The big west coasts event

[usedtobe]

The massive positive anomaly forecast on the super ensemble mean d+11 map below with the positive near the Bering Straits is favorable for heavy precipitation somewhere from northern CA northward into WA as well as the northern Rockies. Such a massive block usually leads to downstream amplification of the trough over the gulf of ak and if it get deep enough and elongates with a positive tilt can bring lots of moisture into the region. Such a feature is often associated with the development of atmospheric rivers which can lead to really big events. There are a couple of analogs, Dec 1955 and NOvember 1973 which both produced very heavy rainfall as far south as sfo. It's way too early to know whether this will be one of those rare cases or just a plain event where the strong fetch produces above normal precipitation but it;s sure worth watching.

The 1955 analogs is shown below. Note where the positive anomaly is located and compare it to the forecast map above. The big difference so far is in the shape of the trough east and southeast of that feature.

http://www.hpc.ncep....les/Page312.htm

The November 1973 analog is not that good of one as the positive anomaly is not as far north. It will be interesting to watch how the trough develops in response to the hugely negative epo ridge. So much for my foray into the western forum.

[BlizzardWx]

Thanks for your thoughts, post again soon! I hope I can get in on the storm track.

[Cheeznado]

Models showing an extremely strong signal of a dangerous situation setting up for CA and OR- a week or more of strong SW flow and pretty high heights- means high snow levels and good upslope in mtns and foothills. Both Euro and GFS have very high rain totals- here is the GFS for 96 hours ending at 228:

Serious flooding looks like a distinct possibility.

[thewxmann]

Areas in northern California and Oregon could have some issues as it has been pretty wet lately in those areas.

[Srain]

Impressive totals via the 12Z GFS:

[usedtobe]

Impressive totals via the 12Z GFS:

I mentioned potential for a heavy precipitation event in my interesting analog thread that no one looked at or seemed interested in. The D+11 was saying that this had the potential to be a big event from Ca northward to WA.

http://www.americanw...resting-analog/

[Chinook]

Areas in northern California and Oregon could have some issues as it has been pretty wet lately in those areas.

Areas near the coast have already had 5" to 20" in the last 14 days, which really wet. Only relatively high elevations have gotten snow in the PNW. You can check it out at http://water.weather.gov/precip/

[Srain]

I mentioned potential for a heavy precipitation event in my interesting analog thread that no one looked at or seemed interested in. The D+11 was saying that this had the potential to be a big event from Ca northward to WA.

http://www.americanw...resting-analog/

Merged the threads, Wes. Sorry I missed it... It does look like a significant event and I've pinned the thread.

[Srain]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

848 AM EST SUN NOV 25 2012

VALID 12Z THU NOV 29 2012 - 12Z SUN DEC 02 2012

...PROLIFIC PRECIPITATION PATTERN THIS PERIOD FOR THE PACIFIC

NORTHWEST, NORTHERN CALIFORNIA, AND THE NORTHERN INTERMOUNTAIN

REGION...

PRELIMINARY UPDATE...

USED THE 00Z/25 ECENS MEAN AS A BASE TO UPDATE THE PRELIMINARY

FRONTS AND PRESSURES FOR DAYS 3 THROUGH 7, ADDING VALUE NEAR THE

MAJOR LOW PRESSURE SYSTEMS TO DEPICT MORE REALISTIC GRADIENTS. THE

ECENS MEAN HAS BEEN THE STEADIEST PIECE OF GUIDANCE OF THE LOT FOR

MANY DAYS NOW, SO TOOK THE BROADEST SYNOPTIC CUES FROM IT. THE

00Z/25 ECENS AND GEFS MEANS BOTH INDICATED THAT THE WAVE OFF THE

NORTHEAST COAST DAY 3 MIGHT BE CLOSER TO THE MAINE COAST THAN THE

DETERMINISTIC MODELS SUGGEST, SO EXTENDED THE 1016 MB ISOBAR

WESTWARD. OVER THE WEST, THE CONSTANT SOUTHWESTERLY FLOW AND THE

PROXIMITY OF THE MEAN LONGWAVE TROUGH JUST OFFSHORE ENSURES A

STEADY PRODUCTION OF LOWER ELEVATION RAINS AND MOUNTAIN SNOWS FROM

THE NORTHERN HALF OF CALIFORNIA TO WASHINGTON AND EASTWARD TO THE

CONTINENTAL DIVIDE. MANY INCHES OF RAINFALL ARE LIKELY OVER FAR

NORTHERN CALIFORNIA, WITH SNOWFALL IN THE FEET FOR THE CENTRAL AND

NORTHERN SIERRA NEVADA, THE CASCADES, THE BITTERROOTS, AND THE

WYOMING PORTION OF THE ABSAROKA RANGE. OVER THE CENTRAL STATES,

THE DETERMINISTIC IS BULLISH WITH AN OUTBREAK OF SHOWERS DURING

THE PERIOD, WITH THE EUROPEAN CENTRE GUIDANCE MUCH LESS SO. THE

ECENS MEAN INDICATES AT LEAST LIGHT RAINFALL THERE, SO WILL

REFLECT THAT IN THE QPF AND POPS.

