168
FXUS63 KBIS 200940
AFDBIS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BISMARCK ND
340 AM CST FRI DEC 20 2024
KEY MESSAGES
- LIGHT SNOW IS EXPECTED ACROSS NORTHWEST AND CENTRAL NORTH
DAKOTA LATE THIS MORNING THROUGH THE EARLY EVENING. THERE ARE
MEDIUM CHANCES FOR AT LEAST ONE INCH OF SNOW IN THE FAR
NORTHWEST AND NORTH CENTRAL, BUT VERY LOW CHANCES FOR UP TO 2
INCHES.
- THERE IS A 20 TO 40 PERCENT CHANCE OF LIGHT FREEZING RAIN
ACROSS MUCH OF WESTERN AND CENTRAL NORTH DAKOTA ON SUNDAY.
- A WARMING TREND IS EXPECTED THIS WEEKEND THROUGH EARLY NEXT
WEEK, WITH ABOVE NORMAL TEMPERATURES FAVORED STATEWIDE FROM
CHRISTMAS EVE THROUGH THE END OF THE WEEK.
DISCUSSION
ISSUED AT 339 AM CST FRI DEC 20 2024
ARCTIC HIGH PRESSURE IS ANALYZED FROM SOUTHWEST MANITOBA THROUGH
EASTERN NORTH DAKOTA EARLY THIS MORNING UNDER NORTHWEST FLOW ALOFT.
TEMPERATURES HAVE FALLEN INTO THE SINGLE DIGITS AND TEENS BELOW ZERO
ACROSS ALL OF WESTERN AND CENTRAL NORTH DAKOTA. A SHORTWAVE CROSSING
THE CANADIAN ROCKIES THIS MORNING WILL INDUCE WARM AIR ADVECTION AND
ISENTROPIC ASCENT SPREADING SOUTHEASTWARD FROM SOUTHEAST
SASKATCHEWAN INTO NORTHWEST, CENTRAL, AND SOUTHEAST NORTH DAKOTA
LATER THIS MORNING THROUGH THE EARLY EVENING. AS A RESULT, ENSEMBLES
ARE VERY CONSISTENT WITH VERY LIGHT QPF RANGING FROM AROUND A
HUNDREDTH OF AN INCH SOUTH CENTRAL AND SOUTHEAST TO NEAR A TENTH OF
AN INCH FAR NORTHWEST AND NORTH CENTRAL. WITH FORECAST SNOW RATIOS
AROUND 15:1, IT FOLLOWS THAT THERE IS A MEDIUM CHANCE FOR EXCEEDING
1 INCH OF NEW SNOW ACCUMULATION TODAY IN PARTS OF FAR NORTHWEST AND
NORTH CENTRAL NORTH DAKOTA. THERE IS ESSENTIALLY NO MODEL SUPPORT
FOR GREATER THAN 2 INCHES, BUT THINK THERE IS A CHANCE SNOW-TO-
LIQUID RATIOS COULD END UP A LITTLE HIGHER THAN FORECAST, AND THERE
IS ALSO A POTENTIAL FOR TRANSIENT MID LEVEL FRONTOGENESIS THAT COULD
PROVIDE BRIEF ENHANCEMENTS TO SNOWFALL RATES. THE DEPARTING SURFACE
HIGH PRESSURE WILL TURN WINDS SOUTHEASTERLY AT AROUND 10 TO NEAR 20
MPH THIS AFTERNOON, STRONGEST IN THE WEST. TODAY WILL BE WARMER THAN
THE DAYTIME HOURS WERE YESTERDAY, BUT HIGHS WILL REMAIN BELOW
AVERAGE, RANGING FROM AROUND 10 EAST TO 25 SOUTHWEST.
HEIGHT RISES ARE FORECAST TRAILING THE DEPARTING SHORTWAVE ON
SATURDAY AS UPPER LEVEL RIDGING OVER THE WESTERN CONUS SHIFTS
SLIGHTLY EASTWARD. THIS SHOULD ALLOW FOR A NOTABLE UPTICK IN HIGH
TEMPERATURES, ESPECIALLY IN THE SOUTHWEST WHERE THE NBM SHOWS MEDIUM
TO HIGH CONFIDENCE IN LOWER TO MID 40S FOR HIGHS. DETERMINISTIC
MODELS AND ENSEMBLE MEAN 500 MB VORTICITY HINT AT A DEAMPLIFYING
SHORTWAVE CROSSING THE REGION SATURDAY AFTERNOON, BUT NO
PRECIPITATION IS EXPECTED AT THIS TIME.
