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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Agreed. Can't last for too long this time, though, with temps rising again by the end of the week.
Currently 57 degrees, temp hasn't moved much in the last couple of hours.
Yeah, out with the NAM! 73.2F in Germantown, MD right now. I want to go out and play, but am dutifully working.
Nice Picture, P. Is that from the Hudson Line?
I guess old man winter is about to wake up from his nap. 22 tomorrow night, brrr
That incoming wind storm looks pretty intense. Gusts could surpass 60 mph.
This warmth will soon be a distant memory.
...STRONG THUNDERSTORMS IN
LITCHFIELD COUNTY IN NORTHERN CONNECTICUT...
AT 643 PM EST...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
THUNDERSTORM CAPABLE OF PRODUCING UP TO HALF INCH HAIL...WINDS UP TO
50 MPH...AND HEAVY RAIN. THIS STORM WAS LOCATED NEAR NEW
PRESTON...OR ABOUT 8 MILES NORTHEAST OF NEW MILFORD...AND MOVING
EAST AT 65 MPH. ANOTHER STORM WAS LOCATED NEAR NEW HARTFORD...AND
MOVING EAST ALSO AT 65 MPH.
THUNDERSTORMS WILL BE NEAR...
THOMASTON AND 7 MILES SOUTH OF TORRINGTON BY 655 PM EST
TERRYVILLE AND 9 MILES SOUTH OF NEW HARTFORD BY 700 PM EST
Current:
Sacandaga Lake, Broadalbin, New York (PWS)
Mostly Cloudy
43.5 °F
Mostly Cloudy
Windchill: 36 °F
Humidity: 52%
Dew Point: 27 °F
Wind: 18.0 mph from the West
Wind Gust: 20.0 mph
Pressure: 29.61 in (Falling)
Visibility: 10.0 miles
we get two snow events the first sun mon maybe into tuseday near half a foot or more then thurs second with another 6 inchest follow by another warm up till major event to ring in march
right now it is 62F with windy conditions and a full moon
Looking forward to more snow Monday!
I have a high gust of 38mph and the sustained winds are 18mph
... High Wind Warning in effect from 5 am to 9 PM EST Saturday...
The National Weather Service has issued a High Wind Warning. It is
in effect from 500 am until 900 PM Saturday. The high wind watch
is no longer in effect.
* Locations... the High Wind Warning is in effect for northeastern
Maryland, Delaware, much of eastern Pennsylvania and much of New
Jersey.
* Hazards... gusty to locally damaging winds are expected on
Saturday. Numerous tree limbs and utility lines will be knocked
down. A few weak trees may also fall.
* Winds... northwest sustained at 20 to 35 mph with gusts up to 60
mph.
* Timing... the highest wind speeds and gusts are expected to occur
between 10 am and 5 PM on Saturday. However, a brief period of
gusts to 40 or 45 mph is anticipated this evening.
* Impacts... widespread power outages are possible. Downed trees
and tree limbs could damage structures and vehicles. The winds
will make it difficult to drive high profile vehicles,
especially on bridges and overpasses.
Precautionary/preparedness actions...
A High Wind Warning means that a hazardous high wind event is
expected. Sustained wind speeds of at least 40 mph or gusts of 58
mph or more can lead to property damage.
(Official primary NWS stations only)
* = Broken
(T) = Tied
Charlotte NC: 78 (T)
Greensboro NC: 78*
Greenville SC: 78*
Raleigh NC: 78*
Arlington [DC] VA: 77*
Danville VA: 77 (T)
Richmond VA: 77 (T)
Dulles VA: 75*
Lynchburg VA: 75 (T)
Wallops Island VA: 73*
Atlantic City NJ: 72*
Roanoke VA: 72 (T)
Trenton NJ: 72 (T)
Newark NJ: 71*
Reading PA: 71*
Harrisburg PA: 69 (T)
Philadelphia PA: 69 (T)
Allentown PA: 67*
N. Queens [LAG] NY: 67*
Pittsburgh PA: 66*
S. Queens [JFK] NY: 66*
Williamsport PA: 66*
Blacksburg VA: 65 (T)
Islip NY: 60*
Binghamton NY: 58*
------------------------------------------------- ----
Looks like the second low for Monday won't be as heavy as the GFS and ECMFW were projecting a few days ago. They were hinting that low #2 would get cranking from IN and east, doesn't look like it gets cranking until well off the coast. As far as model guidance, it looks like there will be a light to possibly localized moderate snowfall of 1"-5" somewhere in the mid-atlantic.
You guys have a high wind warning out there too. MD + N VA also.
warnings have been issued
Warnings
City of Toronto
Friday 18 February 2011
Wind warning
..West winds gusting to 100 km/h this evening and overnight..
