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                   Wednesday, March 17, 2010 - 12:55PM CDT

March 19-20 forecast - update 4

By DAN ROBINSON
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Posted 12:55PM CDT Wednesday: It's becoming clearer that this system's potential will be curtailed both Friday and Saturday, primarily due to the effects of the too-strong cold front rapidly plowing south and east. The front will likely undercut the low levels before or just after convection can get going, virtually shutting off even a thunderstorm threat. In addition, moisture will be lacking and instability non-existent. Upper support is shown weaker and further south. All indicated precip is squarely post-frontal, which usually means a lightning-less band of rain (and in this case, mixing with/changing to snow in places). The only area that may still hold some severe storm/tornado potential is far to the south near the Gulf coast, where weak instability in the sheared environment may allow some convection to organize before the cold front reaches it. However, this is too far outside of my expedition range to consider.

In March, it's pretty typical for one or two necessary severe weather ingredients to fail to materialize during any given setup, but in this case, all factors are looking totally shot in terms of strength and timing. As such, this will be my last update on this system unless the outlook changes significantly in favor of thunderstorms. Ironically, the cold side of this system may end up being the bigger story. The GFS' 00z runs were showing around 5 inches of snow in the St. Louis area, though the 12z run has backed off of that. Accumulation chances are better to the west and north, as well as to the east in the Appalachians. Spring snowstorms have a somewhat better chance at producing thundersnow, which will probably become my main subject of interest during this storm.

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Posted 12:46PM CDT Tuesday: Still not much of a change in the past two model runs (00z and 12z Tuesday). The NAM model can now 'see' Friday, and it is in decent agreement with what the GFS has been showing - aside from keeping the tiny instability axis a little further west into the Texas panhandle and the low slightly north. Saturday's Midwest event looks to be a squall line by all accounts, possibly a strong one depending on how the moisture situation plays out. I don't think tornadoes are quite eliminated from the picture yet, depending on how much instability can be realized near the low. However, I'm definitely less optimistic about this being anything more than a local storm observation at this point.

The cold front associated with the system will be blasting southward, plummeting temperatures behind it to well below freezing before the main precipitation area of the system moves out. As the models have been warning of for several days now, this spells a potentially significant snow event for the Plains and Midwest region late Saturday into Sunday, moving into the Eastern US Monday. The GFS is showing totals over a foot in parts of Kansas. Any snow cover should be short-lived for areas along and south of I-70, as temps rebound sharply into the 40s and 50s each day.

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Posted 4:09PM CDT Monday: The last two runs of the GFS and Euro have kept the same general upper air pattern for Friday and Saturday. The concerns I have with the setup are the strength of the dynamics and the potential for limited instability. The low is shown rapidly deepening into Saturday - this would favor more rain and cloud cover, with very little instability to work with in the afternoon. If instability is realized, the forcing may be too strong for isolated supercells, instead favoring a squall line early in the day. Gulf SSTs, according to NOAA bouy data, have actually dropped in the past week. Immediately prior to the flow turning southwesterly aloft, a northwesterly flow pattern will have been in place over the Gulf for several days. This makes it improbable that we will get much moisture return in time for the arrival of the dynamics of the main event on Saturday.

Friday still holds some potential in the Plains, with a sliver of weak instability still indicated by the GFS in western Oklahoma. However, wind fields may not be as strong on Friday afternoon to support much of a threat. Either way I have no plans to cover in the Plains, as the threat level is below my 'extended range chase threshold' - and with parameters stronger on Saturday closer to St. Louis, I would likely not be able to make it back here in time to observe storms both days.

The GFS also continues to indicate a potential snow event on the back side of the low for parts of Kansas, Nebraska and Iowa.

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Posted 1:39PM CDT Sunday: I've been watching, with some interest, an upper trough shown by the past few runs of the GFS and Euro models to affect the Plains and/or Midwest next weekend. The GFS had been the faster of the two models, showing the system being a Friday event - while the Euro kept it a later show, around the Saturday afternoon time frame. The latest run of the GFS has placed the event more on Saturday in the Mississippi Valley - and if this actually comes to pass as shown, it could be a very potent outbreak.

Keep in mind that this is still 5-6 days out, a long time when dealing with models - and the details/specifics are nearly impossible to nail down this far out. However, let's look at the possible scenario painted by the latest GFS run (12z today), which would place an expedition target somewhere from southern Illinois down through western Tennessee and Kentucky.

First, a trough with strong mid/upper jet streak, overtop of a nose of dewpoints in the 50s, and a strong surface low with backing winds in the warm sector.

All of these are solid ingredients for a tornado event if any semblance of this pattern actually takes place. Again, this 'target' could a.) shift 500 miles in any direction as the event draws closer, b.) end up as a non-tornadic squall line due to forcing being too strong or c.) vanish altogether. At any rate, it is definitely something to watch for next weekend. A winter storm is also possible on the back side of this system as it moves east. I'll be posting more updates on the forecast as the potential event draws closer.

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