Friday, December 17, 2004

AN INSIDER'S LOOK AT THE SOURCE DATA
FOR YOUR PUBLIC FORECASTS

Note to hungry powderhounds: If you want current details on the Solstice Snowstorm, scroll down to the end of this post for the "Hourly Estimates" on specific details of snowfall rates and accumulations.

Note to nervous nellies: I know you are uneasy about this forecast because the Weather Service, Weather Channel and everyone else keeps changing their forecast. I am not going to change my forecast based on every whisper in the wind. The forecast stands, and if I am wrong, I will freely admit why and move on to the next storm.

This map is the projected "Mean Sea Level Pressure" (MSLP), Precipitation and upper level air currents for the period 1 PM to 6PM Monday. This data is put out by the National Weather Service and reorganized into a readable format by the Accuweather.com Professional site. This information is from a program called the "GFS" (Global Forecast System) and is one of the primary computer models the pros use when they talk about "our latest computer models indicate...." Forecasters all over the U.S. utilize this model as one of their sources, whether it is Norm Lewis, Marty Bass, Dave Roberts, Glenn Hurricane Schwartz, Al Roker or Foot's Forecast.

A simple key will help you understand what it means:



The Blue Line: The "540 thickness line" is an indication of how compressed the upper atmosphere is, as well as a general indicator of the ever famous "rain/snow line." As a general rule of thumb, any precip inside the blue line is mostly snow. Precip near the line would be snow/sleet mixed, and precip close to the line is rain/snow mixed. As you can see, the entire I-95 corridor is well inside the blue line, which is located way offshore by 1 PM Monday.

The Colors: Precipitation over a 6 hour period. Remember that the liquid to snow ratio is usually 1:10. The widespread yellow is a trace up to 1/10th of 1 inch. For snow, that translates into mostly flurries or in a very cold air mass, light snow. The green is .10 to .25 (up to 1/4 inch liquid precip). That translates into 1.0-2.5 inches snow. The purple inside the green is 2.5 to .5 (1/4 to 1/2 inch liquid precip). Now we are talking noticeable accumulating snow of 2.5 to 5 inches. You get the idea by now and reading the chart below, you can see that the brighter colors translate into higher precip amounts.

The Solid Lines: These are lines of equal surface air pressure, measured in isobars. Air/wind will generally move about parallel with those lines. Around a low the air moves counterclockwise, and the isobars over all of the northeast are all running north to south. That means a direct discharge of stiff northerly winds and very cold winds blowing straight south behind the storm from eastern Canada to North Carolina. The closer together the lines, the stronger the wind will be.

The Dashed Lines: These are upper air current indicators. When they are bowed south like you see, it means upper air trough of low pressure, and generally cold air. When the dashed lines have lots of space between them and are arched north, it means upper air ridge and generally warm weather.


NOW YOU HAVE THE SAME TOOLS AS THE PROS, and the forecaster of this site.
What can reading this map tell you about the likely weather for Sunday night into Monday?

1. Snowfall for the DC-Baltimore area on west and northwest will not be significant.
Overall: 2" in Frederick County, 3" in Baltimore County, 4" in Harford County and more as you head north and east. Earlier maps showed large areas of green in central MD, southeast PA, indicating 2.5" to 5.0" hence the reason the general snow forecast for central MD is 2-5"

2. Philly, New Jersey and New England will get the bulk of the snow, as you see the storm rotating north. Those areas might end up with 4-6 inches under this arrangement. Central PA from Williamsport on south and west (including State College, Altoona, Pittsburgh) will not see direct accumulations from this storm above 3 inches, but lake effect could easily push that total higher in the aftermath Monday night-Tuesday.

3. Daytime temperatures on Monday will be plenty cold for snow, so it will be powdery and not wet. Cold air will rush in behind the storm, blowing the snow around quite a bit.

SO WHAT IS GOING TO HAPPEN WHEN AND WHERE, AND HOW MUCH.