A second cold front associated with a strong low pressure system over Northern Ontario is advancing across Southern Ontario this evening. West winds just ahead and behind the front will increase to 60 km/h with gusts to 100 this evening before diminishing somewhat early Saturday morning.
Regions south and east of Northern Lake Huron and Georgian Bay is experiencing light snow and northwesterly winds 40 km/h with gusts up to 70. Even though the snowfall is not heavy, many areas are and will experience visibilities reduced to less than 0.5 km in blowing snow and icy stretches of highways into Saturday morning. As colder air floods over the lakes and the wind shifts to more northwesterly Saturday morning, lake effect snow bands will form. Snow squall watches and warnings are not anticipated at this time as the meandering nature of the lake effect bands will limit amounts at any given location..However this will result in sudden very low visibility at times in snow and blowing snow continuing over many localities all the way into Saturday afternoon. Motorists should expect poor winter driving conditions due to low visibility in snow and blowing snow especially in areas near Northern Lake Huron and Georgian Bay. Travel plans should be adjusted accordingly.
00Z GFS just killed low #2 while 00z NAM just exploded it for 12" - 18" in the southern half of PA just like the GFS & ECMFW wanted two days ago. Obviously not buying the NAM at the moment. Would have to wait and see on future runs and other model support before buying anything close to that.
Plus, the NAM lays that 12"-18" down in a 12-hour period Monday night and Tuesday morning. Suspicious about some sort of feedback problems. We'll see if it continues for the 06Z and 12Z runs.
Winds are strong out there this morning, 27 mph at the moment with gusts over 40.
...SNOW SQUALLS IN DUTCHESS AND LITCHFIELD COUNTIES...
AT 738 AM EST...SNOW SQUALLS WERE DETECTED BY RADAR. THESE SNOW
SQUALLS WERE LOCATED ALONG A LINE EXTENDING FROM 13 MILES SOUTHWEST
OF KENT TO LITCHFIELD...OR ALONG A LINE EXTENDING FROM PAWLING TO
TORRINGTON...AND MOVING SOUTHEAST AT 30 MPH.
SNOW SQUALLS WILL BE NEAR...
THOMASTON AND TERRYVILLE BY 750 AM EST
WOODBURY CENTER...WOODBURY AND OAKVILLE BY 755 AM EST
SNOW SQUALLS PRODUCING A 15 TO 30 MINUTE PERIOD OF HEAVY SNOW AND
NEAR WHITE OUT CONDITIONS CAN BE EXPECTED. GENERALLY AN INCH OR LESS
OF SNOW WILL ACCUMULATE...BUT THE SEVERITY OF THE SNOW SQUALLS WILL
SIGNIFICANTLY REDUCE VISIBILITY AND CAUSE EXTREMELY HAZARDOUS DRIVING
CONDITIONS.
We had lightning in Plainville. Last night was interesting, full moon, partially shrouded in clouds with heat lightning and snowy fields. Really odd as I have never seen anything like that before.
Lucky!
That 00Z Radar site NAM snowfall output map is a little too zoomed in. If zoomed out, it shows 12"-18" over the southwest half of PA from Low #2 but NAM doesn't go far enough out to show the heavy snow extention any farther east. There's not gonna be that much snow anyway so it really doesn't matter.
It looks like there will be two lows, the first that tracks across northern Pennsylvania and through Poughkeepsie, the second low tracks through the central or southern mid-atlantic. The GFS, NAM, ECMFW, CMC all show this scenario. There is some tight consensus regarding low #1, but still uncertainty in strength and exact location of low #2.
With low #1, the 06Z GFS targets the southern tier of NY, the northern tier of PA and northwestern NJ with 6"-12". With low #2, it targets the Del-Mar-Va with a bullseye of 6"-8". It puts southeastern PA and parts of southern NJ in a snow hole between the two systems.
With low #1, the 06Z NAM targets the southern tier of NY, the northern tier of PA and extreme northwestern NJ with 10"-18". With low #2, it targets southwestern PA with 6"-12" and Maryland and Delaware with 6"-10". It puts east central PA and central NJ in a snow hole between the two systems.
That 00Z NAM radar site snowfall overlay is showing the accumulations from low #1 over northern PA etc... with accumulations from low #2 over southern PA. The 00Z NAM brought the two low pressures close to one another and therefore it looks like snowfall from 1 low instead of 2 seperate ones.
The 00Z GFS never developed low #2, which could happen, but brought it back on the 06Z focusing on the Del-Mar-Va region.
Viewing: 1551 - 1601
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