791
FXUS02 KWBC 091849
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 PM EDT TUE MAY 09 2023
VALID 12Z FRI MAY 12 2023 - 12Z TUE MAY 16 2023
***FLASH FLOODING A GROWING CONCERN ACROSS PORTIONS OF TEXAS
FRIDAY AND INTO THE WEEKEND WITH A MODERATE RISK OF EXCESSIVE
RAINFALL ON SATURDAY, AND AN EARLY SEASON HEATWAVE ACROSS THE
PACIFIC NORTHWEST***
..SYNOPTIC SCALE OVERVIEW
A SLOW MOVING LOW PRESSURE SYSTEM SITUATED OVER THE
CENTRAL/NORTHERN PLAINS ON FRIDAY, ASSOCIATED WITH A UPPER LEVEL
LOW, WILL TRACK ACROSS THE MIDWEST STATES AND GRADUALLY WEAKEN AS
THE UPPER SYSTEM OPENS UP. EXPECT THIS SYSTEM TO PRODUCE
WIDESPREAD RAIN ACROSS THE DAKOTAS AND EASTERN PORTIONS OF WYOMING
AND MONTANA, WITH MOISTURE ALSO EXTENDING TOWARD THE EAST COAST
ALONG A LEADING FRONT. THE COLD FRONT EXTENDING SOUTH FROM THE
INITIAL PLAINS SYSTEM WILL LIKELY 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 SHOULD DEVELOP ACROSS MUCH OF CENTRAL TEXAS GOING INTO THE
WEEKEND. MEANWHILE, AN UPPER RIDGE STARTING TO BUILD OVER THE
NORTHWESTERN U.S. AND INTO BRITISH COLUMBIA AND ALBERTA ON FRIDAY
WILL CONTINUE TO STRENGTHEN THROUGH THE WEEKEND AND MAY REMAIN
CENTERED OVER WESTERN CANADA INTO THE FIRST PART OF NEXT WEEK.
THE RIDGE WILL PROMOTE SEVERAL DAYS OF MUCH ABOVE NORMAL
TEMPERATURES OVER THE NORTHWEST, WHILE ULTIMATELY LEADING TO
INCREASED DOWNSTREAM UPPER TROUGHING ACROSS EASTERN NORTH AMERICA
AND A RETURN TO COOLER CONDITIONS WITH LOWER HUMIDITY FOR THE
BEGINNING OF NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
A 00Z/06Z OPERATIONAL MODEL COMPOSITE PROVIDED A GOOD
REPRESENTATION OF THE OVERALL PATTERN EVOLUTION THROUGH THE FIRST
HALF OF THE PERIOD THAT COVERS FRIDAY INTO THE WEEKEND. THE
BUILDING NORTHWEST U.S. INTO SOUTHWESTERN CANADA UPPER RIDGE
SHOULD CLOSE OFF A HIGH OVER CANADA BY EARLY SUNDAY, WHILE THE
UPPER LOW DRIFTING INTO THE NORTHERN HALF OF THE PLAINS SHOULD
OPEN UP WITH SOME NORTHERN STREAM ENERGY DROPPING DOWN FROM CANADA
POSSIBLY INTERACTING WITH LOW'S REMAINING ENERGY. THERE IS DECENT
AGREEMENT ON A WEAK SOUTHERN STREAM IMPULSE/UPPER LOW REACHING THE
SOUTHERN HIGH PLAINS BY EARLY SATURDAY BUT A LOT OF SPREAD
DEVELOPS FOR ONE OR MORE UPPER LOWS THAT MAY RETROGRADE OVER PARTS
OF THE WEST, UNDERNEATH THE UPPER HIGH CLOSING OFF TO THE NORTH.
12Z MODELS ADD TO THE UNCERTAINTY WITH SOME NOTABLE CHANGES IN
SPECIFICS RELATIVE TO THE 00Z/06Z RUNS. ELSEWHERE TYPICAL DETAIL
UNCERTAINTIES BY THE LATTER HALF OF THE PERIOD THAT GOES THROUGH
NEXT TUESDAY FAVORED ADDING 06Z GEFS/00Z ECMWF MEAN INPUT TO THE
OPERATIONAL RUNS. THE PRIMARY ADJUSTMENT IN THE LATEST GUIDANCE
CLUSTERING WAS FOR A FARTHER SOUTH PROGRESSION OF A COLD FRONT
INTO THE NORTHERN TIER FROM SOUTHERN CANADA EARLY NEXT WEEK. ALSO
OF NOTE, ADDING THE 12Z RUNS HAVE ACTUALLY INCREASED THE SPREAD
FOR THE ULTIMATE SURFACE PATTERN FROM THE PLAINS INTO THE EAST.
