North American Mesoscale Model Verification on January 26-27...

By: Zachary Labe , 10:34 PM GMT on January 29, 2011

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The North American Mesoscale Model (NAM) is a high resolution model courtesy of the National Center for Environmental Prediction. Another very common and referenced name for the model is the Weather Research and Forecasting Model (WRF). They are the same model and run out to 84 hours. The purpose of the model is to run a higher resolution determining mesoscale features that often cannot be picked up by the global models due to their larger scale. The NAM can be run on an 80km resolution with a broad view of the United States, or 40km which allows the viewer to zoom in on a localized region to put up on mesoscale features. Also a very high resolution form of the NAM exists using a 12km parametric and can zoom into different states. The model is released four times a day at 0z, 6z, 12z, and 18z. Keep in mind that time is zulu or more commonly known as greenwich mean time.

Computer models are critical to forecasting the weather as they use parametric and other mathematical equations to derive the current and predicted state of the atmosphere using a physical and chemical explanation as the background for the predictions. The NCEP is associated with the National Oceanic Atmospheric Administration (NOAA) to be the developer of these computer models. Each meteorological organization worlwide uses their own developed computer models to serve as a basis for the forecasts. Without computer models, we are not able to derive predictions on the weather greater than 24 hours out.

Like all computer models, the NAM shows all layers of the atmosphere in maps determining positioning and speed of the jet stream, surface precipitation amounts (quantitive precipitation forecast QPF), temperatures throughout the entire core of the atmosphere, convective indices, simulated radars, etc. Due to the higher resolution of the NAM, several problems do exist. It often produces convective elements in many middle latitude cyclones and these are known as convective feedback problems. This is where it developed these unstable regions and often associates them as surface lows causing the surface depiction to be distorted. This was a problem in the recent prediction of the January 26-27 winter storm and will be noted below. Also the NAM has a tendency to produce higher than normal precipitation amounts. I typically find myself cutting totals nearly by 30% as shown on the NAM. Finally the NAM has a bias to overamplify a low pressure and close them off to early particularily on eastern United States cyclogenesis off the coast.

The January 26-27 winter storm featured many model headaches do to some interesting variables. Early on it appeared likely for a low pressure to track up inland along the coastal plain. There was a lacking 50/50 low, unfavorable position of the western trough axis, neutral NAO, natural baroclinicity along the coastline, and stale antecedent cold air mass. This would produce rain along I-95 with heavy snows inland. The 500mb synoptic pattern showed very strong signals for this time of setup with most computer model guidance also in support. The GGEM and ECMWF led the pack with the heavy snows from I-81 on westward. The GFS suffered major problems with varying solutions for each run. But then the computer models began to delay the storm. In fact it was delayed nearly 48-60 hours from the original starting time. This caused a different scenario to unfold. The lacking high pressure to the north was still a problem as the anticyclone zoomed east-northeast, but now a high pressure and associated shortwave out ahead of it allowed sunk east-southeast across the Midwest and western Great Lakes. This acted as a 'kicker' helping to push the cyclonegenesis farther off the coast. This in turn allowed for a colder scenario along with precipitation to occur farther east. Therefore a turn of events allowed for a major I-95 snowstorm with 6in+ totals from Washington DC to Boston. This was very fortunate for snow lovers in that corridor due to pure luck given the poor synoptic setup.

Given the NAM's high resolution, it often overanalyzes prognostics post hour 60. It tends to enhance QPF, overamplify lows, and pick up on mesoscale features that really do not exist. Therefore I typically throw out hours 60-84. It would be a rare event where you would find the NAM 84 hour verifying anywhere close to accurate. But in the near term range, the NAM does an excellent job locating temperature thermals, QPF ranges, and picking up on mesoscale features; coastal fronts, enhanced convection, deformation bands, etc. But in this recent storm, it suffered a plethora of problems and even the 6 and 12 hour surface maps had poor verification especially in the QPF department.

Let us first look at the actual accumulated precipitation totals for the storm.

Given this is a 24 hour accumulated precipitation amount, about .01-.1in of additional precipitation fell south of the Mason-Dixon line in the previous 24 hours.

