Archive for January, 2007

5:30 Update…
January 31, 2007

Snow continues to fall across all of NE OK and NW AR. Many locations have already picked up over an inch of snow. The snow will continue to fall through 7 p.m. An additional .25″ to 1.00″ will be possible this evening. We’ll see a lot of the moderate snow taper off to flurries after midnight, with more light snow developing by Thursday morning across the entire area.

South throughout the River Valley, we’re seeing some snow mixing in with sleet. The snow/sleet mixture is still accumulating on the roads. Accumulations between a dusting to an inch have been reported across the River Valley, with an additional .10″ to .25″ possible this evening.

More updates to come!

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Latest Update…3:30
January 31, 2007

Here’s our latest thinking on snowfall totals. I have increased most of northwest Arkansas to 2 to 4 inches of snow. Portions of Benton county into NE OK may see between 3 and 5 inches of snow by the end of Thursday. Travel continues to get worse, as snow is falling all throughout the area. My thinking remains the same across the River Valley, with 1 to 2 inches possible by Thursday afternoon.

Winter Update….
January 31, 2007

The River Valley into SE OK is reporting roads that are becoming slick and hazardous, especially along the bridges and overpasses. Over the next couple of hours look for light to moderate snow to continue, likely covering the roads and creating issues. 1 to 2 inches is still a good bet by Thursday afternoon.

NE OK and NW AR will see more of a moderate steady snow through 5 p.m. This will only cause more road issues. Expect between 1 and 2 inches of snow toward the evening, with additional snow possible overnight. I’ll update my snowfall map coming up soon…more snow accumulation will be possible across sections of NE OK and NW AR.

Here’s the Latest
January 31, 2007

Roads all across NW AR and NE OK are becoming slick and hazardous. Snow has been accumulating on roads, creating numerous accidents. The State Police discourage travel in Benton county at this time due to the amount of car accidents. Between 1/4″ to 1/2″ of snow has already accumulated north of I-40, and we’re expecting additional snows into the evening and overnight. 2 to 4 inches is still a good bet.

The River Valley is also seeing snow starting to accumulate; however, it is very fine and dry, but that will change over the next several hours. Be very careful driving. 1 to 2 inches is not out of the question by Thursday across the River Valley.

Keep checking the blog for more updates!

The Model Trend, COLDER!
January 31, 2007

I have looked over the 0Z data, and the NAM model continues to take a colder approach to this midweek system. I have included my latest precipitation forecast map, now, it may not look like a lot of changes were made, but let me explain….

I moved the “dusting to an inch” designation further south throughout LeFlore and Scott counties. With the colder solution, I needed to adjust the position of the totals. Now, I want to see if the next two runs support the cold temperatures aloft. If they do, I will likely need to add more accumulation across the River Valley. At this point it may need to bumped up between 1 and 2 inches. I verbally mentioned on my 10 p.m. broadcast that those totals may need to be higher.

I feel good about totals across NE OK and NW AR. I think it’s safe to say that a swath of 2 to 4 inches will be very common north of I-40. Some locally higher totals could be an issue, but the majority of that area will see several inches of snow. I’m sure the National Weather Service will issue a Winter Weather Advisory for most counties north of I-40 on Wednesday, but my hunch is that they will hold off on any warnings. I think we all remember the Weather Service during the last event, so I suspect they will continue with more of a conservative approach.
More light snow looks to be an issue for Friday as another wave rolls across the area. We’ll watch for an additional dusting to an inch for Friday. More to come.
Meteorologist Justin Povick will have another update Wednesday morning. Let it Snow!

Winter Weather Update….
January 30, 2007

The computer models are starting to trend more to a colder solution. Which model do you choose, the NAM or GFS? The GFS is wetter than the NAM, but the NAM nailed the last winter event, so I’ll stick with persistence for this next system for Wednesday and Thursday.

