062
FXUS21 KWNC 181914
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 18 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE OVER ALASKA AND A MID-LEVEL TROUGH MOVING
ACROSS THE NORTH-CENTRAL CONTIGUOUS U.S. (CONUS) WILL PUSH COLD CANADIAN AIR
INTO THE NATION’S MIDSECTION FOR THE FIRST HALF OF WEEK-2 BEFORE MODERATING.
MOISTURE FROM SEVERAL SOURCES AND SURFACE LOW PRESSURE COULD BRING THE FIRST
SIGNIFICANT SNOWFALL OF THE SEASON TO SOME AREAS FROM THE NORTHERN ROCKIES
EASTWARD THROUGH THE PLAINS AND UPPER MIDWEST, ESPECIALLY THE HIGHER
ELEVATIONS. MEANWHILE, A MOIST LOW-LEVEL FLOW FROM THE GULF OF MEXICO COUPLED
WITH SURFACE LOW PRESSURE OVER THE GREAT PLAINS MAY LEAD TO HEAVY PRECIPITATION
IN MUCH OF THE PLAINS, MISSISSIPPI VALLEY, AND ADJACENT SOUTHEAST AS WELL AS
STRONG WINDS FOR MUCH OF THE CENTRAL CONUS. CONDITIONS ARE EXPECTED TO MODERATE
DURING THE LAST HALF OF WEEK-2, WITH NO HAZARDS POSTED FOR THIS TIME PERIOD.
HAZARDS
SLIGHT CHANCE FOR MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
AND CENTRAL PLAINS AND THE UPPER MISSISSIPPI VALLEY, THU-SUN, OCT 26-29.
SLIGHT CHANCE FOR HEAVY SNOW FOR HIGHER ELEVATIONS OF THE NORTHERN ROCKIES, THE
NORTHERN AND CENTRAL PLAINS, AND THE UPPER MIDWEST, THU-FRI, OCT 26-27.
SLIGHT CHANCE FOR HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHERN AND CENTRAL
PLAINS, THE LOWER AND MIDDLE MISSISSIPPI VALLEYS, AND THE ADJACENT SOUTHEAST,
THU-FRI, OCT 26-27.
SLIGHT CHANCE FOR HIGH WINDS FOR MUCH OF THE PLAINS, THU-FRI, OCT 26-27.
DETAILED SUMMARY
FOR SATURDAY OCTOBER 21 - WEDNESDAY OCTOBER 25:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY OCTOBER 26 - WEDNESDAY NOVEMBER 01: AT THE START OF WEEK-2, THE
SYNOPTIC PATTERN AS REFLECTED BY 500-HPA HEIGHT ANOMALIES IS FAIRLY AMPLIFIED,
FEATURING STRONG RIDGING ACROSS ALASKA NORTH OF A MID-LEVEL TROUGH OVER THE
CENTRAL NORTH PACIFIC. FARTHER EAST, A SHORTWAVE TROUGH IS CENTERED OVER THE
ROCKIES IN THE PROCESS OF TRAVERSING THE NATION’S MIDSECTION, AND A MODERATE
MID-LEVEL RIDGE COVERS EASTERN NORTH AMERICA. A VERY COLD SURFACE AIR MASS OVER
NORTHWESTERN CANADA WILL BE PUSHED EASTWARD BY THE STRONG ALASKAN MID-LEVEL
TROUGH, THEN PULLED SOUTHWARD INTO CONUS BEHIND THE 500-HPA TROUGH EJECTING OUT
OF THE ROCKIES
DURING WEEK-2, THE PATTERN IS EXPECTED TO REMAIN RELATIVELY STATIC OVER THE
NORTH PACIFIC, BUT RELATIVELY PROGRESSIVE FARTHER EAST FROM THE ROCKIES INTO
THE ATLANTIC. BY THE END OF THE PERIOD, THE TROUGH IN THE CENTRAL CONUS HAS
PUSHED EASTWARD AND WEAKENED, RESULTING IN A LESS-AMPLIFIED BUT FULL-LATITUDE
TROUGH WITH AN AXIS NEAR THE EASTERN NORTH AMERICAN COAST OR SLIGHTLY INLAND.
