514  
FXUS02 KWBC 301606  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1205 PM EDT TUE APR 30 2019  
 
VALID 12Z FRI MAY 03 2019 - 12Z TUE MAY 07 2019  
 
...HEAVY RAIN/FLOODING THREAT FRI/SAT FOR THE SOUTHERN  
PLAINS/LOWER MISSISSIPPI VALLEY...  
 
   
..OVERVIEW
 
 
UPPER RIDGING OVER THE NORTHEASTERN PACIFIC WILL REMAIN STATIONARY  
DURING THE MEDIUM RANGE PERIOD WHICH WILL DRIVE A SPLIT FLOW  
ACROSS THE LOWER 48. FEATURES IN THE FORECAST WILL BE THE NORTHERN  
STREAM OUT OF NW CANADA AND AN INCOMING CLOSED LOW IN THE SOUTHERN  
STREAM INTO CALIFORNIA AROUND SUNDAY. RIDGING OFF THE SOUTHEAST  
COAST WILL WOBBLE IN PLACE ALLOWING FOR WAVY FRONTS ALONG THE EAST  
COAST AS SYSTEMS EXIT INTO THE ATLANTIC.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
IN THE SOUTHERN STREAM (OFF THE WEST COAST), THE GFS/CANADIAN AND  
THEIR ENSEMBLE MEANS WERE QUICKER THAN THE ECMWF/ECMWF ENSEMBLE  
MEAN TO MOVE THE UPPER LOW INTO CALIFORNIA EITHER LATE SATURDAY OR  
EARLY MONDAY AS THE TIMING DIFFERENCE EXCEEDED 36 HRS BETWEEN THE  
QUICKER/SLOWER SOLUTIONS. 00Z UKMET WAS ABOUT 2/3RDS THE WAY TO  
THE SLOWER ECMWF FROM THE GFS AND THIS WAS PREFERRED GIVEN: 1) THE  
SEPARATION FROM EITHER AN UPSTREAM KICKER OR THE NORTHERN STREAM  
IN THE PAC NW/GREAT BASIN, 2) TENDENCY OF THE GFS TO BE TOO QUICK  
TO BRING CLOSED LOWS EASTWARD IN WEAK PATTERNS AND 3) CONTINUITY  
WHICH FAVORED THE SLOWER SIDE OF THE ENSEMBLES.  
 
OVER TEXAS, TAIL END OF A FRONT WILL LINGER ON FRIDAY AND THEN  
SLOWLY MOVE SOUTHEASTWARD ON SATURDAY AS A WAVE MOVES ALONG THE  
BOUNDARY INTO THE SOUTHEAST THEN OFF THE MID-ATLANTIC COAST  
SUNDAY. THIS WILL PROVIDE A FOCUS FOR THE HEAVY RAIN POTENTIAL  
(PARTLY INTO THE SHORT RANGE PERIOD) ALONG AND EAST OF I-35.  
 
IN THE NORTHERN STREAM, CONSENSUS APPROACH WAS MAINTAINED WHICH  
RESULTED IN ONLY A FEW CHANGES TO FRONTAL TIMING/PLACEMENT. SYSTEM  
MOVING OUT OF THE DAKOTAS ON SATURDAY WILL LIFT NORTHEASTWARD  
TOWARD HUDSON BAY BUT THE ECMWF/UKMET BECAME MUCH QUICKER THAN  
MOST OF THE ENSEMBLES WHEREAS THE GFS/CANADIAN WERE MORE IN LINE  
WITH THE ECMWF ENSEMBLE MEAN. PREFERRED TO RELY MOSTLY ON THE  
ECMWF ENSEMBLE MEAN AND CONTINUITY BY SUN-TUE VIA A LARGER BLEND.  
CONFIDENCE DECREASED BY NEXT MON/TUE IN THE LATITUDE OF THE FRONT  
IN THE EAST GIVEN A FLATTER FLOW ALOFT AND SPREAD IN ENSEMBLES.  
WESTERN UPPER LOW IS FORECAST TO PUSH PAST THE DESERTS AROUND THE  
MIDDLE OF NEXT WEEK PER CURRENT TIMING.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE FRONT OVER TEXAS WILL TAP POOLED MOISTURE OVER ITS SURFACE  
BOUNDARY ALIGNED PARALLEL TO THE UPPER FLOW FRIDAY WHICH SUPPORTS  
A THREAT OF HEAVY RAIN/THUNDERSTORMS AND CELL TRAINING/LOCALLY  
ENHANCED RUNOFF ISSUES AS EMBEDDED HARD-TO-TIME WEAKER SHORTWAVES  
MOVE THROUGH. MORE DEFINED SHORTWAVE SHOULD CARRY THE FRONT  
SOUTHEASTWARD SATURDAY AND MOVE THE RAIN THREAT EASTWARD, FOCUSED  
MOSTLY ALONG THE CENTRAL GULF COAST SUNDAY. RAIN THREAT REMAINS  
INTO NEXT WEEK AS THE FRONT LINGERS ALONG THE GULF COAST. THIS  
WILL GENERALLY KEEP TEMPERATURES CLOSER TO OR JUST BELOW TYPICAL  
VALUES FOR EARLY MAY.  
 
PACIFIC CLOSED LOW WILL MOVE INTO CA EARLY NEXT WEEK WHICH WILL  
SPREAD SCATTERED SOUTHWEST-TO-ROCKIES SHOWERS/CONVECTION AS HEIGHT  
FALLS MOVE EASTWARD. MOISTURE ANOMALIES MAY ONLY BE +1 TO +1.5  
SIGMA FROM NORMAL PER THE GUIDANCE AND OTHER INGREDIENTS MAY BE  
MARGINAL AT BEST, BUT EXPECT AT LEAST ISOLATE SHOWERS/STORMS  
ESPECIALLY OVER THE TERRAIN FROM THE SIERRA ACROSS THE GREAT BASIN  
NEXT WEEK. TEMPERATURES WILL TREND TOWARD COOLER THAN AVERAGE AS  
CLOUD COVER AND PRECIPITATION INCREASE.  
 
IN THE EAST, MODEST RAIN WILL ACCOMPANY/PRECEDE FRONTAL PASSAGES  
WITH HIGHER AMOUNTS SOUTH OF ABOUT 40N IN THE WARMER AIRMASS.  
NORTHERN SYSTEM WILL BRING MORE FOCUSED PRECIPITATION (SOME SNOW  
IN THE WEST AT HIGHER ELEVATIONS) AS THE LEAD COLD FRONT  
DISSIPATES SATURDAY TO BE OVERTAKEN BY A TRAILING SYSTEM MOVING  
INTO THE GREAT LAKES SUNDAY. BAROCLINIC ZONE ALONG THE FRONT MAY  
PERIODICALLY FOCUS CONVECTION CONTINGENT UPON MESOSCALE  
ENHANCEMENTS. TEMPERATURES WILL BE ABOVE AVERAGE AHEAD OF AND  
SOUTH OF FRONTAL BOUNDARIES BY ABOUT 5-10 DEGREES F.  
 
FRACASSO  
 
ADDITIONAL 3-7 DAY HAZARDS INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, 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/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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