WEATHERAmerica Newsletter, Sunday, November 18, 2007 at 10:05 P.M. ET

[Larry_Cosgrove]

TODAY'S FUN LINKS: Rutgers University Climate Lab :: Global Snow Lab

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
STRONG WINDS
(Pressure Gradient Derived With Orographic Influences)
S ID....N NV....N, C CA

STRONG WINDS
(Pressure Gradient Derived)
W, C QC


HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)

Isolated Locations In
N CA
(QPF 1 - 2")


FROZEN PRECIPITATION OUTLOOK
(potential for accumulation of ice rime, freezing rain, sleet or more than 3" total snowfall within the next 24 hours)

Isolated Locations In
C AB....C SK....W ON....NU Hudson Bay Islands....N QC
(Snow; 3 - 6")

Scattered Locations In
WA....OR....N CA....NW NV....ID....NW WY....MT
(Snow; 4 - 12"; Higher Amounts Above 7000 Feet)


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)



Panhandle Hook Storm To Bring Snow To The Upper Midwest, Thunderstorms To Dixie....

The presence of a fairly deep 500MB trough over the western two-thirds of North America during the near term favors the "Panhandle Hook" scenario put forth recently by the numerical models. However, there is still some question on the exact path of a shortwave ejecting from the lower Front Range on Day 3. If the subtropical high over the Bahamas is stronger than predicted, the path of the cyclone will be through the western or central Great Lakes (the "B" path). Should the heat ridge be located a but further east and not be as amplified, then a traditional trajectory through the Ohio Valley into Quebec would occur. This latter case, of course, implies snow or ice chances for the Milwaukee WI - Chicago IL corridor while also bringing potentially heavy rain and severe thunderstorms to parts of the drought-ridden Old South and Corn Belt. As of this writing, a track from Oklahoma into lower Michigan seems likely to pan out, somewhat of a middle ground that may allow for a period of stronger convection in much of Dixie on Day 3.

....Chicago Snow Chances: A Close Call

Though by no means a certainty, there is a fair chance that the Chicago WI....IL....IN metro vicinity could see a period of snow from the storm slated to pass close to the region within the next 72 hours. Since 850MB and surface temperatures at the onset of precipitation will be well above freezing, it seems probable that the precipitation will start as rain. Many of the numerical models put the path of the upper low and 500MB vorticity maximum directly over the Windy City, which is NOT favorable for an all-snow or even a heavy-snow event. But the snow shield will come close to the Lake Michigan southwestern shoreline, perhaps as far south and east as an Elgin IL to Racine WI line by the end of the episode.

"Blue Norther" Cold Spell

As the aforementioned storm begins to move into the Corn Belt at midweek, the cPk regime building over the northern High Plains will drain abruptly into Texas. The rapid displacement of mild values by the colder air will produce increasing winds and a fairly sudden cooling in the Lone Star State. This "Blue Norther" may reach as far south as the Bay of Campeche within 120 hours.

Yes, Another Santa Ana Event

The combination of the digging 500MB shortwave over the Great Plains and increased ridging along the West Coast is likely to bolster anticyclogenesis across the Intermountain Region on Wednesday. The presence of a strong cell of high pressure in Wyoming should then trigger a fast, downslope easterly to southeasterly flow of air into California. Thus, another "Santa Ana" or "Canyon Wind" event will sweep through the Golden State, with warmer than normal temperatures, very low dewpoints, and bursts of high winds. This incendiary mixture may stimulate fire hazards into C and S CA which may last into next weekend.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Intense Storm Sequence Over Northern Pacific Ocean Prevents Blocking....

If a blocking signature were to arise over western Canada or Alaska, an intense mA or mAk vortex would become stationary between the Aleutian Islands and Hawaii. While 500MB heights are forecast to be lower than normal through most of the next 15 days (in some cases much so), the progressive nature of the storm track from Mongolia into the eastern Pacific Ocean does NOT favor the formation of a Thumb, Rex, or Omega block at higher latitudes close to the West Coast. So while intrusions of colder air will make a mark on North America in the medium range, the lack of a classic blocking signature means that the nature of the regimes will be transient. And the mean storm track almost certainly will be from the lower or middle Great Plains into the St. Lawrence Valley or upper Appalachia, affording occasional warm-ups into the Eastern Seaboard.

....But NOT Short-Term Amplification With Occasional Strong Mid-Latitude Cyclones

November is traditionally a tough time to forecast the path of storms and eastward expanse of cold air. With a persistent mean trough at 500MB over the Intermountain Region and Great Plains, however, it seems likely that storms of the Panhandle Hook and Colorado/Trinidad type will be recurrent through the medium range. Shortwaves falling in between the ridge axes over the eastern Pacific Ocean and the Bahamas will pull down significant shots of cA and cPk air, and it is probable that some of the colder values will reach into the Deep South and Eastern Seaboard (albeit for short durations).

The Heat Ridge Over The Bahamas Is Very Significant!

No matter what NWP scheme that you peruse, every panel in the medium range keeps showing a strong subtropical high in the vicinity of the Bahamas. Since we know that high-latitude blocking signatures are unlikely to form during the longer term, the heat ridge will have profound impacts on both the trajectory of storms (deflect to the WEST of the Appalachian Mountains) and temperature (assures rapid warm-ups in between cold intrusions) before November 28.


EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)


EPO/PNA Ridge Complex Will Bring Taste Of Winter To Eastern Two-Thirds Of North America....
With GFS ensemble data continuing to point toward a positive 500MB height anomaly over the eastern Pacific Ocean, Alaska, and a portion of the Arctic Sea (remember: this anomaly represents ridging and NOT blocking), teleconnections favor a broad circumpolar vortex over northern Hudson Bay. Since there is no accompanying -NAO signature, the spread out trough associated with the gyre will not be overly amplified. The growing snowpack in Canada should act to chill the source region, and each new shot of colder values will be accompanied by snows in and near the Great Lakes region.

True record cold outbreaks occur when the vortex drops over, or below, James Bay. While repeated intrusions of cA, IcA, and cPk regimes are likely along and to the right of the Rocky Mountains, the resultant temperature spreads will be cold without being extreme. Item to note: if the forecast jet stream configuration verifies, the arrival of very low dewpoints in the Deep South will allow for record low minima and also prevent precipitation in the drought stricken locales.

....While Western U.S. Is Warm, Windy, And Dry
The pronounced ridging pattern across the eastern Pacific Ocean, although not a blocking signature, will be aligned in such a way as to perhaps shut down the volatile storm sequence exiting Russia, Mongolia, and Manchuria during the last week of November and the first few days of December. Placement of a 500MB anticyclonic axis just to the left of the West Coast favors higher than normal surface pressures from the Yukon Territory southward through most of the Intermountain Region. Any shortwave digging to the right of the ridge will aid in anticyclogenesis in the Front Range or the Bonneville Basin, which would stimulate production yet again of a Santa Ana/Canyon Wind event in California. At the very least, the western states and provinces should have a prolonged episode of warm, dry conditions in the extended period.


Prepared by Meteorologist LARRY COSGROVE on
Sunday, November 18, 2007 at 10:05 P.M. ET

Disclaimer:

The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2007 by Larry Cosgrove

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