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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offically @ cashtown 1s

snow 0.2"

lquid equiv. 0.13"

currently 23f
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was checking Chicago's NWS site and they actually have a video storm briefing. WOW! I wish our NWS would at least do that. I've never seen NYC NWS do anything close to that for storms.

Link
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HUH,3-7 inches today, another 1-2 inches of snow tonight, and 1-3 inches of snow/sleet tomorrow, but we are under an advisory and not a warning. Same exact forecast as other areas that are under a winter storm warning. They did this to us last storm. I don't get it.
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ugh...Pennsylvania. the schools are so messed up. Blizz - is it going to be that bad today that schools will be closed? Throughout the area schools are closed, but not in Palmyra. 2 hour delay. what a joke. a school maybe 5 miles from your school Blizz and they are going to school.
Member Since: January 26, 2007 Posts: 72 Comments: 6233
What is the afternoon commute supposed to be like in Harrisburg today?
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@ 6:51 am

Cloudy, 26.5F, Wind NE 5mph

Not much of a problem here. Light coating of light snow. Doesn't look like much is heading this way today. Lehigh Valley / Allentown looking better.
Member Since: December 17, 2009 Posts: 0 Comments: 796
Just started snowing up here. It was 11 degrees as I was driving in this morning in Sutton, MA. Snow is coming down hard here at Smithfield RI where I work. Already a coating on the parking lot and the roads are starting to cover over.
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not much snow down here. looks to be a heavy layer of sleet and fzrain. already a report of wires down near caledonia state park
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SREF/GFS are quite warm and get the Lower Susquehanna Valley to near 40F for tomorrow, while the NAM keeps KMDT at or below freezing. Not sure what to believe, but typically the models underestimate cold air damming from what I have seen before. We shall see though, it will be close.
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1.25in of snow here this morning.

Quoting 717WeatherLover:
PENNLIVE is advertising 40 with rain for Wednesday afternoon. I'm not really sure where they are getting THAT from! Hmmm, even my National Weather Service says rain and upper 30's. I see that they are getting this from the GFS which has the 0 surface line goine way north.

I thought that the GFS was known to do a terrible job with cold air damming and thus not the primary tool for this situation. It would be very terrible if they prepared everyone for rain and we had heavy freezing rain instead.

Blizz is off of school today so once he gets up I am sure he will chime in on the matter. At least the school didn't play games with this!

Yep, I just do not buy the GFS surface temperatures. We shall see though.
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PENNLIVE is advertising 40 with rain for Wednesday afternoon. I'm not really sure where they are getting THAT from! Hmmm, even my National Weather Service says rain and upper 30's. I see that they are getting this from the GFS which has the 0 surface line going way north.

I thought that the GFS was known to do a terrible job with cold air damming and thus not the primary tool for this situation. It would be very terrible if they prepared everyone for rain and we had heavy freezing rain instead.

Blizz is off of school today so once he gets up I am sure he will chime in on the matter. At least the school didn't play games with this!
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28F freezing drizzle and a thin layer of ice on the driveway (& street?) here in Colesville, MD.
Member Since: October 24, 2005 Posts: 0 Comments: 82
Quoting TheRasberryPatch:


that will really make the ice rock hard unless the winds pick up strong and dry out the rain, but I doubt it


535AM and a bunch of white and fluffy "little or no accumulation" as, the latest NWS said at 245AM, is piling up pretty fast.

Moderate to moderately heavy snow temperature 24F, north of Hershey PA
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Well very light coating of pellets, drizzle now with temperatures above freezing since 4am. Currently 33.6 with a dewpoint over 25 degrees so pretty saturated air so were going to see rain all day. Still calling for significant freezing event tonight but I doubt it will get cold enough here at the shore. I'll send you some of this warm air ConnecticutWXGuy if you want!
Member Since: January 1, 2006 Posts: 0 Comments: 358
Looks like the models have trended the storm north, is that what everyone else sees?
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wow, supposed to get 20-25 inches here in New Hampshire. Sweet!
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Light snow has commenced. 19 degrees. Naugatuck CT
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Very light snow w/ a thin coating so far in the Old City section of Philadelphia.
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Snow is beginning to add up. Nearly 1/2" npw.
Member Since: January 20, 2009 Posts: 0 Comments: 1495
Light snow here now, but radar shows this to be a small area, moving ENE, and should be past me by 3AM, then quite a break in the precip. unless something else forms. The main batch is well back in PA. Temp. is up 1/2 degree to 24.1. Light NE wind. LT Stamford CT.
Member Since: January 31, 2009 Posts: 0 Comments: 7067
Light grapenuts falling in West Chester, PA.
Member Since: October 19, 2005 Posts: 0 Comments: 217
Very light snow flurries just began here, temp. up to 23.6F, barometer 30.38F slowly.Calm winds. Just a quick note about the possible storm for FRi/Sat. one local met here used the dreaded words, "snow showers" only, nothing much out of this next storm threat, he said. LT Stamford CT.
Member Since: January 31, 2009 Posts: 0 Comments: 7067
Quoting HeavySnow:


Copycat. Are you sure that's not mint and berries?


I'm surprised you didn't say that earlier. Did you miss my igloo? Haha.
Member Since: January 20, 2009 Posts: 0 Comments: 1495
Quoting Hoynieva:
Graupel here in Brooklyn now - 29 degrees.