CISCO

[usedtobe]

Steve, the pattern with the big positive anomaly near the Aleutian Islands or the Bering strait almost always crushes northern CA and brings very anomalous moisture flux into the region. The NAM is predicting 850 MF anomalies of over 5 SD.

Before I retired I put together this website with helpf from Rich Grumm. It documented the 20 biggest rainfall events since 1950 in greater San Francisco, Sacramento, Monterey region. They almost all had an anomaly pattern like the 500h pattern I showed at the start of this thread. That pattern ended up leading to a strong jet impinging into the area and really anomalous 850 moisture flux. I always thought it was one of the more useful things I did while working that no one ever used much. When you see the 500 pattern showing up consistently for a few days on the CPC D+11 superensemble mean. Look out.

http://www.hpc.ncep....major_rains.htm

[Cheeznado]

The latest Euro precip total just north of SFO thru 180 hours has 10+ inches, about 8 in SFO.

[Srain]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

341 AM EST MON NOV 26 2012

VALID 12Z FRI NOV 30 2012 - 12Z MON DEC 03 2012

===================================================================

...INTENSE WESTERN U.S. RAIN/SNOW STORM LASTING DAYS 3 THROUGH 7...

===================================================================

...SYNOPSIS...

THROUGH THE EXTENDED PERIOD...THE LARGE SCALE PATTERN REMAINS

INTACT...WITH STORES OF COLD AIR AND ASSOCIATED LARGE SCALE

TROUGHING OVER NORTHEASTERN NORTH AMERICA...EASTERN ASIA...AND OFF

THE WEST COAST OF NORTH AMERICA. THIS PATTERN APPEARS RELATIVELY

STABLE...WITH MEDIUM WAVELENGTH RIDGING IN PLACE OVER THE MIDDLE

TO NORTH ATLANTIC...AND FROM THE NORTH POLE TO THE WESTERN

ALEUTIANS. SHORTWAVE ENERGY BOTH TOPPING AND UNDERCUTTING THE

ALEUTIANS RIDGE WILL ROTATE AROUND THE TROUGH ALONG THE WEST COAST

OF NORTH AMERICA...ACTING TO SLOWLY MIGRATE THE TROUGH INLAND.

THIS INCREMENTAL NUDGING OF THE LARGER SCALE TROUGH WILL RESULT IN

AN EXTENDED PERIOD OF HEAVY RAIN AND SNOW ACROSS THE NORTHWEST

QUARTER OF THE U.S...WITH IMPACTS FELT ESPECIALLY IN NORTHERN

CALIFORNIA AND SURROUNDING AREAS.

...MODEL CHOICE...

THE OPERATIONAL RUNS OF THE GFS/ECMWF WERE DEEMED TO BE GOOD IN

THE SHORT RANGE...AND THIS CARRIES THROUGH EARLY DAY 4...THROUGH

WHICH WE USED A NEAR EVEN BLEND OF THE TWO. THE GFS WAS FAVORED

MORE HEAVILY FOR ITS HANDLING OF A DEEPENING LOW OVER EASTERN

CANADA ON DAYS 3-4. AFTER 12Z/30...HOWEVER...WE DROP THE

GFS...WHICH LOSES SIGHT OF AN IMPORTANT SHORTWAVE EJECTING FROM

THE WEST COAST TROUGH. THIS SHORTWAVE WAS ENTERING THE WESTERN

ALEUTIANS THIS MORNING...AND HAD BEEN BETTER AND MORE CONSISTENTLY

HANDLED BY THE ECMWF IN RECENT DAYS. THE WAVE WILL LIKELY DRAG A

PACIFIC FRONT ACROSS THE U.S. THE 00Z ECMWF AND 12Z EC ENSEMBLE

MEAN ARE VERY SIMILAR WITH RESPECT TO THIS FEATURE AND A SECOND

IMPORTANT SHORTWAVE THAT ARRIVES ON THE WEST COAST ON DAYS 6-7. WE

USED A SIMPLE BLEND OF THE EUROPEAN OPERATIONAL AND ENSEMBLE MEAN

FORECASTS TO CONSTRUCT THE DAYS 5-7 SURFACE PRESSURES. WEIGHT

TOWARD THE ENSEMBLE MEAN INCREASES BY DAY 7 TO TONE DOWN THE

NORTHERN ROCKIES LEE LOW INDUCED BY THE DAY 6-7 WAVE. THE

OPERATIONAL RUN DOES APPEAR A BIT OVERLY DEVELOPED WITH THIS WAVE

EARLY IN ITS LIFETIME AS IT DIPS OUT OF THE NORTHERN STREAM IN

CANADA/ALASKA...BUT THE EC ENSEMBLE SUPPORTS THE PRESENCE AND

TIMING OF THE WAVE.