FARTHER WEST, 500-HPA HEIGHT ANOMALIES ACROSS ALASKA SHOULD DROP TO NEAR NORMAL
NORTH OF THE MID-LEVEL TROUGH IN THE CENTRAL NORTH PACIFIC, WHICH PERSISTS AND
EXPANDS EASTWARD SOMEWHAT. MID-LEVEL HEIGHTS INCREASE TO THE EAST OF THIS
TROUGH, LEAVING A MID-LEVEL RIDGE OVER OR JUST INLAND FROM THE WESTERN CONUS
COAST.
A VERY COLD AIR MASS OVER NORTHWESTERN CANADA WILL BE PUSHED EASTWARD BY THE
STRONG ALASKAN MID-LEVEL TROUGH, THEN PULLED SOUTHWARD INTO CONUS BY SURFACE
HIGH PRESSURE BEHIND THE 500-HPA TROUGH EJECTING OUT OF THE ROCKIES. THE GEFS
AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS) BOTH INDICATE AT LEAST A 20%
CHANCE FOR MINIMUM TEMPERATURES TO FALL BELOW THE 10TH PERCENTILE AND BELOW 20
DEG F FOR PORTIONS OF THE NORTHERN AND CENTRAL CONUS. THE GEFS PET CONTINUES TO
BRING UNSEASONABLY COLD WEATHER FARTHER SOUTH THAN THE ECMWF PET, BUT THE TWO
ARE IN CONSIDERABLY BETTER AGREEMENT NOW THAN YESTERDAY. WITH GUIDANCE
CONTINUING TO SIGNAL THIS COLD OUTBREAK, A SLIGHT RISK FOR MUCH BELOW-NORMAL
TEMPERATURES IS AGAIN POSTED FOR PORTIONS OF THE NORTHERN AND CENTRAL PLAINS
AND THE UPPER MISSISSIPPI VALLEY, VALID FOR OCT 26-29. THIS IS TWO DAYS LONGER
THAN YESTERDAY, BASED ON A SLOWER DETERIORATION OF THE CENTRAL CONUS TROUGH
THAN EXPECTED YESTERDAY. THE COLD AIR OUTBREAK MAY DAMAGE SOME OF THE WINTER
WHEAT CROP ACROSS THE REGION, WHICH IS SENSITIVE TO TEMPERATURES BELOW 15 DEG.
F. THE MID-LEVEL TROUGH AND SURFACE HIGH PRESSURE ARE FORECAST TO WEAKEN BY THE
MIDDLE OF WEEK-2, ENDING THE COLD WEATHER HAZARD.
HIGH SURFACE PRESSURE UNDER THE MID-LEVEL RIDGE IS EXPECTED TO COVER THE
EASTERN CONUS EARLY WEEK-2 WHILE HIGH SURFACE PRESSURE MOVES INTO THE NORTHERN
CONUS. BETWEEN THESE SYSTEMS, LOW SURFACE PRESSURE IS FORECAST TO DEVELOP,
BRINGING MOIST AIR FROM THE GULF OF MEXICO INTO A BROAD AREA STRETCHING ACROSS
SOUTHERN AND CENTRAL SECTIONS OF THE PLAINS AND MISSISSIPPI VALLEYS, AND THE
ADJACENT ROCKIES AND SOUTHEAST. MEANWHILE, MODEL SOLUTIONS ALSO DEPICT A
TROPICAL SYSTEM MOVING FROM THE EASTERN PACIFIC INTO MEXICO AND EVENTUALLY
PARTS OF THE WESTERN AND CENTRAL CONUS, PROVIDING ANOTHER SOURCE OF MOISTURE.
