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 TheRasberryPatch:
light snow falling or is it sleet. it's sleet. a light coating on the ground

temp 25F
dewpt. 14.9F that is really low
barometer 1029.5mb and falling
winds calm if any wind out of the East

Campbelltown, PA 5 miles east of Hershey

sounds like a scary next couple of days Blizz. hope and pray it stays sleet


It is not sleet, it is graupel currently. It forms as small snow pellets/balls and is convective in nature. Therefore it only occurs with strong lift and high upper vertical velocity (UVV) rates. The precipitation has been convective in nature so far and very cellular. As the stratiform precipitation moves in overnight, it will change to a light snow before changing to sleet and freezing rain as the upper level thermals rise towards morning. H85s are quiet cold right now.
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light snow falling or is it sleet. it's sleet. a light coating on the ground

temp 25F
dewpt. 14.9F that is really low
barometer 1029.5mb and falling
winds calm if any wind out of the East

Campbelltown, PA 5 miles east of Hershey

sounds like a scary next couple of days Blizz. hope and pray it stays sleet

Member Since: January 26, 2007 Posts: 72 Comments: 6263
@ 9:37pm

27.7F, 18D, Wind SSE 6mph, Light Sleet/Light Snow mixture

Light mix of sleet/snow started at 9:37pm while I was outside.

No sleet expected here tonight. Imagine this will turn to light snow as column cools a little.
Member Since: December 17, 2009 Posts: 0 Comments: 796
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Quoting Blizzard92:
Sorry, no formal forecast blog for this storm. I just want to sit back and watch it unfold for once. But I still will be analyzing, observing, and happy to answer questions.


I remember hearing up at my Penn State visit that classes are rarely if ever canceled. They have heated sidewalks, lol.


I was there for my freshman year in 93/94, and we had a huge snowstorm that shut down the campus, I can't remember when exactly though. All I can remember was the lines of students facing each other across one of the quads for the most EPIC SNOWBALL FIGHT I have ever witnessed and participated in. I got hit in my left ear, which sent the lenses shooting out of my glasses. Luckily, my eagle-eyed gf at the time was able to find my clear lenses in white snow! Took a while for the snow to melt out of my ear though...

Also, I don't know if they still have the covered walkways between the dorms, but you shouldn't climb up there and use them as a vantage point. Got in trouble for that...
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Yes, there is the potential given signals from the ECMWF/GFS for another winter storm towards Friday/Saturday.
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blizz, i know this storm now is critical, but do you see the possibilities for this weekend and what is showing out in the southwest after this current storm departs---I am pretty sure I am reading this right---that there might be something that sneaks up the coast
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Light snow falling. 29F. Bellmawr, NJ. Fitting way to end January - About 25" for the month, very snowy for January.
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409. Finky
Blizz,

Do we have any other larger storms coming in the near future?
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Quoting Finky:


Does that mean the same for Adams County? BTW..Just started to snow here just north of Hanover.

Oh, it will be a very impressive ice for probably most of the Lower Susquehanna Valley, but areas just to your north by 20mi or so will get hit a bit harder.
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3-5 inches for me in Southern Ma. This one is shaping up to be a walk in the park for extreme coastal New England. I hear Southern VT may get 3 feet...Wow
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406. Finky
Quoting Blizzard92:
0z NAM is a near devasting ice storm along and north of the Pennsylvania turnpike. .75-1in of freezing rain for Harrisburg on northward. Looks like York and Lancaster may escape the worst of it rising to 33F or so.


Does that mean the same for Adams County? BTW..Just started to snow here just north of Hanover.
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0z NAM is a near devasting ice storm along and north of the Pennsylvania turnpike. .75-1in of freezing rain for Harrisburg on northward. Looks like York and Lancaster may escape the worst of it rising to 33F or so.
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404. bwi
Looks like it's going to get very cold middle of next week.
Member Since: September 4, 2007 Posts: 0 Comments: 1393


For those keeping track at home, I-70 in Pennsylvania follows Breezewood down towards Northern Maryland.
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Ground is already quite slick from the sleet that has fallen. The dopes here refused to salt the sidewalks in advance.

A whole lot of ankle snappin' awaits.
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Quoting Blizzard92:
Sorry, no formal forecast blog for this storm. I just want to sit back and watch it unfold for once. But I still will be analyzing, observing, and happy to answer questions.


I remember hearing up at my Penn State visit that classes are rarely if ever canceled. They have heated sidewalks, lol.


Classes are rarely cancelled, but the heated sidewalks DONT WORK. They have steam things every so often on the sidewalks but they warm up an area of like one foot by one foot and are scattered to like one a block, so they end up being useless.
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Sorry, no formal forecast blog for this storm. I just want to sit back and watch it unfold for once. But I still will be analyzing, observing, and happy to answer questions.

Quoting Snowlover2010:


Come on. Give me over a foot of snow and 1.0in freezing rain. I wanna get out of classes Wednesday. HAHA.

I remember hearing up at my Penn State visit that classes are rarely if ever canceled. They have heated sidewalks, lol.
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Quoting Blizzard92:
Please everyone continue to post your location every single time you make even the smallest of outside observation. It makes it easier for all of us; thankyou!


absolutely nothing to report here......Roseland, NJ :)
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Quoting Blizzard92:

I did not say that at all. 2-4in of snow; 1-3in of sleet; .25in of freezing rain


Come on. Give me over a foot of snow and 1.0in freezing rain. I wanna get out of classes Wednesday. HAHA.
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Quoting Gaara:

Found it..
LGA GFS 18Z:
Link


Wow, we're kind of dumb. It was right on that same page the entire time.
Member Since: January 20, 2009 Posts: 0 Comments: 1517
Please everyone continue to post your location every single time you make even the smallest of outside observation. It makes it easier for all of us; thankyou!
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Quoting Blizzard92:

The only two ways we avoid the ice storm is if we get more sleet, which is possible. But I think the main accumulations of sleet will be towards State College. And obviously if it is warmer than forecast, then it changes to plain rain. But somehow I think this cold air will be hard to dislodge.


where would you draw a line that will at some extended period of time get plain ole rain??? please extend in jersey
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Quoting Snowlover2010:


So you dont think it will be that bad up in State College?

