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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How do you think we'll far in State College?
So now that my little corner of central MA is having another foot (or two?) of snow added to its already 36-40" snowpack, with another noreaster possible this this weekend, it begs the question: Is it possible that we're in for a few more weeks of winter storms every few days? What are the long term models saying about this conveyor belt?
4.5" and the snow is picking up a bit at this moment.
21.1 °F
Snow Freezing Fog
Windchill: 21 °F
Humidity: 84%
Dew Point: 17 °F
Wind: 0.0 mph
Wind Gust: 0.0 mph
Pressure: 30.25 in (Steady)
Visibility: 0.5 miles
up to 26.3F
Campbelltown, PA
do you expect the temps to continue to climb and then stabilize by evening?
Yeah, you know where I stand on this issue!
light Snow
14.5 °F
Windchill: 7 °F
Humidity: 88%
Dew Point: 12 °F
Wind: 5.0 mph from the NNE
Wind Gust: 6.0 mph
Pressure: 30.39 in (Falling)
Visibility: 0.5 miles
31F
Overcast
Misty, dreary day
Sporadic icing, still expecting overnight accumulation of ice through Wednesday afternoon.
Seems like a dud in NYC so far, snowed pretty good to our north, but only about a half inch of snow/sleet and it has been drizzling at around 30 degrees for the past couple of hours. Don't see how we are going to get the 5" forecast by NWS considering that the storm will likely be rain when the main event gets here. Ideas?
I expect them to rise and then actually fall maybe a degree or two after sunset.
You won't get any more snow probably. I never understand why Upton had that much snow forecast. There will be some freezing rain tonight, but I think you change to plain rain relatively quickly. New York City is not a good location for ice storm formation.
You can't win em all! We've been snow hogs here! Speaking of which, are we getting our snow back on Saturday?
Edit: this was pulled from the NWS full loop radar but its basically useless. Might load one frame if that right now. So overwhelmed by traffic I'd imagine
Also noticed how forecast totals have dropped across the board from chicago to areas in the northeast. I'm guessing something must have changed?
Springfield, MA
Hey Blizz, could you tell me where in New Jersey you expect there to be serious icing issues tomorrow? Debating about planning a "snow day"
Probably only northwest New Jersey for the severe iciing.
... Ice Storm Warning in effect until 6 PM EST Wednesday...
... Winter Storm Warning is cancelled...
The National Weather Service in Upton has issued an Ice Storm
Warning... which replaces the Winter Storm Warning that was in
effect.
* Locations... interior portions of the lower Hudson Valley and
northeast New Jersey.
* Hazards... freezing rain and sleet.
* Accumulations... a 1/4 to 1/2 an inch of ice accumulation... by
Wednesday afternoon.
* Timing... snow has tapered off... and occasional light freezing
rain and freezing drizzle will fall this afternoon through this
evening. Widespread freezing rain and sleet will overspread the
region after midnight tonight and will continue through late
Wednesday morning. Freezing rain and sleet will taper off
through Wednesday afternoon.
Good call. I think several NWS offices should do this and changing from winter storm warning to ice storm warning for areas expecting only freezing rain tonight.
Since this is a two part system, perhaps they are waiting to issue the warning until you get closer to the second wave of precipitation towards early tomorrow morning.
I am like 5 miles east of 287 at 500 ft. I feel like I am right on the border, no?
It's possible. Ice storm warning for parts of NY and Ct. As tired as I am of snow, I'll take the snow over freezing rain any day.
Stay safe everyone.
what are you expecting from this storm? rain to move in by late afternoon? then rain with below freezing temps all night into early tomorrow morning? then falling temps and cold the rest of the week?
We are starting to have freezing rain here, too.
25 degrees
Freezing drizzle
0.1" of sleet fell overnight
0.15" of ice now on top of sleet from freezing rain
Ended up getting no snow so far with this storm.
Yep, you got it right. If we do not get above 30F, then I think a pretty significant ice storm is possible. Although the storm in the west is already stronger than models indicated. Not sure if that will draw up more warm air from the south or not. Currently my winds are out of the east.
Sounds good to me.
Cranford, NJ
They need to get it fixed. This is a historic blizzard going to occur in the Midwest including some major cities along with significant iciing from Indianapolis to central Pennsylvania.
Boy I really hope they cancel our class tonight.
Brooklyn, NY
No need for any winter storm warning. Tonights primary precipitation type will be freezing rain for areas north of the Mason-Dixon line and south of I-80. I cannot rule out some sleet, but that will probably be more towards I-80 or a heavier burst of precipitation.
I sort of wish they hadn't linked to HPC off the main weather.gov homepage. Usually only weather dorks go there, but now it's all gummed up!
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