...IMPACTS...

PERSISTENT ONSHORE FLOW AND ORGANIZED DEEP LAYER LIFT BROUGHT ON

BY THE RESIDENT WEST COAST TROUGH WILL LEAD TO MEMORABLE AND

POTENTIALLY HAZARDOUS AMOUNTS OF RAIN AND HIGHER ELEVATION SNOW.

THE PRESENCE OF THE UPPER TROUGH AND ITS CONNECTION TO

SUB-TROPICAL MOISTURE OFFERS PREDICTABILITY IN SPITE OF THE

SHORTWAVE DIFFERENCES...AS THE GFS AND ECMWF SHOW NEARLY IDENTICAL

PRECIPITATION AMOUNTS IN SPITE OF THEIR DIFFERENCES ALOFT. BETWEEN

THURSDAY AND MONDAY BOTH MODELS PRODUCE 10 TO 15 INCHES OR MORE OF

LIQUID EQUIVALENT ACROSS NORTHERN CALIFORNIA... SOUTHWEST

OREGON...AND OTHER SCATTERED HIGH ELEVATIONS OF THE PACIFIC

NORTHWEST AND NORTHERN ROCKIES.

ELSEWHERE...LOW AMPLITUDE WESTERLY FLOW WILL MAINTAIN PRIMARILY

PACIFIC AIR MASSES OVER MUCH OF THE CONUS...WITH ARCTIC AIR

CONFINED TO THE NORTH/NORTHEAST. PRECIPITATION IS FORECAST TO BE

GENERALLY UNREMARKABLE OUTSIDE OF THE PACIFIC STORM...ALTHOUGH

SOME CLIPPER SYSTEM AND LAKE EFFECT SNOWS WILL CONTINUE UP NORTH.

BURKE

[Srain]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

342 AM EST TUE NOV 27 2012

VALID 12Z SAT DEC 01 2012 - 12Z TUE DEC 04 2012

==================================================================

= EXTREME PRECIPITATION AMOUNTS FOR NRN CALIFORNIA INTO SUNDAY =

==================================================================

GENERAL FLOW PATTERN & MODEL PREFERENCE

=======================================

LATE THIS WEEK, THE FLOW ACROSS THE LOWER 48 WILL FLATTEN AS TWO

OUT-OF-PHASE STREAMS OF THE WESTERLIES MOVE ACROSS THE COUNTRY.

THE DEEP TROUGH ACROSS THE EASTERN PACIFIC IS EXPECTED TO FINALLY

MOVE INTO THE WEST COAST SUNDAY AND PROGRESS TO THE MIDDLE OF

NORTH AMERICA TUESDAY MORNING. UPSTREAM, A REBUILDING POSITIVE

ANOMALY OVER THE BERING SEA/SIBERIA WILL ENSURE THAT TROUGHING IN

THE EASTERN PACIFIC RELOADS DURING THE MIDDLE OF NEXT WEEK.

THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE WERE IN GOOD

AGREEMENT EARLY THIS MORNING, WITH ONLY MINOR DETAIL ISSUES NOTED.

TO DEAL WITH THE LINGERING UNCERTAINTY, THE PRESSURES WERE BASED

ON A 40/30/30 BLEND OF THE 00Z UKMET/00Z GFS/00Z ECMWF THROUGH

SUNDAY BEFORE SWITCHING TO A 50/50 COMPROMISE OF THE 00Z ECMWF/00Z

GFS THEREAFTER.

