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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Probably around .25in for the immediate Gettysburg area for freezing rain; maybe a tad more.
Still trying to learn! Thanks so much! Could get ugly then where I live....nothing but trees! Ugh!
That chart in comment #596 is pretty neat and shows what all of the models predicted the surface temperatures to be versus what is actually happening. As you can see, we are running a tad colder. Here is the link for a bigger picture of the chart...Link
How bad is .25" of ice?
.5"?
Currently 32.4 degrees and overcast.
Welcome - I'm on the Monmouth/Ocean border. I remember a warning back in the mid 70's when the area was hit with a very bad ice storm. Lost a lot of trees that year.
Albany and Boston in total disagreement on snow totals...
Springfield, MA
i believe you have as much water as is in your pipe from the well. that is what someone once told me. I could be wrong. I know you have at least as much as is in your holding tank.
Hahaha... somehow I had a feeling they did not work too well.
Now you know why Blizz refused to go to State College and decided Cornell was better suited. hahahaaha
I've been reading reports of all rain for the York, PA area. Do you think there will be any ice accumulation overnight? I commute from Harrisburg to York so I'm really worried about being stuck here while the weather down there isn't bad at all.
Blizz - the storm is certainly taking its time getting here. has the time frame been extended for the length of the storm? looking at the radar the Low is still in Illinois. at what point does it transfer its energy to the coast, or does it?
As for the "reports" you have been reading. Don't buy it! One of the first things you will see is that Blizz's forecasts are much better than the talking heads on local stations. The people who claim all rain are looking (and like to always look at) the GFS models which do not do a good job with the cold air damming. York will certainly have less ice than Harrisburg but it does not look to be all rain.
The difficulty you have may be getting out of Harrisburg. If Harrisburg sees the .5"+ that are forecast of freezing rain we may well be looking at an emergency declaration issued in the area that requires "non-essential" vehicles to stay off the road.
If you can get there you will be able to get back as this should end by mid-day and the afternoon will be spent cleaning up.
cchamp6- Your area is some freak microclimate region, lol, but I am thinking heavy sleet is possible for your area given a warm layer aloft and the H85 0C line just to your north.
TheRasberryPatch- HRRR model shows the heaviest out of here before 630am. The damage will be done tonight. It is a quick mover with most of the precipitation ahead of the front; no coastal transfer this time around. QPF is not overly high probably right around .75-.9in.
717WeatherLover- 28.3F here currently with light freezing rain developing on the WGAL radar. This lighter ice will accumulate very well and may actually be worse than the heavier batch.
Wundermap of the comma
Right now just frizzle and a very minor ice coat on trees, cars, and awnings.
GOES East
I don't remember the last time I saw that well defined of a classic "comma"! WOW!
You are correct, lol.
dewpt 27.4F
Campbelltown, PA
welcome Starlight
What's it take, in ice inches, to take down a power line??
and mine is decreasing. now at 28.1F
Cloudy, 27.0F
Temperature slowly but steadily falling from a high of 30.5 at 2:00pm.
Forecast:
Freezing rain, worst 1am - 7am, 0.38" additional glaze ice, Low 28F (note that it's already 27F)
Rain 8am - 3pm, above freezing by 8am, 0.40" plain rain, high 39F.
From Mount Holly Discussion...I suggest that those living along and northwest of a Coatesville PA - Doylestown PA - Somerville NJ - Morristown NJ - Caldwell NJ line be prepared with battery powered equipment for possible power outages Wednesday morning if this is an all freezing rain event through 9 am Wednesday.
BTW, their Warnings / Advisories / Grids / Forecasts all match up now. They must be extremely busy...
Pretty sure there's no exact number on that one, haha.
Figured as much. Thanks. Hope it takes more than half an inch.
Minor ice coat? In my area of Brooklyn it's pretty thick.
The precip will be freezing rain and at times sleet most of the night until changing over to plain rain sometime in the morning hours.
I love your new picture philly. Hopefully your folks will be okay.
Good luck to them and everyone else in the path of this wicked ice maker.
Using 18z GFS 1000mb temperatures as a best EST of surface temperature as it seems
to be a good fit at 23z. Also... the GFS 975mb temperatures keep extreme
NW New Jersey and NE PA subfreezing through the entire storm. As a fwiw...at
1pm...could see Sussex in northwest New Jersey sitting at 30f while Cape May NJ Around 50
and Georgetown DE between 55-60 for an hour or 2. We will run slightly colder
temperatures for the ridge area in an update shortly. Am not using the NAM
verbatim as it might be a tad on the cold side.
of interest: maximum modeled pressure falls east of the Apps at 12z Tuesday by
both 12z GFS/NAM over del Bay. That flags a wind in most of New Jersey
staying NE instead of turning east prior to 12z...thereafter
turning light northwest midday. Exception Cape May County. At 930 PM may
add thunder to the forecast for Wednesday morning. Tt are huge...would start
with del and adjacent CW for an event between 12z-15z there...isolated severe
anyone? GFS ml cape only to 200j east of Delaware-Maryland-Virginia at 18z but its Worth
a further review. Also... can see wind gusts 35-40 kts in vicinity
Cape May to southern del Wednesday eve?
If I remember cchamp, I live near you in the freak microclimate region
New Milford CT
light frozen precip
24.1 °F
Overcast
Windchill: 24 °F
Humidity: 85%
Dew Point: 20 °F
Wind: 0.0 mph
Wind Gust: 1.0 mph
Pressure: 30.19 in (Steady)
Visibility: 4.0 miles
current temp 28f
current dew 28f
dewpt. 26.5F
light mist
Campbelltown, PA
Wedge of warm air seeping up from the southwest in the southern half of the western 1/3 of Pennsylvania. Should be the only part of PA above freezing right now. 40F creeping into extreme southwest PA.
In the teens across the Northern Tier and northeastern Poconos of PA.
Hurrican Schwartz lol. I chatted with him online, sent me his "Philadelphia Area Weather Book."
As for the right coast: Front is shifting further northward. I think I'll stay with my earlier prediction. Temps in some places that are right under the advancing arm of the storm are in the 50s. This entrained warm air can really foul up even what seem like easy calls at the last moment. This is not an easy call, Therefore, I shall step out rather cautiously on my front stoop in Brooklyn tomorrow morning.
Good luck to all of you to the north and west. Looks like a doozy.
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