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predicting imminent rainfall

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I am interested in whether or not you can predict rainfall using data available from a pws, not hours ahead but maybe 5 mins in advance.

not sure what use it may be but its more of a challenge.

what sort of variables would be needed from a pws to give a better than 50.50 call of rain?

Divide the current 10 minute average wind speed by 12. That gives the approximate wind run in miles for 5 minutes.

Estimate the current average wind direction.

Take the weather station that direction and that distance and set it down.

When the station gets wet, it'll rain at the original location in about 5 minutes.


Seriously, I don't know of anything a PWS can do with the granularity required for that kind of prediction. Around here (central California coast), I've noticed that in classic winter storms, (our rainy season), the barometer drops and drops, and it's windy. The rain generally doesn't start until the baro bottoms out and the wind dies down.

But trying to predict rain from those 2 observances is going to be pretty hard. I can get a better idea from looking at radar, or other close by stations if the rainfall is hard enough.

If it's really important, I'd consider a more elegant solution to what I described above. Figure out the prevailing directions that the storm comes from, and the average speed at which they travel.

Put one of those Hydreon RG-11 Optical Rain Gauges at the calculated location, set to the most sensitive "It's raining" setting, and use the relay contacts to signal you some how.

I'm curious, what's your definition of high ground level dew point?

--- Quote from: Old Tele man on August 11, 2011, 06:13:09 PM ---Two things are needed: (1) falling barometric pressure, and (2) high ground level dew point temperature.

I'm not sure exactly *how* but cloud level (meaning elevated atmospheric dew point temperature) also comes into play, because of the lapse rate of temperature with increasing altitude.

It's possible for "rain" to occur below a cloud, but no water to actually reach the ground, ie: virga. That occurs when the surface air is much drier (low dew point temp) than the upper air (high dew point temp).

--- End quote ---

Around here normally you are only assured of getting rain when it hits your head.  Thunderstorm cells are very fickle and even watching radar can give you a false idea if it is actually going to rain in a location.

Can't tell how many times I've watched GRLevel3 radar and seen a cell approaching only to have it be done before it gets there, or skip over and regenerate past our location.

We have a location just north of us by about 3-4 miles that due to a little ridge in front of it, gets a lot of the moisture that passed over us and dumps on them on a very regular basis.  Great lightning shows from that area as well.

Its also no uncommon for a cell that ends up generating rain doesn't have any measurable rain from it leading up to when it actually starts raining.

I know a lot of places in the country when you see certain cloud types, its going to rain and rain over a wide area, but we get a lot smaller cells normally in the desert unless it is a fairly large storm.

thanks, I think I may study

pressure tendency
air temp minus dew point
the above tendency
relative humidity
rh tendency

any others?


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