FXUS65 KSLC 051758 AAA
AFDSLC
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
1058 AM MST SAT DEC 5 2009
.SYNOPSIS...A COLD TROUGH MOVING SOUTH INTO THE AREA TODAY WILL
FORM A CLOSED LOW OVER NORTHERN UTAH TONIGHT. THE LOW SHIFTS EAST
SUNDAY AS ANOTHER COLD TROUGH FORMS OVER NORTHERN CALIFORNIA. THIS
TROUGH WILL CROSS THE GREAT BASIN SUNDAY NIGHT THROUGH TUESDAY.
&&
.DISCUSSION...UPDATED TO ISSUE AN ADVISORY FOR ZONES 2/3 AND
SOUTHWEST WYOMING. LIGHT QPF BUT COLD CONDITIONS SUPPORT HIGHER
THAN TYPICAL SNOW RATIOS. ALSO...SIGNIFICANT ROADWAY ISSUES
EXPECTED AS TEMPERATURES ARE IN THE MID TO UPPER 20S.
WATER VAPOR LOOP SHOWS A WEAK UPPER LOW SINKING SOUTH
FROM IDAHO...WITH A REINFORCING WAVE DROPPING SOUTH ACROSS WESTERN
CANADA. A RIDGE IS AMPLIFYING ACROSS THE EASTERN PACIFIC. 400-250MB
ACARS WIND OBSERVATIONS SHOWS A WESTERLY 80-120KT JET ACROSS THE
GREAT BASIN AND INTO THE CENTRAL ROCKIES. 12Z 700MB ANALYSIS SHOWS A
COLD FRONT DROPPING TOWARD NORTHERN UTAH FROM NORTHERN NEVADA AND
SOUTHERN IDAHO. COLD ADVECTION IS OCCURRING LOCALLY WITH -7C AT KSLC
WITH -11C UPSTREAM AT KLKN. 12Z KSLC RAOB INDICATES 5-10C OF WARMING
BELOW 700MB WITH 3-5C OF COOLING ABOVE 700MB DURING THE PAST 24
HOURS. A VEERING SIGNATURE WAS PRESENT.
GOES/RUC PRECIPITABLE WATER INDICATE VALUES ARE BELOW A TENTH OF AN
INCH ACROSS THE REGION.
COMBINATION OF METAR REPORTS AND WEB CAMS ARE SHOWING ACCUMULATING
SNOWFALL ACROSS THE CACHE VALLEY AND LOGAN SUMMIT THIS MORNING. THE
AMBIENT DRY AIRMASS MADE IT A TOUGH CALL...AND IT APPEARS THAT
DYNAMICAL FORCING IS ENOUGH TO OVERCOME DRY CONDITIONS. HAVE
THEREFORE RAISED POPS SIGNIFICANTLY ACROSS NORTHERN UTAH AND
SOUTHWEST WYOMING TODAY FOR A HIGH PROBABILITY OF A LOW QPF EVENT.
AM CONCERNED ABOUT ROAD CONDITIONS AS EVEN THE MINOR SNOWFALL MAY
ACCUMULATE ON COLD ROADWAYS...WHICH ARE RUNNING IN THE LOWER 20S
THIS MORNING. HAVE ISSUED A SHORT TERM FORECAST TO ADDRESS THIS.
CYCLONE IS EXPECTED TO DEVELOP AND DEEPEN ACROSS EASTERN UTAH
TONIGHT ALONG THE BOUNDARY...INDUCING MOIST NORTHEASTERLY FLOW
ACROSS WYOMING. MAY NEED A SNOW ADVISORY AS SNOW RATIOS WILL BE
QUITE HIGH IN THIS COLD AIRMASS. WILL SEE HOW 12Z GUIDANCE AND
OBSERVATIONAL TRENDS COME TOGETHER. ALSO EXPECTING TO ADDRESS THE
MONDAY-TUESDAY SYSTEM A LOT MORE CLOSELY WITH THE AFTERNOON PACKAGE.
OTHERWISE TWEAKED TODAY'S TEMPERATURES/WINDS/SKY COVERAGE BASED ON
RECENT TRENDS.
PREVIOUS DISCUSSION IS BELOW...
THIS SYSTEM WEAKENS AND LIFTS OUT TO THE NE SUN AS A NEW COLD LOW
DEVELOPS OVER NRN CA. THIS FEATURE THEN PULLS THE LARGE COLD UPPER
TROF WESTWARD SUN NIGHT WHERE IT PHASES WITH A WEAK BUT MOIST LOW
BREAKING UNDER THE RIDGE IN THE GULF OF ALASKA AND CREATES A HEAVY
PRECIP EVENT FOR SRN AND CENTRAL UT MON AND MON NIGHT.
SYSTEM IS NOW FORECAST TO BE A BIT WARMER SO ACCUMULATING SNOW IN
DIXIE IS LESS LIKELY ALTHO SOME MIGHT FALL. HEAVY SNOW IS LIKELY
ELSEWHERE AND WILL FRESHEN UP THE EXISTING SPS. CONFIDENCE IS
INCREASING IN THE LIKELIHOOD OF THIS HAPPENING AND NEXT SHIFT WILL
LIKELY NEED TO ISSUE A WINTER STORM WATCH IF MODELS REMAIN
CONSISTENT.
SYSTEM MOVES EAST TUE THEN GFS AND EC DIVERGE ON HOW STRONG THE
SRN STREAM IS FORECAST TO BE BREAKING UNDER THE ALASKA HIGH AND
HOW FAST A SRN STREAM SHORT WAVE CAN MOVE INTO THE WEST.
HAVE KEPT POPS IN FOR WED IN DEFERENCE TO THE EC BUT BOTH DO HAVE
PRECIP FOR THU INTO FRI.
&&
.AVIATION...CIGS WILL LOWER RAPIDLY FROM THE NORTH LATE THIS MORNING
AT THE KSLC TERMINAL...FALLING BELOW 6000FT AGL BETWEEN 17-19Z.
THERE IS A 80 PERCENT OF LIGHT SNOW AT THE TERMINAL THIS
AFTERNOON...WHICH WOULD RESULT IN LOW-END MVFR TO HIGH-END IFR
CONDITIONS. WINDS ARE EXPECTED TO REMAIN FROM THE NORTH...BUT WILL
LIKELY BE VARIABLE NORTHEAST AT TIMES AS THE CLOUD BANK NEARS THE
TERMINAL FROM THE NORTH MID MORNING.
&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...WINTER WEATHER ADVISORY UNTIL 11 AM SUNDAY FOR UTZ002-UTZ003.
WY...WINTER WEATHER ADVISORY UNTIL 11 AM SUNDAY FOR WYZ021.
&&
$$
PUBLIC...ROGOWSKI/WILENSKY
AVIATION...MERRILL
FOR MORE INFORMATION FROM NOAA/S NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY (ALL LOWER CASE)
