Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 210956
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
256 AM MST SUN DEC 21 2014

.SHORT TERM...TODAY THROUGH MONDAY NIGHT
ISSUED AT 255 AM MST SUN DEC 21 2014

WATER VAPOR IMAGERY AND UPPER AIR ANALYSIS SHOWS WEAK TROUGH SITTING
ACROSS THE NRN PLAINS ATTM AS A STRONG +140KT JET STREAK PUSHES INTO
THE PACIFIC NW COAST. SURFACE ANALYSIS SHOW DEEPENING LOW PRESSURE
ACROSS BRITISH COLUMBIA AND ALBERTA SLIDING SE. FORCING ASSOCIATED
WITH JET VORT MAX WILL BRING PRECIP TO THE AREA LATE THIS MRNG INTO
THE AFTN HOURS. WITH TEMPS RISING TODAY INTO THE UPPER 30S TO AROUND
50...PRECIP WILL BE RAIN.

UPPER TROUGH WILL QUICKLY DEEPEN TONIGHT AS UPPER LEVEL ENERGY DIVES
DOWN THE BACKSIDE. STRENGTHENING LOW PRESSURE WILL DROP INTO WRN ND
THIS EVNG...THEN CNTRL SD LATER TONIGHT. COLDER AIR WILL FILTER IN
FROM THE N/NW...WITH GUSTY NW WINDS DEVELOPING OVERNIGHT. RAIN
ACROSS THE AREA WILL CHANGE TO SNOW LATE TONIGHT AS THERMAL PROFILES
COOL...BUT SNOW WITH LITTLE ACCUMULATION. STEEP LOW LEVEL LAPSE
RATES AND HIGH RH WILL RESULT IN UPSLOPE PRECIP...WITH RAIN CHANGING
TO SNOW THIS EVNG. MODELS NOW SHOWING SYSTEM BECOMING A STALLED AND
STACKED UPPER LOW ON MONDAY. TIGHT PRESSURE GRADIENT ACROSS THE AREA
WILL BRING STRONG NW WINDS TO THE AREA. MODELS BEGINNING TO SHOW A
WETTER SOLUTION ACROSS THE PLAINS WITH WRAP AROUND PRECIP DEVELOPING
SOMEWHERE OVER THE WRN TO CNTRL SD PLAINS. HAVE INCREASED POPS QUITE
A BIT TO REFLECT THIS...WHICH ALSO INCREASES EXPECTED SNOW AMOUNTS
OVER THE PLAINS MONDAY. BUT IT STILL LOOKS LIKE TEMPS MAY RISE
ENOUGH TO OR ABOVE FREEZING TO HINDER SNOW ACCUMULATION. THINK
PLAINS WILL SEE AN INCH OR TWO OF SNOW DURING THE DAY. NW WINDS OF
25 TO 35 MPH ARE EXPECTED...WHICH WHEN COMBINED WITH SNOW WILL COULD
PRODUCE AREAS OF BLOWING SNOW. AT THIS TIME THINKING IS A WINTER
WEATHER ADVISORY MAY BE NEEDED...BUT THIS WOULD DEPEND ON HOW MODELS
RESOLVE SNOW AMOUNTS.

UPSLOPE SNOW WILL CONTINUE ALL DAY MONDAY AND THROUGH MONDAY NIGHT
ACROSS THE NORTHERN TO CENTRAL BLACK HILLS AND BEAR LODGE MOUNTAINS.
ATTM IT LOOKS LIKE SNOWFALL AMOUNTS OVER THE HIGHER BLACK HILLS WILL
BE IN THE 6 TO 12 INCH RANGE. WITH A MORE WESTERLY COMPONENT TO THE
WIND...HIGHEST SNOWFALL TOTALS ARE EXPECTED TO BE AROUND THE
CHEYENNE CROSSING TO ONEIL PASS INTO THE FOUR CORNERS AREA. WILL
ISSUE A WINTER STORM WATCH FOR THESE AREAS STARTING TONIGHT AND
CONTINUING UNTIL NOON TUESDAY.