Hourly estimates of weather conditions for I-95 corridor from DC to NYC and Central MD/PA.

SUNDAY
12 PM Cloudy, breezy, a passing flurry and light rain mixed.
6 PM Overcast, light rain snow in western MD and PA
9 PM Overcast, flurries and intermittent light snow in central MD / PA

MONDAY
12 AM Light rain and flurries give way to light snow in central MD and eastern PA
2 AM Snow becomes light to moderate

4 AM Light to moderate snow continues...2 or more inches in most areas
6 AM Moderate snow with a few brief squalls through the area as coastal low moves north...2-3" on ground. North winds increase, blowing the snow around.
9 AM Moderate snow. South of PA/MD line, 3" on the ground. North and east: 4-5"
12 PM Tapering from moderate back to light. Very windy and cold, highs upper 20's
3 PM Light snow and then flurries with increasing north winds as low moves toward New England.
6 PM Accumulations: Another .5 to 1" in central MD to an area average of 4" between DC and PA/MD line.
Western MD/PA...2-3 inches, more in higher elevations due to lake effect snow.
Central PA...2 inches as bulk of storm passes to east. By this time, New England has begun to bear full force of storm, where accumulations north and east of NYC could top 12 inches.

TUESDAY
Cooler and windy behind the storm, temps in the 30's PA and 40's MD/VA.

AND WHAT ABOUT SCHOOL?
This is a slight tweak from the Thursday post.

Metro Baltimore and DC: Rain changing to snow Sunday, and falling temps overnight Sunday. Nervous nellies take note...with an on-going snow event early Monday morning, then yes, schools will start with a 2-hour delay, then re-evaluate at 6:30. Plows will have ample time to clear the snow as comes to an end Monday afternoon, thus schools will be OPEN on Tuesday 12-21, no delay.

Philadelphia and suburbs (Chester, Delaware, Montgomery): The farther north and east, the heavier accumulations are anticipated. With a potential of 4-6", I expect all PHL public and parochial schools will go with a 2-hour delay, as well as most suburban schools. Very wind and very cold. Highs in the upper 20's Monday and Tuesday. Yes that's you T/E and Great Valley. More likely that New Jersey schools will be closed.

Central PA: A delay is not likely as bulk of snow is to your east.

New York City: Delays are likely, but it takes at least 6” to close all NYC schools. But this is a powdery snow, not wet. Winds whipping through the cavernous city will create temporary whiteout conditions.

Boston: Rain at first, then change over to snow as storm lifts north and axis tilts westward. More likely you'll miss school on Tuesday.

Interior New York and New England: You're used to big snow, so fuggetaboutit. Although it will be bitterly cold with highs in the teens and wind chills well below zero.

NEXT IMPORTANT TOPIC...UNDERSTANDING THIS FORECAST SITE

If you only want weather details, look above. If you are new to this site and want some verification of it's accuracy and reliability, the post below aims to explain how the site operates, with some humor thrown in for good measure. As the winter storm season arrives, there is likely to be an increase in the number of people asking: “What is the address of that forecast website?” So new viewers should understand what this site IS and IS NOT.

Foot's Forecast....IS NOT a “Wishcast" site where the forecast is based on what I want to happen. It also IS NOT designed to criticize or support the decisions of any government or school agency on whether or not they should or should not have opened or closed their facilities based on inclement weather.

Foot's Forecast IS designed to give public viewers the opportunity to see the science behind weather forecasting, as well as compete with professional forecasters on the outcome of predicted events. Large scale organizations such as the NWS and Weather Channel will not make specific predictions about a certain area far in advance, that is not their role. This site however, utilizes the very same upper air and observation data as the big agencies, and makes a call based on experience with past weather patterns and a knowledge of how the atmosphere behaves under certain conditions.

