000
FXUS63 KBIS 221939
AFDBIS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BISMARCK ND
239 PM CDT FRI OCT 22 2021
SHORT TERM
(THIS EVENING THROUGH SATURDAY)
ISSUED AT 239 PM CDT FRI OCT 22 2021
BREEZY TO WINDY CONDITIONS EXPECTED ON SATURDAY.
UPPER AIR ANALYSIS IS HIGHLIGHTED BY A MODERATELY AMPLIFIED
PATTERN OVER THE WESTERN CONUS, WITH A DEEP PACIFIC TROUGH
CROSSING THE WEST COAST AND A RIDGE DOWNSTREAM THROUGH THE
NORTHERN ROCKIES. ANTI-CYCLONIC FLOW OVER THE NORTHERN PLAINS WAS
EVIDENT ON WATER VAPOR IMAGERY AS SEVERAL VORTICITY LOBES WERE
EXITING TO THE EAST. SUBSIDENCE AND POSSIBLY THE INFLUENCE OF
EASTERLY UPSLOPE WINDS HAVE MAINTAINED CLOUDS THIS AFTERNOON IN
THE CENTRAL PORTION OF THE STATE, AND WILL LIKELY LINGER IN SOME
FORM THROUGH THE AFTERNOON. PRESSURE FALLS OVER THE FAR WEST DUE
TO A DEEPENING SURFACE LOW OVER THE NORTHERN HIGH PLAINS HAVE
CREATED BREEZY CONDITIONS IN THE NORTHWEST, AND WILL INCREASE
WINDS THIS EVENING ACROSS MORE OF THE WEST AND CENTRAL AS THE
SURFACE LOW SHIFTS EAST.
SATURDAY WILL FEATURE BREEZY CONDITIONS ACROSS THE WEST AND
CENTRAL AS A TIGHT PRESSURE GRADIENT AND MODERATELY STRONG 850MB
WINDS WILL PROVIDE SOUTHEAST WINDS OF 15 TO 25 MPH. THE HIGH END
WIND POTENTIAL WILL DEPEND ON BOUNDARY LAYER MIXING, WHICH IS
UNCERTAIN GIVEN THE UNFAVORABLE FLOW DIRECTION (SOUTHEAST) AND THE
POTENTIAL FOR CLOUD COVER LIMITING INSOLATION. THE SATURDAY
AFTERNOON WINDS WITH THIS FORECAST WERE NUDGED ABOVE NBM, BUT
DOESN'T QUITE GET TO ADVISORY CRITERIA YET. OTHERWISE PARTLY TO
MOSTLY SKIES AND HIGHS IN THE 40S WILL BE THE OTHER WEATHER
CONDITIONS, WITH CHANCES FOR PRECIPITATION INCREASING SATURDAY
NIGHT.
LONG TERM
(SATURDAY NIGHT THROUGH FRIDAY)
ISSUED AT 239 PM CDT FRI OCT 22 2021
A BROAD AREA OF MID LEVEL SHORTWAVE ENERGY WILL BEGIN TO SPREAD OVER
THE NORTHERN PLAINS SATURDAY NIGHT. THE ASSOCIATED FORCING
MECHANISMS (Q-VECTOR CONVERGENCE, PVA, WEAK ISENTROPIC ASCENT)
SHOULD PROVIDE ENOUGH LIFT TO GENERATE AN AREA OF LIGHT
PRECIPITATION MOVING FROM WEST TO EAST ACROSS WESTERN AND CENTRAL
NORTH DAKOTA SATURDAY NIGHT INTO SUNDAY AFTERNOON. THESE
PRECIPITATION CHANCES ARE SUPPORTED BY NUMEROUS ENSEMBLE DATASETS.
SOME MODEL SOUNDINGS SHOW A DEEP ISOTHERMAL LAYER NEAR ZERO DEGREES
CELSIUS AND/OR A LACK OF MOISTURE IN ICE-PRODUCING LAYERS ALOFT
(COLDER THAN -10 C). BUT GIVEN FORECAST SURFACE AIR TEMPERATURES AT
OR ABOVE FREEZING, AND WARMER GROUND TEMPERATURES AT THIS TIME OF
YEAR, FREEZING RAIN IS NOT AN EXPECTED TYPE. SOME SNOW CANNOT BE
RULED OUT, MAINLY ACROSS NORTH CENTRAL NORTH DAKOTA WHERE THERMAL
PROFILES ARE SLIGHTLY COOLER. BUT AGAIN, NO IMPACTS ARE EXPECTED.
DECREASING CLOUDS IN FAR WESTERN NORTH DAKOTA SUNDAY AFTERNOON COULD
ALLOW HIGHS TO REACH THE 50S. OTHERWISE, 40S ARE EXPECTED.
