Detailed Model Analysis

[Air Force Met]

OK...morning update here with the models geared for those in Southeast Texas wishing for snow ( me included...I won't lie).

First the Canadian is bearish taking most of the cold air east. Thickness values only drop to about 550 here. Since it is not even remotely hinting at a freezing/frozen event...we'll stop with it there.

Second...the Euro.

MON 00Z 19-DEC 5.9 555
TUE 00Z 20-DEC 3.2 552
WED 00Z 21-DEC 3.9 550
THU 00Z 22-DEC 2.2 546

850 mb temps are again above freezing (after one round of going back down) and thicknesses are high).

Third...the GFS...everyone's "NEW" favorite model. This is from the 00z run. What I did was run a cross-section analysis from KGLS-KACT (Waco). I'll put the times on the left and then list what I found.

126 HR - VT 20/06Z: 546 thickness line near CLL. Nose of below freezing temps starting at about 1500'-2000' near north Houston which increases to near 1000' on up near Waco. Low RH's at this time. Winds out of NE. Slight upward vertical velocities.

132 HR - VT 20/12Z: 546 thickness line now near north Houston. 540 line near Dallas. Below freezing from Houston to ACT. Isn't below freezing all the way up until ACT. Snow melt thickness (the amount of above freezing temp it takes to melt snow) is B/W ACT and CLL and is at about the 800 MB level. Sleet possible all the way down to Houston. However, no precip forecasted. Very low RH's at this time and downward vertical velocities. RH's as low as 40% at 850MB.

135MB - VT 20/15Z: Cold air aloft continues to be reinforced. SFC temps below freezing to neat down town. Snow melt level at near Woodlands at 800 MB with sleet line just north of GLS. However, continued low RH's mean no precip.

138HR - VT 20/18Z: 546 line near Houston. 540 line near OK/TX border. SFC temps warming into low 30's. Some warm air moving into mid levels due to east wind (Remember This!) . Snow melt level near Conroe/Huntsville, sleet level near north Houston but still very low RH's near 800MB and downward vertical motions. RH's in mid levels are increasing

141HR - VT 20/21Z: SFC temps near 34-36. Above freezing until about 1000' and then below freezing. RH's increasing above 6000'. Some sleet possible Montgomery county if precip were to fall...although 50% RH's at 800-900mb make this unlikely.

144HR - VT 21/00Z: 546 line hugging coast, 540 line still far to the north. SFC temps 34-36...closer to low 40's near GLS. Sleet thickness starts north Houston, snow starts south of Waco. RH's coming up but still low near sfc to about 3000'. NCEP model page is showing precip at this time, but there is still a lot of dry air in the lower 3000'. It's not saturated until you reach 8000-9000'. Winds are out of the west above 6000'. I wouldn't plan on a lot of that reaching the ground. Temps in mid levels are coming up in response to the west winds. Where 850 MB temps of near -1C were found over Houston, they are now about +1.5C. CLL: -2C to -1C; 820MB temps near HOU of +0.2C to near 2.2C

147HR - VT 21/03Z: Column starts to moisten. 80% RH line is just above the sfc with lower RH's in the lower 500'. Believe it or not the GFS has coldest temps near north Houston. Not sure why that is (will discuss temps later). Cold tongue of sleet possible near Woodlands northward but no snow as mid layer of warm air continues to grow and mix it’s way into the lower layers. Mid layer temps have decreased somewhat…but overall size haze grow. Thickness of the above freezing layer went from being 100MB deep over north Houston to 150MB deep in 3 hours…but it cooled every so slightly…but not that much.

150HR – VT 21/06Z: RH’s continue to increase and I believe some evaporative cooling is having some effect on the column as the snow melt level is now just north of CLL/Huntsville. SFC temps below freezing north of I-10, so freezing rain/sleet possible north of that line. Sleet band will be very narrow because area that is above freezing with below freezing aloft is only about 20 miles wide. At this time the model is putting down heaviest precip near CLL and south of there. If this verifies it will be freezing rain and sleet…probably sleet because the column is about 3000’ thick below freezing…more than enough to refreeze. The above freezing part near CLL and south toward Hempstead/Brenham/Navasota/even Sealy is thick enough to melt snow…but with some additional evaporative cooling there may be a mix.

153HR – VT 21/09Z: Precip continues to do it’s work of cooling the atmosphere as the last of the low RH’s are driven out south of ACT. Freezing line still near I-10. Snow melt level now moved south of the CLL area. However, vertical velocities are starting to decrease. What precip is falling should be snow south of CLL, mix of sleet and snow near Conroe with freezing rain and sleet north of I-10.

