A Short Term Forecast is issued only during significant weather. Generally, the Short Term Forecast will discuss current thunderstorms, showers, fronts, fog, and wind. If these phenomena become more of a threat to life or property, appropriate watches, warnings, or advisories will be issued. The Short Term Forecast is intended to replace the Special Weather Statement during weather events. Special Weather Statements will still be issued for weather events that are more than 6 hours away.

EXAMPLE SHORT-TERM FORECAST (NOW, FPUS46)

LAXNOWLAX
TTAA00 KLAX 011754
SHORT TERM FORECAST
NATIONAL WEATHER SERVICE OXNARD CA
1000 AM PST FRI NOV 1 2000

CAZ041-046-047-054-059-087-088-020000-
LOS ANGELES COUNTY-
1000 AM PST FRI NOV 1 2000

.NOW...
LOW CLOUDS AND FOG WILL BURN OFF TO THE COAST BY MIDDAY. SKIES OVER LA COUNTY THIS AFTERNOON WILL BE MOSTLY SUNNY INLAND AND PARTLY CLOUDY ALONG THE COAST. HIGH TEMPERATURES WILL BE IN THE LOWER 70S AT THE COAST...THE LOWER 80S DOWNTOWN AND IN THE MOUNTAINS...AND MID 80S TO MID 90S OVER THE VALLEYS.
$$

CAZ040-044-045-053-020000-
VENTURA COUNTY-
1000 AM PST FRI NOV 1 2000

.NOW...
LOW CLOUDS AND FOG WILL BURN OFF THE COAST BY MIDDAY.
$$

First of all, what is a pressure gradient? The term "gradient" implies a difference. Therefore, simply put, "pressure gradient" means "pressure difference." Meteorologists use pressure gradients in many ways, but primarily for determining wind speed and direction. Without going into too much detail or technicalities, in the upper atmosphere, air generally flows clockwise around areas of high pressure and counter-clockwise around areas of low pressure. However, at the surface, due to friction and other smaller scale effects, air flows more from high pressure to low pressure.

In Southern California, pressure gradients are critical in forecasting the weather. When reading weather related documents, you will often see Meteorologists refer to "onshore" and "offshore" flow. Onshore flow refers to air flow from the water onto the shore. In other words, west to east. A majority of the time, the pressure gradients in Southern California are onshore. Why is this? Well, that would take a little while to explain in detail. However, to put it simply, climatologically speaking, there is an area of high pressure in the eastern Pacific and low pressure in the desert southwest. The low pressure in the desert intensifies during the day because when the air heats up, it rises, causing the air pressure at the surface to lower. This increases the pressure gradient. When the gradient increases, so does the wind. This is why we so often get afternoon sea-breezes at the coast and why it seems to increase as the day goes along. The mountains provide a very effective block, keeping the cooler marine air to the west. However, the breezes due filter through some of the passes, which is why the Antelope Valley gets breezy in the afternoons.

"Offshore" flow occurs when the pressure is higher over the land than over the ocean, often resulting in north-east winds over Southern California. This is also referred to as the Santa Ana effect, or the Santa Ana winds. The terrain here often enhances the offshore breezes because as the winds are forced through the narrow canyons, they increase in speed. This is referred to as the Bernoulli effect. This is why during Santa Ana's, some places will have winds exceeding 50 mph and others will have almost nothing.

Having said that, what do the three columns in the table represent? The first column is the actual difference in the pressure between the two listed stations, in millibars (mb). So, for example, a 3.2 mb. gradient means that the surface pressure at LAX (Los Angeles International Airport), is 3.2 mb. higher than at (DAG) Barstow-Daggett airport. If the value was -3.2 (negative 3.2 mb.), that would mean the pressure at DAG is 3.2 mb. higher than at LAX.

The second column and third columns are a little more complicated, but equally as important. The second column is the difference between the gradient today and the gradient yesterday, at the same time. This is also referred to as the 24 hour change. So for example, if the number in the first column today at 16z is 3.2, and the number yesterday at 16z was 2.4, then the value in the second column today would be 0.8. To put it another way, if the gradient at 16z today was 3.2, and the second column showed a 0.8, then that would mean that the gradient yesterday at 16z was 2.4. Meteorologists use the second column to determine trends; that is, whether the gradient is becoming more onshore or offshore. Using the above example, the 0.8 would indicate that the gradient is becoming slightly more onshore (an onshore trend). A negative value would imply an offshore trend. Keep in mind, though, that an offshore trend doesn't necessarily imply offshore flow. It can simply imply a weaker onshore flow.

The third column is even more complicated. To put it as simply as possible, it is the difference in the difference. In mathematical terms, it is an acceleration, or the second derivative. More definitively, it is the difference between the 24 hour change now and the 24 hour change 3 hours ago. So, for example, if the 24 hour change at 16z was 1.4 and at 13z is was 2.4, the third column value would be -1.0. What does this tell us? It tells us whether the change is increasing or decreasing. If, for example, we have offshore flow, and the third column is negative, this would tell us that the offshore flow is intensifying (becoming more negative).

Here is an example:

• 46054 2518 / 3224 / 15-17 / 57 / 54 / 1018.3 W. SANTA BARBARA CHNL

• 46054 - Station ID number
• 2518 - Observation time, day/hour (UTC) (in this case, Feb. 25 at 1800Z)
• 3224 - Wind direction and speed (here, from 320 degrees at 24 kts.)
• 15-17 - Swell height and period (here, 15 ft at 17 second intervals). If there is a third number, that will be the direction from which the swells are travelling (26 would represent 260 degrees).
• 57 - Sea surface temperature in degrees F.
• 54 - Air temperature in degrees F.
• 1018.3 - Air pressure in millibars.
• W. SANTA BARBARA CHNL - Name of buoy or site of observation.

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