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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Quoting Hoynieva:
.

If there are I don't know where to find them gaara. I can only find data lists of numbers which I assume need software to decipher. Would be nice to see these graphs from all models.

I'm usually not hoping for rain, and I really would like a nice storm for you guys, I just need it to rain wed afternoon. Sorry all. No worries either, there will be another one or two storms next week...and the next...and the next...


I still hate you.....just sayin
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Up here in NW VT they're saying 12-18", mostly from the second system coming through on Wednesday.

Blizz, are you thinking of doing a snow totals prediction map?
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Member Since: July 15, 2006 Posts: 178 Comments: 56059
Member Since: July 15, 2006 Posts: 178 Comments: 56059
Member Since: July 15, 2006 Posts: 178 Comments: 56059
Member Since: July 15, 2006 Posts: 178 Comments: 56059
thanks CtWxGuy

Down here in Clinton not looking forward to ANY ice...Let it be all snow. I'll dump it in my neighbors yard.......
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Tinkahbell - Very unlikely. You might get 1-3" before changeover, but then it's mostly rain.

Gaara - the only concern there are the roofs. I'm actually worried about my balcony now. I'm not sure how much weight it can hold and with the 2.5' currently on there plus a full compost bin, I'm hoping it's ok. I'd shovel it off but I can't get out the door...tried earlier and it seemed the door was about to break.


Member Since: January 20, 2009 Posts: 0 Comments: 1521
Quoting WalesMass:
CtWX: Where do you think the freezing rain (ice storm?) vs. mostly snow line is going to be? Obviously somewhere in CT/MA, but where?



sorry for the delay, I've been busy all day trying to get my car started. Looks like fuel froze in the lines but now I'm starting to wonder if it's something worse. Anywho, this is a really tough call right now. It seems that the models have trended slightly colder throughout the levels and are a tad colder than what most official forecasts depict. Right now I think the major ice storm happens in southern half of CT, with another blockbuster snowstorm for northern half of CT and points north. But if the levels continue to be projected slightly cooler as we have seen in the last few runs (remember, it looked like most of CT would change to plain rain 36 hours ago), there would likely be surprise higher snowfall totals but less ice. I'm tending to lean towards the colder scenarios. It is possible the ice storm is confined to the immediate coast and just some sleet pellets mixing in north of I-95, if more of a colder trend ensues.
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358. Gaara
Quoting Hoynieva:
.

If there are I don't know where to find them gaara. I can only find data lists of numbers which I assume need software to decipher. Would be nice to see these graphs from all models.

I'm usually not hoping for rain, and I really would like a nice storm for you guys, I just need it to rain wed afternoon. Sorry all. No worries either, there will be another one or two storms next week...and the next...and the next...


I'm rooting for as much rain as possible because there's really no room to put any more snow right now. Will try to make a contest post before midnight.
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Is there a possibility that this stuff will NOT turn to rain in Central DE?
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.
Quoting Gaara:



I like this, but I'm trusting the NAM less and less right now. There is no equivalent of these graphs for the GFS, correct?

I like comparing KBDR to KHVN (and KBDL, to a lesser extent), too.


If there are I don't know where to find them gaara. I can only find data lists of numbers which I assume need software to decipher. Would be nice to see these graphs from all models.

I'm usually not hoping for rain, and I really would like a nice storm for you guys, I just need it to rain wed afternoon. Sorry all. No worries either, there will be another one or two storms next week...and the next...and the next...
Member Since: January 20, 2009 Posts: 0 Comments: 1521
355. Gaara
Quoting Hoynieva:
More rain, less sleet and snow :)




I like this, but I'm trusting the NAM less and less right now. There is no equivalent of these graphs for the GFS, correct?

I like comparing KBDR to KHVN (and KBDL, to a lesser extent), too.
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Quoting breald:


I'm not even under a watch, but we have a parking ban starting tomorrow afternoon..LOL


There's your 3" - 6" snow with ice Breald. NWS Issued an advisory for you. Obviously your Warning criterea is higher than mine.
Member Since: December 17, 2009 Posts: 0 Comments: 796
More rain, less sleet and snow :)

Member Since: January 20, 2009 Posts: 0 Comments: 1521
lol we're all gonna die!!!!
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Winter Storm Warning

Statement as of 4:52 PM EST on January 31, 2011

... Winter Storm Warning in effect from 6 am Tuesday to 1 am EST
Thursday...

