Upcoming cold for the South (Models, Maps, Discussions)

[Tyler]

Thanks Portastorm.

To determine surface temps using 850 temps. I think a pro would no better than me. But, usually you would convert celsius to fahrenheit (duh), then add a couple of degrees to that, I think. So if, we have 850 temps of -6 to -8 like the Euro wants Sunday morning, then lows would be roughly 21-25 degrees. But that would be under perfect radiational cooling, no winds, no clouds.

[drezee]

LIMITATIONS

Forecasting the surface high using the 850-mb temperature has been a popular forecasting technique. First I will go over the limitations of using this method:

1. Method does not work on cloudy days or days with afternoon precipitation

2. There is a tendency that temperature will be higher than predicted on days the wind is light and will be lower than predicted on days the wind is strong. This is because the low level wind effects the depth of mixing.

3. Method assumes air is mixed only between the surface and 850 mb. If the air mixes to a height significantly above or below 850 mb the technique will not work accurately.

4. Method does not account for elevation. High elevation regions have a greater chance of mixing air that is above 850 mb.

5. Daylight hours effect accuracy. There is a significant difference in daylight hours between the warm and cool season.

6. Method only works in a barotropic atmosphere. Fronts or differential advection will contaminate technique.

7. Method does not work well in regions with complex topography or near mesoscale temperature gradients such as coastal areas, very hilly areas, and areas near large lakes.

The method works best in locations near sea level, in the warm season, on barotropic days, with flat topography, on moderate windy cloud-free days. If any of these conditions are not met then take that into account on the temperature prediction.

THE METHOD

The method itself is very easy. Method needs to be done on a Skew-T to ensure accuracy. From the morning sounding, note the 850-mb temperature. Take a parcel of air at 850 mb using the 850-mb temperature and bring it dry adiabatically to the surface. The temperature of this parcel after it is brought to the surface will estimate the high temperature for the day.

IS THIS METHOD BETTER THAN USING MOS?

The only way to know is to try the method each day for a year and compare it to the MOS values and actual high. With experience in using the 850-mb technique you will be able to "tweak" the value you attain each day to take into consideration the limitations given at the start of this essay. On days with active weather, the 850-mb technique and MOS have problems. Your greatest skill in outforecasting MOS will be on days with active weather (frontal passages, precipitation).

[wxman22]

Heres my temp forcast NOTE: I could be a little to cold or a little to warm on the temps I'll change the forcast when needed.:

[Tyler]

Actually, that may be exactly right. I would agree with it.

[PTrackerLA]

While I agree that the European looks cold I'm basing my opinions on how the NWS is pretty much splitting the differences between the warmer GFS and the colder European. We'll see what happens but if you think we have extreme cold coming this weekend (teens/low 20's for my area) you're likely wrong. 55/30 isn't all that big of a deal for the second week in February.

Excuse me? Your putting words in my mouth. Where in the world did I say extreme cold!? All I said is it will likley be colder than what the GFS is showing, MUCH COLDER. Also, highs will likely struggle to reach 50, and stay stuck in the 40's. Not sure about mid 50's like you say, unless clouds clear, which they most likely will not.

I really hate it when people put words in my mouth.

I said IF you thought it would be that cold, never said you said it. There were speculations it could be extremely cold about a week ago but it's clear that won't be the case anymore. Enjoy your light freeze.

[Tyler]

I wouldn't call mid 20's a "light freeze".

[CajunMama]

If it hits the mid 20's near daybreak...yeah it would be a light freeze.

[PTrackerLA]

Well that's a good 6 or 7 degrees colder than the current forecast. Hope you're right about the major temp busts this weekend.

Side note: This is the first time I can remember not having to cut back our Elephant ear plants and (some variety that looks similar) in our backyard due to a freeze. If it doesn't happen this weekend it would appear that they're safe!

[southerngale]

GFS: Houston sees +1 850 temps
Euro: 850 temps -6 to -8. We are stuck in the 40's all weekend vs. 60's GFS.

Don't you guys get it? The NWS is leaning more towards the pathetic inconsistent GFS, THATS why ya'lls forecasts are so warm. They are going the inbetween route, which is what you should do. But now, this is model disagreement is getting ridiculous.

