#2466 Postby srainhoutx » Mon Dec 18, 2017 11:12 am
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1102 AM EST MON DEC 18 2017
VALID 12Z THU DEC 21 2017 - 12Z MON DEC 25 2017
...ABOVE AVERAGE CONFIDENCE IN THE POTENTIAL FOR AN OUTBREAK OF
ARCTIC AIR ACROSS THE CENTRAL U.S. BY THE WEEKEND...
...OVERVIEW...
MODELS CONTINUE TO LOCK ON TO A SIGNIFICANT AMPLIFICATION OF THE
FLOW PATTERN THROUGH CHRISTMAS MORNING - RIDGE EXTENDING FROM THE
WEST COAST TO THE ARCTIC AND A MEAN TROUGH FROM THE HUDSON BAY
AREA TO THE DESERT SOUTHWEST/MEXICO. THIS SETUP WILL FAVOR COLD
AIR TO SURGE SOUTHWARD... WHICH WILL CREATE A SHARP THERMAL
GRADIENT ACROSS THE SOUTHERN TIER STATES AS WARM, MOIST AIR LIFTS
NORTHWARD (THANKS TO A STRENGTHENING SUB-TROPICAL RIDGE OFFSHORE
THE SOUTHEAST COAST). THIS BOUNDARY WILL MORE OR LESS REMAIN
ACROSS THE SOUTHERN PLAINS/GULF STATES THROUGH THE EXTENDED PERIOD
AND WILL ACT AS A FOCUS FOR WIDESPREAD PRECIPITATION... WITH
OCCASIONAL WINTER WEATHER NORTH OF THE BOUNDARY AND A LONG
DURATION RAIN ALONG AND SOUTH OF THE FRONT. BRIEF PERIODS OF
HEAVIER RAINFALL MAY BE POSSIBLE. SHORTWAVE ENERGY PASSING THROUGH
SOUTHERN CANADA AND THE NORTHERN U.S. WILL KEEP THE UPPER
MISSISSIPPI VALLEY TO THE NORTHEAST COLD AND SNOWY. NORTHWESTERLY
WINDS WILL LIKELY INDUCE LAKE ENHANCED SNOW FOR THE FAVORED
UPSLOPE REGIONS IN NEW YORK AND MAINE.
...GUIDANCE/PREDICTABILITY ASSESSMENT...
OVERALL THE DETERMINISTIC AND ENSEMBLE MEANS ARE IN STRONG
AGREEMENT THAT A LARGE SCALE CHANGE WILL OCCUR OVER THE NEXT
WEEK...ALTHOUGH DIFFERING IN REGARDS TO THE TIMING AND LOCATIONS
OF INDIVIDUAL SHORTWAVES. THE ECWMF AND THE GFS HAS CONTINUED TO
TREND FASTER WITH THE EASTWARD PROGRESSION OF THE TROUGH. THE CMC
WAS REMAINED SLOW WHICH APPEARED TO BE AN OUTLIER. GUIDANCE BEGAN
AS A BLEND OF THE 00Z ECWMF AND 06 GFS...AS THE TROUGH AMPLIFIES
THE DEGREE OF SPREAD AMONGST GUIDANCE WIDENED AS WELL. FOR
CHRISTMAS EVE AND CHRISTMAS THE BLEND HAD A HEAVIER WEIGHTING OF
THE 06Z ENSEMBLE MEAN AND LESSER WEIGHTING OF THE EC/GFS.
...WEATHER HIGHLIGHTS/THREATS...
A BAND OF SNOW IS FORECAST TO TRACK FROM THE CENTRAL PLAINS TO
THE NORTHEAST AS A LOW PRESSURE PASSING THROUGH. THE HIGHEST
ACCUMULATIONS ARE FAVORING THE UPPER MISSISSIPPI VALLEY/UPPER
GREAT LAKES. SNOW WILL ALSO BE POSSIBLE OVER NEW ENGLAND FRI/SAT
WHERE WARM ADVECTION WILL BE THE LIKELY MECHANISM. THERE REMAINS
SOME SPREAD OVER EXACTLY HOW STRONG THIS SYSTEM WILL BE, AND THUS
FOR PRECISE INTENSITY OF SNOW/WINDS.
THE AREA EXTENDING FROM LOWER MS/TN VALLEYS INTO THE SOUTHERN
APPALACHIANS HAS THE HIGHEST PROBABILITY OF SEEING THE HEAVIEST
RAINFALL DURING THE MEDIUM RANGE. THIS WILL BE FROM A COMBINATION
OF THE COLD FRONT CROSSING THE REGION LATER IN THE WEEK, AND THEN
WITH OVERRUNNING PRECIP NORTH OF THE FRONT THAT SHOULD SETTLE NEAR
THE GULF/CAROLINA COASTS BY THE WEEKEND.
THE CENTRAL U.S. WILL HAVE A SIGNIFICANT DROP IN TEMPERATURES
LATER IN THE WEEK. A VAST AREA WILL HAVE MAXIMUM TEMPERATURES
FALLING FROM 10-20 DEG F ABOVE NORMAL TO 20-30 DEG F BELOW NORMAL
BY NEXT WEEKEND. RELATIVE WARMTH OVER THE EAST WITH SOME ANOMALIES
EXCEEDING +10 DEG F SHOULD EXTEND THROUGH SAT BEFORE BEGINNING TO
FADE BY SUN-MON WITH FRONTAL PASSAGE.
CAMPBELL
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