KDMX wx discussion

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1211 PM CST SUN NOV 29 2015

.UPDATE...
ISSUED AT 545 AM CST SUN NOV 29 2015

WINTER WEATHER ADVISORY FOR SOUTHERN IOWA HAS BEEN EXTENDED
THROUGH 9 AM THIS MORNING TO ACCOUNT FOR THE POSSIBILITY OF
ADDITIONAL FZRA SHOWERS THROUGH THE FIRST PART OF THE MORNING.
IADOT REPORTS CONTINUE TO SHOW PARTIALLY ICE COVERED ROADS IN THE
SOUTH AND RECENT SPOTTER REPORTS ALSO CONFIRM SOME LOCALIZED
ICING. LATEST REGIONAL RADAR TRENDS SUGGEST THAT THE BAND OF
PRECIPITATION MIGHT BE STARTING TO SHIFT SOUTHEASTWARD...AND IF
THIS TREND CONTINUES AND ROADS IMPROVE...THE ADVISORY MAY BE
LIFTED EARLY.

&&

.SHORT TERM /TODAY/...
ISSUED AT 352 AM CST SUN NOV 29 2015

UPPER LEVEL FLOW PATTERN HAS BEGUN AMPLIFYING THIS MORNING AS AN
ELONGATED 500MB BARBELL LOW OVER THE WESTERN CONUS BECOMES MORE
LONGITUDINALLY ORIENTED IN RESPONSE TO A SHORTWAVE WRAPPING ALONG
THE LOW`S SOUTHERN FLANK. THIS HAS ALLOWED MOISTURE AND CLOUDS TO
BEGIN STREAMING BACK INTO IOWA IN THE FORM OF A 4 TO 6 KFT CLOUD
DECK. THIS RETURNING MOISTURE HAS ALSO FUELED SCATTERED SHOWERS
ACROSS THE SOUTHERN CWA. THE LACK OF ICE IN THE COLUMN AND SURFACE
TEMPS HOVERING BETWEEN 29 AND 32 MEANS PRECIP HAS OSCILLATED BETWEEN
RAIN AND FREEZING RAIN THROUGHOUT THE NIGHT WITH SOME ICE IMPACTS TO
ROADS BEING REPORTED BY THE IADOT. THE DRY 950-800 MB WEDGE SHOWN BY
THE 00Z OAX/DVN RAOBS APPEARS TO HAVE COMPLETELY SATURATED AS FAR
NORTH AS I-80 BY NOW BASED ON SURFACE OB VERIFICATION OF KDMX RADAR
ECHOES.

UPSTREAM RADAR TRENDS IN KANSAS AND OKLAHOMA INDICATE THAT SCATTERED
RA/FZRA SHOWERS WILL PERSIST THROUGH MUCH OF THE MORNING IF NOT
LONGER...AND HAVE MAINTAINED SLIGHT CHANCE TO CHANCE POPS ACROSS THE
SOUTH AND SOUTHWEST FOR TODAY AS A RESULT. GIVEN THE OVERCAST SKIES
AND WEAK NE/E FLOW...TRENDED DOWN HIGHS FOR TODAY CLOSER TO WHAT WAS
SEEN YESTERDAY ACROSS THE CWA...VERY SIMILAR TO WHAT IS BEING
MODELED BY THE CONSRAW. THE RESULT IS HIGHS ONLY A FEW DEGREES
WARMER THAN CURRENT OBS...WHICH FOR SOUTHWEST IOWA TRANSLATES INTO
HIGHS OF 31 TO 33 DEGREES. FREEZING RAIN WILL REMAIN A BIT OF A
CONCERN THROUGHOUT THE DAY IN THESE AREAS...BUT GIVEN THE LACK OF
ANY REAL CAA AND LOW QPF AMOUNTS...ICE ACCRUAL SHOULD BE MINIMAL.
LATENT WARMING AND ALBEIT MINIMAL SOLAR INSOLATION DURING THE DAY
MAY ALSO BE ENOUGH TO TIP THE SCALES IN FAVOR OF RAIN OVER FREEZING
RAIN. WILL THUS LET THE ONGOING WINTER WEATHER ADVISORY EXPIRE AFTER
SUNRISE AND NOT REISSUE FURTHER NORTH AND WEST...BUT THIS SHOULD
NONETHELESS BE WATCHED THROUGHOUT THE DAY.

