IS THERE A SUMMER DROUGHT BUILDING FOR SE USA/ LOWER MIDDLE ATLANTIC?
It seemed only few weeks ago that every one was complaining about how wet the pattern was --especially when it came to the weekends over the Middle Atlantic region and the East Coast in general. That perception was accurate because we were stuck in a pattern that with featured a lot of rain and especially during the weekends. But over the past 30 to 60 days the pattern has begun to change and the number of rain events are decreasing in terms of frequency and the rainfall amounts are also going down. In fact up until the big rains over Virginia and portions of the North Carolina on yesterday May 31, some data suggested a minor drought area was beginning to develop over the Southeastern states into Virginia.
This image shows the total rainfall over the past 30 days up to and including the 31st of May. Notice that in northeastern Pennsylvania the rainfall amounts have been between 6 to 10 inches and widespread rainfall amounts of 5 to 8 inches across New Jersey southeastern New York... the rest of Pennsylvania... all of Maryland.... and portions of Western Virginia... and Central Ohio. Clearly that area has not seen below normal rainfall over the past 30 days. But to the south you will see that much of central and eastern North Carolina has seen rainfall under 1.5 to 2.0" over the past 30 days which is somewhat below normal. In between we have areas of western North Carolina, the Virginia Piedmont, and central Virginia with the rainfall amounts are range of 2-4. " Again a lot of that fell just on May 31.
This next image shows the same sort of 30 day precipitation but it's based upon the percentage of normal. Again over much of southern and southwestern Virginia as well as most of North Carolina... eastern Tennessee and eastern Kentucky rainfall amounts are range anywhere from 50 to 90% of normal. And those big rainy areas over Pennsylvania, Maryland, New Jersey, and interior Southeast New York these rains were about 150 to 200% above normal rainfall.
here is the 60 day total rainfall and it shows widespread 8 to 10 inch rains across most of southern New England.... eastern New York State ..New Jersey ...Pennsylvania ...Maryland... northwest Virginia ...most of Virginia... Ohio... and portions of Kentucky. The green area represents somewhat lighter than normal rainfall with rainfall amounts between 4 and 6 inches mostly over central and southern Virginia the central and Eastern North Carolina
In looking at the rainfall anomalies over the past 60 days of the southern counties of Virginia east of Lynchburg as well as central and eastern North Carolina has seen 50 to 90% of normal rainfall. Meanwhile Western North Carolina as well as southwest Virginia and most of West Virginia and the northern third Virginia into Washington, DC and western and Central Maryland have all seen between 110 to 150% above normal rainfall.
With respect to actual rainfall amounts relative to normal... the numbers listed as inches not percentage we see that most of New Jersey... southeastern New York ...Delaware ...Pennsylvania ...Maryland and northwest Virginia have all seen between 2 to 6 inch rains above normal. Meanwhile again much of eastern and South Central Virginia were dry than normal.
This next image shows the overall 500mb pattern for North America MAY 12-13. The key feature here to notice the enormous trough over the Midwest and the persistent blocking pattern over Greenland.
This trough moved to the East Coast on the 13th bringing with it significant rain. But since then some areas in the Middle Atlantic have had not seen rainfall over 1.0" since May 13.
But after that the pattern began to change. This image shows the jet stream pattern for May 16 and we can see the deep trough that was over the Midwest has now moved off the East Coast and another deep trough is moving into the West Coast. In between these two big troughs we can see a strong ridge developing over the plains the Midwest. The development of that ridge ensured that the pattern would turn somewhat drier over the Middle Atlantic region since there is no way in this kind of pattern for systems coming in from the Pacific Ocean two across the country and make it to the East Coast up.
Next here is the overall 500mb pattern for My 18th. The strong block over Greenland continues to be very quite noticeable on all the weather maps all we can clearly see a ridge has developed over the eastern CONUS .The results is that these big weather systems coming in from the Pacific Ocean will not be able to cross the he Rockies and reach the Plains.
n May 21 we can clearly see one massive trough over the West Coast and the Pacific Northwest into the Rockies and the equally strong ridge over the Southeastern states.