The National Weather Service in Taunton has issued a Winter Storm
Warning for heavy snow... which is in effect from 6 am Tuesday to
1 am EST Thursday. The Winter Storm Watch is no longer in effect.

* Locations... western and central Massachusetts... as well as
interior Essex County in Massachusetts.

* Hazard types... heavy snow.

* Accumulations... 18 to 24 inches of snow.

* Timing... snow will overspread this region late tonight... likely
affecting the Tuesday morning commute. Snow continues through
the day... followed by a brief break Tuesday evening. More snow
then should arrive late Tuesday night into Wednesday. It is
possible for some sleet to mix with the snow south of Route 2 in
Massachusetts Wednesday afternoon.

* Impacts... significant snow and sleet accumulations will make
travel on untreated surfaces treacherous. The additional snow...
or even worse heavier sleet... could further strain weaker
structures and tree limbs. Power lines may also be downed...
resulting in isolated power outages.

* Winds... north 5 to 15 mph with gusts up to 25 mph.

* Temperatures... around 20 for high temperatures Tuesday and
Wednesday. Zero to 10 above for lows Tuesday night.

* Visibilities... one half mile at times.
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350. DM125
Quoting baba17:
dm125

where do you live?


I live in the Wilkes Barre area of PA
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dm125

where do you live?
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348. bwi
Radar filling in this evening. Just in time to get pelted by sleet on my evening ride home. But so far no precip outside my office window in DC.
Member Since: September 4, 2007 Posts: 0 Comments: 1407
347. Gaara
Is it just me or are the NAM and GFS diverging wildly on the rain/snow line on the 18z runs? GFS looks a lot cooler, even aloft.
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346. DM125
SNOW TOTALS ARE BEING INCREASED. Weather.com has put my area into the 12+ region.
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Quoting SilverShipsofAndilar:
Does anyone have any thoughts on the morning/afternoon commutes? Looks like Blizz is afk for a while.


Harrisburg - I don't want to drive TO work if I cannot later drive FROM work.
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SSA where are you from again?


Also, The NWS has the albany area pegged as the hotspot right now with a forecast of 24 in...i guess it all just depends on where the rain/sleet/snow lines set up. Either way this looks to be a monster
Member Since: February 23, 2010 Posts: 0 Comments: 685
Does anyone have any thoughts on the morning/afternoon commutes? Looks like Blizz is afk for a while.
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Quoting shipweather:


I just started at MDT (AKA HIA) if someone else did on here too I'd be surprised. Tomorrow will be a mess and then again Wednesday with possibly some residual on Thurs.


Sorry Silver and Ship. I knew it was someone who had an S in their name. Hope it's going well, Ship. As for the HIA instead of MDT, I slipped out of my weather warrior/aviation nut modes for a second there. Gosh, I'm all confused this evening! You'd think I already slipped on the ice and hit my head or something.
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Quoting 900MB:


Keeper- That is quite an artic blast on day 10 of your model!
febuary is known to be the month to be worse of winter with snow ice and deep cold and it looks like that is what it will be
Member Since: July 15, 2006 Posts: 178 Comments: 56059
340. DM125
The temperature for Wednesday was 35 degrees and now it is 31 degrees. What does this mean?
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Quoting zotty:
Is it better for the trees and power lines to have snow come first, and then the ice?  I would imagine that's the case, but I'm not sure.  
Is it supposed to be a windy system?  


Quite the opposite. The snow takes in the rain even better. No Splash if you know what I mean.

It will be a really devastating storm for some. Be very careful all.
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Quoting 717WeatherLover:


Didn't you just start at HIA? How's it going?


No, not I.
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Quoting 717WeatherLover:


Didn't you just start at HIA? How's it going?


I just started at MDT (AKA HIA) if someone else did on here too I'd be surprised. Tomorrow will be a mess and then again Wednesday with possibly some residual on Thurs.
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336. bwi
HPC pulling the ice forecast further south. Was 10 percent at PA/MD border yesterday, then a little south of the border this morning, now down well south of DC. Bad trend.
Member Since: September 4, 2007 Posts: 0 Comments: 1407
Quoting SilverShipsofAndilar:


My worst fear is being trapped in the office overnight Tuesday and possibly Wednesday.


