Thursday, January 20, 2011

An Orgy of Wintry Weather

   Over the next seven days winter weather enthusiasts will likely get their fill. Three weather events will affect Eastern NY state and much of the Northeast. They are: a moderate, for some marginally heavy snow tonight through Friday 21 January, followed by extreme cold  Friday night through next Monday, and then the threat for a long duration, possibly major winter storm next Tuesday through Wednesday (25th-26th January).
   Tonight and tomorrow's snowfall is associated with an area of low pressure over the Northern Great Lakes and an arctic cold front that trails southwest from the low across the Upper Midwest. There is also another wave of low pressure and associated frontal activity that is moving across the Tennessee Valley. This low is forecast to move off the Mid-Atlantic Coast tonight and track northeast towards southeastern New England by tomorrow morning. This low will spread snow across Upstate NY by tonight through Friday morning.

The following maps are a forecast for Friday morning 7am 21st January of surface pressure, 850 millibar temperatures (degrees C) and Quantitative Precipitation Forecast for the 6 hour period ending at 7am. The first map is from a computer model called the UK, the second from another model called the GFS and th last map is from a model called the NAM. The QPF scale (amount of precipitation) is on the right hand side of each map. As far as the location of the low pressure area they are all fairly close. For Eastern NY during winter when a low tracks close to the East end of LI or Rhode Island, this is a very good track for Eastern NY to get some of its best snowfall.

The model data from 7am this morning is indicating the following QPF amounts for Albany for the entire upcoming event : UK and NAM- 3/10th of an inch, GFS a bit more than 4/10ths of an inch. A "technique" for forecasting QPF that I use on occasion (for simplicity I'll call it the Penn Method: ) is indicating amounts also near 3/10ths of an inch.

During a typical winter storm that produces snow one would take the forecast QPF and multiply it by ten to get a snowfall amount. However, when the atmosphere is cold, like in this impending storm, the multiplying factor is greater. Since 850 millibar temperatures will be at or below minus 10C for most locations during this storm the snow will be more "fluffy". So for tomorrow's event the multiplying factor is 15:1. Thus 3/10ths multiplied by 15 yields 4.5 inches of snow for Albany.

Since QPF forecast for areas SW though SE of Albany is forecast to be 4/10ths to a half inch then snowfall amounts across the Southern Catskills, the Hudson Valley south of Hudson NY (this would include Kingston and Poughkeepsie), east to Litchfield County CT,as well as the extreme southern portions of Columbia County NY and Berkshire County, MA) will be near 6 to 7 inches.

Areas NW through NE of Albany will have QPF amounts less 3/10ths of an inch so snowfall amounts will be less (but snow ratios will be a bit higher). So for areas north of Clifton Park west through the Mohawk Valley 3-6 inches of snow is expected.

The snow will arrive across Eastern NY between 8pm (Catskills) and 11pm (Lake George). By 7 am Friday morning here could be 2-4 inches on the ground from areas west of to Albany and points east and south. 1-2 inches is possible to the north of Albany.

Model data is also indicating a period of moderate to heavy snow for a couple of hours either side of 6am for the Catskills, to Albany and the Berkshires and points south. During this approximate 4 hour period snowfall rates of 1/2-1 inch/per hour are possible. The steady snow will likely taper off from west to east between 11am and 2pm Friday.

However, there could be another "sneaky" period of snow showers during tomorrow afternoon as the arctic cold front moves across Eastern NY State. The GFS is indicating a period of strong vertical motion and a pronounced winds shift around 4-5 pm in Albany.

This front could be like a summertime cold front that produces strong thunderstorms but instead of thunderstorms this one could produce a brief period of moderate to heavy snow shower perhaps even a heavy snowsquall. Now if these heavy snow showers or squalls do occur there could be some localized additional accumulations of an inch or so in places, along with reduced visibilities and falling temperatures (causing some quick freeze up perhaps of wet and/or snow covered roads, sidewalks and other surfaces).

After this front moves though the arctic air will move in and stick around through Monday.

As for the cold weather, data is indicating that come Sunday and Monday, Albany will experience the coldest days that we have seen in quite a few years.

Daytime highs (for Albany) will only be in the middle or upper DIGITS and Saturday and Sunday night will likely be quite a bit below Zero, it is possible that on Sunday night Albany's low temperature will be between minus 10 and minus 15! And during this same time areas north towards Glens Falls could be near -20 degrees!

During this time temperature anomaly data across NY State is near -3 Standard Deviations (SD) below normal. A rule of thumb that I use is for every 1 SD that equates to about 7 to 8 degrees away from normal. In this upcoming case we are talking about temperatures of 20-24 degrees below normal. So for a quick forecast of expected high and low temperatures all you have to do is subtract the 20-24 degrees BELOW normal from the normal high and low.

Here are the normal high,low and average as well as records for the period January 22 (Sat) through January 22 (Mon):
22 31 12 22 44 0 62 1906 B20 1984 5 1943 46 1906 54.0 1906 B3.5 1984
23 31 12 22 44 0 64 1906 B20 1970 4 1976 40 1933 50.0 1906 B5.5 1976
24 31 12 22 44 0 54 1999 B17 1948 B1 1882 39 1953 45.5 1967 B7.5 1882

To add to the cold there will be some wind at times, especially over the higher terrain of Eastern NY and Western New England that could cause some dangerous wind chills.

Now we move on to the Monday through Wednesday period..right now the only model that is going for a winter storm is the ECMWF. For the past several days it has been pretty insistent on this scenario.

I'll have more tomorrow as to why I think the EC is the model of choice for next week's potential storm.

We'll see how this pans outs..

Sunday, January 16, 2011

Forecast Upper Air Soundings for the Tues 18 Jan Winter Weather Event

The individual white charts that I posted below are called "soundings" or SkewT's; in this case a forecast sounding from a particular model (in this case the UK) for both a particular location and specific time.

In simplicity it is a plot of air temperature (the red line) and dew point temperature (the green line) versus height. Height is plotted not in feet but millibars. Along the right vertical axis going up is the forecast wind speed (in knots) and direction (in degrees). On the direction shaft you will see lines called barbs)a full barb is - to 10kts, a 1/2 barb is 5kts and pennants are 50 kts. So for example if you had a pennant followed by two full barbs and a half-barb the wind speed would 75kts (NOTE: I use knots on my forecast sounding data, some plots use meters/sec)

The far right data blocks show forecast instability parameters, shear and helicity/holicity values as well as layer thickness'. Thickness is basically the distance between two pressure levels. The greater the distance between levels the higher the thickness and the warmer the air. The less the distance between the layers the "colder" the air. Thickness data can be useful in determining precipitation type (or p-type). I'll post a brief write-up on how you can use thickness info to forecast p-type in another blog update.

More on SkewT's can be found here: How to read a SkewT and here More on SkewT