AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
606 PM CST THU NOV 19 2015
.SHORT TERM /TONIGHT/...
ISSUED AT 359 PM CST THU NOV 19 2015
SURFACE HIGH WILL PUSH EAST ACROSS MISSOURI...KEEPING CONDITIONS DRY
ACROSS IOWA OVERNIGHT. FORECAST CONCERN OVERNIGHT BECOMES WINDS AND
TEMPERATURES. WINDS REMAIN STRONG AND GUSTY ACROSS THE CWA
THROUGHOUT THE AFTERNOON WITH GOOD MIXING. GUSTS WILL BEGIN
DIMINISHING...AND HAVE DIMINISHED ACROSS THE SOUTH HOWEVER SUSTAINED
WINDS REMAIN NEAR OR ABOVE ADVISORY CRITERIA AND WILL KEEP ADVISORY
AS IS FOR NOW. MAY NEED TO CANCEL EARLY IF DOWNWARD TREND
CONTINUES...ESPECIALLY CLOSER TO SUNSET. HAVE DROPPED OVERNIGHT LOWS
A FEW DEGREES WITH CLEAR SKIES AND WINDS BECOMING LIGHT...ESPECIALLY
IN THE WEST. EXPECT WAA TO RETURN LATE IN THE PERIOD IN THE SOUTH
AND WEST...AND CLOUDS WILL INCREASE AFTER 06Z ACROSS CENTRAL IOWA
HELPING TO LIMIT COOLING. HOWEVER...REMAIN ON THE HIGH SIDE OF
GUIDANCE AND MAY BE TOO WARM.
.LONG TERM /FRIDAY THROUGH THURSDAY/...
ISSUED AT 359 PM CST THU NOV 19 2015
PRIMARY FOCUS THROUGH THE PERIOD IS FOCUSED ON THE STORM SYSTEM
FOR FRIDAY AND FRIDAY NIGHT THEN THE COLD FOR THE WEEKEND.
SIGNIFICANT SHORT WAVE ENERGY WILL INTENSIFY OVER THE
INTERMOUNTAIN WEST TONIGHT THEN RACE EAST/SOUTHEAST ON FRIDAY. LEE
SIDE CYCLOGENESIS WILL COMMENCE THROUGH THE DAY FRIDAY WITH THE
LOW CENTER DRIVING SOUTHEAST ACROSS THE WESTERN PLAINS WHILE AN
INVERTED TROUGH EXTENDS NORTH INTO NORTHERN MISSOURI WITH A
SECONDARY LOW DEVELOPING IN THAT REGION. INTENSE THETA-E ADVECTION
WILL COMMENCE FRIDAY MORNING THROUGH A DEEP LAYER. WHILE IT IS A
DEEP LAYER OF ADVECTION...THE PRIMARY FORCING ASSOCIATED WITH THIS
SYSTEM IS IN THE 850-700 MB LAYER AND USED THIS AREA OF THETA-E
ADVECTION AS A FOCUS FOR THE MOST INTENSE PRECIPITATION AREA AND
FOCUS FOR THE WINTER STORM WATCH. IN ADDITION...A STRONG REGION OF
QG FORCING WILL ARRIVE DURING THIS PERIOD PRODUCING IMPRESSIVE
VERTICAL ASCENT DUE TO BOTH KINEMATIC AND THERMODYNAMIC PROCESSES.
HIGH PRESSURE RIDGE MOVING ACROSS THE STATE INTO FRIDAY MORNING
WILL LEAVE A VAST AMOUNT OF DRY AIR IN PLACE BELOW 850 MB. WILL
NEED TO RELY ON PRECIPITATION LOADING BELOW THIS LAYER FOR
SATURATION IN THE LOW LEVELS AND ACCUMULATING PRECIPITATION ONSET.
GIVEN THE FORCING...THIS PROCESS SHOULD OCCUR QUICKLY NORTH OF
HIGHWAY 30 WHILE DRY AIR MAY LINGER A BIT LONGER SOUTH OF THIS
REGION. POSSIBLE PRECIPITATION DOES NOT BEGIN IN THE DES MOINES
METRO UNTIL MID TO LATE AFTERNOON.
OTHER FACTORS INCLUDE CURRENT WARM SURFACES INCLUDING ROADWAYS.
COLD WINDS TODAY LIKELY HAVE PUSHED THESE SURFACES INTO THE 30S
AND 40S. COLD TEMPERATURES AND COOLING DURING MODERATE/HEAVY SNOW
ON FRIDAY WILL QUICKLY DROP THESE TEMPERATURES THEREFORE DO NOT
EXPECT ANY GREAT REDUCTIONS IN SNOW AMOUNTS DUE TO SFC MELTING.
FARTHER SOUTH...NEAR SURFACE LAYER MAY LINGER AND MAY DELAY THE
RAIN TO SNOW TRANSITION PHASE AND LIMIT SNOWFALL AMOUNTS THERE.
