIDE.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR HANCOCK.
MO...WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR CLARK-
SCOTLAND.
LE/SHEETS
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