A stationary front draped across Oklahoma and Arkansas will cause for a wet and stormy pattern over the next few days. The next 4 days or so look to have the potential for severe thunderstorms somewhere in the southern plains. The greatest threat will most likely be Sunday for the OK/AR region.

An outbreak of severe weather is likely across the area Thursday and Thursday night with the threat of hail, damaging winds and tornadoes. The Storm Prediction Center in Norman, OK currently has a Moderate Risk out for much of eastern Oklahoma, southeast Kansas and far northwestern Arkansas.

An negatively tilted trough is expected to plunge down into the southern plains from Colorado by tomorrow afternoon. Strong upper air divergence southeast of this trough will strengthen a surface low pressure in southeast Kansas and northern Oklahoma. This low pressure will drag a dry line across  Oklahoma tomorrow afternoon and will be the focus of storm initiation throughout the day.
Below is the 500mb height and wind plot at 4pm tomorrow. (NAM)

Below is the surface pressure and wind plot at 4pm tomorrow. (NAM)

Below is the surface dew point plot at 4pm tomorrow. (NAM) Is shows the 

The direction shear will be very supportive of supercells tomorrow across the mentioned area. As well as the CAPE values (2000j/kg +). The SPC's SREF's supercell composite parameter is impressive along the OK/AR border tomorrow at 7pm along with the significant tornado parameter. Below is the supercell composite paramter bullseyed over the AR/OK border.

Below is the significant tornado parameter also bullseyed on the AR/OK border.

Overall, there is a very large threat of severe weather across eastern Oklahoma from 3pm - 10pm. Supercells with very large damaging hail, strong winds and tornadoes will be the threat. Later in the day, 7pm-12am, the threat will shift eastward into Arkansas. If the storms stay isolated, the tornado threat will continue into Arkansas. This could be a very dangerous situation. Keep your eyes on the quickly changing weather conditions tomorrow afternoon.

David

An area of low pressure at the surface located in Texas will drape a warm front across the Arlatex today. This warm front will be the ignition for strong to severe thunderstorms this afternoon and tonight. The greatest chance for severe weather (hail, wind and tornadoes) appears to be south of I-40 centered around Texarkana.

This setup has prompted the SPC to issue a slight risk out for the area, including a 5% tornado prob and a 30% hail prob.

Hopefully I will be able to make it out this afternoon to chase so check my storm chasing blog later for more details. STORM CHASING BLOG

The Spring Equinox isn't until Sunday the 20th, but it sure feels like Spring outside! Trees are beginning to bloom, the grass is beginning to green, and it was 62F when I left for work this morning! 
Of course, the word "Spring" is almost synonymous to the beginning of severe weather season to most people and this might actually be the case this year as severe weather looks to impact the plains this weekend and early next week. A large trough in the jet stream is currently making its way onto the West Coast and will be the cause for numerous days of possible severe weather in the middle of the country. The GFS and ECMWF are pretty similar with their evolution of this trough throughout the weekend and early next week and both have been pretty consistent as well. However, the NAM (which only reaches out to 0Z Monday) has been really lacking in moisture return and really kills Saturday and Sunday. The GFS did trend a little towards the NAM on the 12Z run though it's still not as weak with moisture in the TX Panhandle as the NAM.
I guess, we'll just wait and see...

It was a beautiful day in the southern plains today.  Above average temperatures made for an early gift of Spring in February!  This time last week, parts of northern Oklahoma had two feet of snow on the ground and low temperatures in the -20's!  The all-time record low temperature for Oklahoma was recorded last week in Nowata, Oklahoma (north of Tulsa) at -31F.  Today, they reached an amazing 79F degrees on February 10th! If your math is a bit sketchy, that's a 110 degree swing!
This warm spell does have a slight kink in the forecast.  Next week high temperatures will only get into to 50's on Monday and Tuesday with a quick visit with freezing the night in between.

