SW LA/SE TX Winter 2013-2014

[BigB0882]

Cranking as in going fast or as in producing a lot of moisture? Hopefully the latter.

[vbhoutex]

Jeff's morning email:

***High impact winter storm likely on Tuesday***

Discussion:

Arctic cold front is surging across TX the morning and is currently nearing College Station. Upstream temperatures over north TX have fallen below freezing with very strong N winds gusting to over 35mph. Even colder surge of air is currently pushing through OK with temperatures in the 10’s. Boundary will move off the coast around midday with strong N winds developing across the region. High temperatures near 60-65 in the next few hours will fall quickly into the 40’s this afternoon and 30’s by early to mid evening under strong cold air advection. A dry air mass will begin to moisten overnight into early Tuesday as lift increases within the cold arctic dome entrenching over the area. Lift appears to come from passing base of trough over the Midwest with precipitation breaking out along the I-35 corridor near Austin and spreading ESE into SE TX on Tuesday. From this I will go into the details.

Temperatures:

Upstream air mass is very cold and fairly dry. Expect temperatures to fall to freezing by around midnight tonight over our N and NW counties with the freezing line progressing southward early Tuesday and reaching the coast by mid morning on Tuesday. Combined effect of cold air advection and “wet bulbing” or falling precipitation cooling the air temperature toward the dewpoint looks effective in bringing much of the area below freezing in the 600am-1000am period on Tuesday. Onset of precipitation will likely help to enhance the cooling effect and drive temperatures lower during the day instead of upward. By mid afternoon temperatures could be in the mid to upper 20’s across much of the area. Very cold Wednesday morning…could be looking at hard freeze criteria with temperatures bottoming out in the upper 10’s north to low/mid 20’s elsewhere.

P-type:

Skip down for P-type, “is reasoning”

“I have spent a good deal of time pouring over the forecast model soundings this morning and analyzing various winter P-type techniques to attempt to nail down the P-type for this event. I focused on the snow aspect first using the 1000-850mb thickness values in the SREF ensemble guidance. Given a warm nose aloft I was looking for a value between roughly 1300m and 1275m. SREF guidance shows the 1300m line near HWY 105 around 600am Tuesday, but only sags it very slowly southward during the day reaching the I-10 corridor around 21Z (300pm). A fairly well defined and thick warm nose is noted on both the GFS and NAM forecast soundings roughly about 6,000ft thick. There is room within this warm nose for evaporative cooling, but it appears the mid levels do not cool much while it moistens. I have always been worried about that pesky warm nose in the mid levels and I am not sure it can be overcome to produce snow. The forecast soundings look very much like a freezing rain and sleet profile especially from HWY 105 southward to the coast. The warm nose does shrink and the entire temperature profile comes very close to freezing/sub-freezing in the 300-600pm time period Tuesday and this is when a changeover might occur further southward across the metro Houston area. However the soundings also show significant drying and precipitation ending in the 600-900pm time period so the window for snow production along and south of I-10 is fairly small.”

North of HWY 105: sleet and snow becoming all snow

North of US 59 (SW) and I-10 (E) of Houston: freezing rain mixed with sleet, possibly changing to all sleet and snow

Coast to US 59 (SW) and I-10 (E): freezing rain changing to freezing rain and sleet mix, possible light snow near end of event

Soundings and surface air temperatures really support a freezing rain and sleet event for the southern 2/3rds of SE TX. Northern counties could start as a very brief period of freezing rain, but will likely transition quickly over to snow and remain snow for most of the duration.

Timing:

Of great importance with this event is the onset timing of the precipitation as the area is facing some hard decisions today. Models are is fairly good agreement on the precipitation onset although some of the most recent guidance is trending just a touch faster so I have windowed out times instead of using hard points due to some uncertainty.

N of Hwy 105: 400-600am onset lasting until 300-600pm

N of US 59: 800-1000am onset lasting until 500-800pm

Coast northward: 1000am-noon onset lasting until 800-1000pm

Conditions will quickly deteriorate within about an hour or two of the onset of precipitation at a given location.

Accumulation:

Accumulation is strongly tied to P-type with snow producing more accumulation and sleet/freezing rain less. The major difference between this event and the event last week is that surface air temperatures appear to be about 2-3 degrees colder which will make for very effective ice formation. Additionally it appears temperatures will be at or below freezing the entire time the precipitation process is occurring helping to foster greater accumulation amounts.

Other item to keep in mind is the potential for some banding of the precipitation as is sometimes common is more snow situations which can generate locally heavy snowfall rates and localized higher amounts. This is nearly impossible to determine until it is underway, but the last 3 snow events in SE TX have all had meso scale snow bands which produced greater than expected accumulations. I see some of the shorter range guidance attempting to resolve some of this banding…but do not put much faith it their locations at this point.

Hwy 105 north: 1-3 inches of snow. A trace - .05 of an inch of ice

US 59 north: .10 to .20 inch of ice/sleet. .50 -1.5 inches snow

Coast to US 59: .10 to .30 inch of ice/sleet. Dusting of snow possible

Matagorda Bay area: .05-.10 inch of ice accumulation.

