SPRING 2005 SEASONAL FORECAST
The East Spring weather pattern for 2005 is going to feature above normal precipitation over large portions of the Plains and the Midwest and rather warm and dry conditions across the Southeast which will begin to extend up into the Northeast US by the end of the season and the approach the summer. Over the western US the dry conditions that the Pacific Northwest is seen for much of the Winter will see some intervals all of above normal precipitation which will alleviate the dry conditions to some degree... but by late in the season interior sections all of the Pacific Northwest as well as the Rockies the overall pattern will turn drier.
REVIEW OF THE WINTER 2004-05...
The winter all of 2004 --05 was a mild the winter across much of the Plains with the Lower Plains seeing adequate moisture but little snow cover... the Pacific Northwest saying significant precipitation early then a rather dry second half the winter... and intervals of rather distinct wet pattern over southern California which contributed to the rather wet and mild winter over the Lower Plains and the Delta... the eastern US (East of the Mississippi river) experienced several intervals of very distinct above and below normal temperatures which developed in 15 day cycles -- Much above Normal DEC 1-15 JAN 1-15 FEB 1-15 and periods of Below Normal temps DEC 16-31.... JAN 16-31... FEB 16 - MARCH 14.
WxRisk is in the process of doing a extend to review of the winter of 2004 --05... but as we take a look at some of the sea surface temperature maps we can see that the winter was dominated by several key features... which I call sea surface temperature anomaly (SSTA) couplets or SSTAC.
MID DEC 2004
Tthe first map shows the SSTA from December 17 and not highlighted for key areas for consideration.
AREA A = was a SSTAC that developed from northern Japan into the East coast of Siberia...
AREA B = was a SSTAC over central Pacific Ocean
AREA C = was a large pool of warm SSTA that developed from the north central Atlantic into the area around Greenland Iceland and the Davis Straits... and finally AREA D= was a SSTAC that developed off the Southeast coast of Brazil and eastern Argentina into the south central Atlantic... in a manner with very similar to what we saw last season.
The two critical areas for North America in winter 2004-05 turned out to be AREA B & C... which dominated the winter pattern across most of North America. The development of large pool of warm SSTA at 160-150 West longitude which was really part of a SSTAC... was thought by some forecasters likely to shift slowly with time reaching the West coast of North America by the middle the winter which would develop a +PNA pattern (strong Ridge on the West coast = warm & dry ...and deep trough on the East coast meaning cold and wet). However this pool of warm water movement to the West coast of North America was delayed by several weeks ... and the development of large pool of cold water off the Baja coast was able to sustain a trough along the southwest coast of North America during the first portions of the winter . It was this trough that was able to sustain several major early season winter storm systems into central and southern California as well as the Great Basin and the first time in several winters.
The other major facet of this winter was the development of a large pool of the warm SSTA in the north central Atlantic that extended into the area around Greenland Iceland and the Davis Straits. There several efforts by several different weather forecast services to portray this pool of warm water in the north central and Northeast Atlantic as a "classic warm pool of SSTA configuration " that one sees around Greenland during the winters months that have featured high latitude blocking known as the Greenland block (-NAO). However that really was an nothing more than a feeble attempt of spin or hype to essentially force a colder pattern for the eastern US which was not going to happen during the first half the winter.
Indeed the data clearly shows that when there are "High latitude blocking event " in the Jet stream over Greenland ... which sometimes extend into Labrador or the Davis Straits... there is a distinct SSTA configuration in an and around the area of Greenland. This is SSTA configuration features a large pool of warm SSTA in the area but also features a large area of rather cold SSTA just to the south in the north central or northeast Atlantic Ocean. In other words there is a Couplet of warm and cold SSTA located in close proximity to each other in and around Greenland and it has been theorized that it is this couplet of warm & cold SSTA that supports prolonged episode of Greenland Blocking . But as you can plainly see from the mid December sea surface temperature maps that is not the case... instead we see a large extension of warm water covering the Atlantic Ocean from the subtropical regions all the up towards Iceland.
During this period we saw the excessively warm temperatures that dominate most of the eastern US as well as the increased storminess over the West Coast. Note how AREA B... the pool of warm SSTA located over the central Pacific had weakened which meant that the strong Ridge in the eastern Pacific and the trough along the West coast was going to become weaker. Also note that AREA C ...the large area of Warm SSTA in the central north Atlantic actually slide SW to the East coast which ended up strongly supporting the Ridging over the eastern US. In AREA D the large area couplet of warm and cold water in the far southeast Pacific... and warm SSTA off the Brazil.... and cold pool in the south central Atlantic set up a very rainy pattern for much of Argentina and Southeast Brazil.
As the pattern turned colder over the eastern US the pool of warm water -- AREA C -- was so located just off the East Coast which meant that the Ridge which had been situated over the Southeast US in the first 15 days of January was not far away. It was this close proximity of the very warm SSTA to the East Coast as the primary reasons why WXRISK forecasted a return to very mild conditions again in the first 15 days of February even though there was overwhelming data from many of the medium-range models that a much colder pattern was going to locked in the early February (i.e. the over hyped and somewhat unreliable CDC NAO ensemble index). Since most forecasters do not pay any significant attention to the SSTA configurations... I was pretty much alone in forecasting this development which fortunately for me turn out to be correct. Note that AREA B saw a strengthening or increase in the warm SSTA in the east central Pacific with a somewhat eastward shift since mid December.
