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January 20-21 Snow/Sleet/Ice/Rain event OBS


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Nice updated disco from LWX

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC

903 PM EST FRI JAN 20 2012

.SYNOPSIS...

LOW PRESSURE WILL BRING A WINTRY MIX OF PRECIPITATION TO MID

ATLANTIC REGION OVERNIGHT AND SATURDAY...FOLLOWED BY A COLD FRONT

SATURDAY NIGHT. HIGH PRESSURE WILL BUILD IN FROM THE NORTH SUNDAY

INTO EARLY MONDAY...WITH LOW PRESSURE PASSING TO THE SOUTH. A SECOND

COLD FRONT WILL MOVE ACROSS THE REGION MONDAY NIGHT INTO TUESDAY.

&&

.NEAR TERM /THROUGH SATURDAY/...

SFC HIPRES OVR NY/VT THIS EVNG...XTNDG SWD TO THE DELMARVA.

MEANWHILE...LOPRES IN THE TN VLY SET TO MV NEWD. AMS ACRS CWFA

VERY DRY...SPCLY NRN 2/3RDS. THEREFORE PCPN DVLPG JUST S/SW OF

CWFA /TRIGGERED BY WAA/ WHICH WL BE ADVECTING INTO AREA TNGT WL

PROMPT WET BULB COOLING. SPOTTER REPORTS JUST STARTING TO COME

INTO THE WFO...MOST OF WHICH HV BEEN SNW. /CHO REPORTED PL./ LWX

00Z RAOB BACKS UP THOSE REPORTS...BUT RNK RAOB HINTS AT WHATS TO

COME.

LTST GDNC STILL PTS TO THE SFC LOW PASSING OVR THE XTRM SRN CWFA

/OR JUST TO THE S/. COLD AIR WL BE ABLE TO DRAIN SWD DOWN THE

COAST...ENHANCED BY EVAP OF INIT PCPN. MEANWHILE...H8 WAA WL

RESULT IN A WARM AIR INTRUSION...RESULTING IN PTYPE OF SN...THEN

SN/PL...MIXING WITH AND EVENTUALLY CHGG TO FZRA. N-CENTRL AND

NERN MD REMAIN THE COLDEST...SUGGESTING SNW WL BE MORE PROLONGED

AND ICE MORE LMTD IN THOSE LOCATIONS. LTL CHG FM PRVS ACCUM

FCST...MOST LOCATIONS SEEING AN INCH OR TWO OF SNOW... BUT UP TO 3

INCHES ACRS THE N.

BY 12Z SAT...FZRA WL BE THE MOST PREVALENT PTYPE...W/ TEMPS NEAR

32F /RA/ SRN FRINGE OF CWFA /WAA WORKING TO SFC/ AND PL HANGING

ON ACRS NRN MD AND IN THE NRN SHEN VLY/PTMC HIGHLANDS...THX TO CAD.

WARM AIR WL BE WORKING NW DURING THE DAY SAT WHILE PCPN PUSHES SE.

ICE ACCUMULATIONS ARND A TENTH-INCH...HIGHER NW.

LTST GDNC SUGGESTING SOMEWHERE IN THE NWRN/WRN CWFA MOST AT RISK

FOR BIG ICE ACCUMS. TEMPS/DEWPTS ALNG/W OF THE APPALACHIANS AS

WELL AS THE NOSE OF H8 TEMPS SUGGESTS THE MTN RDGS MOST AT RISK.

WL BE FCSTG UP TO TWO-TENTHS FOR THE NRN SHEN VLY/PTMC HIGHLANDS.

AT THIS POINT...BIGGEST UNCERTAINTY REMAINS HOW MUCH OF EACH PTYPE

WL FALL...AS WELL AS IF ACCRETION WL BE SUFFICIENT. FOR ONE

THING...WRF GDNC /BACKED BY HRRR/ CONSISTENTLY SUGGESTING PCPN

MAY COME IN TWO DISTINCT PDS-- FIRST W/ THE H8 WAA /THRU ABT

08Z/...THEN A RESPITE/LULL THRU DAWN WHEN A PD TRIGGERED BY PVA WL

ARRIVE. WL HOLD ONTO HEADLINES AS IS FOR NOW. QPF LOOKS TO BE

HIEST IN FAR NWRN CORNER FO CWFA. WL CONT TO MONITOR...AND UPGRADE

IF/WHEN REQD.

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