.LONG TERM...TUESDAY THROUGH SATURDAY
ISSUED AT 255 AM MST SUN DEC 21 2014

AMPLIFIED PATTERN EXPECTED IN THE PERIOD AS THE EARLY WEEK LONG
WAVE TROUGH SPURS MASSIVE RIDGING INTO GREENLAND. THIS WILL FAVOR
A TEMPORARY BLOCK TO THE FLOW PATTERN AND ALLOW A CP/MODIFIED
ARCTIC AIRMASS INTRUSION INTO THE CONUS PER WAVE TRAIN OF TROUGHS
EMANATING FROM THE NORTHERN PAC UPPER LOW. THIS COLD INTRUSION
WILL ARRIVE WITH A DECENT/STRONG SHORTWAVE EXPECTED IN THE WED
NIGHT-THUR PERIOD...WITH ANOTHER SHOT OF COLD AIR LIKELY NEXT
WEEKEND PER WESTERN NOAM RIDGING AND FRONT SIDE NORTHERN STREAM
IMPULSES. AFTER A MILD DAY WED...BELOW NORMAL TEMPS CAN BE
EXPECTED FOR THE REMAINDER OF THE PERIOD.

TUE-WED...LINGERING MORNING SNOW SHOWERS OVER THE BH AND FAR
SCENTRAL CAN BE EXPECTED AS THE EARLY WEEK SYSTEM EXITS THE REGION.
HAVE WENT WITH HIGH POPS OVER THE NORTHERN BH. CLOSED MID LEVEL
CIRCULATION WILL LIKELY SUPPORT A SLOWER EXIT OF THIS SYSTEM...WITH
SNOW LINGERING THROUGH AT LEAST MORNING. DRY WEATHER EXPECTED TUE NIGHT
THROUGH WED...WITH INCREASING CHANCES FOR PRECIP WED NIGHT INTO
THUR MORNING. COLDER CONDS EXPECTED TUE GIVEN ONGOING CAA WITH
STRONG GUSTY NW WINDS. 20 TO 30 MPH WINDS CAN BE EXPECTED TO
CONTINUE ON THE SD PLAINS FOR MUCH OF TUES.

THUR-SAT...STRONG COMPACT SPLITTING IMPULSE WILL QUICKLY MOVE INTO
THE REGION WED NIGHT SUPPORTING INCREASING CHANCES FOR SNOW. COUPLED
UPPER LEVEL JET STRUCTURE WILL LIKELY ENHANCED LIFT...FAVORING A
DECENT FGEN BANDED SNOW EVENT ACROSS A PORTION OF THE FA. CURRENT
PROGS SUGGEST MOST OF THE FA COULD SEE SNOW THUR ALTHOUGH N-S
ADJUSTMENT CAN BE EXPECTED IN SUBSEQUENT MODEL RUNS. INCREASED POPS
GIVEN RUN-RUN CONSISTENCY AND STRONG LIFT ASSOCIATED WITH THIS SYSTEM
COUPLED WITH SUFFICIENT MOISTURE. CURRENT PROGS SUGGEST 1-3 INCHES
POSSIBLE...WITH CERTAINLY HIGHER AMOUNTS IN ANY PERSISTENT BANDS.
GIVEN STRONG MERIDIONAL COMPONENT TO THE LL FLOW...HAVE CUT TEMPS
THUR-SAT...WHICH MAY NOT BE ENOUGH IF A DEEPER CENTRAL CONUS UPPER
TROUGH IS REALIZED.

&&

.AVIATION...FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING
ISSUED AT 255 AM MST SUN DEC 21 2014

VFR CONDITIONS WILL CONTINUE THROUGH 18Z ON SUNDAY. A STRONG
CDFNT MOVING INTO THE REGION SUNDAY AFTN WILL BRING SCT -SHRA AND
MVFR CIGS. CONDITIONS WILL DETERIORATE FURTHER SUNDAY
NIGHT...ESPECIALLY OVER NERN WY AND THE NRN BLKHLS AS SNOW
DEVELOPS.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER STORM WATCH FROM THIS EVENING THROUGH TUESDAY MORNING FOR
     SDZ024-028.

WY...WINTER STORM WATCH FROM THIS EVENING THROUGH TUESDAY MORNING FOR
     WYZ057.

&&

$$

SHORT TERM...13
LONG TERM...JC
AVIATION...JC





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