Is it really going to rain?

[Mrs. Mosher]

all next week? We are finally getting to come for the summer and I'm afraid we will be rained out. What are your thoughts?

[SIKSUKR]

Where did you get the impression it is going to rain here all week.If it was from watching the generic Weather Channel forecast maps that might show a rain illustration on each day,I would not put much stock in that.Most of the time those illustrations are showing a chance of thunderstorms which are waaaay too hard to forecast more than a day and especially harder for localized rain.Don't despair and worry as it will probably be just fine.

[CanisLupusArctos]

Next week is looking a lot cloudier and wetter than this week but remember we are talking about *next week*. Weather forecasts lose a lot of detail accuracy beyond 48 hours. By that I mean the details are usually incorrect beyond 48 hours so you have to read the extended forecast with a mind toward generalization. For example if they say there's a chance of a major rain storm 4 days away, it means there will most likely be a major rainstorm somewhere near us between day 3.5 and 4.5 (the same is true of winter storms.)

Right now I would just look at the generalized forecast for next week which says it'll be cloudier and wetter than this week... and that's not saying "awful" because this month (until now) has been abnormally dry. We'll be coming up on high fire danger again if we don't start getting some rainfall.

What SIKSUKR said is true about summertime weather. We get a lot of localized weather in summertime. Not like winter when a storm is big enough to affect everyone all at once. Summertime weather brings a storm in one town while the bordering town has sunny with "rumbles of thunder from somewhere."

Remember, a 50% chance of showers (in summer) usually means literally that showers will be scattered around the area in such a way that 50% of all points in the area get a shower, while the other 50% do not. If there are enough showers to cover 50% of the state, that probably means the rest of the state will be covered by clouds, because "dry" clouds tend to extend outward from rain areas. That picture improves with a 30% chance of showers because if there are only enough showers to cover 30% of the area, the "dry" clouds extending from those showers are probably not going to be able to cover more than another 30% (or so... depends on situation.) Therefore a 30% chance of showers leaves a few locations with no sign of rain at all.

When predicting large-area storm systems (like nor'easters) the percentages take a slightly different meaning. For example, a "60% chance of snow" refers to the historical/statistical probability that the whole system will follow the predicted track. That means the computers and their human interpreters believe there is a 40% chance that the nor'easter will follow a different track. Usually they know what the "plan B" track would be, but they seldom have the time to talk about "plan B." That is when it helps to know your weather, because you can spot the "plan B forecast" and recognize it whenever it starts to happen, while everyone else is screaming that the forecast is wrong.

To encourage you about next week, I will point out that June had a LOT of cloudy, rainy forecasts but most of them were 40% chances or less. We got a lot of rain (over 5 inches) for the month, but it all came from hit-or-miss showers and thunderstorms. There was plenty of sun mixed in, though very few days were sunny from beginning to end. We had our share of storms at the lake in June, but one of my friends in Dover last night told me they've had a lot of storm threats but no direct hits yet. It's all chances and probability -- meteorological Las Vegas.

[Rose]

Remember, a 50% chance of showers (in summer) usually means literally that showers will be scattered around the area in such a way that 50% of all points in the area get a shower, while the other 50% do not.

From the National Weather Service Operations Manual, the Probability of Precipitation (POP) forecast is "The likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The time period to which the POP applies must be clearly stated (or unambiguously inferred from the forecast wording) since, without this, a numerical POP value is meaningless." (http://www.weather.gov/wsom/manual/archives/NC118411.HTML#z8-31)

So if the POP forecast for Laconia is 30%, then there is a 30% chance that Laconia will see precipitation.

[meteotrade]

Its nearly impossible to get a "washout" week in late July like you'd see in June or September. I think shower/t-storm risk is higher than normal, and temps will probably run a few degrees below normal for the balance of the week... but don't expect day after day of rain and cool temps.

[Orion]

all next week? We are finally getting to come for the summer and I'm afraid we will be rained out. What are your thoughts?


.......um...... no.

[chipj29]

Just remember this little formula. If you see the POP (Probability of Precipitation), there is an inverse to that. PONP (Probability of NO precipitation).
If it says there is a 30% POP/chance of rain, that means there is a 70% PONP.

OK I just made that up...but it is true. To me, anyway. ;)

[CanisLupusArctos]

From the National Weather Service Operations Manual, the Probability of Precipitation (POP) forecast is "The likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The time period to which the POP applies must be clearly stated (or unambiguously inferred from the forecast wording) since, without this, a numerical POP value is meaningless." (http://www.weather.gov/wsom/manual/archives/NC118411.HTML#z8-31)

So if the POP forecast for Laconia is 30%, then there is a 30% chance that Laconia will see precipitation.