Low temperatures on Feb 10, 2011

High temperatures on Feb 17, 2011

Yes, another major winter storm will impact the southern plains tomorrow evening and during the day Wednesday. A trough in the jet stream which has been set up for the past few weeks continues to bring down arctic air. A disturbance will ride it down and cross over the southern plains. As it does, precipitation will move into Oklahoma from Kansas in the evening hours following a strong cold front. This front could possibly create strong enough forcing to cause heavy bands of precipitation. This will all move in coverage from the northwest to the southeast. 

Liquid to Snow ratios look to be pretty high with this storm across Oklahoma. 15:1 will be normal with possibly higher ratios. Arkansas will see just a bit lower ratios during peak precipitation rates.

If the NAM were to verify, .5" of precip for central OK would call for about 7" of snow and 0.75" of precip for western AR would call for near a foot of snow.

The GFS however paints a bit of a different picture (below). .5" of precipitation for central OK is still seen but western AR is looking at around .25". That would be a measly 4" of snow. 

The NWS currently has the entire state of Oklahoma (minus 4 counties) under a Winter Storm Warning including a few counties in Arkansas. This warning area is expected to grow tomorrow.

Right now, I'm going with a mixture of the two models with around 6" for western AR and 8" for central OK. Higher totals locally are possible however within convective bands. High winds during snowfall and afterwards will result in amazing drifting so measurements will be difficult to obtain. If you do measure, be sure the area is far enough away from obstructions to get an accurate measurement.

After a week of feeling as though we were in Minnesota, we are finally thawing out a bit this weekend. Significant melting has taken place with highs in the upper 30's and low 40's. This will continue to be the case tomorrow and Tuesday. Today's rain may change over to a quick snow late tonight. Less than a half an inch is expected in Norman and less than an inch in Greenwood. Once Tuesday arrives, we'll seem to be having a case of deja vu as winds out of the north start to pick up and the temperature plummets. Rain Tuesday will change over to snow as the arctic air moves into the area in Norman. Precipitation will begin a bit later for western Arkansas though this precip will probably be all snow Tuesday night and Wednesday. 
My preliminary snowfall totals forecast is a general 6-8" for Norman and 4-6" for western Arkansas. 
I know everyone LOVES pretty colorful maps, so here's the NWS-Tulsa's snowfall map for the week. This does include tonight's snowfall as well.

The system is still sitting off the coast so this forecast is pretty introductory. Expect another blog post some time tomorrow evening regarding this potential winter storm.

Visit my 2/1/2011 photo album for pictures as well as a NWS snowfall graphic and Mesonet data from the recent winter storm!

After a gorgeous Friday and Saturday with high's in the 70's, reality will set back in this week as Old Man Winter makes his presence known. An upper level storm system currently over California will dip down into the southern US and come across Oklahoma and Arkansas. With it, winter weather could be a major issue for Oklahoma, Kansas, Missouri, Arkansas, and many more states to the northeast.

As the system passes by, cold air wrapping around the low will quickly change rain over to freezing rain and/or sleet and then snow. The timing of this cold air and the location of the heaviest precipitation is key for where the highest snow totals will be and where the heaviest freezing rain and sleet occurs.
By 3:00am CST Tuesday, freezing rain and/or sleet will already begin to fall across central Oklahoma. This will change over to snow for the OKC metro some time before 6:00am. This will quickly increase in coverage and intensity as the low deepens and strengthens. The band of precipitation will dump heavy snow across the central and northeast parts of the state as well as respectable sleet accumulations in northeastern Oklahoma. Some freezing rain mixed in is possible as well. Totals ranging from 4 inches to 12 inches are to be expected stretching from Lawton, OK northeastward through Oklahoma with the highest totals in the northeast part of the state. Below is the 3hr precipitation image at 6am Tuesday with the surface temperatures in Fahrenheit over-layed.

Precipitation forming in Texas will also track into eastern Oklahoma and Arkansas. The timing of the freezing air at the surface looks to hold off just long enough for the precipitation to move out before much accumulation occurs for the majority of western Arkansas. Some changeover to sleet and snow is possible for western Arkansas before it completely moves out. The image below is for 3pm on Tuesday. Heavy rain will be falling in western Arkansas but the freezing line will cross over just in time for the heavy precipitation to taper off and not allow much snow accumulation. Sleet and freezing rain is possible before 1-2" of snow for the Arkansas River Valley.