156HR – VT 21/12Z: The 540 line is finally near Houston but the precip is done. RH’s starting to decrease above 8000’ in northern areas. Only upward vertical velocities are near the coast…which means precip over. Freezing line still near I-10. Snow melt level is now near Houston…but precip is over…500mb shortwave has passed and so has any chance of snow south of Conroe.

162HR – VT21/18Z: Vertical velocities all downward and RH’s are bone dry above 850MB. Some RH’s are still high at the sfc…but that is low clouds hanging back….especially around GLS. Temps are above freezing from the SFC all the way up to about 2000’ everywhere. Anyone near ACT/CLL that received any accumulation expect it to be gone by late afternoon as temps warm into the low 40’s.
*****
OK…that was the detailed analysis of the GFS model. Now…I want to say that was the analysis of what the MODEL was saying…not what Nelson Lee was saying. I need to clear that up. I am not saying there will be snow in Conroe and Freezing rain/sleet north of I-10…that’s what a cross-sectional analysis of the GFS was saying. I also want to point out that the cross section was only one slice…from GLS to ACT. I will do different slices on later runs but for SE Texans…GLS to ACT covers a lot of us.

Now the reason I do cross-sectional analysis is this: I noticed a lot of reference to the 850 mb temp, thickness values and snow. That’s fine and all…but unless you really see what the atmosphere is dealing you…you can’t make a snow call on those values alone. You never know if that 1500’ warm layer is poking in at 800 MB – 700 MB to melt that snow and turn it into sleet (which it is)…or if it is sitting at 1500’ above the sfc and it’s just below freezing at 860MB…which it is also on occasion near CLL. That’s why you need cross sections if you are forecasting for a large area…or forecast skew-t’s if you are looking at one station.

So the finding: 1st – Rarely in the upcoming event is the column cold enough aloft to support snow south of CLL without it melting at some point except at the very end of the period when the precip is ending.
2nd – and more troubling – The temps seem too cold. One thing I noticed while running the cross sectional analysis is that the sfc temps of the GFS seemed almost the same…or within a degree or two from Waco through CLL through Houston. Now…for those of you who live in SE Texas…you know that that is rarely…if ever the case. They are always a good 10 degrees or more colder than us. However, on the cross sections…the sfc temps were almost the same along the whole length of the slice. For example. On 20 Dec @ 21Z, ACT is 02C, CLL is 02C, HOU is 01C. On 21 Dec 00Z, ACT has 01C, CLL is at 01C and HOU is at 01C. By 03Z, Act is 01C, CLL 00C, HOU00C.

So…before precip starts…and any of the column can cool through evaporative cooling (which is the only way it can really happen)…Waco starts off with a warmer temp by about 2F than Houston in the middle of Tuesday afternoon. It stays like that with the temps uniform from north to south…with very little deviation and actually keeps a cold pocket near north Houston. I have a hard time buying that…so I think the temps in SE Texas are probably a little too cold…by about 1-2C.

I’ll add more as it comes in.

[Air Force Met]

Now I'll request something...please don't quote my post...or at least not all of it...the page will be too long

If you want to quote something...copy paste it or edit it...so we won't have to wade through a massive long post to see a sentence.

Thanks

[gboudx]

Very informative AFM, and thanks for taking the time to provide it to us. I know you are focused on SETX, but if you have any comments on what us up here in DFW can expect we'd appreciate it. I understand if you prefer to keep the focus on SETX.

[Brent]

Great analysis.

[Air Force Met]

Case in point. Current Temps:

ACT - 39
CLL - 42
HOU - 47

And this is after a Fropa...and is actually a little "tighter" spread than normal. Earlier today is was 29/36/44 which is more like it....about 15 degrees.

[Portastorm]

All I want to say is thanks so much for taking your time to do such a detailed analysis for us! Very interesting stuff.

[Air Force Met]

Very informative AFM, and thanks for taking the time to provide it to us. I know you are focused on SETX, but if you have any comments on what us up here in DFW can expect we'd appreciate it. I understand if you prefer to keep the focus on SETX.

I'll make ONE exception...then it's back to your room young man

Nothing. You're getting coal for Christmas

Seriously...Doesn't look like GFS is putting down much precip at all. If it does fall...very light...maybe a flake. Noticed your local guys are callnig for highs in the low 40's with lows barely near 30. Now for the folks in SETX...if that's what Dallas is calling for...what does that say about us?

