Significant Winter Storm Expected for Oklahoma 2/1

Winter Weather Discussion for February 1, 2011.
Issued:  01/29/2011.

*Significant Winter Storm to Impact Oklahoma*
*Prepare now for this winter storm*

Highlights:

–  Winter Storm Watch for all of northeast Oklahoma effective Monday through Wednesday morning.  Sleet and snow of 4-10 inches is likely within the watch area.

–  Winter weather is likely across much of Oklahoma with the main impacts east of I-35 on Tuesday.  Additional watches are likely on Sunday.  Current thinking is for an ice storm in areas south of I-40 and east of I-35, initially ice then quickly sleet to snow from Oklahoma City to Tulsa, and finally initially sleet turning to snow north of I-44.  The potential for snow amounts greater than 10 inches does exist.

–  Bitterly cold temperatures and strong winds will result in dangerously low wind chills across the state on Wednesday.  Thursday won’t be much better.

Discussion:

Complex forecast continues for this issuance.  Models are developing some agreement regarding the upper system relative to timing and placement.  There are some important differences between them with respects to precipitation duration and amounts.

Northern stream shortwave trof will drop south out of Canada as the Hudson Bay low continues to spin to the east.  This will dislodge an Arctic airmass out of central Canada and send it screaming southward.  A weak front is approaching Oklahoma this evening and will move southward into the state, with temperatures a little below normal on Sunday.  To the west a shortwave trof will drop southward along the west coast and form into a closed low.  This feature will shift quickly east on Monday and move into Oklahoma on Tuesday.  Previously mentioned Arctic front will blast through Oklahoma Tuesday morning as precipitation begins ahead of the southwest U.S. system.

00z NAM is reasonable and is close to the GFS, there is no reason to split hairs as the event is still 60 hours out.  The hard part is knowing what phase of water will occur as the event starts and where.  Current thinking is all snow is likely in northwest Oklahoma and the panhandle with 2-5 inches likely.  In the Oklahoma City to Tulsa area the event will start around 1am Tuesday morning as brief freezing rain and then sleet.  During the early morning hours the sleet will transition to snow.  Total amounts look to be in the 6-8 inches with near 10 inches in northeast Oklahoma.  Higher amounts are possible.  Southeast of this area freezing rain will last longer, turning to sleet and eventually snow.  Total amounts of snow and sleet will be in the 4-6 inches range with potentially one inch of ice.  In far southeast Oklahoma, a cold rain will turn to freezing rain and then briefly to sleet and all snow.  Ice accumulations above one inch are possible with 2-4 inches of sleet and snow.  This is all a wild ass guess right now, but 21 years of operational experience to back it up.

I remain a little concerned about the warm nose on all models to be in southeast Oklahoma with +4C at 819mb.  While I know the column will cool due to lift, I just don’t want to ignore the freezing rain potential.  Especially since Arctic cold fronts almost always move faster than the models predict.

This appears to be a high-end event and enhanced wording will likely be required tomorrow.  I arrived at this decision as a function of the amount of precipitation expected, areal coverage, and aftermath.  Models are likely under-doing wind speeds on the backside of the surface low-850mb.  00z NAM shows 50-55kts at 850mb Tuesday evening and this may indeed increase.  This concern feeds into the expected wind chill and impact on infrastructure coated with ice.

Additional winter weather watches are likely Sunday morning and much of Oklahoma will likely be in some form of a watch or advisory by late Sunday.

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