Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3
000
FXUS02 KWBC 200657
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
156 AM EST TUE FEB 20 2018

VALID 12Z FRI FEB 23 2018 - 12Z TUE FEB 27 2018

...CLASHING AIR MASSES SET STAGE FOR AN ARKLATEX TO TN/OH VALLEYS
HEAVY RAINFALL THREAT...
...ARCTIC AIR DROPS THROUGH THE WEST/CENTRAL U.S. WITH RECORD
WARMTH ACROSS THE EAST...


...PATTERN OVERVIEW...

A BROAD UPPER RIDGE INITIALLY EXTENDING FROM THE GULF OF MX INTO
THE SOUTHWESTERN ATLANTIC WILL CONTINUE TO ERODE AS A SERIES OF
UPSTREAM SYSTEMS EFFECTIVELY LOWER HEIGHTS OVER THE EASTERN U.S. A
BLOCKING PATTERN IS IN PLACE OVER THE EASTERN PACIFIC WITH A STOUT
UPPER RIDGE REMAINING FIXED OVER THE REGION. THIS RESULTS IN A
STEADY STREAM OF SOUTHWARD PROPAGATING SHORTWAVES WHICH RACE DOWN
THE WESTERN COAST OF NORTH AMERICA AND EVENTUALLY INTO THE
WESTERN/CENTRAL STATES. THE INITIAL SYSTEM MOVING FROM THE DESERT
SOUTHWEST TOWARD THE GREAT PLAINS ON FRIDAY/SATURDAY SHOULD BE A
PATTERN CHANGER AS IT WILL FINALLY SHIFT A QUASI-STATIONARY
BOUNDARY ACROSS THE ARKLATEX/OZARKS REGION DOWNSTREAM. THIS COLD
FRONT IS EXPECTED TO EXIT THE EASTERN SEABOARD BY LATE SUNDAY
WHILE THE BOUNDARY STALLS ALONG THE IMMEDIATE GULF COAST INTO
EARLY NEXT WEEK. ELSEWHERE...A PAIR OF ADDITIONAL SYSTEMS LOOM
OVER THE WEST COAST...EACH KEEPING ACTIVE PRECIPITATION OVER THE
REGION ACCOMPANIED BY RATHER LOW EXPECTED SNOW LEVELS.


...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES...

WHILE GLOBAL MODEL DIFFERENCES ARE EVIDENT DURING THE PERIOD...A
PATTERN SHIFT WILL BE UNDERWAY AS THE PERSISTENT SUBTROPICAL RIDGE
OVER THE SOUTHEASTERN U.S. BEGINS TO BUCKLE. CONSIDERING THE
INITIAL BAND OF HEIGHT FALLS EJECTING OUT OF THE DESERT
SOUTHWEST...MULTI-DAY ENSEMBLE SPAGHETTI PLOT COMPARISONS HAVE
SHOWN A MARKED DECREASE IN SPREAD. HOWEVER...THERE IS PLENTY OF
NOISE IN THE PLOTS INTO THE WEEKEND WITH EMERGING TIMING/AMPLITUDE
DIFFERENCES. ABUNDANT MOVEMENT IS NOTED AMONG THE 00Z GUIDANCE
WHICH HAS DECREASED THE SPREAD FROM THE FORMER CYCLE. BY
24/1200Z...THE PREVIOUSLY OUTLYING 12Z CMC/UKMET HAVE MADE
QUICKER/SLOWER ADJUSTMENTS...RESPECTIVELY...TO BRING BETTER MODEL
CLUSTERING TO THE TABLE. HOWEVER...FURTHER SHORTWAVE DETAIL
DIFFERENCES FURTHER COMPLICATE THE FORECAST OVER THE OH
VALLEY/GREAT LAKES REGION. DEPENDING ON THE AMPLITUDE OF THIS
FEATURE...A RATHER DEVELOPED CYCLONE MAY CROSS THE OH VALLEY/GREAT
LAKES ON SUNDAY MORNING. ENSEMBLE LOW PLOTS INDICATE QUITE A BIT
OF SCATTER WITH OPERATIONAL RUNS SHOWING VERY LITTLE CONSISTENCY
IN PLACEMENT AND INTENSITY. MOST NOTABLY...THE 00Z GFS IS NOW THE
DEEPEST SOLUTION FAVORING A 984-DM LOW OVER LAKE MI ON 25/1200Z.
FURTHER...THE 12Z ECMWF APPEARED TO BE AN OUTLIER OF SORTS AS IT
SITS WELL EAST OF ITS ENSEMBLE MEAN WITH ONLY ABOUT 5 OF ITS 50
MEMBERS IN THE NEIGHBORHOOD. EVENTUALLY THIS ENTIRE TROUGH WILL
SWEEP OFFSHORE WITH THE FRONTAL ZONE EXITING BY ROUGHLY SUNDAY
EVENING.

