856
FXUS63 KBIS 130950
AFDBIS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BISMARCK ND
450 AM CDT SAT SEP 13 2025
KEY MESSAGES
- SEVERAL ROUNDS OF SCATTERED SHOWERS AND THUNDERSTORMS THROUGH
THE MIDDLE OF NEXT WEEK.
- MEDIUM TO HIGH PROBABILITIES FOR AT LEAST AN INCH OF RAIN
ACROSS THE WESTERN HALF OF THE STATE THROUGH THIS WEEKEND.
- ISOLATED SEVERE THUNDERSTORMS POSSIBLE THROUGH THIS EVENING,
AND AGAIN SUNDAY AFTERNOON AND EVENING.
- TEMPERATURES FAVORED TO REMAIN NEAR TO ABOVE AVERAGE THROUGH
THE MIDDLE OF NEXT WEEK.
DISCUSSION
ISSUED AT 449 AM CDT SAT SEP 13 2025
AN ELONGATED UPPER LEVEL LOW IS SPINNING WITHIN A LONGWAVE TROUGH
OVER THE WESTERN CONUS EARLY THIS MORNING. A NORTHERN CIRCULATION IS
FORECAST TO CUTOFF FROM THE BASE OF THE TROUGH LATER TODAY, WITH
THAT AREA OF CYCLONIC VORTICITY WRAPPING INTO ITS OWN CLOSED LOW AS
IT EJECTS POLEWARD OFF THE SOUTHERN ROCKIES THROUGH THE CENTRAL AND
NORTHERN PLAINS LATE TONIGHT THROUGH SUNDAY NIGHT. THIS SETUP
COMBINED WITH HIGHLY ANOMALOUS ATMOSPHERIC MOISTURE PARAMETERS WILL
CONTINUE TO BRING SEVERAL ROUNDS OF SHOWERS WITH ISOLATED TO
SCATTERED THUNDERSTORMS ACROSS WESTERN AND CENTRAL NORTH DAKOTA
THROUGH THE WEEKEND. THE TIME PERIODS WITH THE GREATEST EXPECTED
COVERAGE/HIGHEST PROBABILITIES OF SHOWERS ARE 1) THIS MORNING AND
AFTERNOON AS A 700 MB LOW OVER NORTHERN WYOMING SUPPLEMENTS DCVA AND
2) SUNDAY AFTERNOON AND EVENING WITH THE ARRIVAL OF THE MID/UPPER
LOW ORIGINATING FROM THE BASE OF THE LONGWAVE TROUGH. TONIGHT LOOKS
TO BE A DRIER PERIOD, BUT WE ARE STILL CARRYING A 20 TO 30 PERCENT
CHANCE OF RAIN AT MOST LOCATIONS THROUGH THE NIGHT GIVEN THE PATTERN
AND AVAILABILITY OF MOISTURE. DETAILS ON TIMING, PLACEMENT, AND
INTENSITY OF SHOWERS AND STORMS WILL CONTINUE TO HAVE VERY LOW
PREDICTABILITY.
A RISK FOR ISOLATED SEVERE STORMS REMAINS IN THE FORECAST THROUGH
EARLY THIS EVENING, AND RETURNS FOR SUNDAY AFTERNOON AND EVENING.
THERE IS LITTLE TO NO PROJECTED CHANGE IN THE OVERALL CAPE/SHEAR
PARAMETER SPACE THROUGH SUNDAY EVENING, WITH MUCAPE CONSISTENTLY
AROUND 500-1500 J/KG AND EFFECTIVE BULK SHEAR AROUND 25-35 KTS. THE
HIGHER ENDS OF THESE RANGES ARE FAVORED TO BE MORE OVER CENTRAL
NORTH DAKOTA, AND LOWER TO THE WEST. HAIL LARGER THAN QUARTERS AND
WIND GUSTS HIGHER THAN 60 MPH SEEM LESS LIKELY TO OCCUR THIS
WEEKEND, BUT THIS PATTERN HAS PROVEN TO BE FULL OF SURPRISES THUS
FAR. SURFACE-BASED CONVECTION ALSO SEEMS UNLIKELY. INTERESTINGLY,
THE HRRR AND RAP ERODE CIN IN SOUTHWEST NORTH DAKOTA THIS AFTERNOON,
BUT THEIR FORECAST SOUNDINGS SHOW A SUSPICIOUSLY WELL-MIXED, VERY
SHALLOW BOUNDARY LAYER UNDERNEATH A DEEP LAYER OF HIGH RH.
NEVERTHELESS, THE 00Z HREF DID PAINT ITS MOST CONCENTRATED UH TRACKS
OVER SOUTHWEST AND SOUTH CENTRAL NORTH DAKOTA, BUT WITH A TEMPORAL
PEAK OF LATE MORNING/EARLY AFTERNOON. ANOTHER POTENTIAL HAZARD TO
KEEP IN MIND DURING THE AFTERNOON HOURS TODAY AND SUNDAY IS A NON-
ZERO POTENTIAL FOR NON-SUPERCELL TORNADOES/LANDSPOUTS, AS SEVERAL
INGREDIENTS THAT FAVOR THEIR DEVELOPMENT ARE FORECAST TO BE PRESENT.
