Sunday, March 20, 2011

Progressive Pattern This Week

It's been a while since I actually looked at a week-long forecast of what's up in the weather, so I thought I'd begin the week by taking a quick look at that.

A look at tonight's 00Z 300mb analysis shows the upper-air pattern:
Fig 1 -- 300mb objective analysis valid 00Z, March 21, 2011.  From the HOOT website.
Features of note include the rather deep trough just off the California coast, with smaller shortwaves over eastern Montana and the Dakotas and also just off the northeast coast.  In examining the wind pattern, it seems that the jet streaks really aren't that organized--there's lots of wind maxima floating around out there.  It should be no surprise, then, that the surface map tonight looks just as disorganized.
Fig 2 -- Surface objective analysis of temperatures (colors) mean sea-level pressure (contours) and winds (barbs)  at 00Z, March 21, 2011.  From the HOOT website.
There are a few scattered low-pressure centers that don't seem to be doing much.  There's a low up in northern Minnesota that seems to be associated with the leading edge of those shortwaves coming out of the Dakotas that I mentioned before.  Another surface low is analyzed in central Colorado--but is there really much upper-air support for that one?  The same goes for the low pressure center off the northwest Oregon coast--that looks to be right under the middle of the upper-level trough--a bad place to be if you're a low since you're away from the jet streaks and divergence aloft.  So based on the upper-air and surface analyses given, I'm really not seeing too much in terms of low-pressure centers strengthening and becoming big concerns--for now.

However, there's a lot of other things going on in the surface map above that I am interested in.  Note that even though there are no strong low pressure centers, there are decently strong gradients of pressure--particularly across the southern plains and up into the Ohio River valley.  These pressure gradients are helping to drive those strong, southerly winds we see in the surface analysis.  Those winds are helping to advect warm, moist air up from the Gulf of Mexico.  You can see in the surface analysis how far that warm air has moved up already.  In fact, there even looks to be a front stretching across the central plains, through northern Illinois and along the Ohio River.

But, that "front" doesn't seem to be associated with any low-pressure center.  You could argue that it's associated with that low in Colorado or maybe even the remnants of an occluding low over Minnesota.  But these look pretty iffy...

Without a low-pressure center really driving this front along, we'd expect this to be a stationary front--one that's not going to move much until the winds start organizing themselves better.  To do that they're going to need upper-level divergence to come in and start spinning up a surface low.  What would do that?  How about that trough moving in from the west coast?  Here's the GFS forecast for Monday:
Fig 3 -- GFS 24-hour forecast of 300mb  winds (shaded) and heights (contours) for 00Z, Tuesday (Monday evening) March 22, 2011.  From the HOOT website.
That deep trough from the west coast is forecast to move in with an organized, strong jet streak on its leading side.  The exit region of that jet streak (we could get finicky with the details and say the left-exit region, but that's more difficult to pin down...and besides, the area between a trough and a ridge is generally a divergent region aloft...) should bring some healthy divergence aloft to the central Rockies.  And, just like that, surface pressures are forecast to fall in that region...
Figure 4 -- GFS 24-hour forecast of surface temperature (colors), mean sea-level pressure (contours),  and winds (barbs) for 00Z, Tuesday (Monday evening) March 22, 2011.  From the HOOT website.
There's an area of forecast lowering pressures extending from Montana down through eastern Colorado, corresponding to the exit region of that jet streak.  Note how in 24-hours the GFS has not moved that frontal boundary at all--without a surface low to organize the wind field, that boundary won't move.

However, there is a curiosity (at least to me) that I can't fully explain.  We have that stationary front sitting there for some 24+ hours over the central United States.  It has a pretty good temperature gradient across it--mid-60s to the south of the front and 30s north of the front.  Since our winds aloft are connected to the temperature gradients below (via the thermal wind relation), I would have expected the upper-level winds to have picked up a bit over the frontal boundary.  Yes--they do pick up a little bit in this GFS forecast over the front in the 300mb image above, but not in a very organized way directly over the frontal boundary.  This puzzles me a bit, and makes me slightly suspicious of the upper-air forecast from the model.  Just a curiosity.