Didn't you just start at HIA? How's it going?
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This is going to be a mess. I'm interested, but also concerned.
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333. 900MB
Finally a NWS updated forecast for NYC, and an uptick! Now calling for 4-8" and temps steady in upper 20's between tonight and Weds morning. Then mid 30's and rain to snow Weds and Weds eve with little or no add'l accumulation. I am assuming a new winter storm watch or warning with updated info should be issued any time now.
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Quoting 717WeatherLover:


I know what you mean. Once again I think about the school superintendents balancing the not using up snow days and keeping kids safe scales. Your worst nightmare has got to be the thought of having the students trapped at school overnight!


My worst fear is being trapped in the office overnight Tuesday and possibly Wednesday.
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Being in SO MD and the forcast of rain with maybe a few slick spots early morning is looking good. Will be watching and hoping for the best for all of you in the path of this one. I have family in or around Chicago, Detroit, Pittsburgh and Worcester.....I hope they keep power! Some of the roads are already tunnel like for the large amounts of snow.
Stay Safe..
Member Since: September 10, 2001 Posts: 2 Comments: 486
Quoting zotty:
I guess the way to play this storm is to not shovel until it is all over.  That way the ice just goes on top of the snow, and then shovel both out Thursday morning.  
I'd give PPs left one for a snow blower...


Meh, don't know about that at least with single stage snow throwers ice crusted snow doesn't throw so well. Anyone know if 2 stagers are any better with snow that has a .5" crust of ice on it? Wouldn't want to waste PP's left one if it wouldn't work anyway!
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Quoting SilverShipsofAndilar:


What are you thinking about morning/evening commutes tomorrow? I thought there was a chance for an ice glaze overnight tonight but even if that doesn't happen will it be impossible to get home from Harrisburg later in the afternoon?


I know what you mean. Once again I think about the school superintendents balancing the not using up snow days and keeping kids safe scales. Your worst nightmare has got to be the thought of having the students trapped at school overnight!
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A message from the NWS on their Facebook page....and yes, I am a friend of the NWS for NYC on Facebook!!!

US National Weather Service New York NY
Because of the widespread interest in NWS weather forecasts and warnings for the winter storm, we are experiencing delays on our web pages. System administrators are working to resolve the issues as quickly as possible. We apologize for the inconvenience.
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92, what are your thoughts on a major ice storm for extreme Southern New England?
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326. zotty
Is it better for the trees and power lines to have snow come first, and then the ice?  I would imagine that's the case, but I'm not sure.  
Is it supposed to be a windy system?  
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325. zotty
I guess the way to play this storm is to not shovel until it is all over.  That way the ice just goes on top of the snow, and then shovel both out Thursday morning.  
I'd give PPs left one for a snow blower...
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Blizz -

Mid - Hudson Valley looks like it could get hammered - combination of everything that will eventually add up to something really ugly I'm afraid...

We recently got upgraded to a winter storm warning and I'm pretty worried about the integrity of the roof for my house and barn...

Not cool.

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Quoting breald:
NOAA must be getting a lot of hits today. I can't get into the regional forecast.


Well I've been on it all day hahah.



So this looks to be snow tomorrow and then ice at night and of course all snow for northern MA/southern VT?
Member Since: February 23, 2010 Posts: 0 Comments: 685
Quoting Blizzard92:

Good call by them. Some of us may even see a few inches of sleet accumulation. This storm is going to be nasty.


What are you thinking about morning/evening commutes tomorrow? I thought there was a chance for an ice glaze overnight tonight but even if that doesn't happen will it be impossible to get home from Harrisburg later in the afternoon?
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321. DM125
All together from this storm, they are calling for 5-10 inches of snow and .5 of ice accumulations here in the Wilkes-Barre/Scranton area.
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320. 900MB
Quoting Blizzard92:

Good call by them. Some of us may even see a few inches of sleet accumulation. This storm is going to be nasty.


Blizz- We haven't had an updated forecast here in NYC in 12 hours. Think there will be any big changes?
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319. 900MB
Quoting KEEPEROFTHEGATE:


Keeper- That is quite an artic blast on day 10 of your model!
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318. bwi
WWA for DC, for light freezing rain tonight and tomorrow. Temps have stayed at or below freezing so far today, as predicted.
Member Since: September 4, 2007 Posts: 0 Comments: 1407
Quoting Blizzard92:

Good call by them. Some of us may even see a few inches of sleet accumulation. This storm is going to be nasty.


Any predictions yet?
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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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