The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
Air Force Met wrote:txagwxman wrote:
Crazy weather...but it should end next weekend for a while.
I've got lots of lessons learned for this past event. I am putting a bust review together and will be glad to share when its completed.
I already know where the forecast when wrong...and looking via hind site...it was very obvious that SE TX would see a freezing rain event...even with the available guidance.
BASICALLY: I saw the negatives...and dis-regarded them because the models themselves were putting down snow. The GFS/NAM/Euro/GEM were all putting out snow depths of varying degrees. I falsely assumed (and that is because I don't know how the models do the calculating) that the "snow" parameters were built into the models and that THEY would QC themselves...and if certain parameters were not met...then they wouldn't put a 1" snow depth over an area. I assumed the model would know the critical thicknesses. That's why when I noticed that critical temps and critical thicknesses were not being met...I assumed I wasn't reading something right.
Well...I was reading it right and the model was ignoring it. That makes it even more frustrating. It's one thing to blow a forecast because you didn't see something. Its another thing to blow it when you know what the rules of thumb are saying...and you ignore them because the model said something different. THAT'S being a model reader and I've always hated it when forecasters do that (since so many today have lost the true skills of forecasting and instead read the models).
Another critical value that I saw lacking...spoke about it here and talked at length to Jeff about it on Tuesday/Wednesday (and with several other mets as well) was the -8C critical temp rule of thumb. That rule says "If you have a melting layer, then the temp of the cold layer below MUST be -8C somewhere in the column. If not, freezing rain is the result."
None of the models showed that -8C mark at anything below 600MB...and above 500-600mb...it was dry...so no snow being made there. THEN...even when it was moist...the melting layer at 850-700 was thick enough to melt the snow above it...and there was no -8C below it...so it fell as sleet and freezing rain. If the snow melts...you must reach -8C for it to re-freeze into snow. If not...it remains super-cooled and liquid until it reaches the ground...or it turns to sleet.
So yes...a very frustrating forecast...especially given that temps were never below -8C...and the critical thicknesses of 1000-500, 850-500, and 700-500 (the last two are VERY important if you have a warm layer...which we did) were all above the threshold. And in this instance...the last 2 were spot on if you took a blend of them. ALSO...using the partial thickness table...the Rules state...if you have a 1000-850 thickness of 129-131 DM...and a 850-700 thickness of >154 DM, you should expect "Freezing rain, with sleet near 154DM on the 850-700."
At 06z, Houston had thicknesses of 129 and 155. So...freezing rain mixed with sleet...
At 12Z...127 and 154. That rules states: "Freezing rain/freezing drizzle and sleet" with weak upper level forcing and little/no CAA at the surface.
I will have more in my bust review...but the best thickness values for this event (due to the warm layer) was the 850-500 thickness. It was almost spot on...
A look at the soundings at LCH/CRP points out the obvious reasons why ice was observed and not snow. Initially, the column was saturate at LCH through a very deep layer. However, looking at the CRP 00z sounding shows drying in the mid/upper levels. This drying in the -12 to -18 C layer result in an absence of dendrites (ice crystals) which means you don't see much, if any, snow. Both soundings showed warm layers aloft which would preclude snow despite the presence of any dendrites. The models typically underforecast the strength of the warm nose. Very common model error.
00Z
After 00z, the soundings became very dry aloft, which further removed the presence of dendrites. The 12z CRP sounding terminated early but even as far east as LCH the column was drying out from the top down. The CRP sounding was now at or below freezing throughout the column but a pronounced warm nose was still see at LCH (=freezing rain/sleet).
12Z
Look at the drying that is very apparent on the water vapor loop from 04/00z to 04/13z.
http://tinyurl.com/4u6m2x8