WEATHER IMPACTS

===============

STARTING WEDNESDAY, HEAVY PRECIPITATION WILL BEGIN IN EARNEST

ACROSS NORTHERN CALIFORNIA, ONLY TO INCREASE IN INTENSITY THURSDAY

THROUGH SUNDAY BEFORE WANING THEREAFTER AS THE RESPONSIBLE UPPER

TROUGH MOVES INTO THE WEST AND THE ONSHORE FLOW FROM THE PACIFIC

DECREASES. AREAL AVERAGE LIQUID EQUIVALENT (LEQ) AMOUNTS OF 10-15

INCHES ARE STILL ANTICIPATED FOR THE FAVORED TERRAIN ACROSS

NORTHERN CALIFORNIA, WITH LOCAL LEQ AMOUNTS OF 20+ INCHES

POSSIBLE. THIS WOULD LEAD TO EXTREME SNOW AMOUNTS (POSSIBLY WELL

IN EXCESS OF TEN FEET) ACROSS THE HIGHEST TERRAIN OF THE SISKIYOUS

AND NORTHERN SIERRA NEVADA, WHICH COULD LEAD TO AVALANCHES WELL

AFTER THE SNOW HAS ENDED. LESSER, THOUGH STILL SIGNIFICANT,

PRECIPITATION AMOUNTS ARE EXPECTED ACROSS THE PACIFIC NORTHWEST

(WHERE AREAL AVERAGE LEQ AMOUNTS OF 4-8 INCHES ARE POSSIBLE) AND

NORTHERN CONTINENTAL DIVIDE (WHERE AREAL AVERAGE LEQ AMOUNTS OF

3-5 INCHES ARE ANTICIPATED).

AS THE EASTERN PACIFIC TROUGH MOVES OUT OF THE WEST INTO THE

PLAINS, SOME GULF MOISTURE SHOULD BE TAPPED LEADING TO MODERATE TO

HEAVY RAINS FROM EASTERN TEXAS THROUGH THE MID-MISSISSIPPI VALLEY

INTO THE GREAT LAKES FROM MONDAY INTO NEXT TUESDAY.

ROTH

[usedtobe]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

342 AM EST TUE NOV 27 2012

VALID 12Z SAT DEC 01 2012 - 12Z TUE DEC 04 2012

==================================================================

= EXTREME PRECIPITATION AMOUNTS FOR NRN CALIFORNIA INTO SUNDAY =

==================================================================

GENERAL FLOW PATTERN & MODEL PREFERENCE

=======================================

LATE THIS WEEK, THE FLOW ACROSS THE LOWER 48 WILL FLATTEN AS TWO

OUT-OF-PHASE STREAMS OF THE WESTERLIES MOVE ACROSS THE COUNTRY.

THE DEEP TROUGH ACROSS THE EASTERN PACIFIC IS EXPECTED TO FINALLY

MOVE INTO THE WEST COAST SUNDAY AND PROGRESS TO THE MIDDLE OF

NORTH AMERICA TUESDAY MORNING. UPSTREAM, A REBUILDING POSITIVE

ANOMALY OVER THE BERING SEA/SIBERIA WILL ENSURE THAT TROUGHING IN

THE EASTERN PACIFIC RELOADS DURING THE MIDDLE OF NEXT WEEK.

THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE WERE IN GOOD

AGREEMENT EARLY THIS MORNING, WITH ONLY MINOR DETAIL ISSUES NOTED.

TO DEAL WITH THE LINGERING UNCERTAINTY, THE PRESSURES WERE BASED

ON A 40/30/30 BLEND OF THE 00Z UKMET/00Z GFS/00Z ECMWF THROUGH

SUNDAY BEFORE SWITCHING TO A 50/50 COMPROMISE OF THE 00Z ECMWF/00Z

GFS THEREAFTER.

WEATHER IMPACTS

===============

STARTING WEDNESDAY, HEAVY PRECIPITATION WILL BEGIN IN EARNEST

ACROSS NORTHERN CALIFORNIA, ONLY TO INCREASE IN INTENSITY THURSDAY

THROUGH SUNDAY BEFORE WANING THEREAFTER AS THE RESPONSIBLE UPPER

TROUGH MOVES INTO THE WEST AND THE ONSHORE FLOW FROM THE PACIFIC

DECREASES. AREAL AVERAGE LIQUID EQUIVALENT (LEQ) AMOUNTS OF 10-15

INCHES ARE STILL ANTICIPATED FOR THE FAVORED TERRAIN ACROSS

NORTHERN CALIFORNIA, WITH LOCAL LEQ AMOUNTS OF 20+ INCHES

POSSIBLE. THIS WOULD LEAD TO EXTREME SNOW AMOUNTS (POSSIBLY WELL

IN EXCESS OF TEN FEET) ACROSS THE HIGHEST TERRAIN OF THE SISKIYOUS

AND NORTHERN SIERRA NEVADA, WHICH COULD LEAD TO AVALANCHES WELL

AFTER THE SNOW HAS ENDED. LESSER, THOUGH STILL SIGNIFICANT,

PRECIPITATION AMOUNTS ARE EXPECTED ACROSS THE PACIFIC NORTHWEST

(WHERE AREAL AVERAGE LEQ AMOUNTS OF 4-8 INCHES ARE POSSIBLE) AND

NORTHERN CONTINENTAL DIVIDE (WHERE AREAL AVERAGE LEQ AMOUNTS OF

3-5 INCHES ARE ANTICIPATED).