Sleet and snow are possible toward Wednesday evening, especially north of I-40. The snow could change over to rain and or freezing rain toward Thursday morning south of I-40 before turning back over to snow/sleet for Thursday afternoon.

The 18Z NAM is slightly colder than it’s earlier run from this morning. I have included the chance for some minor snow/sleet accumulations from a dusting to an inch throughout the River Valley at this point. SE OK and SW AR could see some periods of freezing rain with this event, so be advised of some minor icing south of I-40 for Thursday morning. The column of air is forecast to be frozen north of I-40 throughout the event, so 2 to 3 inches of sleet and snow are forecast across a good majority of NE OK and NW AR. There may be a window for slightly higher accumulations in Delaware, Benton, and McDonald counties.

I’ll have another update coming out after 10 p.m. Please log in and post your comments.

Latest Thinking…Wednesday-Thursday Storm System
January 30, 2007

It has been very interesting watching the models since last week. The GFS has been hinting at some sort of winter storm for about the last 7 days now, and now the models are starting to come into some sort of agreement. The GFS and the NAM are similar with the timing of the precipitation. The OZ runs suggest the onset of the precipitation beginning by late Wednesday afternoon into the evening as the first wave moves overhead. The atmosphere will need some time to moisten up and recover from the dry northerly flow aloft on Tuesday, so that’s why we’re going with Wednesday night for the start. The main trough will then swing through by Thursday afternoon, creating another surge of precipitation throughout Thursday afternoon.

As always the challenge to winter weather forecasting hinges on the position of the cold air. At this time the best chance for accumulating snow/sleet will remain north of I-40. The column of the atmosphere at the start of the precipitation looks to be frozen. Sleet and or snow will likely be an issue for Wednesday night across the majority of the area; however, the OZ NAM is indicating some warm air advection on a stout 850 mb feed from the Gulf, and this could change over the snow across the River Valley to a mix of freezing rain and rain by Thursday morning.

NE OK and NW AR is looking to see snow for Wednesday night, changing to sleet by Thursday morning, with a change back over to snow by Thursday afternoon as the precipitation starts to wind down. If the warm air advection is stronger, there maybe a shot for some freezing rain on Thursday morning as well.

This is our preliminary precipitation forecast map. We’ll be updating the map and our blog entries regularly throughout this event. Please log onto our blog and post comments and questions.

Closed Lows, Dynamics & A Chilly Week!
January 29, 2007

Forecast models have a tough time handling closed Lows. Put that Low over the Pacific, like we presently have and the lack of real data available over water makes modeling solutions that much weaker. That being said, confidence in the long range forecast remains less than grand. The one certain thing is that there is a lot of cold air just to our north that will be continually funneled into the central part of the country by a succession of waves over the upcoming week. Throw some moisture into the mix and we’ve got snow, though timing when, where as well as how much is proving to be quite the challenge.

Cold air strengthens and aids in Cyclogenesis or Low pressure formation. We recently received an email, questioning how a shot of arctic air effects development and it’s a simple as that. A more formal definition from a case study entitled Useful Relationships Between 500 mb Features and Major Freeze Events in the Lower Rio Grande Valley of Texas is posted below.

“The differential horizontal advection of temperature produces the major amplitude changes in a traveling wave. The 500 mb height will fall at a point where cold air advection exists with increasing strength downward; a common situation with a cold front. The strong low-level cold air advection of an Arctic outbreak will produce a rapid deepening of the 500 mb trough; similarly, a pronounced deepening of a 500 mb trough on a prog chart indicates strong surface cold air advection. The 1000-500 thickness and the inferred thickness advection may be used, with caution, to forecast the Arctic outbreak with the related 500 mb features.”

Here’s the link to the paper:

http://www.srh.noaa.gov/ssd/techmemo/tm88.htm

The above takes me back to my senior year at Plymouth State University and every forecaster’s most favorite class…(possible pun intention), Calculus based Dynamics I and II 😉 I can still hear my professor say in layman’s words… “A blast of arctic air is like a shot in the arm to a developing low”. OK …enough about Dynamics, but this is the crux of what cold air can do, in some cases sending ordinary events into the record books. For snow, we want this!