Here are the preceeding NAM total QPF forecasts...


(January 24; 18z) (January 25; 6z)


(January 25; 12z) (January 25; 18z)


(January 26; 0z) (January 26; 6z)

As you can see, the NAM had a lot of variance with the northwestern sharp precipitation gradients. These tight gradients this year have been caused by the rapid intensification of the coastal lows allowing the heaviest moisture to be confined closer to the center of circulation. Also in this instance, a very cold and dry air mass along with associated cold front was quickly advancing southeast across the Great Lakes and was even picked up on by the 700mb RH charts. This allowed the flow out of the northwest to dry up some moisture for areas more inland.

The NAM simulated radar vs. the forecast QPF did not match up. Often the NAM simulated radar showed the heavier mesoscale bands lining up in northern Maryland and southern Pennsylvania where as it only showed total QPF to be .25in-.5in. In fact looking at total verification, the NAM did very well for its simulated radar.


(Actual NEXRAD National Radar) (6z NAM January 26 Simulated Radar)

The NAM did seem to have a hold on the 500mb map showing the negatively tilted trough producing the coastal low along with the placement of the upper level low and associated shortwave kicker just to the west.


(18z NAM January 24; 500mb) (0z NAM January 26; 500mb)

In general the differences in the 500mb maps were very subtle with just a general strengthening in the closed 500mb low, which verified a tad north of the January 24 18z model run.

The NAM did a very excellent job in identifying mesoscale band using the UUV/700mb RH charts.

(6z NAM January 26; 700mb)

It indentifed the enhanced snow growth over southeastern Pennsylvania up through New Jersey and New York City. The problems with the NAM generally existed in the QPF fields. The model likely suffered a very convective feedback issues in QPF totals. This is why it is important to note other maps than surface maps to help locate the heaviest precipitation. The 700mb map screamed that snow totals would be farther inland with the enhanced deformation band and UUV rates. In general the high resolution models handled this the best with the HRR scoring an amazing victory for QPF along with the ECMWF. The GFS/NAM did a very poor job for QPF, but as noted above QPF does not always tell the story. Sometimes it is important to note other layers of the atmosphere to help make a forecast. This point is why many forecasters missed the boat. Many forecasters (especially broadcoast meteorologists) are drawn to the easy to understand QPF/surface maps, but one has to look at all layers of the atmosphere to make a prediction.

Lower Susquehanna Valley Doppler...

(Courtesy of WGAL)

"Here northeast of Harrisburg 2010-2011 winter statistics"
(Snow Stats)
Current Snow Cover- 0-3in
Monthly Total (November)- Trace
Monthly Total (December)- 0.6in
Monthly Total (January)- 18.90in
Monthly Total (February)- 1.35in
Seasonal Total- 20.85in
Winter Weather Advisories- 7
Winter Storm Warnings- 2
Ice Storm Warnings- 0
Blizzard Warnings- 0
Freezing Rain Advisories- 1
Winter Storm Watches- 2

(Temperature Stats)
Lowest High Temperature- 23.7F
Lowest Low Temperature- -1.7F
Wind Chill Advisories- 0
Wind Chill Warnings- 0

(Snow Storms Stats)
First accumulating snow - December 10 - 0.50in of snow
Clipper light snow - January 7-8 - 2.25in then another 1in of snow
Double Barrel Low - January 11 - 4.5in of snow
Coastal Low - January 17-17 - 1.8in of snow/sleet
Arctic Front - January 20-21 - 2.1in of snow
Upper level/coastal low - January 26 - 5.75in of snow
Two clippers - January 28-29 - 1.5in of snow

Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011... (Blizzard92)
.4-.5in of freezing rain
Ice Storm 2011...
Ice Storm 2011 (Blizzard92)
Melting begins...
Ice Storm 2011
Ice Storm 2011 (Blizzard92)
Melting begins...
Ice Storm 2011

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Not sure if anybody heard of the supercooled warm rain process, but it is freezing rain within a column of saturated air that is dry from about 700mb up and can be completely below freezing, with no warm layer. Happens out in the plains states fairly often and is a real headache to forecast. I can't say that i've ever seen it in the northeast, fortunately.