Thank you for the correction. My previous understanding was explained to me a few years ago by a NWS meteorologist... who apparently should spend more time reading the manual.

[Rose]

Thank you for the correction. My previous understanding was explained to me a few years ago by a NWS meteorologist... who apparently should spend more time reading the manual.

This is one I have to keep refreshing my mind on. Thankfully, the NWS has plenty of this stuff online.

[CanisLupusArctos]

This is one I have to keep refreshing my mind on. Thankfully, the NWS has plenty of this stuff online.

Is it a public link or is the manual only on internal servers? I always thought that stuff wasn't public but if it is, it would be interesting to read it sometime.

[Rose]

Is it a public link or is the manual only on internal servers? I always thought that stuff wasn't public but if it is, it would be interesting to read it sometime.

I found the portion relating to POPs and included the link in post #4 using Google. Snipping bits off that link I found the NWS Directives System (http://www.weather.gov/directives/index.htm), which I haven't gone through yet.

Federal Meteorological Handbooks come in handy. You can find them at the Office of the Federal Coordinator for Meteorology (http://www.ofcm.gov/) under Publications.

Now, probably the coolest site is MetEd (http://www.meted.ucar.edu/index.htm), which I haven't had any free time to explore much. You have to register, but I think it's available to everyone.

[viands]

Dear Mrs. Mosher,

On behalf of the entire board, our apologies, you were right. It is possible. :(

The Board

[meteotrade]

Dear Mrs. Mosher,

On behalf of the entire board, our apologies, you were right. It is possible. :(


Unreal. I posted earlier that a washout week in late July is nearly impossible... and this will be the closest thing to it that I've ever seen! Amazingly persistent southerly flow continues over the next few days. Today's probably the best day out of the next few- at least some breaks of sun and low t-storm chances. WED/THU will have widespread tropical downpours. Finally a nice day on FRI.

[chipj29]

Unreal. I posted earlier that a washout week in late July is nearly impossible... and this will be the closest thing to it that I've ever seen! Amazingly persistent southerly flow continues over the next few days. Today's probably the best day out of the next few- at least some breaks of sun and low t-storm chances. WED/THU will have widespread tropical downpours. Finally a nice day on FRI.
I wouldn't call this a washout week at all. I have been on vacation for the last 12 days, with 6 remaining. Sure we have had some storms and rain showers, but nothing I have cancelled plans over. I have had plenty of outdoor fun, and will enjoy the next 6 days, even if we do get a couple of storms.

[jerseyonbear]

We arrive this weekend for two weeks we have waited for since last summer. It is much nicer with sunny days when 4 little ones can be outside and not under one roof. A couple of days of Old Maid and Crazy Eights is enough. Looking forward to sun fun and relaxation. Haven't checked the weather long range.We have had a real heat wave in New Jersey.

[SIKSUKR]

Not a washout week but this one has me backtracking on my earlier post.What a strange pattern we are in.Usually when we have t-storms,they are associated with a cold front that moves through and replaces the humid air with more dry canadian air.To have day upon day of humidity along with almost a solid week of daily t-storms is very unusual.

[meteotrade]

I wouldn't call this a washout week at all. I have been on vacation for the last 12 days, with 6 remaining.

1) Last week was a top ten week... a real winner. Unfortunately many folks, including Mrs. Mosher, are not as fortunate as you to have 18 days at the lake! (must be nice... ;) )

2) I'm pretty sure you'll be singing a different tune by Thursday night... today and tomorrow look downright miserable. Siksukr's right... this is a very atypical pattern for late July.

Jerseyonbear, next week looks better but not perfect... trough is still on top of us so temps will probably continue a bit below normal although rain chances are lower. The following week will likely turn warmer.

[chipj29]

I can def. agree that this weather has not been typical. The humidity is unbearable. The only plans I have cancelled so far on my vacation was last Friday. The kids and I were scheduled to go to Water Country for the day. The day turned out to be hot and sunny...but we had to cancel as my son broke his hand on Thursday. :(

[Sman]

My neighbor always says a rainy day at the lake is better than a rainy day at home...some truth to that but it depends how much money you have spent on your vacation and how many kids you have, for those of you on vaca, its time for bloody Mary maybe :confused:

[chipj29]

I stand corrected on the rain not cancelling any plans. The last couple of days have been lame at best. I had not 1 but 2 softball games cancelled last night. Tonights game is almost sure to be rained out. Oh well, vacation is winding down, and I needed a couple down days anyway. As long as the sun comes out over the next 3 days I will be good.