LOOKING FARTHER UPSTREAM...OPERATIONAL MODELS CONTINUE TO STRUGGLE
WITH HEIGHT FALLS SWINGING THROUGH THE PACIFIC NORTHWEST ON SUNDAY
MORNING. AS WAS THE CASE WITH THE PRECEDING SYSTEM...THE 00Z
CMC/UKMET HAVE MADE POSITIVE ADJUSTMENTS WITH NOTABLE SLOWING
TRENDS. THIS AT LEAST BRINGS THE CORE OF LOWER HEIGHTS BACK INTO
THE PACIFIC NORTHWEST ON 25/1200Z. ENSEMBLE MEMBERS ARE RATHER
SCATTERED ABOUT SO COULD SEE FUTURE ADJUSTMENTS IN LATER MODEL
RUNS. THIS SHORTWAVE SHOULD EJECT EASTWARD INTO THE NORTH-CENTRAL
U.S. BY EARLY MONDAY WHICH MAY SPIN UP ANOTHER AREA OF LOW
PRESSURE. THE 00Z GFS/12Z ECMWF AGREE ON A WAVE OVER IA ON
26/1200Z BUT PREVIOUS RUNS HAVE NOT BEEN CONSISTENT AS THE GFS HAS
BEEN PLACING EMPHASIS ON A SYSTEM OVER SOUTH-CENTRAL CANADA
RATHER. ELSEWHERE...AN EVEN MORE IMPRESSIVE UPPER LOW TAKES AIM ON
THE NORTHERN CA/OR COAST ON TUESDAY WITH 500-MB HEIGHT DEPARTURES
POSSIBLY EXCEEDING 2 TO 2.5 STANDARD DEVIATIONS. ACCOMPANYING THIS
SYSTEM WOULD BE RATHER COLD 1000-500 MB THICKNESSES...POTENTIALLY
BELOW 530-DM. WHILE OPERATIONAL MODELS AGREE ON THE FEATURE...THEY
VARY WITH PLACEMENT AND WOULD EXPECT FUTURE CHANGES GIVEN IT IS A
DAY 6/7 FORECAST.

INITIALLY LEANED ON A MULTI-OPERATIONAL MODEL CONSENSUS OF THE 18Z
GFS/12Z ECMWF WITH THE 12Z UKMET FOR DAY 3/FRIDAY BEFORE SLOWLY
ADDING ENSEMBLE MEANS TO THE PICTURE. TRIED TO LOWER CONTRIBUTIONS
FROM THE 12Z ECMWF ON DAY 5/SUNDAY GIVEN THE QUICKER OH
VALLEY/LOWER GREAT LAKES WAVE. EVENTUALLY MOVED TOWARD A FULLY
ENSEMBLE-BASED APPROACH INTO NEXT WEEK GIVEN GROWING DIFFERENCES
ACROSS THE BOARD. AN EVEN SPLIT OF THE 18Z GEFS/12Z ECMWF ENSEMBLE
MEANS WAS UTILIZED.


...WEATHER HIGHLIGHTS/THREATS...