Check out an example sounding.

Link

Member Since: Posts: Comments:
Quoting Snowlover2010:
So it seems weird to me that Berks is at 24, Lancaster is at 27, and yet it is 29 up here at State Colllege(Oh and we are supposed to be the ones getting sleet). Dunno just seems a little weird to me.


That is because the surface temperature has nothing to do with what falls from the sky. It is what is happening on the way down that decides the dropplet will end up frozen or liquid when it hits.
Member Since: December 18, 2009 Posts: 0 Comments: 270
Quoting Snowlover2010:
So it seems weird to me that Berks is at 24, Lancaster is at 27, and yet it is 29 up here at State Colllege(Oh and we are supposed to be the ones getting sleet). Dunno just seems a little weird to me.

Keep in mind for sleet that surface temperatures do not matter. It is a matter of how thick the cold layer is aloft. Given your higher latitude and elevation, the cold layer is a bit more aloft allowing for the potential for sleet. Don't get me wrong, I still think significant freezing rain is likely for State College.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
Not sure I care for this new layout. I need to prepare for change, not have it thrust at me when I'm least expecting it...

alas...i'll get used to it.
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So it seems weird to me that Berks is at 24, Lancaster is at 27, and yet it is 29 up here at State Colllege(Oh and we are supposed to be the ones getting sleet). Dunno just seems a little weird to me.
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yeah, that's odd as hell what just occurred.
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Thanks, Blizz!
Member Since: December 18, 2009 Posts: 0 Comments: 270
Quoting PalmyraPunishment:


From my 3rd story window everything looks like glass.


Maybe it is. I heard there was a glass warning out for your area.
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Really? Wunderground just HAD to release a new version of the blogs during the biggest storm of the year. Wow. Talk about crappy timing.
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Quoting 717WeatherLover:
I noticed that the difference in the temperature and dew point on Blizz's weather station is now greater than 1. The difference had been steady at one for quite a while. Someone correct me if I am wrong. I thought that if there is a greater difference between temp and dew point that it was better for cooling the column as the precipation started. Still trying to get the ideas all straight in my head with this concept.

Any help, anyone?

Correct. That is evaporational cooling where saturation tries to occur at the dewpoint.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
I noticed that the difference in the temperature and dew point on Blizz's weather station is now greater than 1. The difference had been steady at one for quite a while. Someone correct me if I am wrong. I thought that if there is a greater difference between temp and dew point that it was better for cooling the column as the precipation started. Still trying to get the ideas all straight in my head with this concept.

Any help, anyone?
Member Since: December 18, 2009 Posts: 0 Comments: 270
Quoting weathergeek5:
wow the site suddenly changed for me.


Same. We must have just experienced an update. Interesting.
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Looks like we are off the hook here in Central DE. They have canceled our freezing rain advisory. Temp 35 degrees at the base as of now.
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Met Ed already has around 1500 people throughout pennsylvania without power, just wait til the heavy precip moves in.
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wow the site suddenly changed for me.
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We're getting a sleet/freezing rain mix here currently.
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It's now raining quite steadily, almost to the point where you could call it a "downpour" here in Camp Hill.

From my 3rd story window everything looks like glass.
Member Since: January 31, 2007 Posts: 9 Comments: 2250
yep it includes me.
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Freezing rain - Germantown, MD - 31F (high today was 34F).
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Quoting Blizzard92:
CTP just really raised totals for ice accretions tonight...

I still think it is too high for the Laurel Highlands and State College region, but not high enough in the northern Dauphin County latitude going west to east.


One thing possibly pushing up total in State College is the freezing rain we got from the front running precip earlier this evening at like 6:30. Prob got .05in of ice at least out of it.
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Not good...

Ice storm warning now for metropolitan Philadelphia...
Member Since: December 14, 2007 Posts: 285 Comments: 15140
746. RkTec
Emmaus, PA

23.5 degrees
IP/ZR mix falling

0.5-0.75" of ice in the forecast.
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745. 900MB
Looks like the sh*t storm starts here in NYC in about an hour. Holding steady at 27 degrees.Should be interesting. I am thinking more ice/sleet and frozen schazzle on the ground, trees, etc..than expected.
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Quoting Blizzard92:

I still think the sleet will prevent major ice accretions towards your region. I have heard there has been nearly .2in of sleet recently in State College.



