North American Mesoscale Model Verification on January 26-27...
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
Reader Comments
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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.
There's your 3" - 6" snow with ice Breald. NWS Issued an advisory for you. Obviously your Warning criterea is higher than mine.
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...
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.
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.
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.
Down here in Clinton not looking forward to ANY ice...Let it be all snow. I'll dump it in my neighbors yard.......
Blizz, are you thinking of doing a snow totals prediction map?
I still hate you.....just sayin
get a grip or so help me you will be grounded for a month hahahaa
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.
I can feel your love across the cyber stars tonight.
Heavy, Heavy snow blob(not band) south of chicago as the storm begins to wind up.
Looks awesome!
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.
I drove from Arlington to Germantown, flurries Falls Church to 270, light snow on 270 to Gaithersburg, then light snow in Germantown.
Found it..
LGA GFS 18Z:
Link
here's my snow/ice map. Probably will need further fine tuning.
We have light snow in northern Prince George's county.
hahaha nah i'm cool. i wanted to incite a riot and it didn't happen. bummer.
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.
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
where would you draw a line that will at some extended period of time get plain ole rain??? please extend in jersey
Wow, we're kind of dumb. It was right on that same page the entire time.
Come on. Give me over a foot of snow and 1.0in freezing rain. I wanna get out of classes Wednesday. HAHA.
absolutely nothing to report here......Roseland, NJ :)
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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