Anyhow, by Tuesday morning, the GFS (if it is to be believed) moves the upper-level trough out over the plains with a strong jet streak on its eastern side.
Fig 5 -- GFS 36-hour forecast of 300mb  winds (shaded) and heights (contours) for 12Z, Tuesday, March 22, 2011.  From the HOOT website.
However, notice that the "tilt" of the trough axis is changing.  In the first (and to some extent the second) upper-air maps shown above, if we were to draw a trough axis through the trough, it would be oriented roughly straight north-south.  This is called having a "neutral" tilt.  Now in the image above on Tuesday morning, if we were to draw the same axis, it would be tilted, going from northwest to southeast.  This is called being "negatively" tilted.  As the tilt of a trough axis changes, so do the locations all the forcings associated with it (i.e. the jet streaks).  At the time above, however, we see a nice, cyclonically-curved jet streak and would expect a strong surface low to be forming underneath the divergent exit region, somewhere over the central plains.  And that's exactly what the GFS is doing...
Figure 6 -- GFS 36-hour forecast of surface temperature (colors), mean sea-level pressure (contours),  and winds (barbs) for 18Z, Tuesday,  March 22, 2011.  From the HOOT website.
Now that stationary front that is STILL sitting across the central plains and across the Ohio River valley finally has a low-pressure center that can take over its dynamics and start advecting it around.  Or will it?  Regardless, the wind field is now being profoundly directed by the surface low.  This is enhancing convergence along that pre-existing front and also along a dryline and cold front in western Texas and New Mexico, respectively.  Aloft we know that a divergent region of a jet streak is moving overhead, particularly in the eastern and southern plains area.  With convergence below and divergence aloft, the stage is set for thunderstorms to start firing up on Tuesday.

One interesting aspect of this potential severe weather event on Tuesday is that the stationary front has been sitting there for over 48 hours now.  This low-pressure center did not have to do a whole lot of work to advect warmth (and moisture) north from the Gulf of Mexico--it's just sitting there already.  Thus we'd expect thunderstorms to be rather widespread as such a large area will have had time to bring moisture and warmth northward.

In accordance with this pattern, the SPC has a day-three slight risk out from the southern plains up into the southern Great Lakes:
Fig 7 -- SPC three-day convective outlook valid for 12Z Tuesday through 12Z Wednesday.  From the SPC website.
And, we now see, at least on the large scale, why they have this region highlighted.  Though I highly encourage you to read their discussion for an excellent description of what they're looking at.

However, the dynamics start changing by Wednesday.  Remember how the trough was becoming more negatively tilted?  The GFS forecasts it to continue to do so on Wednesday:
Fig 8 -- GFS 60-hour forecast of 300mb  winds (shaded) and heights (contours) for 12Z, Wednesday, March 23, 2011.  From the HOOT website.
This has the effect of what I call "flat-lining" the jet streak (that's not a technical term--just something I use) and making the winds more zonal (more west-east).  Our once-curved jet streak becomes a more straight jet streak, with divergence assumed in the left exit region, or over the southern Great Lakes.  Since we connect the winds aloft with temperature gradients below, we might expect another west-east oriented pseudo-stationary front to have set up underneath and parallel to the jet streak.  Here's the GFS forecast for the surface at that time:
Figure 9 -- GFS 60-hour forecast of surface temperature (colors), mean sea-level pressure (contours),  and winds (barbs) for 12Z, Wednesday,  March 23, 2011.  From the HOOT website.
We see the surface low right where we would expect it to be--in the southern Great Lakes region underneath the divergent exit region of the jet.  We don't exactly see a west-east oriented frontal boundary (yet...).  We do see that the cold front is not a very strong one at all--the temperature and wind gradients across it are very weak.  It also is sharply tilted and not parallel to the jet aloft--the thermal wind response may be trying to pull the front more horizontal to be in line with the jet streak aloft.  Furthermore, the cold front doesn't even really extend up to the surface low anymore.  This hints that the low is occluding and dying.  As the trough aloft becomes so negatively titled, it more or less loses its identity as a trough and, with it, the ability to direct the upper-air pattern in a way favorable to the surface low.  So, the surface low starts dying off...

Just to show how nicely (at least in this case) the thermal wind response can work (at least in the model), here is the GFS forecast for the surface pattern by Wednesday evening.
Figure 10 -- GFS 72-hour forecast of surface temperature (colors), mean sea-level pressure (contours),  and winds (barbs) for 00Z, Thursday (Wednesday evening),  March 24, 2011.  From the HOOT website.
 The surface low has drifted off to the east but remains diffuse and unorganized.  The frontal boundary has also become diffuse but it is now once again oriented in a west-east direction--in agreement with the jet aloft. Such is the power of the thermal wind adjustment.  Of course, if the temperature gradient continues to weaken (which, judging by the winds, it looks like it will...), that will feed back and in turn weaken the jet aloft.  It's all one big dynamic give and take.

So that's my take on the large-scale dynamics for the first half of this week.  Lots to look forward to...

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