670
FXUS02 KWBC 041839
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
238 PM EDT MON AUG 04 2008
VALID 12Z THU AUG 07 2008 - 12Z MON AUG 11 2008
A PERSISTENT RIDGE OVER NUNAVUT/NORTH-CENTRAL CANADA THIS PERIOD
/2 TO 3 STANDARD DEVIATIONS ABOVE NORMAL/ FAVORS TROUGHING IN
NORTHEASTERN NORTH AMERICA AND A TROUGH NEAR THE WEST COAST. THE
MODELS AGREE ON THIS PATTERN...AND AMONGST THEMSELVES SHOWED A
HIGH LEVEL OF AGREEMENT...UNTIL THE 12Z GFS BECAME AVAILABLE.
CONSIDERING THE AMPLIFYING FLOW PATTERN...THIS LEADS TO HIGHER
THAN NORMAL CONFIDENCE IN THE FORECAST TODAY. THE UPPER LOW NEAR
THE PACIFIC NORTHWEST IS FORECAST TO HAVE STANDARDIZED ANOMALIES
OF 3-4 DEVIATIONS BELOW NORMAL AT 500 HPA BY LATE SATURDAY. 00Z
ECMWF WAS USED INTO FRIDAY MORNING...BEFORE BLENDING IN THE 00Z
NCEP ENSEMBLE MEAN. THE 12Z GFS WAS NOT CONSIDERED EAST OF THE
110TH MERIDIAN DUE TO ITS QUESTIONABLE LOOK TO THE PATTERN ACROSS
CANADA/GREAT PLAINS/EAST. THIS CHOICE WAS MADE TO TAKE INTO
ACCOUNT UNCERTAINTY OF THE POSITIONING OF THE LOW NEAR THE
NORTHEAST...WHERE THE 00Z ECMWF IS AN EASTERLY OUTLIER...DESPITE
GOOD AGREEMENT ALOFT. THIS KEPT GOOD CONTINUITY.
NORTHWEST...
THE UPPER LOW NEAR THE PACIFIC NORTHWEST IS FORECAST TO HAVE
STANDARDIZED ANOMALIES OF 3-4 DEVIATIONS BELOW NORMAL AT 500 HPA
BY LATE SATURDAY. THE HIGH ANGLE OF THE AUGUST SUN WILL LEAD TO
LOW LEVEL TEMPERATURE ANOMALIES OF 1-2 STANDARD DEVIATIONS BELOW
CLIMATOLOGY. THIS SYSTEM ALOFT WILL DRIVE A PACIFIC COLD FRONT
INLAND...WHICH CLEARS THE NORTHERN CONTINENTAL DIVIDE BY EARLY
NEXT WEEK. ONSHORE FLOW SHOULD LEAD TO MODERATE RAINS ACROSS THE
OLYMPIC PENINSULA AND CASCADES AFTER FRONTAL PASSAGE.
SOUTHWEST...
THE UPPER LEVEL INVERTED TROUGH/ENERGY WHICH SPAWNED EDOUARD WILL
CONTINUE MARCHING WESTWARD THROUGH NORTHERN MEXICO...LEADING TO A
SURGE OF MONSOON RAINS ACROSS THE SOUTHWEST THIS WEEKEND INTO
EARLY NEXT WEEK.
PLAINS/MID-MISSISSIPPI VALLEY/SOUTHEAST...
A FRONTAL ZONE REMAINS QUASI-STATIONARY IN THE SOUTHERN PLAINS
THROUGH THE PERIOD...ACTING AS A FOCUS FOR CONVECTION AS LOW LEVEL
INFLOW INTERCEPTS THE BOUNDARY FROM THE GULF OF MEXICO. MODERATE
TO HEAVY RAINS ARE EXPECTED FROM THE FRONT RANGE OF THE COLORADO
ROCKIES EAST-SOUTHEAST THROUGH SOUTHERN MISSOURI/ARKANSAS INTO THE
SOUTHEAST AS POTENTIAL CONVECTIVE VORTICES SURF THE SOUTHERN EDGE
OF THE WESTERLIES AT THE BASE OF THE EASTERN TROUGH. LOCAL
AMOUNTS OF OVER 5 INCHES ARE POSSIBLE IN THIS REGION FROM THURSDAY
INTO NEXT MONDAY.
NORTHEAST...
PREVIOUS RUNS OF THE GFS WERE AN OUTLIER WITH THE STRENGTH OF THE
KEY SHORTWAVE OVER THE GREAT LAKES DAY 3 THAT LEADS THE
AMPLIFICATION OF THE EASTERN TROUGH...SO IT WAS DISCOUNTED. THE
12Z RUN DECIDED TO LEAVE THAT PIECE OF ENERGY UP IN NORTHERN
CANADA...LEADING TO A DIVERGENT SOLUTION ACROSS CENTRAL AND
EASTERN NORTH AMERICA. THE UPPER TROUGH IS FORECAST TO BE 4
STANDARD DEVIATIONS BELOW NORMAL IN THE 500 HPA HEIGHT FIELD
/SIMILAR TO THIS TIME IN AUGUST 2004 BUT SHIFTED 5-10 DEGREES
FARTHER EAST/. THE HIGH ANGLE OF THE AUGUST SUN WILL KEEP DAYTIME
SURFACE TEMPERATURES FROM REACHING AS HIGH OF A DEPARTURE FROM
NORMAL IN THESE AREAS. STILL...IT WILL BE MILDER THAN NORMAL
ACROSS THE EAST THIS WEEKEND. A PROLONGED ATLANTIC FETCH COMBINED
WITH A STRONG SYSTEM ALOFT SHOULD LEAD ANOTHER BOUT OF MODERATE TO
HEAVY RAINS IN THE NORTHERN APPALACHIANS THIS PERIOD.
ROTH
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