Copycat. Are you sure that's not mint and berries?
Member Since: July 7, 2004 Posts: 18 Comments: 2989
Graupel here in Brooklyn now - 29 degrees.
Member Since: January 20, 2009 Posts: 0 Comments: 1495
Quoting Blizzard92:
Temperatures for Harrisburg crash to 18F Wednesday night with 35mph gusts of wind. That will be oh so lovely.


that will really make the ice rock hard unless the winds pick up strong and dry out the rain, but I doubt it
Member Since: January 26, 2007 Posts: 72 Comments: 6233
Member Since: July 15, 2006 Posts: 165 Comments: 52262
What could be in store Friday-Saturday? (Saturday is the 5th, that's just too ironic following the blizzard of 2/5/10)
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I have had snow and graupel lightly for a couple of hours. A very light dusting at most. 27 degrees in Annandale, VA. WWA in effect until tomorrow noon.
Member Since: July 7, 2004 Posts: 18 Comments: 2989
Quoting Blizzard92:

Temperatures aloft are above freezing. The entire column of the atmosphere must be below freezing to get snow.


From my meteorology class last summer (Thx Dr. Hilliker!):



I realize graupel isn't on there, but I believe it falls under the snow category. Please correct me if I'm wrong. Cheers~
Member Since: October 19, 2005 Posts: 0 Comments: 217
just heard on the TWC that Obama is delaying his trip to state college till thursday---Good Call by the President---no use in traveling in bad weather
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Quoting wxgeek723:


LOL didn't know you had one of those comical sarcastic sides Blizz.

Still light snow. They're more like evil little ice flakes actually.

Lol, actually I really am an extremely sarcastic person but in a comical sense as you put it.
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Quoting Blizzard92:
Temperatures for Harrisburg crash to 18F Wednesday night with 35mph gusts of wind. That will be oh so lovely.


LOL didn't know you had one of those comical sarcastic sides Blizz.

Still light snow. They're more like evil little ice flakes actually.
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Total flash freeze of whatever may have slushed up.
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Yep, nothing like a little wind to finish many of those ice encased trees. Gonna be a messssss!!!
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Quoting Blizzard92:
Temperatures for Harrisburg crash to 18F Wednesday night with 35mph gusts of wind. That will be oh so lovely.


What's the wind chill on that one? Close to -5F?
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Temperatures for Harrisburg crash to 18F Wednesday night with 35mph gusts of wind. That will be oh so lovely.
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December 16, 2007
Harrisburg, PA

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Quoting breald:
How can temps be below freezing, but we still get rain?

Temperatures aloft are above freezing. The entire column of the atmosphere must be below freezing to get snow.
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How can temps be below freezing, but we still get rain?
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Just got the call from my mom. Schools in Huntingdon County have shut down for tomorrow already. Wimps.

Light graupel here in Camp Hill.
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426. RkTec
Emmaus, PA - Lehigh County

25.3 degrees
Light Graupel
dusting
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Interestingly enough, I looked up our temperature profile for the December 16, 2007 major ice storm here in the Lower Susquehanna Valley. We hovered around 30-31F most of the heavy freezing rain during the night. This event will be a tad colder for the first half of the precipitation and at night too. Also we were only under a winter weather advisory, which I remember complaining about then, lol.
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Thanks Blizz!!!
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PengSnow- Yep, Pittsburgh will definitely turn to plain rain. Given the geographic location, the warm air rushes up the western edge of the Appalachians.

TheRasberryPatch- Absolutely correct. It will be interesting to see if we get any snow tonight. Mesoscale models say 3-5in, but global models suggest very little.
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Quoting Blizzard92:

Someone needs to do some digging on Harrisburg ice storm history. I think the sweet spot will be right along Blue Mountain up to about Selinsgrove. Northern Dauphin, Perry, northern Franklin, Juniata, northern Lebanon, southern Schuylkill, southern Northumberland, and Snyder will be an absolute mess likely. I heard they already have outside state electrical companies on standby.


I hope it stays north of me. I thought that might have been graupel, but it was so small and not white more clearish.

btw - is this the overrunning that we are getting from now until tomorrow afternoon and then the main storm comes in late afternoon and evening?
Member Since: January 26, 2007 Posts: 72 Comments: 6233
Talked to Friends in Pittsburgh and they are saying the radio stations are calling for a possibility of 1 inch of ice tomorrow night-however I am looking at the nws and their calling for rain---will there be a change for them
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Quoting onoweather:
blizz- looks like we're in the "sweet spot" for this ice storm if thats what you want to call it. I'm glad I have a wood stove. Do you remember the last time we had an inch of ice? The worst I storm that I can recall is the one from a few years ago.

Someone needs to do some digging on Harrisburg ice storm history. I think the sweet spot will be right along Blue Mountain up to about Selinsgrove. Northern Dauphin, Perry, northern Franklin, Juniata, northern Lebanon, southern Schuylkill, southern Northumberland, and Snyder will be an absolute mess likely. I heard they already have outside state electrical companies on standby.
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Two factors to consider:

A majority of the ice storm will be occuring at night. This will allow no influence of solar radiation allowing for a significant accretion of ice on all surfaces.

With heavy rain rates, freezing rain accretion will be a tad more difficult as marginal temperatures around 30-32F make it difficult for heavy freezing rain to accumulate. This is because of the release of latent heat and absent supercooled raindrops. Still though, heavy freezing rain will probably accumulate greatly as this will fall at night.
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blizz- looks like we're in the "sweet spot" for this ice storm if thats what you want to call it. I'm glad I have a wood stove. Do you remember the last time we had an inch of ice? The worst I storm that I can recall is the one from a few years ago.
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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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About Blizzard92

Cornell University- Atmospheric Sciences Student; Central PA SKYWARN Storm Spotter; American Meteorological Society Member; PA CoCoRaHS Branch Member

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Linglestown, PA
Elevation: 520 ft
Temperature: 24.2 °F
Dew Point: 15.2 °F
Humidity: 68%
Wind: Calm
Wind Gust: 9.0 mph
Updated: 10:37 AM EST on January 18, 2014

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