If large scale computer models project a .25 to .5 equivalent amount of liquid in the atmosphere at a certain time, and the upper level temperatures are such that snow is the likely type of precipitation, then the call is made for a 2 inch to 5 inch snowfall. That is how I arrive at a snowfall prediction days before the other guys. But then you ask:

"WHY IS IT THAT OTHER FORECASTS SEEM TO CHANGE EVERY 3-6 HOURS?"

In becoming interested with a weather forecast which calls for snow, some people inadvertently fall victim to PRE-STORM PARANOIA. The symptoms are:

1. You start thinking about and making all kinds of plans for what wonderful things you will be able to get done on a particular day in the future because the forecast indicates your school or place of business might close for inclement weather.

2. You start to tell your spouse or significant other about all your wonderful plans for what you think you’ll be able to get done on this particular day in the very near future.

3. You begin altering pre-established plans for the time period immediately before and after the supposed inclement weather event. (Well, that meeting we had planned for Monday... you know, I mean it's looking like the weather will be....)

AND… the most significant symptom of PRE STORM PARANOIA

4. You begin checking the online weather forecast for the particular day in question, at least 5 times a day, seeing if you can find out “the latest.”

THE PURPOSE OF THIS QUEST IS: You are, quite consciously, trying to find meteorological justification for continuing your high expectations that the vast, complex, highly inter-connected, world-wide atmospheric thermodynamics will all cooperate in just the right order to enable you to allow your special plans to come true: Such as decorate the tree, catch up on grading or assignments, shop for presents, get the oil changed, shovel the walk, walk the dog, and sleep in, all on the same day.

AND THEN THE FORECAST CHANGES. So in your multi-hour checking of the Weather Channel, or Accuweather, or the Weather Service, or even Foot’s Forecast, you discover that the forecast changes slightly about every 6 hours. And with the forecast change comes a wide range of rollercoaster type emotions, usually correlated to the predicted amount of snow to fall. In the morning, the forecast calls for snow and wind, then by noon, it has changed to flurries. By dinner, you notice with horror that it just calls for cloudy and windy. Then the next morning, the storm is back and the forecast is for more snow than you hoped, then it’s gone again by the next evening. AND YOU WONDER… “Why does it keep changing like that?”

The medium and long term forecasts you read on the Weather Channel, NWS, Accuweather are mostly determined by an automatic computer reporting feature. In Washington, DC at the National Meteorological Center (NMC), vastly complex computer programs collect observation data from all over the world every second of the day. Every 12 hours, the computer then processes all that data into a program called the Global Forecast System (GFS). A “new run” of the GFS computer modeling program is conducted every 6 hours. The short term (1-7 day runs) occur first, followed by the 8-14 day runs, which occur 6 hours later. Thus forecasters all over the world get “the latest” scoop on what the computer sees coming down the road. There are a variety of other forecast models, run by different agencies, such as the European, the UKMET, The Air Force has one, so does the Navy, and so on.

The problem is that the information from the GFS is often automatically spewed right out to public sites, such as the Weather Channel local forecast. And you, the public viewer, end up seeing the forecast vacillate back and forth between rain or snow or wind or flurries, or no storm at all, all in the same day.

THAT’S WHERE FOOT’S FORECAST COMES IN

The forecast on this site is designed to digest all the various data sources, and synthesize it all into one precise statement that answers the question people want to know: “WHAT WILL HAPPEN WHEN, AND WHERE, AND HOW MUCH.” While I will admit when my forecast was wrong, I update the projections less frequently because it is not necessary to change it every 3 hours just because the computer said so.

So the nickname for this site has been: "The Final Word in Weather"

If the call for our first storm does not come true, I will explain why it didn’t, and see you on Monday. If it does come true, you can roll over to the cool side of the pillow, and dream some more about everything you’ll be getting done on “that particular day” you’re so worried about. Let's all hope that dream comes true.

NEXT UPDATE: A weekend overview Friday evening. Updates on the weekend will be mid-morning, mid afternoon and late evening. And be sure to check back in Sunday evening as we all watch and see what really happens.

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