That's why I have a problem with the GFS numbers...I think they are too cold. Apparently so does the Dallas office because of the GFS is basically putting down highs in the low 30's for us in Tuesday...well...right now there's a whole lot of people not buying it.

[Air Force Met]

All I want to say is thanks so much for taking your time to do such a detailed analysis for us! Very interesting stuff.

Just for those nice words (and the fact I was going to do one any way ) I'm gonna run one from GLS to AUS.

hehe

[gboudx]

Thanks dude. I've noticed the DFW NWS backing off the past 2 days in terms of precip, and now temps as well. Doesn't bother me though. I'd rather it not be cold, if there's no snow.

[Extremeweatherguy]

the 6Z GFS run looks even a tad cooler, and still shows that threat of snow in northern Houston. This has been pretty consistant.

[CaptinCrunch]

Local Mets have backed off the frozen precip for now and have forecasted temps to be about 8 degrees higher than what they were yesterday. How ever you really wont know until the next 0z model run if there going to flip back to a colder forecast, my gut feeling is that it will flip back and the NWS will scramble to update the forecast to refelect the flip-flopping models.

[Air Force Met]

the 6Z GFS run looks even a tad cooler, and still shows that threat of snow in northern Houston. This has been pretty consistant.

And as I said in the analysis...it's too cool. The current slice I'm working on shows near constant surface temps from Austin to Houston. That means the GFS is is showing the same sfc temps in an area from Austin to Waco to CLL to Houston. Now...everyone of us can tell you that is not going to happen because it doesn't happen.

Also...you need to read my analysis again...the GFS doesn't show snow to near north Houston. There is a warm layer tucked in between the layers you are not looking at. read my analysis and post. You cannot just look at 850 temps and thickness values. That is a good quick-look but not good enough. There is a warm layer caused by the west winds that is sufficient to melt any snow that is falling and it extends to CLL. It is even more pronounced on the new slice (GLS-AUS). BY the time the warm layer disappears enough for it to be snow...the precip has ended...vertical velocities are downward and RH's are falling. That's what the GFS is saying. The warm layer over north Houston is 5000' thick at 06Z, 2800' thick at 09Z and finally freezing at 12z...at which time the precip is done because vertical velocities are downward and RH's above 700MB are drying out.

[Air Force Met]

OK…Austin to GLS…starting at 18Z.

20/18Z – Again uniform temps near freezing…and again this seems to be a problem with the GFS on this run. Just for grins I decided to run it at 144 hrs from CRP to DAL. It’s got BIG problems. Dallas is 3C, Waco 2C, CRP 2C. The coldest spot is near Victoria at +0.3C and Dallas is the warmest. Any questions? Doesn’t this strike any other Texans as ODD?

Anyway, RH’s high above 7000’. 80% west of La Grange. Still downward VV’s. Warm layer is 1500’ deep over AUS @ 6000’ increasing to 3000’ over GLS b/w 5000-8000’.

21/00Z – GLS now 4C (still to cold in my opinion…39 for Galveston?) Above 0C layer is near Ft Bend County. Below 0C layer of 2-4k’ deep is from Sealy to Bastrop with 1C sfc temps. Warmest part of the tongue is over Houston at 2.5C, near La Grange it’s 1C but 4k’ feet thick. RH’s thickening.

21/03Z – West winds aloft from 4000-9000’ have warmed temps from 2/3000’ to 7000 above freezing. Over Austin the layer is 3500’ thick with a max temp of +1C. SFC temp is -1C Sleet range given freezing level is at 3000’. Sleet thickness is somewhere near Eagle Lake. Snow Grains possible closer to Austin. VV’s just east of Austin, precip decreasing.

21/06Z – Slight thickening of the warm layer but not a whole lot of change. Vertical velocities have moved east now near Columbus. RH’s decreasing over Austin, precip should be ended. Large slice of 1C from SE of AUS to the Coast which means sleet to El Campo…and with temps above freezing…cold rain every where SE of that. No SFC temps below 0C.

21/09Z – VV’s Near GLS Above Freezing at SFC from Austin SEWARD Precip ending near coast, Ft Bend, Harris, Brazoria counties. Above Freezing everywhere one this slice from Sealy SEWARD. Sealy NWWARD is below freezing above the sfc to about 3000’…then above freezing again.

So…same problem with temps…I don’t see how we are the same temp as Dallas, Waco, Austin, College Station.

[Air Force Met]

Take a look at the 12Z GFS. Whereas 543 thickness values were abround houston when there was precip...now on the new run they are about 549. 850mb temps over HOU are about 4-5C up from 1C at 06Z, and up to 5C at 12Z...up from 0C.

