WEATHERAmerica Newsletter, Saturday, March 22, 2008 at 8:30 P.M. ET

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)
 
Snow For Easter? If Not Snow, Then Cold! 
 

While it is really not that rare to have an Easter holiday with snow in the Midwest or Northeast (especially with the date occurring in late March this year), it is a tad unusual to be talking about the possibility for frozen precipitation reaching into Dixie. It could happen on Sunday, what with the 0 C isotherm reaching as far south as the Interstate 10 corridor. With so much cold air and vorticity aloft, I would not be surprised if some locations in the Ohio Valley and middle Appalachia receive accumulations of the white stuff. The cold air will likely be a factor in forecasts east of the Mississippi River through Tuesday morning.
 
Warmer Air Will Build Northward From Mexico
 

Of course, this being spring, cold air cannot be pervasive across the entire U.S. True to form, build-ups of warmer (cT) air have occurred periodically in the past two weeks over Mexico. Some of the hotter readings occasionally reach the Southwest and parts of the lower Great Plains. This should be the case on Tuesday, with the warm and dry domain slowly pushing north and east ahead of a large storm complex along the West Coast. Parts of NM and W TX have a decent shot at reaching 100 deg F on the afternoon of March 25.
 
Pacific Northwest About To Get Hit Again 
 

There is a particularly menacing storm lurking offshore from the Pacific Northwest, and this feature should make landfall over Vancouver Island BC on Sunday night or Monday morning. Snow elevation levels are forecast to be fairly low ahead of the storm, so the coastal and inland ranges of BC, WA, OR and eventually ID are likely to see a good dump of snowfall to start the new week. Rain and wind will also be an issue in the coastal and valley locations. In time this disturbance could have a profound impact on much of the lower 48 states during the medium range.
 

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
 
Is There A Violent Spring Storm In Your Future?
 

There may actually be TWO important storms to target the U.S. in the medium range, if satellite views and the operational GFS output from 12z and 18z are taken into consideration. While the latest ECMWF model data suggest no extreme conditions from the disturbances over the eastern Pacific Ocean (which are VERY impressive looking on GOES WEST images), the ensemble runs of bother American and European schemes are nothing less than scary, showing a full-latitude impact from a storm sequence between the 96 and 192 hour time frames.
 
Keep in mind that unlike previous systems, the emerging threats in play are 1) severe thunderstorms and tornadoes and/or 2) heavy rainfall with flooding. Give the GFS version credit for doing better than the ECMWF panels with recent systems; the tornado and flood situations, followed by sharp attacks of cold, have been well documented by the American equation and its variants. Surface convergence is a key factor with risks from the Great Plains into the Midwest and Old South; note a perfect quadra-convergence of cP, cT, mT and IcP values on Days 5 - 7. If I lived in the communities hit by torrential rains last week, I would be very concerned about the danger of a severe+flood event arising later next week.
 
Volatile Temperature Swings Are Indicated (Again)
 

In what may be blamed as an effect of a "La Nina vs. Madden Julian Oscillation battle" (see below), the U.S. will again be visited by a rather extreme temperature change next week. The cT and mT regimes ahead of the aforementioned major storm will be replaced by a biting cA domain. If model forecasts are correct, that colder air mass will itself be short-lived, replaced by warming ahead of yet another storm at the start of the extended period.
 
Will Blocking In The NAO Position Keep The Cold In Place? 
 

There is strong agreement among all of the numerical models and the respective ensemble members of a period of blocking in the NAO position (in this case Greenland and Iceland) during most of the medium range and extended period. While a positive height anomaly of this type is NOT a universal suppressant of the jet stream at this time of year, through April such a ridge can maintain cold profiles over the Midwest and Northeast. So while the southern tier of the U.S. may see meaningful periods of warming, temperature trends from the upper Missouri Valley into New England and the Mid-Atlantic states will remain mostly below seasonal normals through April 7.
 

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
 
Western Pacific Ocean Storm Sequence Still The Main Factor In Longer Term