A HIGHER AMPLITUDE SHORTWAVE IS FORECAST TO CROSS THE NORTHERN
ROCKIES SATURDAY NIGHT AND REACH THE NORTHERN PLAINS BY SUNDAY
EVENING. THE CURRENT SUITE OF ENSEMBLE GUIDANCE SHOWS UNCERTAINTY IN
THE NORTH-SOUTH PLACEMENT OF ITS TRACK. BASED ON QPF THRESHOLD
PROBABILITIES, THE MORE FAVORED SOLUTION APPEARS TO BE A MORE
SOUTHERN TRACK, WITH ONE LOWER-MEMBERSHIP CLUSTER THAT CONTAINS A
MAJORITY OF GEFS MEMBERS SHIFTED DISTINCTLY TO THE NORTH. THOSE
PROBABILITIES ARE NOT OVERLY IMPRESSIVE, WITH ONLY LOW TO MEDIUM
CHANCES FOR EXCEEDING 0.05 INCH. HOWEVER, ANY QPF COULD BE
PROBLEMATIC AS THERE IS A STRONG SIGNAL FOR FREEZING RAIN AS A
DOMINANT PRECIPITATION TYPE. ALL ENSEMBLE SYSTEMS SHOW VERY HIGH
PROBABILITIES OF 850 MB TEMPERATURES ABOVE FREEZING UNTIL AT LEAST
LATER IN THE DAY SUNDAY WHEN COLDER AIR BEGINS TO WRAP AROUND THE
BACKSIDE OF THE WAVE. THERE IS ALSO HIGH CONFIDENCE IN MAXIMUM
SURFACE TEMPERATURES ON SUNDAY REMAINING BELOW FREEZING NORTH AND
EAST OF THE MISSOURI RIVER, AND AREAS TO THE SOUTH AND WEST COULD
STILL SEE PRECIPITATION START AS FREEZING RAIN IN THE MORNING.
AS ALLUDED TO ABOVE, COLDER AIR WILL BEGIN TO WRAP AROUND THE
SHORTWAVE AND GRADUALLY TRANSITION PRECIPITATION TYPES OVER TO SNOW
SUNDAY EVENING, ALTHOUGH MUCH OF THE PRECIPITATION COULD BE OVER BY
THEN. THERE IS AN INTERESTING LOW-PROBABILITY OUTCOME BEING
PRESENTED BY DETERMINISTIC GUIDANCE THAT INVOLVES A MESOSCALE POCKET
OF HEAVIER SNOW DRIVEN BY DYNAMIC COOLING, STEEP MID AND UPPER LEVEL
LAPSE RATES, AND CONDITIONAL SYMMETRIC INSTABILITY. THIS IS
SOMETHING THAT MAY NEED TO BE MONITORED AS CAMS REACH INTO THE LATE
SUNDAY TIME FRAME, BUT AS OF NOW THE OCCURRENCE OF THIS SCENARIO
CARRIES A LOW PROBABILITY, AND THERE IS NO PREDICTABILITY WHATSOEVER
AT THIS TIME RANGE OF ITS POTENTIAL PLACEMENT.
THE NBM HIGH TEMPERATURE SPREAD ON MONDAY IS MUCH LARGER THAN ANY
OTHER DAY OF THE EXTENDED PERIOD. THIS APPEARS TO BE TIED TO THE
TIMING OF TRANSIENT RIDGING, OF WHICH AROUND 60 PERCENT OF GLOBAL
ENSEMBLE MEMBERS, INCLUDING ALL BUT 2 ECMWF MEMBERS, FAVOR A QUICKER
AND THEREFORE WARMER SOLUTION. BUT EVEN A COLDER OUTCOME WOULD
LIKELY KEEP TEMPERATURES WITHIN 5 TO 10 DEGREES OF NORMAL HIGHS IN
THE 20S. ENSEMBLES THEN RECONVERGE ON A SPLIT FLOW PATTERN BRINGING
PERSISTENT MILD PACIFIC AIR TO THE REGION THROUGH THE CHRISTMAS
HOLIDAY AND POSSIBLY LASTING TO THE END OF THE CALENDAR YEAR. FROM
CHRISTMAS EVE THROUGH THE END OF THE WEEK, EVEN THE 25TH PERCENTILE
OF NBM HIGH TEMPERATURE GUIDANCE SUPPORTS AT LEAST 5 TO 10 DEGREES
ABOVE NORMAL, WITH SEVERAL DAYS ABOVE FREEZING IN THE SOUTHWEST.
EVEN THOUGH DETERMINISTIC AND ENSEMBLE MEAN HEIGHT AND VORTICITY
FIELDS IMPLY A MORE ACTIVE PATTERN FOR THE SECOND HALF OF THE WEEK,
THE PROBABILITY OF ANY MEASURABLE PRECIPITATION NEXT WEEK REMAINS
LOWER THAN 20 PERCENT AT THIS TIME.
AVIATION /06Z TAFS THROUGH 06Z SATURDAY/
ISSUED AT 339 AM CST FRI DEC 20 2024
VFR CONDITIONS ARE MOSTLY EXPECTED THIS MORNING, ALTHOUGH SOME
PATCHY FOG AND/OR LOW CLOUDS CANNOT BE RULED OUT ACROSS PARTS OF
NORTHWEST AND CENTRAL NORTH DAKOTA. LIGHT SNOW WITH LOW VFR TO
MVFR CEILINGS AND MVFR TO IFR VISIBILITY IS EXPECTED TO MOVE
SOUTHEASTWARD THROUGH NORTHWEST, CENTRAL, AND INTO SOUTHEAST
NORTH DAKOTA LATER THIS MORNING THROUGH THIS EVENING. THE
HIGHEST PROBABILITIES FOR CATEGORICAL RESTRICTIONS ARE FROM KMOT
TO KJMS, WITH AREAS FROM AROUND KXWA TO KBIS HAVING LOWER
PROBABILITIES. WINDS WILL BECOME SOUTHEASTERLY AROUND 10-15 KTS
THIS AFTERNOON.
BIS WATCHES/WARNINGS/ADVISORIES
NONE.
DISCUSSION...HOLLAN
AVIATION...HOLLAN
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