A weak upper level low was spinning up along the Northern CA coast this morning well NW of Mammoth and at the same time a weak upper jet was spreading some high cloudiness over our region.    New 12Z 250mb Nam 212 shows strong divergence over the sierra from Southern Mono County south over the Northern portion of the Southern Sierra between 2100Z Tuesday and 00Z Wednesday. However the 700MB RH has only between 40% and 50% RH, so there is little moisture to work with tomorrow afternoon instability. This is a case that if we had the kind of moisture that we had a week or two ago, during the monsoon SE flow, we would be looking at heavy rain with a system like this. This time only the slight chance of a few showers or thunder boomer late in the afternoon and early evening.

The over all upper pattern is changing from the peak of the Summer a few weeks ago.  There is in process a westward shift in the key anomalies over the northern CONUS with the coldest weather retrograding toward the northern Great Plains instead of the Great Lakes and the upper ridge that has been over the pacific NW back toward 140west.  So far the Monsoon is focused over the 4 corner states and should remain well east of the Eastern Sierra.

Looking at the prospect’s for some rainfall from an eastern pacific tropical storm, as long as the Subtropical continental upper high is extended far enough west, the likelihood of significant rain bearing subtropical moisture getting this far north is very remote…..However, with that said, the odds will begin to increase by the very end of the month of August and especially into September as the subtropical continental high begins to weaken climatically.


Interesting article from WSI




Toms Corner:

The models appear to be flip-flopping a lot with the position of low off Ca coast next 3 days which is to be expected with any weak closed low. However the 12Z GFS is now giving this low the boot ejecting it inland on Tuesday with resulting PVA and deformation in NE quadrant kicking up the thunderstorm potential over the Sierra by Tuesday afternoon. The 00z EC constructively phases CA lo with subtropical low moving n from Baja but retrogrades dumbelling low to nw while stronger trough digs into nrn Rockies. 12Z GFS on the other hand destructively phases with Baja low which helps spin up stronger west to east upper jet which in turns helps to eject Ca low ewd Tuesday. In a majority of cases I’ve seen with tropical or subtropical lows in Ern Pacific there is more of a tendency toward destructive phasing which tracks energy ewd south of our area which usually diminishes the chances of seeing a tropical energy connection this far north. Watch the trend of the 200 mb isotach forecast on the SJSU site as this phasing occurs and the resulting upper level divergence field. You will typically see a rapid spin up of jet speeds and divergence as the tropical low comes closer or merges with the northern lattitude low or trough. The rapid increase in upper divergence will result in a large plume of high and sometimes mid level level moisture that you typically see on the IR and Wv satellite loop eminating from the NE quadrant of the tropical system.  The lower level moisture at 700 mb and below will usually stay with the parent low level circulation which often times dissipates over colder waters. The mistake many forecasters make is believing the high level moisture plume seen on WV loop will somehow initiate thunderstorms over the Sierra but more often it cuts off solar radiation and decreases instability, needed for deep convection.  What can happen as seen a few weeks ago is the upper jet that is spun up from this subtropical connection will proving a favorable upper divergence pattern over a pre-existing area of unstable air (positive CAPES and lifted indices at zero or less) and if enough low or mid level convergence occurs to trigger this instability then deep convection will occur. So when these tropical moisture surges appear on satellite loop you have to ask if they are high level debris clouds supported by upper divergence or is it the actual low circulation being constructively phased with jet intact moving north which would support a convergence/divergence couplet and a true deep layer tropical connection.  Its all in the details best seen on the SJSU and SPC graphics.