EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 258 AM EDT SUN JUN 25 2017 VALID 12Z WED JUN 28 2017 - 12Z SUN JUL 2 2017 ***PATTERN OVERVIEW AND MODEL PREFERENCES*** THE MEDIUM RANGE PERIOD WILL BEGIN ON WEDNESDAY WITH A QUASI-ZONAL PATTERN ACROSS THE CONTINENTAL U.S. AS THE EASTERN U.S TROUGH AXIS EXITS THE COAST AND LOW AMPLITUDE RIDGING ALOFT BUILDS IN. LOWERING MID LEVEL HEIGHTS ACROSS THE WESTERN HALF OF THE NATION IN RESPONSE TO THE DISTURBANCE MOVING EASTWARD ACROSS SOUTHERN CANADA WILL SUPPRESS THE UPPER LEVEL RIDGE SOUTHWARD INTO MEXICO. BY NEXT WEEKEND, UPPER LEVEL TROUGHING WILL LIKELY BECOME ESTABLISHED OVER THE NORTH-CENTRAL U.S. ALONG WITH COOLER TEMPERATURES AND SIGNIFICANT RAINFALL. IN TERMS OF MODEL PREFERENCES, A BLEND OF THE OPERATIONAL UKMET, GFS, AND ECMWF WAS INCORPORATED FOR THE FIRST HALF OF THE FORECAST PERIOD. NO CMC WAS USED ANYWHERE IN THE FORECAST GIVEN ITS EARLY DEPARTURES FROM THE MODEL CONSENSUS. IT WAS TOO AMPLIFIED WITH THE EAST COAST TROUGH AND INDICATED LESS IN THE WAY OF TROUGHING ACROSS THE INTERMOUNTAIN WEST COMPARED TO THE MODEL CONSENSUS. THE GFS REMAINS A DEEPER SOLUTION ACROSS THE CENTRAL U.S. WITH RESPECT TO THE TROUGH FROM SOUTHERN CANADA, BUT STILL CLOSE ENOUGH TO BE INCORPORATED INTO THE FORECAST BLEND. INCREASING PERCENTAGES OF THE GEFS AND EC MEANS WERE USED FOR NEXT WEEKEND WHEN UNCERTAINTIES RELATED TO MESOSCALE DETAILS BECOME MORE SIGNIFICANT. ***SENSIBLE WEATHER*** THE UPPER LEVEL LOW MOVING EASTWARD ACROSS SOUTHERN CANADA, ALONG WITH ITS SURFACE LOW AND TRAILING COLD FRONT, APPEARS TO BE THE MAIN PLAYER IN TERMS OF PRECIPITATION DURING THIS PERIOD. AS THIS DISTURBANCE REACHES THE NORTHERN PLAINS AND UPPER MIDWEST LATER IN THE WEEK, NUMEROUS HEAVY SHOWERS AND STORMS ARE LIKELY TO DEVELOP, ESPECIALLY ALONG AND AHEAD OF THE TRAILING COLD FRONT AND NEAR A SURFACE LOW. MODELS CONTINUE TO STRONGLY ADVERTISE AN AXIS OF HEAVY RAIN FROM EASTERN KANSAS AND NEBRASKA TOWARDS THE GREATER CHICAGO AREA, WITH SOME SOLUTIONS INDICATING IN EXCESS OF 5 INCHES OF RAIN ACROSS PARTS OF IOWA AND NORTHERN MISSOURI THROUGH FRIDAY. AREAS OF EXCESSIVE RAINFALL CAPABLE OF PRODUCING FLASH FLOODING APPEAR INCREASINGLY LIKELY, AND FUTURE EXCESSIVE RAINFALL OUTLOOKS FROM WPC WILL HAVE ADDITIONAL INFORMATION AS THE EVENT ENTERS THE SHORT-RANGE PERIOD. IT IS ALSO INTERESTING TO NOTE THAT MONSOONAL MOISTURE IS BEGINNING TO INCREASE OVER WESTERN MEXICO, AND THE NORTHERN EXTENT OF THIS MAY REACH INTO SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO AT TIMES. HOT WEATHER IS FORECAST TO CONTINUE FOR THE LOWER ELEVATIONS OF THE DESERT SOUTHWEST, ALBEIT SLIGHTLY COOLER COMPARED TO THE EXTREME VALUES OF LATE OWING TO THE WEAKENING OF THE UPPER RIDGE. A HIGH QUALITY AIRMASS WITH PLEASANT TEMPERATURES IS EXPECTED ACROSS THE EASTERN U.S. FOR THE MIDDLE OF THE WEEK WITH A CANADIAN SURFACE HIGH OVER THE REGION. HOWEVER, THE HEAT AND HUMIDITY IS FORECAST TO RETURN TO CLOSE OUT THE WORK WEEK WITH SOUTHERLY FLOW INCREASING AHEAD OF THE APPROACHING FRONTAL SYSTEM TO THE WEST. D. HAMRICK