289
FXUS02 KWBC 051903
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
302 PM EDT FRI MAY 05 2023
VALID 12Z MON MAY 08 2023 - 12Z FRI MAY 12 2023
..OVERVIEW
A QUASI-ZONAL UPPER LEVEL FLOW PATTERN WITH A BROAD, LOW-AMPLITUDE
UPPER RIDGE OVER THE OHIO VALLEY AND AN ARRIVING UPPER TROUGH NEAR
THE WEST COAST IS EXPECTED FOR MONDAY. THIS TROUGH IS FORECAST TO
AMPLIFY FURTHER AND CLOSE OFF A LOW THROUGH MIDWEEK IN THE
INTERMOUNTAIN WEST, WHICH SHOULD BUILD RIDGING DOWNSTREAM ACROSS
THE PLAINS, WHILE A TROUGH AXIS REMAINS ATOP THE NORTHEAST. THIS
PATTERN SHOULD SLOWLY TRANSLATE EAST BY LATE WEEK, WITH THE
WESTERN TROUGH/LOW SHIFTING INTO THE HIGH PLAINS AND RIDGING
ACROSS THE EAST-CENTRAL U.S. PUSHING THE TROUGH AXIS OUT OF THE
NORTHEAST EVENTUALLY. WARM TO HOT TEMPERATURES ARE LIKELY EAST OF
THE ROCKIES IN RESPONSE TO THE UPPER RIDGE ALOFT, WHILE SHOWER AND
THUNDERSTORM CHANCES ARE LIKELY TO INCREASE ACROSS THE CENTRAL
U.S. FOR THE LATTER HALF OF NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE AGREES WELL ON THE OVERALL SYNOPTIC PATTERN
DESCRIBED ABOVE THROUGH THE EARLY PART OF THE PERIOD. IN TERMS OF
SMALLER-SCALE FEATURES, THE 00Z/06Z MODEL GUIDANCE VARIED SOMEWHAT
WITH THE POSITION OF A SHORTWAVE IN THE NORTH-CENTRAL
U.S./SOUTH-CENTRAL CANADA MONDAY-WEDNESDAY, WITH THE 00Z ECMWF
PERHAPS HOLDING ONTO A SMALL CLOSED LOW FEATURE TOO LONG.
DIFFERENCES ARE ALSO EVIDENT WITH SURFACE FRONTAL POSITIONS IN THE
EASTERN HALF OF THE U.S. BUT WITH THE OVERARCHING FEATURES IN
PLACE, THE UPDATED WPC FORECAST UTILIZED A BLEND OF THE 00Z/06Z
GFS AND 00Z ECMWF AND CMC EARLY ON. BY MID- TO LATE WEEK, MODELS
HAVE WAFFLED A BIT WITH HOW EARLY TO CLOSE OFF A LOW IN THE WEST
WITHIN THE TROUGH AXIS (MOST END UP HAVING A CLOSED LOW BY
THURSDAY) AND WITH ITS TRACK EASTWARD. THE 00Z/06Z MODEL GUIDANCE
WAS REASONABLY AGREEABLE WITH THE TROUGH'S TIMING TRENDING A
LITTLE FASTER, AS WELL AS SHOWING GOOD CONSENSUS ON THE DOWNSTREAM
RIDGING, SO A MAJORITY DETERMINISTIC MODEL BLEND WAS ABLE TO BE
USED THROUGH THE PERIOD BUT INCLUDING SOME ENSEMBLE MEAN GUIDANCE.
HOWEVER, THE INCOMING 12Z MODEL CYCLE SHOWS SOME DIVERGENCE IN THE
TROUGH'S TIMING/TRACK EAST ALONG WITH ITS ASSOCIATED SURFACE LOW,
WITH THE GFS SPEEDING UP AND THE ECMWF SLOWING DOWN. SO THERE
COULD CERTAINLY BE MORE SHIFTS IN THE FORECAST GOING FORWARD.
