Analyzed 21Z Mon Aug 21, 2017
Valid 00Z Tue Aug 22, 2017
Valid 06Z Tue Aug 22, 2017
Valid 12Z Tue Aug 22, 2017
Valid 18Z Tue Aug 22, 2017
Valid 00Z Wed Aug 23, 2017
Valid 12Z Wed Aug 23, 2017
Valid 00Z Thu Aug 24, 2017
Valid 12Z Thu Aug 24, 2017
Valid 12Z Fri Aug 25, 2017
Valid 12Z Sat Aug 26, 2017
Valid 12Z Sun Aug 27, 2017
Valid 12Z Mon Aug 28, 2017
+ Short Range Forecast Discussion (Day ½-2½)
+ Medium Range Forecast Discussion (Day 3-7)
Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Mon Aug 21 2017
Valid 00Z Tue Aug 22 2017 - 00Z Thu Aug 24 2017
...Flash flooding and severe weather possible from portions the
Upper/Middle Mississippi Valley to New England through Tuesday
...Flash flooding and severe weather possible from portions of the
Central/Southern Plains to the Mid-Atlantic morning...
...Heavy rainfall will continue across the Southern Rockies
A surface low pressure system moving through the central U.S. will
be the focus for showers and thunderstorms over the next few days.
The leading area of low pressure will lift through the Upper
Midwest/Great Lakes into eastern Canada by midweek while a cold
front moves south and east through the central and eastern U.S.
Strong to severe thunderstorms will be capable of producing heavy
rainfall over portions of the Southern/Central Plains, Mississippi
Valley, Mid-Atlantic and Northeast as the system passes through.
Areas with heavy rainfall will have an increased risk for flash
flooding. The Storm Prediction Center has highlighted parts of the
Northern Plains and Upper Mississippi Valley as having a slight
risk for severe weather on Tuesday... and from the Ohio Valley to
the Northeast on Wednesday. Areas of excessive rainfall have been
highlighted by WPC from Nebraska to Wisconsin on Tuesday... and
Oklahoma and Arkansas on Wednesday.
Parts of the Southern Rockies are getting showers and
thunderstorms this afternoon as an upper level disturbance
interacts will monsoonal moisture. Rain will be heavy at times and
may lead to flash flooding.
Scattered to isolated showers will spread across California and
the Great Basin region as a slow-moving upper low approaches the
West Coast. A slow-moving upper low off the California coast
should give way to scattered to isolated shower activity across
portions of California and the Central Great Basin early this
week. The highest rainfall amounts are forecast for the Sierra
Nevada mountain range.
Afternoon thunderstorms will develop along the central and eastern
Gulf Coast over the next few days. Precipitation amounts are
expected to increase as an inverted trough pushes eastward toward
the east coast of Florida. Expect much cooler temperatures behind
the cold front as it sweeps eastward. Meanwhile, warmer than
normal temperatures will occur in the Pacific Northwest and
Graphics available at
+ Additional Links
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1205 PM EDT MON AUG 21 2017
VALID 12Z THU AUG 24 2017 - 12Z MON AUG 28 2017
MODELS AND ENSEMBLES SHOW GENERALLY GOOD AGREEMENT WITH RESPECT TO
THE LARGE SCALE PATTERN DURING THE MEDIUM RANGE PERIOD. THERE
REMAIN SOME FORECAST/FLOW DETAILS TO WORK OUT...ESPECIALLY TOWARDS
THE END OF THE PERIOD...BUT OVERALL MODELS DO SHOW BETTER THAN
AVERAGE AGREEMENT ON MOST SYSTEMS.
THE UPPER LEVEL TROUGH INITIALLY OVER THE GREAT LAKES AND THE
NORTHEAST IS SLOW TO MOVE OFFSHORE LATER THIS WEEK AND THE
WEEKEND. THERE IS GENERALLY GOOD AGREEMENT AMONGST EVEN THE
OPERATIONAL MODELS EXCEPT THE 00Z CMC WHICH APPEARS SLIGHTLY
QUICKER IN MOVING THE TROUGH OFFSHORE. OUT WEST...MODELS CONTINUE
TO SHOW GOOD AGREEMENT WITH A SHORTWAVE ENTERING THE PACIFIC
NORTHWEST/VANCOUVER DAY 3/THURSDAY AND CLOSING OFF AN UPPER LOW AS
IT PROGRESSES EASTWARD THROUGH WESTERN CANADA REACHING THE WESTERN
HUDSON BAY SHORE BY DAY 6/SUNDAY. AS THE UPPER LOW LIFTS MORE
NORTHWARD ON DAY 6 AND 7...THE SOUTHERN EDGE OF THE LARGER SCALE
TROUGH SPLITS AND MOVES EAST-SOUTHEASTWARD INTO THE UPPER
MIDWEST/GREAT LAKES BY DAY 7. IN BETWEEN THESE TWO SYSTEMS...A
STRONG RIDGE BUILDS AND THERE IS SOME DISAGREEMENT AS TO THE
STRENGTH OF THIS RIDGE AXIS WHICH MAY HAVE SOME INFLUENCE ON THE
PROGRESSION OF THE CLOSED LOW TO THE WEST BY THE END OF THE PERIOD.
