WEATHERAmerica Newsletter, Saturday Evening, March 29, 2008 at 9:20 P.M. CT

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
SEE ATTACHMENTS

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

Isolated Locations In
N TX....OK....AR....MO....KS....NE....IA....IL....IN ....KY....TN....NC....SC
(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
MB....ON
(Snow; 3 - 5")

Isolated Locations In
S BC....WA....OR....ID....W MT....W WY.....W CO....UT....NV....CA High Sierra
(Snow; 4 - 12"; Above 3000 Feet)

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

This Time, A Severe Weather Threat....

The disturbance now entering the Intermountain Region should undergo leeside secondary cyclogenesis over Colorado within the next 24 hours. Deepening along a fairly impressive area of surface convergence (cPk/cT/mT), the low will be accompanied by explosive growth in vertical motion across the central and lower Great Plains. But instead of taking a track through the Ohio Valley and creating a winter weather scenario, this feature will almost surely move northeastward into the Great Lakes before exploding across the Laurentian Shield late Tuesday.

You may note the isolated or pocketed character of maximum UVVs about the warm front and the lower right quadrant. This phenomena almost always depicts severe weather with supercell formation, and may indicate the potential for an outbreak of severe weather from the Arkansas and Red River Valleys into sections of the Corn Belt and Tennessee Valley between late Sunday afternoon and the wee hours of Tuesday. Instability forecasts of the warm sector are a little stronger than normal. But with such powerful upper dynamics and an open Gulf of Mexico moisture source, the risk of intense convection across much of the Old South and Midwest will certainly be increasing as we start the new work week.

....With Relatively Little Snowfall

While there is still abundant cold air across Canada and the northern U.S., warmer values are gaining control. Add to that fact the structure of the latest disturbance (trough type), and you can see why the threat for new heavy snowfall will be much more limited with the passage of the early week storm. Snowfall may be locally heavy with a cold vortex in central Canada, as well as portions of the Intermountain Region and Black Hills. The principle concern for inclement weather, however, is for flooding and severe weather.

Cold Vs. Warm Is Evenly Divided....For Now
A fairly flat, west-to-east balance in colder vs. warmer temperatures is forecast to continue through the next few days. With the formation of a strong -NAO blocking signature across the Davis Strait and Greenland, however, and incoming strong storms from the Pacific ocean, colder readings may win out across much of the eastern half of the U.S. during the medium range. Conversely, intense heat taking shape in northern Mexico may make inroads into Texas and the Southwest next week.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Round 2 From The Sequence Of Major Storms

The very active pattern across the U.S., with regard to storms, is expected to continue for the foreseeable future. The next system to have an impact on North America has two components (polar and subtropical jet stream) and is ticketed to arrive along the West Coast (bringing perhaps the last steady rainfall of this spring to California) in about three or four days. Undergoing redevelopment over Colorado, the disturbance should pull up hot/dry and warm/humid values against its incoming cold intrusion from western Canada. The result may be something closely akin to the forecast for the near term: lots of severe thunderstorms and heavy rain. Other impressive disturbances in the Orient could bring even more convective impacts to the nation by Days 9 and 10.

One Last Cold Intrusion?

The numerical models show extensive blocking in the NAO position during the medium range. Since such a strong storm complex will be moving into a mean trough and vortex across eastern North America, the potential exists for important cold air advection into the lower 48 states. This being spring, the cPk or cA intrusion probably will not be long lasting or as widespread as what was seen just two weeks ago. Still, temperature outlooks for the Midwest and Northeast should be skewed "below normal" through at least April 7.

Watch The Mexican Heat Source!

You may have noticed a repeating pattern this spring. Intense heat builds across the deserts of northern Mexico, then shoots northward into Texas and the High Plains. This trend keeps showing up over and over in medium and extended range prognoses charts, and gives us a clue to the upcoming summer. A Sonoran heat ridge should start to show up along the Mexican border, and in coming weeks will gain in strength and latitude across the Intermountain Region. This implies an early, and hot, summertime for California and the American Southwest, with the anticyclone likely getting the entire western U.S. in its searing grip by early July.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

How Long Will Blocking Signatures Hold Up Over Canada And Greenland?

It would be unwise to predict a long-lasting positive height anomaly in the NAO (or any other) position, given the time of the year and the powerful, progressive storm sequence that is taking aim at North America from the Orient. Still, the ensemble means of the various computer outlooks suggest that a strong Rex signature will lock in over the Davis Strait into Greenland between April 5 and 11. As long as this ridge complex is present, temperatures across parts of the West, Great Lakes, and Northeast are likely to stay below normal.

The Dying La Nina And Other Important SST Anomalies

Finally, there is some admittance from some of the governmental weather services around the world that the La Nina episode is indeed weakening (when in truth the fall of the -ENSO regime was evident in mid-February). The colder SST configuration in the Pacific Basin is moderating rapidly now, but is still exerting an impact on sensible and apparent weather across the Northern Hemisphere. The principle result of the cold hydrothermal anomaly is the vigorous and active polar jet stream stretching from the Ural Mountains into central North America, which is forecast to continue through mid-April before weakening.

While much of the waters of the western Atlantic Basin have been relatively cool, the Gulf of Mexico has recently warmed. With a disturbance drifting westward through the Bahamas in the next few days, we may see a possibility (if not in near time, in the not too distant future) for some kind of subtropical feature to draw near the Southeast or Gulf Coast. The massive Saharan heat ridge across northern Africa has brought about warm anomalies in the Cape Verde vicinity. A caveat, however, to those who think that the boost in se surface readings heightens chances for hurricanes: the very power of the anticyclone has dampened down impulses along the ITCZ, and dust injections have been common through most of the Sahel (as well as to the north and east). Bottom line: it will be tough during the first part of summer for any "Cape Verde" type disturbance to survive into the more hospitable waters of the equatorial Atlantic Ocean.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, March 29, 2008 at 9:20 P.M. CT

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 2008 by Larry Cosgrove

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