646
FXUS02 KWBC 210651
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 AM EDT MON OCT 21 2024
VALID 12Z THU OCT 24 2024 - 12Z MON OCT 28 2024
..GENERAL OVERVIEW
A COLD FRONT IS FORECAST TO EXIT THE EAST COAST BY THURSDAY MORNING
AND BRING A RETURN TO MORE SEASONABLE TEMPERATURES AFTER THE RECENT
WARM SPELL, WITH A LARGE SURFACE HIGH SETTLING IN ACROSS THE
EASTERN U.S. BEHIND IT. A SECOND FRONTAL BOUNDARY IS ALSO EXPECTED
TO DROP SOUTH ACROSS THE MIDWEST AND OHIO VALLEY BY THE WEEKEND,
REINFORCING THE AUTUMNAL AIRMASS AND MAINLY DRY WEATHER. OUT WEST,
A PACIFIC COLD FRONT IS LIKELY TO REACH THE PACIFIC NORTHWEST COAST
LATE FRIDAY OR EARLY SATURDAY, BRINGING A RETURN TO RAIN AND
MOUNTAIN SNOW FROM NORTHERN CALIFORNIA TO WASHINGTON STATE. A MORE
ROBUST PATTERN CHANGE IS ON THE DISTANT HORIZON FOR THE WEST AS THE
UPPER LEVEL TROUGH LIKELY AMPLIFIES FURTHER GOING INTO EARLY NEXT
WEEK, AND A STRONG COLD FRONT PASSING THROUGH THE REGION.
..GUIDANCE/PREDICTABILITY ASSESSMENT
COMPARED TO LAST NIGHT, THERE HAS BEEN IMPROVEMENT IN THE LEVEL OF
MODEL AGREEMENT WITH THE 00Z GUIDANCE, WITH THE SPLIT FLOW OVER THE
EASTERN PACIFIC BECOMING BETTER RESOLVED. AT THE TIME OF
FRONTS/PRESSURES ISSUANCE, THE 12Z CMC WAS AN OUTLIER SOLUTION WITH
THE BUILDING WESTERN U.S. TROUGH SUNDAY AND INTO MONDAY, AND DID
NOT HAVE MUCH ENSEMBLE SUPPORT, SO IT WAS PHASED OUT OF THE
FORECAST BLEND BY LATE SATURDAY. HOWEVER, THE NEW 00Z RUN IS MORE
COMPATIBLE WITH THE 00Z ECMWF AND CLOSER TO THE ENSEMBLE MEANS. THE
GFS IS SLOWER WITH ARRIVAL OF THE TROUGH ACROSS THE WEST BY MONDAY,
AND A LITTLE SLOWER THAN THE GEFS MEAN. TO START, A GENERAL BLEND
OF THE DETERMINISTIC GUIDANCE WORKED WELL AS A STARTING POINT ON
THURSDAY, AND ENSEMBLE MEANS WERE GRADUALLY INCREASED TO ABOUT HALF
BY MONDAY.
..WEATHER/HAZARDS HIGHLIGHTS
MOST OF THE COUNTRY SHOULD BE VOID OF HEAVY RAINFALL AS MOISTURE
WILL BE LIMITED DESPITE A FEW FRONTAL BOUNDARIES AND TROUGHS
PASSING THROUGH. WARM AIR ADVECTION ACROSS THE MIDWEST ON THURSDAY
AROUND THE BACKSIDE OF THE SURFACE HIGH SHOULD SUPPORT MAINLY LIGHT
TO MODERATE SHOWERS AHEAD OF AN APPROACHING COLD FRONT FROM THE
NORTHERN PLAINS, AND THIS WILL LIKELY WEAKEN FURTHER AS IT REACH
THE NORTHEAST TO CLOSE OUT THE WORK WEEK. SOME SHOWERS ARE ALSO
LIKELY FOR PORTIONS OF THE FLORIDA PENINSULA WITH NORTHEASTERLY
FLOW IN PLACE.
THE EXCEPTION WILL BE WESTERN WASHINGTON AND OREGON WHERE ONSHORE
FLOW AND TERRAIN ENHANCEMENT WILL TEND TO INCREASE RAINFALL
PROSPECTS ACROSS THIS REGION, MAINLY NEXT WEEKEND AS THE MOISTURE
PLUME AHEAD OF THE PACIFIC FRONT MOVES INLAND. A FEW INCHES OF
RAINFALL WILL BE POSSIBLE OVER THE FRIDAY NIGHT- MONDAY MORNING
TIME PERIOD FOR THE COASTAL RANGES AND THE WESTERN SLOPES OF THE
CASCADES. HEAVY SNOW IS POSSIBLE FOR THE HIGHEST MOUNTAINS.
HOWEVER, THE RAINFALL TOTALS SHOULD BE LIMITED ENOUGH DURING THE
DAY 4 AND 5 PERIOD (THURSDAY AND FRIDAY) TO WARRANT KEEPING THE
EXCESSIVE RAINFALL OUTLOOKS FOR THOSE DAYS BLANK FOR NOW. THERE IS
STILL SOME DISCREPANCY IN THE GUIDANCE FOR THE ARRIVAL OF THIS
RAINFALL EVENT, SO STAY TUNED TO FUTURE FORECASTS.
IN TERMS OF TEMPERATURES, MOST OF THE COUNTRY SHOULD EXPERIENCE
ABOVE AVERAGE TEMPERATURES DURING THIS FORECAST PERIOD, AND THIS
ESPECIALLY HOLDS TRUE ACROSS THE PLAINS AND THE DESERT SOUTHWEST,
WHERE HIGHS COULD BE 10-20 DEGREES ABOVE AVERAGE MOST DAYS, WITH
THE GREATEST ANOMALIES ON SUNDAY-MONDAY ACROSS THE CENTRAL AND
NORTHERN PLAINS. THIS WILL EQUATE TO HIGHS IN THE MIDDLE 60S TO
UPPER 70S FOR THOSE REGIONS. READINGS CLOSER TO LATE OCTOBER
CLIMATOLOGY ARE LIKELY FOR THE EAST COAST STATES IN THE WAKE OF THE
COLD FRONTS. THINGS WILL LIKELY TREND TO COOLER THAN USUAL FOR THE
PACIFIC NORTHWEST BY MONDAY AS THE UPPER TROUGH BUILDS AND MORE
CLOUDS/PRECIPITATION DEVELOPS.
HAMRICK
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC MEDIUM
RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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