MODELS AND ENSEMBLES ALSO CONTINUE TO SHOW GENERALLY GOOD
AGREEMENT THAT THE REMNANTS OF TROPICAL STORM HARVEY WILL CROSS
THE BAY OF CAMPECHE AND MOVE GENERALLY WEST/NORTHWEST INTO MEXICO
OR MAYBE EVEN FAR SOUTHERN TEXAS BY DAY 5. THE 06Z GFS TENDS TO
WANT TO LIFT THE REMNANTS INTO TEXAS WHILE THE 00Z ECMWF MAINTAINS
AND MORE WESTWARD TRACK INTO EASTERN AND NORTHERN MEXICO. WHILE
THE FORECAST AT THIS POINT LIES SOMEWHERE IN THE MIDDLE...THE
EXACT TRACK OF THE REMNANTS AND INTERACTION OF THIS MOISTURE WILL
A LINGERING BOUNDARY ALONG THE GULF COAST BY THE END OF THE PERIOD
WOULD HAVE IMPACTS ON SENSIBLE WEATHER ACROSS THIS REGION.
FOR ALL SYSTEMS...THE WPC FORECAST USES A BLEND OF THE OPERATIONAL
06Z GFS/00Z ECMWF WITH THE 00Z GEFS/ECMWF ENSEMBLE MEANS WITH MORE
OPERATIONAL INFLUENCE ON DAYS 3 AND 4...SLOWLY PROGRESSING TO A
MOSTLY ENSEMBLE MEAN BLEND BY DAY 7.
...SENSIBLE WEATHER HIGHLIGHTS...
EXPECT SHOWERS AND THUNDERSTORMS TO FOCUS ACROSS THE SOUTHEAST AND
THE GULF COAST ALONG A LINGERING STATIONARY BOUNDARY. FLORIDA
SHOULD ALSO SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE ALONG
THE COAST INTERACTS WITH THIS FRONTAL BOUNDARY. TO THE
NORTH...SHOWERS AND THUNDERSTORMS WILL DEVELOP ACROSS THE CENTRAL
U.S. AND THE NORTHERN PLAINS/UPPER MIDWEST AHEAD OF UPPER
TROUGHING IN WESTERN MEXICO AND EVENTUALLY DROPPING INTO THE
NORTHERN TIER. SHOWERS AND THUNDERSTORMS SHOULD ALSO DEVELOP
ACROSS THE SOUTHERN ROCKIES LATE IN THE WEEK WITH RETURN FLOW
REACHING THE SOUTHERN PLAINS BY THE WEEKEND.
ONE PLACE OF UNCERTAINTY IS WITH THE REMNANTS OF TROPICAL STORM
HARVEY AS IT MOVES INTO EAST CENTRAL MEXICO BY FRI/SAT. MOISTURE
ON THE NORTHERN FRINGE OF THE SYSTEM MAY BRING THE POTENTIAL FOR
HEAVY RAINFALL ACROSS SOUTH TEXAS AND INTO THE RIO GRANDE VALLEY.
THE MAGNITUDE AND HOW THIS PLAYS OUT THOUGH STILL STRONGLY DEPENDS
ON EXACTLY WHERE THE REMNANTS TRACK AND HOW THE MOISTURE INTERACTS
WITH THE LINGERING FRONT ALONG THE GULF COAST.
REGARDING TEMPERATURES ACROSS THE COUNTRY...WITH THE UPPER TROUGH
ACROSS THE GREAT LAKES/OHIO VALLEY/NORTHEAST...TEMPERATURES WILL
BE SEVERAL DEGREES COOLER THAN NORMAL LATER THIS WEEK AND THROUGH
THE WEEKEND. STRONG RIDGING OUT WEST WILL ALSO LEAD TO WARMER THAN
AVERAGE TEMPERATURES ACROSS MUCH OF THE WESTERN U.S. AND THE GREAT