724
FXUS02 KWBC 151917
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
317 PM EDT WED MAY 15 2024
VALID 12Z SAT MAY 18 2024 - 12Z WED MAY 22 2024
...SOUTHERN TIER HEAVY RAINFALL THREAT POSSIBLY LINGERING INTO THE
WEEKEND OVER THE SOUTHEAST...
..OVERVIEW
BEHIND AN UPPER TROUGH AND SURFACE SYSTEM PROGRESSING EASTWARD
FROM THE MISSISSIPPI VALLEY SATURDAY ONWARD, GUIDANCE SHOWS A
SYSTEM CROSSING SOUTHERN CANADA AND THE NORTHERN TIER OF THE LOWER
48 DURING THE WEEKEND. THEN GUIDANCE IS CONSISTENT IN PRINCIPLE
REGARDING ESTABLISHMENT OF WHAT MAY BECOME A FAIRLY AMPLIFIED MEAN
TROUGH ALOFT OVER THE WEST. WHILE A LOT OF EMBEDDED DETAILS ARE
STILL UNCERTAIN, THE FORECAST PATTERN EVOLUTION SHOULD SUPPORT A
TRANSITION OF RAINFALL EMPHASIS FROM THE SOUTH/EAST TO START THE
WEEKEND TOWARD DEVELOPMENT FROM THE NORTHERN HALF OF THE PLAINS
INTO THE MIDWEST AND CONTINUING EASTWARD BETWEEN SUNDAY AND THE
MIDDLE OF NEXT WEEK. MEANWHILE THE WESTERN TROUGH WILL LIKELY BRING
COOLER TEMPERATURES TO THE REGION AND AT LEAST SOME SCATTERED
PRECIPITATION, INCLUDING HIGH ELEVATION SNOW IN THE NORTHERN
ROCKIES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
EVEN AS THE PERIOD BEGINS SATURDAY, THERE ARE SOME MODEL/ENSEMBLE
DIFFERENCES ACROSS THE EAST-CENTRAL U.S. WITH THE UPPER TROUGH AND
WEAK SURFACE LOW. AMONG AN OVERALL SLOWER TREND IN THE GUIDANCE
OVER THE PAST DAY OR TWO, GFS RUNS REMAIN ON THE SLOWER SIDE OF THE
ENVELOPE IN TRACKING THESE FEATURES EAST COMPARED TO NON-NCEP
MODELS AND AI/ML MODELS. THE CMC MAY BE A GOOD MIDDLE GROUND IN
POSITION AS THE ECMWF IS ON THE EASTERN SIDE. INTO NEXT WEEK, THERE
IS AN INCREASING MODEL SIGNAL FOR THIS ENERGY AND THE SURFACE LOW
TO LINGER IN THE WESTERN ATLANTIC NEAR THE OUTER BANKS OR SO. THE
MOST RECENT 12Z ECMWF NOW SHOWS THIS AS WELL AFTER MORE PROGRESSIVE
EARLIER RUNS.
WITH THE SATURDAY-SUNDAY SOUTHERN CANADA/NORTHERN TIER U.S. SYSTEM
RESOLVED FAIRLY WELL, THE NEXT FEATURE OF INTEREST IS THE
DEVELOPING WESTERN U.S. UPPER TROUGH. MULTIPLE ROUNDS OF SHORTWAVE
ENERGY SHOULD ARRIVE INTO THE NORTHWEST DURING THE WEEKEND INTO
NEXT WEEK, WITH ONE OR MORE UPSTREAM FEATURES POSSIBLY HELPING TO
DEEPEN/REINFORCE THE OVERALL TROUGH NEXT WEEK. THIS BRINGS INTO
PLAY THE TYPICAL LOW-PREDICTABILITY ISSUE OF WHEN/HOW INDIVIDUAL
SHORTWAVES WILL EJECT FROM THE MEAN TROUGH AND GENERATE WELL-
DEFINED SURFACE WAVES EAST OF THE ROCKIES. ADDING TO THIS, A
SOUTHERN STREAM UPPER LOW WILL LIKELY REACH INTO THE SOUTHWEST BY
EARLY IN THE WEEK AND MAY CONTRIBUTE TO LOW PRESSURE DEVELOPMENT
DOWNSTREAM. AT LEAST GENERALLY, MODELS SHOW A COUPLE OF ROUNDS OF
CENTRAL U.S. SURFACE LOW DEVELOPMENT MONDAY AND TUESDAY WITH A
NORTHEAST TRACK BY WEDNESDAY BUT WITH LOW- MODERATE CONFIDENCE IN
THE DETAILS. TIMING AND STRENGTH OF UPSTREAM ENERGY DIVING THROUGH
THE WEST SIDE OF THE TROUGH NEXT WEEK IS ALSO UNCERTAIN. THE 12Z
ECMWF NOW CLUSTERS WITH THE GFS RUNS IN SHOWING A CLOSED UPPER LOW
NEARING OR ATOP THE PACIFIC NORTHWEST BY MIDWEEK.
THE WPC FORECAST WAS BASED ON A BLEND OF THE 06Z GFS AND 00Z ECMWF
AND CMC EARLY IN THE PERIOD, AND GRADUALLY INCREASED THE ENSEMBLE
MEANS COMPONENT TO HALF BY DAY 6 AND OVER HALF DAY 7 AS THE SPREAD
INCREASED. THIS GENERALLY MAINTAINED GOOD CONTINUITY WITH THE
PREVIOUS FORECAST THOUGH WITH A SLIGHTLY SLOWER/LINGERING WESTERN
ATLANTIC LOW.
