Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 170659
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST WED JAN 17 2018

VALID 12Z SAT JAN 20 2018 - 12Z WED JAN 24 2018

***PATTERN OVERVIEW***

THE MEDIUM RANGE PERIOD WILL BEGIN ON SATURDAY WITH BROAD ZONAL
FLOW ALOFT OVER THE NORTH CENTRAL U.S. AND A LARGE TROUGH OVER THE
WESTERN THIRD OF THE NATION AND A SMALLER SCALE TROUGH NEAR THE
GULF OF MEXICO.  THIS WESTERN U.S. TROUGH WILL INDUCE SURFACE
CYCLOGENESIS OVER THE PLAINS AND BECOME THE DOMINANT WEATHER STORY
DURING THIS FORECAST PERIOD, ESPECIALLY SUNDAY AND INTO MONDAY.
AFTER BRIEF RIDGING ALOFT OVER THE WEST, UNSETTLED CONDITIONS ARE
EXPECTED TO RETURN TO THE WEST COAST AS TWO SEPARATE STORM SYSTEMS
AFFECT THE REGION.


***GUIDANCE ASSESSMENT AND PREFERENCES***

MODELS AND ENSEMBLES ARE IN RELATIVELY GOOD AGREEMENT ON SATURDAY
WITH THE OVERALL SYNOPTIC SCALE PATTERN.  THE 12Z ECMWF BECOMES
SLOWER THAN THE OTHER SOLUTIONS BY SUNDAY WITH THE MAJOR STORM
SYSTEM EXITING THE ROCKIES ON SUNDAY AND THIS TREND BECOMES MORE
NOTEWORTHY GOING INTO MONDAY.  IT IS ALSO WELL TO THE SOUTHWEST OF
ITS 12Z ENSEMBLE MEAN BY THIS TIME, DECREASING CONFIDENCE IN ITS
SOLUTION.  THE ECMWF IS ALSO NOTABLY DIFFERENT BY DEPICTING A
STRONG NORTHERN STREAM SYSTEM OVER SOUTH CENTRAL CANADA TUESDAY
MORNING WHERE THE MODEL CONSENSUS IS FAVORING A SURFACE HIGH.
THERE WAS BETTER CLUSTERING WITH THE CMC, GFS, GEFS MEAN, AND EC
MEAN, SO THESE SOLUTIONS WERE GIVEN THE GREATEST WEIGHTING IN THE
FORECAST PROCESS.  THE UKMET INDICATED MORE UPPER LEVEL RIDGING
OVER THE WESTERN U.S. BY MONDAY IN CONTRAST TO THE MODEL
CONSENSUS, WHICH INDICATES MORE OF A LOW AMPLITUDE TROUGH AT THAT
TIME.

WITH THE DEPARTURE OF THE CENTRAL-EASTERN U.S. STORM SYSTEM, MODEL
SPREAD BECOME MORE SIGNIFICANT OVER THE CENTRAL AND WESTERN PARTS
OF THE NATION AND DECREASING CONFIDENCE IN ANY ONE PARTICULAR
DETERMINISTIC SOLUTION.  THEREFORE A GREATER PERCENTAGE OF THE
GEFS AND EC MEANS WAS INCORPORATED BY DAYS 6 AND 7 FOR PLACEMENT
OF THE MAIN SYNOPTIC FEATURES.


***SENSIBLE WEATHER***

AN ACTIVE WEATHER PATTERN CAN BE EXPECTED FOR THE NATION`S
MIDSECTION BEGINNING ON SUNDAY AS A STRONG SURFACE LOW DEVELOPS
OVER THE CENTRAL AND NORTHERN PLAINS.  THERE WILL BE TWO MAIN
FACETS TO THIS STORM SYSTEM, WITH THE FIRST BEING SCATTERED TO
NUMEROUS SHOWERS AND THUNDERSTORMS FROM EASTERN TEXAS TO THE OHIO
VALLEY AS DEEP GULF OF MEXICO MOISTURE SURGES NORTHBOUND AHEAD OF
THE COLD FRONT.  THE SECOND WILL BE THE LIKELIHOOD OF A
SIGNIFICANT WINTER STORM WITH POSSIBLE BLIZZARD CONDITIONS OVER
PARTS OF THE DAKOTAS, MINNESOTA, AND WISCONSIN WHICH WOULD BE
UNDER THE DEFORMATION ZONE OF THE MATURING CYCLONE.  IN ADDITION
TO THIS, HEAVY RAIN AND COPIOUS MOUNTAIN SNOW WILL REMAIN IN THE
WEATHER HEADLINES FOR THE WESTERN U.S. AS A COUPLE OF PACIFIC
SYSTEMS MAKE THEIR WAY INLAND, WITH A POSSIBLE ATMOSPHERIC RIVER
EVENT ON SUNDAY FOR NORTHERN CALIFORNIA.

THE MOST PRONOUNCED ANOMALIES FOR TEMPERATURES WILL BE IN THE WARM
SECTOR OF THE CENTRAL U.S. STORM SYSTEM.  OVERNIGHT LOWS IN
PARTICULAR MAY BE 15 TO 25 DEGREES ABOVE NORMAL OVER MUCH OF THE
PLAINS AND MIDWEST THIS WEEKEND, AND THEN REACHING THE EAST COAST
STATES BY NEXT MONDAY.  HIGH TEMPERATURES SHOULD NOT BE AS
ANOMALOUS BUT STILL 10 TO 20 DEGREES ABOVE NORMAL OVER SOME AREAS.
 COLDER WEATHER RETURNS TO THE PLAINS AFTER THE COLD FRONT MOVES
THROUGH.

HAMRICK

$$




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