WEATHERAmerica Newsletter, Saturday Evening, April 5, 2008 at 8:00 P.M. CT

by LarryCosgrove | April 5, 2008 at 05:28 pm

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WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
ISOLATED Severe Thunderstorms
(Microbursts, Large Hail)
FL Peninsula

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
N, C MO....IA....SE MN....W WI

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

Scattered Locations In
FL
(QPF 1 - 4")

Isolated Locations In
Coastal BC, WA, OR
(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
BC....WA....OR....N CA....N NV....N UT....NW CO....W WY....W MT
(Snow; Above 4000 Feet; 4 - 12")

Scattered Locations In
S ND....N SD....N, C MN....W ON
(Snow; 4 - 12")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Precursor To "The Big Show" Across The Midwest

A low which is exiting the Front Range of WY will be small but strong, producing an arc of heavy snowfall across the Upper Midwest with some heavy to severe thunderstorms in the upper Mississippi Valley. Note the slow northward advance of moist unstable air into impressive upper dynamics, implying that the greatest danger for tornadoes and hail will be in parts of IA and MO on Sunday. This feature will set up a frontal structure from the TX/OK Panhandle region into the Great Lakes which will be a focus for thunderstorm formation (and partially a track mechanism) for a critical storm threat occurring April 7 - 12 (see below).

Pattern Of Cold, Unstable Conditions Across The West Heralds A Major Change

The two disturbances in the respective polar and subtropical jet streams approaching the West Coast will not phase. So the threat for a critical weather event lies further down the road, with the impulses in the Gulf of Alaska and east of Hawaii (see more below in the MEDIUM RANGE OUTLOOK). These disturbances will merely be a sort of "appetizer" to the "main course", providing some rains along the coastal communities from BC into N CA and snow or small thunderstorm cells through the interior of the West. But the gradual lowering of temperature should give a clue to the nature of a wintry pattern over the Intermountain Region, with the convective risks increasing in California and the Desert Regions.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
Winter's Last Gasp, Spring's Hammer Blow: Storm Of April 8 - 11

The chain of disturbances along the polar jet stream over the northern Pacific Ocean is beginning to interact with the subtropical moisture axis stretching from the International Dateline into California. While the first couplet of storms may produce a moderate convective and precipitative impact on the Mississippi and Ohio Valleys, the second grouping (2 and b on the satellite image) is going to produce an array of wild weather affecting most of the lower 48 states and a good chunk of central and eastern Canada.

The storms will begin to phase over the Intermountain Region, then take a "Panhandle Hook" track into the Great Lakes, passing very close to the Minneapolis-St. Paul MN metro in the process. The air mass clash and jet stream configuration practically screams "severe weather", and the presence of so much cold and unstable air to the left of the path of lowest pressure promises abundant snowfall through much of the Intermountain Region, northern High Plains and parts of Manitoba, Ontario and Quebec as well.

The European and American models are in agreement on the potential for a notable, perhaps historic, weather event. The Reno NV; Salt Lake City UT, and Denver CO metropolitan areas could be hit with a classic spring blizzard with strong winds, thunder, and heavy wet snowfall. And the threat for a vicious impact for severe thunderstorms with heavy rain, tornadoes, large hail and damaging winds looms large for an area from TX, OK, and KS into the Ohio and Tennessee Valleys.

Stay tuned!

Chilly, Stormy Trend For Much Of Midwest, Northeast After April 12....

Another strong storm will follow the "big blast" during the middle of the week. This system will follow a path just below the Canadian border, offering perhaps one last chance at an important snowfall in portions of the Upper Midwest, Ontario and Quebec. The most dramatic effect will be to re-introduce cold air to the Great Lakes and Northeast (in fact, the cPk regime may reach as far south as the Tennessee Valley and Piedmont), along with persistent cloudy, raw, and wet conditions above the Ohio River and Mason-Dixon Line.

....While Warmth Returns To The Southern Tier Of U.S.

With the mean storm track slowly reverting to the north after April 12, the potential for warming (through ridging) will increase in the southern and western tier of the U.S. Hot and dry air will make its first appearance in the Mojave Desert (see graphic above) on April 14, and begin to shift north and eastward into the central and lower Great Plains at the start of the extended period.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

A Quieter View Of The Western Pacific Ocean....

There has been a noticeable, if slight, shift in weather across the Orient which should have a direct impact on conditions across North America in the extended period. While a chain of disturbances is still active from China to near the International Dateline, the size and strength of the impulses has declined. The previously-impressive connection to equatorial thunderstorm bands has also vanished, with the most convection noted south of the equator through New Guinea and Java. Ridging over Oceania is building west to Vietnam and east toward Johnston Atoll. So while chances for precipitation will remain through the U.S. between April 16 and 21, these systems should not carry the major impacts expected in the medium range forecast.

....And An Early "Look-See" At The Equatorial Atlantic Ocean And Africa

A major change has occurred in the structure and strength of the long-dominant Saharan heat ridge. During the winter the anticyclone was extremely well-organized and covered an area from the Mediterranean Sea into the equatorial regions of Africa (making for a very dry, dusty Sahel and Arabian Peninsula). In the past week, however, the ridge complex has weakened and become aligned more north to south from Provence into Niger. At the same time, the previously dormant Inter Tropical Convergence Zone has sprung to life, with four massive impulses bringing heavy rainfall and strong thunderstorms from the Rift Valley westward to just below the Cape Verde Islands. Westerlies are still pronounced at low latitudes, so there is no imminent threat of a freak tropical cyclone formation in the equatorial Atlantic Basin. But as the flow aloft becomes more diffuse and allows for easterlies to take over in about 2. 5 months), and warmer than normal waters prove hospitable for warm-core cyclogenesis and convection, things could get interesting in the tropics!

Prepared by Meteorologist LARRY COSGROVE on
Saturday, April 5, 2008 at 8:00 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.

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