..WEATHER/HAZARDS HIGHLIGHTS
AS OF DAY 4 SATURDAY-SATURDAY NIGHT, THE UPPER TROUGH/SURFACE
SYSTEM MOVING THROUGH THE EAST WILL SPREAD AN AREA OF RAINFALL
ACROSS THE SOUTHERN TWO-THIRDS OR MORE OF THE EAST. THE EXCESSIVE
RAINFALL OUTLOOK COVERING THIS PERIOD DEPICTS A MARGINAL RISK AREA
EXTENDING FROM NORTHERN FLORIDA INTO THE CENTRAL APPALACHIANS AND
PARTS OF THE MID-ATLANTIC. THERE SHOULD BE FAVORABLE MOISTURE AND
INSTABILITY WITHIN THIS AREA BUT SO FAR THE GUIDANCE, EVEN AS
SIMPLE AS ENSEMBLE PROBABILITIES OF EXCEEDING AN INCH, HAS NOT
CLUSTERED PARTICULARLY WELL FOR HEAVIEST ACTIVITY. THE PRIMARY
REGION OF INTEREST FOR THE POSSIBILITY OF AN EVENTUAL SLIGHT RISK
UPGRADE WOULD BE IN THE VICINITY OF NORTHERN FLORIDA INTO SOUTHERN
ALABAMA/GEORGIA IF GUIDANCE SHOWS BETTER CLUSTERING, GIVEN WET
ANTECEDENT CONDITIONS AND WHAT SIGNALS CURRENTLY EXIST FOR HIGH
RAIN RATES. SOME RAINFALL COULD LINGER ALONG PARTS OF THE EAST
COAST INTO SUNDAY IF THE SLOWER SIDE OF THE ENVELOPE VERIFIES FOR
THE SYSTEM AFFECTING THE REGION, BUT DETAILS REMAIN VERY UNCERTAIN.
MEANWHILE THE GREATER RAINFALL EMPHASIS WILL SHIFT INTO THE
CENTRAL PLAINS/MIDWEST REGION AROUND THE START OF THE WEEK, IN
ASSOCIATION WITH SHORTWAVE ENERGIES EJECTING FROM THE MEAN TROUGH
AND DEVELOPING SURFACE LOW PRESSURE SYSTEMS. DURING THE DAY 5
(SUNDAY- SUNDAY NIGHT) ERO PERIOD, GUIDANCE SHOWS MIXED SIGNALS
REGARDING THE MAGNITUDE/LOCATION OF CONVECTIVE DEVELOPMENT WITHIN
THE CENTRAL PLAINS/MIDWEST REGION. AS SOME GUIDANCE INDICATES
MAXIMUM AMOUNTS FROM NEBRASKA/SOUTHEAST SOUTH DAKOTA TO MINNESOTA
WHILE SOME ARE WELL SOUTH FROM KANSAS TO MISSOURI, CONTINUE TO
DEPICT NO RISK AREA IN THE ERO AT THIS POINT BUT EXPECT THAT A
MARGINAL RISK WOULD BE NEEDED IF A MORE COHERENT GUIDANCE SIGNAL
ARISES. ANOTHER ROUND OF PERHAPS HEAVIER RAINFALL AMOUNTS IS LIKELY
OVER PORTIONS OF THE NORTHERN HALF OF THE PLAINS AND MISSISSIPPI
VALLEY INTO MONDAY-TUESDAY OF NEXT WEEK, AFTER THE ERO PERIOD.
MEANWHILE, PORTIONS OF THE WEST, ESPECIALLY NORTHERN AREAS, MAY
BEGIN TO SEE SCATTERED PRECIPITATION BY THE WEEKEND OR EARLY NEXT
WEEK AND CONTINUING INTO MIDWEEK AS UPPER TROUGHING SETTLES OVER
THE REGION. SOME PRECIPITATION MAY BE IN THE FORM OF SNOW IN THE
HIGH ELEVATIONS OF THE NORTHERN ROCKIES.
EXPECT SOUTH TEXAS TO SEE MULTIPLE DAYS OF HAZARDOUS HEAT DURING
THE PERIOD WITH HIGHS PERSISTENTLY RUNNING 10-15F ABOVE NORMAL WITH
MAX HEAT INDEX VALUES POSSIBLY REACHING AT LEAST 110F. HIGHS NEAR
OR OVER 100 DEGREES COULD STRETCH FARTHER NORTH ACROSS THE SOUTHERN
HIGH PLAINS AT TIMES. SOUTHERN FLORIDA MAY ALSO EXPERIENCE
HAZARDOUS HEAT DURING THE WEEKEND WITH HIGHS LOCALLY UP TO 5-10F
ABOVE NORMAL AND SIMILAR MAX HEAT INDEX VALUES, FOLLOWED BY
CONTINUED ABOVE NORMAL BUT SLIGHTLY LESS EXTREME READINGS. BOTH
AREAS MAY SEE DAILY RECORD HIGHS.
THE SOUTHERN TWO-THIRDS OF THE WEST WILL BE QUITE WARM ON SATURDAY
BUT THEN THE DEVELOPING UPPER TROUGH WILL LEAD TO A COOLING TREND,
WITH NORTHERN PARTS OF THE WEST SEEING NEAR TO BELOW NORMAL HIGHS
FROM THE WEEKEND ONWARD AND AREAS TO THE SOUTH TRENDING TO A MIX OF
SLIGHTLY ABOVE/BELOW NORMAL HIGHS. AREAS FROM THE CENTRAL/SOUTHERN
ROCKIES INTO THE GREAT LAKES WILL TEND TO SEE ABOVE NORMAL
TEMPERATURES DURING THE PERIOD. CLOUDS AND RAINFALL SHOULD KEEP
HIGHS ON THE COOL SIDE OVER THE MID-ATLANTIC DURING THE WEEKEND.
TATE/RAUSCH
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page