A PROGRESSIVE PATTERN WILL BRING AN AMPLIFIED, TRANSIENT RIDGE OVER
THE NORTHERN PLAINS ON MONDAY AS A POWERFUL TROUGH DIGS INTO THE
WESTERN CONUS. THE MAIN EXPECTED IMPACT FROM THIS SETUP IS STRONG
SOUTHERLY SURFACE WINDS. ENSEMBLE DATASETS THAT COMPARE CURRENT
OUTPUT TO CLIMATOLOGY (E.G., NAEFS AND ECMWF SITUATIONAL AWARENESS
TABLES) HIGHLIGHT WESTERN NORTH DAKOTA FOR SURFACE TO 850 MB WINDS
NEAR THE 99TH PERCENTILE, AND DETERMINISTIC GUIDANCE IS CONSISTENT
IN SHOWING 900-800 MB WINDS AROUND 40-50 KTS. A CAPPING INVERSION
ABOVE THE BOUNDARY LAYER AND THE LACK OF A WESTERLY COMPONENT TO THE
WIND FIELD COULD DAMPEN BOUNDARY LAYER MIXING AND KEEP SURFACE GUSTS
BELOW THE BOUNDARY LAYER MAXIMUM. HOWEVER, SUSTAINED SURFACE WIND
SPEEDS TEND TO OVER-PERFORM GUIDANCE IN THESE SETUPS. THIS PERIOD
WILL NEED TO BE MONITORED FOR POTENTIAL HEADLINES AS PROBABILITIES
FOR ADVISORY CRITERIA WINDS CONTINUE TO SHOW AN INCREASING TREND.
THE AFOREMENTIONED CAPPING INVERSION WILL ALSO KEEP TEMPERATURES
COOLER THAN WHAT IS TYPICALLY SEEN IN THIS HEIGHT PATTERN AND
THERMAL PROFILE, THOUGH THEY SHOULD STILL BE NEAR NORMAL FOR LATE
OCTOBER.
THE DEEP WESTERN CONUS TROUGH IS FORECAST TO EJECT INTO THE GREAT
PLAINS BY WEDNESDAY, WITH A NEGATIVELY TILTED AXIS SHOWING UP IN
ENSEMBLE MEAN HEIGHT FIELDS. THE MOST UNCERTAIN ASPECT OF THIS
SYSTEM IS TIMING, WITH CLUSTER ANALYSIS SHOWING ECMWF ENSEMBLE
MEMBERS FAVORING A SLOWER SOLUTION. REGARDLESS OF THE TIMING,
ENSEMBLE SYSTEMS SHOWS HIGH CONFIDENCE IN PRECIPITATION CHANCES,
WHICH CAN BE CONFIRMED BY AN ANALYSIS OF FORCING MECHANISMS IN
DETERMINISTIC GUIDANCE. NBM TEMPERATURE GUIDANCE SUGGESTS MOST, IF
NOT ALL PRECIPITATION WILL FALL AS RAIN, WITH ONLY ISOLATED MIXES OF
SNOW POSSIBLE DURING EARLY MORNING HOURS. DESPITE THE OVERALL
LOWERING OF HEIGHTS ALOFT, STRONGER BOUNDARY LAYER MIXING MAY
ACTUALLY RESULT IN WARMER SURFACE TEMPERATURES THROUGH THE MIDDLE OF
THE WEEK. IT IS ALSO LIKELY TO REMAIN WINDY AT TIMES THROUGH MID
WEEK, PARTICULARLY WITH THE PRESSURE RISES BEHIND THE MAIN TROUGH
AXIS. UNCERTAINTY IN THE SYNOPTIC PATTERN GROWS BY THE END OF THE
WEEK, BUT A TRANSITION TO NORTHWEST FLOW ALOFT WITH AN UPPER LEVEL
RIDGE OVER THE ROCKIES SEEMS TO BE A MORE LIKELY OUTCOME AT THIS
TIME.
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z SATURDAY AFTERNOON)
ISSUED AT 1226 PM CDT FRI OCT 22 2021
GENERALLY VFR CONDITIONS THROUGH THE TAF PERIOD, THOUGH CHANCE OF
REDUCED CATEGORIES EARLY SATURDAY MORNING. SOUTHEAST WINDS WILL
GRADUALLY INCREASE THIS EVENING IN THE WEST, BECOMING 15-25 KTS AT
KDIK-KXWA AFTER 10Z. WITH SOUTHERLY WINDS DRAWING UP LOW LEVEL
MOISTURE, MVFR CIGS MAY DEVELOP IN SOME ARES, MORE LIKELY
SOUTHWEST. CONFIDENCE IN LOCATION AND COVERAGE IS LOW AT THIS
TIME, BUT WILL NEED MONITORING. SOUTHEAST WINDS BEGIN TO INCREASE
CENTRAL LATER SATURDAY MORNING.
BIS WATCHES/WARNINGS/ADVISORIES
NONE.
SHORT TERM...AE
LONG TERM...MH
AVIATION...AE
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page