283
FXUS63 KDMX 230008
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
608 PM CST WED JAN 22 2020
UPDATE
ISSUED AT 604 PM CST WED JAN 22 2020
HOLDING ONTO THE ADVISORY MAINLY EAST OF I35 AS STILL SOME
ENHANCED RETURNS AND POOR TRAVEL CONDITIONS IN THE EASTERN AREAS
OF THE REGION. TEMPERATURES THERE SLIGHTLY COLDER AND PLOW CAMS
STILL SHOW SNOW COVERED CONDITIONS ALONG THE INTERSTATE AND
SECONDARY ROADS IN EAST CENTRAL IOWA. WILL EXPIRE AT 9 PM.
DISCUSSION
/TONIGHT THROUGH WEDNESDAY/
ISSUED AT 309 PM CST WED JAN 22 2020
THE PRIMARY CONCERNS WILL BE FOG TRENDS INTO THURSDAY MORNING IN
ADDITION TO CONTINUED WINTRY PRECIPITATION TRENDS THROUGH FRIDAY
WITH CONFIDENCE REMAINING ON THE LOW SIDE. OUR CURRENT SCENARIO
HAS FAIRLY WIDESPREAD DEFORMATION ZONE PRECIPITATION DRIVEN BY A
NOTED SHORT WAVE/PV ANOMALY ESSENTIALLY OVERHEAD, WHICH SHOWS UP
NICELY IN RADAR AND VARIOUS SATELLITE IMAGERY PRODUCTS. THE ONLY
PROBLEM TODAY HAS BEEN THAT MODELS HAD POORLY HANDLED THE THERMAL
STRUCTURE, BEING TOO COOL, RESULTING IN MORE MIX/LIQUID THAN THE
PREVIOUSLY ANTICIPATED SNOW EARLIER TODAY. WE SHOULD BE BACK TO
MAINLY SNOW THROUGH SUNSET NOW HOWEVER WITH DECENT BUT BRIEF
SNOWFALL RATES INTO THE EVENING COMMUTE. THIS APPEARS TO BE DRIVEN
BY SUPERSATURATION IN THE DENDRITIC GROWTH ZONE /RAP/ PRODUCING
STELLAR DENDRITES, WITH AN 800M ISOTHERMAL LAYER JUST BELOW 0C PER
THE GFS HELPING NEAR SURFACE AGGREGATION.
THESE KINEMATIC DEFORMATION ZONE LIFT CONTRIBUTIONS SHOULD
DIMINISH DURING THE EVENING HOWEVER AND BE REPLACED BY LATE
ARRIVING WARM/THETA-E ADVECTION THERMAL FORCING SOUTH AND EAST
INTO THE NIGHT. THIS WILL KEEP LIGHT PRECIP GOING IN THOSE
LOCATIONS. THE PRECIP TYPE MAINLY LOOKS TO BE LIGHT SNOW, ALTHOUGH
MUCH LIKE TODAY SOME WINTRY MIX MAY OCCUR AT TIMES DUE TO
AN INTERMITTENT LACK OF ICE INTRODUCTION DURING THE WEAKER LIFT.
THE FORCING/OMEGA IS WEAK, AND PARTICULARLY NOT MUCH IN THE
DENDRITIC GROWTH ZONE. THUS THE CURRENT WINTER WEATHER ADVISORY
WILL BE ALLOWED TO EXPIRE. SNOW RATES SHOULD NOT REACH CRITERIA,
AND TRAVEL IMPACTS HAVE BEEN MINIMAL DUE TO ROAD TEMPS AND
DEWPOINTS AT OR ABOVE FREEZING SO ANY ADDITIONAL LIQUID COMPONENT
OF A WINTRY MIX SHOULD NOT AMOUNT TO MUCH.