THE BIG STORY WILL REMAIN THE HEAVY RAINFALL EXPECTED FROM THE
ARKLATEX UP THROUGH THE OZARKS AND INTO THE OH VALLEY. WITH
POSSIBLY 4 TO 6 INCHES OF RAIN POSSIBLE BEFORE THE START OF THIS
PERIOD...ANY ADDITIONAL PRECIPITATION MAY EXACERBATE FLOODING
ISSUES. WHILE AMOUNTS ARE TAME COMPARED TO THAT DURING THE SHORT
RANGE...RIVERS WILL LIKELY BE AT VARIOUS FLOOD STAGES WHICH MAY
POSE HAZARDOUS CONDITIONS TO SURROUNDING LOCATIONS. REGARDING
EXPECTED RAINFALL...THE HEAVIER AMOUNTS SHOULD BE SHIFTING AWAY
FROM THE ARKLATEX AND MOVING INTO THE OH/TN VALLEYS WHERE SOME OF
THE BETTER DYNAMICS WILL RESIDE. EVENTUALLY THE SYSTEM WILL EXIT
THE EAST COAST WITH SOME TRAILING BAND OF LIGHT/MODERATE RAIN
EXTENDING OVER THE GULF COAST INTO EARLY NEXT WEEK. THERE ARE SOME
HINTS OF ENHANCED ACTIVITY PUSHING TOWARD THE UPPER TX/LA COAST
BUT DETAILS REMAIN NEBULOUS. REGARDING WINTRY PRECIPITATION...A
RIBBON OF SNOW MAY FALL FROM THE UPPER MIDWEST INTO INTERIOR NEW
ENGLAND. OUT WEST...A WET PERIOD IS LIKELY OVER THE PACIFIC
NORTHWEST INTO THE UPPER INTERMOUNTAIN WEST. SNOW LEVELS SHOULD
CONTINUE TO FALL WITH THE LOWERING THICKNESS VALUES. THE EARLIER
MENTIONED UPPER LOW COULD BRING SOME HEAVIER PRECIPITATION TO
AREAS OF NORTHERN CA BY MONDAY WITH SNOW POTENTIALLY HUGGING THE
COAST GIVEN THE MAGNITUDE OF COLD AIR ALOFT.

WITH SPRINGTIME MOISTURE RETURN LIFTING UP FROM THE LOWER MS
VALLEY INTO THE EASTERN U.S...RATHER MILD OVERNIGHT TEMPERATURES
WILL PREVAIL AS FORECAST ANOMALIES RANGE FROM 20 TO 30 DEGREES
ABOVE CLIMATOLOGY. THIS WILL PARTICULARLY BE THE CASE OVER THE
MID-SOUTH INTO THE TN/LOWER VALLEYS AS OVERNIGHT READINGS REMAIN
IN THE UPPER 50S TO LOWER 60S. MANY DAILY TEMPERATURE RECORDS
COULD BE BROKEN GIVEN SUCH WARM OVERNIGHT LOWS EXPECTED.
CONDITIONS WILL REMAIN MILD DURING THE DAYS AS WELL ALTHOUGH
RECORDS SHOULD BE BROKEN MORE ON AN ISOLATED BASIS. ON THE
CONTRARY...COLD WEATHER WILL MAINTAIN ITS STRANGEHOLD OVER THE
WESTERN/CENTRAL STATES. WHILE THE SHARPEST DEPARTURES FROM AVERAGE
ARE FORECAST OVER THE NORTHERN ROCKIES...GENERALLY IN THE 20 TO 30
DEGREE RANGE...THE ENTIRE REGION WILL BE THE CHILLY SIDE. NEAR
FREEZING TEMPERATURES SHOULD CONTINUE TO BE A DAILY OCCURRENCE
EACH MORNING OVER THE PACIFIC NORTHWEST AND EXTENDING INTO VAST
SECTIONS OF CA. WHILE THE MAJOR CITY CENTERS SHOULD STAY ABOVE
FREEZING...MANY OF THE COOLER VALLEY LOCATIONS CAN EXPECT A FREEZE.


RUBIN-OSTER


$$





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