The ingredients are available for measurable snow this week; it will just take the right proportions at the right time. There has been better consistency for a late week into the early weekend snow event versus a mid week one at this point.

We’ll be updating this tricky situation frequently.

Winter Weather Update….
January 27, 2007

Bottom line, temperatures will really drop late Saturday night into Sunday. On Saturday, a wave of low pressure will provide the lift for some light rain across SE OK and SW AR, with a wintry mix likely throughout NE OK and NW AR. Precipitation amounts look to be fairly light, mainly a few tenths of an inch areawide. We’re not expecting major accumulations north of I-40, possibly a dusting at the most. The River Valley will see rain, and then slowly transition by the late afternoon to a mix of rain and snow. A cold rain is anticipated south of the River Valley.

Right now, a blocking pattern is taking hold of the West Coast, allowing polar air to spill south. Until the block is removed, we’ll continue to see reinforcing shots of cold air come our way. The GFS continues to be active next week with another storm system coming into focus for Tuesday or Wednesday. The cold air won’t be in question, so snowfall would be likely, but right now the time of the event is not clear. Keep up with the 40/29 Super Doppler Storm Team this weekend for the latest.

SNOW LATE NEXT WEEK!?
January 26, 2007

It’s always wise to jump off of something solid and real versus a computer model. So what does that mean exactly might you ask? Well, in the world of weather forecasting there’s both real and simulated data, the later which is often referred to as “the models”, in weather office lingo. The above map is a real sampling of the atmosphere at a pressure level of 500 millibars (mb). This correlates to about 18,000 feet. We’ll start here with the real.

Near the Gulf of California there are two closed isobars or lines of equal pressure that make a circle. This feature is the “cut-off closed low” that Drew highlighted on both the five and six o’ clock shows, and also our chance of a little rain and snow late on Saturday. The numbers on the isobars on the whole map correlate with the height at which the pressure is 500mb. The two around the Gulf of California Low happen to be 558 and 570. These numbers are listed in decameters, so for the 570 height contour we would give this a height of 5700 meters (m). After a little quick math, this correlates to a height of 18701 feet (ft).

Looking upstream or to the West, the next feature of interest is a Low that just about has a closed contour around it off the coast of Northern California. It is this storm system that is prognosticated to move through our area by the second half of next week. The dip in the isobars just above this Low, to the east of Alaska, represents a shot of cold air that will spill in here early on Sunday. This is a precursor for any hopeful snow event. The more cold air we have the better and the way things are looking a few successive shots will be moving in.

Back to the Low off the coast of California. Long range models have this system (music please) spinning and spinning and not moving much and then spinning some more and then finally breaking up somewhat in a few pieces that look to scoot through the central plains next week. There is a reason for this. Back to the 500mb map. The east coast is presently within a deep trough of Low pressure. The central Atlantic is dominated by a strong ridge of high pressure and lastly the Mediterranean has a strong Low pushing in. This pattern of Low, High, and Low is a blocking pattern known as an Omega block. An Omega block thwarts westward movement and with this present set-up a building of cold air over much of the country can be expected.

So with assumed cold air place, finally things can get interesting! The map in color above is a simulated state of the atmosphere for Friday February 2nd at noon. It’s cold, snowy and very much a picture of winter across much of the country. This latest run of the GFS (18Z) at this time, already had a system that passed through our area late Wednesday into Thursday but this image highlights a Low forming south of Colorado with yet another push of cold air. This one will have jet stream energy to work with and just may be the one to watch out for!

Will things change? You bet! Will the models continue to hint at a legitimate winter scenario? Say your prayers!

But, will it be turning COLD? Bank on it!

THINK SNOW!