Prognostic Meteorological Discussion
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FXUS02 KWBC 280541
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
140 AM EDT FRI JUL 28 2017

VALID 12Z MON JUL 31 2017 - 12Z FRI AUG 04 2017

...PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES...

THE LONG WAVE PATTERN ACROSS THE CONUS WILL BE RELATIVELY STABLE
THROUGH THE MEDIUM RANGE, WITH PERSISTENT TROUGHING ACROSS THE
EASTERN HALF OF THE COUNTRY, AND AN ANOMALOUS UPPER-LVL RIDGE
AMPLIFYING ACROSS THE GREAT BASIN (500 HPA HEIGHT ANOMALIES
POTENTIALLY REACH +2 STANDARD DEVIATIONS BY THE MIDDLE OF NEXT
WEEK). DETERMINISTIC SOLUTIONS AS WELL AS ENSEMBLE MEANS SHOW GOOD
CONSENSUS WITH RESPECT TO THIS LONG WAVE FLOW PATTERN.
TELECONNECTIONS ASSOCIATED WITH A PERSISTENT NEGATIVE HEIGHT
ANOMALY ACROSS THE GULF OF ALASKA FURTHER SUPPORTS WESTERN U.S.
RIDGING AND TROUGHING IN THE EAST. GUIDANCE HAS VARIED FROM RUN TO
RUN WITH RESPECT TO THE CHARACTER OF THE EASTERN TROUGH, THE
RESULT OF SMALL SCALE FEATURES IN THE FLOW WITH LOW
PREDICTABILITY. MANY OF THESE FEATURES ORIGINATE IN THE NERN
PAC/GULF OF ALASKA DURING EITHER THE SHORT RANGE OR EARLY MEDIUM
RANGE.

INITIALLY ON DAYS 3-4 (MON-TUE) THERE IS SOME SPREAD AMONG THE
GUIDANCE WITH RESPECT TO A WAVE OF LOW PRESSURE ALONG A PERSISTENT
STATIONARY FRONTAL BNDRY EXTENDING FROM THE NRN GULF OF MEXICO
INTO THE ATLANTIC OCEAN OFF THE EAST COAST. THE ECMWF HAS BEEN
PERSISTENT IN SHOWING A STRONGER SURFACE LOW CLOSER TO THE
MID-ATLANTIC COAST, WHILE THE GFS/CMC/UKMET ARE A BIT MORE
SUPPRESSED. ECMWF ENSEMBLE MEMBERS PROVIDE RELATIVELY STRONG
SUPPORT FOR THE DETERMINISTIC ECMWF, WITH A FAIR DEGREE OF
CLUSTERING OF SOLUTIONS, WHILE THE GEFS AND CMC ENSEMBLE MEMBERS
ARE MUCH MORE DISBURSED. GIVEN THE HIGH RUN-TO-RUN CONSISTENCY OF
THE ECMWF, ALONG WITH SUPPORT FROM ITS ENSEMBLE, WHILE ALSO TAKING
INTO ACCOUNT THAT TRACKING FRONTAL WAVES TOO CLOSE TO THE
MID-ATLANTIC COASTLINE IS OCCASIONALLY A BIAS OF THE ECMWF, THE
WPC FORECAST WAS INITIALLY BASED PRIMARILY ON A BLEND OF THE
ECMWF/GFS ALONG WITH THE ECENS MEAN/GEFS MEAN, WITH SLIGHTLY MORE
WEIGHTING TOWARD THE EC/EC MEAN SOLUTIONS. DESPITE THE SPREAD WITH
RESPECT TO THE SURFACE LOW, MODEL SOLUTIONS GENERALLY SHOW GOOD
CONSENSUS WITH RESPECT TO THE SURFACE FRONT LINGERING AND DRIFTING
SLOWLY NORTHWARD ACROSS THE GULF COAST THROUGH MUCH OF THE MEDIUM
RANGE. VARIOUS SOLUTIONS SHOW THE POTENTIAL FOR ADDITIONAL WAVES
OF LOW PRESSURE ALONG THE FRONT, BUT AT THIS TIME THEY GENERALLY
APPEAR WEAK AND HIGH DISBURSED AMONG ENSEMBLE GUIDANCE, AND THUS
SHOW VERY LOW PREDICTABILITY.

