Wednesday, January 09, 2013

Why This Pattern Concerns Me... Heavy Rain/Ice


Where do I start?  There's just so much going on with our weather over the next couple of weeks using the words "active" and "challenge" doesn't do it justice.  Heavy rain, near record highs, snow, ice, and arctic air... there's so much on our plate it's like Thanksgiving dinner and we're all going back for seconds and thirds whether we like it or not.

First of all, the rain is here and it's going to be heavy at times.  This should end Thursday and temperatures COULD get out of hand and challenge daily record highs.  Read the post below for those numbers.  Then all eyes will turn to Saturday and an arctic boundary moving into the state.

All the data points to multiple rounds of rainfall through next week as the flow aloft is conducive to bringing several disturbances overhead.  This southwest flow aloft (I'll show you below) is warm and moist, while at the surface, heavy and dense arctic air settles into the state.  

There's good news and bad news.  The good, all of the data points to subfreezing air at the surface staying across north central and northwest Arkansas (not good if you're reading this from there).  Remember, the flow aloft is warm and moist so this will be more ice than snow up there.  The data also suggests that while it's much colder for central and southern Arkansas, temperatures will hold just above freezing.

The bad... I don't trust the data right now.  It could always be right, but we have seen time and time again the modeling underestimating low level arctic air.  It underestimates its intensity and its extent to the south.  99 out of 100 times ( a little exaggeration) that's what happens.  If you get dry, low level arctic air in here, evaportative cooling can take temperatures down even further than what the models say.  So that's what concerns me, the ice could go further south than the models project.  Remember, this isn't our forecast right now, but I use this blog to express my thoughts about upcoming big weather events.  

Here are the maps below showing what I'm looking at...

This is the upper level pattern early next week from the European.  See the area of high pressure aloft in the Pacific?  The flow around that is delivering shots of colder air into the central U.S.  The trough over the southwestern United States bends the flow so that it's out of the southwest here.  This brings multiple ripples in the jet stream overhead and produces several rounds of precipitation on top of the cold air at the surface.  See the ridging over the Atlantic.  This is helping to produce the flow aloft parallel to the surface front thus creating a very slow moving boundary.
The data is very consistent in keeping the subfreezing air across northern Arkansas.  That's the white line you see here.  Everything along and north of that line would be frozen.  However, like I said, I'm not trusting this right now.  Any movement to the south or even north will affect our forecast.  This suggests a cold rain for much of the state except the far north.
Here's another point to consider.  The GFS is laying down a snowpack early next week mainly north of Arkansas.  With more rounds of cold air coming in from the north, the surface temperature could be impacted by air moving over these snowfields.  The data will not pick up on this so we must consider this when making a forecast.
This is 10 day rainfall from the 00Z GFS.  That's more than 6 inches across much of the mid south.

The next run of the GFS, 6Z, shows basically the same thing.
HPC rainfall amounts for the next 7 days shows 6''+ over the southeastern half of the state.

2 comments:

Scott Reed said...

Thanks so much for all your hard work. How is this rain going to effect the drought? Should we expect the same type of drought conditions as we did in 2012?

Anonymous said...

What does your gut tell you about the weekend weather possibilities?