David Goines

A shortwave trough is approaching the area from the Rockies and will bring with it colder temperatures and will squeeze out some precipitation as well. Ever since the system came onshore from the Pacific, each model run has trended a bit further south. As a result, the forecasted snowfall totals for areas such as Fort Smith and Greenwood have increased over time. This really wasn't an instance where people were just being conservative and not wanting to put out high amounts, it was really just a product of computer model reliance as well as the lack of data because of the storm system being over the Pacific (where there aren't many places to launch radiosondes). Now that there is more data available for the models, they have all (well, most) come into agreement on a more southerly, moist and cooler storm system.
Below is a 500mb vorticity chart showing the shortwave trough currently over the Rockies on the left and the 24 hour forecasted 500mb vorticity chart showing the trough at noon Thursday.


(CLICK ON THE THUMBNAILS  TO BRING UP A LARGER IMAGE)

Precipitation should begin as rain for the Fort Smith area around 4am as sleet and then transition over to snow closer to noon. Below is the 21, 24 and 27 hour forecasted 3 hour precipitation totals. The left image shows light precip at 9am in Fort Smith while the middle and right image show higher precipitation rates at noon and 3pm.

These images show the water equivalent precipitation over three hours. The snow to water ratio will be higher than normal for our area during this event because of dendrite growth. The image below is a diagram showing snow flake growth based on temperature. Notice the larger snowflakes are dendrites that form between -10C and  -20C. Below is also a sounding in Fort Smith at noon tomorrow that shows saturation in this -10C to -20C area which means snowfall ratios will be around 12:1 or even 15:1. So that means .1 inch of precipitation will output 1.2 inch or even 1.5 inch of snow. This means snow totals could be rather high because of "fluffy" snow.

Here is my snowfall totals prediction. My totals are admittingly a bit bullish  because I'm going out on a limb and guessing that the 0Z models will trend even far southward. 3 inches will probably be the norm for the RV but 5 inches could be possible locally. I'll post another update later tonight after the 0Z runs come out and change accordingly.

David Goines

With the big "arctic blast" only a few days away now, you would think it would make for an easier forecast. That's not necessarily the case, especially with this being a winter system. The track of a storm system can mean the difference in snow or no snow for a given area. This event has two systems that will be snow producers so the uncertainty is doubled. 
The first wave will be Sunday evening into Monday affecting a region from around Texarkana eastward to the Atlantic coast. This could dump snowfall totals nearing half a foot for the Texarkana area. The farther north into Arkansas, the less the totals will be for Sunday evening. Expect about an inch in Fort Smith. Little accumulation is expected for the OKC metro with around one inch as well.
Now on to the wave Monday evening into Tuesday. The affected area looks to be northern Oklahoma into Kansas where snowfall accumulations will be 2 to 4 inches. A little freezing drizzle could mix in with snow during the day Monday in the metro but it shouldn't cause any problems with just a dusting to 1" of accumulation expected. The Fort Smith area could also see a dusting of snow on Monday.
After the precipitation is all said and done, the temperatures will drop like a rock. Highs won't get above freezing for Tuesday and Wednesday and low temperatures Tuesday night into Wednesday will be in the low teens and possibly the single digits. This is not unprecedented cold weather, (as was forecasted earlier this week) but it is the coldest air of this winter. As I said earlier, a slight deviation in storm track could mean for differing snowfall accumulations so this forecast will most likely change. But the current consensus is that both the Norman area and the Fort Smith area will only end up with about an inch of snow when it's all over with. Norman may experience some freezing drizzle as well. 

The NAM is a bit farther south with the first system and a bit farther north with the second system when compared to the GFS. This would mean for less snow for the I-40 corridor in Oklahoma and Arkansas.

Below is tonight's total snowfall model by the GFS (1/8/11 0Z run) using the Evan Kuchera algorithm. Courtesy of Earl Barker's website (www.wxcaster.com).

Below is tonight's total snowfall model by the NAM (1/8/11 0Z run) using the Evan Kuchera algorithm. Courtesy of Earl Barker's website (www.wxcaster.com).