PRECIP SHOULD SPREAD NORTHWARD BY MID AFTERNOON AS DEEPENING 290-
300K ISENTROPIC ASCENT DEVELOPS OVER THE STATE AS THE AFOREMENTIONED
500 MB WAVE BEGINS TO LIFT OUT OF THE FOUR CORNERS REGION. THIS WILL
BE THE START OF OUR NEXT SIGNIFICANT WINTER STORM COVERED IN THE
LONG TERM SECTION BELOW.

.LONG TERM /TONIGHT THROUGH SATURDAY/...
ISSUED AT 352 AM CST SUN NOV 29 2015

PRIMARY CONCERN IN THIS PERIOD WILL OBVIOUSLY BE FROM TONIGHT INTO
TUESDAY FOLLOWING THE PROGRESSION OF CURRENT ROCKIES LONG WAVE
TROUGH/CLOSED LOW. CONFIDENCE IN OVERALL PROGRESSION HAS INCREASED
SOMEWHAT...BUT SENSIBLE WEATHER DETAILS ARE STILL NOT HIGH OVERALL.
AT PERIOD ONSET THIS EVENING CURRENT SHORT WAVES DIVING INTO THE
TROUGH THROUGH CA AND JUST OFF THE SRN CA COAST WILL QUICKLY
AMPLIFY AS THEY REACH NE/KS TOWARD DAYBREAK. CURRENT REGIME WITH
LIFT AND MOISTURE CENTERED AROUND H7 OR 3KM WILL INTENSIFY AND
DEEPEN THROUGH THE NIGHT WITH H85/H7 OR 285-295K UVM INCREASING.
THIS MOISTURE AND LIFT WILL MAINLY BE BELOW -10C/3KM
HOWEVER...WITH LITTLE TO NO ICE INTRODUCTION...SO THE FREEZING
RAIN POTENTIAL PRECEDING THE CHANGE TO SNOW HAS BECOME MORE
WORRISOME. THERE COULD VERY WELL BE AT LEAST A TENTH OF AN INCH OF
ICE ACCUMS BY EARLY MON MORNING DUE TO A DEEP LAYER OF SUPERCOOLED
DROPLETS BEFORE THE FORCING DEEPENS SO HAVE MOVED THE WATCH
STARTING TIME TO 06Z RATHER THAN 12Z. TEMPS SHOULD BE JUST BELOW
FREEZING IN MANY AREAS AND IN FACT RAW MODEL SURFACE TEMP
METEOGRAMS ESSENTIALLY DEPICT NEAR FLAT LINE TRENDS INTO
TUE...WHICH CONTINUES TO MAKE SENSE WITH PLENTY OF LOW CLOUDS AND
TRUE COLD FRONT NOT PASSING THROUGH IA UNTIL TUE NIGHT AS THE
SYSTEM PULLS AWAY. THUS WILL CONTINUE TO MINIMIZE THE DIURNAL
TREND STAYING TOWARD OR BEYOND THE WARM AND COOL ENDS OF MOS LOWS
AND HIGHS RESPECTIVELY. THE SUB-FREEZING DEPTH MAY APPROACH 2500FT
TONIGHT...BUT THE 0-2KM MIN TEMP IS NOT BELOW -5C. WITH PRECIP
STAYING ALL LIQUID TO START...IT WILL BE DIFFICULT TO RE-FREEZE
INTO SLEET VS WHAT WOULD OCCUR IF THE PROCESS WAS MELTING SNOW RE-
FREEZING.

