Only Thing to "Fall' on Friday Will Be Raindrops

The Sun’s away

And the bird estranged;

The wind has dropped,

And the sky’s deranged;.....

This morning 9/22/2011 at sunrise

......Summer has stopped.

And some are very frustrated about seeing it go so fast. My sentiments exactly.

Grrrr...

Fall begins at 5:05 A.M. EDT, Friday morning. The autumnal equinox is defined as the point at which the Sun appears to cross the celestial equator from north to south. The word equinox means “equal night”: Night and day are about the same length of time. In addition to the near equal hours of daylight and darkness, the equinox is a time when the Sun’s apparent motion undergoes the most rapid change. Around the time of the equinox, variations in the position on the horizon where the Sun rises and sets can be noticed by alert observers from one day to the next. Eventually, it will be rising and setting along the southern horizons as we progress toward December 21-22 as the first day of the Winter Solstice approaches. 

TODAY: Deeper layer moisture is entering the picture as expected, although this still does not mean widespread rains due to lack of any boundaries and few triggers. Ample daytime heating though will give rise to  respectable upward motion (vertical velocities). The models are again a bit out of whack, with even the short term RUC rotating an  inverted mid-level trough from south to north during the day across South and Central Florida. This, in turn, would change steering currents from SSE to nearly SSW-SW by late today. Do not believe that will occur to such an extreme, but continued SSE-S Flow seems more likely, with winds closer to the ground remaining ESE-SE. This would progress the east coast sea breeze inland for the focusing mechanism for the peak storm period after 3-4pm west of I-95. If winds aloft do become more southerly, then outflow or even back building could work storms toward US1 north of Vero Beach by 5pm, to possibly the beaches from Daytona and north. Best hedged bet relies on storms remaining in the interior at this point though.

Up to this point this morning, it appears some showers with lightning from time to time generated from eddies off the western Bahamas,   worked into Palm Beach County. Since earlier this morning, they have been working up the east coast, with some renegade showers reaching the coast further north toward Flagler County. The showers seem to be increasing off shore now that the sun has risen. With that, perhaps the RUC has something there with the more SSE-S Flow to develop later in the day

 . 

But for now, it's probably best to rely on persistence, since there is nothing evident that would preclude a persistence forecast of past model trends. What is happening out in the Gulf is interesting though. It appears much of the activity there is being provoked by divergent Southern Branch jet stream winds...which will remain over the Gulf for at least 2 more days. These are shown further down in this post.

Showers along the East Coast should work north and possibly increase after 10AM. Some could contain lightning, more so after 11AM to noon. If so, outflow from these near the coast and just offshore combined with the prevalent ESE-SE flow at the surface will send boundaries inland for greater storm coverage. Storms could start to occur earlier today than in the past two days., possibly peaking between 3-5pm.

FRIDAY-SUNDAY: Not any change in line of thought for this time frame with an increased chance of thunderstorms   most anywhere. The pattern will eventually evolve as to end the morning rain showers near the coast or at least put a lid on the amount of them, with afternoon storms forming by early to mid-afternoon and ending near sunset. Storm motion will be slow, but they should work northward and  a bit toward the east, especially on Saturday and Sunday.

It was noted that a La Nina pattern was evolving or had evolved in August in a post over a week ago. What does that can mean as we work into winter? 

In regard to the tropics, tropical storms and hurricanes have hit Florida during La Nina events in October. So far, it looks unlikely to occur, at least through the first week of October. There has been, coincidentally since the La Nina Event was made public, a developing southern branch jet stream running across Southern California toward Florida...it is quite apparent still in model guidance..and is expected to remain in place to varying degrees over the Gulf and near Florida for quite some time to come.

 Here is the forecast for today's jet stream and the forecast of October 6  

 

A general  discussion concerning La Nina years follows.  Note that from these images above and then  provided below, that a La Nina like  pattern seems to already be established. It does not mean this will always be the pattern all the time, just the more prevalent one. 

.La Niña conditions returned in August 2011 due to the strengthening of negative sea surface temperature (SST) anomalies across the eastern half of the equatorial Pacific Ocean (Fig. 1). With the exception of the far westernmost Niño-4 region, all of the latest weekly Niño index values were –0.5oC or less . Also supporting the return of La Niña conditions was the strengthening of the below-average subsurface oceanic heat content anomaly in response to increased upwelling and further shoaling of the thermocline across the eastern Pacific Ocean

The better model performance, combined with the historical tendency for significant La Niña episodes (as in 2010-11) to be followed by relatively weaker La Niña episodes, leads to increased confidence that La Niña will persist into the winter. While it is not yet clear what the ultimate strength of this La Niña will be, La Niña conditions have returned and are expected to gradually strengthen and continue into the Northern Hemisphere winter 2011-12.

The Climate Prediction Center defines. . ."La Niña conditions" as existing when: A one-month negative sea surface temperature anomaly of -0.5C or less is observed in the Niño-3.4 region of the equatorial Pacific Ocean (5ºN-5ºS, 120ºW-170ºW) and an expectation that the 3-month Oceanic Niño Index (ONI) threshold will be met AND An atmospheric response typically associated with La Niña is observed over the equatorial Pacific Ocean Across the contiguous United States, temperature and precipitation impacts

associated with La Niña are expected to remain weak during the remainder of the Northern Hemisphere summer and early fall, and to generally strengthen during the late fall and winter. 

During September-November 2011, there is evidence that La Niña favors an increased chance of above-average temperatures across the mid-section of the country, and an increased chance of above-average precipitation across the Pacific Northwest

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html

La Niña episodes are associated with three prominent changes in the

wintertime atmospheric flow across the eastern North Pacific and North

America. The first is an amplification of the climatological mean wave

pattern and increased meridional flow across the continent and the eastern

North Pacific. The second is increased blocking activity over the high

latitudes of the eastern North Pacific. The third is a highly variable

strength of the jet stream over the eastern North Pacific, with the mean jet

position entering North America in the northwestern United States/

southwestern Canada.

Accompanying these conditions, large portions of central North America

experience increased storminess, increased precipitation, and an increased

frequency of significant cold-air outbreaks, while the southern states

experiences less storminess and precipitation. Also, there tends to be

considerable month-to-month variations in temperature, rainfall and

storminess across central North America during the winter and spring

seasons, in response to the more variable atmospheric circulation throughout

the period

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/nawinter.shtml