FARTHER WEST THE PRIMARY CONCERN WILL LIKELY TURN TO FOG
OVERNIGHT, WHICH MAY BE DENSE. HUMIDITIES ARE ALREADY HIGH WITH
NIGHTFALL AND STEADY STATE LOW LEVEL MOISTURE NEAR THE SURFACE
TROUGH. THIS CONCEPTUAL MODEL IS ALSO SUPPORTED BY VARIOUS RAW AND
MOS GUIDANCE SOURCES, WHICH SUGGEST DENSE FOG WELL INTO THE
MORNING. HAVE ADDED PLENTY OF FOG WORDING AND WILL INTRODUCE A
DENSE FOG ADVISORY ALONG AND WEST OF I-35 FOR STARTERS BUT FURTHER
EXPANSION EASTWARD MAY BE NECESSARY.
LOOKING AHEAD TO THURSDAY NIGHT FOG MAY BE A SIMILAR STORY, ALBEIT
A BIT FARTHER EAST AS THE INVERTED TROUGH PROGRESSES COINCIDENT
WITH THE MO SURFACE LOW DEVELOPMENT. PRECIP WILL ALSO BEGIN TO
BECOME MORE PREVALENT AGAIN AS THE CURRENT NORTHERN ROCKIES PV
TROUGH INCREASES MAINLY KINEMATIC LIFT FROM SOUTHERN IA THU NIGHT
INTO EASTERN IA BY FRI. THIS WILL GENERATE ONE LAST ROUND OF LIGHT
SNOW WITH CUMULATIVE AMOUNTS NOW LESS THAN PREVIOUSLY ADVERTISED,
LITTLE NW TO AS HIGH AS 3-4 INCHES SOUTHEAST.
THE OUTLOOK FOR THE WEEKEND HAS CHANGED LITTLE WITH DRY
CONDITIONS AND NORMAL TO JUST ABOVE NORMAL TEMPERATURES. A MINOR
CONCERN WILL BE ONE LAST SHORT WAVE DROPPING INTO THE NORTHWEST
FLOW AROUND SUNDAY, BUT FOR NOW BOTH FORCING AND SATURATION ARE
SPOTTY AND NEVER REALLY PHASED. AFTER MORE FAIR WEATHER AND SHORT
WAVE RIDGING THROUGH MONDAY THE MODELS ARE IN DECENT AGREEMENT,
CONSIDERING THE TIME RANGE, BRINGING ANOTHER TROUGH FROM THE
CENTRAL AND SOUTHERN PLAINS INTO THE MID MS VALLEY TUESDAY INTO
WEDNESDAY. THIS WILL BRING ANOTHER POTENTIALLY PROLONGED PRECIP
EVENT WITH FORCING AND MOISTURE IN PLACE ACROSS IA FOR A FEW DAYS.
ROUGH EXAMINATION OF THE THERMAL STRUCTURE SUGGESTS THIS WOULD
PRIMARILY BE SNOW.
AVIATION
/FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING/
ISSUED AT 607 PM CST WED JAN 22 2020
MANY CONCERNS REMAIN WITH LOW CIGS, LINGERING PRECIPITATION AND
EVENTUAL DENSE FOG THAT MAY PERSIST THROUGH MOST OF THURSDAY
AFTERNOON. WILL NEED TO EVALUATE THE ENDING TIME OF LOW VSBY FOR
THE 06Z PACKAGE. OTHERWISE, LIFR/IFR CONDITIONS WITH PERIODS OF
LIGHT SNOW, FOG AND PATCHY FREEZING DRIZZLE EXPECTED. /REV
DMX WATCHES/WARNINGS/ADVISORIES
DENSE FOG ADVISORY UNTIL NOON CST THURSDAY FOR IAZ004>007-
015>017-023>026-033>037-044>048-057>060-070>073-081>083-092>094.
WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR IAZ027-
028-038-039-049-050-061-062-074-075-084>086-095>097.
UPDATE...REV
DISCUSSION...SMALL
AVIATION...REV
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