AS THE EASTERN PACIFIC TROUGH MOVES OUT OF THE WEST INTO THE

PLAINS, SOME GULF MOISTURE SHOULD BE TAPPED LEADING TO MODERATE TO

HEAVY RAINS FROM EASTERN TEXAS THROUGH THE MID-MISSISSIPPI VALLEY

INTO THE GREAT LAKES FROM MONDAY INTO NEXT TUESDAY.

ROTH

The event is starting to look more and more like some of the big analog events, 1955, 1964 though none except Dec 1995 were this early in the season if I remember correctly. Usually the early season biggies are further north.

[donsutherland1]

The event is starting to look more and more like some of the big analog events, 1955, 1964 though none except Dec 1995 were this early in the season if I remember correctly. Usually the early season biggies are further north.

Great thread, Wes. It certainly appears impressive, especially so early in the season.

[usedtobe]

Great thread, Wes. It certainly appears impressive, especially so early in the season.

My memory let me down. Two of the 20 biggest events were during November (1973, 1950), and 5 were during Dec (1964, 1955, 1981, 1995 and 2002). The 1964 and 1955 events were really big ones with similar anomaly patterns to this event. That's what drew me to the event.

[snownut]

I have a new interest in watching west coast weather patterns as I have my sights set on moving to Bend OR in a few years. The rain shadow keeps Bend in a high desert climate (around 12" annual) so watching to see how much falls there over the next week. Thank for the thread!

[baroclinic_instability]

These height anomalies are just incredible.

[BlizzardWx]

I mentioned potential for a heavy precipitation event in my interesting analog thread that no one looked at or seemed interested in. The D+11 was saying that this had the potential to be a big event from Ca northward to WA.

http://www.americanw...resting-analog/

Hey, I posted because I liked it.

Still looking like a big time soaker. Too bad we don't have many members in that area.

[Srain]

The webcam shots from Mount Shasta Ski Park are going to be awesome. This may well end a multi year drought that has plagued N California and fill lakes that have all but dried up over the past 10 years. Concerns of severe flooding with mud slides/mud flows in burn areas across Northern California is increasing and snow totals in the higher terrain are going to be most impressive. Totals that snow lovers back E can only dream about in their wildest dreams. My concern is the Sacramento River may well flood. There is a lot of prime agriculture area that could be affected with this multi day event.

[Srain]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

331 AM EST WED NOV 28 2012

VALID 12Z SUN DEC 02 2012 - 12Z WED DEC 05 2012

==============================================================

= VERY WET PATTERN FOR NORTHERN CALIFORNIA WINDS DOWN SUNDAY =

= RENEWED HEAVY PRECIPITATION FOR THE NORTHWEST NEXT TUE/WED =

==============================================================

GENERAL FLOW PATTERN/MODEL PREFERENCE

=====================================

THE GENERAL FLOW PATTERN IS CONTROLLED BY A QUASI-STATIONARY POLAR

VORTEX OVER NUNAVUT AND A POSITIVE MID-LEVEL HEIGHT ANOMALY ACROSS

EASTERN SIBERIA AND THE BERING SEA. RENEWED TROUGHING ACROSS THE

EASTERN PACIFIC COMBINED WITH THE POLAR VORTEX ACROSS NORTHERN

CANADA ESCORTS AN AMPLIFIED LONGWAVE TROUGH THROUGH THE WESTERN

AND CENTRAL UNITED STATES WITH TIME. THE GUIDANCE IS IN

EXCEPTIONALLY GOOD AGREEMENT, INCLUDING THE GEFS/ECMWF ENSEMBLE

MEANS. AS SUCH, THE PRESSURES WERE BASED ON A 40/30/30 BLEND OF

THE 00Z UKMET/00Z GFS/00Z ECMWF THROUGH MONDAY BEFORE SWITCHING TO

A 50/50 BLEND OF THE 00Z GFS/00Z ECMWF SOLUTIONS. THIS BLEND

MAINTAINED GOOD CONTINUITY.

