Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 240505
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1204 AM EST SAT FEB 24 2018

VALID 12Z TUE FEB 27 2018 - 12Z SAT MAR 03 2018

...HEAVY RAIN/FLOOD THREAT FOR SATURATED SOILS/SWELLING RIVERS
ACROSS PORTIONS OF THE MID-MISSISSIPPI VALLEY AND MID-SOUTH...


...PATTERN OVERVIEW...

THE FLOW PATTERN ACROSS NORTH AMERICA WILL AT LEAST PARTIALLY
REORGANIZE DURING THE MEDIUM RANGE. A BUILDING UPPER RIDGE ACROSS
THE HIGH LATITUDES OF THE NORTH ATLANTIC WILL PROMOTE DEVELOPMENT
OF A REX BLOCK ALONG APPROXIMATELY 30 DEG W  BY DAY 3 (TUE).
THROUGH THE MEDIUM RANGE, THE POSITIVE HEIGHT ANOMALY CENTER WILL
GRADUALLY SHIFT WESTWARD, REACHING THE LABRADOR SEA BY NEXT
FRI-SAT, WITH THE AXIS OF THE REX BLOCK DIPOLE SHIFTING WEST AS
WELL. MEANWHILE, POSITIVE HEIGHT ANOMALIES WILL ALSO BUILD NORTH
FROM THE PACIFIC ACROSS ALASKA AND INTO THE ARCTIC OCEAN. THE
IMPLICATIONS OF THIS HEMISPHERIC PARTIAL REALIGNMENT FOR WEATHER
ACROSS THE CONUS ARE SOMEWHAT UNCLEAR. TELECONNECTIONS WITH
RESPECT TO TEMP/PRECIP ANOMALIES ACROSS THE CONUS ARE HIGHLY
SENSITIVE TO THE EXACT POSITION OF THE NORTH ATLANTIC RIDGE. IF
THIS NEGATIVE NAO/AO PATTERN CAN PERSIST FOR LONG ENOUGH, IT WOULD
FOSTER A COLDER AND DRIER PERIOD FOR THE EASTERN SEABOARD,
ALTHOUGH LIKELY BEYOND THE END OF THIS FORECAST PERIOD. OUT WEST,
THE IMPLICATIONS SEEM CLEARER, WITH STRENGTHENING RIDGING ACROSS
THE NORTH PAC/ALASKA SUPPORTING PERSISTENT TROUGHING FROM THE WEST
COAST TO THE NORTHERN ROCKIES. ADDITIONALLY, RISING HEIGHT APPEAR
LIKELY ACROSS THE SOUTH CENTRAL U.S., AS THE SUBTROPICAL RIDGE
SHIFTS WEST ACROSS THE GULF OF MEXICO.


...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...

MODELS/ENSEMBLE ARE IN DECENT AGREEMENT IN SHOWING AN UPPER LOW
BRIEFLY CUTTING OFF ACROSS THE SOUTHWESTERN U.S. TUE/TUE NIGHT,
BEFORE OPENING INTO A WAVE AND QUICKLY MOVING EAST ACROSS THE
CENTRAL/EASTERN STATES WED-THU. SOLUTIONS CONTINUE TO DIVERGE AS
TO HOW QUICKLY THE LOW BEGINS TO OPEN AND MOVE EAST. THE ECMWF HAS
SHOWN THE POOREST RUN-TO-RUN CONTINUITY, AND AFTER SPEEDING THE
SYSTEM UP WITH THE 00Z FRI RUN, THE 12Z RUN SLOWED THE SYSTEM DOWN
AGAIN RATHER SIGNIFICANTLY. THE GFS HAS BEEN CONSISTENTLY ON THE
FASTER SIDE OF THE SPREAD WITH HOW QUICKLY THIS OCCURS, WHILE THE
CMC HAS REMAINED ON THE SLOW SIDE OF THE SPREAD. OF THE MODEL
GUIDANCE AVAILABLE FOR THIS FORECAST, THE 12Z UKMET SEEMED THE
MOST MIDDLE GROUND SOLUTION FOR FOR THE TIMING OF THIS SYSTEM AS
IT EXITS THE SOUTHERN ROCKIES AND CROSSES THE CENTRAL U.S.
WED-THU. AS THE UPPER SHORTWAVE REACHES THE OH VALLEY ON THU,
MODELS GENERALLY AGREE THAT SOME DEGREE OF PHASING OCCURS WITH
ADDITIONAL NORTHERN STREAM ENERGY, RESULTING IN A RAPID
AMPLIFICATION AND DEVELOPMENT OF AN UPPER LOW, ALONG WITH PERHAPS
A RELATIVELY DEEP SURFACE CYCLONE ACROSS THE MIDWEST/ GREAT LAKES.
THE ECMWF SOLUTION HAS VARIED FROM RUN-TO-RUN, BUT SEEMS TO BE
TRENDING TOWARD A DEEPER SURFACE LOW ACROSS THE MIDWEST ON THU,
MORE LIKE THE GFS/UKMET. THE FORECAST WAS INITIALLY BASED HEAVILY
ON A MULTI-MODEL DETERMINISTIC BLEND DURING DAYS 3-5, INCLUDING
THE 12Z ECMWF/UKMET AND 18Z GFS. GIVEN THE ABOVE CONSIDERATIONS,
THE UKMET WAS WEIGHTED MORE HEAVILY THAN THE ECMWF OR GFS THROUGH
MUCH OF THIS TIME PERIOD.