ADDITIONAL SHORTWAVE ENERGY WILL TRAVERSE THE NORTHERN TIER DURING
THE MEDIUM RANGE, WITH A SURFACE COLD FRONT FORECAST TO MOVE FROM
THE NORTHERN ROCKIES ON MON TO THE GREAT LAKES AND NORTHEAST BY
WED AND THU, RESPECTIVELY. AS DISCUSSED ABOVE, MUCH OF THIS ENERGY
ORIGINATES IN THE NORTH PAC OR GULF OF AK, SPILLING OVER THE TOP
OF THE BROAD RIDGE EXTENDING FROM THE WESTERN U.S. INTO WESTERN
CANADA, AND MODELS SHOW A FAIR AMOUNT OF RUN-TO-RUN VARIABILITY
WITH RESPECT TO BOTH TIMING AND AMPLITUDE. THUS, THE FORECAST WAS
GRADUALLY TRANSITIONED TO HEAVIER ENSEMBLE WEIGHTING OVER TIME,
INCLUDING THE ECENS, GEFS, AND NAEFS.


...WEATHER HIGHLIGHTS...

ABUNDANT FLOW OF MONSOONAL MOISTURE ALONG WITH MULTIPLE RELATIVELY
SMALL-SCALE UPPER-LEVEL IMPULSES TRACKING AROUND THE ANOMALOUS
RIDGE CENTERED OVER THE GREAT BASIN WILL SUPPORT CONTINUED
POTENTIAL FOR AREAS OF HEAVY CONVECTION, ESPECIALLY FROM THE SRN
ROCKIES EASTWARD TO THE SRN PLAINS. GEFS STANDARDIZED ANOMALIES
INDICATE POTENTIAL FOR PWAT ANOMALIES OF 2+ STANDARD DEVIATIONS
ACROSS THESE AREAS EARLY NEXT WEEK, SUPPORTING THE POTENTIAL FOR
HEAVY RAINFALL IN ANY CONVECTION THAT OCCURS. CONVECTIVE ACTIVITY
WILL EXTEND INTO PORTIONS OF THE SOUTHWEST AS WELL, ALBEIT MORE
SCATTERED IN NATURE. THE CORE OF THE UPPER RIDGE WILL SUPPRESS
CONVECTIVE CHANCES ACROSS MUCH OF THE GREAT BASIN, HOWEVER. THE
PERSISTENT AND WAVY FRONTAL BNDRY SHOULD CONTINUE TO PROVIDE FOCUS
FOR AREAS OF POTENTIALLY HEAVY CONVECTION ACROSS PORTIONS OF THE
GULF COAST AND THE SOUTHEAST. ENERGY AND ASSOCIATED SURFACE
FRONTAL FEATURES ACROSS THE NRN TIER MAY BRING AREAS OF POTENTIAL
CONVECTION AS WELL, BUT MODELS CURRENTLY SHOW LITTLE IN THE WAY OF
A COHERENT SIGNAL WITH RESPECT TO ANY HEAVY CONVECTION.

THE WRN RIDGE ALOFT WILL SUPPORT WELL ABOVE NORMAL TEMPS FROM
INLAND PARTS OF NRN-CNTRL CA AND PAC NW INTO THE NRN ROCKIES, WITH
MULTIPLE DAYS OF PLUS 5-15F ANOMALIES. OVER THE EAST EXPECT MEAN
TROUGHING ALOFT TO MAINTAIN NEAR TO BELOW NORMAL TEMPS FOR A
MAJORITY OF THE PERIOD.


RYAN

$$





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