OTHERWISE...EXPECT THE RAIN TO SNOW TRANSITION TO OCCUR QUICKLY
WITH VERY COLD TEMPERATURES ALOFT AND NEAR SFC WETBULBS FALLING TO
NEAR FREEZING RAPIDLY. OVERALL...EXPECT A SIGNIFICANT EARLY SEASON
WINTER STORM EVENT WITH POTENTIAL FOR WIDESPREAD 6 TO 10 INCHES OF
SNOW THROUGH FRIDAY NIGHT. GIVEN THE FORCING AND UPPER LEVEL LAPSE
RATES ABOVE 700 MB...CANNOT DISCOUNT LOCALIZED
CONVECTION/THUNDER SNOW BUT HAVE NOT INCLUDED AT THIS TIME. SHOULD
HAVE A PERIOD OF 1 INCH PLUS SNOWFALL RATES ESPECIALLY 3 PM TO 9
PM. HAVE LEFT THE NORTHERN AND SOUTHERN FRINGES AS A WATCH FOR
NOW SINCE THERE IS STILL POTENTIAL FOR A SHIFT IN EITHER DIRECTION
AND WILL LET FOLLOWING SHIFTS TO FURTHER EVALUATE. NORTHERN WINDS
OF 15 TO 25 MPH FRIDAY NIGHT CAUSE SOME AREAS OF BLOWING AND
DRIFTING OF SNOW. LIKELY TO RESULT IN TREACHEROUS TRAVEL
ESPECIALLY DUE TO FIRST EVENT OF YEAR AND EVERYONE ADJUSTING BACK
TO WINTER DRIVING CONDITIONS.
FOR WINTER HEADLINES...HAVE TWO SEGMENTS FOR NOW TRYING TO CAPTURE
TIMING. CERTAINLY MAY NEED TO MASSAGE THIS AS THE EVENT UNFOLDS.
VERY COLD CONDITIONS WILL FOLLOW SATURDAY AND COULD HAVE LOWS NEAR
ZERO SATURDAY NIGHT AS HIGH PRESSURE RIDGE MOVES OVERHEAD.
TEMPERATURES SHOULD START TO RISE ABOVE FREEZING OVER THE SOUTH
SUNDAY AND ALL LOCATIONS BY MONDAY.
&&
.AVIATION...20/00Z
ISSUED AT 550 PM CST THU NOV 19 2015
MAIN CONCERN IS ONSET OF SNOW/RAIN. CIGS LOWER TO MVFR BY 18Z IN
AT KFOD THEN NORTHEAST TO KALO BY 21Z...THEN CRASH HARD TO
IFR/LIFR THROUGH END OF PERIOD. -RA AND -SN WILL GIVE WAY TO SN BY
00Z MOST AREAS. WINDS INCREASE FROM SOUTHEAST TO NEARLY 12KTS WITH
GUSTS 16KTS AFT 18Z EAST AND SOUTHEAST. /REV
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY UNTIL 6 PM CST THIS EVENING FOR ADAIR-ADAMS-
APPANOOSE-AUDUBON-BLACK HAWK-BOONE-BREMER-BUTLER-CALHOUN-CARROLL-
CASS-CERRO GORDO-CLARKE-CRAWFORD-DALLAS-DAVIS-DECATUR-EMMET-
FRANKLIN-GREENE-GRUNDY-GUTHRIE-HAMILTON-HANCOCK-HARDIN-HUMBOLDT-
JASPER-KOSSUTH-LUCAS-MADISON-MAHASKA-MARION-MARSHALL-MONROE-PALO
ALTO-POCAHONTAS-POLK-POWESHIEK-RINGGOLD-SAC-STORY-TAMA-TAYLOR-
UNION-WAPELLO-WARREN-WAYNE-WEBSTER-WINNEBAGO-WORTH-WRIGHT.
WINTER STORM WARNING FROM 2 PM FRIDAY TO 6 AM CST SATURDAY FOR
AUDUBON-BLACK HAWK-BOONE-BREMER-BUTLER-CERRO GORDO-DALLAS-
FRANKLIN-GRUNDY-GUTHRIE-HAMILTON-HARDIN-JASPER-MARSHALL-POLK-
POWESHIEK-STORY-TAMA.
WINTER STORM WATCH FROM FRIDAY AFTERNOON THROUGH LATE FRIDAY
NIGHT FOR ADAIR-CASS-MADISON-MAHASKA-MARION-WARREN.
WINTER STORM WARNING FROM 10 AM FRIDAY TO 6 AM CST SATURDAY FOR
CALHOUN-CARROLL-CRAWFORD-GREENE-HANCOCK-HUMBOLDT-KOSSUTH-PALO
ALTO-POCAHONTAS-SAC-WEBSTER-WRIGHT.
WINTER STORM WATCH FROM FRIDAY MORNING THROUGH LATE FRIDAY NIGHT
FOR EMMET-WINNEBAGO-WORTH.
&&
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