Monday, February 11, 2013

Lots of Weather To Watch This Week.


1:40 PM Monday Update... I really have no change in my thinking for Tuesday night and into Wednesday morning.  I still think it's the higher elevations of the Ozarks that could see some slushy accumulations on grassy surfaces.  I still like the idea of a dusting in the lower elevations with up to 2'' of wet snow in the higher elevations.  When you look at the weather maps, it looks like a perfect set up for a significant snowfall, but surface temperatures will be problematic... for snow lovers.  This will likely be a situation where an intense snowfall rate will be needed to lower temperatures a few degrees and this is where accumulations will more likely occur.  

Temperatures in central Arkansas will likely stay well above freezing, but it will still be possible to see a few flakes fly Wednesday morning.  I will once again remind you that any change to the track and/or intensity will change this forecast so stay tuned.  These upper level lows are very well known for throwing one or two surprises.

Here's a winter weather guidance map from HPC.  Read an update on the weekend situation further below...

HPC shows exactly what I'm thinking.  This indicates a high chance for at least 1'' of snow in the blue area of northwest Arkansas.
Now onto the weekend.  The GFS still indicates the chance for significant snow Friday night into Saturday morning while the Euro does NOT.  At this point, I just can't buy the GFS idea on this and will stick to the European.  Both agree on a quick hitting blast of colder air.  Nothing we haven't seen before though.  Let's watch the GFS forecast high for Saturday.  Last night it said 49.  The run this morning says 47.  I still think there's a good chance many locations won't get out of the 30s Saturday with wind chill values in the teens and 20s.  Read more on the AM post below.

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 Waking up this Monday morning and examining all the data, as I always do with my cup of java, several things jump out at me.

First of all, I took the chance for rain today out of the forecast for southern Arkansas.  Like Richard Marx said, "I Should Have Known Better". Yes, I am once again showing my age and my love for 80s music, or was that early 90s?  It happens so many times these fronts stall just south of the state and moisture overruns the boundary producing rain further north than what the models indicate.  That's happening this morning.  While rain will stay confined to the south, clouds will make an appearance from time to time today further north.  That's why I couldn't go mostly sunny today and put some cloud cover in the forecast.

Next, the system Tuesday.  As I have said, it looks like mainly rain.  However, 90% of the time, these upper level systems can bring a surprise or two and that's certainly possible.  Unlike the Christmas Day storm, there is NO arctic air for this storm to work with so it must bring it's own cold air.  The only way you can get a changeover to snow is with the upper level system.  Lift generated by this cold pocket aloft can cool the column of air in the atmosphere enough for snow to make it to the ground near its track.  This is most likely to happen across the northern two rows of counties Tuesday evening.  However, because there is NO arctic air in place, surface temperatures will be just above freezing.  It can still snow despite this and if the intensity of the snow is great enough, the temperature can cool further and some accumulation can occur.  For this reason, I think far northern Arkansas can see a dusting in the lower elevations and up to 2'' in the higher elevations.  Remember, we're talking about the northern 2 rows of counties.  While rain could change over to snow further south even into portions of central Arkansas, accumulations appear very unlikely.  With all that said, any change in the track and intensity can change this forecast. Like I said, these upper level systems are well known for throwing curve balls.

Now onto the end of the week.  I just can't buy the Global Forecast System (GFS) temperature output.  It shows a high Friday of 52 and Saturday 49.  There is one heck of a trough coming into the central United States with a strong push of cold air.   I don't think it's anything we haven't seen already this winter, but I just wanted to call to your attention these numbers and watch how bad the GFS will likely perform with those highs.  I think it's very possible not to get out of the 30s Friday and Saturday with very breezy conditions creating wind chill readings in the teens and 20s.  Again, nothing we haven't already experienced, but let's watch and see how the GFS performs.  Also, this cold air will not hang around.  It should be in and out within just 2 or 3 days.

Also, the GFS and the Euro show a wave of energy producing some snowfall late Friday into Saturday.  The GFS is more aggressive with some of its runs while the Euro is not.  THIS IS NOT AN IDEAL SET UP FOR ANYTHING SIGNIFICANT!  I can see a situation where snow flurries and/or snow showers quickly move through the region.

Lots going on.  Here are the maps explaining all I have talked about above.

This is the NAM valid Wednesday morning at 6AM.  The blue blob is snow and you can see how that is confined to northern Arkansas.  Look at the blue dashed circle over that area.  That's the reflection of the upper level low bringing it's own cold air.  Notice how the frozen precipitation is all within that area.  Any change in the track and/or intensity can change the forecast.
Let's fast forward to Friday at midnight.  That's one heck of a trough over the central United States.  See that area of yellows over Oklahoma and Arkansas.  That's upper level energy and that's why the models indicate some light snow possible at this time.  This set up is not ideal for significant snow, but can still generate some.  With a trough like this, I don't see temperatures as warm as the models indicate for late week.
The 6Z GFS is going crazy with precip Friday night/Saturday morning.  I think this is overdone.  Notice the blue dashed "540 line" is well south.  This would be all snow and LOTS of it.  However, I still think this is not likely to verify.
This is the 00Z run of the GFS and it's more realistic with just a quick hitting wave of flurries or snow showers in the cold air Friday night.  Notice the precip. amounts are much less than above.
And the Euro has a similar idea so I think it's entirely possible for a few flakes to fly late Friday night, but at this time, it does not look like a big storm whatsoever, BUT let's watch it!!!!  If the wave of energy digs further south and west and taps into more Gulf moisture, then it's a different ballgame.  However, I have not seen any data supporting that idea and it would be "wishcasting" at this point.  But hey, it could happen.  It's 5 days away right? LOL

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