THE  850  TEMPERATURE RULE EXPLAINED

  Long before  there  the  development  of well known MOS    forecast   products --  such as  ETA MOS...  GFS /AVN MOS...  NGM MOS   for  temperature foreacting...    Old time forecasters  had used several rules or  techinques   for  coming up  with temperature forecasts.  I have made reference  to the  850   Temperature Rule  several times.   Here is  the 850   Temperature   as it as presented to me  by  Dr Stan the Man   Gedzelmann of CCNY and    Modified  by  Jim Noblie M.S,   of  Fleetweather's  Forsensic  Meteorology  Division (and all around  good egg).

The most important  thing to know    regarding the 850 Temperature Rule is that  in its basic form  the  850   Temperature  rule  you will get the   MAX  POSSIBLE  Temperature... but NOT necessarily  the  ACTUAL   temperature    OR  even  a temperature that  you should  Forecast.      In the Advanced  section   there will be some modifiers  to   consider that  wil help you come up with a  more viable forecast temperature.  

THE BASIC  850 RULE

STEP 1   to   Figure the   Morning Low temperature   for the   Day / Time in question  use   FORECASTED 850  Tt at 12z    from the    various  NWP model (s)  of  your choice     To  figure  the POSSIBLE MAX       daytime  temperature   use the    FORECASTED 850 Tt       valid   at 00z

Because   the effect on air masses by  solar radiation     changes due to the seasons...  the  850  Rule    involves adding   various numerical values   to the   850 temperature that  results in  the  MAX  POSSIBLE    TEMPERATURE  (MPT).  The  850 Temperature   Rule works best between in the  area   from  30N   to    44 N  Latitude.

EXAMPLES

      ACTUAL    850  TEMPS              
SURFACE    TEMP  PLOTS                

MODIFIERS

DOWNLSOPE or  COMPRESSION   
This is  only a   factor  in  certain  areas of  the  nation-- mainly    down wind or to the East  of the  CONUS  3  mountain ranges. 
The  Numerical Values   in  Degrees  Celisus
  SEE  map.

When  considering these  next two Moidifiers     --  DRY GROUND   and   EXCESSIVELY  WET   GROUND.... the following   source of  data  can be used

previous Month rainfall       30  Day  Actual Rainfall                                90   Day  Actual  Rainfall      
DROUGHT monitor             short  term  drought  (experimental)            PALMER  drought index  

EXCESSIVELY  DRY GROUND   --    when the following   criteria    are  reached   during the  warm months   (MAY 15   to OCT 15)     a
Modifier   of  +1 degree   or   +2  degree C...       can be added.

EXCESSIVELY WET GROUND  If  the area  / location under  consideration has  seen  excessive Rain fall ...      then wet  Ground conditions  can   affect  Max temps  during the  warm months     AND     cool season   months.    

STRONG  WAA /    CAA 

Major changes in Air mass    overnight  and / or during the day   is a common  occurrence.   Sometimes these   air masses can   bring  about significant  problems     in temperatures forecasting .   In the  Old  days when  Model   guidance  was available  only in 12 hours increments  the    affect of  significant   WWA   (warm air advection)  or  CAA  (Cold air advection)     would have to be  estimated.   With the  usage of    numerous     highly   detailed short range   and    Medium Range   models  in  6 hr   time increments....  such    things  as  strong   WAA   or  CAA   no longer have to be estimated.    Using   6 hr  time  increments   is  very    helpful in figuring   WAA  / CAA.

MODERATE   WAA  ==   850 temps  that rise  3-4   degrees    C  in  6 hour  time   increments -- +2   to Max  Possible   Temps 
STRONG   WAA  ==   850 temps  that rise  5 -7   degrees    C  in  6 hour  time  increments --  + 3   to Max  Possible  Temps 

MODERATE  CAA  ==   850 temps  that DROP   3-4   degrees    C  in  6 hour  time   increments --    Minus 2   to Max   Possible  Temps
STRONG   CAA  ==   850 temps  that DROP  5    to  7  degrees   C  in  6 hour  time   increments --  Minus  3  to Max   Possible  Temps 

SNOW ON THE  GROUND  over    3 inches   =   minus   1 degree  C

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