Also, if the Euro is correct, we'd be in the mid 20's sunday morning...

This is the worst model disagreement I've seen in a long time.

And the forecast here now calls for 29° - not too far off.

It will definitely be colder than it has been, but nothing earth shattering. It seems like it will be closer to what you expect in the middle of winter.

Mid 20's is record territory for Houston... Not sure about Beaumont!

Actually, I just checked. It looks like 25° is the record low for February 12th (Sunday)...several years with that temp, the most recent 1981. It just didn't seem that cold for February.

However, NWS here just upped the forecasted low of 29° to 31° as well as the high for Saturday. The trend is warmer, at least with them.

[jkt21787]

I really think now that the next week or two (at the most) will be the 'last hurrah' of winter. Hope something you want happens in that time.

BTW I know several will not be happy with what I say and will try to disprove or discredit me, which is fine. Its just my opinion and I could be wrong too.

[Tyler]

If it hits the mid 20's near daybreak...yeah it would be a light freeze.

You call that a light freeze? Ok. People can have different definitions.

[Portastorm]

OK, back to the temps discussion. This morning in Austin, at least downtown, we hit low 40s and the 12z 850 mb temp was 4 degrees.

If the Euro is right and at 6 a.m. Sunday were at -6 or -8 degrees with the 850 mb temps, that would mean a low of 25 or less, maybe even low 20s. Wow! That would be something.

[Portastorm]

If it hits the mid 20's near daybreak...yeah it would be a light freeze.

You call that a light freeze? Ok. People can have different definitions.

To me, a light freeze is 30 degrees. A "freeze" would be anything at 25 degrees or thereabouts with a major freeze being anything below 20.

[Tyler]

At the aiport, it said you guys hit 29.

http://www.srh.noaa.gov/data/obhistory/KAUS.html

You guys would probably be around 20 next weekend, under ideal radiational cooling.

[Portastorm]

At the aiport, it said you guys hit 29.

http://www.srh.noaa.gov/data/obhistory/KAUS.html

You guys would probably be around 20 next weekend.

Let me tell you about that. The ASOS station at the airport is notorious around these parts for really being off from the rest of the real world. I don't know if it's because it is in a depressed (geographically speaking) area or what but the reading from Camp Mabry is much more reliable and in keeping with other observed temps in the city.

[Tyler]

Oh ok, didn't know that! Thats the same situation with the Conroe observation site north of Houston. Conroe is always around 5 or 10 degrees cooler than surrounding observation sites. They say its becuase its elevation is a bit lower, located in a "soup bowl".

[Johnny]

Does their look to be ideal radiational cooling for this coming up Saturday night into Sunday morning? From what I am seeing it doesn't look that way. Clouds will hang tough through most of the day Saturday and start clearing out Saturday night. That's a big IF. Also the wind will be a factor with a steady northerly flow.

[Tyler]

Does their look to be ideal radiational cooling for this coming up Saturday night into Sunday morning? From what I am seeing it doesn't look that way. Clouds will hang tough through most of the day Saturday and start clearing out Saturday night. That's a big IF. Also the wind will be a factor with a steady northerly flow.

Ya, thats what will most likely happen. Monday morning will likely have ideal radiational cooling.

[Portastorm]

Does their look to be ideal radiational cooling for this coming up Saturday night into Sunday morning? From what I am seeing it doesn't look that way. Clouds will hang tough through most of the day Saturday and start clearing out Saturday night. That's a big IF. Also the wind will be a factor with a steady northerly flow.

I would guess that you folks in southeast Texas may have a more difficult time shedding the clouds than we would in south central Texas. Of course, this all depends on the timing of the front and how that short wave disturbance acts as it moves across our area.

Way too many unknowns or "little knowns" at this point.

Personally I'm maybe not quite as bullish as Tyler or Extreme are about the temps but I'm equally convinced that the GFS is going to be too warm this weekend.

[gboudx]

Aren't "light" or "hard" freezes determined by the number of hours an area remains below 32, and not based solely on the actual air temp?