Wednesday, May 17, 2006

Indian Ocean/Indonesian Convection -- Great Plains Severe Convection Relationship?

Our latest weather-climate discussion for the ESRL/PSD MJO international web site has been posted dated May 17, 2006. Please see

to view it.

The following is taken from Section 2 of this discussion called “Predictive Insights”, with some wording changes to reflect the most updated information. Also, the following is a link to our recently accepted paper by MWR which discusses the GSDM.

SSTAs across most of the central and east equatorial Pacific are within .5 deg C of normal, with SSTs ranging from ~26C near 100W to 29C and warmer just west and southwest of the date line. SSTs of 29C and higher also extend from the southwest Pacific into the IO. At depth, anomalies around plus 1-2C extend from 50-250m east-west along the equatorial cold tongue, meaning a slightly deeper than normal oceanic thermocline. SSTAs from the IO into the west Pacific are at least plus .5-1C, with the Pacific horseshoe experiencing ~ plus 1-2C. The Caribbean into much of the NTA also has SSTAs ~ plus 1-2C. Latest prediction from CPC expects ENSO-neutral conditions to prevail during the next 3-6 months (see latest TAO data here, ESRL/PSD data here ). The following are links to ENSO discussions.

At this time the signal from the MJO is weak with the WH upper tropospheric divergence signal re-emerging into the IO. Satellite imagery (eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa; other imagery here ) still has enhanced convection extending from the east Pacific ITCZ across tropical South America and into especially central Africa. However, during the last few days there has been a rapid increase in the tropical convection across the very warm SSTs of the central IO particularly near the Bay of Bengal. Tropical convection is less organized from the west Pacific toward Southeast Asia (with the important exception of Typhoon Chanchu; see here). Convection has also become a little better organized along the SPCZ over the warm South Pacific SSTs east of Australia. Suppression, which has been over the IO for a few weeks, has abruptly shifted into the northwest Pacific Ocean (latest 3-day averages of OLR total and anomalies, and other data here ).

Statistical and numerical models of the MJO (see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools) generally support the notion of enhanced convection continuing to organize across the IO from tropical Africa by about early week 2, with suppression generally north of Indonesia. SSTs have also remained slightly above average across portions of the central and southern Indian Ocean (~ plus 1-2C), which can be a precursor to a re-emerging MJO into the EH. Finally, SSTs are ~ plus 1-2C above normal across the tropical southwest Pacific, as well as the subtropical North and South Pacific and Atlantic Oceans. These regions will also contribute tropical convective forcing at times. The seasonal cycle will play a role to whatever evolutionary route the tropical convective forcing takes during the next few weeks.

Since April 28th the interplay between SSTs, tropical forcing and circulation response-feedbacks have produced a situation best described by GSDM Stage 3. Recently, with the intense tropical convection returning to the IO, the global circulation is believed to be transitioning to GSDM Stage 4. Plots of recent daily mean 150mb and 250mb daily mean vector wind anomalies clearly present a signal of twin subtropical centered anticyclones ~ 0 deg. longitude as well as the lingering pair just east of the date line. Twin anticyclones, as part of the lower latitude wave trains which characterize GSDM Stage 4, are also developing ~120E (Typhoon Chanchu is also a contributor). The EH subtropical cyclones have all but disappeared. The North Pacific Jet has retracted (with amplification) during the last several days.

AAM anomalies are ~.5 standard deviations above the 1968-1997 climatology, with anomalous zonal mean westerly flow across particularly the SH tropical and subtropical atmospheres, and the midlatitudes of both hemispheres. Anomalous zonal mean easterly flow exists along ~30N and 30S, as well as the NH polar latitudes. The latter are actually a residual of the "bursting anticyclones" that started in March 2006.

AAM tendency has again become positive (~ 2 standard deviations; see plot ), with much of that due to positive torques from north-south mountain ranges including those in East Asia and the Andes (see plot). The frictional torque remains negative, and will remove the added zonal mean westerly due to the mountains (see plot ). In the context of the GSDM, right now we are seeing a submonthly component helping to maintain existing anomalies. However, with the IO convection intensifying, we believe Stage 4 is most probable by the end of week 1, with a transition to GSDM Stage 1 during the week 2-3 time scale, particularly should the tropical forcing move into Indonesia. Of course, the details (timing, location and amplitude) are unclear after roughly day 3.

For the PNA sector, and particularly the CONUS, a ~140W trough, ~110W ridge and eastern USA trough still looks like a good bet for much of week 1. However, these synoptic features are expected to progress by the end of this period as the circulation evolves into GSDM Stage 4. Nearly all models and their ensembles have locked onto this prediction, especially the CDC and NCEP ensembles. The current high latitude retrogression of anticyclonic circulation wind anomalies extending from the North Atlantic Ocean (leading to a negative NAO) into eastern Canada will be a contributor to the weather across the CONUS not only for week 1, but afterwards as well.

