Tuesday, September 6, 2011

A steadfast pattern for the week ahead

I thought I'd do a quick look at what the GFS model is saying for the weather over the next week.  Let's start with this morning's analysis.
GFS 500mb analysis from 12Z, Sept. 6, 2011.  Annotations by LM.
This is this morning's 500mb analysis from the GFS model.  We can see the remnants of Tropical Storm Lee as marked by the deep trough with a small cutoff area in the south.  Lee's remnants are interacting with a frontal boundary draped through the east coast states.  From the thermal wind relation, we know that surface temperature gradients (like fronts) imply strong winds aloft parallel to the front axis.  Therefore, we can infer that there is a front across eastern New England and the mid-Atlantic states from this jet streak aloft.  Lee has brought with it very deep tropical moisture, which is helping to fuel some incredible rainfall amounts up and down the east coast.  Here's this morning's precipitable water analysis from the GFS:
GFS precipitable water analysis at 12Z, Sept. 6, 2011.
That swath of dark greens running up the east coast indicates precipitable water values in the 2-inches-plus range--large amounts of water in the atmosphere.  No wonder we're seeing such flooding rains.

Further to the southeast and just coming onto the map is Hurricane Katia.  As far as Katia goes, it may be a powerful category three storm right now, but ensemble track forecasts don't show it making landfall.
ECMWF ensemble track forecast (50 members) for Katia as of 12Z, September 5, 2011.
However, dangerous rip currents and some high swells will definitely be possible due to Katia's approach, even if it remains well off-shore.

Get familiar with this overall synoptic pattern, though--it's going to stick with us.  Particularly with respect to the idea of a large ridge over the western US and a trough over the Mississippi and Ohio River valleys.  Here's the GFS 500mb forecast for Thursday morning:
GFS 54 hour forecast of 500mb heights and winds, valid 12Z, Thursday, Sept. 8, 2011.
We can see Hurricane Katia spinning off of Cape Hatteras, staying well off shore.  However, I'm kind of suspicious of this upper-air pattern from the GFS as we don't often see big, circular cutoff lows hovering around the Ohio River valley and the upper midwest.  But, the ECMWF is showing something somewhat similar (though the trough is a little more compact).  Regardless, low heights aloft mean cooler air aloft, which in turn means a more unstable atmosphere. A more unstable atmosphere is susceptible to vertical motion and, consequently, clouds and rain.  As long as that trough lingers, there will be daily threats of rain for that area.  Now, the moisture available to work with won't be like what we are seeing now on the east coast.  Here's the GFS precipitable water forecast for the same time:
GFS 48 hour forecast of precipitable water valid 12Z, Sept. 8, 2011.
We can see the copious amounts of tropical moisture that Katia is bringing with it off the coast.  However, in the Indiana-Ohio area, while there is still elevated precipitable water values, nothing is nearly that deep.  As such, there will be moisture to work with, but not enough to get really intense rainfall.

Meanwhile, out west, things look pretty dry.  Being under a ridge, general subsidence will prevail everywhere and we should see lots of clear skies, warm temperatures, and little chance of rain.

Let's fast forward to Sunday.  Here's the 500mb forecast from the GFS again.
GFS 120-hour forecast of 500mb heights and winds valid 12Z Sunday, Sept. 11, 2011.
We still are forecast to have the same overall pattern--trough in the east with big ridge in the west.  But note that the character of the trough has changed. It's no longer a cutoff trough--the contours around it are not closed circles.  Instead, the trough has become an open-wave trough again.  Open-wave troughs tend to be more progressive in moving along than cutoff troughs, which tend to stick around for a while.  So, this gives us some hope that the trough might move on by next week, hopefully bringing an end to the perpetual chances of showers throughout the midwest.

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