WEATHERAmerica Spring 2007 Season Forecast

[Larry_Cosgrove]

Overview

There are no outstanding solar or geologic influences seen or forecast to affect conditions around North America through the following 120 days (see Solar Terrestrial Activity Report and Global Volcanism Program | Volcanic Activity Reports). Two climatic trends stand out as clues to the likely development of weather in the lower 48 states during the calendar spring season (for simplicity noted as March 1 through June 30). That is, the lack of any high-latitude blocking (with the demise of the recent +PNA signal) and the end of the El Nino episode in the equatorial Pacific Basin. An expected transition to La Nina by June will likely result in the elimination of the ongoing series of subtropical jet streams (and the late development of the classic +ENSO split flow in coming days). With regression of the upper level flow, it appears that the heat ridge complex noted over the Greater Antilles is set to expand into the Southeast by late spring.

Temperature

There has been a persistent subtropical high over the past five months, generally located in the vicinity of the Bahamas, FL and the Greater Antilles (in repressed phase over the Sargasso and Caribbean Seas). With latitude regression and unification of the polar westerlies, the tendency may be for a slow increase in temperatures across the Old South with colder values across the western third of the nation and along the Canadian border. If the current pattern of no high-latitude blocking continues, the orientation of the jet stream will probably range from semizonal and split to -PNA (responding to the ongoing tendency for mA or mP vortex formation in the Gulf of Alaska).

Precipitation

With the suspected elimination of deep equatorial fetch mechanisms (the persistent subtropical jet streams noted throughout the Northern Hemisphere this winter), the favored type of precipitation event will change from stratiform, synoptic scale to convective in the mesoscale. The process will start slowly; evidence the strong mTw flow with a storm southwest of Baja CA. But as more systems emerge from the northern Pacific Ocean (digging into the Southwest then ejecting eastward), rainfall will revert to higher latitudes. With increased ridging over FL, then building toward TN by the end of calendar spring, Dixie will start the season very wet, but likely turn quite dry in June. The Pacific Northwest, Great Plains, and much of the Midwest should also be very wet and increasingly stormy. And while the Northeast looks to be relatively dry, that pattern may change as we head into summer and shortwaves skirmish the region along the then-unified polar westerlies.

Goodbye El Nino, Hello La Nina

A trend readily noticeable on SST imagery (see
ORAD/MAST Satellite Sea Surface Temperatures (SST) Anomaly Animations and GODAS Temperature Anomaly/Subsurface SST) of the Pacific Ocean shows a fairly dramatic cooling trend occurring in sector 3,4. While the overall look of hydrothermal profiles is still a +ENSO, the breakdown of the warm episode is underway. Past modeling of this El Nino phase suggests a return to neutral or even a cool anomaly (La Nina) by the start of summer 2007. The transition should be gradual; note that SOI and MEI relationships have been erratic this winter and not yet showing a strong tendency for cooling in the middle of the Pacific Basin.

What can we surmise about how apparent weather will turn out across the U.S.? Using past ENSO + to - arrays can help. The most recent "flip" sequence occurred in 1998; other examples that may prove useful are 1995 and 1973. You can notice one constant in those spring to summer periods: a relatively strong positive height anomaly tending to build north and west from the Bahamas and Greater Antilles. This action helps to relocate the southern branch of the polar westerlies further and further inland, to the point that by June a unified stream is found near or just below the Canadian border. In the process, heat and drought tend to take over the Deep South while the middle latitudes show more rainfall with a retreating cP/mT boundary. Keep in mind that the timing of the -ENSO arrival appears to be different than the analogue years presented, which may render absolute precipitation and thermal comparisons relatively useless.

Late Season Winter Weather Threats

The lack of any pronounced blocking signatures during the first half of this winter is probably a good enough reason for the lack of cold and frozen precipitation in the eastern third of the nation. And while the past month has been colder and record snows have been seen in lake-related favored spots, the imminent collapse of the +PNA regime suggests a return to milder values across much of the U.S. to the right of the Rocky Mountains, if only for the medium range period. Since NO dual blocking scenarios have emerged and NONE are forecasted by indices used by the CPC, it figures that those frozen precipitation and cold intrusion events that do arise will be of a transitory nature. Still, with the return of a split flow configuration during the last month of calendar winter, it would be unwise to rule out a major wet snow episode or shock cold sequence. The most likely places for frozen types would be in the digging phase (western states) and phasing/linkage stage (Appalachia and some locations in the Northeast). Remember that most snowmaking systems in later winter are dependent on elevation, nocturnal presence, and strong vertical motion associated with the storm core.

Severe Thunderstorm And Tornado Potential

In what may be a steady transition from an El Nino to a La Nina, with a resurgent heat ridge and the elimination of the subtropical jet stream, conflict will result from an increased presence of mTw regimes with impulses from the southern branch of the polar westerlies. This spring may see a highly active convective pattern following this collision. Typically, the threat for supercells would start near the Gulf Coast in March, revert to much of Dixie in April, then occupy a more traditional "Tornado Alley" (Great Plains through the Corn Belt) in May. If we use the analogue of 1998, then by the end of spring the convective risk zone would stretch along the Canadian border into the Northeast, with the possibility remaining for damaging outbreaks of tornadoes, microbursts, and hail.