..WEATHER/HAZARDS HIGHLIGHTS
GRADUALLY INCREASING MOISTURE ACROSS THE EASTERN HALF OF THE
NATION, ALONG WITH SOME WEAK SHORTWAVE DISTURBANCES ALOFT, WILL
TEND TO RESULT IN SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS
DEVELOPING FROM THE DEEP SOUTH TO THE SOUTHERN GREAT LAKES. A
MARGINAL RISK FOR THE DAY 4 ERO (MONDAY-MONDAY NIGHT) HAS BEEN
MAINTAINED FOR PORTIONS OF THE MID-MISSISSIPPI VALLEY INTO THE
OHIO VALLEY AND LOWER MICHIGAN, WITH A MINOR EXTENSION EASTWARD
PER LATEST MODEL GUIDANCE. THERE MAY BE MORE THAN ONE AXIS OF
HEAVY RAINFALL WITHIN THIS REGION BUT THE PLACEMENT REMAINS QUITE
UNCERTAIN FOR NOW. SOME RAIN IS FORECAST TO SHIFT INTO THE EASTERN
SEABOARD ON TUESDAY, BUT BEGINNING LATE TUESDAY AND BEYOND, THE
CENTRAL U.S. SHOULD BE THE MAIN FOCUS FOR HIGHER RAINFALL AMOUNTS
AS MOISTURE LEVELS INCREASE AHEAD OF THE AMPLIFYING WESTERN
TROUGH. FOR DAY 5/TUESDAY, A MARGINAL RISK OF EXCESSIVE RAINFALL
WILL BE INTRODUCED ACROSS MUCH OF TEXAS, THE FIRST OF LIKELY
MULTIPLE DAYS OF INCREASED CONVECTION THERE WITH AMPLE MOISTURE
AND INSTABILITY IN PLACE. FARTHER NORTH, A MARGINAL RISK IS IN
PLACE FOR PORTIONS OF THE CENTRAL PLAINS WHERE THERE IS SOME
AGREEMENT FOR HEAVY RAIN THAT COULD CAUSE ISOLATED FLASH FLOODING.
RAIN AND STORMS ARE LIKELY TO EXTEND SOUTHEAST OF THERE, BUT
MODELS HAVE NOT NARROWED IN ON A LOCATION/AXIS FOR IMPACTFUL
STORMS YET, AS IT WILL DEPEND ON SMALL-SCALE FRONTAL POSITIONS AND
OTHER MODEL DIFFERENCES. LOOKING AHEAD TO WEDNESDAY AND THURSDAY,
A BROADER EXPANSE OF MODERATE TO LOCALLY HEAVY RAINFALL SHOULD
CONTINUE FROM TEXAS TO THE DAKOTAS IN ASSOCIATION WITH THE
EJECTING WESTERN U.S. STORM SYSTEM. THERE WILL LIKELY BE ENOUGH
COLD AIR ALOFT TO SUPPORT LOWERING SNOW LEVELS ACROSS PORTIONS OF
THE NORTHERN ROCKIES, AND LATE SEASON SNOW IS PROBABLE FOR THE
HIGHER ELEVATIONS OF WYOMING AND MONTANA DURING THIS TIME.
IN TERMS OF TEMPERATURES, HIGHS WILL LIKELY BE RUNNING 5 TO 15
DEGREES ABOVE EARLY MAY AVERAGES FROM THE CENTRAL/SOUTHERN PLAINS
TO THE OHIO VALLEY TO START THE WORKWEEK, AND THEN REACHING INTO
THE MIDWEST STATES GOING INTO THE MIDDLE OF THE WEEK. PORTIONS OF
TEXAS INTO THE DEEP SOUTH COULD REACH OR EXCEED 90 DEGREES. IT
SHOULD INITIALLY BE ABOVE AVERAGE ACROSS THE EASTERN U.S. BEFORE A
COLD FRONT USHERS IN A COOLER AND DRIER AIRMASS FOR THE
WEDNESDAY-FRIDAY TIME PERIOD. IT WILL LIKELY BE SLIGHTLY BELOW
AVERAGE ACROSS PORTIONS OF THE WEST COAST STATES AND THE
INTERMOUNTAIN WEST IN ASSOCIATION WITH THE UPPER TROUGH, BUT
OVERNIGHT LOWS SHOULD BE CLOSER TO AVERAGE HERE AND GENERALLY
ABOVE AVERAGE EAST OF THE ROCKIES.
TATE/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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