Northern Rockies Heavy Snow: 9-10 November 2012

While the EC storm at the end of the week exits, a new one will be taking shape over the northern Rockies and into the northern Plains. The Billings, MT NWS Forecast Office highlighted the storm for the end of the week in their Area Forecast Discussion from last night.

000

FXUS65 KBYZ 051047

AFDBYZ

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE BILLINGS MT

347 AM MST MON NOV 5 2012

.LONG TERM...VALID FOR WED...THU...FRI...SAT...SUN...

CONFIDENCE ABOVE NORMAL THAT A SHARP CHANGE TO WINTRY WEATHER IS

IN STORE FOR THE FORECAST AREA. CONFIDENCE IN DETAILS ARE STILL

AVERAGE...INCLUDING HOW MUCH SNOWFALL CAN BE EXPECTED AND JUST

HOW COLD TEMPERATURES WILL TURN BY THE WEEKEND.

WEDNESDAY WILL LIKELY BE THE WARMEST DAY OF THE WEEK FOR MANY

LOCATIONS WITH DOWNSLOPE WIND AND PREFRONTAL WARMING. THE CAVEAT

BEING INCREASING CLOUDS WHICH MAY LIMIT WARMING A BIT. SW FLOW

ALOFT WILL BEGIN TO PUSH MOISTURE INTO THE FORECAST AREA WITH A

FEW SHOWERS POSSIBLE. WE DO HAVE THE FIRST PUSH OF COLDER AIR

SLIDING INTO THE FORECAST AREA FROM THE NORTH WEDNESDAY NIGHT

INTO THURSDAY. THIS WILL CREATE AN OVERRUNNING PATTERN IN THE WEST

FOR SCATTERED RAIN AND SNOW SHOWERS. BY THURSDAY NIGHT A STRONG

UPPER JET WILL NOSE INTO SOUTHWEST MONTANA PLACING THE LEFT FRONT

QUADRANT OVER OUR WESTERN ZONES. THIS WILL INDUCE STRONG WARM AIR

ADVECTION OVER OUR WESTERN ZONES AND AIDED BY THE OVERRUNNING

VERTICAL PATTERN SHOULD RESULT IN SUBSTANTIAL LIFT. THUS PERIODS

OF HEAVY SNOW ARE EXPECTED IN THE WEST LATE THURSDAY INTO THURSDAY

NIGHT. THIS INCLUDES PARK...WHEATLAND AND SWEET GRASS COUNTIES. ON

FRIDAY THE FOCUS OF THE BEST DYNAMICS AND LIFT SHIFTS EAST INTO

THE CENTRAL ZONES INCLUDING BILLINGS. AS THE TROUGH PUSHES THROUGH

THE ROCKIES ANOTHER SURGE OF COLD AIR...THIS TIME ARCTIC IN

NATURE...WILL PUSH DOWN THE FRONT RANGE AND INTO OUR FORECAST

AREA. PRESSURE RISE AND GRADIENTS ASSOCIATED WITH THIS FRONT

SUGGEST A SIGNIFICANT POTENTIAL FOR BLOWING SNOW AND NOTABLE WIND

CHILLS. SO AT THIS TIME...THE WORST WINTER CONDITIONS FOR BILLINGS

ARE POINTING TO LATE FRIDAY THROUGH FRIDAY NIGHT. THE EMPHASIS OF

THE STORM TURNS TO NE MONTANA AS WE GO INTO THE WEEKEND. THERE IS

POTENTIAL FOR A TROWAL FEATURE TO IMPACT FALLON COUNTY SATURDAY

CREATING BLOWING SNOW AND SIGNIFICANTLY REDUCED VISIBILITY...BUT

CONFIDENCE IN THIS REMAINS LOW AT THIS TIME.

SO THE QUESTION WE OFTEN WILL GET NOW IS HOW MUCH SNOW AND HOW

COLD WILL IT GET. QPF FIELDS SUGGEST 6 TO 18 INCHES OF SNOW IN

THE HIGH COUNTRY...AND POSSIBLY NARROW BANDS OF 6 TO 12 INCHES

POSSIBLE AT LOWER ELEVATIONS FROM BILLINGS WEST. CIPS ANALOG

GUIDANCE SHOWS MEAN SNOWFALL HITTING THE BEARTOOTHS WITH OVER A

FOOT OF SNOW FALL AND NEARBY AREAS FROM FROM THE FOOTHILLS TO

JUDITH GAP AND BILLINGS WITH AS MUCH AS 4 TO 8 INCHES. RAW MODEL

GUIDANCE SUGGEST HIGHS BY SATURDAY IN THE TEENS ON THE

PLAINS...WITH A WARM UP BEGINNING SUNDAY INTO MONDAY. PLEASE NOTE

THESE NUMBERS ARE MEANT TO GIVE SOME IDEA OF THE POTENTIAL FOR

THIS SYSTEM...BUT UNCERTAINTY REMAINS AS WE ARE STILL FOUR OR FIVE

DAYS OUT...AND EVEN A MODEST VARIATION IN THE POSITION OF THE JET

STREAM CAN CHANGE THE EXPECTATIONS AND PLACEMENT OF HEAVIEST

SNOWFALL A GREAT DEAL. BT

&&

A couple GFS forecast maps valid on 20121109/1200V120 are located below. They show a strong early-season system across the northern Rockies with plenty of cold air across southern and southwestern Canada.

The analog impact guidance from the GFS 20121104/1200F120 forecast shows the potential for a high-impact event for the northern Rockies. Across the higher terrain of southern Montana, mean COOP snow values exceeding 12" and high probabilities of 12" or more of snow are shown where about a third of the top analogs have snowfall in the mountains exceeding 12".

One of the most similar analogs with respect to the low-level thermal fields, 4 December 1998, is shown below.

During this historical event, more than a foot of snow was common in the higher elevations with snowfall extending into northeastern MT as well.

The impact guidance suggests that strong winds may not be a concern, but it is important to note that the GFS forecast has a stronger system in low levels than the majority of the analogs.  There are low probabilities of wind gusts in the area exceeding 30 kts.  This is supported by one of the more similar analogs with respect to the strength of the system and low-level wind fields, 5 October 2007, where gusts are in the 20-30 kts range.

The analog run that this blog post was written from is available here.