WEATHER IMPACTS

===============

HEAVY RAINS, HIGH ELEVATION SNOWS, AND WINDY CONDITIONS ARE

EXPECTED TO REIGN SUPREME ACROSS OREGON AND NORTHERN CALIFORNIA

INTO SUNDAY BEFORE LONGWAVE TROUGHING MOVES INLAND, WHICH LOWERS

THE STRENGTH OF THE ONSHORE FLOW AND DRIES UP THE ATMOSPHERIC

RIVER OF MOISTURE INTO THE REGION. STORM TOTAL AREAL AVERAGE

LIQUID EQUIVALENT (LEQ) AMOUNTS OF 10-15 INCHES ARE STILL

ANTICIPATED ACROSS NORTHERN CALIFORNIA, WITH LOCAL AMOUNTS OF OVER

20 INCHES POSSIBLE, WITH VERY HEAVY SNOWS RESTRICTED TO THE

HIGHEST ELEVATIONS. THESE RAINS WILL DO MORE THAN END THE

MODERATE DROUGHT ACROSS NORTHERN CALIFORNIA AND REFILL AREA

RESERVOIRS. THESE TYPE OF PRECIPITATION TOTALS ACROSS THE RUGGED

TERRAIN OF THE REGION COULD LEAD TO MUDSLIDES AND LANDSLIDES.

LESSER, THOUGH STILL SIGNIFICANT, PRECIPITATION TOTALS ARE

EXPECTED ACROSS THE NORTHERN CONTINENTAL DIVIDE AND PACIFIC

NORTHWEST AS WELL. SEE THE HPC QPF GRAPHICS ONLINE FOR A

BREAKDOWN OF THE PRECIPITATION EXPECTED OVER THE NEXT FIVE DAYS

ACROSS THE WEST. SEE PRODUCTS FROM NATIONAL WEATHER SERVICE

OFFICES ACROSS THE WEST FOR WATCHES AND WARNINGS ISSUED FOR THIS

WELL-ADVERTISED SYSTEM DURING THE SHORT RANGE PERIOD.

ONCE THE TROUGH EMERGES INTO THE PLAINS, MODERATE TO HEAVY RAINS

ARE EXPECTED TO SPREAD FROM THE WESTERN GULF COAST INTO THE

MID-MISSISSIPPI VALLEY, DEEP SOUTH, AND MID-SOUTH FROM TUESDAY

THROUGH WEDNESDAY. WHETHER OR NOT HEAVY RAINS MAKE IT INTO THE

GREAT LAKES TUESDAY HAS BECOME A QUESTION MARK, AND IS HIGHLY

DEPENDENT UPON WHETHER OR NOT ORGANIZED CONVECTION HANGS ON LONGER

ACROSS THE GULF COAST WHICH WOULD CUT OFF INFLOW INTO PLACES

FARTHER NORTHEAST (SEEN IN THE ECMWF BUT NOT THE GFS SOLUTION).

STAY TUNED.

AS RENEWED TROUGHING MOVES SLOWLY EASTWARD THROUGH THE EASTERN

PACIFIC, A NEW ROUND OF HEAVY RAINS IS EXPECTED ACROSS THE PACIFIC

NORTHWEST AND NORTHERN CONTINENTAL DIVIDE TUESDAY THROUGH

WEDNESDAY. THE SOUTHWEST AND CENTRAL HIGH PLAINS APPEAR TO BE

HIGH AND DRY THROUGH THE PERIOD, PROTECTED BY PERSISTENT RIDGING

AND DOWNSLOPE FLOW RESPECTIVELY.

ROTH

[aslkahuna]

The webcam shots from Mount Shasta Ski Park are going to be awesome. This may well end a multi year drought that has plagued N California and fill lakes that have all but dried up over the past 10 years. Concerns of severe flooding with mud slides/mud flows in burn areas across Northern California is increasing and snow totals in the higher terrain are going to be most impressive. Totals that snow lovers back E can only dream about in their wildest dreams. My concern is the Sacramento River may well flood. There is a lot of prime agriculture area that could be affected with this multi day event.

This type of pattern with an intense Pineapple Express is usually not conducive to heavy snow below 7000 feet as the air is tropical and the freezing level is high. Fortunately the analog being discussed in November 1955 and not December 1955 as the period of 18-28 December 1955 resulted in massive rainfall amounts in CA north of Point Concepcion and arguably the worst floods of the 20th Century in Central and Northern CA with 78 killed and 350 million 1955 dollars damage in a State with less than 1/2 the current population. Twenty four hour rainfall amounts in excess of 20 inches fell in the Coast Ranges from Big Sur north and total amounts over 10 days reached 100 inches or more in the coast ranges. Heavy November snow in the Sierra was melted as the Z level was at 10k ft.

Steve

[Srain]

PRELIMINARY EXTENDED FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

324 AM EST THU NOV 29 2012

VALID 12Z MON DEC 03 2012 - 12Z THU DEC 06 2012

...ANOTHER ONSLAUGHT OF HEAVY PRECIPITATION FOR THE

NORTHWEST/NORTHERN CALIFORNIA EARLY TO MID NEXT WEEK...