Impacts:

Expect some fairly significant impacts especially to travel starting Tuesday morning and lasting into at least midday Wednesday. Surface temperatures are colder with this event so much of the precipitation will freeze and not be “wasted”. Suspect the bridges and overpasses will go pretty quickly…even if they have been treated with the anti-ice agent. Could even see some problems with the surface streets by late Tuesday and especially Tuesday night with temperatures falling into the lower 20’s.

As for power outages…still think we are below the needed threshold to get widespread outages. Some of the more “wet” model guidance would push areas along and south of US 59 into the .30-.40 inch of ice accumulation range which could be enough to cause some issues…but this is likely the high end worst case with a more moderated ice accumulation of .10-.30 of an inch. N winds of 15-20mph could add some to the ice effect with additional pressure.

Aviation travel is going to be affected with required de-icing procedures on departing aircraft. Expect ice accumulation on control surfaces prior to take-off and moderate to heavy icing through about 6,000 ft in mixed phase precipitation before breaking into the mid level warm nose.

Next update will be with the issuance of the Winter Storm Warning.

[windnrain]

http://www.weather.com/encyclopedia/winter/overrun.html

"overruning" appears to produce heavy precipitation totals.

[PTrackerLA]

Where is the cold air? As of 1pm it's 57 here and Shreveport is still only 52. Not exactly what I would call a sharp arctic front. Makes me wonder if we'll even reach freezing tonight.

[ROCK]

Where is the cold air? As of 1pm it's 57 here and Shreveport is still only 52. Not exactly what I would call a sharp arctic front. Makes me wonder if we'll even reach freezing tonight.

Dallas is 37F...OKC is 27F already.....wait for it...

[BigB0882]

I don't think we are supposed to be down to freezing until about 4am or so. I don't remember any models showing us dropping until tonight.

[PTrackerLA]

Ah I see teens in NW Arkansas so that's what will be coming down. Awaiting a Winter Storm Warning to be issued for my area at anytime, trigger has been pulled for SE LA already. 18z NAM starting to roll now...

[LaBreeze]

Where is the cold air? As of 1pm it's 57 here and Shreveport is still only 52. Not exactly what I would call a sharp arctic front. Makes me wonder if we'll even reach freezing tonight.

PTrackerLA, what do you think is the outlook for Lafayette and Vermilion Parishes? Ice for sure - but any snow? How much ice are you thinking we'll get? I respect your opinion and thank you in advance.

[BigB0882]

Oh god I can't take any more models. The stress is too much!

[thatwhichisnt]

This should be an interesting run..

[ROCK]

looks plenty wet to me...

[southerngale]

School districts around here have already cancelled school for tomorrow.

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[ROCK]

School districts around here have already cancelled school for tomorrow.

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same here in SE Texas....

[PTrackerLA]

From what I see out to 30 hrs NAM still on track for a high impact even here tomorrow. The simulated radar shows some heavy echoes right over my area. Many may not be familiar with snows but a lot of times precip forms in bands and it's not out of the question to see 1"-2" an hour if you get stuck under a heavy band. I think we're going to have quite a bit of freezing rain but hopefully a nice burst of snow on the back end...perfect for a snowball fight!

[PTrackerLA]

Where is the cold air? As of 1pm it's 57 here and Shreveport is still only 52. Not exactly what I would call a sharp arctic front. Makes me wonder if we'll even reach freezing tonight.

PTrackerLA, what do you think is the outlook for Lafayette and Vermilion Parishes? Ice for sure - but any snow? How much ice are you thinking we'll get? I respect your opinion and thank you in advance.

Latest NAM still pegging our area for .25"-.50" of liquid precip. I'm hopefully only a quarter inch tops of ice and the rest snow. I think we'll all see at least some flakes but at this point we almost have to wait for the precip to start falling tomorrow morning to get a good idea. I stocked up on food (and booze) at lunch so I'm ready for whatever !

[windnrain]

LSU classes were just canceled. Took them 3 minutes after the winter storm warning.

Euro shows 2" of snow in BR. I definitely think models are converging on that we are at least going to get a decent snow!

[Jagno]

All Calcasieu Parish schools are cancelled for Tuesday and Wednesday. My youngest graduated several years ago and they still call me.

I just got back from Sam's and it was worse than Christmas Eve in that place. People were stocking up as though we are going to be without electricity for a month.

So, what's the word for the Lake Charles area on the new runs? Timing?

[BigB0882]

Oh man, I really thought people wouldn't think to stock up on groceries. We have NOTHING to eat in our house. Hopefully there is enough bread and stuff to have something on hand if we lose power.

[southerngale]

Oh man, I really thought people wouldn't think to stock up on groceries. We have NOTHING to eat in our house. Hopefully there is enough bread and stuff to have something on hand if we lose power.

Go shopping!

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[BigB0882]

I am going straight after school lets out but it may not be until about 4:30 depending on traffic. I will grab whatever I can find to give us something in case of outages.