WEATHERAmerica Newsletter, Sunday Evening, April 20, 2008 at 5:40 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)
Some Thunderstorms May APPROACH Severe Limits
NE VA....DC....MD....DE....Cape May County S NJ

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

Scattered Locations In
NE VA....DC....MD....DE....Cape May County S NJ
(QPF 1 - 4")

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

Numerous Locations In
SE BC....AB....SK
(Snow; 4 - 48"; Blizzard C AB, C SK)

Isolated Locations In
Coastal BC....WA....OR....ID....W MT....NW WY
(Snow; Above 1500 Feet; 4 - 12")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

500MB Cutoff Low Brings Lots Of Rain To Potomac Valley, Chesapeake Bay, And The Delmarva

The large closed low over the Ohio Valley is expected to dig east-southeastward during the next 72 hours into the VA Capes. In the process, the deep moisture axis between the upper disturbance and a high pressure cell in the Maritime Provinces will help to set up a band of recurrent heavy (sometimes convective) rainfall from the Delmarva Peninsula into the headwaters of the Potomac River. A mesoscale event (no important rain will fall 100 miles north or south of the precipitation maximum), the disturbed area should drift into the Gulf Stream waters later Tuesday after dumping as much as 4 inches of moisture from NE VA into the MD Eastern Shore.

Heat Builds Across The Southern, Central Great Plains....

Despite all of the images of cold and winter that are present in parts of the West, summer is in fact regrouping over the Desert Regions and the lower Rio Grande Valley. Hot temperatures will be the rule in much of the Southwest and lower Great Plains, and with time the warmer air will overspread most of the Midwest and Dixie states. The dry character of the air mass may allow the 100 deg isotherm to advance into the Red River Valley (TX and OK) on Day 3.

....While Winter Hangs Tough Over Pacific Northwest And Western Prairie Provinces

The abundance of very cold air over the western Canada, the Pacific Northwest, and the West Coast is well illustrated by the satellite image above. Also note the vortices present near Vancouver Island and over the Gulf of Alaska. The nearer feature is expected to bring a blizzard to portions of BC, AB, and SK during the following 24 hours. As for the more distant storm, a landfall over the Willamette Valley is expected Tuesday night with another surge of heavy rainfall and unusually low elevation snows from S BC to as far south as N CA and as far east as the Teton Range.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Gale Center May Pose Threat To Bermuda

The low which is currently bringing a heavy rain threat to portions of the Mid-Atlantic region may prove hard to get rid of. Many of the numerical models suggest that the closed upper low will wobble south and eastward, then be re-energized by a shortwave retrogressing from the Maritime Provinces on Friday. The result: yet another gale center over the Sargasso Sea. The low is forecast to pass close to Bermuda before turning northward, kicking up surf and eventually bringing a period of heavy rain and wind to portions of Nova Scotia and Newfoundland later in the coming weekend.

Pacific Ocean Storm Sequence May Bring Many Extremes To Lower 48 States....

The satellite images of the eastern Pacific Ocean continue to show the formation of yet another subtropical jet stream from near the International Dateline to Mexico. Linking with this wind stream is a storm positioned to the right of a Bering Sea vortex. While the storm will impact the northwestern states as near as Wednesday, the combined effects of both the broad disturbance and the southern branch moisture/energy influence will likely affect the Great Plains and Midwest in four or five days. Surface convergence of four air masses, combined with a rapidly deepening, negative-tilt storm strongly suggest a severe weather outbreak over the Corn Belt on Friday.

....As Could The "Dual Blocking" Scenario" Shown By The ECMWF Scheme

The displays of high-latitude blocking at 500MB over or near North America are very impressive on many of the computer models, especially the latest run of the ECMWF scheme. While having a Rex or Omega signature present does not strictly imply more storms and colder temperature profiles in the U.S. during spring (unlike winter), there will be more localized wet and cold anomalies. With the dual block arrangement on European model panels during the medium range, teleconnections favor the following types of apparent weather in the U.S.:

1) Colder than normal over the Pacific Northwest, Great Lakes, and Northeast

2) Potential for CAD wedging along the Piedmont and Atlantic Coastal Plain as far south as the Atlanta GA metro

3) Intense heat from Southern California eastward to the lower Mississippi Valley

4) Multiple severe weather and flooding rain events across the Corn Belt

5) Repeated heavy stratiform rains in the lower Great Lakes and Mid-Atlantic regions

In other words, a "rock and roll" weather pattern for much of the nation after April 24.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

La Nina Continues To Disappear

The La Nina episode which played a significant, but not dominant, role in the winter season of 2007-2008 is now largely a thing of the past. A thin ribbon of cooler hydrothermal anomalies still exists in the ENSO 3,4 sector. But that segment is moderating, and warmer than normal SSTs are the rule from the Ecuador coastline to about 130 W Longitude.

While it is too early to say if a warm phase (El Nino) event will get underway over the equatorial Pacific Basin, it can be said that the influence of the La Nina anomaly is waning. The subtropical jet stream which keeps appearing from the International Dateline to Mexico may well continue into the summer. But the hard-hitting sequence of storms that played the biggest influence during the low sun period will likely soon be history. With disturbances in the westerlies becoming weaker and tracking further to the north, it will be only a matter of time before the (Sonoran and Bermudan) heat ridges of summer rise up again.

Troublesome Possibilities Seen On Satellite Imagery Of Western Pacific And Indian Oceans

While the ENSO component of long-range forecasting suggests that summer-type conditions in North America are not that far off in the future, the satellite imagery covering Asia, the Indian Ocean and the western Pacific Ocean paint a truly "spring-like" picture. The vaunted storm sequence that has characterized the 500MB longwave pattern since November of 2007 is still showing signs of life, with three fairly impressive impulses in the series from near Japan to the Tien Shan Range. Adding to the concern that this cluster of cyclones will have a major impact on the lower 48 states and Canada is a flare-up of convection over Indonesia and Malaysia (part of the Madden-Julian Oscillation) and a formative tropical disturbance moving westward above Papua-New Guinea. Either one of these features could energize the northern branch jet stream component, much as was the case with Typhoon Neoguri last week. The arrival of each area of low pressure will probably translate to three cases of severe weather in the Great Plains, Midwest and Mid-South between April 29 and May 6.

Prepared by Meteorologist LARRY COSGROVE on
Sunday Evening, April 20, 2008 at 5:40 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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