BY MONDAY MORNING THE AFOREMENTIONED SHORT WAVES WILL PRODUCE A DEEP
LAYER OF KINEMATIC FORCING INTO THE MO VALLEY SUPPLEMENTED BY
WEAKER THERMODYNAMIC CONTRIBUTIONS. THERE WILL BE A PERIOD WHERE
LIFT IN THE DENDRITIC GROWTH ZONE /DGZ/ IS HIGH WITH ELEVATED SNOW
RATIOS...BUT THIS WILL NOT LAST LONG. MODELS DO NOT AGREE ON THE
DETAILS...BUT ALL SUGGEST SOME FORM OF A DRY SLOT AND/OR WEDGE OF
LIMITED LIFT WILL PUSH INTO IA DURING THE DAY. THIS MAY MAKE ICE
INTRODUCTION PATCHY AND LOWER SNOW RATIOS QUITE A BIT...AND ALSO
MAKE PRECIP TYPES PROBLEMATIC. MODELS SUGGEST LOW STATIC STABILITY
DURING THIS TIME AS WELL...SO WHAT PRECIP OCCURS MAY BE MORE
SHOWERY WITH VARIED TYPES AND INTENSITIES. THE SHORT WAVE FORCING
WILL EXIT INTO THE NIGHT WITH THE DEFORMATION ZONE PRECIP LIKELY
STAYING NORTH AND WEST OF IA. DEEP MOISTURE WILL REMAIN IN
PLACE...BUT WITH ONLY PATCHY SUPPORTING LIFT NEAR THE CORE OF THE
UPPER LOW. THIS WILL RESULT IN A PROLONGED PERIOD OF MAINLY LIGHT
SNOW ACCUMS LINGERING THROUGH THE NIGHT AND INTO AT LEAST EARLY
TUE. AS HINTED AT ABOVE...ALL THIS UNCERTAINTY HAS RESULTED IN
LEAVING THE WATCH IN PLACE UNTIL DETAILS BECOME MORE CLEAR.
CONSIDERED UPGRADING TO A WARNING...BUT CURRENT ICE ACCUMS ARE
BELOW WARNING CRITERIA...AROUND A TENTH OR SO OVER THE NW HALF OF
IA...AND WHILE SNOW AMOUNTS EVENTUALLY REACH THREE TO EIGHT FROM
CENTRAL TO NW IA /NW OF DES MOINES/ THEY DO NOT OCCUR WITHIN THE
TIME CRITERIA...6 INCHES IN 12HRS OR 8 IN 24HRS. THE MAIN TAKE
AWAY FROM THIS EVENT LOOKS TO BE THE POTENTIAL FOR TRAVEL PROBLEMS
OVER THE NW HALF OF IA STARTING LATER TONIGHT AND INTO AT LEAST
MONDAY EVENING DUE TO A PROLONGED SNOWFALL OF VARIED RATES FALLING
A TOP PRECEDING FREEZING RAIN. 0-2KM TEMPS LOOK TO HOVER BETWEEN
-5C TO 1C SO THE SNOW WILL BE HEAVY...WET AND STICKY...DIFFICULT
TO REMOVE. WINDS DO NOT LOOK SUFFICIENT TO CAUSE ANY TREE CONCERNS
HOWEVER.

THE REMAINDER OF THE FORECAST STILL LOOKS FAIRLY UNEVENTFUL WITH
GRADUALLY MODERATING TEMPERATURES LIKELY REBOUNDING BACK TO OR JUST
ABOVE NORMAL. OUR TROUBLESOME SYSTEM WILL PASS THROUGH THE GREAT
LAKES TUE OR WED WITH FAIRLY LIGHT FLOW INTO THE MIDWEST...THE
WESTERLIES WELL TO OUR NORTH. THE NEXT SEQUENCE OF A WRN TROUGH MAY
BRING SOME PRECIP INTO NEXT WEEKEND...BUT IT DOES NOT LOOK
SIGNIFICANT BEING FAIRLY PROGRESSIVE.

&&

.AVIATION...29/18Z
ISSUED AT 1208 PM CST SUN NOV 29 2015

VFR CONDITIONS FOR MUCH OF THE AREA WILL PERSIST THIS AFTERNOON
BUT MVFR TO IFR CEILINGS WILL MOVE NORTHWARD INTO CENTRAL IOWA
LATER THIS AFTERNOON INTO TONIGHT AS SYSTEM LIFTS NORTHWARD INTO
THE FORECAST AREA. AREAS OF RA/FZRA ARE EXPECTED TO OVERSPREAD THE
STATE TONIGHT WITH WIDESPREAD IFR TO LIFR DEVELOPING AND
CONTINUING ACROSS ALL AREAS INTO MONDAY AS SYSTEM PASSES THROUGH
STATE.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM MIDNIGHT CST TONIGHT THROUGH LATE MONDAY
NIGHT FOR ADAIR-AUDUBON-BOONE-CALHOUN-CARROLL-CASS-CERRO GORDO-
CRAWFORD-DALLAS-EMMET-FRANKLIN-GREENE-GUTHRIE-HAMILTON-HANCOCK-
HARDIN-HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-SAC-STORY-WEBSTER-
WINNEBAGO-WORTH-WRIGHT.

&&