054
FXUS02 KWBC 100655
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
254 AM EDT WED MAY 10 2023
VALID 12Z SAT MAY 13 2023 - 12Z WED MAY 17 2023
***FLASH FLOODING A GROWING CONCERN ACROSS PORTIONS OF TEXAS THIS
WEEKEND WITH A MODERATE RISK OF EXCESSIVE RAINFALL ON SATURDAY,
AND AN EARLY SEASON HEATWAVE ACROSS THE PACIFIC NORTHWEST***
..SYNOPTIC SCALE OVERVIEW
THE LOW PRESSURE SYSTEM OVER THE MIDWEST ON SATURDAY, ASSOCIATED
WITH A DECAYING UPPER LEVEL LOW, WILL CONTINUE WEAKENING GOING
THROUGH THE WEEKEND, WITH A SWATH OF MODERATE TO HEAVY RAIN ACROSS
PORTIONS OF THE UPPER MIDWEST. THE TRAILING COLD FRONT FROM THIS
STORM SYSTEM IS EXPECTED TO SLOW DOWN CONSIDERABLY ACROSS TEXAS,
AND IN COMBINATION WITH SHORTWAVE ENERGY ALOFT AND COPIOUS
MOISTURE TRANSPORT FROM THE WESTERN GULF OF MEXICO, AREAS OF HEAVY
RAIN ARE BECOMING LIKELY ACROSS MUCH OF CENTRAL TEXAS GOING INTO
THE WEEKEND. MEANWHILE, THE UPPER RIDGE CONTINUES TO BUILD ACROSS
THE NORTHWESTERN U.S. AND INTO BRITISH COLUMBIA AND ALBERTA, WITH
MUCH WARMER TEMPERATURES BECOMING WIDESPREAD. IN RESPONSE TO THE
BUILDING RIDGE WILL BE A DOWNSTREAM TROUGH THAT DEVELOPS FURTHER
FROM THE GREAT LAKES TO THE NORTHEAST AND MID-ATLANTIC REGION WITH
A RETURN TO COOLER CONDITIONS AND LOWER HUMIDITY FOR THE BEGINNING
OF NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE 00Z MODEL GUIDANCE SUITE HAS A GOOD OVERALL SYNOPTIC SCALE
DEPICTION OF THE UPPER LEVEL AND SURFACE PATTERN FOR THE WEEKEND,
WITH ABOVE AVERAGE CONFIDENCE FOR MOST AREAS SO A
MULTI-DETERMINISTIC MODEL BLEND SUFFICES AS A STARTING POINT IN
THE FORECAST PROCESS. THE MODELS AGREE WELL ON A STRONG UPPER
RIDGE BUILDING INTO SOUTHWESTERN CANADA, WASHINGTON AND OREGON,
AND THE UPPER LOW OVER THE DAKOTAS EVOLVING INTO AN OPEN WAVE THAT
MERGES WITH THE WESTERLIES. THERE IS ALSO BETTER MODEL CONSENSUS
ON THE AXIS AND MAGNITUDE OF THE HEAVY QPF EXPECTED OVER TEXAS
THIS WEEKEND, AND THE GFS HAS A BETTER SIGNAL FOR A WEAK UPPER LOW
OVER WESTERN TEXAS ON SATURDAY. LOOKING AHEAD TO MONDAY, THERE
ARE SOME TIMING DIFFERENCES WITH THE SHORTWAVE BUILDING ACROSS
THE EASTERN U.S. AND ALSO FOR THE SECOND REINFORCING SHORTWAVE
DROPPING SOUTH FROM CENTRAL CANADA ON TUESDAY. TO ACCOUNT FOR THE
INCREASING MODEL DIFFERENCES FOR THE BEGINNING OF NEXT WEEK, USE
OF THE ENSEMBLE MEANS INCREASED TO ABOUT 50% FOR THE END OF THE
FORECAST PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
A HIGH IMPACT HEAVY RAIN EVENT IS LOOKING INCREASINGLY LIKELY
ACROSS MUCH OF CENTRAL AND SOUTHERN TEXAS GOING INTO THE UPCOMING
WEEKEND, IN ADDITION TO THE RAIN ALREADY EXPECTED DURING THE SHORT
RANGE PERIOD. A FAVORABLE COMBINATION OF A STALLING FRONT,
MULTIPLE WEAK SHORTWAVE IMPULSES PASSING ALOFT, SUSTAINED DEEP
MOISTURE FLUX FROM THE WESTERN GULF OF MEXICO, AND APPRECIABLE LOW
LEVEL INSTABILITY SOUTH OF THE FRONT WILL SET THE STAGE FOR
MULTIPLE MESOSCALE CONVECTIVE SYSTEMS WITH TRAINING THUNDERSTORMS
AND HIGH RAINFALL RATES. THIS UNSETTLED PATTERN GOING INTO THE
DAY 4 PERIOD SATURDAY WILL LIKELY RESULT IN 2-5 INCHES OF RAINFALL
WITH LOCALLY HIGHER TOTALS POSSIBLE ACROSS MUCH OF CENTRAL AND
SOUTHERN TEXAS. SINCE THIS HEAVY RAINFALL ON DAY 4 WILL BE
FALLING ON ALREADY HIGHLY SATURATED GROUNDS FROM THE PREVIOUS
DAYS, A MODERATE RISK REMAINS IN EFFECT ACROSS PORTIONS OF
SOUTH-CENTRAL TEXAS, INCLUDING THE GREATER SAN ANTONIO AREA.
