Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201002
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
402 AM CST THU NOV 20 2014

.SHORT TERM /TODAY/...
ISSUED AT 400 AM CST THU NOV 20 2014

HIGH PRESSURE WILL DOMINATE THE WEATHER TODAY. THE BAND OF CLOUDS
ON THE BAROCLINIC RIBBON EXTENDING FROM NERN MT THROUGH SRN IA
WILL CONTINUE TO GRADUALLY THIN AS THE AIRMASS CONTINUES TO DRY.
TEMPERATURES WILL STRUGGLE TO RISE TODAY UNDER THE SHALLOW
INVERSION AND WEAKENING GRADIENT. TEMPS WILL CONTINUE TO FALL INTO
THE MID MORNING WITH RECOVER NOT ALL THAT MUCH HIGHER THAN THE
PRESENT READINGS ALTHOUGH HIGH PRESSURE AXIS WILL PASS BY
NIGHTFALL...THERE WILL BE ENOUGH MIXING TO BRING DOWN THE NWLY
WINDS FROM ALOFT THROUGH THE DAY WITH A RAPID DIMINISH IN WINDS
TOWARD 00Z.

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
ISSUED AT 400 AM CST THU NOV 20 2014

SURFACE RIDGE WILL SLIDE EAST OF THE CWA LATE TONIGHT WITH WAA
COMMENCING EARLY FRIDAY MORNING. NUDGED UP LOWS 1 TO 2 DEGREES
OVER THE NORTH AND STABILIZED HOURLY TEMP TRENDS AFTER 06Z WITH
THE IDEA THAT THE SOUTHERLY WINDS WILL NEGATE ANY FURTHER COOLING
OF THE SNOWPACK-MODIFIED AIRMASS. STRONG WAA CONTINUES THROUGH
THE DAY ON FRIDAY WITH LOW STRATUS CLOUDS FILLING IN DURING THE
DAY.

THERE IS SOME MODEL DISAGREEMENT WITH REGARDS TO THE ONSET OF
PRECIPITATION ON FRIDAY NIGHT...BUT FREEZING RAIN/DRIZZLE IS STILL
EXPECTED FROM 00Z TO 12Z SATURDAY. FORECAST SOUNDINGS REMAIN
SATURATED BELOW 800MB WITH A LARGE +5 TO +7 C INVERSION UP THROUGH
THIS LAYER. THE SHALLOW MOISTURE PROFILE AND BROAD/WEAK
THERMODYNAMIC FORCING LENDS CREDENCE TO A MORE DRIZZLE TYPE EVENT.
LATEST GUIDANCE HAS SURFACE TEMPS WARMING SLIGHTLY FASTER THAN
EARLIER RUNS AND THIS HAS RESULTED IN THE MODELED FREEZING RAIN
ENDING SOONER THAN PREVIOUSLY FORECAST. HOWEVER...WITH GROUND
TEMPS STILL QUITE COLD...IT IS POSSIBLE THAT FREEZING PRECIP MAY
OCCUR EVEN WITH AIR TEMPS A COUPLE OF DEGREES ABOVE FREEZING. THIS
POTENTIAL WILL NEED TO BE EVALUATED IN UPCOMING SHIFTS.

NAM/SREF SOUNDINGS ACROSS THE CWA MAINTAIN A SATURATED PROFILE
BELOW 900MB THROUGHOUT THE DAY ON SATURDAY. INTRODUCED PATCHY FOG
WORDING AND RAISED CLOUD COVER FOR THIS TIME FRAME AS THE WARM
MOIST AIR IS ADVECTED OVER THE COOLER GROUND. A SURFACE CYCLONE
WILL DEVELOP OVER TEXAS ON SATURDAY AFTERNOON AS A SHARP UPPER
LEVEL SHORTWAVE DIVES ACROSS THE SOUTHWESTERN STATES AND EJECTS
NORTHEASTWARD LATE ON SATURDAY. THE SURFACE LOW DEEPENS FROM
1000MB TO 980 MB BETWEEN 00Z SUN AND 00Z MONDAY AS IT TRACKS UP
INTO THE WESTERN GREAT LAKES REGION. THERE MAY BE A BREAK IN THE
PRECIP IN IOWA ON SATURDAY AFTERNOON AND EVENING BEFORE THIS
SYSTEM ARRIVES...BUT MAINTAINED POPS DUE TO THE PERSISTENT NEAR
SURFACE SATURATED LAYER AND SEVERAL REGIONS OF 925MB THETA-E
ADVECTION THAT LIFT THROUGH DURING THIS TIMEFRAME AND COULD STILL
GENERATE SOME PRECIP.

THE EC TAKES THE SURFACE CYCLONE ON A SLIGHTLY MORE WESTWARD TRACK
COMPARED TO THE GFS ON SUNDAY...BUT IN EITHER CASE MODERATE TO
EVEN SHORT PERIODS OF HEAVY RAINFALL ARE POSSIBLE OVER THE
EASTERN TWO THIRDS OF IOWA ON SUNDAY WITH PWATS AROUND 0.75 TO 1
INCH. SUBFREEZING AIR WILL SPILL INTO IOWA LATE ON SUNDAY AND
CHANGE ANY REMAINING PRECIP BACK OVER TO SNOW. AS THE UPPER LEVEL
WAVE LIFT NORTHEASTWARD IT WILL INTERACT WITH A SECOND CLOSED LOW
DROPPING SOUTHEAST FROM ALBERTA. THIS WILL LEAD TO THE FORMATION
OF A COMPLEX DOUBLE BARREL LOW THAT WILL PERSIST OVER THE NORTHERN
PLAINS INTO TUESDAY. UNDERSTANDABLY...MODEL SOLUTIONS CONTINUE TO
DIFFER ON THE EVOLUTION AND STRUCTURE OF THIS LOW AND TIMING THE
ARRIVAL OF SNOW ASSOCIATED WITH THE INVERTED TROUGH WRAPPING
AROUND THE LOW IS SOMEWHAT DIFFICULT. A TIGHTENING PRESSURE
GRADIENT BEHIND THE DEPARTING SURFACE LOW WILL GENERATE STRONG
WINDS AND BLOWING SNOW ON MONDAY AFTERNOON. THIS SHOT OF COLD AIR
WILL NOT LAST LONG AS WARMER AIR RESURGES NORTHWARD FOR THE MIDDLE
OF THE WEEK AHEAD OF A CLIPPER SYSTEM DIVING THROUGH THE REGION.

&&

.AVIATION...20/06Z
ISSUED AT 1139 PM CST WED NOV 19 2014

STRATUS CONTINUES TO THIN ACROSS CENTRAL IOWA. KDSM MAY HAVE A
BRIEF PERIOD OF MVFR CIGS BEFORE THE STRATUS DEPARTS. OTHERWISE
WIDESPREAD VFR IS FORECAST. WEST TO NORTHWEST WINDS WILL PREVAIL
THROUGH THURSDAY THEN GRADUALLY BECOMING SOUTH TO SOUTHEAST
THURSDAY EVENING.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...MS NOV 14
LONG TERM...SKOW
AVIATION...DONAVON



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