THESE FACTORS CREATE A HEAVY PRECIPITATION RISK ACROSS THE MIDDLE AND SOUTHERN
PARTS OF THE CENTRAL CONUS, WITH THE PRESSURE GRADIENT BETWEEN THE SURFACE LOW
AND HIGH PRESSURE SYSTEMS ALSO BRINGING A STRONG WIND THREAT TO A LARGE PORTION
OF THE PLAINS. BOTH HAZARDS SHOULD WANE AFTER THE FIRST COUPLE DAYS OF THE
PERIOD AS WEATHER SYSTEMS PROGRESS EASTWARD AND WEAKEN.
THE AFOREMENTIONED MOISTURE INFLUXES AND THE COLD AIR OUTBREAK EXPECTED IN THE
CENTRAL CONUS WILL COMBINE TO BRING THE POSSIBILITY OF HEAVY SNOW TO A BROAD
AREA ACROSS THE NORTHERN INTERMOUNTAIN WEST, THE NORTHERN HALVES OF THE ROCKIES
AND PLAINS, AND THE UPPER MIDWEST EARLY IN THE PERIOD. THE HIGHER ELEVATIONS
ARE MOST AT RISK OF HEAVY SNOWFALL, BUT THERE IS ENOUGH RISK THAT THE FIRST
SIGNIFICANT SNOWFALL OF THE SEASON COULD OCCUR OVER A MUCH BROADER AREA TO POST
A THREAT FOR A SLIGHT RISK OF HEAVY SNOWFALL ACROSS A SIGNIFICANT PORTION OF
THE LOWER ELEVATIONS IN THE ROCKIES AS WELL AS THE NORTHERN HALF OF THE PLAINS
AND THE UPPER MIDWEST. ACROSS THE PLAINS AND UPPER MIDWEST, THE SNOW WATER
EQUIVALENT (SWE) PET TOOL BASED ON THE GEFS SHOWS INCREASED CHANCES FOR A
SIGNIFICANT AMOUNT OF SNOW, BUT LOW ODDS THAT SWE WILL REACH 0.5 INCH OR
HIGHER. SEVERAL DETERMINISTIC MAPS, INCLUDING THE 12Z CANADIAN AND THE LAST FEW
RUNS OF THE GFS, DROP SEVERAL INCHES OF SNOW ON THE LOWER ELEVATIONS EAST OF
THE FRONT RANGE. IN CONTRAST, THE EUROPEAN DETERMINISTIC MODELS AND ENSEMBLES
HAVE NOT BEEN AS SUPPORTIVE OF HEAVY SNOWFALL. IN ANY CASE, THE THREAT WILL
WANE AFTER THE FIRST COUPLE DAYS OF THE PERIOD AS THE COLD AIR MODERATES AND
THE INFLUX OF MOISTURE DECLINES.
ACROSS ALASKA, A STRONG MID-LEVEL RIDGE COVERING THE STATE IS EXPECTED TO
SLOWLY RELAX AS WEEK-2 PROGRESSES, LEAVING THE STATE WITH NEAR-NORMAL 500-HPA
HEIGHTS NORTH OF A CENTRAL NORTH PACIFIC MID-LEVEL TROUGH BY THE END OF THE
PERIOD. THIS WOULD FAVOR RELATIVELY TRANQUIL WEATHER ACROSS MOST OF THE STATE,
WITH GENERALLY ABOVE-NORMAL TEMPERATURES STATEWIDE. ABOVE-NORMAL PRECIPITATION
SHOULD BE LIMITED TO PARTS OF THE ALEUTIANS, THE SOUTHWESTERN COAST OF MAINLAND
ALASKA, AND NORTHWESTERN PARTS OF THE STATE, BUT THERE IS LITTLE CHANCE FOR
TOTALS TO REACH HAZARDS THRESHOLDS. A SURFACE STORM AROUND THE MIDDLE OF WEEK-2
COULD ENHANCE WINDS ALONG PART OF THE SOUTHERN COASTLINE, BUT TOOLS STRONGLY
FAVOR WIND SPEEDS REMAINING BELOW THREATS THRESHOLDS.
FORECASTER: RICH TINKER
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