FROM DAY 6 ONWARD, SPREAD BEGINS TO INCREASE DRAMATICALLY. THIS IS
LIKELY DUE, AT LEAST IN PART, TO THE HIGHER LATITUDE FLOW PATTERN
REORGANIZATION DISCUSSED ABOVE. WHILE MODELS/ENSEMBLES ARE IN
GENERAL AGREEMENT ON THE DESCRIBED PATTERN EVOLUTION, ANY LARGE
SCALE DIFFERENCES AT THAT TIME SCALE CAN RESULT IN QUITE
SIGNIFICANT SMALLER SCALE DIFFERENCES. MODELS SUGGEST THAT ANOTHER
ROUND OF SURFACE CYCLOGENESIS IS POSSIBLE ALONG/OFF THE EAST COAST
ON FRI, BUT SOLUTIONS REMAIN WIDELY VARYING WITH RESPECT TO THE
SPECIFICS OF ANY POTENTIAL LOW PRESSURE SYSTEM. FARTHER WEST,
CONFIDENCE IS A BIT HIGHER THAT ADDITIONAL UPPER-LEVEL ENERGY WILL
RAPIDLY AMPLIFY  AS IT TRAVERSES THE EASTERN PERIPHERY OF THE
UPPER RIDGE, ALONG THE WEST COAST FROM THU NIGHT ONWARD. SOME
SOLUTIONS SHOW HEIGHTS QUICKLY FALLING ACROSS THE NORTHERN
INTERMOUNTAIN WEST/GREAT BASIN BY FRI NIGHT/SAT, WHILE OTHERS KEEP
A MORE CONSOLIDATED UPPER LOW JUST OFF THE WEST COAST, WITH LESS
MOVEMENT INLAND. 500 HPA ENSEMBLE SPAGHETTI PLOTS SHOW A VERY
LARGE AMOUNT OF SPREAD ALONG THE WEST COAST BY DAY 7, SO PREFERRED
TO LEAN HEAVILY ON ENSEMBLE MEANS VERY LATE IN THE PERIOD. THUS,
THE WPC FORECAST SHOWED AN INCREASE IN ENSEMBLE MEAN (12Z
ECENS/NAEFS) WEIGHTING ON DAY 6, WITH DETERMINISTIC SOLUTIONS
ELIMINATED ENTIRELY BY DAY 7.


...WEATHER HIGHLIGHTS/THREATS...

PERSISTENT TROUGHING ALONG THE WEST COAST WILL KEEP CONDITIONS
UNSETTLED THERE, WITH THE THREAT FOR HEAVY PRECIPITATION
INCREASING BY MID TO LATE NEXT WEEK AS THE NEXT TROUGH/UPPER LOW
DIGS ALONG THE WEST COAST. HEAVY PRECIPITATION IS POSSIBLE FROM
THE CASCADES SOUTHWARD INTO NORTHERN & CENTRAL CA IN THE WAKE OF A
COLD FRONT THAT PUSHES INLAND.  PERSISTENT TROUGHING AND
PRECIPITATION WILL KEEP TEMPERATURES BELOW AVERAGE ACROSS MUCH OF
THE WEST THROUGH NEXT WEEK.

WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY, TN VALLEY,
AND PERHAPS THE SOUTHEAST/OHIO VALLEY FROM LATE TUE THROUGH THU
NIGHT AS MOIST LOW-LEVEL SOUTHERLY FLOW WITHIN A CYCLONE`S WARM
CONVEYOR BELT SPREADS AMPLE MOISTURE NORTHWARD OVER A RETURNING
WARM FRONT. FARTHER NORTH, A LOW PRESSURE SYSTEM ON
WEDNESDAY/THURSDAY TRACKING ACROSS THE CENTRAL
PLAINS/MIDWEST/GREAT LAKES WOULD BRING CHANCES OF SNOWFALL TO
PORTIONS OF THOSE REGIONS, ALTHOUGH CONFIDENCE IS LOW IN THE
SPECIFICS AT THIS POINT. HEAVY RAIN WILL ALSO BE POSSIBLE ACROSS
PORTIONS OF THE MID-ATLANTIC OR NORTHEAST THU-FRI, WITH SNOW
POSSIBLE FOR NEW ENGLAND. CONFIDENCE IS LOW AS TO EXACTLY WHERE
THE HEAVIEST PRECIP WOULD FALL, HOWEVER, WHICH WILL BE DEPENDENT
ON THE SPECIFICS OF ANY SURFACE CYCLOGENESIS THAT OCCURS AS THE
SYSTEM CROSSES THOSE REGIONS. BROAD POSITIVE HEIGHT ANOMALIES WILL
KEEP TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN
U.S., WITH HIGHS REACHING DOUBLE DIGITS ABOVE AVERAGE ACROSS
PORTIONS OF THE MID-MS VALLEY/MIDWEST BY TUESDAY. COOLER
TEMPERATURES WILL PREVAIL ACROSS MUCH OF THE CENTRAL/EASTERN U.S.
BY LATE NEXT WEEK IN THE WAKE OF A SURFACE COLD FRONT.

RYAN

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