AREA D saw a significant weakening all of the SSTAC in and around the southern portions of South America but the rather wet pattern continued for much of Argentina and Brazil.
EARLY & MID FEB 2005:
AREA B while sliding east ever so slowly... saw a significant weakening again of the pool of warm SSTA in the east central Pacific and this allowed for the Ridging in the Jet stream to weaken... which tended to support or favor a more normal or mild pattern across the CONUS. AREA C saw the pool of very warm SSTA slide East back out to the central Atlantic Ocean which supported the Ridge developing in the east central Atlantic up towards Iceland and Greenland and did NOT support a sustain interval of a Greenland Blocking (negative phase of the NAO) . Given these features and how they had shifted from JAN 22... it is not surprising that the first 15 days of February saw more mild temperatures across the CONUS.
By Mid FEB a pool of somewhat colder than normal SSTA that began to develop off the Southeast coast of the US / Bahamas in early FEB began to increase in size and intensity ...which lead to the formation of a SSTAC with the pool of warm water in the north central Atlantic. This was the first time all winter we have began to see the classic cold / warm SSTA COUPLET that one looks for to see the sustained intervals of High Latitude blocking such as a Greenland / Block (a.k.a. negative NAO).
AREA D saw a change in the configuration of this SSTAC... the pool of cold water in the far southeastern Pacific had shifted further out away from the West coast of South America and the resulting change in the SSTAC was a pool of cold water in the south central Atlantic situated next to an increasing pool of warm water. This was nearly identical to the sea surface temperature anomaly configuration from this time last year which was a major factor in the development of the widespread drought over eastern Argentina and southeastern Brazil.
METEOROLOGICAL ISSUES SPRING 2005
1. WEAKENING EL NINO The link here clearly shows the moderate to weak El Niņo which we have been experiencing for most of the winter is now coming to end. Keep in mind that the El Niņo criteria is defined by the SSTA in the ENSO regions known as 3.4 (there are four regions with the El Niņo "area" which cover the equatorial Pacific from the coast of Peru out to the Dateline). However the climatology of weak El Niņo's events that last or end in the last Winter into the Spring is very clear and shows a distinct bias for rather colder than Normal Spring season over the middle and eastern portions of the US as well as Spring season which features above normal precipitation for the central US.
2. THE BLOCK OVER EASTERN CANADA
The high latitude blocking pattern which has developed in the middle of February and continued into the second week of March can be seen here. However all the last week the Greenland block which was located over Quebec Canada.... was seen sliding to the West while at the same time High pressure continue to develop over Alaska. This westward movement of this blocking feature in the Jet stream over Eastern Canada is referred to in the weather business as "Retrogression".
This Retrogression is one of the reasons why I continue to show above normal heights moving across central and western Canada over next 45 days. And with the Blocking feature we will continue to see strong support for an enhanced Pacific Jet stream bringing in weather systems ( short waves) into the West coast and support a colder than normal pattern over the Midwest and New England.
Eventually the blocking feature will move towards the West coast of Canada and at that point I believe we will see a split flow develop in the Jet stream pattern by the middle of the season.
3. POSSIBLE CHANGES IN SSTA PATTERNS.... we have finally seen the large area off warm pool of SSTA slide East and reach the West Coast of Canada. This pool of warm water normally supports the appearance of a Ridge in the Jet stream over western North America and a trough over the eastern US ( a.k.a. the +PNA pattern). However and this particular case the pool of warm SSTA is fairly weak and has lost a lot of its intensity over the last several weeks. As a result the ridge on the West Coast is not going to be very strong and the energy from the Pacific Jet should be strong enough to pierce the Ridge with considerable frequency. If the ridge was stronger I would be forecasting a much drier pattern over Pacific Northwest but because of the weakness in the warm SSTA pool off the West coast of Canada ... I see a Above Normal precipitation pattern for the Pacific Northwest in the Spring 2005 season.
In addition the strong pool of warm SSTA in the north central Atlantic is s till there and with a Ridge re-developing over western Europe into the UK ... this supports a return to the pattern we saw DEC 1-FEB 15... adjusted for the season of course In other words a + NAO.... strong Ridging off the SE coast .... the mean trough over the Plains and WCB... storm track from the central Plains across the Midwest into Northern NW England.
4 CURRENT SOIL MOISTURE PATTERNS
There is been quite a bit of research done in seasonal climatology and forecasting a little last 10 to 15 years regarding the connection between excessively wet soil and excessively dry conditions. There is an old saying in the weather business that when in drought forecaster out and when you are in a wet pattern continued forecaster but pattern. That idea has some validity to it especially when the SSTA the show little if any significant change.
From the National Drought monitor web site
we can see 2 relatively new products from CPC -- the short-term and long-term drought Indicators as well as the long-term Palmar Drought index. The CONUS is divided up into climo regions and as we can see the areas over the Ohio valley / ECB as well as the central and Lower Plains appear to be fairly Wet while there are large Dry areas over the Pacific Northwest the High Plains and northern Rockies. In order to use the soil moisture as a hint or indicator of what future pattern might resemble one must keep in mind that its not as simple as the soil being slightly moist or slightly dry but prolonged areas of Much Above for Much Below Normal moisture levels which cover significant portions of the region.
From these indicators and maps of the current soil moisture areas across the Conus CPC has run the soil moisture climatology model which is reasonably accurate Model out to 3 months. And as you can see the forecasted soil moisture is a continuation of the current pattern and in my opinion has implications for the overall seasonal pattern look like in the spring of 2005.