Prognostic Meteorological Discussion
Issued by NWS

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3
000
FXUS02 KWBC 191521
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1119 AM EDT SAT AUG 19 2017

VALID 12Z TUE AUG 22 2017 - 12Z SAT AUG 26 2017

...OVERVIEW...

NORTHERN HEMISPHERE WILL DO A LITTLE ATMOSPHERIC SHUFFLING NEXT
WEEK WHICH WILL BE FORCED BY STRONG BLOCKING IN THE NORTH ATLANTIC
AND INCREASED TROUGHING INTO THE NORTHEAST. RIDGING IN THE PAC NW
WILL ONLY TEMPORARILY YIELD AS TROUGHING FROM THE GULF OF ALASKA
TRIES TO DIP INTO SOUTHERN BRITISH COLUMBIA. THIS WILL BRING A
COLD FRONT EASTWARD AND SOUTHWARD, USHERING IN COOLER AIR (NEAR TO
A FEW DEGREES BELOW AVERAGE) BY NEXT WEEKEND IN THE GREAT LAKES
AND NORTHEAST.


...GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO THE
LARGE-SCALE PATTERN, WITH A COUPLE AREAS OF MODERATE SPREAD
SURROUNDING THE DETAILS. THE EXACT EVOLUTION OF THE UPPER LOW
CROSSING EASTERN CANADA TUE-WED STILL REMAINS SLIGHTLY MURKY.
WHILE GUIDANCE SHOWS GOOD AGREEMENT WITH RESPECT TO THE
TIMING/AMPLITUDE OF THE LONG WAVE TROUGH, SOME DIFFERENCES REMAIN
WITH RESPECT TO THE INTENSITY AND TIMING OF THE UPPER LOW. THIS IN
TURN LEADS TO SOME VARIATION WITH RESPECT TO THE DEPTH/TIMING OF
THE SURFACE LOW, BUT DIFFERENCES IN THE TIMING OF THE TRAILING
COLD FRONT FARTHER SOUTH ACROSS THE OH/TN VALLEYS AND THE EASTERN
U.S. ARE RELATIVELY SMALL. THE 06Z GFS DEEPENED THE UPPER LOW MORE
QUICKLY THAN THE 00Z RUN, WHICH RESULTS IN A SURFACE LOW THAT IS
DEEPER AND A BIT FARTHER WEST RELATIVE TO THE 00Z RUN. SOME
RUN-TO-RUN VARIATION WAS STILL NOTED IN BOTH THE GFS AND THE
ECMWF, WHICH REDUCES CONFIDENCE A BIT AS THE SPECIFICS OF ANY
INDIVIDUAL MODEL OR RUN.

SPREAD CONTINUES TO DECREASE WITH RESPECT TO THE UPPER LOW MOVING
FROM THE GULF OF AK INTO WESTERN CANADA AND POSSIBLY THE PAC NW BY
THE MIDDLE OF THE WEEK, AS THE IMPACTS/EVOLUTION OF THE SYSTEM`S
MERGER WITH THE REMNANTS OF TYPHOON BANYAN BECOME CLEARER. GIVEN
THE AMPLIFYING NORTHERN STREAM FLOW PATTERN, MODELS HAVE SETTLED
TOWARD A SOMEWHAT SLOWER PROGRESSION THAN MUCH OF THE GUIDANCE
SHOWED A COUPLE DAYS AGO. THE SYSTEM IS NOW WELL-REPRESENTED BY A
BLEND OF THE GFS/ECMWF AND THEIR RESPECTIVE ENSEMBLE MEANS.

ADDITIONALLY, MODELS HAVE TRENDED SLIGHTLY WEAKER WITH THE
SUBTROPICAL RIDGE BUILDING INTO THE SOUTHEASTERN/SOUTH CENTRAL
U.S. BY DAYS 6-7, WHICH SEEMS TO RESULT IN A SLIGHT NORTHWARD
SHIFT AMONG MANY OF THE MODELS AND ENSEMBLE MEMBERS IN THE
EVENTUAL TRACK OF WHAT IS NOW TROPICAL STORM HARVEY BY THE TIME
THE SYSTEM REACHES THE BAY OF CAMPECHE. A SUBSTANTIAL PORTION OF
SOLUTIONS STILL CONTINUE TO TAKE THE SYSTEM ON A MORE WESTWARD
TRACK, HOWEVER.

THE WPC FORECAST WAS BASED HEAVILY IN A BLEND OF THE DETERMINISTIC
00Z ECMWF AND THE 00Z/O6Z GFS RUNS INITIALLY, KEEPING A SLIGHT
MAJORITY DETERMINISTIC THROUGH DAY 6, WITH INCREASING ENSEMBLE
MEAN (00Z ECENS/06Z GEFS) WEIGHTING BY DAY 7.


...WEATHER HIGHLIGHTS...

TUE-THU - FRONT WILL PUSH EASTWARD AND DROP SOUTHWARD AS THE SFC
LOW LIFTS INTO SOUTHERN CANADA. TEMPERATURES WILL WARM AHEAD OF
THE BOUNDARY WELL INTO THE 80S OR NEAR 90 INTO NEW ENGLAND (WITHIN
A FEW DEGREES OF RECORD HIGHS) AND STAYING IN THE 90S AND LOW 100S
OVER TEXAS. PRECIPITATION COULD BE HEAVIEST THROUGH THE
LOWER/EASTERN LAKES AS THE SFC LOW LIFTS NORTHEASTWARD. FLORIDA
SHOULD ALSO SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE MOVES
THROUGH THE STATE WED/THU.

THU-SAT - PAC NW WILL SEE AN INCREASE IN SHOWERS AS THE COLD FRONT
PUSHES INLAND BUT AMOUNTS SHOULD BE GENERALLY LIGHT. COOLER
TEMPERATURES ARE EXPECTED WITH THE INCREASE IN CLOUD COVER BUT
ONLY BRIEFLY. FARTHER SOUTH, COMBINATION OF THE WEAKENING UPPER
LOW/TROUGH PUSHING INTO SOUTHERN CALIFORNIA AND THE UPPER RIDGE
FOCUSING OVER TEXAS SHOULD SUPPORT INCREASED SHOWERS/CONVECTION
OVER THE FOUR CORNERS REGION, MOSTLY OVER NEW MEXICO AND SOUTHERN
COLORADO PER THE ENSEMBLES.


RYAN/FRACASSO


$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.