Prognostic Meteorological Discussion
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000 FXUS02 KWBC 260659 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 259 AM EDT MON JUN 26 2017 VALID 12Z THU JUN 29 2017 - 12Z MON JUL 3 2017 ***PATTERN OVERVIEW*** A QUASI-ZONAL FLOW PATTERN AT THE BEGINNING OF THE FORECAST PERIOD ON THURSDAY IS FORECAST TO BECOME MORE AMPLIFIED BY THE WEEKEND WITH A BROAD TROUGH OVER THE GREAT LAKES REGION AND ANOTHER TROUGH OVER THE INTERMOUNTAIN WEST. A LOW AMPLITUDE UPPER RIDGE IS LIKELY ACROSS THE GREAT PLAINS BEHIND THE EASTERN U.S. TROUGH. A COUPLE OF WELL-DEFINED SURFACE LOWS ALONG A FRONTAL ZONE SITUATED OVER THE CENTRAL U.S. WILL LIKELY BE ASSOCIATED WITH WIDESPREAD SHOWER AND THUNDERSTORM ACTIVITY, ESPECIALLY FOR THE END OF THIS WEEK. ***MODEL PREFERENCES*** IN TERMS OF MODEL PREFERENCES, A BLEND OF THE OPERATIONAL UKMET, GFS, AND ECMWF WAS INCORPORATED FOR THE FIRST HALF OF THE FORECAST PERIOD. ALTHOUGH THE CMC WAS CLOSER TO THE MODEL CONSENSUS COMPARED TO YESTERDAY, IT INDICATED MORE UPPER LEVEL RIDGING ACROSS THE WESTERN U.S. BY NEXT WEEKEND, SO IT WAS NOT INCORPORATED INTO THE FORECAST. 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. THIS HAS BEEN THE TREND OVER THE PAST SEVERAL MODEL CYCLES FOR THE GFS IN BEING MORE PRONOUNCED WITH THIS FEATURE. THE ECMWF IS SLOWER WITH THE PROGRESSION OF THIS FEATURE ACROSS THE GREAT LAKES. TOWARDS THE END OF THE FORECAST PERIOD, THERE IS A HINT IN THE ECMWF AND CMC GUIDANCE FOR A WEAK CUT-OFF LOW DEVELOPING OFF THE COAST OF CALIFORNIA, BUT THE SENSIBLE WEATHER IMPACTS SHOULD BE NEGLIGIBLE. 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, WILL BE THE MAIN PLAYER IN TERMS OF PRECIPITATION DURING THIS PERIOD. AS THIS DISTURBANCE REACHES THE NORTHERN PLAINS AND UPPER MIDWEST BY THURSDAY NIGHT, NUMEROUS HEAVY SHOWERS AND STORMS ARE LIKELY TO DEVELOP, ESPECIALLY ALONG AND AHEAD OF THE TRAILING COLD FRONT AND NEAR A SURFACE LOW ALONG THE FRONT. 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 DURING THE THURSDAY TO FRIDAY TIME PERIOD, AND FUTURE EXCESSIVE RAINFALL OUTLOOKS FROM WPC WILL HAVE ADDITIONAL INFORMATION AS THE EVENT ENTERS THE SHORT-RANGE PERIOD. MONSOONAL MOISTURE FROM WESTERN MEXICO MAY REACH INTO SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO AT TIMES, ENHANCING AFTERNOON AND EVENING SHOWER AND THUNDERSTORM ACTIVITY. AFTER A FEW DAYS OF A HIGH QUALITY AIRMASS WITH PLEASANT TEMPERATURES AND LOW HUMIDITY, A RETURN TO SUMMER-LIKE HEAT AND HUMIDITY IS FORECAST FROM THE OHIO VALLEY TO THE MID-ATLANTIC AND NORTHEAST. HIGHS IN THE LOWER 90S ARE LIKELY EAST OF THE APPALACHIANS BY FRIDAY, WHICH IS SEVERAL DEGREES ABOVE NORMAL. COOL WEATHER SHOULD CONTINUE ACROSS MINNESOTA, WISCONSIN, AND MICHIGAN AS THIS AREA REMAINS UNDER THE INFLUENCE OF THE UPPER LEVEL TROUGH. HOT AND DRY CONDITIONS ARE EXPECTED TO PERSIST ACROSS THE LOWER ELEVATIONS OF THE DESERT SOUTHWEST, BUT LESS THAN THE IMPRESSIVE HEAT WAVE LAST WEEK GIVEN LESS RIDGING ALOFT. D. HAMRICK $$

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