All Weather News

Thunderstorms, Tornadoes, Ice Storms, Avalanches

6 Jan 2009, 5:47 pm
HAMweather Advisory Center
HAMweather Advisory Center

HAMweather Radar Center Southeastern U.S.
HAMweather Radar Center Southeastern U.S.
HAMweather Advisory Center
HAMweather Advisory Center
HAMweather Advisory Center
HAMweather Advisory Center

Snow in Chehalis, WA credit: Jude3 - komonews.com
Snow in Chehalis, WA credit: Jude3 - komonews.com

The thunderstorm threat we discussed yesterday has evolved in the deep south, with a tornado watch that extends from southeastern Mississippi through the midlands of Alabama to the northeastern extent of the state. Shear exhibits the same trends as discussed, where vertical profiles favor linear formations with embedded supercells and the possibility of tornadoes due to low level shear even with the marginal thermodynamics. The threat will continue through the evening as the line continues to move east, with the additional threat of flash flooding.

At the time of this writing, our Advisory Center (note top left) shows flash flood warnings, areal flood warnings, and flash flood watches for most of Mississippi and Alabama where rainfall totals have averaged between 2-4″ with locally higher amounts. Reports of roadways becoming impassable have already surfaced, please do not try to drive through them if you are in the area.

The mixed precipitation of freezing rain / sleet we discussed yesterday is continuing on its forecast trek through the northeast. Freezing rain advisories and ice storm warnings have popped up in the forecast zones. A sample ice storm warning appears as follows:

–[snip]–
AN ICE STORM WARNING REMAINS IN EFFECT UNTIL 1 PM EST WEDNESDAY….

WINTRY PRECIPITATION WILL CONTINUE ACROSS THE REGION THROUGH THE AFTERNOON HOURS. FOLLOWING A BRIEF PERIOD OF SNOW AND SLEET. A PROLONGED PERIOD OF SLEET WILL OCCUR DURING THE VERY LATE AFTERNOON AND EVENING HOURS BEFORE GRADUALLY MIXING WITH AND CHANGING TO FREEZING RAIN LATER TONIGHT. A SIGNIFICANT
ACCUMULATION OF ICE BETWEEN ONE QUARTER AND ONE HALF OF AN INCH IS LIKELY OVERNIGHT…
–[snip]–

Heavy snows will continue for the northern Rockies over the next few days as Pacific moisture continues to funnel into the region. As mentioned yesterday snow levels are at higher levels (6k or so), however it is very likely that accumulations will surge past the foot mark, approaching two feet. Previous accumulations and high freezing levels have cause avalanche warnings to be issued for much of the pacific northwest, with the unusual use of the word “many” in the warning below:


–[snip]–
…AVALANCHE WARNING FOR EXTREME AVALANCHE DANGER TUESDAY AFTERNOON THROUGH WEDNESDAY AFTERNOON…

INCREASINGLY HEAVY SNOW OR RAIN RISING FREEZING LEVELS AND
INCREASING WINDS SHOULD SPREAD TO MOST AREAS LATER TUESDAY MORNING AND AFTERNOON. THIS SHOULD BE FOLLOWED BY HEAVY TO VERY HEAVY RAIN OR SNOW RELATIVELY HIGH FREEZING LEVELS AND STRONG WINDS TUESDAY NIGHT AND WEDNESDAY. SUCH WEATHER SHOULD SEVERELY STRESS THE 20 TO OVER 50 INCHES OF RECENT LOWER DENSITY AND GENERALLY WEAKER SNOW RECEIVED SINCE THE LAST MAJOR WARMUP ON DEC 31ST. CURRENTLY UNSTABLE SNOWPACK CONDITIONS SHOULD WORSEN CONSIDERABLY OVER THE NEXT 36
HOURS WITH EXTREME AVALANCHE DANGER DEVELOPING ABOVE ABOUT 4 TO 5000 FEET AND GENERALLY HIGH DANGER CONTINUING OR INCREASING AT LOWER ELEVATIONS.

MANY NATURAL OR HUMAN TRIGGERED SLIDES SHOULD BECOME CERTAIN TUESDAY AFTERNOON THROUGH WEDNESDAY ALONG WITH AN INCREASING POTENTIAL FOR SOME LARGE DESTRUCTIVE AVALANCHES. SUCH AVALANCHES MAY INVOLVE MOST OF THIS SEASONS SNOWPACK AND RANGE UP TO 6 TO 10 FEET OR MORE. ALTHOUGH THE GREATEST LIKELIHOOD OF CLIMAX AVALANCHES RELEASING TO THE GROUND IS EXPECTED ALONG THE CASCADE EAST SLOPES AND THE NORTHEAST WASHINGTON CASCADES THE OVERWHELMING AMOUNT OF STRESS ON
BUREID WEAK LAYERS SHOULD PRODUCE VERY DANGEROUS CONDITIONS AND RELATIVELY LARGE AVALANCHES IN A VARIETY OF LOCATIONS.

AS A RESULT BACK COUNTRY TRAVEL IN AVALANCHE TERRAIN SHOULD BE AVOIDED TUESDAY AFTERNON THROUGH WEDNESDAY EVENING AND TRAVELERS SHOULD CONFINE TRAVEL TO RELATIVELY FLAT TERRAIN WELL AWAY FROM AVALANCHE PATH RUNOUTS.

NOTE THAT NO CONTROL WORK IS PERFORMED OUTSIDE OF HIGHWAY CORRIDORS AND SKI AREA BOUNDARIES SO EXPECT BACK COUNTRY CONDITIONS WHEN LEAVING SUCH AREAS. THIS STATEMENT WILL BE UPDATED AS CONDITIONS WARRANT. PLEASE VISIT WWW.NWAC.US FOR DETAILS.

–[snip]–


Please be sure to view our Advisory and Radar Centers for the latest updates and developments.

As always stay tuned to your favorite weather outlet, stay informed, and stay safe!

cheers,

–patrick

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