GFS trending warmer from 00Z to 12Z.

I want to add...it looks like the GFS is about a day or two behind the Euro. The Euro showed cold...then so did the GFS. The Euro adjusted...and now maybe the GFS is...I'll wait until the 00Z run to see for sure.

[Portastorm]

AFM, after reading your take on the GLS-AUS slice ... looks like I'll keep my snowplow in the garage.

[Air Force Met]

AFM, after reading your take on the GLS-AUS slice ... looks like I'll keep my snowplow in the garage.

Yeah...that might be wise. I was gonna break the snowmobile out myself but I'm gonna pass. What ticks me off is my 6 year old hear the dread "S" word on the news last night...he hears everything...and now won't listen to dad who keeps telling him it ain't happening. He's still mad because we didn't DRIVE up to College Station a couple of weeks ago when they were getting flurries.

[Extremeweatherguy]

Take a look at the 12Z GFS. Whereas 543 thickness values were abround houston when there was precip...now on the new run they are about 549. 850mb temps over HOU are about 4-5C up from 1C at 06Z, and up to 5C at 12Z...up from 0C.

GFS trending warmer from 00Z to 12Z.

I want to add...it looks like the GFS is about a day or two behind the Euro. The Euro showed cold...then so did the GFS. The Euro adjusted...and now maybe the GFS is...I'll wait until the 00Z run to see for sure.

Ok as of right now I will back off a little on my snow idea and accept that it may only be a cold rain in Houston . Hopefully we do get some sort of winter weather before the winter is done though; and may be, just may be something will still happen with this system. Who knows...strange things happen all the time.

[Portastorm]

AFM, after reading your take on the GLS-AUS slice ... looks like I'll keep my snowplow in the garage.

Yeah...that might be wise. I was gonna break the snowmobile out myself but I'm gonna pass. What ticks me off is my 6 year old hear the dread "S" word on the news last night...he hears everything...and now won't listen to dad who keeps telling him it ain't happening. He's still mad because we didn't DRIVE up to College Station a couple of weeks ago when they were getting flurries.

I totally understand.

My 2 boys -- ages 7 and 12 -- go nuts where the "S" word is mentioned. I have deliberately said nothing about next week for fear that their hopes would be crushed. Kinda glad I didn't say anything now.

[Air Force Met]

Something also tells me that JB's "a day that could be as wintery as any we have seen in many a year with most station north of I-10 with some form of frozen precip and the chance of the frozen precip reaching the gulf coast Monday night. This is the start of what I think could turn into one of the great pre-Christmas winter events from the south in many a year" is going to fail to come to pass.

As I was explaining earlier in the week, the cold air is not there to pour down in this direction. It's not built up in Alaska and what cold air that is up there is in Northern Canada and with the upper flow that is setting up..it's primed to be sent eastward. That's why I was a little surprised with some talk about this event being like 1899 and 1989. That's just plain ludicrous. The cold air that could be tapped is cold...but it's not that cold. It takes more than patterns to make an outbreak happen. You actually have to have something to work with...like cold air and building high pressures in the Alaskan frontier...eastern slopes. The 1899/1983/1989 events were nation wide events with upper air patterns that tapped into Siberian air...that is not going to happen this time...and I think in the next day you will see JB jump off of it...although quitely.

[Air Force Met]

OK…analysis from the 12Z run…this of course is just for fun since it’s pretty plain to see what happened by just looking at the NCEP charts. But then again maybe not.

21/00Z – 21/12Z The changes in the model are two-fold throughout the period. The first part makes sense…the second part doesn’t.

Part one. The temps throughout the column increase. Temps are above freezing from 6000’ at Waco to near 9000’ at GLS. B/W 03-09Z there is a cold layer of near freezing to below freezing from about 1000’ to about 3000’ from near ACT to IAH.

Part Two. Now here’s the part that doesn’t make any sense. The model puts a lot more precip down. However, RH values are lower than the 00Z run. Vertical velocities are higher…but RH’s…especially 2000=6000’ are a lot lower…in some cases below 60%. This makes no sense given heavy precip on the south side especially. The RH’s should be over 90% but they are not.

I’ll run it again later to make sure all the data is in...like the RH values…but something doesn’t makes sense on this run. What I would expect to happen is the RH’s to rise and the freezing level to lower to near 3000’-5000’. Like I said…I’ll give it an hour or two and re-run it to ensure all the data got in.