GENERAL FLOW PATTERN/MODEL PREFERENCES

======================================

TWIN POLAR VORTICES NEAR SOUTHERN ALASKA AND OVER NUNAVUT/THE

CANADIAN ARCHIPELAGO THROUGH THE MEDIUM RANGE PERIOD SHOULD KEEP

THE FLOW PATTERN PROGRESSIVE ACROSS THE MID-LATITUDES OF THE

EASTERN PACIFIC AND NORTH AMERICA THIS PERIOD. THE BIG QUESTION

IS HOW PROGRESSIVE. WITH THE INITIAL LONGWAVE TROUGH MOVING

THROUGH NORTH AMERICA, THE 00Z GFS IS THE QUICKEST TO THE MOVE IT

EASTWARD WHILE THE 00Z UKMET IS THE SLOWEST/MOST SOUTHWEST. ALL

IN ALL, THE 00Z ECMWF KEPT THE BEST CONTINUITY FROM DAY-TO-DAY,

AND IS CLOSEST TO THE ENSEMBLE MEAN SOLUTIONS LATE IN THE PERIOD.

A COMPROMISE OF THE 00Z GFS/00Z ECMWF WAS REASONABLE THROUGH

TUESDAY BEFORE SWITCHING TO A 00Z ECMWF/00Z GEFS MEAN SOLUTION FOR

THIS SYSTEM FOR MID NEXT WEEK.

TROUGHING MOVING FROM THE EASTERN PACIFIC INTO THE NORTHWEST ALSO

SHOWS SOME PROGRESSION ISSUES. WHILE OVERALL THE 00Z GFS IS

SLOWER AND A BETTER FIT TO THE ENSEMBLE MEANS, THE NORTHERN PIECE

OF ENERGY AT THE APEX OF THIS TROUGH IS MORE PROGRESSIVE IN THE

GFS SCENARIO, WHICH ULTIMATELY LEADS TO A MORE PROGRESSIVE SURFACE

SYSTEM AND FRONT OVER THE NORTHERN HIGH PLAINS. THE 00Z GEFS MEAN

SHOWS A BIT MORE DEFINITION THAN THE ECMWF ENSEMBLE MEAN SOLUTION

DOES, SO THOUGHT IT MORE USABLE. A COMPROMISE OF THE 00Z

ECMWF/00Z GEFS MEAN APPEARS BEST HERE, THOUGH DID INCORPORATE SOME

IDEAS THAT A SLOWED DOWN 00Z GFS WOULD HAVE PROVIDED.

WEATHER IMPACTS

===============

SUNDAY IS THE LAST DAY OF THE FIRST ROUND OF VERY HEAVY

PRECIPITATION AND WIND FOR NORTHERN CALIFORNIA/OREGON BEFORE THE

LONGWAVE TROUGH MOVES EASTWARD, SPREADING LESSER PRECIPITATION

THROUGH THE INTERMOUNTAIN WEST/CENTRAL ROCKIES MONDAY. AS THE

SYSTEM EMERGES INTO THE PLAINS, A MOISTURE TAP SETS UP FROM THE

GULF OF MEXICO, SPREADING MODERATE TO HEAVY RAINS FROM EASTERN

TEXAS INTO THE MID-MISSISSIPPI VALLEY AND MID-SOUTH MONDAY NIGHT

INTO TUESDAY, CENTRAL APPALACHIANS TUESDAY, WITH LIGHTER

PRECIPITATION FOR THE GREAT LAKES AND NORTHEAST TUESDAY INTO

WEDNESDAY.

MEANWHILE, HEAVY PRECIPITATION FOCUSES UPON THE PACIFIC NORTHWEST

INTO NORTHERN CALIFORNIA ONCE AGAIN FROM LATE MONDAY INTO THURSDAY

BEFORE PROGRESSING ACROSS THE NORTHERN CONTINENTAL DIVIDE AND

CENTRAL ROCKIES WEDNESDAY INTO NEXT THURSDAY. WITH THIS ROUND OF

PRECIPITATION, 2-4 INCHES AREAL AVERAGE LIQUID EQUIVALENT

PRECIPITATION (WITH LOCAL AMOUNTS NEXT 6 INCHES) FALLING IN THE

FORM OF VALLEY RAINS AND MOUNTAIN SNOWS IS EXPECTED, WHICH COULD

EXACERBATE ISSUES CAUSED BY THE MUCH HEAVIER PRECIPITATION

EXPECTED BETWEEN NOW AND SUNDAY. STAY TUNED.