Been basically all freezing rain here so far. Little sleet mixing but not accumulating.
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CTP just really raised totals for ice accretions tonight...

I still think it is too high for the Laurel Highlands and State College region, but not high enough in the northern Dauphin County latitude going west to east.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
Quoting P451:


Hey TT.

Fielding a guess here: Pressure must have gone from a slow steady decrease to a suddenly more rapid decrease.



Yeah maybe. She's a huge wuss and would be extra sensitive to that sort of thing.
Member Since: December 17, 2009 Posts: 0 Comments: 796
Quoting Snowlover2010:
Not sure bout tonight. Temps see to be really rising. Up to 28.8 in State College.

I still think the sleet will prevent major ice accretions towards your region. I have heard there has been nearly .2in of sleet recently in State College.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
740. 900MB
Quoting Dabbio:
900MB,

Try weather.com Current Surface if this link does not work for you:

Link

Thanks!
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@ 10:20 pm

26.9F, Fog

First convective looking freezing rain showers knocking on the door.

Mount Pocono is 20F with light freezing rain
Member Since: December 17, 2009 Posts: 0 Comments: 796
Not sure bout tonight. Temps see to be really rising. Up to 28.8 in State College.
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Quoting 717WeatherLover:


Can't you just rub your magic 8 ball and tell us if it warms up tonight? JK, I fully understand the complex nature of the battles of the different layers of warm and cold air that is taking place tonight. It just cracks me up how many people still want you to tell us if it will happen.

Hahaha... yep, I try to provide all of the options and highlight one as the most likely forecast according to my thoughts. That is essentially what forecasting is.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
I hold a lot of faith in the P-type of the WGAL radar that Blizz has on his blogs. I never noticed before tonight that you really can't tell the difference between sleet and freezing rain on it which I suppose makes sense if you think about the radar signature through the atmosphere.
Member Since: December 18, 2009 Posts: 0 Comments: 270
I'll tell you what cracks me up 717....that after the second....and then third....and then fourth....major storm did folks start to understand weight of precipitation on structures. You're right! Regardless of what the forecast will be, why not just simply prepare for what is happening?? Simple math, simple logic. And now we're about to be whacked with number six....
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Rain, rain. Go away. Come again, some other day. We have fog. The end. (Dover, DE)
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Quoting Blizzard92:

I highly doubt we have school tomorrow. An ice storm of this potential will cause major problems through tomorrow evening with widespread power outages and tree damage. Now if we have temperatures warm up tonight, then this eliminates the threat of course.


Can't you just rub your magic 8 ball and tell us if it warms up tonight? JK, I fully understand the complex nature of the battles of the different layers of warm and cold air that is taking place tonight. It just cracks me up how many people still want you to tell us if it will happen.
Member Since: December 18, 2009 Posts: 0 Comments: 270
Only 0.25in of freezing rain adds nearly 500lbs to the length of a typical powerline
Member Since: December 14, 2007 Posts: 285 Comments: 15140
Quoting 717WeatherLover:


Yea, I'm sure that 2 hour delay tomorrow will give everyone time to get the roads and schools set for you guys to go to school. Sounds like they took heat today for not using the 2 hour delay to see how the roads turned out.

I highly doubt we have school tomorrow. An ice storm of this potential will cause major problems through tomorrow evening with widespread power outages and tree damage. Now if we have temperatures warm up tonight, then this eliminates the threat of course.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
Already have some precip falling again here. It's a very light sleet and mist.

Brooklyn, NY
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Freezing Rain:Link

Precipitation starts in cold air aloft as ice crystals/snowflakes that have formed via heterogeneous nucleation, deposition, and ice multiplication and grown through aggregation and riming. Crystals then fall through a melting (warm) layer that is sufficiently deep or warm enough to completely melt the crystals to water drops. The drops then become supercooled as they fall through a subfreezing (refreezing) layer near the surface and freeze on contact if ground objects are colder than 0 C (32 F). A typical freezing rain sounding is shown in Figure 2.