I did not say that at all. 2-4in of snow; 1-3in of sleet; .25in of freezing rain
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Sleeting in Camp Hill...
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Quoting Blizzard92:

The only two ways we avoid the ice storm is if we get more sleet, which is possible. But I think the main accumulations of sleet will be towards State College. And obviously if it is warmer than forecast, then it changes to plain rain. But somehow I think this cold air will be hard to dislodge.


So you dont think it will be that bad up in State College?
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Test #2 for the HRRR model. It is showing a quick 3-6in of snow tonight for much of central Pennsylvania. The last storm it was extremely accurate. Let's see how it does with this bold snow prognostic.
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Quoting PalmyraPunishment:
on the serious though, i know this is a serious event and could turn out to be a severe ice storm. i hope everybody's making sure they have their ducks in a row in the event you have to go out into it.

i'm taking off of work wednesday to prevent even needing to be anywhere between tuesday evening (after work) and whenever the worst is over and the "all clear" is sounded.

still wishing this wasn't a threat for ice.

The only two ways we avoid the ice storm is if we get more sleet, which is possible. But I think the main accumulations of sleet will be towards State College. And obviously if it is warmer than forecast, then it changes to plain rain. But somehow I think this cold air will be hard to dislodge.
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on the serious though, i know this is a serious event and could turn out to be a severe ice storm. i hope everybody's making sure they have their ducks in a row in the event you have to go out into it.

i'm taking off of work wednesday to prevent even needing to be anywhere between tuesday evening (after work) and whenever the worst is over and the "all clear" is sounded.

still wishing this wasn't a threat for ice.
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wow HPC thinks less ice and more snow for SNE? interesting :)
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Quoting TheRasberryPatch:


get a grip or so help me you will be grounded for a month hahahaa


hahaha nah i'm cool. i wanted to incite a riot and it didn't happen. bummer.

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Hi all, just giving some stats for my house in Stamford CT. High today hit 31F Low this morning was 9F, now temp. is down to 19F , high thin overcast moving in from the West and SW. Baro. is 30.40" and steady.
Member Since: January 31, 2009 Posts: 0 Comments: 7855
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384. bwi
Winter weather advisory extended further south into MD and VA. Temps a little cooler than expected, I think (we're at 29), so what ever falls will stick and create slippery travel. Precip amounts look very light though, so probably no issues as long as it stays sleet or snow. Plus radar looks pretty sparse at this point. I think local NWS was taken aback by the disastrous commute/road situation on the last storm -- the one where they didn't hoist the winter storm warning until the last minute. So now they might be being a little extra cautious for what may be a non-event tonight anyways.

We have light snow in northern Prince George's county.
Member Since: September 4, 2007 Posts: 0 Comments: 1393
Good evening folks. Someone between I-80 and the Mason-Dixon line will see a severe ice storm with power outages until the weekend. This is a dangerous ice situation.
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here's my snow/ice map. Probably will need further fine tuning.
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381. 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...

Found it..
LGA GFS 18Z:
Link
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Quoting 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.


I drove from Arlington to Germantown, flurries Falls Church to 270, light snow on 270 to Gaithersburg, then light snow in Germantown.
Member Since: January 9, 2011 Posts: 0 Comments: 138
Quoting TheF1Man:
It's so quiet on here! almost 3000 posts last week and now only 300? em>


a couple of reasons....the 4 days before the event Blizz put out the new blog talking about it and a lot of people were on the bridge deciding if they should jump with every new model.
Member Since: January 26, 2007 Posts: 72 Comments: 6263
It's so quiet on here! almost 3000 posts last week and now only 300? Looks like we're lacking in the blog endurance department...

Heavy, Heavy snow blob(not band) south of chicago as the storm begins to wind up.

Quoting anduril:
Link Seeing this in full res is just amazing.


Looks awesome!
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weather.gov is soooooooo slow at the moment. They should switch on a backup server or something for these big storms. It's the 3rd time this winter their server has been over-loaded. The site has been unusable, at times, during the last hour.
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Link Seeing this in full res is just amazing.
Member Since: August 29, 2006 Posts: 0 Comments: 172
^that radar picks up a whole lot of virga^
Member Since: January 20, 2009 Posts: 0 Comments: 1517
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Thanks, Dan. It's refreshing to know an anonymous human in the vast cyber cosmos has such passionate hate for another anonymous human.

I can feel your love across the cyber stars tonight.
Member Since: January 20, 2009 Posts: 0 Comments: 1517
hmm I don't like those 18z temps at 2m. Would like to see that 0C line further south by like 50 miles.

850mb temps still would suggest mostly frozen precipitation for most of CT though. We'll have to see if it really gets above freezing at the surface that far north.
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Quoting PalmyraPunishment:
lol we're all gonna die!!!!


get a grip or so help me you will be grounded for a month hahahaa
Member Since: January 26, 2007 Posts: 72 Comments: 6263
Sleet moving into Newville, PA.
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Live in the LSV Just north of the City of York. Been hearing this will be mostly rain come Tuesday into Wednesday is this true??
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The double whammy for the northern mid-Atlantic states indicated by the chart posted by Keeper at #362 is pretty interesting. Is there going to be such a sharp distinction between the storms that some people will drop their guard?
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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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