Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 240615
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
214 AM EDT MON JUL 24 2017

VALID 12Z THU JUL 27 2017 - 12Z MON JUL 31 2017

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

THE FORECAST CONCLUDING THE MONTH OF JULY WILL KEEP AN UPPER RIDGE
IN PLACE ACROSS THE DESERT SOUTHWEST TOWARD THE SOUTHERN/CENTRAL
HIGH PLAINS. THIS HAS SEEMINGLY BEEN ITS PREFERRED LOCATION FOR A
LARGE PORTION OF THE SUMMER. TO THE NORTH...A SINUSOIDAL WAVE
PATTERN WILL BE FEATURED WITH A BROAD LONGWAVE TROUGH EXTENDING
OVER THE NORTHEASTERN U.S. WHILE THE STRONGEST HEIGHT FALLS SHOULD
RESIDE NORTH OF THE U.S./CANADIAN BORDER...MODELS CONTINUE TO
SUGGEST TRAILING SHORTWAVE ENERGY WILL LIKELY SPIN DOWN FROM THE
UPPER MIDWEST AND POSSIBLY CUT OFF ACROSS THE OH VALLEY/CENTRAL
APPALACHIANS/MID-ATLANTIC THIS WEEKEND. THIS IDEA WAS BEING HINTED
AT YESTERDAY BY MANY OF THE GLOBAL ENSEMBLE MEMBERS. BUT IT
APPEARS A MAJORITY OF THE AVAILABLE MODEL PRODUCTION SUITE
SUPPORTS THIS ASPECT OF THE FORECAST. THE ACTUAL EVOLUTION VARIES
CONSIDERABLY AMONG THE GUIDANCE AS WELL AS BETWEEN THEIR
RESPECTIVE RUNS. FOR EXAMPLE...THE 18Z/12Z GFS WERE SLOWER TO
EJECT THE FRONTAL ZONE/ATTENDANT SURFACE WAVES BUT THE LATEST 00Z
CYCLE MOVES IT FARTHER OFFSHORE...SIMILAR TO THE 12Z ECMWF.
LIKEWISE...THE 00Z CMC MADE A SIMILAR ADJUSTMENT TO ITS FORECAST.
ELSEWHERE...A MEAN UPPER TROUGH SHOULD INHABIT THE NORTHEASTERN
PACIFIC WITH A SERIES OF VORTICITY CENTERS SPINNING OFF TOWARD THE
PACIFIC NORTHWEST/BRITISH COLUMBIA. GIVEN THE PRESENCE OF
DOWNSTREAM RIDGING...MANY OF THESE IMPULSES SHOULD LIFT INTO
WESTERN CANADA WHILE SHEARING IN THE PROCESS.

GIVEN DECENT MODEL AGREEMENT BETWEEN THE 18Z GFS/12Z ECMWF THROUGH
DAY 5/JULY 29...WAS ABLE TO UTILIZE MUCH OF THEIR FORECASTS WITH
SOME INCLUSION OF THEIR RESPECTIVE ENSEMBLE MEANS AS WELL AS THE
12Z NAEFS MEAN. BEYOND THIS PERIOD...DIFFERENCES IN THE HANDLING
OF THE UPPER TROUGH PINCHING OFF OVER THE EASTERN U.S. LED TO
FURTHER INCLUSION OF ENSEMBLES THEREAFTER. OF NOTE...THE SHIFT IN
THE 00Z GFS SEEMS TO PLACE ITS FRONTAL FEATURES VERY NEAR THE
MANUAL FRONTS/PRESSURES FORECAST SENT EARLIER. TOWARD THE END OF
THE PERIOD...WAS STILL ABLE TO HOLD ON TO 30 PERCENT OF THE
DETERMINISTIC 18Z GFS/12Z ECMWF GIVEN SIMILAR PORTRAYAL OF THE
LARGE-SCALE PATTERN.


...WEATHER HIGHLIGHTS...

WHILE THE HOT SPOT IN THE FORECAST SHOULD BE ACROSS THE DESERT
SOUTHWEST WITH AFTERNOON HIGHS EXCEEDING 110 DEGREES...THE MOST
ANOMALOUS WARMTH WILL BE ACROSS THE PACIFIC NORTHWEST INTO THE
UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. EXPECTED HIGHS SHOULD
BE AT LEAST 10 DEGREES ABOVE AVERAGE AS HIGHS SOAR WELL INTO THE
90S OVER INLAND LOCATIONS WHILE READINGS CLOSER TO THE 80S ARE
LIKELY ALONG AND WEST OF I-5. ACROSS THE EASTERN U.S...SOME AREAS
WILL BE AT OR SLIGHTLY BELOW CLIMATOLOGY GIVEN THE PRESENCE OF THE
UPPER TROUGH AND ASSOCIATED RAINFALL CHANCES.

GIVEN THE PRESENCE OF THE UPPER RIDGE ACROSS THE FOUR CORNERS
REGION...AMPLE MONSOONAL MOISTURE WILL CONTINUE TO CHURN ITS WAY
NORTHWARD INTO THE DESERT SOUTHWEST. ADDITIONALLY...ANY MOISTURE
EMERGING FROM DECAYING TROPICAL CYCLONES IN THE EASTERN PACIFIC
WILL NEED TO BE ACCOUNTED FOR AS WELL. AREAS OF THE FOUR CORNERS
REGION SHOULD BE QUITE WET WITH CONVECTION GRADUALLY SLIDING
EASTWARD THROUGH THE HIGHER TERRAIN AND EVENTUALLY INTO THE
ADJACENT GREAT PLAINS. LOOKING TO THE EASTERN HALF OF THE
NATION...IT SHOULD REMAIN UNSETTLED ALONG THE FRONTAL ZONE WHICH
MAY BE SLOW MOVING AT TIMES GIVEN THE LINGERING UPPER TROUGH.


RUBIN-OSTER



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