MSLP change versus 5appp group change

[Mike Bender]

The app group uses station pressure (not corrected for altitude) while 
if you compute the 3 hour slp (assuming you mean mslp) from the grids 
you could be getting a combo of things.  Standard MSLP uses a reduction 
to sea level plus an "R" reduction that uses the 12 hour temperature 
change average (temp now + temp 12 hours ago / 2).  If you get a strong 
cold front, which we have had lately, or thunderstorms in the summer, 
the R value can get a little crazy.  Also, be aware that the models use 
different reduction routines (the old eslp from the eta is a good 
example), so things will not be pure.  Oh, since I have been away from 
field observations for a while, I am not sure what ASOS uses in its' 
computations of these values anymore.

Mike Bender



Arthur A. Person wrote:
> Hi...
> 
> This isn't a gempak question directly but relates to data used by gempak.
> 
> I was looking at surface METAR data today and came upon some perplexing 
> discrepancies between observed 3-hour differences in reported mean 
> sea-level pressure (e.g. P(12Z)-P(09Z)) versus the reported 3-hour 
> tendencies (5appp groups reported in the METAR remarks sections).  Most 
> stations do not exhibit the discrepancy, but a few show significant 
> differences.  I first noticed this in some case data from last July and 
> decided to see if it shows up in todays data, and it does.  For example:
> 
> KDUJ 040856Z AUTO 27013KT 10SM BKN023 OVC030 M06/M10 A3016 RMK AO2
>      SNB04E44 SLP234 P0000 60000 T10611100 58002
> 
> KDUJ 040956Z AUTO 26014KT 10SM BKN021 OVC026 M07/M11 A3017 RMK AO2
>      SLP238 T10671106
> 
> KDUJ 041056Z AUTO 27013KT 10SM BKN021 OVC028 M07/M11 A3018 RMK AO2
>      SLP242 T10671106
> 
> KDUJ 041156Z AUTO 26014KT 6SM -SN BKN016 BKN026 OVC037 M08/M11
>      A3018 RMK AO2 SNB1059 SLP244 P0000 60000 T10781111 11039
>      21078 51005
> 
> The 12Z-09Z SLP difference is 1024.4-1023.4 = +1.0 mb/3hrs, whereas the 
> 12Z 5appp group is 51005 or +0.5 mb/3hrs, a 0.5 mb discrepancy.
> 
> Similarly,
> 
> METAR KAVP 040854Z 34009G14KT 10SM BKN037 OVC085 01/M04 A3006 RMK
> AO2 PK WND 230243/0803 SLP182 T00061044 58010 $
> 
> METAR KAVP 040954Z 35013G22KT 10SM SCT029 BKN036 OVC046 00/M07 A3007
> RMK AO2 SLP187 T00001067 $
> 
> METAR KAVP 041054Z 33012KT 10SM CLR M01/M09 A3007 RMK AO2 SLP186
> T10111094 $
> 
> METAR KAVP 041154Z 33011KT 10SM BKN034 OVC055 M01/M08 A3009 RMK AO2
> SLP192 T10111078 10006 21011 53007 $
> 
> The 12Z-09Z SLP difference is 1019.2-1018.2 = +1.0 mb/3hrs, whereas the
> 12Z 5appp group is 53007 or +0.7 mb/3hrs, a 0.3 mb discrepancy.
> 
> The case I was looking at from July had discrepancies as large as 0.9 mb.
> 
> I suspect this is some sort of "feature" of ASOS related to the 
> "characteristic" of the changing pressure which tries to capture a more 
> immediate tendency than a simple 3-hour difference, but if true, that's 
> a departure from what weather forecasters are used to seeing.  Does 
> anyone know the real reason why the ASOS 5appp tendencies don't always 
> agree with the change in the reported pressures?  Are there any 
> documentation links you could point me to?
> 
>                             Thanks...
> 
>                               Art
> 
> Arthur A. Person
> Research Assistant, System Administrator
> Penn State Department of Meteorology
> email:  person at meteo.psu.edu, phone:  814-863-1563
> 
> 
>