The Quick and Dirty: Forecasting Surface Temperatures

Lows bottomed out in the mid 50s across the Salt Lake Valley earlier this morning, but they have already climbed into the low 60s as of 0900 MDT. Skies are mostly clear, with only a few high stratocumulus, and winds are light out of the south. These are all cues - but not guarantees - that it will be a warm one out there today. Based on conditions this morning, let's attempt to determine how warm it will get today (4/20/2026). 

If we look at the mid-levels (~ 700 mb), 700 mb temps are near 5 C at 3100 m ASL according to the SPC mesoanalysis (figure below), which is fairly mild air and points to a warm day ahead. But why are we looking at the 700 mb level? How does 700 mb temperatures - more than 1800 m above the surface - predict temperatures at the surface? 

SPC 1500Z 700 mb Analysis: heights (m), winds, temperature, and RH (> 70%)

Over the higher elevations across the Western U.S., 700 mb air has a lot of influence on conditions at the surface. A quick overview of how this is the case: diurnal heating of the Earth's surface creates buoyant eddies (thermals - bubbles of rising air) that help mix heat, moisture, and momentum in the lower parts of the atmosphere. The layer in which this mixing due to buoyant eddies occurs is called the Planetary Boundary Layer or more precisely the Convective Boundary Layer (CBL). Similar to how a fan mixes air within a room in your home, these eddies mix air through a layer of the atmosphere. Rising and sinking motions created by mixing allow for air from different levels to be transported in the vertical, hence the 700 mb air (on days like today) can be mixed down to the surface. 

Schematic of the evolution of the CBL adapted from Stull, R.B. (1988)

Although this exchange of air is an adiabatic process (no heat is transferred into or out of the system), this is not an isothermal process - temperature of air does not remain constant. Therefore, the 5 C air from 700 mb will no longer be 5 C by the time it descends to the surface. This is due to a thermodynamic process involving the relationship between temperature, pressure, and volume , which you may remember from freshman level chemistry or physics - PV = nRT. Through some differential calculus - that will not go through here - this relationship/equation PV = nRT is derived and combined with the first law of thermodynamics to formulate the dry adiabatic lapse rate (DALR) of the atmosphere. Interestingly, the DALR comes out to be 9.8 C/km (the same number as the acceleration due to gravity). In plain language, this means that as air sinks it warms by 9.81 degrees (C or Kelvin) and as it rises it cools by the same rate. For convenience and quick calculations, a DALR of 10 C/km is commonly used.

So let's put this information all together to forecast surface temperature from the 700 mb temperature. Below, I will give a short and dirty tutorial on how to forecast surface high temperatures. The questions we want to answer and the pieces of information we know:

But wait, let's make a few corrections to refine this forecast...

There are many assumptions that are made using this method for quickly calculating surface temperatures. One major assumption is that 700 mb temperatures will remain constant through the day. This is likely not going to always be the case, and it is necessary to investigate how the 700 mb temperature may change through the day. Today, there's a good chance that the 700 mb temperature will warm 1-2 degrees C by this afternoon (some marginal warm air advection). 

So if we redo the above calculations with a 700 mb temp equal to 6 C: 

76.6-78.6 + (1 C = 1.8 F) = 78.4-80.2 F

We get a forecasted surface high temperature of 78.4-80.2 F!

Today's surface high temperature is forecasted be near 80 F for the SL Valley

Let's cross check our forecasted surface temperature against the NWS forecast :

NWS Forecasted High Temperatures (F) for 4/20/2026

Looks like our rule of thumb for forecasting surface temperatures worked well. The NWS has a forecasted high of 80 F for SLC today, which matches closely with our forecast using 700 mb temps. 

Anyway, that is the quick way to forecast surface high temperatures when you have clear and relatively calm large scale conditions. On days with precipitation or more active conditions, this rule of thumb breaks down. Now go forth and forecast...