THE GREATEST IMPACT FROM SHOWERS AND STORMS THROUGH THE WEEKEND IS
NOW LIKELY TO BE LOCALLY EXCESSIVE RAINFALL. THERE IS STILL A 50 TO
70 PERCENT CHANCE FOR AT LEAST ONE INCH OF RAIN ALONG AND WEST OF
HIGHWAY 83 THROUGH SUNDAY NIGHT, AND THIS DOES NOT TAKE INTO ACCOUNT
WHAT HAS ALREADY FALLEN. HOWEVER, THERE WILL BE A HIGH SPATIAL
VARIABILITY TO FINAL RAINFALL TOTALS GIVEN THE CONVECTIVE
INFLUENCES. THE CONTEXT OF THIS IS EXEMPLIFIED BY THE HREF LOCALIZED
PROBABILITY-MATCHED MEAN QPF, WHOSE 24-HOUR TOTALS RANGE FROM AS LOW
AS A FEW HUNDREDTHS OF AN INCH TO AS HIGH AS 5 INCHES. WE HAVE
ALREADY SEEN THIS IN OBSERVED DATA AS WELL, WITH POCKETS OF 2-6
INCHES ESTIMATED BY MRMS SCATTERED ACROSS A LARGE AREA WITH AN
ESTIMATE OF LESS THAN A TENTH OF AN INCH. ANY LOCATION THAT SEES
TRAINING CONVECTION OR MULTIPLE ROUNDS OF HEAVY RAIN THROUGH THE
WEEKEND HAS A RISK FOR LOCALIZED FLOODING.
HIGH TEMPERATURES THIS WEEKEND ARE LIKELY TO BE AROUND 70 TO 80. IT
WILL ALSO REMAIN UNUSUALLY HUMID FOR MID-SEPTEMBER WITH DEWPOINTS IN
THE 60S FOR MUCH OF THE AREA. THE EXCESSIVE MOISTURE AND RAINFALL
COULD ALLOW FOG TO DEVELOP PRETTY MUCH ANYWHERE TONIGHT INTO SUNDAY
MORNING.
THERE IS STILL TIMING UNCERTAINTY ON THE DEPARTURE OF THE DEEPER
MID/UPPER LOWER THAT ORIGINATES FROM THE BASE OF THE LONGWAVE
TROUGH, WITH ENSEMBLE CLUSTERS RANGING FROM AS EARLY AS MONDAY
MORNING TO AS LATE AS MONDAY EVENING. UNTIL THE SYSTEM FULLY MOVES
INTO CANADA, CHANCES FOR SHOWERS WILL REMAIN IN THE FORECAST, BUT
THE PROBABILITY FOR THUNDERSTORMS LOOKS MUCH LOWER ON MONDAY.
HOWEVER, A TRAILING SHORTWAVE THAT IS FORECAST TO DIG INTO THE
PACIFIC NORTHWEST ON SUNDAY COULD JUST AS SOON RETURN CHANCES FOR
RAIN BY LATE TUESDAY. THIS SHORTWAVE LOOKS TO BE MUCH WEAKER, AND IT
COULD END UP STALLING OR MEANDERING AROUND THE NORTHERN PLAINS TO
WESTERN GREAT LAKES. ENSEMBLES DO SEEM CONSISTENT ON A BROAD
TROUGHING PATTERN OVER THE REGION FOR THE MIDDLE OF THE WEEK, WITH
TEMPERATURES COOLING CLOSER TO AVERAGE VALUES OF HIGHS IN THE LOWER
TO MID 70S AND LOWS IN THE 40S. ALL ENSEMBLE CLUSTERS THEN SHOW AT
LEAST A BRIEF PERIOD OF RIDGING TOWARD THE END OF THE WEEK, ALBEIT
WITH CONSIDERABLE TIMING AND AMPLITUDE DIFFERENCES.
AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
ISSUED AT 116 AM CDT SAT SEP 13 2025
SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DRAMATICALLY INCREASE IN
COVERAGE DURING THE OVERNIGHT HOURS, IMPACTING ALL TAF SITES EXCEPT
KJMS. CEILINGS WILL DECREASE QUITE DRAMATICALLY AS THUNDERSTROMS
OVERSPREAD THE REGION, WITH MVFR PREVAILING. THERE IS ALSO THE
POTENTIAL FOR FOG IN THE NORTH, WHICH COULD IMPACT KXWA AND KMOT. IF
IT WERE TO FORM IT WOULD SIGNIFICANTLY REDUCE VISIBILITY INTO
IFR/LIFR. CEILINGS WILL SLOWLY IMPROVE SATURDAY, WITH POCKETS OF IFR
IN THE MORNING GIVING WAY TO PREVAILING MVFR BY MID-AFTERNOON.
SHOWER/STORM COVERAGE WILL ALSO DECREASE THROUGHOUT THE DAY. WITH
RECENT PRECIPITATION AND LIGHT WINDS SATURDAY NIGHT, FOG WILL BE A
CONCERN, BUT THAT THREAT LOOKS TO BE JUST OUTSIDE OF THE CURRENT TAF
PERIOD.
BIS WATCHES/WARNINGS/ADVISORIES
NONE.
DISCUSSION...HOLLAN
AVIATION...BR/WFO FGF
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