Warming Temps on the Way Thursday into Friday….Developing Closed Low to Bring First Widespread Spring Thunderstorm Outbreak as early as Sunday into Tuesday….Westerlies may Make a Run for the Coast by Mid Month…..
Wednesday May 1, 2013
Posted at 10:08 am by Howard
The Dweebs are pretty excited about the prospects for some Spring Time Thunder and showers beginning Sunday, then into Tuesday. This mornings 12z GFS deterministic run has an upper cyclonic center developing off the Northern Ca coast as early as Sunday morning. As it slides SW…It couples with an older Cut Off, currently trapped underneath an upper high to the north. (REX BLOCK) The old cut off should offer some additional moisture as it phases with the cooler, quasi continental short wave headed through the pacific northwest this Friday.
As the over land upper jet of 90knots rolls off the Northern Ca. coast….it spins up a new quasi cutoff which then couples, and thus opens the old cut-off low to release its moisture. Again as explained below in my old discussion, the high May sun angle along with good upper divergence aloft and our elevated heat source should all help to put on a pretty good show between Sunday PM and Tuesday. The upper jet is expected to be positioned into the South Central Ca. coast…favorable for good upper divergence over the Central Sierra. The upper low is expected to eject through Ca Tuesday.
However, you can never trust a cut off with it timing out of here so stay tuned!
Longer Range: Anomalous positive heights will develop over the pacific NW later next week while the wave length open up a bit across the CONUS. Will have to keep an eye out for the westerly’s making a run for the west coast later week 2.
Dr. Howard and the Dweebs………………….:-)
Upper Air Pattern to change the following week for the possibility of Showers and Thunder.
A developing REX block is occurring out at 140W. The REX is highlighted by an upper level low below “latitude wise” an upper high to the north. This pattern, at this Longitude, this time of the year, keeps the westerly’s to our north and often times our weather fair. However, the block is expected to break down late next weekend with the upper cut-off drifting toward California. At the same time, a short wave dropping SE from the pacific north west may either diverge to the Rt, thereby coupling with it, or eject it through Southern Ca to our south. The position or exact track of the upper low will be important to the forecast for our weather next week.
At this time, confidence is low because of the distance in time. However, with antecedent warmth of the upcoming weekend and the possibility of increasing divergence and cooling aloft, an unstable atmosphere may very well develop over the elevated heat source of the Sierra. Showers and thunder for the high country is a good possibility beginning Sunday PM the 5th through the 7th or 8th of May.
Longer Range Thoughts:
The Dweebs note that the QBO is still very neutral in its index and as is often the case, blocking over near the Greenland area persists. The current forecasts of the more significant teleconnections of the AO and NAO are neutral to negative and the PNA index forecast is of a weakly positive sign into May. So more of the same weather is expected for some time to come. Warmer than normal over the Far west and southwest and Colder than normal over most areas of the Midwest and wetter then normal over parts of the east and southeast. Will the Midwest have a slower start to their growing season?
Dr Howard and the Dweebs……………………………….:-)