ROTH

[Srain]

Wes started this thread on November 18th in the Central/Western sub forum and since this will likely be the major weather headline event for the upcoming period it is worthy of main page discussion/exposure. Low elevation flooding rainfall will linger into next week with the onslaught of Pacific moisture with higher elevation snows across California and the Pacific NW. High winds as well as very large surf will also be part of this multi storm event that should linger into the middle of next week.

[am19psu]

Max snowfall on this event? 120"? 150"?

[griteater]

From Mt. Shasta Ski website...

The Guinness Book of World Records reports Mount Shasta received the most snowfall ever in a single snowstorm: 189 inches (15 ¾ feet) February 13th to the 19th, 1959.

[Srain]

Here are some pictures I took back in January 2008 of the Northern California Region. One of the biggest concerns is the flooding aspect from this event. Certainly the region has been in a multi year drought and the lakes and reservoirs will benefit. The other issue may well be the flood potential for the Sacramento River Basin/Sacramento Valley near Redding.

[am19psu]

From Mt. Shasta Ski website...

The Guinness Book of World Records reports Mount Shasta received the most snowfall ever in a single snowstorm: 189 inches (15 ¾ feet) February 13th to the 19th, 1959.

CT Blizz is not impressed.

[usedtobe]

This type of pattern with an intense Pineapple Express is usually not conducive to heavy snow below 7000 feet as the air is tropical and the freezing level is high. Fortunately the analog being discussed in November 1955 and not December 1955 as the period of 18-28 December 1955 resulted in massive rainfall amounts in CA north of Point Concepcion and arguably the worst floods of the 20th Century in Central and Northern CA with 78 killed and 350 million 1955 dollars damage in a State with less than 1/2 the current population. Twenty four hour rainfall amounts in excess of 20 inches fell in the Coast Ranges from Big Sur north and total amounts over 10 days reached 100 inches or more in the coast ranges. Heavy November snow in the Sierra was melted as the Z level was at 10k ft.

Steve

actually, the analog was Dec 1955 as I made a typo and it was based on a D+11 but the pattern also was very similar to 1964. The big difference with those events and this one is the long term drought that preceded this event which will mitigate the run off. . The url I posted has the maps for that event and I posted another that has the maps for the other 20 biggest events since 1950 according to the unified precipitation data which has plenty of shortcomings but did pick out the big flood events. The location of the positive anomaly near the Bering Straits or western AK is a tell for big rain events in CA, but you already know that. As you noted, the snow levels tend to be high with these type events and many Pineapple Express or atmospheric river events (the new jargon) cause flooding problems elsewhere besides ca as snow levels across northern Ut and Idaho tend to be high. They also often end up bringing really warm air east of the rockies.

[Buffalo Bumble]

NWS forecast for Mount Shasta through Sunday. Insane....

Today Snow. The snow could be heavy at times. High near 23. Wind chill values as low as -7. Windy, with a south southwest wind 75 to 85 mph, with gusts as high as 115 mph. Chance of precipitation is 100%. Total daytime snow accumulation of 33 to 39 inches possible.

• Tonight Snow. The snow could be heavy at times. Steady temperature around 21. Wind chill values as low as -7. Windy, with a south southwest wind 90 to 95 mph increasing to 100 to 105 mph in the evening. Winds could gust as high as 115 mph. Chance of precipitation is 100%. New snow accumulation of 37 to 43 inches possible.
  
• Friday Snow showers. The snow could be heavy at times. Temperature falling to around 15 by 4pm. Wind chill values as low as -13. Windy, with a south southwest wind 85 to 95 mph decreasing to 75 to 85 mph in the morning. Winds could gust as high as 115 mph. Chance of precipitation is 100%. New snow accumulation of 23 to 29 inches possible.
  
• Friday Night Snow. The snow could be heavy at times. Low around 14. Wind chill values as low as -13. Windy, with a south southwest wind 70 to 75 mph, with gusts as high as 115 mph. Chance of precipitation is 100%. New snow accumulation of 21 to 27 inches possible.
  
• Saturday Snow. The snow could be heavy at times. High near 19. Windy, with a southwest wind 75 to 80 mph, with gusts as high as 115 mph. Chance of precipitation is 100%. New snow accumulation of 22 to 28 inches possible.
  
• Saturday Night Snow. The snow could be heavy at times. Low around 17. Windy. Chance of precipitation is 100%. New snow accumulation of 29 to 35 inches possible.
  
• Sunday Snow. The snow could be heavy at times. High near 18. Windy. Chance of precipitation is 100%. New snow accumulation of 11 to 17 inches possible.