As the North Pacific Jet retracts and interacts with the retrograding anticyclone, a ridge across the central Pacific possibly extending well into the Arctic is probable as week 2 approaches. A downstream stronger trough should begin evolving just off the USA west coast, linking with a STJ extending into the southwest and south central states. A ridge is probable across the central USA as a trough lingers across the east and northeast states. During the week 2-3 time-scale, an active GSDM Stage 1 pattern may return although farther north than March due to the seasonal cycle. Experience tells us that the numerical models will likely struggle again as the tropical convective forcing comes back into the EH, particularly should a GSDM Stage 4-1 transition occur. In fact, inconsistencies between model forecasts have already been increasing during the last 3-5 days.

The following is a breakdown for weeks 1-3. Links to additional model output is given at the end. My confidence in the following overall scenario is about average to even slightly above average; however, forget that with timing, etc., details. As I will continue to say, statements of probability are needed to maximize skill for weeks 1-3 and beyond.

Week 1 (18-24 May 2006): Overall, still not a very active pattern for much of the USA, at least early on. GSDM Stage 4 is most probable during this time. Roughly the eastern third of the country looks to have below normal temperatures while record maximum temperatures are possible for locations such as the Intermountain west and the Front Range extending into the Southern Plains. Most shower and thunderstorm activity will be east of the Mississippi River, with little precipitation to the west. While the Northeast states gradually dry out, rainfall should increase across the Pacific Northwest. Gulf of Mexico moisture transport will improve by early next week, and the possibility for one or two episodes of strong/severe west-northwest flow thunderstorms for the Northern Plains/Upper Mississippi Valley into the Great Lakes/Ohio Valley is a concern. Relatively isolated high-based severe storms may be possible along the dryline from western Nebraska-Texas Panhandle due to the STJ. Finally, record low minimums and late season frost may also be a concern for at least the upper Great Lakes States.

Week 2 (25-31 May 2006): GSDM Stage 4 would be probable by the start of this period, possibly transitioning to GSDM Stage 1 by the end. As Gulf of Mexico moisture transport continues to improve (SSTs are above average there, ~ plus .5-1C), widespread severe local storms may become an issue for much of the central USA, particularly centered on locations such as Nebraska-Iowa on to the north and northeast. Dryline storms may become quite intense across the western High Plains. Warmth should also spread eastward from the Central and Southern Plains into the Tennessee Valley/Deep South. In fact, portions of the South may see record highs with temperatures in excess of 100F along with surface dewpoints at least into the 60s (vertical mixing may limit magnitudes). Near normal to slightly cool conditions may persist across the east and particularly for the Northeast while below normal temperatures spread across the Pacific Northwest.

Week 3 (1-7 June 2006): A transition from GSDM Stage 4-1 and possibly to Stage 2 is a concern for this period and may continue into week 4. That would mean an active southwest flow storm track pattern for the Plains as troughs come into the western states and then move northeast into the Upper Mississippi Valley. Tropical moisture transport from the Gulf of Mexico should be plentiful by this time (if not much sooner). The weather would be similar to what was observed during March, only farther north and west. A focus for severe local storms may occur from the Northern/Central Plains into the western Great Lakes States. If GSDM Stage 2 (with the tropical convection moving into the west Pacific) does evolve by, say, late week 3 into week 4, troughs digging into the central Rockies and High Plains would be more probable. That would allow the storm track to shift farther southward across the Plains, perhaps leading to a situation we saw ~May 5-9.

Specifically for southwest Kansas, the dry pattern is back (was only interrupted). Much warmer temperatures are likely starting ~ May 18, with highs well into the 90s. Starting this weekend into the following week, there may be a strong STJ to contribute to at least isolated high based thunderstorms across the elevated terrain to our west. As these storms move by, lightning and high wind may be more probable than decent rainfall; however, we will see. The main storm track will be shifted north of us, as climatology alone says. Temperatures are likely to remain well above average, with days in the 90s with even a few locations above 100F. By week 2, should GSDM Stage 1 return, that would suggest troughs coming into the Rockies and then moving northeast into the Upper Mississippi Valley. Severe local storms would likely accompany these. However, with the storm track to our north, that could mean “hot, windy and dry” at times. However, with less momentum to mix down I would think at least dryline storms and attendant rainfall would be possible (as discussed above).

Lets see if GSDM Stage 2 returns sometime during early June, as discussed above. For those really hopeful of getting decent widespread rainfall across southwest Kansas for this upcoming warm season, SSTs have been warming across the west central Pacific during the last couple of weeks. That would favor tropical convective forcing to propagate farther east, possibly favoring more persistent GSDM Stage 2 patterns.

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from Storm Prediction Center not only for severe storms, but also fire weather concerns. Finally, the CPC USA Hazards Assessment for offers additional insights not only for possible week 1 high impact weather, but week 2 as well.

I will be on quite a bit of travel through Friday, May 26. Thus I may not be able to do another writing on this Blog until Memorial Day weekend (which may be quite stormy for a good portion of the USA).

Ed Berry

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