Melting layers greater than 1200 ft deep usually cause complete melting, although for large flakes or for maximum melting layer temperatures less than or equal to 1 C, a deeper melting layer may be required for complete melting.

If the ground temperature is warmer than 0 C but the air temperature is colder than 0 C, then heat conduction may prevent freezing on the ground/streets, but freezing will occur on elevated cold surfaces, i.e., trees, power lines, and cars. If the ground is frozen, then freezing rain can occur despite air temperatures above 0 C (at least for awhile).

Member Since: December 14, 2007 Posts: 285 Comments: 15140
Quoting Blizzard92:
Best idea is to avoid travel. I just went outside and skated across my entire neighborhood road.


Yea, I'm sure that 2 hour delay tomorrow will give everyone time to get the roads and schools set for you guys to go to school. Sounds like they took heat today for not using the 2 hour delay to see how the roads turned out.
Member Since: December 18, 2009 Posts: 0 Comments: 270
Quoting Dabbio:


You've got that exactly right. I was in the midst of (one of?) those little Brooklyn tornadoes this Summer, one touched down right behind my apartment while I, naturally, was looking out the front, and is documented here: Link along with much amusing YouTube commentary. You can imagine that was the highpoint of my career as a weather buff.


Yeah, I've watched some of those videos, but not that one in particular until now. One of my friends who lives in Park Slope had a portion of his roof ripped off at the apt building where he is the super. I went for a walk a few days later through the area of Queens that was hit by one during the same storm and it was even worse. You could actually see the path the tornado took, just sawing off trees half way up the trunk and twisting fences around nearby objects. Hundreds of cars and houses decorated with trees. Definitely some wicked weather last summer.
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Member Since: December 14, 2007 Posts: 285 Comments: 15140
I am worried as hell about weight for just about everybody in this CWA...
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P451 -

I am just south of you in Tarrytown. We had about two inches of snow and sleet and a crust of freezing drizzle.

Looks like we are in for a tough one tonight. I just put down a ton of salt on my driveway and have loaded up on the wood for the potbelly in case we lose power.

Best of luck.
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Quoting Blizzard92:
Best idea is to avoid travel. I just went outside and skated across my entire neighborhood road.


Blizz, will this continue to be a major issue through CT during most of tomorrow? What do you think think the liquid total equivalent will be? Thanks.
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Best idea is to avoid travel. I just went outside and skated across my entire neighborhood road.
Member Since: December 14, 2007 Posts: 285 Comments: 15140
We had very dense fog earlier, raining real good now. The newspaper for tomorrow was printed 6 hours early and is already here.
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Hey all. Just got in from removing snow from complexes and couldn't even get to my other commercial sites. Roads closed because....our multi-story brick Tennis and Rec center has collapsed!!! We haven't even gotten the "big" storm yet and crap is hitting the fan. I just posted two days ago about this worry and now it's no joke. Hey F1, the Trumbull Tennis Center is down....that's a big, sturdy building? This is becoming bad!
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Quoting 717WeatherLover:


We might be able to get there but I doubt we could get home. Do you have room for all of us to stay for a sleep over?


It's a complex. You could just barge into random people's apartments and throw them out of their beds...

All single females 22-30 are welcome to stay in my royal quarters.

Back to the weather.

Camp Hill, PA - 27.3F
Freezing Rain
It's not a drizzle but not a downpour, which as Blizz stated, is better for accumulation. Trees are glazed and sidewalks are coated.

I give electricity 2 hours...
Member Since: January 31, 2007 Posts: 9 Comments: 2250
Quoting crowe1:


Said with a Dr. Evil laugh.
Member Since: July 15, 2006 Posts: 178 Comments: 56141
Quoting PalmyraPunishment:
SilverShipsofSlipperyWhenWet?




I posted this on my facebook earlier!
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Cornell University- Atmospheric Sciences Undergrad; Research Assist.- Onset of Spring Indices Toolbox; Interests- Small spatial scale climatolology

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