THE 12Z UKMET/CMC/ECMWF HAVE BECOME NOTICEABLY MORE SUPPRESSED
WITH THE PLAINS/MIDWEST SURFACE WAVE VERSUS THE PRIOR CYCLE, WHILE
THE 12Z GFS DIFFERENCES ALOFT (SLOWER TO DEVELOP TROUGHING OVER
THE OHIO VALLEY/MID-ATLANTIC) LEAD TO A MUCH SLOWER PROGRESSION
THAN OTHER GUIDANCE, INCLUDING THE 00Z/06Z GFS OR LATEST GEFS
MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
A HIGH IMPACT HEAVY RAIN EVENT IS LOOKING INCREASINGLY LIKELY
ACROSS MUCH OF CENTRAL AND SOUTHERN TEXAS GOING INTO FRIDAY AND
CONTINUING THROUGH MUCH OF THE UPCOMING WEEKEND. 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. A BROAD
SLIGHT RISK AREA OF EXCESSIVE RAINFALL REMAINS IN PLACE FOR THE
DAY 4 PERIOD (12Z FRIDAY-12Z SATURDAY) ACROSS MAINLY CENTRAL TEXAS
TO THE RIO GRANDE WITH A STRONG MODEL SIGNAL FOR 3-6 INCH TOTALS
DURING THIS TIME PERIOD. THE UNSETTLED PATTERN REMAINS UNABATED
GOING INTO THE DAY 5 PERIOD (12Z SATURDAY-12Z SUNDAY) WITH AN
ADDITIONAL 3-6 INCHES LIKELY WITH LOCALLY HIGHER TOTALS POSSIBLE.
SINCE THIS HEAVY RAINFALL ON DAY 5 WILL BE FALLING ON ALREADY
HIGHLY SATURATED GROUNDS FROM THE PREVIOUS DAYS, A COORDINATED
MODERATE RISK WAS INTRODUCED ACROSS PORTIONS OF SOUTH-CENTRAL
TEXAS, INCLUDING THE GREATER SAN ANTONIO AREA, IN THE PREVIOUS
ISSUANCE AND THE LATEST UPDATE PLANS TO MAINTAIN THIS AREA. THERE
WILL LIKELY BE SOME CHANGES TO LOCATION/COVERAGE OF THIS RISK AREA
IN FUTURE FORECAST UPDATES AS GUIDANCE REFINES IMPORTANT DETAILS.
ELSEWHERE ACROSS THE CONTINENTAL U.S., WIDESPREAD RAIN OF VARYING
INTENSITY WILL LIKELY BE ONGOING ACROSS THE DAKOTAS ON FRIDAY IN
ASSOCIATION WITH THE UPPER LOW TRACKING OVER OR NEAR THAT REGION.
THE ASSOCIATED SURFACE WAVE AND FRONT EXTENDING TO ITS EAST SHOULD
THEN FOCUS AN AREA OF RAIN FROM THE MIDWEST INTO PORTIONS OF THE
EAST, BUT WITH LESS CONFIDENCE ON COVERAGE AND TOTALS. MEANWHILE,
ONE OR MORE UPPER LOWS/SHORTWAVES TRACKING UNDERNEATH THE
SOUTHWESTERN CANADA UPPER HIGH MAY HELP TO PRODUCE SCATTERED AREAS
OF RAINFALL OVER THE SOUTHERN THREE-FOURTHS OF THE WEST DURING THE
WEEKEND INTO NEXT WEEK.
THE MOST EXTREME HEAT RELATIVE TO NORMAL WILL BE OVER THE PACIFIC
NORTHWEST, WITH SATURDAY THROUGH MONDAY HAVING THE BEST POTENTIAL
TO SEE HIGHS REACHING 15-25F ABOVE NORMAL. SUCH ANOMALIES MAY
YIELD HIGHS REACHING INTO THE 90S FOR SOME INLAND VALLEY LOCATIONS
AND COULD CHALLENGE DAILY RECORD HIGHS. RECORD WARM LOWS ARE ALSO
LIKELY DURING THIS EVENT AND COULD EXTEND INTO TUESDAY WHEN
DAYTIME HIGHS COULD START TO MODERATE SLIGHTLY OVER WESTERN
WASHINGTON AND OREGON. ON THE OTHER HAND, WIDESPREAD RAIN AND
CLOUDY CONDITIONS WILL PROMOTE MULTIPLE DAYS OF BELOW NORMAL HIGHS
OVER THE SOUTHERN HIGH PLAINS WITH SOME LOCATIONS SEEING HIGHS
10-20F BELOW NORMAL--ESPECIALLY DURING SATURDAY-MONDAY. MORNING
LOWS WILL BE NEAR AVERAGE THOUGH. MUCH OF THE EASTERN HALF OF THE
COUNTRY WILL SEE TEMPERATURES 5-15F ABOVE NORMAL DURING
FRIDAY-SATURDAY BUT THEN A COOLER TREND WILL PUSH IN FROM THE WEST
AND NORTH, CONFINING MOST OF THE ABOVE NORMAL READINGS TO THE
SOUTHEAST THEREAFTER, PERHAPS ASIDE FROM A REBOUND OVER THE
MIDWEST AND VICINITY AHEAD OF A FRONT DROPPING SOUTH FROM CANADA
EARLY NEXT WEEK.
RAUSCH/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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