THERE WILL LIKELY BE SOME CHANGES TO THIS RISK AREA IN FUTURE
FORECAST UPDATES ONCE THE MODEL SIGNAL BECOMES MORE REFINED.
THERE WILL LIKELY BE SOME ADDITIONAL HEAVY RAIN GOING INTO DAY 5
AS WELL ACROSS PARTS OF TEXAS AND OKLAHOMA, AND A SLIGHT RISK IS
PLANNED FROM FAR SOUTHERN KANSAS TO THE RIO GRANDE TO COVER THIS
POTENTIAL, ALTHOUGH THE HEAVIEST RAINFALL SHOULD BE WANING BY THIS
TIME. FARTHER NORTH ACROSS THE UPPER MIDWEST, SOME LINGERING
THUNDERSTORM COMPLEXES ARE A GOOD POSSIBILITY GOING INTO SATURDAY
IN ASSOCIATION WITH THE WEAKENING UPPER TROUGH/SURFACE LOW. DRIER
WEATHER ARRIVES IN TIME FOR MOTHER'S DAY ACROSS MUCH OF THE
NORTH-CENTRAL U.S., AND THEN MORE OF THE CENTRAL/EASTERN U.S. GETS
LESS COVERAGE OF SHOWERS AND STORMS GOING INTO EARLY NEXT WEEK.
THE OTHER STORY THAT WILL BE MAKING WEATHER HEADLINES WILL BE THE
EARLY SEASON HEATWAVE CENTERED OVER THE PACIFIC NORTHWEST STARTING
THIS WEEKEND AND PERSISTING INTO EARLY NEXT WEEK AS THE ANOMALOUS
UPPER HIGH BECOMES ANCHORED IN PLACE OVER SOUTHERN BRITISH
COLUMBIA. THE LATEST TEMPERATURE FORECAST CALLS FOR HIGHS
REACHING THE MIDDLE 80S TO MIDDLE 90S FOR MANY OF THE LOWER
ELEVATIONS IN WASHINGTON AND OREGON, WHICH WOULD EASILY SET DAILY
HIGH TEMPERATURE RECORDS. OVERNIGHT LOWS WILL ALSO BE QUITE MILD
WITH READINGS ONLY FALLING INTO THE 60S FOR MANY OF THESE SAME
AREAS. IT WILL ALSO BE QUITE WARM FOR THE CENTRAL VALLEY OF
CALIFORNIA WITH HIGHS WELL INTO THE 90S THERE AS WELL. ELSEWHERE
ACROSS THE NATION, THE HEAT AND HUMIDITY ACROSS THE CENTRAL AND
EASTERN U.S. IS EXPECTED TO DECREASE SOME GOING INTO EARLY NEXT
WEEK AS A WEAK FRONTAL BOUNDARY CROSSES THE SOUTHERN TIER STATES,
AND A STRONGER COLD FRONT PASSES THROUGH THE GREAT LAKES AND
NORTHEAST BRINGING A MORE REFRESHING AIRMASS. COOLER WEATHER
OWING TO CLOUD COVER AND RAINFALL WILL LIKELY BE THE CASE ACROSS
MUCH OF THE SOUTHERN PLAINS WITH HIGHS RUNNING 5-15 DEGREES BELOW
AVERAGE MOST DAYS.
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES ARE
AT:
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
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