By May 25 we have a huge trough over the West Coast and the Rockies and a equally strong ridge over the Southeastern states.
The development of this persistent ridge over the Southeastern states is one the reasons why the pattern has turned drier especially from the Pennsylvania/ Maryland border southward. Because the ridge is not very amplified -- it still early in the summer season it does not reach very far to the north - and as a result the pattern is still quite wet across portions of the Ohio Valley Pennsylvania New Jersey New York State and New England.
The current upper air pattern shows the deep trough over the eastern CONUS for this weekend. Now the heat wave is ended we have ideal early summer June conditions and will for the next several days.
One one the reasons why I am concerned about the potential for a drought or leased a dry summer it has to do with the sea surface temperatures in the Atlantic Basin.
This image shows the sea surface temperatures anomalies -- relative to normal -- as of May 26. Notice that the ocean water temperatures are quite warm relative to normal off the Southeast US coast and in the Western Atlantic Ocean. There also quite warm relative to normal in the central and northeast portions of the Atlantic Ocean. The seven day temperature trend however show that the water is in the West Atlantic Ocean were cooling off significantly and beginning to warm up in the Atlantic Ocean.
However the new data as of June 1 continues the trend of keeping the West Atlantic Ocean off the East Coast quite warm relative to normal. This extends across the central antic ocean up into the Azores and off the coast of Spain/ Portugal and western France. Meanwhile the far Eastern Atlantic Ocean is continuing to see colder than normal ocean water temperatures.
If we look at the anomaly trends we can continue to see that over the past seven days toward temperatures are of warm significantly off the southeast CONUS coast. Undoubtedly this is due to the extensive ridge and hot conditions over that part of the country and the Western Atlantic Ocean.
The reason why this is important is because the large pools of ocean water temperatures that are warmer than normal can and do impact the prolonged or persistence seasonal pattern. So in this case having a large area of much warmer than Normal Ocean where temperatures in the West Atlantic Ocean off the southeast us coast will tend to favor a persistent ridge to develop over the southeastern U.S. Coast or in the southwest Atlantic for most of the summer. This implies dry than normal conditions and potentially hotter than normal conditions especially if the all Nina continues to weaken and breakdown.
Indeed if we take a look at some of the weather models which will take us into the middle of June and beyond on this Saturday morning extended European ensemble for day 7 ..we see a significant RIDGE over Midwest and two large troughs - along the West Coast and off of New England. Normally this would be a dry pattern for the middle Atlantic and the Southeastern states but there may be a strong upper air disturbance over Arkansas and Louisiana next weekend which could bring that area significant rain and thunderstorms.
It seems a good probability that that disturbance will move east northeast into the Tennessee Valley and the Middle Atlantic states June 9 or 10th. This could lead to another round of moderate or Steven locally heavy rains and some areas over the southeastern states and the Middle Atlantic region.
Longer term the 11 to 15 day CFS mean shows a developing ridge of some intensity over than middle portion of North America . This new pattern which shut off the parade of moisture . dry for while as well as the Middle Atlantic region. New England and the great lakes however would see thunderstorms coming over the top of the ridge.
In the 16 to 20 day pattern the CFS ensemble is even more amplified with a deep trough over the West Coast and a significant ridge over the heart of the Midwest extending as far north as Hudson's bay Canada. And the trough is now well off the East Coast and the Atlantic Ocean. This would be a hot and dry pattern for the lower Midwest as well as the Middle Atlantic states and southeast. Assuming of course that the CFS 16 to 20 day is valid.
So in summary we are not yet had a drought by any stretch of imagination. We have a lot of deep soil moisture reserves but conditions are turning drier will continue to do so as we move deeper into the summer.
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