Prognostic Meteorological Discussion
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594
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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